Comparison of different seismic retrofitting methods

LONDON’S GLOBAL UNIVERSITY

Comparison of different seismic retrofitting methods for RC frames by

Athanasios Vavakas

Supervisor: Professor Dina D’Ayala Academic Year 2015-16

University College London Department of Civil, Environmental & Geomatic Engineering

Submitted in partial fulfilment of the requirements for the degree of Master of Science in Earthquake Engineering with Disaster Management of University College London, September 2016

I hereby declare that this dissertation is all my own original work and that all sources have been acknowledged and that I have followed the academic practices that are stated by University College London regulations.

(Signature)

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Abstract Reinforced concrete structures are one of the main building technologies for ages in many countries through the world, and especially in Mediterranean countries, such as Greece and Italy. Those countries are very prone to earthquake events, however a high percentage of the current building stock, has been designed to resist gravity load only (mostly buildings before 1970s), since they have designed before the introduction of seismic design codes. The need of strengthening those structures to be able to sustain the earthquake loading has become increasingly. One of the most advanced retrofitting methods is the use of fibre reinforced polymers as reinforcement for concrete structures. One of the main advantages which make this method preferable, is the ease in installation and the lightness of the material. Various codes and guidelines have been implemented, providing suggestions about the design of FRP reinforcement and the installation. This study aims to critically assess the design and installation process that are suggested from two of the most advanced codes (Italian & American), to identify differences between the codes and any discrepancies and gaps on the on-site installation and implementation procedures that are provided. To compare the design procedures, a pushover analysis of building that have been designed using earlier designed codes will be performed in SeismoStruct to identify the performance objective and decide the appropriate retrofit strategy. The building's structural elements will be strengthened with FRP reinforcement for both design procedures of the two codes, in order to achieve the performance levels that this structure needs to comply with. The installation and implementation procedures will be critically assessed by carrying out an extensive literature review comparing the practical aspects of the various experimental strengthening methods and the installation suggestions and procedures that are proposed by the current codes.

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Acknowledgements I would like to express my sincere gratitude to the following people: My supervisor, Professor Dina F. D'Ayala, for giving me the opportunity of working with her and her assistance throughout this project. With her experience, knowledge and suggestions, understood from a few meetings what exactly I am most interested about and structured a project, meeting perfectly my needs. Daniel Pohoryles, whose suggestions and assistance during the project was vital for the implementation of this thesis. My MSc programme director, Dr. Carmine Galasso, which make me feel comfortable in this university since day one and his help whenever it was needed. My classmates and brothers, Alvaro, Andres, Fernando, Hassib, Keith, Nicola, Orlando, Pietro, for their help through this year and the amazing times we had together. It was an honour guys. Last, but most importantly, my parents for their unconditionally support and belief in me since the day I was born and for teaching me that I can achieve everything if I just believe in myself.

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Mathematical Notations Uppercase Roman letters Notation

Definition

Ac

cross-sectional area of concrete

Af

area of FRP reinforcement

As1

area of steel reinforcement subjected to tension

As2

area of steel reinforcement subjected to compression

Ec

Young’s modulus of elasticity of concrete

Ef

Young’s modulus of elasticity of FRP reinforcement

Es

Young’s modulus of elasticity of steel reinforcement

FC

confidence factor

MRd

flexural capacity of FRP-strengthened member

MSd

Factored moment

NRcc,d

axial capacity of FRP-confined concrete member

NSd

factored axial force

VRd

shear capacity of FRP-strengthened member

VRd,c

concrete contribution to the shear capacity

VRd,s

steel contribution to the shear capacity

VRd,f

FRP contribution to the shear capacity

VSd

factored shear force

Lowercase Roman letters Notation

Definition

b

width of the section

bf

width of FRP reinforcement

fc

concrete compressive strength (cylindrical)

fccd

design strength of confined concrete

fcd

design concrete compressive strength

fcm

mean value of concrete compressive strength

fctm

mean value of concrete tensile strength

ffd

design strength of FRP reinforcement

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ffdd

design debonding strength of FRP reinforcement (mode 1)

ffdd,2

design debonding strength of FRP reinforcement (mode 2)

ffed

effective design strength of FRP shear reinforcement

ffib

characteristic strength of the fiber itself

ffk

characteristic strength of FRP reinforcement

ffpd

design debonding strength of FRP reinforcement

f1

confining lateral pressure

fc

compressive stress in concrete, psi mean ultimate tensile strength of FRP based on a population of 20 or more tensile tests per ASTM D3039

fc′

specified compressive strength of concrete

fy

yield strength of longitudinal steel reinforcement

fyd

design yield strength of longitudinal steel reinforcement

h

height of the section

keff

coefficient of efficiency for confinement

kH

coefficient of efficiency in the horizontal direction

kV

coefficient of efficiency in the vertical direction

kα

coefficient of efficiency related to the angle a of fibers

lb

bond length

led

optimal bond length

pf

spacing of FRP strips or discontinuous FRP U-wraps

s

interface slip

su

interface slip at full debonding

tf

thickness of FRP laminate

x

distance from extreme compression fiber to neutral axis

κb

efficiency factor for FRP reinforcement in determination of εccu (based on geometry of cross section)

κv

bond-dependent coefficient for shear

κε

efficiency factor equal to 0.55 for FRP strain to account for the difference between observed rupture strain in confinement

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Lowercase Greek letters Notation

Definition

γm

partial factor for materials

γRd

partial factor for resistance models

εo

concrete strain on the tension fiber prior to FRP strengthening

εc

concrete strain on the compression fiber

εccu

design ultimate strain of confined concrete

εco

concrete strain on the tension fiber prior to FRP strengthening

εf

strain of FRP reinforcement

εfd

design strain of FRP reinforcement

εfd,rid

reduced design strain of FRP reinforcement for confined members

εfk

characteristic rupture strain of FRP reinforcement

εfdd

maximum strain of FRP reinforcement before debonding

εs1

strain of tension steel reinforcement

εs2

strain of compression steel reinforcement

εyd

design yield strain of steel reinforcement

η

conversion factor

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Table of Contents Abstract ......................................................................................................................................ii Acknowledgements ....................................................................................................................ii Mathematical Notations ........................................................................................................... iii Chapter. 1

Introduction .......................................................................................................... 2

Chapter. 2

Literature Review................................................................................................. 4

2.1 FRP debonding ................................................................................................................. 4 2.1.1 Overview ................................................................................................................... 4 2.1.2

Plate/sheet end debonding.................................................................................... 5

2.1.3

Intermediate-crack-induced (IC) debonding ........................................................ 6

2.1.4

Mitigation measures ............................................................................................. 7

2.1.5

FRP debonding strengthening according CNR-DT 200 R1/2013 ....................... 7

2.1.6

FRP debonding strengthening according ACI 440.2R-08 ................................... 8

2.1.7

Comparison of the codes ...................................................................................... 9

2.1.8

FRP system installation according ACI 440.2R-08 & CNR-DT 200 R1/2013 . 10

2.1.9

Comparison of the codes .................................................................................... 12

2.1.10

Preparation of concrete surface according ReLUIS 2009.................................. 13

2.2

FRP Flexural strengthening of reinforced concrete elements ................................... 15

2.2.1 Overview ................................................................................................................. 15 2.2.2

Reinforced concrete beams ................................................................................ 15

2.2.3

Reinforced concrete column .............................................................................. 15

2.2.4

Flexural Strengthening according ACI 440.2R-08 ............................................ 16

2.2.6

Flexural Strengthening according CNR-DT 200 R1/2013 ................................ 19

2.2.7

Flexural strengthening of RC beams using anchors .......................................... 22

2.2.8

Comparison of the proposing methods .............................................................. 26

2.3

Flexural Strengthening of RC Columns Using Anchors ........................................... 27

2.3.1

Comparison of the proposing methods .............................................................. 29

2.3.2

FRP flexural strengthening implementation according ACI 440-F ................... 30

2.3.3

FRP flexural strengthening implementation according ReLUIS 2009 .............. 31

2.3.4

Comparison of the implementation procedures ................................................. 33

2.4

FRP confining of reinforced concrete elements ........................................................ 34

2.4.1

Overview ............................................................................................................ 34

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2.4.2

FRP confinement according CNR-DT 200 R1/2013 ......................................... 39

2.4.3

FRP confinement according ACI 440.2R-08 ..................................................... 40

2.4.4

Comparison of Codes ......................................................................................... 40

2.4.5

FRP confinement implementation according ACI 440-F .................................. 43

2.4.6

FRP confinement implementation according ReLUIS 2009 ............................. 44

2.5

FRP shear strengthening of reinforced concrete elements ........................................ 46

2.5.1 Overview ................................................................................................................. 46 2.5.2 Failure modes .......................................................................................................... 48 2.5.3 Shear Strengthening according ACI 440.2R-08 ...................................................... 50 2.5.4

Shear Strengthening according CNR-DT R1/2013............................................ 51

2.5.5

Comparison of Codes ......................................................................................... 52

2.5.6

Shear Strengthening implementation according ACI 440-F .............................. 54

2.5.7

Shear Strengthening implementation according ReLUIS 2009 ......................... 54

2.5.8

Special Anchorage for Shear Strengthening ...................................................... 57

2.6

FRP strengthening design of beam-column joints .................................................... 62

2.6.1 Overview ................................................................................................................. 62 2.6.2 Shear and bond strengthening.................................................................................. 62 2.6.3 Shear strengthening of two-dimensional exterior joints .......................................... 62 2.6.4 beams

Shear strengthening of two-dimensional exterior joints with slabs of transverse 63

2.6.5

Limitations of the proposed methods ................................................................. 64

2.6.6

FRP strengthening of three-Dimensional beam-column joint ........................... 64

2.6.7

Strengthening according CNR-DT 200 R1/2013 ............................................... 67

2.6.8

Joint strengthening implementation according ReLUIS 2009 ........................... 67

2.6.9

Joint strengthening implementation according ACI 440F ................................. 71

Chapter 3. 3.1 3.1.1

Research objectives and Methodology .............................................................. 72

Case study description ............................................................................................... 72 Description of the structure ................................................................................... 72

3.2

Material Properties .................................................................................................... 73

3.3

Actions ...................................................................................................................... 74

3.3.1

Vertical Actions ..................................................................................................... 74

3.3.2

Floor Masses .......................................................................................................... 75

3.3.3

Seismic Loading .................................................................................................... 76

3.3.3.1 Behavior factors for horizontal seismic actions ................................................. 77

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3.3.3.2 Estimation of the fundamental period ................................................................ 77 3.3.3.3 Lateral force method ............................................................................................. 78 3.3.3.3 Base shear force ................................................................................................. 79 3.4

Numerical Model & Pushover Analysis.................................................................... 80

3.4.1

Distribution of the horizontal seismic forces for the Pushover analysis ............... 80

3.4.2

Elastic Response Spectra ....................................................................................... 81

3.5

Static Pushover analysis in SeismoStruct.................................................................. 82

3.5.1

Identification of the Performance level design ...................................................... 83

3.5.2

Level of performance improvement ...................................................................... 84

3.6

Derivation of Capacity Curve ................................................................................... 87

3.6.1

Properties of the equivalent SDoF system ......................................................... 87

3.6.2

Idealization of Capacity Curve .......................................................................... 88

3.7

Transformation to Spectral Acceleration-Spectral Displacement format ................. 91

3.8

Derivation of Inelastic Spectra .................................................................................. 91

Chapter 4. 4.1

Performance of the Original RC building ................................................................. 93

4.1.1 4.2

Results analysis .................................................................................................. 93

Inter-Story Drift ................................................................................................. 94

Strengthening of RC columns (top) using FRP ......................................................... 95

4.2.1

Confinement of columns .................................................................................... 95

4.2.2

Flexural strengthening of columns..................................................................... 98

4.2.3

Shear strengthening of columns ....................................................................... 100

4.2.4

Modelling in SeismoStruct .............................................................................. 101

4.2.5

Performance Criteria ........................................................................................ 102

4.2.6

Capacity Curve of the strengthened structure .................................................. 103

4.2.7 Transformation of MDoF system to an equivalent SDoF system ......................... 104 4.2.8 Performance of the retrofitted structure................................................................. 104 4.2.9 Inter-Story Drift ratios ........................................................................................... 106 4.3

Strengthening of RC columns (bottom) using FRP ................................................ 107

4.3.1

Confinement of columns .................................................................................. 107

4.3.2

Flexural strengthening of columns................................................................... 108

4.3.3

Shear strengthening of columns ....................................................................... 109

4.3.4

Modelling in SeismoStruct .............................................................................. 110

4.3.5

Performance Criteria ........................................................................................ 110

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4.3.6

Capacity Curve of the strengthened structure .................................................. 111

4.3.7 Transformation of MDoF system to an equivalent SDoF system ......................... 112 4.3.8

Performance of the retrofitted structure ........................................................... 112

4.3.9

Inter-Story Drift ratios ..................................................................................... 114

4.4

On-site implementation according ReLUIS 2009 ................................................... 115

4.5

On-site implementation according ACI 440F ......................................................... 116

Chapter 5.

Discussion ........................................................................................................ 118

5.1

Weak Beam-Strong Column Design ....................................................................... 118

5.2

Comparison of design codes ................................................................................... 119

5.3

Implementation procedure....................................................................................... 120

Chapter 6.

Conclusion & Future Recommendations ......................................................... 121

Chapter 7.

References ........................................................................................................ 123

Appendix A. Supplementary Data ............................................................................................ 1 A.1 Damage state tables ............................................................................................................. 1 A.1.1 Damage state tables for column strengthening (Floors 4-6) ..................................... 1 A.1.2 Damage state tables for column strengthening (Floors 0-3) ..................................... 3 A.2 Idealized Capacity Curves ............................................................................................... 5 A.2.1 Idealized Capacity Curves for column strengthening (Floors 4-6) .......................... 5 A.2.1 Idealized Capacity Curves for column strengthening (Floors 0-3) .......................... 7

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List of Figures Figure 2-1: Types of debonding in FRP strengthened RC members (Source: Buyukozturk et al., 2004) .................................................................................................................................... 4 Figure 2-2: Debonding failure modes of an FRP-plated RC beam (Source: Teng & Chen, 2007) .......................................................................................................................................... 5 Figure 2-3: FRP rupture (Source: Teng & Chen, 2007) ............................................................ 6 Figure 2-4: FRP debonding from flexure-shear crack ............................................................... 6 Figure 2-5: shear failure and debonding .................................................................................... 6 Figure 2-6: Maximum force transferred between FRP and concrete (Source: CNR-DT 200 R1/2013) .................................................................................................................................... 7 Figure 2-7: Graphical representation of the guidelines for allowable termination points of a three-ply FRP laminate (Source: ACI 440.2R-08) .................................................................. 10 Figure 2-8: Debonding and delamination of externally bonded FRP systems (ACI 440.2R-08) .................................................................................................................................................. 16 Figure 2-9: Elastic strain and stress distribution (Source: ACI 440.2R-08) ............................ 17 Figure 2-10: FRP flexural strengthening: debonding failure modes (Source: CNR-DT 200 R1/2013) .................................................................................................................................. 19 Figure 2-11: Failure mode of a RC member Strengthened with FRP (Source: CNR-DT 200 R1/2013) .................................................................................................................................. 19 Figure 2-12: Anchor system dimensions (Source: Chahrour & Soudki, 2005) ....................... 22 Figure 2-13: Ductile anchor system (Source: Galal & Mofidi, 2009) ..................................... 22 Figure 2-14: Anchor system dimensions (Source: Pellegrino &Modena, 2009) ..................... 23 Figure 2-15: Proposed hybrid FRP/ductile steel anchor system (Galal & Modifi, 2009) ....... 23 Figure 2-16: Cross section of anchored specimen F4_2Φ9 (Source: Bournas et al., 2015) ... 28 Figure 2-17: Cross section of anchored specimen 3_1.5 (Source: Vrettos et al., 2013) ......... 28 Figure 2-18: Column Retrofit Scheme (Source: ACI 440-F) .................................................. 30 Figure 2-19: Beam Retrofit Scheme (Source: ACI 440-F) ...................................................... 30 Figure 2-20: Cross-section A-A ............................................................................................... 31 Figure 2-21: Cross-section C-C ............................................................................................... 31 Figure 2-22: Flexural reinforcement with a composite beam with end anchorages (ReLUIS, 2009) ........................................................................................................................................ 31 Figure 2-23: RC rectangular column confinement using FRP sheets (Source: Alkhrdaji T., 2015) ........................................................................................................................................ 34 Figure 2-24: RC circular column confinement using FRP sheets (Source: Sika Corporation)34 Figure 2-25: Strengthening Of Circular Columns versus Rectangular Columns (Source: Chauhan & Umravia, 2012) ..................................................................................................... 35 Figure 2-26: Effect of corner radius (Source: Chauhan & Umravia, 2012) ............................ 36 Figure 2-27: Bonded specimens (Source: Pham et al., 2013) .................................................. 38 Figure 2-28: Segment of concrete (Source: Pham et al., 2013) ............................................... 38 Figure 2-29: Confinement of rectangular sections (Source: CNR-DT 200 R1/2013) ............. 42 Figure 2-30: Equivalent circular cross section (Source: ACI 440.2R-08) ............................... 43 Figure 2-31: FRP wrapping for confinement and shear strengthening, Left a): Discontinuous wrapping, Right b): Continuous wrapping (Source: ACI440-F) ............................................. 43

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Figure 2-32: diagonal shear crack in a reinforced concrete beam near its connection to a column...................................................................................................................................... 46 Figure 2-33: Cracking due to shear failure in (RC) laboratory test beam ............................... 46 Figure 2-34: FRP Shear Reinforcement Configurations: (a) Bonded Surface Configurations; (b) FRP Reinforcement Distributions; c) Fiber Orientations; (d) Pseudolsotropic FRP Reinforcement Schemes (Source: Khalifa et al. 1998) ............................................................ 47 Figure 2-35: Shear failure modes of FRP U-jacketed RC beams (Source: Teng, 2001) ......... 48 Figure 2-36: Debonding failure of the U-shaped beam (Source: Ferreira et al. 2016) ............ 49 Figure 2-37: FRP strips with angle β=90̊ ................................................................................. 51 Figure 2-38: FRP strips with angle β between 0̊ and ̊ (CNR-DT 200 R1/2013)...................... 51 Figure 2-39: Cross section of FRP strengthened member using complete wrapping .............. 53 Figure 2-40: Typical wrapping schemes for shear strengthening using FRP laminates (Source: ACI 440.2R-08) ....................................................................................................................... 53 Figure 2-41: Cross section of FRP strengthened member using U-wrapping ......................... 53 Figure 2-42: FRP wrapping for confinement and shear strengthening, Left: Full-wrapping through the T-beam, Right: Field implementation (Source: ACI 440-F) ............................... 54 Figure 2-44: Continuous FRP wrapping in internal beam with fan-shaped anchors (Source: ReLUIS 2009) .......................................................................................................................... 55 Figure 2-46: Discontinuous shear reinforcement with FRP composites of an internal beam (Source: ReLUIS 2009) ........................................................................................................... 55 Figure 2-43: Continuous FRP wrapping in internal beam (Source: ReLUIS 2009) ................ 55 Figure 2-45: Discontinuous shear reinforcement with FRP composites of an internal beam with fan-shaped anchors (Source: ReLUIS 2009) ................................................................... 55 Figure 2-47: Configuration of anchors for beams and dimensions of embedment depth and length of anchorages (left: horizontally placed, right: inclined) (Source: Koutas & Triantafillou 2013) ................................................................................................................... 57 Figure 2-48: FRP Anchor types and dowel stress distribution (Source: Kim & Smith, 2010) 58 Figure 2-49: Left: U-wrap specimen cross section; Right: cross section of specimen strengthened with CFRP L-strips and anchored with CFRP rope (Source: El-Saikaly et al. 2015) ........................................................................................................................................ 58 Figure 2-50: CFRP ropes (Source: El-Saikaly et al., 2015) ..................................................... 58 Figure 2-51: Strengthening schemes proposed by Antonopoulos and Triantafillou (Source: Antonopoulos & Triantafillou, 2003) ...................................................................................... 63 Figure 2-52: Strengthening schemes proposed by Le-Trung et al. (Source: Le-Trung et al. 2010) ........................................................................................................................................ 63 Figure 2-53: Dimensions and application sequence of FRP sheets for one-way exterior joints (Source: Akgüzel et al. 2011) .................................................................................................. 64 Figure 2-54: Details of FRP anchor dowels installed in slab and beam elements (Source: Akguzel & Pampanin, 2012).................................................................................................... 65 Figure 2-55: Dimensions and application sequence of FRP sheets for corner exterior joints of the improved method (Source: Akguzel & Pampanin, 2012) .................................................. 65 Figure 2-56: a) Diagonal bands with unidirectional metallic fabric of intermediate node (external view) b) L-shaped carbon fiber fabric placed at the intersection of quadriaxial beams with the pillar of a corner node (internal view) ............................................................ 67

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Figure 2-57: Quadriaxial balanced fabric of carbon fiber placed in correspondence of the node panel on corner node (the quadriaxial fabric is also disposed on the inner face of the emerging beam, not visible in the figure) ................................................................................ 68 Figure 2-58: Quadriaxial balanced fabric of carbon fiber placed in correspondence of the panel of an intermediate node.................................................................................................. 68 Figure 2-59: Column confinement of corner node .................................................................. 68 Figure 2-60: Column confinement of an intermediate node .................................................... 68 Figure 2-61: Shear reinforcement with U-shaped configuration at the beam ends of an intermediate node ..................................................................................................................... 69 Figure 2-62: Shear reinforcement with U-shaped configuration at the beam ends of a corner node .......................................................................................................................................... 69 Figure 2-63: Laboratory Retrofitted Unit (Akguzel et al., 2008) ............................................ 71 Figure 2-64: Conceptual FRP strengthening detail (Source: ACI 440F, Chapter 13, 2014) ... 71 Figure 3-1: Side view (y-direction).......................................................................................... 72 Figure 3-2: Plan view of structure ........................................................................................... 72 Figure 3-3: Column cross-section (Floors 4-6) ........................................................................ 73 Figure 3-4: Column cross-section (Floors 0-3) ........................................................................ 73 Figure 3-5: Beam Cross-Section .............................................................................................. 73 Figure 3-6: Seismic hazard map of Greece (Source: EAK 2000) ............................................ 76 Figure 3-7: Elastic Acceleration Response Spectrum .............................................................. 82 Figure 3-8: Capacity curve for MDof system .......................................................................... 87 Figure 3-9: Transformation of MDoF into equivalent first-mode SDoF (Source: Filiatrault A. et.al 2013) ................................................................................................................................ 87 Figure 3-10: MDoF and SDoF building capacity curves ......................................................... 88 Figure 3-11: Elastic-Perfectly plastic idealisation of capacity curve of equivalent SDoF system in pushover analysis (Source: EC8-Annex B3) ........................................................... 89 Figure 3-12: Idealized Capacity Curve .................................................................................... 90 Figure 3-13: SDoF idealized and capacity curve graph ........................................................... 90 Figure 3-14: Acceleration – Displacement idealized capacity curve....................................... 91 Figure 4-1: Inelastic Displacement Demand............................................................................ 93 Figure 4-2: Inter-story drift ratio of the building ..................................................................... 94 Figure 4-3: Roughened ends according CNR-DT, Radius 25mm ........................................... 96 Figure 4-4: Detailed roughening profile .................................................................................. 96 Figure 4-5: Roughened ends according ACI, Radius 20mm ................................................... 96 Figure 4-6: Capacity curve for the retrofitted buildings (CNR-DT example) ....................... 103 Figure 4-7: Capacity curve for the retrofitted buildings (ACI example) ............................... 103 Figure 4-8: Inelastic Displacement Demand (CNR-DT) ....................................................... 105 Figure 4-9: Inelastic Displacement Demand (ACI) ............................................................... 105 Figure 4-10: Inter-story drift ratio of the building (ACI example) ........................................ 106 Figure 4-11: Inter-story drift ratio of the building (CNR-DT example) ................................ 106 Figure 4-12: Capacity curve for the retrofitted buildings (CNR-DT example) ..................... 111 Figure 4-13: Capacity curve for the retrofitted buildings (ACI example) ............................. 111 Figure 4-14: Inelastic Displacement Demand (CNR-DT) ..................................................... 112 Figure 4-15: Inelastic Displacement Demand (ACI) ............................................................. 113

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Figure 4-16: Inter-story drift ratio of the building (CNR-DT example) ................................ 114 Figure 4-17: Inter-story drift ratio of the building (CNR-DT example) ................................ 114 Figure 4-18: On-site implementation procedure steps according ReLUIS 2009 ................... 115 Figure 4-19: On-site implementation procedure steps according ACI 440F ......................... 116 Figure 5-1: Seismostruct model for collapse step in top & bottom retrofit case, strong beamweak column mechanism ....................................................................................................... 119 Figure 99: SDoF idealized and capacity curve graph (CNR-DT example) ............................... 5 Figure 100: SDoF idealized and capacity curve graph (ACI example) ..................................... 5 Figure 101: Acceleration – Displacement idealized capacity curve (CNR-DT example) ......... 6 Figure 102: Acceleration – Displacement idealized capacity curve (ACI example) ................. 6 Figure 103 SDoF idealized and capacity curve graph (CNR-DT example) .............................. 7 Figure 104: SDoF idealized and capacity curve graph (ACI example) .................................... 7 Figure 105 : Acceleration – Displacement idealized capacity curve (CNR-DT example) ........ 8 Figure 106: Acceleration – Displacement idealized capacity curve (ACI example) ................. 8

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List of Tables Table 2-1: Experimental information and results.................................................................... 25 Table 2-2: Experimental information ...................................................................................... 27 Table 2-3: Summary of experimental results ........................................................................... 28 Table 2-4: Representative data on FRP-retrofitted RC columns............................................. 36 Table 2-5: Representative data on RC columns retrofitted with concrete cover and FRP ..... 37 Table 2-6: Results of strengthening procedure for both experiments...................................... 60 Table 3-1: Mass distribution of the 6-storey building ............................................................. 75 Table 3-2: Horizontal force distribution on each frame .......................................................... 81 Table 3-3: Performance objectives, damage and damage descriptions for RC buildings ...... 83 Table 3-4: Performance objectives matrix for new buildings (SEAOC 1999) ........................ 84 Table 3-5: NZSEE Risk Classifications and Improvement Recommendations (NZESEE, 2006) .................................................................................................................................................. 85 Table 3-6: Damage states of the Seismostruct model .............................................................. 86 Table 4-1: Inter-story drift values............................................................................................ 94 Table 4-2: Product characteristics .......................................................................................... 96 Table 4-3: Strength Reduction and Material Safety Factors for ACI & CNR-DT .................. 97 Table 4-4: Summary of strengthening results .......................................................................... 98 Table 4-5: NSM bar properties ................................................................................................ 98 Table 4-6: CFRP anchor properties ........................................................................................ 99 Table 4-7: Flexural strengthening moment capacities .......................................................... 100 Table 4-8: Orientation angles used in ACI & CNR-DT ........................................................ 100 Table 4-9: Shear strengthening results .................................................................................. 101 Table 4-10: Strain limit for collapse mechanisms ................................................................. 102 Table 4-11: SDoF system factors ........................................................................................... 104 Table 4-12: Values for the idealized capacity curve derivation ............................................ 104 Table 4-13: Shape factors for CNR-DT & ACI ..................................................................... 107 Table 4-14: Strength Reduction and Material Safety Factors for ACI & CNR-DT .............. 108 Table 4-15: Flexural strengthening moment capacities ........................................................ 109 Table 4-16: Shear strengthening results ................................................................................ 109 Table 4-17: Strain limit for collapse mechanisms ................................................................. 110 Table 4-18: SDoF system factors ........................................................................................... 112 Table 4-19: Values for the idealized capacity curve derivation ............................................ 112 Table 5-1: Maximum base shear for each scheme................................................................. 118 Table 5-2: Codes for FRP shear strengthening models & RC model.................................... 119 Table A-1: Damage states of the Seismostruct model (CNR-DT example) ............................... 1 Table A-2: Damage states of the Seismostruct model (ACI example) ....................................... 2 Table A-3: Damage states of the Seismostruct model (CNR-DT example) ............................... 3 Table A-4: Damage states of the Seismostruct model (ACI example) ....................................... 4

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Chapter 1. Introduction

Chapter. 1 Introduction The majority of the existing building stock, comprises building that do not comply with current seismic codes. Those buildings are usually exhibit deficiencies such as poor detailing, discontinuous load paths and deficiencies on capacity design provisions. That can be also confirmed from the catastrophic earthquakes, which until nowadays, result in structural collapse of structures but most importantly in human injuries and fatalities. The aid of strengthening the existing structures is becoming more pronounced, since devastating earthquakes can occur at any time worldwide. Although, the rehabilitation schemes, might not be cost-effective since many of them require significant time and cause lot of disruption due to the difficulty on their application. In the last decades, intervention methods have been revised and also new methods have been developed, following the developments of the seismic code requirements. One of this methods is the use of Fibre Reinforced Polymers (FRPs) as a strengthening material. It is a composite material made of a polymer matrix reinforced with fibres. The most common fibres are carbon (C), glass (G) or aramid (A) which are bonded together in a matrix made of epoxy, vinyl ester or polyester. They were first used as external retrofit in structures in Japan and Europe, however there has been a vast progress in the last 15 years or so. The application of FRP's for the strengthening of existing structures, is becoming increasingly common because of the various advantages that the material has such as, the high strength-to-weight ratio, the easy installation and handling and the shielding against corrosion. It can be used to various reinforced concrete elements such as beams, column, walls, slabs, etc. in order to enhance the shear, flexural and axial capacity of the elements. FRP's usually used as an externally reinforcement in the form of sheets, plates or anchors and the application requires the use of epoxy resins to bond them in the external surfaces of structural members. The current research aims to analyse the design procedure using fibre reinforced polymers to strengthen an existing structure and identify the on-site application. It also aims on the comparison between the design codes and the site applications. Chapter 2 contains an extensive and critical literary review of books, journal, articles and design codes on the use and effect of fibre reinforced polymers on reinforced concrete elements as strengthening material, as well as, background theory and experimental tests for each

2

Chapter 1. Introduction

strengthening type. Critical analysis and comparison of the implementation and technical procedure is made aiming to determine the practicability of the proposed interventions. The design recommendations & procedure of the two most advanced codes (CNR-DT, ACI 440.2R08) for the design of FRP systems is presented for each strengthening type (flexural, shear, confinement, etc.) of the reinforced concrete elements. This aims to define any differences regarding the design procedure, but also the actual implementation recommendations that are specified from the two codes. Moreover, in order to critically assess the technical implementation procedures that are currently used, two different codes will be used (ReLUIS 2009, ACI 440-F) and compared to define differences or compatibility issues with the suggested design procedures. Chapter 3, involves the strengthening intervention of a real case study building that have been designed using earlier seismic design codes. The building is modelled using SeismoStruct software in order to observe the response of the structure under the new lateral loads and derive the capacity curve. Different strengthening implementations using FRP materials and following the two design codes (CNR-DT & ACI) are made in order to reach the performance objective that have been set by the current codes. Moreover, a detailed guidance for the on-site application procedure of FRP material regarding the suggestions of ReLUIS 2009 and ACI 440-F is described for the strengthened models. Chapter 4&5, are based on results analysis and discussion. First, the strengthened models that designed using the design procedures from CNR-DT and ACI 440.2R-08 respectively, will be compared to assess any differences between the two codes. Secondly, a detailed discussion of the application procedures is going to be made about the potential deficiencies and application differences between the design and implementation procedure.

3

Chapter 2. Literature Review - FRP Debonding

Chapter. 2 Literature Review 2.1 FRP debonding 2.1.1 Overview In concrete members that have been retrofitted with externally bonded FRP systems, is very common to observe debonding failures. They occur in regions of high stress concentrations, due to propagation of cracks and material discontinuities. These are regions that include the ends of the FRP reinforcement and those around the shear and flexural cracks. The most common failure modes of FRP strengthened RC beams, include shear and flexural failure (concrete crushing and FRP rupture), as well as FRP debonding [1]. Therefore the most commonly used codes and guidelines are focus on two major debonding failure modes [2]: 1. Plate/sheet end debonding 2. Debonding caused by shear or flexural cracks. The debonding propagation path that initiated from stress concentrations, depends on the elastic and strength properties of the repair and substrate materials and their interface fracture properties. Figure 2-1 illustrates possible mechanisms of debonding in FRP strengthened RC members. Debonding can take place within or at the interfaces of materials that form the strengthening system. Most of the debonding failures are taking place in the concrete substrate, although more debonding mechanisms can occur, depending on the geometric and material properties. Generally the crack propagation in one of the constituent materials is favoured compared with the interface debonding in design of structural joints. Although, the last is commonly observed, especially in cases of poor surface preparation or application [1].

Figure 2-1: Types of debonding in FRP strengthened RC members (Source: Buyukozturk et al., 2004)

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2.1.2 Plate/sheet end debonding End debonding of FRP plate, can be caused by extensive stress concentration at the FRP plate end that derive from geometrical and flexural stiffness discontinuities. The likeliness of this type of failure to occur depend on many factors [3]. a) At Figure 2-2, debonding can be observed in case of a RC beam with low level of internal steel shear reinforcement. Every of the different plate end debonding modes can occur when the plat length or the width is variable b) At Figure 2-2b a critical diagonal crack is illustrated which can be formed when the distance between the beam support and the plate end is very close, causing debonding failure. c) The critical diagonal crack may occur outside the plate region in cases where the plate end distance is remote. In Figure 2-2d, concrete cover separation can be observed d) Critical diagonal crack debonding before the concrete cover separation Figure 2-2c can happen in cases where the critical diagonal crack debonding failure load is lower than the shear resistance of the original RC beam. e) Figure 2-2e, presents the extreme case of plate end in the pure bending region, where the orientation of the crack is mainly vertical. f) When the plate width is significant smaller than beam's width, the interface between those two materials becomes more critical compared with the interface between the steel tension bars and the concrete. The plate end interfacial debonding that illustrated in Figure 2-2f becomes the critical mode.

Figure 2-2: Debonding failure modes of an FRP-plated RC beam (Source: Teng & Chen, 2007)

5


2.1.3 Intermediate-crack-induced (IC) debonding It is possible for debonding to initiate from a flexural or flexural-shear crack in the high moment region and then spreads towards one of the plate ends as it is illustrated in Figure 2-3. This type of debonding failure is known as intermediate crack (IC) induced interfacial debonding (or simply IC debonding) [3]. Due to this movements of the crack faces, a peeling force is introduced, that trigger debonding. Moreover, the tensile stresses that are transferred from the concrete to the FRP at the crack location introduce high interfacial shear stresses. When the applied loading increases, those stresses are also increase and by the time they reach their critical values, debonding starts to spread towards one of the plate ends (nearer end).

Figure 2-3: FRP rupture (Source: Teng & Chen, 2007)

Another failure mode that observed in beams strengthened in flexure, was due to flexure-shear crack within the shear span of the beam, which leads to debonding of the external FRP reinforcement and shear failure of the beam [4], Figure 2-5. Debonding at flexure-shear cracks may also occur due to bad anchoring of plate ends and unsufficient long shear span of the strengthened beam that does not promote the development of proper bond [4]. In this case it possible that, debonding can spread to the ends of the beam, Figure 2-4.

Figure 2-5: shear failure and debonding

Figure 2-4: FRP debonding from flexure-shear crack

6


2.1.4 Mitigation measures The risk of debonding can be increase by various factors related with the on-site application quality. These also include poor workmanship and the use of inferior adhesives. The application process is very important to eliminate these factors and also guarantee that debonding failure is controlled by concrete [3]. According researches the importance of surface preparation and environmental conditions is high and can significantly affect bond durability. Proper preparation such as rounding the beam edges, levelling and cleaning the surface, prevent moisture, etc. can minimize the debonding of FRP plate [3]. Plate end debonding can be avoided by using mechanical anchors or U-wrapping FRP mechanisms. Moreover, anchors can limit the FRP debonding failure and change the failure mode from debonding to rupture [3].

2.1.5 FRP debonding strengthening according CNR-DT 200 R1/2013 From CNR-DT 200 R1/2013 Section 4.2.2.5., the ultimate value of the force transferred to the FRP system before debonding depends on the length 𝑙𝑏 of the bonded area. The optimal bond length is defined as the length, 𝑙𝑒𝑑 , if exceeded, having no increase in the force trasngerred between the concrete and FRP.

Figure 2-6: Maximum force transferred between FRP and concrete (Source: CNR-DT 200 R1/2013)

7

Chapter 2. Literature Review - FRP Debonding Reference

Equation

Comments

Optimal Bond Length, 𝑙𝑒𝑑 𝜋 2 ∗ 𝛦𝑓 ∗ 𝑛𝑓 ∗ 𝑡𝑓,1 ∗ 𝛤𝐹𝑑 1 √ 𝑙𝑒𝑑 = 𝑚𝑎𝑥 { , 200𝑚𝑚} 𝛾𝑅𝑑∗ 𝑓𝑏𝑑 2

CNR-DT 200 R1/2013: 4.1

Design Bond Strength, 𝑓𝑏𝑑 𝑓𝑏𝑑 =

𝛦𝑓 , modulus of elasticity 𝑡𝑓,1 , thickness of FRP 𝛾𝑅𝑑 , corrective factor

2𝛤𝐹𝑑 𝑠𝑢

𝑓𝑐𝑚 , concrete compressive strength CNR-DT 200 R1/2013: 4.2

Design Fracture Energy

𝛤𝐹𝑑 =

𝑘𝑏 𝑘𝐺 ∗ √𝑓𝑐𝑚 𝑓𝑐𝑡𝑚 𝐹𝐶

CNR-DT 200 R1/2013: 4.3

𝑓𝑐𝑡𝑚 , concrete tensile strength FC, confidence factor

𝑘𝑏 , geometrical corrective factor 𝑘𝑏 = √

2 − 𝑏𝑓 /𝑏 ≥1 1 + 𝑏𝑓 /𝑏

Mode 1 (end debonding) Ultimate design strength, 𝑓𝑓𝑑𝑑

CNR-DT 200 R1/2013: 4.4

𝑓𝑓𝑑𝑑

2 ∗ 𝐸𝑓 ∗ 𝛤𝐹𝑑 1 = ∗√ 𝛾𝑓,𝑑 𝑡𝑓

The ultimate design strength is reduce according the following equation CNR-DT 200 R1/2013: 4.5 CNR-DT 200 R1/2013: 4.6

𝑓𝑓𝑑𝑑,𝑟𝑖𝑑 = 𝑓𝑓𝑑𝑑 ∗

 If, , 𝑙𝑒𝑑 ≤ 𝑙𝑏

 If, , 𝑙𝑒𝑑 ≥ 𝑙𝑏

𝑙𝑏 𝑙𝑏 (2 − ) 𝑙𝑒𝑑 𝑙𝑒𝑑

Mode 2 (intermediate debonding) Ultimate design strength, 𝑓𝑓𝑑𝑑,2 𝑓𝑓𝑑𝑑,2

CNR-DT 200 R1/2013: 4.7

𝑘𝑞 𝐸𝑓 2𝑘𝑏 𝑘𝐺,2 = ∗√ ∗ √𝑓𝑐𝑚 𝑓𝑐𝑡𝑚 𝛾𝑓,𝑑 𝑡𝑓 𝐹𝐶

Maximum Design Strain 𝜀𝑓𝑑𝑑,2

𝑓𝑑𝑑,2 = ≥ 𝜀𝑠𝑦 − 𝜀0 𝐸𝑓

𝑘𝐺,2 , corrective factor 𝑘𝑞 , coefficient that considers load distributions

𝜀𝑠𝑦 , design yield strain 𝜀0 , maximum tensile strain

2.1.6 FRP debonding strengthening according ACI 440.2R-08 The design requirement that provided by ACI 440.2R-08 to mitigate FRP debonding failure modes are provided below.

8

Chapter 2. Literature Review - FRP Debonding Reference

Equation

ACI 440.2R08:13.1.1

FRP debonding 

Mechanical anchorages can effectively increase stress transfer Transverse anchoring with FRP wraps can increase FRP strain at debonding FRP end peeling



ACI 440.2R08:13.1.2

Comments Initiated at flexural cracks, flexural/shear cracks, or both 

Use anchorage (transverse FRP stirrups). Or/and



Locating the curtailment as close to the region of zero moment

If 𝑉𝑢 > 0.67𝑉𝑐 FRP laminates should be anchored with transverse reinforcement. Area of transverse clamping FRP U=wrap reinforcement, 𝐴𝑓𝑎𝑛𝑐ℎ𝑜𝑟

ACI 440.2R08:13-1

ACI 440.2R08:13-2

𝐴𝑓𝑎𝑛𝑐ℎ𝑜𝑟 =

(𝐴𝑓 𝑓𝑓𝑢 )𝑙𝑜𝑛𝑔𝑖𝑡𝑢𝑑𝑖𝑛𝑎𝑙 (𝐸𝑓 𝑘𝑣 𝜀𝑓𝑢 )𝑎𝑛𝑐ℎ𝑜𝑟

Critical Length, 𝑙𝑑𝑓 𝑛𝐸𝑓 𝑡𝑓 𝑙𝑑𝑓 = √ √𝑓𝑐 ′

The available anchorage length of FRP should exceed the value of 𝑙𝑑𝑓

2.1.7 Comparison of the codes ACI 440.2R-08, includes suggestions for the location of cut-off points for the FRP laminate, in order to avoid end peeling failure. Simply supported beams: 

Single-ply FRP laminate should be terminated at least a distance equal to 𝑙𝑑𝑓 after the point along the span corresponding to the cracking moment,𝑀𝑐𝑟 .



The outermost ply should be terminated not less than 𝑙𝑑𝑓 past the point along the span corresponding to the cracking moment



The termination points of the plies should be tapered.

Continuous beams: 

Single-ply FRP laminate should be terminated d/2 or 150mm minimum beyond the inflection point.



The outermost ply should be terminated no less than 150mm beyond the inflection point.

9

Chapter 2. Literature Review - FRP Debonding In both cases, when it comes to multiple-ply applications, each successive ply should be terminated in more than an additional 150mm beyond the previous ply (e.g. 3 plies, terminated at least 450mm)

Figure 2-7: Graphical representation of the guidelines for allowable termination points of a three-ply FRP laminate (Source: ACI 440.2R-08)

On the other hand, it is specified at CNR-DT 200 R1/2013 in Section 4.8.2.2, at least 200mm of anchorage length should be provided for the end portion of FRP systems used for strengthening of RC members. In other case mechanical connectors should be used, that based on proper experimental tests.

2.1.8 FRP system installation according ACI 440.2R-08 & CNR-DT 200 R1/2013 The behaviour of the FRP strengthening system on the RC members is dependent on the concrete substrate. Most of the debonding failures take place in the concrete substrate, so the proper preparation and profiling of the concrete surface is very important to avoid debonding of the FRP system. ACI 440.2R-08 and CNR-DT 200 R1/2013, display procedures and recommendations about the concrete substrate preparation.

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Chapter 2. Literature Review - FRP Debonding ACI 440.2R-08 FRP systems should not be applied to concrete substrates suspected of containing corroded reinforcing steel. Repair of the deterioration should be made, before applying any FRP system. Crack Injection Cracks that have width of 0.3mm or more, can cause delamination or fiber crushing and subsequently affect the performance of the externally bonded FRP system. This type of cracks should be pressure injected with epoxy before FRP installation. For smaller cracks exposed to aggressive environments, resin injection or sealing, to prevent corrosion of existing steel reinforcement, may require. Surface Preparation 1. In order to avoid stress concentrations at corners, they should be rounded to a minimum (13 mm) radius. Then, the roughened corners should be smoothed with putty. 2. The out-of-plane variations, including form lines, should not exceed the 1mm or what is recommended by the FRP manufacturer. Localized out-of-plane variations can be removed by grinding. 3. Surface should be cleaned using abrasive or water-blasting techniques to remove dust, dirt and any other matter that could affect the bond between the FRP and concrete. 4. Holes and voids should be exposed during the surface profiling and should be filled with resin-based putty. 5. Then, the surface should be cleaned and protected before FRP installation. Installation Recommendations  Primer & Putty Primer should be applied to all areas on the concrete surface where the FRP system is to be placed. The applied primer should be protected from dust, moisture and other contaminants before applying the FRP. According to FRP system manufacturers the surfaces should be dry before receiving the FRP system (not applied in wet surfaces).

CNR-DT 200 R1/2013 Deteriorated concrete shall be removed from all damaged areas. To prevent deterioration of the restored concrete, the corroded steel bars should be protected, against further corrosion. Crack Injection Cracks wider than 0.5 mm within solid concrete in the substrate shall be stabilized using epoxy injection methods before FRP strengthening can take place. For concrete surface roughness larger than 10 mm compatible epoxy paste should be used to level. For unevenness larger than 20 mm, specific filling material shall be used. Surface Preparation 1. Sandblasting of the concrete surface should be performed, that would provide a roughness degree of at least 0.3mm. 2. Cleaning of the concrete surface should be performed to remove any dust, dirt and other particles that intervene between the concrete and FRP system. 3. Edges of the corners should be rounded to a minimum radius of 20mm.

Installation Recommendations No installation of FRP should be performed when the environment is very moist. FRP systems shall be installed in humidity and temperature conditions as defined by the materials data sheet.

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Chapter 2. Literature Review - FRP Debonding ACI 440.2R-08, describes the installation procedure of wet layup FRP systems. Those FRP systems are usually installed by hand. 1. Saturating resin should be applied to uniformly to all surfaces where the system is to be placed. Fibers are also impregnated separately using a resin-impregnating machine before placement on the concrete surface. 2. The fibers are placed in the uncured saturating resin, by the way it is recommended by the manufacturer. The entrapped air between layers should be released or rolled out before the resin sets. 3. The consecutive layers of FRP sheets should be placed before the complete cure of the previous layer of resin. 4. When the FRP application is terminated then, maybe the application interlayer surface preparation, such as light sanding or solvent application as recommended by the system manufacturer, is required.

2.1.9 Comparison of the codes Both codes have similar procedure for surface preparation and crack injection. Special attention is taken to avoid deterioration of steel reinforcement and cracks are maintained by the use of epoxy resin. However, CNR-DT 200 R1/2013 suggests that sandblasting of the concrete surface should be performed in order to provide a roughness degree on the surface. On the other hand, ACI 440.2R-08 suggests that primer should be applied to all areas on the concrete surface where the FRP system is to be placed, with thickness recommended by the FRP manufacturer. Moreover, ACI 440.2R-08 describes also the FRP system application procedure and give specific details about the proper application of FRP sheets. On the other hand, CNR-DT 200 R1/2013 does not refer in the FRP system application.

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2.1.10 Preparation of concrete surface according ReLUIS 2009 The implementation of installation procedure of FRP systems in a concrete substrate is described by ReLUIS 2009.

1. Substrate care Firstly, the substrate wrappers of carbon must be level, dry and free of materials that prevent bonding and have sufficient strength

Remove any loose parts and substrate determined and rubbed with appropriate means, depending on the degree and extent of weathering. (Source: ReLUIS, 2009) 2. Rounding of corners The external corners of the beam need to be rounded to prevent the sharp corner edges from causing stress concentrations and premature failure of the FRP wrap. Re-profiling will be done by hand or with suitable non-flying mechanical tools and must guarantee rmin = 25 mm finally, the dust is removed with the use of vacuum cleaner.

(Source: ReLUIS, 2009) 3. Corrosion maintenance The original reinforcement should be maintained for corrosion. Application of two coats of cement mortar-component anticorrosion is suggested to ensure no corrosion of iron.

(Source: Isomat Manual)

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Chapter 2. Literature Review - FRP Debonding 4. Crack Injection The repair of cracks in reinforced concrete elements, must be carried out to restore the structural continuity of each element. The procedures involve the use of products that, trowelled, cast or injected, are able to ensure monolithic adhesion between the two damaged parts and for the entire depth of the lesion. The first steps of the procedure include: 

Dust extraction of the slots with compressed air.  Sealing surface cracks with thixotropic epoxy adhesive Repairing cracks by sealing a spatula or with resin by casting

(Source: ReLUIS, 2009)



Dusting of sand on the adhesive Epoxy thixotropic, still fresh.  Removal of the sand is not anchored by suction. Repairing cracks with resin by injection 

Placement of the injection tubes with thixotropic epoxy adhesive, simultaneously with the operation of sealing  Injecting the epoxy superfluid or hyperfluid bearing the CE  Removal of the injection tubes.  Sealing the holes with thixotropic twocomponent epoxy adhesive. All the procedures must include the use of products that meet the requirements defined by EN 1504-9.


5. Volumetric reconstruction of concrete Volumetric reconstruction for the restoration of the concrete cover of reinforced concrete using trowelling or spraying it with plastering to thicknesses of about 25 - 35 mm for layer. In cases the volumetric reconstruction is performed in correspondence of elements with sharp corners, it will be applied where the FRP reinforcement and the re-profiling (slips) with rmin = 25 mm, will be carried out. (Source: ReLUIS, 2009)

14

Chapter 2. Literature Review - FRP Flexural Strengthening

2.2 FRP Flexural strengthening of reinforced concrete elements 2.2.1 Overview Nowadays, bonding of FRP plates or sheets has become a very popular method for the flexural strengthening of reinforced concrete elements, such as beams, slabs, walls, and columns. Flexural strengthening can be achieved by fastening an FRP strengthening system to the tension face a flexural member. This results in increasing the effective tensile force resultant in the member and thus the moment capacity of the member increases too. However, these methods have disadvantages that can range from the application procedure up to lack of ductility.

2.2.2 Reinforced concrete beams Several studies were performed in the last years, to study the behaviour of retrofitted beams and analysed the various parameters influencing their behaviour, [5] [6] [7] [8] [9] [10]. The results have shown that, the application of FRP sheets is very effective for flexural strengthening of reinforced concrete beams, by drastically increase the flexural capacity and flexural stiffness. The analysis of strengthened members at ULS can be implemented following the procedures for RC members. However, special attention and admeasure should be given in the contribution of FRP and the bond between FRP and concrete during the design and implementation. Attention has been given into investigate and analyse the failure modes that occur in reinforced concrete beams retrofitted with FRP. The most common failure modes that have been identified are: 

Classical failure modes, where the full composite action is maintained between FRP and concrete until concrete crushing or FRP rupture.



Debonding failure modes, consisting in loss of composite action prior to attainment of any of the classical mode.

2.2.3 Reinforced concrete column When performing seismic retrofitting of RC columns, flexural strengthening might needed, especially if the structures are originally designed for gravity loads. Researches have been implemented in order to examine the effect of FRP reinforcement in flexure [11] [12] [13]. The results have shown that providing flexural reinforcement in terms of FRP sheets, can enhance the flexural resistance but also reducing the displacement of the column. So, the

15

Chapter 2. Literature Review - FRP Flexural Strengthening flexural strengthening of columns is needed to satisfy the capacity requirements or ensure the continuation of the longitudinal reinforcement.

2.2.4 Flexural Strengthening according ACI 440.2R-08 2.2.4.1

Reinforced concrete beams

According ACI 440, reinforced concrete beams strengthened with externally bonded FRP reinforcement can exhibit one of the following flexural failure modes: 

Crushing of the concrete in compression before yielding of the reinforcing steel;



Yielding of the steel in tension followed by rupture of the FRP laminate



Yielding of the steel in tension followed by concrete crushing



Shear/tension delamination of the concrete cover (cover delamination); and



Debonding of the FRP from the concrete substrate (FRP debonding).

Figure 2-8: Debonding and delamination of externally bonded FRP systems (ACI 440.2R-08)

However, for the purposes of calculations, the guidelines assume that two failure modes are occurring: 1. Compressive failure of the concrete (Mode 1) 2. Failure of the FRP strengthening system (Mode 2)

16

Chapter 2. Literature Review - FRP Flexural Strengthening The stress in the FRP, internal steel and concrete are required to determine for each of those failure modes, in order to estimate the ultimate bending capacity of the section. In order to calculate the ultimate strength the procedures that followed, should satisfy the strain compatibility and force equilibrium and consider the governing mode of failure. ACI 440.2R08 provides the Figure 2-9 below, which illustrates the internal strain and stress distribution for a rectangular section under flexure at the ultimate limit state.

Figure 2-9: Elastic strain and stress distribution (Source: ACI 440.2R-08)

The design procedure for flexural strengthening of a RC beam is described below. Reference

ACI 440.2R08:10-1

Equation The design flexural strength, 𝜑𝛭𝑛 and the factored moment 𝜧𝒖 , should satisfy the following equation 𝝋𝜧𝒏 ≥ 𝜧𝒖

The strength reduction factor,

ACI 440.2R08:10-5

0.90 𝑓𝑜𝑟 𝜀𝑡 ≥ 0.005 0.25(𝜀𝑡 − 𝜀𝑠𝑦 ) 𝑓𝑜𝑟 𝜀𝑠𝑦 < 𝜀𝑡 < 0.005 𝜑 = 0.65 + 0.005 − 𝜀𝑠𝑦 0.65 𝑓𝑜𝑟 𝜀𝑡 ≥ 𝜀𝑠𝑦 {

Comments 𝝋𝜧𝒏 , refers to the nominal strength of the member multiplied by a strength reduction factor 𝜧𝒖 ,refers to the moment calculated from factored loads 𝜺𝒕 , tensile strain in extreme tension steel at nominal strength

Design Strain of the FRP, 𝜀𝑓𝑑

ACI 440.2R08:10-2

𝑓𝑐′ 𝜀𝑓𝑑 = 0.41√ ≤ 0.9𝜀𝑓𝑢 𝑛𝐸𝑓 𝑡𝑓

ACI 440.2R-

Effective strain level in the FRP, 𝜀𝑓𝑒 𝑑𝑓 − 𝑐 𝜀𝑓𝑒 = 𝜀𝑐𝑢 ( ) − 𝜀𝑏𝑖 ≤ 𝜀𝑓𝑑 𝑐

08:10-3

Existing strain at the substrate, 𝜀𝑏𝑖

𝜺𝒃𝒊 , initial substrate stain 𝒅𝒇 , effective depth of FRP reinforcement

17


𝜀𝑏𝑖 =

ACI 440.2R08:10-11

ACI 440.2R08

𝑚1 (ℎ − 𝑘1 𝑑) (𝐼𝑐𝑟 )1 𝐸𝑐

Effective stress level in the FRP reinforcement, 𝑓𝑓𝑒 = 𝐸𝑓 𝜀𝑓𝑒 Stress in steel, 𝑓𝑠 𝑓𝑠 = 𝐸𝑠 𝜀𝑠 ≤ 𝑓𝑦

Stress in steel can be found from the strain level in the steel using its stress-strain curve

Mode 1: Concrete Crushing Effective strain level in the FRP 𝑑𝑓 − 𝑐 𝜀𝑓𝑒 = 𝜀𝑐𝑢 ( ) − 𝜀𝑏𝑖 ≤ 𝜀𝑓𝑑 𝑐

With φ, 𝜑 = 0.65 +

Strain in Steel reinforcement

𝜀𝑠𝑦

𝑑−𝑐 𝜀𝑠 = 𝜀𝑐𝑢 ( ) ≤ 𝜀𝑠𝑦 𝑐

ACI 440.2R08

Mode 2: FRP Failure Concrete strain at failure 𝑐 𝜀𝑓𝑒 = (𝜀𝑓𝑒 + 𝜀𝑏𝑖 ) ( ) ≤ 𝜀𝑐𝑢 𝑑𝑓 − 𝑐

𝜑 = 0.65 + 𝜀𝑠𝑦

𝑑−𝑐 𝜀𝑠 = (𝜀𝑓𝑒 + 𝜀𝑏𝑖 ) + ( ) ≤ 𝜀𝑠𝑦 𝑑𝑓 − 𝑐

ACI 440.2R08:10-12

𝑐=

08:10-13

0.25(𝜀𝑡 −𝜀𝑠𝑦 ) 0.005−𝜀𝑠𝑦

For < 𝜀𝑡 < 0.005

𝒂𝟏 and 𝜷𝟏 are parameters defining a rectangular stress block in the concrete equivalent to the nonlinear distribution of stress

𝐴𝑠 𝑓𝑠 + 𝐴𝑓 𝑓𝑓𝑒 𝑎1 𝑓𝑐′ 𝛽1 𝑏 

ACI 440.2R-

0.005−𝜀𝑠𝑦

For < 𝜀𝑡 < 0.005

With φ,

Strain in Steel reinforcement

The strain and stress level in the FRP can be found be assuming a neutral axis depth and the internal force equilibrium may be checked using the following equation

0.25(𝜀𝑡 −𝜀𝑠𝑦 )

The nominal flexural strength of the section with FRP external reinforcement is computed using the equation below, 𝛽1 𝑐 𝛽1 𝑐 𝑀𝑛 = 𝐴𝑠 𝑓𝑠 (𝑑 − ) + 𝜓𝑓 𝐴𝑓 𝑓𝑓𝑒 (ℎ − ) 2 2

 

𝜓𝑓 is an additional reduction factor for FRP, with recommended value 0.85 𝛽 𝑐 𝐴𝑠 𝑓𝑠 (𝑑 − 21 ) , steel contribution to bending 𝛽 𝑐 𝐴𝑓 𝑓𝑓𝑒 (ℎ − 21 ), FRP contribution to bending

18


2.2.6 Flexural Strengthening according CNR-DT 200 R1/2013 2.2.6.1

Reinforced concrete beams

According, CNR-DT 200 R1/2013, debonding failure modes for flexural strengthening are classified in the following categories: 

Mode 1 (Laminate/sheet end debonding)



Mode 2 (Intermediate debonding, caused by flexural cracks)



Mode 3 (Debonding caused by diagonal shear cracks)



Mode 4 (Debonding caused by irregularities and roughness of concrete surface)

Figure 2-10: FRP flexural strengthening: debonding failure modes (Source: CNR-DT 200 R1/2013)

The guidelines suggest that, the most frequent failure modes are Mode 1 & 2. Although, recommendations for surface preparation and composite material installation are provided in order to mitigate the risk of the remaining failure modes. According CNR-DT 200 R1/2013: Section 4.2.2.3, the flexural analysis of FRP strengthened members can be implemented, using strain compatibility and force equilibrium methods. The stress at any point in the member must correspond to the strain at that point; the internal forces must balance the external load effects.

Figure 2-11: Failure mode of a RC member Strengthened with FRP (Source: CNR-DT 200 R1/2013)

19

Chapter 2. Literature Review - FRP Flexural Strengthening The design procedure for flexural strengthening of a RC beam according CNR-DT 200 R1/2013, is described below. Reference

Equation

CNR-DT 200 R1/2013:4.13

Flexural design at the ULS of FRP strengthened members requires that both the flexural capacity, 𝑴𝑹𝒅 and factored ultimate moment, 𝑴𝒔𝒅 satisfy the following equation:

Comments

𝑀𝑠𝑑 ≤ 𝑀𝑅𝑑 CNR-DT 200 R1/2013:4.14

The maximum FRP tensile strain 𝜀𝑓𝑘 𝜀𝑓𝑑 = 𝑚𝑖𝑛 {𝜂𝛼 ∗ , 𝜀 } = 𝜀𝑓𝑑𝑑 𝛾𝑓 𝑓𝑑𝑑 Failure Modes

CNR-DT 200

FRP mechanical ratio, μf 𝑏𝑓 (𝑛𝑓 ∗ 𝑡𝑓,1 ) ∗ 𝑓𝑓𝑑𝑑,2 𝜇𝑓 = 𝑓𝑐𝑑 ∗ 𝑏 ∗ 𝑑

R1/2013:4.2.2. 3

Balanced mechanical ratio, μf1-2 CNR-DT 200

𝜇𝑓1−2

R1/2013:13.3

ℎ 𝑑 = − 𝜇𝑠 (1 − 𝑢) 𝜀𝑐𝑢 ∗ 𝜀𝑓𝑑 ∗ 𝜀0 0.8 ∗ 𝜀𝑐𝑢 ∗

Region 1 CNR-DT 200 R1/2013:4.2.2.

Steel in compression, 𝜀𝑠2 = (𝜀𝑓𝑑 + 𝜀0 ) Steel in tension, 𝜀𝑠1 = (𝜀𝑓𝑑 + 𝜀0 )

𝜀𝑐𝑢 (ℎ 𝑥

Concrete in compression, 𝜀𝑐 = 𝜀𝑐𝑢 Steel in compression, 𝜀𝑠2 = 𝜀𝑐𝑢 Steel in tension, 𝜀𝑠1 = 𝜀𝑐𝑢

𝑥−𝑑2 𝑥

𝑑−𝑥 𝑥

Flexural capacity of FRP-strengthened members 0 = 𝜓𝑏𝑥𝑓𝑐𝑑 + 𝐴𝑠2 𝜎𝑠2 − 𝐴𝑠1 𝜎𝑠1 − 𝐴𝑓 𝜎𝑓 Flexural capacity, 𝑴𝑹𝒅

CNR-DT 200 R1/2013:4.16

𝑀𝑅𝑑 =

Failure due to concrete crushing (strain equal to εcu) occurs with the yielding of steel in traction, while the FRP strain has not reached its ultimate value.

Region 2 − 𝑥) − 𝜀0 ≤ 𝜀𝑓𝑑

R1/2013:4.2.2.

R1/2013:4.15

𝑥−𝑑2 (ℎ−𝑥)

𝑑−𝑥

FRP, 𝜀𝑓 =

CNR-DT 200

𝜇𝑓 ≤ 𝜇𝑓1−2 , Failure occurs in Region 1

(ℎ−𝑥)

CNR-DT 200

3(4)

Depending on whether FRP maximum tensile strain,𝜺𝒇𝒅 (Region 1) or concrete maximum compressive strain,𝜺𝒄𝒖 , is reached (Region 2).

𝜇𝑓 > 𝜇𝑓1−2 Failure occurs in Region 2 FRP, 𝜀𝑓 = 𝜀𝑓𝑑 𝑥 Concrete in compression, 𝜀𝑐 = (𝜀𝑓𝑑 + 𝜀0 ) (ℎ−𝑥) ≤ 𝜀𝑐𝑢

3(3)

Flexural failure is assumed to occur when the maximum concrete compressive strain, 𝜺𝒄𝒖 is reached

For both failure modes, the position of the neutral axis, x, is using the translational equilibrium equation

1 [𝜓𝑏𝑥𝑓𝑐𝑑 (𝑑 − 𝜆𝑥) + 𝐴𝑠2 𝜎𝑠2 (𝑑 − 𝑑2 ) 𝛾𝑅𝑑 + 𝐴𝑓 𝜎𝑓𝑑1]

20


2.2.6.2

Reinforced concrete columns

The design procedure for flexural strengthening of a RC column according CNR-DT 200 R1/2013, is described below. Reference CNR-DT 200 R1/2013:11.1

Equation FRP strengthened members subjected to combined bending and axial loading shall be designed as follows

𝑀𝑠𝑑 ≤ 𝑀𝑅𝑑 (𝑁𝑠𝑑 )

Comments 𝑴𝒔𝒅 is the design applied moment 𝑴𝑹𝒅 is the flexural capacity of the strengthened member considering the design axial force 𝑁𝑠𝑑 

Mechanical ratio of tension steel CNR-DT 200 R1/2013:11.2 CNR-DT 200 R1/2013:11.3

CNR-DT 200 R1/2013:11.4 CNR-DT 200 R1/2013:11.5

𝐴𝑠1 ∗ 𝑓𝑦𝑑 𝑓𝑐𝑐𝑑 ∗ 𝑏 ∗ 𝑑

𝜇𝑠 =



Mechanical ratio of FRP system

𝜇𝑓 =

𝑏𝑓 ∗ 𝑡𝑓 ∗ 𝑓𝑓𝑑 𝑓𝑐𝑐𝑑 ∗ 𝑏 ∗ 𝑑

 

Non-dimensional equations reflecting applied loads

𝑁𝑠𝑑 𝑓𝑐𝑐𝑑 ∗ 𝑏 ∗ 𝑑 𝑀𝑠𝑑 = 𝑓𝑐𝑐𝑑 ∗ 𝑏 ∗ 𝑑 2

𝑛𝑠𝑑 = 𝑚𝑠𝑑

The failure mode and corresponding value of the parameter, 𝒎(𝒎𝒓) 𝜼 can be determined as a function of η

CNR-DT 200 R1/2013:Table 11.1

CNR-DT 200 R1/2013:Table 11.8 CNR-DT 200 R1/2013:11.1



𝐴𝑠1 is are of the existing yield reinforcement 𝑓𝑐𝑐𝑑 is the the design strength of confined concrete, b and d the width and effective depth of the FRP strengthened member bf and tf are the FRP width and thickness 𝑓𝑓𝑑 is the FRP ultimate design strength

The parameters, 𝜂𝑖 (0,1,2,3) 𝜂 = 𝑛𝑠𝑑 + 𝜇𝑠 ∗ (1 − 𝑢) + 𝜇𝑓 𝜂0 = −𝜇𝑠 ∗ 𝑢 2 𝑟 𝜂1 = ∗ 3 𝑟+1 1.75 ∗ 𝑟 𝜂2 = 0.8 ∗ 1.75 ∗ 𝑟 + 1 𝜂3 = 0.51 + 𝜇𝑓 ∗ (1 − 𝑟)

Νon-dimensional flexural capacity 𝒎𝑹𝒅 (𝒏𝒔𝒅 ) 1 𝑚𝑅𝑑 (𝑛𝑠𝑑 ) = 𝑚𝑚𝑟 (𝜂) + ∗ [𝜇𝑠 ∗ (1 + 𝑢) + 𝜇𝑓 ] 2

The following equation must satisfied 𝑚𝑅𝑑 (𝑛𝑠𝑑 ) ≥ 𝑚𝑠𝑑

21


2.2.7 Flexural strengthening of RC beams using anchors The use of FRP for flexural strengthening of a beam, can increase the effective tensile force resultant in the member and thereby increase the moment capacity of the member. Although, there are problems associated with FRPs’ total elastic behaviour until ultimate that can result in catastrophic failure when reaching the ultimate capacity of RC beams due to the lack of ductility. Moreover, the premature debonding failure occurring at the ends of bonded FRP plates create difficulties in using this type of strengthening. Investigation have been made from various researchers that in order to examine the effectiveness mechanical anchors for increasing the flexural capacity and ductility of RC beams, as long as, their effect on the mode of failure and full utilization of the high FRP tensile strength [14] [15] [16]. At present, the main guidelines (ACI 440.2R-02 and CNR-DT 200/2004) do not provide indications on the contribution of anchorages to the ultimate capacity of beams. The following experiments have proposed the use of mechanical anchors at the end of the beam as bonding technique. The different anchors that were used are illustrated below. The first anchor consists of one steel plate 160x40x13 mm with one hole in the middle and two threaded 3/8'' holes in its thickness, two steel tensile link members typical coupon samples for steel tension test, four high tensile 3/8'' threaded rods and one heavy duty bolt at each end [14], Figure 2-13. The second anchor consisted of top and bottom 10-mm-thick steel plates fastened together using two M12 tightened bolts [16], Figure 2-12. The third anchor system consisted of a mechanical steel bolted plate anchorages, 240x240 mm with 5 16mm bolts, which were used at both ends [15], Figure 2-14.

Figure 2-13: Ductile anchor system (Source: Galal & Mofidi, 2009)

Figure 2-12: Anchor system dimensions (Source: Chahrour & Soudki, 2005)

22


Figure 2-14: Anchor system dimensions (Source: Pellegrino &Modena, 2009)

The installation procedure of the anchoring system for each experiment is described below: Galal K. & Modifi A. (2009) 1. The FRP sheets are wrapped around two steel plates at theirs ends (with a 180°) and then epoxy bonded through an overlap to the original FRP sheet, usually 150mm. 2. In order to avoid stress concentration and FRP rupture, the steel plates have rounded corners. 3. Through two steel link members one at each side of the beam, the steel plate is linked to an angle that is anchored to the beam support corner, Figure 2-15.

Figure 2-15: Proposed hybrid FRP/ductile steel anchor system (Galal & Modifi, 2009)

Chahrour A. & Soudki K. (2005) The anchoring procedure is not detailed in this case. The FRP strip on the bottom face is clamped at both ends to the mechanical anchors, which are consists of steel plates which are bolted through the beam. Pellegrino C. & Modena C. (2009) 1. Preparation of the concrete surface with an abrasive disk 2. Drilling the concrete for bolts with diameter of 16mm and 100mm length are used to laminate the steel plates.

23

Chapter 2. Literature Review - FRP Flexural Strengthening 3. Application of the two-component epoxy resin (thickness of 2 mm) 4. The laminate is placed on the tensile edge without exerting pressure on the adhesive 5. Introduction of the laminate into the provisional steel anchoring device in the fixed end. 6. Insert the laminate into the mobile provisional steel anchoring device 7. The laminate is inserted into the mobile provisional steel anchoring device, in the mobile end. 8. The hydraulic jack and all provisional anchorages are removed 48 hours after the prestressing process

Table 2-1, it is illustrated that the beams strengthened with partially or fully bonded endanchored FRP strips had higher enhancement in ultimate capacity than the fully bonded beam with no end-anchorage. Generally the deflection of the strengthened beams at ultimate was lower that of the control. As it can be seen in Table 2-1, the ductility of strengthened beams was lower than the control, which justifies their stiff behaviour up to failure. Although the unbonded specimens, have shown the most ductile behaviour [16] [14] [15]. In all the experiments, the control beams failed in the conventional flexural manner of steel yielding followed by crushing of the concrete. The presence of anchorage in the externally bonded FRP system prevented early peel off of the CFRP sheet, which enhanced the T-beam strength and ductility. In the case of bonded FRP strip (FMB), the presence of the anchors prevented debonding of the FRP sheet and, the beam developed its full flexural capacity. Although, the high strains that the bonded FRP sheet was subjected due to the growth and widening of the flexural cracks, resulted in rupture of the FRP [14]. In the second case, all the beams failed by intermediate flexural shear crack-induced interfacial debonding of the CFRP strip. It is also clear that the comparison of the bonded beams performance, shows that smaller unbonded lengths are preferred, because they allow a better utilization of the CFRP strip tensile capacity. Moreover, this experiment did not manage to eliminate the end slip of the FRP sheet [16]. In this case, the unanchored beam shows brittle behaviour due to sudden delamination of the CFRP laminate, starting from the free end and spread to the other end. However, the anchored beam shows similar behaviour, but due to the presence of the anchored system, higher value of the ultimate load. Failure of the end-anchorage device is also observed in this case [15].

24

Chapter 2. Literature Review - FRP Flexural Strengthening Table 2-1: Experimental information and results

Anchorage System Author

Galal & Mofidi (2009)

Chahrour & Soudki (2005)

Pellegrino & Modena (2009)

Beam Dimensions (mm)

3000x280x155

Increase in Load Capacity

Displacement Ductility

Specimen

FRP bond

No Layers

FCO

-

-

-

4.15

FEB

Wet bonded

1

1.04

1.91

FMU

Wet unbonded

FMB

Wet bonded

1 2

Bonded 750mm unbonded length

2000x250x150 4

Failure Mode

External Anchorage

Concrete crushing after rebars’ yield. FRP debonding

1

Hybrid CFRP/steel Ductile anchor

1.21

9.09

Concrete crushing after rebars’ and steel link members’ yield.

1

Hybrid CFRP/steel Ductile anchor

1.27

3.37

FRP rupture

-

1.26

3.5 1.8

Concrete crushing FRP debonding

1

Mechanical anchor

1.24

2.1

FRP debonding

Concrete crushing FRP debonding FRP debonding (failure of anchorage)

1

RC-C RC-N

Bonded

1

-

1.36

N/A N/A

RC-EA

Bonded

1

Mechanical anchor

1.5

N/A

10000x500x300

25


2.2.8 Comparison of the proposing methods The difference in load increase between the beams with bonded and unbonded anchorage was about 5%m, but in case of unbonded beams the increase in ductility was significant compare to the other anchoring methods. Although the FRP sheets that are bonded to the beam, was subjected to high strains due to the growth and widening of the flexural cracks, which resulted in a sudden rupture of the CFRP sheets. On the other hand, this type of failure was avoided in the unbonded beam. Instead concrete crushing after rebar’s’ and steel link members’ yielding occurred. Comparing the application procedures, the advantage of first proposal, is that the mechanically anchored unbonded FRP strengthening system does not require surface preparation or adhesive application, which can reduce the cost and implementation time. Moreover, the application is not complicated and can be implemented without the use of skilled labour. The second application is also convenient, however in case when the beam is casted in the slab, difficulties arise in the implementation of the anchoring, since the steel plates which are bolted through the beam. The drilling of the holes through the beam can be challenging and cause lot of disturbance since both sides of the beams need to be implemented. The application procedure for the third experiment is very detailed and it is difficult to be implemented. Summing up, taking into account the implementation and the effectiveness of each method, the hybrid unbonded CFRP sheet/ductile steel anchor system, might be the most appropriate to use in flexural anchoring of a beam.

26


2.3 Flexural Strengthening of RC Columns Using Anchors In order to enhance flexural capacity of the columns without significant increase of stiffness, researchers are investigating the use of fiber anchors. As it has been mentioned above, fiber anchors have been investigated repeatedly for application regarding the shear strengthening of columns [17], shear strengthening of beams [18] [19] [20] and flexural strengthening of beams. The following experiments are focused on investigating the response of flexural strengthened columns with FRP that are anchored at the columns’ end sections with fiber anchors [11] [12]. In both experiments spike anchors inserted into concrete holes, with their fibers fanned out over the layers of the FRP sheet. In the first experiment [12], wrapping confinement of the column was also provided, whereas in the second [11] only flexural strengthening was used but with a higher number of both unidirectional and bidirectional FRP textiles. The following Table 2-2 presents the information about the anchorage systems that used in both experiments. Table 2-2: Experimental information Flexural FRP strength.

Anchorage System Author

Column Dimensions (mm)

Specimen

Type

Bonding

Control

CFRP fan anchors

-

2_1.5

Vrettos et al. (2013)

1600 x250 x250

Dimensions (mm)

No of Sheets

-

-

-

94.5

1

CFRP fan anchors

Resin

3@ 67mm

63

2_1.0

CFRP fan anchors

Resin

2@ 100mm

63

1

Resin

2@66

56

2

Resin

1

121

2

Resin

1

228

F2_1Φ9 600 x200 x200

2@ 100mm

Weight (g/m)

3_1.5

F2_2Φ6

Bournas et al. (2015)

Resin

No & Distance (one side)

F4_1Φ12 F4_2Φ9 T4_2Φ6R T4_2Φ6 M

CFRP fan anchors CFRP fan anchors CFRP fan anchors CFRP fan anchors CFRP fan anchors CFRP fan anchors

1

14000 x200

4 600x200

Resin

2@66

121

4

Resin

2@66

56

4

Mortar

2@66

56

4

27

Chapter 2. Literature Review - FRP Flexural Strengthening The anchoring procedure for both experiments is described below. 1. The concrete surface on each of the two opposite sides of the column specimens was prepared by cleaning and roughening. Then the FRP sheets were bonded on each of the two opposite sides of the strengthened columns. 2. Holes were drilled into the base of the column. For the first case (250mm depth, 14/16mm diameter) [12, 7] and for the second case (200mm depth, 18mm diameter) [11]. 3. The holes were filled with epoxy to half of their depths 4. The anchorage of the spike anchors was placed into the holes, after applying the FRP sheets on both sides and fanned out over the FRP sheet. 5. For both cases at least another FRP sheet was applied in the columns. In the first case, the columns were wrapped with single layer of FRP, Figure 2-17. The corners of near the base of the columns, were rounded to 25mm to improve the effectiveness of confinement [12]. 6. Although in the second case, the last carbon fiber sheet was applied on the top of the

fan. In the case of 4 FRP sheets the two anchors were fanned out over the first and third layers of the FRP sheet, respectively [11], Figure 2-16.

Figure 2-17: Cross section of anchored specimen 3_1.5 (Source: Vrettos et al., 2013)

Figure 2-16: Cross section of anchored specimen F4_2Φ9 (Source: Bournas et al., 2015)

Table 2-3: Summary of experimental results Specimen

Drift ratio at peak force

Control 2_1.5 3_1.5 2_1.0

3.67% 2.4% 2.79% 2.48% Displacement at peak force (mm) 3.18 3.85 4.99 3.97 2.00 1.25

F2_2Φ6 F2_1Φ9 F4_1Φ12 F4_2Φ9 T4_2Φ6R T4_2Φ6M

Increase of Peak Force (with respect to control) 1.35 1.25 1.19 Increase of Peak Force (with respect to F2_2Φ6) 1.21 1.7 2.1 0.56 0.48

28

Chapter 2. Literature Review - FRP Flexural Strengthening The carbon fiber spike anchors comprise an effective anchorage system in preventing premature delamination of FRP and TRM sheets from concrete surfaces. It is clear from both experiments that the CRFR anchors comprise an effective anchorage system, which increase the flexural resistance of the beam. The displacement at peak force of the column is decreasing, although the use of anchors results in higher stiffness. We also observe that the ''heavier'' the anchor the more effective. The effectiveness of anchors is increasing almost linearly with their weight. From both experiments it can be concluded that, the anchors should be as few and as heavy as possible. The failure of the strengthened columns was due to tensile rupture of the anchors at the cross section of maximum moment. The absence of debonding failure indicates that the epoxy adhesive performed well and the anchor length was sufficient. In the case the mortar was used instead of epoxy, debonding occurred which indicates that epoxy resin is more effective bonding agent than mortar.

2.3.1 Comparison of the proposing methods The application procedure is simple, as it only requires the drilling of the holes at the column base and the bonding of the anchor fans at the first sheet of FRP, preferable using epoxy resin. Although, for multiple layer application, the protruding fibers of the anchors should be fanned out over the different layers in order to ensure the transfer of the tension forces from the FRP sheet into the concrete base. First's experiments column was two times higher that second ones. However, the embedded depth of the anchors was 50mm deeper. So, it is viable to say that, 200-250mm could be a good estimate for the drilling depth, regardless the height of the column. The proposed methods are tested and used in bottom columns only, so it is not clear how the fibers can be anchored in case of upper story columns where beams and slabs exist.

29


2.3.2 FRP flexural strengthening implementation according ACI 440-F ACI 440-F illustrates some structural schemes that have implemented in research programs or mitigation strategies in the field, for flexural strengthening of reinforced concrete beams. According to ACI 440-F, longitudinal FRP sheets along the beams and columns will improve the members' moment capacity and force hinging to form in the beams. In Figure 2-19, is proposed that the longitudinal FRP reinforcement, should be continuous between discontinuous beams. That can be implemented by using thin FRP strips, underside of the beam that pass through the sides of the column and connect the two beams. In case of flexural strengthening of the columns, in order to ensure the continuity of the FRP reinforcement between the top and bottom column, cut-outs must be formed at the corners with longitudinal sheets bonded to the column faces. It is also suggested that, to improve the bond strength of the longitudinal sheets, CFRP wrapping should be applied above and below the beam and slab subassembly.

Figure 2-19: Beam Retrofit Scheme (Source: ACI 440-F)

Figure 2-18: Column Retrofit Scheme (Source: ACI 440-F)

30


2.3.3 FRP flexural strengthening implementation according ReLUIS 2009 According to ReLUIS 2009, flexural strengthening can be achieved with the use of composite materials, arranging unidirectional fabrics or pultruded carbon fiber coated sheets on the recess of the structural element. Shear strengthening of the beams can also be used as end anchoring of the flexural reinforcement, Figure 2-22. In contrast with the currently used codes, ReLUIS, 2009, suggests in the implementation procedure the use of fiber anchorage for flexural strengthening of the beams.

Figure 2-20: Cross-section A-A

Figure 2-22: Flexural reinforcement with a composite beam with end anchorages (ReLUIS, 2009)

Figure 2-21: Cross-section C-C

After the preparation of the concrete substrate that is described in 2.1.10, the application procedure that is proposed from ReLUIS for flexural strengthening is described below.

31

Chapter 2. Literature Review - FRP Flexural Strengthening 1. FRP layer application After applying the resin, the FRP fabric is immediately placed making sure that it is spread on evenly. Using a roller over the fabric, trying to achieve complete impregnation and release the entrapped air

(Source: ReLUIS, 2009) 2. Anchorage application (a) In case of fiber end-anchoring, firstly holes are drilled at the beams ends. After the application of the first layer of FRP, the fibers are then inserted after the application of epoxy resin.

(Source: ReLUIS, 2009) 3. Anchorage application (b) The fibers are then fanned out and bonded in the FRP sheet, using appropriate epoxy resin.

(Source: ReLUIS, 2009) 4. Second layer application The application of a second layer of fabric the same procedure must be carried out by brush or short hair roller, over the previous still fresh adhesive layer

(Source: ReLUIS, 2009) 5. Shear strengthening application Shear strengthening systems can be also used for anchorage. In case of using shear

32

Chapter 2. Literature Review - FRP Flexural Strengthening strengthening systems, after the completion of the application of flexural reinforcement, the FRP is wrapped around the ends of the beams to enhance the shear resistance and also provide anchorage, to avoid end-peeling of the FRP.


2.3.4 Comparison of the implementation procedures Comparing the implementation procedure of flexural strengthening for reinforced concrete beams and columns that ReLUIS, 2009 and ACI 440-F are suggesting, it is clear that the two guidelines propose different methods on the detailing of FRP reinforcement, especially at the intersections between beams and columns. At flexural strengthening of beam, ACI 440-F suggests that the longitudinal FRP reinforcement, should be continuous between discontinuous beams, using thin FRP strips that connect the two beams. Moreover, the use of full-wrapping of the beam is suggested, by making cut-outs on the slab, to enhance the bond strength of the longitudinal sheets. On the other hand, ReLUIS 2009, suggest the use of anchors at the end of the beams in order to avoid the enddebonding. Moreover, shear strengthening should be used in forms of U-wraps at the end of the beams as end anchoring. The columns are also considered in ACI 440-F and suggestions for the application of FRP sheets for flexural strengthening are suggesting. However, ReLUIS 2009, is more focused on beam detailing and is not referring on the flexural strengthening of columns.

33

Chapter 2. Literature Review - FRP Confinement

2.4 FRP confining of reinforced concrete elements 2.4.1 Overview At the event of an earthquake, the lateral cyclic earthquake force can degrade the concrete strength and reinforcing bar, which might result in early column failure. Nowadays, the revised codes have introduced improved design specifications, in order to avoid such deficiencies. However, some of the existing structures that have been damaged or designed with previous codes need to be retrofitted. One retrofit technique to avoid column failure is the use of Fibre Reinforced Polymer (FRP) wraps [21]. Various researches have been implemented [22] [23] [17] [24] [25], to prove that, FRP confining is can be used on columns to achieve: 1) Increase the axial load capacity of a column. 2) Increase the lateral displacement capacity of a column. Moreover, it is also be used to prevent lap-splice failures and delay rebar buckling of columns under seismic actions. Even though the FRP system appears to act as internal steel spiral or ties reinforcement, because in is wrapped around the hoop direction, actually this is not the main purpose. The primary role of FRP wrapping is to provide stability and restrain the longitudinal steel reinforcing bars in the compression member [21]. External confinement with FRP can be applied on both circular and rectangular columns, wrapped with sheets made of unidirectional fibers in the circumferential direction [17]. Figure 2-23 & Figure 2-24 below show circular and rectangular RC columns, confined with FRP on their exterior surfaces. The application if similar with the four sided wrap in shear strengthening.

Figure 2-23: RC rectangular column confinement using FRP sheets (Source: Alkhrdaji T., 2015)

Figure 2-24: RC circular column confinement using FRP sheets (Source: Sika Corporation)

34

Chapter 2. Literature Review - FRP Confinement Experiments have been implemented in order to prove the effectiveness of FRP confinement in RC columns [26] [27] [28]. A comparison between confined and unconfined columns, results in better performances, in terms of ductility and increase of the axial capacity. Moreover, the external confinement of RC columns with FRP wrapping can delay the buckling of rebars, restrain concrete cover spalling and enable compressive concrete strains reach larger values [27]. Experiments have also implemented, that compare the variation of the effects of FRP wrapping, when different shape columns are tested [28] [26]. It has been observed that the FRP wrapping is more effective for circular columns, following by square and rectangular columns. The axial load capacity of the columns was increased due to FRP confinement. The axial load carrying capacity was found to be increased by 159% for circular columns and by 79% &76% for square and rectangular columns respectively [28]. Experiments conducted for square columns, shown that the axial strength can be increased up to 310% comparing to the unconfined specimen, however, depending on various parameters such as (number of wraps, steel reinforcement, dimensions, etc.) [26]. The aspect ratio (h/b) of the column, has also an important role in the axial strength and ductility improvement of the column. As the aspect ratio of the column increases (becomes rectangular column), the axial load capacity and ductility are significantly decreasing [26]. The FRP wrapping resist the axial loads by membrane action which is more effective in circular sections, rather than square or rectangular sections. Square or rectangular column sections consist of corners and long flat sides. The stress concentrations at the corners and the inefficient confinement at the flat sides can cause possible loss of membrane action of the FRP composite and reduction of confinement [29]. Figure 2-25 below illustrates the effective confined cores of circular and rectangular column sections.

Figure 2-25: Strengthening Of Circular Columns versus Rectangular Columns (Source: Chauhan & Umravia, 2012)

35

Chapter 2. Literature Review - FRP Confinement It is suggested that the strength can be improved by rounding the corners of rectangular section [22]. The rounding radius can increase the effectiveness, until a certain amount of threshold. However, the steel ties or spirals that confining the column can limit the rounding of the corner radius [29]. Table 2-4: Representative data on FRP-retrofitted RC columns Height (mm)

Corner Radius (mm)

No of FRP Layers

Axial Load Capacity Increase (with respect to unconfined column)

Authors

Specimens

Section (mm)

Benzaid et al. (2008)

S2R1 S2R2 S2R3

100x100 100x100 100x100

300 300 300

0 8 16

2 2 2

6% 20% 36%

N-0

150x150

800

0

0

-

RF-0

150x150

800

20

3

54%

Hadi M.&Pham T. (2013)

Failure Mode

FRP Rupture FRP Rupture FRP Rupture Concrete Spalling FRP Rupture

From Table 2-4, it can be observed that the FRP confining is more effective in the square sections with the most rounded corners. That occurs because there is a high concentration of stresses at the corners of the sections that are not rounded. The rupture of the FRP jackets was the reason for column failure in all cases. The failure of FRP wraps was observed at or near a corner in all the specimens mainly due to stress concentrations, since the sharp edges of the column were not rounded off. So it is suggested that the sharp corners of the columns should be rounded off [22].

Figure 2-26: Effect of corner radius (Source: Chauhan & Umravia, 2012)

Increase in the axial load-carrying capacity and axial strain was observed in the column with rounded corners, with respect to confined columns with sharped ends [22] [30]. From Table 2-4 we can observe that, the more the column were rounded, the higher axial load capacity.

36

Chapter 2. Literature Review - FRP Confinement The procedure for applied the FRP sheets that was used in the experiments is described below: 

For both experiments the adhesive that was used was a mixture of resin and hardener. Only in [30] is specified that the components were mixed with 5:1 ratio.



The columns were cleaned and dried before the application of the resin.



The adhesive was spread onto the surface of the column and then the first layer of FRP was wrapped directly on the surface.



In order to avoid the voids between the FRP sheet and the concrete surface, a roller was used continuously to remove the entrapped air bubbles.



After the adhesive was spread onto the surface of the first layer of FRP, a second layer was applied.



To avoid debonding of the FRP, each layer was wrapped around the column, having an overlap of 100mm [30] or 1/4 of the perimeter [22].

Researchers have also proposed that the effectiveness of FRP confinement can be increased for square and rectangular columns by changing the cross-sectional shape [31] [30].

The

techniques are used to strengthen an existing square column by circularising with four precast segmental circular concrete covers and then wrapping them with FRP. Table 2-5 below illustrates the results of different tests. All the sections are square (150x150mm) with height of 800mm. Table 2-5: Representative data on RC columns retrofitted with concrete cover and FRP No of FRP Layers

Axial Load Capacity Increase (with respect to unconfined column)

Ductility Increase (with respect to unconfined column)

Authors

Specimens

Height of Segment (mm)

Hadi M.&Pham T. (2013)

N-0

0

0

-

RF-0 CF-0

0 31

3 3

2.2 4.05

R-0

0

-

-

C40-0 C80-0

31 31

3 3

3.6 3.7

3.6 2.7

Concrete Spalling FRP Rupture FRP Rupture Concrete Spalling FRP Rupture FRP Rupture

C100-0

31

3

3.85

3.4

FRP Rupture

Hadi M., Doan L. & Pham T. (2013)

9.5 4.9 -

Failure Mode

The circular concrete covers that were used were cast in the lab and then cleaned to be smooth. In both experiments the four segments were bonded in the columns, using as adhesive a mix of epoxy resin. In the first experiment [30] the adhesive consists of microsphere blend mixed with

37

Chapter 2. Literature Review - FRP Confinement epoxy resin with 2:1 ration. In the other experiment [31], it was consisted of epoxy resin, slow hardener and silica microsphere with a ratio of 5:1:10. Then the specimens were wrapped with the FRP layers. From the behaviour of the confined columns and the failure modes we observe that concrete core and the additional concrete covers worked as a composite material to failure under concentric loads. The confined columns failed in a brittle manner with rupture of FRP and crushing of the concrete covers [31] [30]. Even the proposed strengthening technique to modify the square sections to a circular before wrapping with FRP, is very effective, the cost and the implementation time might be prohibited. The segmental covers are casted in factory, however their application depend on the location and orientation of the column. Also the cost is increased with the use of FRP sheets. The axial load capacity increase is around 1.5-2 times higher when using segmental covers, compared with using only FRP sheets. Although, there is an increase in the stiffness of the columns and as result, the structure. In case of specimen RF-0 where only FRP sheets used to confine, we can observe a significant increase in ductility, compared to the other specimens [30]. So, it might be more beneficial to use only confinement with FRP, since also the cost and implementation time will be significant lower.

Figure 2-28: Segment of concrete (Source: Pham et al., 2013)

Figure 2-27: Bonded specimens (Source: Pham et al., 2013)

38


2.4.2 FRP confinement according CNR-DT 200 R1/2013 The procedure of confinement of reinforced concrete members according the Italian guidelines is presented below. Reference

Equation

Comments

CNR-DT R1/2013: 4.29

𝑁𝑠𝑑 ≤ 𝑁𝑅𝑐𝑐,𝑑

FRP confinement is performed to ensure that this equation is met

CNR-DT R1/2013: 4.30 CNR-DT R1/2013: 4.5.2.(6)

𝑁𝑅𝑐𝑐,𝑑 =

1 ∗ 𝐴𝑐 ∗ 𝑓𝑐𝑐𝑑 + 𝐴𝑠 ∗ 𝑓𝑦𝑑 𝛾𝑅𝑑

Design strength,𝑓𝑐𝑐𝑑 , of confined concrete 2/3

𝑓1,𝑒𝑓𝑓 𝑓𝑐𝑐𝑑 = 1 + 2.6 ∗ ( ) 𝑓𝑐𝑑 𝑓𝑐𝑑

In order for the confinement to be effective

2/3

CNR-DT R1/2013: 4.31 CNR-DT R1/2013: 4.33 CNR-DT R1/2013: 4.37

CNR-DT R1/2013: 4.34

𝑓𝑐𝑐𝑑

𝑓1,𝑒𝑓𝑓 = 𝑓𝑐𝑑 [1 + 2.6 ∗ ( ) 𝑓𝑐𝑑

]

Effective confinement lateral pressure,𝑓1,𝑒𝑓𝑓 , 1 𝑓1,𝑒𝑓𝑓 = 𝑘𝑒𝑓𝑓 ∗ 𝑓1 = 𝑘𝑒𝑓𝑓 ∗ ( ∗ 𝜌𝑓 ∗ 𝐸𝑓 ∗ 𝜀𝑓𝑑,𝑟𝑖𝑑 ) 2

𝑓1,𝑒𝑓𝑓 > 0.05 𝑓𝑐𝑑

𝑓1 is the confinement lateral pressure

Reduced design strain of the composite, 𝜀𝑓𝑑,𝑟𝑖𝑑

𝜀𝑓𝑑,𝑟𝑖𝑑 = 𝑚𝑖𝑛{𝜂𝛼 ∗ 𝜀𝑓𝑘 /𝛾𝑓}; 0.004

Coefficient of effectiveness, 𝒌𝒆𝒇𝒇

𝑘𝑒𝑓𝑓 = 𝑘𝐻 ∗ 𝑘𝑉 ∗ 𝑘𝑎

𝑘𝑉 =vertical effective coefficient 𝑘𝐻 =horizontal effective coefficient 𝑘𝑎 =inclined effective coefficient For continuous wrapping 𝒌𝑽 = 𝟏

Discontinuous Wrapping CNR-DT R1/2013: 4.35

2

𝑝𝑓′ 𝑘𝑉 = (1 − ) 2𝑑𝑚𝑖𝑛

𝑝𝑓′ =clear spacing between the strips

39


2.4.3 FRP confinement according ACI 440.2R-08 The procedure of confinement of reinforced concrete members according the ACI guidelines is presented below. Reference ACI 440.2R08:12-3

Equation Maximum confined compressive strength of concrete, 𝑓𝑐𝑐′

𝑓𝑐𝑐′ = 𝑓𝑐′ = 𝜓𝑓 3.3𝑘𝑎 𝑓𝑙 Maximum confinement pressure , 𝑓𝑙

ACI 440-F: 3-8

ACI 440.2R08:12-5

ACI 440.2R08:12-6

𝑓𝑙 =

2𝐸𝑓 𝑛𝑡𝑓 𝜀𝑓𝑒 2𝐴𝑠ℎ 𝑓𝑦 + 𝐷 𝑠ℎ 𝐷′

Comments a reduction factor of 𝜓𝑓 = 0.95 shape factor 𝑘𝑎 𝐴𝑠ℎ , existing transverse reinforcement bar area 𝑠ℎ , spacing 𝑓𝑦 , yield strength

Effective strain level in the FRP at failure, 𝜀𝑓𝑒

𝜀𝑓𝑒 = 𝑘𝜀 𝜀𝑓𝑢 Maximum compressive stain in the FRP confined concrete, 𝜺𝒄𝒄𝒖

𝜀𝑐𝑐𝑢 = 𝜀𝑐′ (1.50 + 12𝑘𝑏

𝑓1 𝜀𝑓𝑒 0.45 ( ) ) 𝑓𝑐′ 𝜀𝑐′

It should be limited to 𝜀𝑐𝑐𝑢 ≤ 0.01 in order to prevent excessive cracking and the loss of concrete integrity

2.4.4 Comparison of Codes 2.4.4.1

Continuous and Discontinuous wrapping

ACI 440.2R-08, is taking into account only confined RC members with continuous FRP wrapping. The number of FRP plies, n, that are going to be used can be obtained by: 𝑛=

𝑓1 √𝑏 2 + ℎ2 𝜓𝑓 2𝐸𝑓 𝑡𝑓 𝜀𝑓𝑒

However, the ACI 440-F provide an equation for discontinuous wrapping to obtain the number of plies, 𝒏∗ . FRP strips are placed at certain spacing with the total volume of FRP jacket to maintained by 𝑛∗ =

𝐿𝑝 𝑛 𝑛𝑓 𝑤𝑓

40

Chapter 2. Literature Review - FRP Confinement Where: 

𝑛∗ = 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑝𝑙𝑖𝑒𝑠 𝑖𝑛 𝑡ℎ𝑒 𝑑𝑖𝑠𝑐𝑜𝑛𝑡𝑖𝑛𝑢𝑜𝑢𝑠 𝑤𝑟𝑎𝑝𝑝𝑖𝑛𝑔



𝑛 = 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑝𝑙𝑖𝑒𝑠 𝑓𝑜𝑟 𝑐𝑜𝑛𝑡𝑖𝑛𝑢𝑜𝑢𝑠 𝑤𝑟𝑎𝑝𝑠



𝐿𝑝 = 𝑙𝑒𝑛𝑔ℎ𝑡 𝑜𝑓 𝑝𝑙𝑎𝑠𝑡𝑖𝑐 ℎ𝑖𝑛𝑔𝑒



𝑤𝑓 = 𝑤𝑖𝑑𝑡ℎ 𝑜𝑓 𝑤𝑟𝑎𝑝𝑠



𝑛𝑓 = 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑑𝑖𝑠𝑐𝑜𝑛𝑡𝑖𝑛𝑜𝑢𝑠 𝑤𝑟𝑎𝑝𝑠

It also must be taken into account that: 1. Width of the FRP strips should not be less than ¼ the smaller section dimension. 2. Spacing of FRP strips must not exceed 6 bar diameters 𝑑𝑏 of the longitudinal reinforcement. On the other hand CNR-DT 200 R1/2013 in Section 4.5.2.1(6), takes into account both continuous and discontinuous wrapping and define different values for the coefficient of vertical efficiency, 𝑘𝑣 

Continuous wrapping, 𝑘𝑣 = 1



Discontinuous wrapping, 𝑘𝑉 = (1 − 2𝑑

𝑝𝑓′ 𝑚𝑖𝑛

2

)

Where: 

𝑝𝑓′ = 𝑐𝑙𝑒𝑎𝑟 𝑠𝑝𝑎𝑐𝑖𝑛𝑔 𝑜𝑓 𝐹𝑅𝑃 𝑠ℎ𝑒𝑒𝑡𝑠



𝑝𝑓 = 𝑐𝑒𝑛𝑡𝑒𝑟 − 𝑡𝑜 − 𝑐𝑒𝑛𝑡𝑒𝑟 𝑠𝑝𝑎𝑐𝑖𝑛𝑔 𝑜𝑓 𝐹𝑅𝑃 𝑠ℎ𝑒𝑒𝑡𝑠



𝑑𝑚𝑖𝑛 = 𝑚𝑖𝑛𝑖𝑚𝑢𝑚 𝑐𝑟𝑜𝑠𝑠 − 𝑠𝑒𝑐𝑡𝑖𝑜𝑛 𝑜𝑓 𝑡ℎ𝑒 𝑚𝑒𝑚𝑏𝑒𝑟

Reduction of the confinement effectiveness due to the diffusion of stresses between two subsequent wrappings shall be considered. 𝑝𝑓′ ≤ 𝑑𝑚𝑖𝑛 in the case of discontinuous wrapping.

2.4.4.2

Overlapping

To ensure the effectiveness of FRP confinement in case of complete section wrapping, ACI 440.2R-08, suggests minimum overlapping lengths for the FRP plies. The manufacturer should provide overlap lengths for the FRP system, however it is recommended that an overlap length of at least 6 inches (around 150mm) should be used.

41

Chapter 2. Literature Review - FRP Confinement On the contrary, CNR-DT 200 R1/2013, does not specify any previsions about the overlapping length. Despite the suggestions, the overlapping lengths that were used in the experiments that were mentioned below, were 100, mm [30] or 1/4 of the perimeter [22]. Also, there termination points of the plies have not been tapered.

2.4.4.3

Rounding of square and rectangular sections

CNR-DT 200 R1/2013 in Section 4.5.2.1.2(3), taking into account the arch effect that forms within the concrete cross section and affects the effectiveness of confinement. The value of the corner radius, suggested 𝑟𝑐 ≥ 20𝑚𝑚 . So, the horizontal efficiency,𝑘𝐻 , is taking into account the arch effect and can be expressed by: 𝑘𝐻 = 1 −

𝑏 ′2 + ℎ′2 3𝐴𝑔

Figure 2-29: Confinement of rectangular sections (Source: CNR-DT 200 R1/2013)

For noncircular cross sections, ACI 440.2R-08 is Section 12.1.2, provide shape factors 𝑘𝑎 , 𝑘𝑏 that are dependent on: 

The cross-sectional area of the effectively confined concrete 𝐴𝑒 (which is depends on the radius of corners 𝑟𝑐 )



The side-aspect ratio ℎ/𝑏

42


Figure 2-30: Equivalent circular cross section (Source: ACI 440.2R08)

ACI 440.2R-08 in Section 13, suggest a minimum value of 13mm for the radius of corners 𝑟𝑐 , to be used when the sheet is wrapped around outside corners. From Table 2-4, we observe that the maximum value of 𝑟𝑐 that was used in the experiments was 20mm which corresponds to the minimum value of CNR-DT 200 R1/2013. It has been proved that using higher values for the radius of corners, the confinement becomes more effective.

2.4.5 FRP confinement implementation according ACI 440-F A schematic representation of the FRP layout according ACI 440-F, is presented below. The Figure 2-31a), illustrates the discontinuous wrapping application on the column, where FRP sheets with spacing between them, confined the column. In Figure 2-31b), the continuous application of the FRP sheets in the column is presented. In order to avoid the formation of weak spot, it is suggested by the code, that for multiple plies application the overlaps should be staggered around the section of the member. It is also stated, that a minimum of two plies should be used in order to ensure proper confinement response of the plastic hinge.

Figure 2-31: FRP wrapping for confinement and shear strengthening, Left a): Discontinuous wrapping, Right b): Continuous wrapping (Source: ACI440-F)

43


2.4.6 FRP confinement implementation according ReLUIS 2009 On site implementation procedure according to ReLUIS 2009 for confinement is similar with the Shear strengthening implementation. Compared with the codes, ReLUIS 2009, suggest a minimum of 25mm for column corner rounding 𝑟𝑐 , instead of 20mm or 13mm that the CNRDT 200 R1/2013and ACI 440.2R-08 are suggesting accordingly. Furthermore, specifications or suggestions about the overlapping length of the FRP plies or information about the minimum number of FRP plies that should be used, are not provided in ReLUIS 2009, in contrast with the ACI440-F. The application procedure of FRP confinement system that is suggested by ReLUIS 2009, is illustrated below.

1. Rounding the corners The external corners of the beam need to be rounded to prevent the sharp corner edges from causing stress concentrations and premature failure of the FRP wrap. Re-profiling will be done by hand or with suitable non-flying mechanical tools and must guarantee rmin = 25 mm finally, the dust is removed with the use of vacuum cleaner.

(Source: ReLUIS 2009) 2. Preparation of the substrate (a) In a clean and dry concrete surface , using a roller or a brush, a subsequent application of two-component epoxy primer superfluid is made, for the treatment of the support in a clean and dry concrete surface. Width of the treated strip should be equal to the width of the composite strip that is going to be assembled.

(Source: ReLUIS 2009)

44

Chapter 2. Literature Review - FRP Confinement 3. Preparation of the substrate (b) Thixotropic epoxy filler is applied in the concrete substrate with the same width as the sheet that is going to be applied, with the use of flat trowel, in order to standardize and regularize completely even the smallest irregularities on the surface

(Source: ReLUIS 2009) 4. First layer application Application of the first layer of epoxy adhesive of medium viscosity. The application impregnating the fabric with uniform thickness around 0.50 mm, must be applied with a brush or short-haired roller on the layer. The width of the treated strip is equal to the width of the composite strip to be mounted

(Source: ReLUIS 2009) 5. FRP fabric installation The fabric is placed with good tension and constant pressure, with the plastic rolls over to anoint with resin surface of the element to achieve complete impregnation and release of entrapped air. Brush up several times over the impregnated fabric the metal roller, to eliminate the any air bubbles formed during the machining and to stretch the fibers of the fabric band according to its warping. (Source: ReLUIS 2009) 6. Second layer application If a second layer of FRP fabric is applied, the above procedure is repeated, over the previous still fresh adhesive layer, in a uniform thickness of about 0.50 mm to complete coating of the fabric band.


45

Chapter 2. Literature Review - FRP Shear Strengthening

2.5

FRP shear strengthening of reinforced concrete elements

2.5.1 Overview This chapter is dealing with shear strengthening systems design, of reinforced concrete flexural members loaded by transverse loads. The non-prestressed concrete members, reinforced with steel reinforcing bars and only considered. The chapter below, presents the design procedures of ACI 440.2R-08, ACI 440-F (Draft) and CNR-DT 200 R1/2013. It is also focused on the technical implementation and application of the shear strengthening that the above codes are suggesting, but also compares the application procedure that is suggested by ReLUIS (2009). There is a particular need for shear strengthening of reinforced concrete beams in building and structures which were designed and constructed according to older design provisions that lack of proper earthquake design details such as proper anchorage and reinforcement of the sections.

Figure 2-32: diagonal shear crack in a reinforced concrete beam near its connection to a column

Figure 2-33: Cracking due to shear failure in (RC) laboratory test beam

There is a wide variety of possible FRP shear reinforcement configurations that have been proposed in order to increase the shear capacity of reinforced concrete members through FRP retrofit. As illustrated in Figure 2-34 [32], some of those option include bonding to the sides of the beam, U-wrapping, and full wrapping. Shear strengthening can be rather difficult and complicated because of the wide variety and configurations of FRP shear reinforcement, which affect the design and the shear contribution of FRP sheets [1] [2]. FRP is also characterized by high strength in the direction of the fiber orientation. So fibers can be oriented in directions in which they best reinforce shear cracks or it can be also effective to be oriented in two perpendicular directions [32].

46


Figure 2-34: FRP Shear Reinforcement Configurations: (a) Bonded Surface Configurations; (b) FRP Reinforcement Distributions; c) Fiber Orientations; (d) Pseudolsotropic FRP Reinforcement Schemes (Source: Khalifa et al. 1998)

Experimental results evidence that FRP reinforcements of rectangular cross section provide the greater shear strengthening effectiveness [33]. All the three techniques above, have shown increase in the strength of the beam member. Specifically, the most efficient in strength enhancement is the completely wrapping of the section, followed by the three-sided U-wrap and finally the two-side bonding of the beam. The FRP system can be installed as a continuous sheet along the span length of a member or discrete strips with some spacing between them. It has been found from experiments that retrofitted beams with continuous U-wrap textile FRP, had better load deflection behaviour when compared to U-wrap strips [33]. Using continuous textile FRP it is possible to avoid brittle failure of beams because they carried huge deflections before failure and warn before the possible collapse [33]. Experimental results on shear strengthening of RC beams have shown an increase in the ultimate load carrying capacity of beams up to 50% [33] which however, depends on: 

Configuration of the FRP shear reinforcement (fully wrap, U-wrap, etc.)



Distance of FRP sheets (continuous, discrete strips)



FRP material (carbon, glass, etc.)



Number of FRP layers

47


2.5.2 Failure modes Shear failures are typically brittle and should be avoided as a failure mode for concrete members reinforced or strengthened with FRP. They are classified in two types [34] [35]: 

Shear tension failures with FRP rupture (Figure 2-35)



Shear tension failures with FRP debonding (Figure 2-36)

Figure 2-35: Shear failure modes of FRP U-jacketed RC beams (Source: Teng, 2001)

According to experimental results, the modes of failure depend on the shear reinforcement index 𝜌𝑓𝑣 𝐸𝑓 , where: 

𝜌𝑓𝑣 is the ratio of FRP shear reinforcement to the effective cross-sectional area.



𝐸𝑓 is the modulus of elasticity of the FRP stirrups.

As the value of shear reinforcement index increases, the shear capacity in shear–tension increases and the mode of failure changes from shear-tension to shear compression [34]. The existing experiments have shown that almost all beams with wrapped FRPs and some beams bonded with U jackets failed by shear or diagonal tension with FRP rupture [35] [36]. The failure mode of FRP strengthened beams in shear tension or diagonal tension is similar to the respectively shear tension and diagonal tension failures in conventional RC beams. Firstly, vertical flexural cracks occurring that originate from the tension face. Inclined crack that occurs near the support can propagate up to the loading point and in some instances it is possible that diagonal crack form abruptly [35]. As the diagonal crack widens, it can lead to failure that involves also debonding of the FRP material along the line of the diagonal shear crack in the concrete. Moreover, available tests, show that all beams with FRP bonded on sides only and also beams bonded with U-wraps,

48

Chapter 2. Literature Review - FRP Shear Strengthening failed by debonding of the FRP from the concrete [37] [36], Figure 2-36. In this case the beam will fail very quickly in a brittle mechanism, because they are not able to redistribute the forces as soon as the FRP starting to unstuck [37]. So, the bond strength between FRP and concrete is the main parameter in this mode.

Figure 2-36: Debonding failure of the U-shaped beam (Source: Ferreira et al. 2016)

In practise however, the beams cannot fully wrapped around all the sides because they are always cast in the slabs, so the top surface of the beam is under the slab area. Therefore, the presence of a RC slab allows only the three sides of the beam to be exposed. Consequently, shear strengthening can be implemented by wrapping the FRP system around the three sides of the member or by bonding to the two sides of the member [33]. It is suggested that the engineer should have good knowledge for the FRP strengthening system that will be used by the contractor.

49


2.5.3 Shear Strengthening according ACI 440.2R-08 Reference

ACI440.2R08:11-1

Equation

Comments

The design shear strength of a concrete member strengthened with an FRP system should exceed the required shear strength.

𝜑 = 𝑠𝑡𝑟𝑒𝑛𝑔𝑡ℎ 𝑟𝑒𝑑𝑢𝑐𝑡𝑖𝑜𝑛 𝑓𝑎𝑐𝑡𝑜𝑟

𝜑𝑉𝑛 ≥ 𝑉𝑢 ACI 440.2R08:11-1

Shear capacity of an FRPstrengthened concrete member

ACI 440.2R08:11.3

𝜑𝑉𝑛 = 𝜑(𝑉𝑛 + 𝑉𝑠 + 𝜓𝑓 𝑉𝑓 )

ACI 440.2R08:11-3

Shear contribution of the FRP shear reinforcement

ACI 440.2R08:11-4

ACI 440.2R08:11-5

𝑉𝑓 =

𝑉𝑛 = existing shear capacity of the concrete 𝑉𝑠 = shear capacity of the existing steel shear reinforcement 𝑉𝑓 = shear capacity of the FRP strengthening system

Three-sided FRP U-wrap - 𝜓𝑓 = 0.85 Fully-Wrapped sections - 𝜓𝑓 = 0.95

𝐴𝑓𝑣 𝑓𝑓𝑒 (sin 𝑎 + cos 𝑎)𝑑𝑓𝑣 𝑠𝑓

𝐴𝑓𝑣 = 2𝑛𝑡𝑓 𝑤𝑓 Tensile stress in the FRP shear reinforcement at nominal strength 𝑓𝑓𝑒 = 𝜀𝑓𝑒 𝐸𝑓

ACI 440.2R08:11-6a

Maximum effective strain in the FRP strengthening system at failure

ACI 440.2R08:11-6b

𝜀𝑓𝑒 = 0.004 ≤ 0.75𝜀𝑓𝑢

ACI 440.2R08:11-7 ACI 440.2R08:11-8 ACI 440.2R08:11-9 ACI 440.2R08:11-10

Bond-reduction coefficient

ACI 440.2R08:11-11

Shear strength provided by reinforcement

𝜀𝑓𝑒 = 𝑘𝑣 𝜀𝑓𝑢 ≤ 0.004 𝑘𝑣 =

𝑘1 𝑘2 𝐿𝑒 ≤ 0.75 11900𝜀𝑓𝑢

Dimensional variables used in shear strengthening calculations for repair, retrofit, or strengthening using FRP laminates



For completely wrapped members

 Three or two sided wrapped 23300 Active bond length, 𝐿𝑒 = 0.58 (𝑛𝑓 𝑡𝑓 𝐸𝑓 )

𝑘1 = (

2/3 𝑓𝑐′

27

)

𝑑𝑓𝑣 − 𝐿𝑒 𝑓𝑜𝑟 𝑈 − 𝑊𝑟𝑎𝑝𝑠 𝑑𝑓𝑣 𝑘2 = 𝑑𝑓𝑣 − 2𝐿𝑒 𝑓𝑜𝑟 𝑡𝑤𝑜 𝑠𝑖𝑑𝑒𝑠 𝑏𝑜𝑛𝑑𝑒𝑑 { 𝑑𝑓𝑣

𝑉𝑠 + 𝑉𝑓 ≤ 0.66√𝑓𝑐′ 𝑏𝑤 𝑑

The reinforcement limit for the sum the shear strengths is limited based on ACI 318-05 section 11.5.6.9 and is stated on the equation below

50


2.5.4 Shear Strengthening according CNR-DT R1/2013 According to CNR-DT R1/2013:4.3.2, shear strengthening is achieved by applying one or more layers of FRP material externally bonded to the surface of the member to be strengthened. The external FRP reinforcement can be applied discontinuous, with gaps between the strips of continuous, with strips adjacent to each other.

Figure 2-37: FRP strips with angle β=90̊

Figure 2-38: FRP strips with angle β between 0̊ and ̊ (CNR-DT 200 R1/2013)

Reference

Equation

𝑉𝑆𝑑 = 𝑑𝑒𝑠𝑖𝑔𝑛 𝑎𝑝𝑝𝑙𝑖𝑒𝑑 𝑠ℎ𝑒𝑎𝑟 𝑓𝑜𝑟𝑐𝑒

CNR-DT R1/2013:4.3. 3.1 (1) CNR-DT R1/2013: 4.18

Comments

𝑉𝑆𝑑 ≤ 𝑉𝑅𝑑

𝑉𝑅𝑑 = 𝑠ℎ𝑒𝑎𝑟 𝑐𝑎𝑝𝑎𝑐𝑖𝑡𝑦

Shear capacity

𝑉𝑅𝑑,𝑠 = 𝑠𝑡𝑒𝑒𝑙 𝑐𝑜𝑛𝑡𝑟𝑖𝑏𝑢𝑡𝑖𝑜𝑛 𝑡𝑜 𝑠ℎ𝑒𝑎𝑟 𝑐𝑎𝑝𝑎𝑐𝑖𝑡𝑦

𝑉𝑅𝑑 = 𝑚𝑖𝑛{𝑉𝑅𝑑,𝑠 + 𝑉𝑅𝑑,𝑓 , 𝑉𝑅𝑑,𝑐 }

𝑉𝑅𝑑,𝑓 = 𝐹𝑅𝑃 𝑐𝑜𝑛𝑡𝑟𝑖𝑏𝑢𝑡𝑖𝑜𝑛 𝑡𝑜 𝑠ℎ𝑒𝑎𝑟 𝑐𝑎𝑝𝑎𝑐𝑖𝑡𝑦 𝑉𝑅𝑑,𝑐 = 𝑐𝑜𝑛𝑐𝑟𝑒𝑡𝑒 𝑐𝑜𝑛𝑡𝑟𝑖𝑏𝑢𝑡𝑖𝑜𝑛 𝑡𝑜 𝑠ℎ𝑒𝑎𝑟 𝑐𝑎𝑝𝑎𝑐𝑖𝑡𝑦

CNR-DT R1/2013: 13.16 CNR-DT R1/2013: 13.17

𝑉𝑅𝑑,𝑐 = 0.9𝑑𝑏𝑎𝑐 0.5 𝑉𝑅𝑑,𝑠 = 0.9𝑑

𝑓𝑐𝑑 (cot 𝑔𝑎 + cot 𝑔𝜃) 1 + cot 𝑔2 𝜃

𝛢𝑠𝑤 𝑓 (cot 𝑔𝑎 𝑠 𝑦𝑤𝑑

+ cot 𝑔𝜃) sin 𝑎

𝐴𝑠𝑤 , area of steel stirrups S, spacing of steel stirrups 𝑓𝑐𝑑 , design concrete strength 𝑓𝑦𝑤𝑑 , design steel stirrups strength

FRP contribution to the shear capacity CNR-DT R1/2013: 4.19

𝑉𝑅𝑑,𝑓 =

𝑏𝑓 1 0.9𝑑𝑓𝑓𝑒𝑑 2𝑡𝑓 (cot 𝜃 + cot 𝛽) 𝛾𝑅𝑑 𝑝𝑓

 

𝑏𝑓 , width of the FRP strips 𝑝𝑓 , spacing of the FRP strips

51

Chapter 2. Literature Review - FRP Shear Strengthening CNR-DT 200 R1/2013:4.3. 3.1 (3)

50 mm ≤ 𝑏𝑓 ≤ 250 mm 𝑏𝑓 ≤ 𝑝𝑓 ≤ 𝑚𝑖𝑛{0.5𝑑, 3 ∗ 𝑏𝑓, 𝑏𝑓 + 200 𝑚𝑚}

For external FRP reinforcement in the form of discrete strips, strips width, bf (mm), and centre- to-centre spacing between strips, 𝑝𝑓 (mm) shall not exceed the following limitations

2.5.5 Comparison of Codes 2.5.5.1 Environmental conditions considerations ACI 440.2R-08 specifies in section 9.3.3, that in case of complete wrapping, the concrete section should be assessed for the effects of environmental conditions, such as steel corrosion, freezing, vapour pressure, moisture, etc. Moisture can be trapped under the wrap and cause degradation of the interface, substrate, or the FRP composite. It is also specified that means that allow moisture to escape from the concrete structure should be provided. On the other hand, CNR-DT R1/2013, does not specify any provisions or special attention for any effects of environmental conditions to the members that are fully wrapped.

2.5.5.1

Wrapping Configurations

ACI 440.2R-08:11.2 suggests, 3 types of FRP wrapping to be used in rectangular beams or columns and are illustrated in Figure 2-40. 

Completely wrapping of the FRP system is the most efficient wrapping scheme. It is most commonly used, where access to all four sides of the structural element is possible, such as columns. In case of beam elements, access to all sides is limited and completely wrap of the beam is impractical. It is suggested by ACI 440.2R-08:11.2 that other wrapping FRP systems shall be used, such as three-side wrap (U-Wrap) or two-side wrap.



U-wrap, is the most efficient scheme after the completely wrap and is commonly used in T-beams where access to all sides is restricted.



2-side wrap is the least efficient scheme, used when the FRP system cannot be wrapped around either the top or the bottom of the section.

52


Figure 2-40: Typical wrapping schemes for shear strengthening using FRP laminates (Source: ACI 440.2R-08)

Only 2 layouts are considered in CNR-DT 200 R1/2013, fully wrapping and U-wrapping. 2side bonding application is not allowed, due to potential debonding problems occurring under seismic reverse actions.

Figure 2-39: Cross section of FRP strengthened member using complete wrapping

Figure 2-41: Cross section of FRP strengthened member using U-wrapping

In order to avoid debonding of the end portions in case of U-wrap strengthening, CNR-DT 200 R1/2013 suggests in section 4.3.2(4) the use laminates/sheets and/or bars installed in the direction of the members longitudinal axis. Moreover, U-wrap strengthening can be considered equivalent to that of completely wrapped members, if the effectiveness of those devices is proven. However, there not any specifications or any considerations about the anchorage of shear reinforcement in the compression zone. ACI 440.2R-08, follows a more conservative approach and suggests that for non-fully wrapped FRP systems strains are limited by a shear bond-reduction coefficient. The additional capacity reduction factor, 𝜓𝑓 , is used on the FRP contribution and is taken as: 

𝜓𝑓 = 0.85 for three-sided FRP U-wrap



𝜓𝑓 = 0.95 for fully-Wrapped sections

53


2.5.6 Shear Strengthening implementation according ACI 440-F According to ACI 440.2R-08 and ACI 440-F fully wrapping of the section is the most effective shear FRP configuration. It can been applied without any disruption in concrete columns, however in beams because of the slab-beam connection in the full wrapping may not be possible. Therefore, ACI 440-F suggest that discrete strips should be used in order to fully wrap the section. In order to implement that, slits should be made in the slab along the side of the beam to anchor effectively the three-sided systems in the adjoining slab or wall. It is also stated that, special attention should be taken to avoid damage of the existing steel bars normal to the span of the beam.

Figure 2-42: FRP wrapping for confinement and shear strengthening, Left: Full-wrapping through the T-beam, Right: Field implementation (Source: ACI 440-F)

2.5.7 Shear Strengthening implementation according ReLUIS 2009 ReLUIS 2009, provides application procedures of FRP shear reinforcement on beams. It includes the application of discontinuous reinforcement (Figure 2-46 & Figure 2-44) or continuous (Figure 2-45). ReLUIS 2009 also suggests, the use of fan-shaped anchors in cases that is possible. From Figure 2-43 & Figure 2-46 we observe that ReLUIS 2009, suggests installation of drilled-in fan-shaped anchors by drilling holes up to the slab level.

54


Figure 2-44: Discontinuous shear reinforcement with FRP composites of an internal beam (Source: ReLUIS 2009)

Figure 2-45: Continuous FRP wrapping in internal beam (Source: ReLUIS 2009)

Figure 2-46: Discontinuous shear reinforcement with FRP composites of an internal beam with fan-shaped anchors (Source: ReLUIS 2009)

Figure 2-43: Continuous FRP wrapping in internal beam with fan-shaped anchors (Source: ReLUIS 2009)

The first steps of FRP application are similar between the different applications (shear, flexural, etc.). Firstly, the substrate wrappers of carbon must be level, dry and free of materials that prevent bonding and have sufficient strength. Remove any loose parts and substrate determined and rubbed with appropriate means, depending on the degree and extent of weathering. The external corners of the beam need to be rounded to prevent the sharp corner edges from causing stress concentrations and premature failure of the FRP wrap. Re-profiling will be done by hand or with suitable non-flying mechanical tools and must guarantee rmin = 25 mm finally, the dust is removed with the use of vacuum cleaner. In case of cracks in reinforced concrete elements, structural repair should be carried out, with the use of injectable epoxy resins, depending on the size of the crack to ensure monolithic adhesion between the two damaged parts and for the entire depth of the lesion. The geometry of the element but also levelling of the substrate are restored using mortar with controlled shrinkage thixotropic.

55

Chapter 2. Literature Review - FRP Shear Strengthening 1. Preparation of the substrate In order to optimize the adhesion efficiency of the overall system of FRP reinforcement and the support, through the preparation of the substrate (elimination of irregularities in the concrete surface at the interface), proceed by shaving and levelling through direct reporting of thixotropic epoxy filler, for the regularization of the support surface in about the application of the product must be performed on still "fresh primer "(if present) with a notched trowel in a thickness of about 1-2 mm. Next smoothing the adhesive with a flat trowel, in order to standardize and regularize completely even the smallest irregularities on the surface. (Source: ReLUIS 2009) 2. First layer application Application of the first layer of epoxy adhesive of medium viscosity. The application impregnating the fabric with uniform thickness around 0.50 mm, must be applied with a brush or short-haired roller on the layer. The width of the treated strip is equal to the width of the composite strip to be mounted. (Source: ReLUIS 2009) 3. FRP fabric installation Positioning of bands of fabric immediately after application of the first layer of primer, taking care to lay them without wrinkling, to achieve complete impregnation and release the entrapped air. Penetration of the adhesive and the resin through the fibers (impregnation) should by achieved with suitable metal roller.

(Source: ReLUIS 2009) 4. Second layer installation The application of a second layer of fabric the same procedure must be carried out by brush or short hair roller, over the previous still fresh adhesive layer, in a uniform thickness of about 0.50 mm to complete coating of the fabric band.


56


2.5.8 Special Anchorage for Shear Strengthening In order to improve the effectiveness of the externally applied FRP in shear strengthening of T-beams, anchorage systems can be provided. The current codes are not clearly define any anchorage mechanisms that can be used in shear strengthening. As it is discussed in 2.5.4, ACI 440-F propose that in order to apply effective shear FRP, fully wrap of the section should be made by cutting the slab along the side of the beam. The revised Italian guidelines CNR-DT 200 R1/2013, refer that ‘by using laminates/sheets and/or bars installed in the direction of member longitudinal axis, the behavior of U-wrap can be considered equivalent to that a completely wrapped member, provide the effectiveness of those devices in proven.’ However, they are do not propose any anchorage system of configuration. Anchorage systems can usually consist of metallic elements (such as plates and bolts, etc.) or FRP anchors. Although the metallic anchors are effective they have some major drawbacks such as compatibility with jacket materials, high weight and require protection against corrosion [19]. Experiments have been made to examine the performance of anchors in different orientations, spacing and material. Their main purpose is to prevent or delay debonding failures. The authors of this paper have examine the performance of anchors placed horizontally (in the web) versus vertically (in the slab), the number and spacing of anchors and the role of carbon versus glass fibers in the anchors [19]. In the Figure 2-47 below we can observe the different anchor orientations that proposed. It is mentioned that inclined anchor that placed into the concrete had angle approximately equal to 25° with respect to the vertical. The purpose of that is to minimize the difficulty and cost of drilling vertical holes inside slabs exactly at the corners where they meet the web.

Figure 2-47: Configuration of anchors for beams and dimensions of embedment depth and length of anchorages (left: horizontally placed, right: inclined) (Source: Koutas & Triantafillou 2013)

57

Chapter 2. Literature Review - FRP Shear Strengthening With these anchoring methods Figure 2-48, the anchor Type A are subjected to pull-out forces where the tension forces are transferring from the FRP sheet into the slab and anchor Type B to shear forces where the tension forces are transferring from the FRP sheet below the concrete slab into the web [19] [18].

Figure 2-48: FRP Anchor types and dowel stress distribution (Source: Kim & Smith, 2010)

The suggested effective embedment depth after which the capacity of the anchor no longer increases has been found 100mm [38]. Pull-out tests have shown that as the embedment depth increases the average bond strength decreases [39]. The results of the experiment show that the spike anchors can provide viable solutions towards enhancing the shear resistance of RC T-beams. It has been found that there was at least 39% increase of the strength of T-beam with anchors comparing to the one without. Moreover, anchors placed inside the slab (almost vertical), are much more effective than those placed horizontally inside the web and by increasing the number of anchors in the shear span, there was also increase in shear resistance [19]. Other experiments have tried to achieve effective full-wrap scheme by anchoring the U-wrap strengthening scheme in order to avoid premature debonding failure [20].

Figure 2-50: CFRP ropes (Source: ElSaikaly et al., 2015)

Figure 2-49: Left: U-wrap specimen cross section; Right: cross section of specimen strengthened with CFRP Lstrips and anchored with CFRP rope (Source: El-Saikaly et al. 2015)

58

Chapter 2. Literature Review - FRP Shear Strengthening With this method, CFRP ropes were inserted through holes that were drilled at the web-flange intersection and flaring their ends into the free ends of the U-wrap CFRP L-strips. The results of the study show that CFRP rope can prevent the debonding failure mode of the CFRP L-strips plates and convert the strengthening scheme from a U-wrap to a fully wrapped strengthening scheme [20]. Both anchoring mechanisms were consisted from a bundle of FRP fibers. In the second case the fibers were held together by a thin tissue net, which was removed during the installation procedure [20]. The procedure that was used in order to insert the anchors for both experiments is described below. 

In both experiments the initial steps of the procedure include the preparation of the concrete surface, by roughening using a grinding machine and then cleaning with compressed air to remove dust and loose particles.



It is stated that the beam edges were rounded to a radius equal to 20 mm [19].



The epoxy resin was then applied and the sheets were placed against the epoxy- resin coating. The holes were drilled in the points that was indicated and then cleaned with compressed air and water. In the first experiment, holes of 12mm diameter and 70mm deep were drilled, and then filled with epoxy up to half of their depths [19]. In the second experiment holes of 15mm diameter were drilled, through the web of the beams. Before inserting the CFRP ropes in the holes, they were impregnated for 30 minutes in a low-viscosity epoxy [20]. After the ropes were inserted, the hole was partially filled with an epoxy adhesive.



In both experiments the anchors were inserted in the holes and then extended in the overlap length that was proposed (80mm, 120mm).



Finally the anchors were pressed in the strips in order to ensure proper bond between the two surfaces [19] [20]. For the first experiment, it is also stated that the second layer of FRP was then applied [19].

59

Chapter 2. Literature Review - FRP Shear Strengthening Table 2-6: Results of strengthening procedure for both experiments

Authors

Testing Beam

Dimensions (mm) Koutas L. & Triantaf illou T. (2013)

1750 x300 x300

Shear Span (mm)

600

Anchorage

Specimen

Configur ation

Hole diameter (mm)

Embedd ed Depth (mm)

Distanc es (mm)

CON

-

12

70

-

U2C

-

12

70

-

Inclined

12

70

167

Horizont al

12

70

167

Inclined

12

70

100

-

-

-

-

-

175

U2CAN3Cin U2CAN3Ch U2CAN5Cin

SO

ElSaikaly G. et al. (2014)

4520 x508 x406

FRP Layer s

S1-LS 1050

No Anchora ge No Anchora ge

2 CFRP 2 CFRP 2 CFRP 2 CFRP 2 CFRP

Increase in Shear Resistanc e

Failure Mode

DC

1.4 2.0 1.5 2.12

DC,D DC, D, A.R DC,D. A.P DC, D, A.R -

S1-LSRope

Horizont al

15

Through Slab

260

S3-LSRope

Horizont al

15

Through Slab

175

Steel LStrips Steel LStrips Steel LStrips

D. 2.41 C.C. 3 C.C. 3.32

Where:  D.C. - Diagonal cracking  D. - Debonding of FRP  A.P. - Anchor pullout  A.R. - Anchor rupture  C.C. – Concrete Crushing Even though the results of the experiment are characterized as preliminary the practical field implementation of this method is debatable. From Table 2-6, we observe that the spacing between the anchors is very close. So, in cases of applying the FRP sheets in all over the length of the beam, it might be difficult and not cost effective to drill holes in such close distances through the beam. The above methods also, require the drilling of holes with large diameters, which means that more epoxy resin will also be used. Moreover, special attention should be taken in order not to damage the existing steel bars in the span of the beam when drilling the holes. Specifically in the case that require the holes to pass through the width of the beam in order to insert the

60

Chapter 2. Literature Review - FRP Shear Strengthening anchorage, is difficult to have such large holes in such close distances without affecting the longitudinal or the tension reinforcement of the beam. So, in both cases the anchorage and accordingly the FRP strips (if strips are applied) are limited to be placed between the spacing of the internal steel hoops. Moreover, in the application procedure for the experiment [19], is stated that the anchors were inserted after the first layer of FRP wrap was applied, so through the layer. However, this type of application can reduce the effective area of the FRP, which can damage the wrap under loading.

61

Chapter 2. Literature Review - FRP Beam-Column Joint Strengthening

2.6 FRP strengthening design of beam-column joints 2.6.1 Overview Reinforced concrete structures that have designed, especially before 1970s have shown a particular vulnerability, during their exposure to earthquakes. Serious structural deficiencies that arise from lack of capacity design approach and/or poor detailing of reinforcement. In several post-earthquake studies, it has been reported that the main cause of building collapse was failure of beam-column joints, especially exterior joints. Fiber reinforced polymers can be used to strengthen the existing beam-column joints and enhances their flexural or shear capacities. The main aims of the FRP strengthening are [40] [41]: 

Provide anchorage to the joint that improves the bond of the bottom reinforcement bars of the beam.



Satisfy the strong-column weak-beam design and move the plastic hinge away from the column.



Protect the beam critical regions against shear failure.



Improve shear capacity of joints.

2.6.2 Shear and bond strengthening The purpose of those retrofit schemes are to improve the beam bar bond-slip capacity and increase the shear strengthening of the joint. To achieve that, FRP sheets were placed along the beam bottom face and along the inside face of the bottom column and also U-wraps were used to achieve the shear strengthening of the beams [42]. In other cases full wrapping of the column was used as long as diagonal placed U-wraps at an angle of 30° [43] [44]. The results shown, increase of the strength and change of the failure mode. Moreover, the joint shear capacity can be improved.

2.6.3 Shear strengthening of two-dimensional exterior joints Many experiments have been implemented on shear strengthening of two-dimensional exterior joints [45] [46] [47]. Using FRP sheets or strip at the top and/or bottom face of the beam and the column in different numbers, configurations (X-shaped fibres at 45°, U-shaped, etc.) and geometries, the behaviour of the joint was examined, but also in case of presence of a transverse beam [45] [47]. Moreover, effectiveness of the use of FRP for the strengthening of beam–

62

Chapter 2. Literature Review - FRP Beam-Column Joint Strengthening column connections was also examined, when full wrapping of beams and columns was applied [46].

Figure 2-51: Strengthening schemes proposed by Antonopoulos and Triantafillou (Source: Antonopoulos & Triantafillou, 2003)

Figure 2-52: Strengthening schemes proposed by LeTrung et al. (Source: Le-Trung et al. 2010)

The results have shown better performance of specimens with sheets comparing those strengthened with strips, since the anchorage of strips is more difficult and might require the use of mechanical anchorage [45] [47]. By using more FRP layers the strength and energy dissipation of the specimens was increased, however not proportionally. Furthermore, the presence of a transverse beam decreased the effectiveness of the FRP retrofit. In case of full wrapping of beam and column, the joint was strengthened effectively and the damage was relocated into the beam [46].

2.6.4 Shear strengthening of two-dimensional exterior joints with slabs of transverse beams The shear strengthening of exterior/corner joints has been investigated. Experiments have been conducted taking account the effect of slab and transverse beam. FRP retrofit schemes were used in order to improve the flexural and shear capacity of the columns. In case of specimen with slab the beam and column were fully wrapped with FRP sheets by drilling slots in the slab. That resulted in increasing the energy dissipation of the joint and in some cases preventing joint shear failure and moving the mechanism of failure to top column flexural hinging.

63


2.6.5 Limitations of the proposed methods The majority of the experiments did not take account the existence of slab or transverse beam in the joint. Some experiment suggest the full wrapping of columns and beams, however if slab is present this is impossible without making cut-outs in the slab. Also, in cases where Xwrapping of the joints is used, the structures would undergo under significant changes, since large cuts through the slab and transverse beams would require. Also, it is questionable how those strengthening schemes can be implement in cases where transverse beams and 4-sided interior beam-column joint are used.

2.6.6 FRP strengthening of three-Dimensional beam-column joint To derive more realistic conclusions about the effect of FRP strengthening on threedimensional (3D) beam-column joints with the presence of floor slabs, the experiment of a reinforced concrete frame building that tested on the shake table at the University of Canterbury was selected [48]. The specimen consists of two 3-storey 2-bay asymmetric frames in parallel, and is 1/2.5 scaled version of the original prototype. The strengthening procedure is illustrated in the following figures. Moreover, the same researcher propose an improved retrofit method, tested on four 2/3 scale three-dimensional corner beam-column joint test specimens and is also illustrated below [49].

Figure 2-53: Dimensions and application sequence of FRP sheets for one-way exterior joints (Source: Akgüzel et al. 2011)

64


Figure 2-55: Dimensions and application sequence of FRP sheets for corner exterior joints of the improved method (Source: Akguzel & Pampanin, 2012)

Figure 2-54: Details of FRP anchor dowels installed in slab and beam elements (Source: Akguzel & Pampanin, 2012)

FRP strengthening application proccedure: 

Step 1: Installation of vertical FRP sheets on the exterior sides of the column faces to enhance its flexural capacity.



Step 2: Horizontal FRP sheet installation on the joint region



Step 3 & 4: Vertical and horizontal FRP laminates were installed inside the column and beam faces.

65

Chapter 2. Literature Review - FRP Beam-Column Joint Strengthening 

Step 5: In order to increase the flexural and the shear strenght of beam and slab elements, wrapping of the corner joint of the beams with FRP sheets, starting from the bottom face of the slab and extended to the top of the slab.

In case of one-way exterior joints, a horizontal layer of FRP sheet is also attached on the top of the slab inside the column to increase the flexural strength of the slab (Step 7,Figure 2-53) 

Step 6: Column confinement with FRP



Step 7 & 8, (9): FRP anchor dowels were installed in the beam and slabs to avoid early FRP debonding problems and also strengthen the laminates against buckling under compression loads

The procedure of anchor preparation is described below: 1. The FRP anchors were prepared by twisting the strips of FRP sheets, folding into two and epoxying into pre-drilled holes in the beams and slab. 2. The anchors are plugged into the holes, during the application of epoxy resin. 3. Anchor fans outside the holes are glued to the FRP sheets that have already applied on beam and joint faces. Finally, cutouts were made in the slab reinforcement in the perimeter of FRP layers that have been anchored into the top of the slab, with the purpose of reducing the over-strength effects and deformation demand from the adjacent slab element [49]. The results of the 2/3-scaled tests, shown that minor damage observed on the joint and no debonding of the FRP occurred in the beam and column of the retrofitted specimens [48]. Also, an increase of strength up to 40% and enhancement of displacement ductility was observed on the retrofitted specimen compared to the original specimen. However, the conclusions of the full scale tests demonstrated that the global behaviour of the structure, cannot improved when only the exterior joints are retrofitted [49]. So it is essential that full-scale experiments that also retrofit the interior joints will carried out. Moreover, in both experiments the researchers do not specify the reason of selecting the specific number of anchors on the beam-column joints, to prevent debonding. The procedure of cutting the slab in order to reduce the over-strength effects is not identified and could be challenging and effect the behaviour of the slab, since it will require cutting of the slab reinforcement.

66


2.6.7 Strengthening according CNR-DT 200 R1/2013 CNR-DT 200 R1/2013 do not provided specific design procedure for the design of FRP reinforcement, to strengthen the beam-column joint. However at clause 4.7.2.3.2, it is suggested that capacity design criteria should be followed, to prevent the formation of plastic hinging in the column before the beam. According to clause 4.7.2.1.4 (1), ''Beam-column joints of RC members can be effectively strengthened with FRP only when FRP reinforcement is applied with the fibers running in the direction of principal tensile stresses and provided the FRP reinforcement is properly anchored''. The FRP strengthening is not considered effective, when the FRP reinforcement is not properly anchored. It is also suggested, that the maximum tensile strain for FRP reinforcement should not be larger than 4%.

2.6.8 Joint strengthening implementation according ReLUIS 2009 Interventions for the strengthening of the unconfined beam-column joints, mainly for the joints on the perimeter of the structure, or the corner (corner joint) or in front (intermediate joint) are provided by ReLUIS 2009. It is suggested that the proposed interventions can increase the shear strength of the beams and pillars in their ends, where the maximum ductility occurs and prevent the collapse mechanism. In order to increase the resistance capacity of the joint panel capacity, inclined metal fabrics as shown in Figure 2-56a, should be used. Also L-shaped carbon fiber fabric quadriaxial bands should be placed at the intersection of the beams with the pillar, Figure 2-56b.

Figure 2-56: a) Diagonal bands with unidirectional metallic fabric of intermediate node (external view) b) L-shaped carbon fiber fabric placed at the intersection of quadriaxial beams with the pillar of a corner node (internal view)

67

Chapter 2. Literature Review - FRP Beam-Column Joint Strengthening Increase in the node panel shear strength The increase of the node panel shear strength can be achieved, in the case of using composite materials, by means of carbon quadriaxial fabric arrangement (as shown in Figure 2-57 for corner node and in Figure 2-58 for intermediate node)

Figure 2-57: Quadriaxial balanced fabric of carbon fiber placed in correspondence of the node panel on corner node (the quadriaxial fabric is also disposed on the inner face of the emerging beam, not visible in the figure)

Figure 2-58: Quadriaxial balanced fabric of carbon fiber placed in correspondence of the panel of an intermediate node.

Confinement of the ends of the pillars The confinement of column ends, allows increase in the shear strength and the deformation capacity. The confinement can be achieved by wrapping the columns with unidirectional carbon fabric, Figure 2-60 & Figure 2-59.

Figure 2-59: Column confinement of corner node

Figure 2-60: Column confinement of an intermediate node

68

Chapter 2. Literature Review - FRP Beam-Column Joint Strengthening Increase in the shear strength of the beam ends The benefits of the shear strengthening of beams have been described in SECTION X before. Although in order to achieve that, U-wrapping with carbon fibers can be used as anchorage, as shown in Figure 2-61 & Figure 2-62.

Figure 2-62: Shear reinforcement with U-shaped configuration at the beam ends of a corner node

Figure 2-61: Shear reinforcement with Ushaped configuration at the beam ends of an intermediate node

The application fiber-reinforced composite made from high-strength steel fibers (SRP) in the form of high-strength steel wires in unidirectional fabric (ropes) is described below. The surface preparation follows the same procedure that have been described in the previous chapters.

69


1. Fabric Application After the surface preparation and the fabrics are placed immediately after application of the levelling shaving, laying them without wrinkling.

(Source: ReLUIS 2009) 2. Metal Connector installation Metal connectors (nails) are installed, in order to fix the fabric in adherence to the surface a.c. support and for keeping the correct positioning of the fabric during the execution of the subsequent application stages. Insert the resin through the fibers (impregnation) in order to penetrate the thixotropic epoxy putty in the fabric


3. Application of the second layer of epoxy Application of the second layer of epoxy grout thixotropic, for the impregnation of the fabric, taking care to fully cover the fibers. The application must be performed on the first layer still "fresh" with a notched trowel in a thickness of around 1-2 mm and subsequent smoothing of the epoxy grout with a flat spatula, in order to level and stabilize up to the complete coating and impregnation of the fabric, in order to avoid accidental contact between steel fibers (SRP) and carbon (FRP). (Source: ReLUIS 2009) To remove the air trapped on the fabric, brushing with a metal roller should happen. 4. Second FRP layer application For the application of new layers of overlapping fabric it is necessary that the previous steps will be repeated as many times, as there are layers to be applied. If the application is terminated, fine sand is applied on the last resin layer, in order to ensure the future adhesion of the resins suitable for subsequent processing to completion.

70


2.6.9 Joint strengthening implementation according ACI 440F Suggestions for the joint strengthening using FRP are provided by ACI 440F, based on experimental evidence [45] [50] [44]. Those experiments can provide guidance and information about determining if an FRP system can be used to enhance the performance of unconfined joints. The layout and detailing of FRP depends on the geometry of the existing joints and the number of member framing in to it. To resist the seismic force, FRP reinforcement in both directions is required. Flexural Strengthening With the purpose of increasing the flexural capacity and the shear strength of the joint, vertical FRP laminates and NSM bars can be installed on the sides of the column. In latest draft version of the ACI-440 F (Chapter 13), it is suggested that in seismic applications within plastic hinge regions, confinement of the flexural FRP reinforcement should be made using FRP strips that completely wrap around the perimeter of the section to enhance the resistance against debonding of the flexural reinforcement. Shear Strengthening In case of exterior joint a horizontal laminate wrapped around the exterior face of the joint is used to increase the joint shear strength and prevent the expulsion of a concrete wedge, Figure 2-63. The anchorage length of the horizontal FRP layer into the beam is limited by the specified length that is provided by the FRP manufacturer. Additional strips of FRP can be used in the beams and columns to anchor the main FRP laminates. In case of floor system existence, localized cutting of the slab might be required to complete wrap the beam with FRP. Away from the plastic hinge region, U-wraps or other anchorage systems should be used to provide anchorage to the FRP flexural reinforcement. In Figure 2-64, we observe the proposed layout of a joint strengthened with FRP with the transverse FRP wraps run continuously through the joint, which may require cut-outs at the corners of the slab

Figure 2-63: Laboratory Retrofitted Unit (Akguzel et al., 2008)

Figure 2-64: Conceptual FRP strengthening detail (Source: ACI 440F, Chapter 13, 2014)

71

Chapter 3. Research Objectives & Methodology

Chapter 3. Research objectives and Methodology The main objective of this thesis is the comparison between the design according to the currently used codes and the actual implementation on the construction site in order to point out any discrepancies and compatibility issues between the design and the construction procedure. To achieve the objectives that have been set, the present study is based on three phases. In the initial stage, with the critical literature review, the differences between the design codes and the on-site implementation were identified. In the second phase of the project, the case study will be modelled using SeismoStruct software. Pushover analysis is performed to predict the expected response of the building under earthquake loading. In order to evaluate the performance of structure, the capacity spectrum of the structure is compared to the seismic demand spectrum. Different strengthening implementations using FRP materials and following the two design codes (CNR-DT & ACI) are made in order to reach the performance objective that have been set by the current codes. Finally, analysis and discussion of the derived results will be implemented.

3.1

Case study description

3.1.1 Description of the structure The structure of the case study is defined as a 6-floor reinforced concrete building, located in the island of Crete in south Greece. The building constructed in 1970s, according to the earlier seismic National Codes, considering both gravity and seismic loads. The building plan is consisted of 3 frames and 6 bays with global dimension of 24 m in x-direction and 10 m in ydirection. The height of all storeys is 3 m. 1

1

2

3

4

5

6

2

3

4

5

6

7

7 3m

3m

3m

3m

3m

3m

A

3m

X

3m B

Y

3m 250x250mm 3m

3m

4m C

3m 350x350mm

3m D

3m 3m

Figure 3-2: Plan view of structure

3m

3m

3m

3m

3m

Figure 3-1: Side view (y-direction)

72

Chapter 3. Research Objectives & Methodology The structural system consists of beams and columns. The dimensions of the construction elements are: 

Slab with 150mm thickness.



Beams: 25 cm width x 45 cm depth



Columns (Floor 1-3): 35 cm x 35 cm



Columns (Floor 4-6): 25 cm x 25 cm 3 F 16 Bars

250mm

350mm 4 F 16 Bars

25mm

F 8/200mm

450mm

2 F 16 Bars

F 8/200mm

F 8/200mm

250mm

350mm 2 F 16 Bars

25mm 250mm

Figure 3-5: Beam Cross-Section

2 F 16 Bars

3 F 16 Bars

Figure 3-4: Column cross-sectionFigure 3-3: Column cross-section (Floors 0-3) (Floors 4-6)

3.2 Material Properties The structure is made of reinforced concrete class C20/25 with a specific weight of 25 KN/m³. The elasticity of steel is taken as Es = 200GPa and the specific weight of Mild250 steel as 78.5 KN/m³. Concerning the structural material adopted, using the safety factors from EC2, § 2.4.2.4, 𝛾𝑐 = 1.5 and 𝛾𝑠 = 1.15, for concrete and steel strength respectively and the coefficient 𝑎𝑐𝑐 = 1 taking account of long term effects on compressive strength, the following mechanical properties will be considered in the design process. Concrete 𝑓𝑐𝑘 = 20 𝑀𝑝𝑎 𝑓𝑐𝑚 = 28 𝑀𝑝𝑎 𝑓𝑐𝑡𝑚 = 2.2 𝑓𝑐𝑑 = 13.33 𝑀𝑝𝑎 Steel 𝑓𝑦𝑘 = 250 𝑀𝑝𝑎 𝑓𝑦𝑑 = 217 𝑀𝑝𝑎

73


3.3

Actions

3.3.1 Vertical Actions In a seismic design situation the vertical actions (permanent loads “G” and variable-live loads “Q”) have to be taken into account. The permanent loads “G” are represented by the self-weight of the structure and additional permanent load. For later load the uniformly distributed load equal to 2.00 kN/m2 is assumed The variable-live loads are, in a seismic design situation, reduced with a factor of 2i = 0.3 (EN 1990/Table A.1.1).

Loading

References Dead Loads

Design loads from Materials kN/m

kN/m2

General Floor finishes

2.00 Imposed Loads kN/m2

Imposed Load

-

2.00

Concrete Self-Weigh

24.99 kN/ m3

Steel Self-Weight

76.97 kN/ m3

Ultimate limit State (ULS)

Permanent actions

1.35

ϒG, sup EC1 & NA.1 Variable actions ϒQ, sup

1.5

The floor systems consist of a reinforced concrete slab supported on a rectangular grid of beams. The load to floor beams is distributed using triangular distribution, assuming that each of the beams along the edges of the slab carries the same triangular load.

74


3.3.2 Floor Masses The seismic action effects computed considering the masses associate to the following gravity load combination:

𝐺𝑘 + ∑(𝜓𝐸𝑖 𝑄𝑘𝑖 ) 𝑖 The floor masses are determined according to (EN 1998-1/3.4.2) 

Complete masses resulting from the permanent load (self-weight of the structure + 2 𝑘𝑁/𝑚2



Masses from the variable-live load are reduced using the factor 𝜓𝐸𝑖 = 𝜑𝛹2𝑖



Factor 𝛹2𝑖 amounts to 0.3 in the case of an office building (EN 1990 § Table A.1.1).



Factor 𝜑 is equal to 1.0 for the roof story and 0.5 for other story’s (EN 1998-1 § 4.2.4, Table 4.2).

Table 3-1: Mass distribution of the 6-storey building

Floor Level

Wfloor (kN)

1

2248

Mfloor (tonnes) 229

2

2248

229

3

2185

223

4

2122

216

5

2122

216

6

2056

210

Total

12981

1323

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3.3.3 Seismic Loading The building is located in Crete, Greece. The Greek seismic hazard map is presented by seismic building code “EAK-2000”. Seismic zonation has been based on ground acceleration values with 10% probability of exceedance in 50 years, i.e., 475years mean return period. According to Figure 3-6, Crete is considered as Seismic Zone II with PGA=0.24g for Ground Type A.

Figure 3-6: Seismic hazard map of Greece (Source: EAK 2000)

According to EC8, Part 1 , Clause 3.2.2.2 Note (2), If the earthquakes that contribute most to the seismic hazard defined for the site for the purpose of probabilistic hazard assessment have a surface-wave magnitude, Ms, not greater than 5,5, it is recommended that the Type 2 spectrum is adopted. Because Greece is a high seismicity area we choose Type 1 Elastic spectrum. Ground Type

S

Tb (s)

Tc (s)

Td (s)

A

1.0

0.15

0.40

2

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3.3.3.1

Behavior factors for horizontal seismic actions

Ductility class is Medium Ductility (DCM) The upper limit value of the behaviour factor q, introduced in 3.2.2.5(3) to account for energy shall be derived for each design direction as follows: 𝑞 = 𝑞0 ∗ 𝑘𝑤 ≥ 1.5 

The basic value of the behavior factor, 𝑞0 for frame system, dual system, coupled wall system is 3,0 au⁄a1 (EC8: Part1 § Table 5.1)



The value au⁄a1 for multistory, multi-bay frames or frame-equivalent dual structures is 1.3 (EC8: Part1 § 5.2.2.2 (5))



The factor 𝑘𝑤 = 1,00, for frame and frame equivalent dual systems (EC8: Part1 § 5.2.2.2 (11))

So 𝑞 = 3 ∗ 1.3 ∗ 1 = 3.9

3.3.3.2

Estimation of the fundamental period

Fundamental period of vibration period T1 of the building (EC8-1 § 4.33.21)

T1 = Ct H3/4 Where: 

Ct = 0.075 for moment resistant space concrete frames.



H is the height of the building from foundation.

So: T1 = Ct H3/4 T1 = 0.075 x 18 ¾ T1 = 0.655 s According Eurocode 8 § 3.2.2.5 Note (4) For the horizontal components of the seismic action the design spectrum, Sd(T), shall be defined by the following expressions: 2 𝑇 2.5 2 0 ≤ T ≤ 𝑇𝐵 ∶ 𝑆𝑑 (𝑇) = 𝑎𝑔 ∗ 𝑆 ∗ [ + ∗ ( − )] 3 𝑇𝐵 𝑞 3 𝑇𝐵 ≤ T ≤ 𝑇𝑐 ∶ 𝑆𝑑 (𝑇) = 𝑎𝑔 ∗ 𝑆 ∗

2.5 𝑞

77

Chapter 3. Research Objectives & Methodology 2.5 𝑇𝑐 𝑎𝑔 ∗ 𝑆 ∗ ∗[ ] 𝑞 𝑇 𝑇𝑐 ≤ T ≤ 𝑇𝐷 ∶ 𝑆𝑑 (𝑇) = { ≥ 𝛽 ∗ 𝑎𝑔 𝑇𝐷 ≤ T ∶ 𝑆𝑑 (𝑇) = {

2.5 𝑇𝑐 𝑇𝐷 ∗[ 2 ] 𝑞 𝑇 ≥ 𝛽 ∗ 𝑎𝑔

𝑎𝑔 ∗ 𝑆 ∗

In this case T1 = 0.655 s so, 𝑇𝑐 ≤ T ≤ 𝑇𝐷 = 0.4 ≤ 0.655 ≤ 2

3.3.3.3 Lateral force method According to EC8-1 § 4.3.3.2.1(1, this type of analysis may be applied to buildings whose response is not significantly affected by contributions from modes of vibration higher than the fundamental mode in each principal direction. It should be satisfied that:

𝑇1 ≤ { In this case since, 𝑇1 = 0.655 ≤ {

4 ∗ 𝑇𝑐 2.0 𝑠

4 ∗ 0.4 so this method can be used. 2.0 𝑠

For the different importance classes, the design ground acceleration presented below: 𝑎𝑔 = 𝛾1 ∗ 𝑎𝑔𝑅 Where:  

Ag: is the design ground acceleration on type A ground γ1: importance factor

From Table 1.2.1 for Building category III - γ1= 1.2 𝑎𝑔 = 𝛾1 ∗ 𝑎𝑔𝑅 = 1.2 ∗ 0.24𝑔 = 0.288𝑔

𝑆𝑑 (𝑇) = 0.288𝑔 ∗ 1 ∗

2.5 0.4 ∗ = 0.93𝑠 3.9 0.655

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3.3.3.3

Base shear force

According to EC8 part 1 § 4.3.3.2.2 (1), the seismic base shear force Fb, for each horizontal direction in which the building is analysed, shall be determined using the following expression:

𝐹𝑏 = 𝑆𝑑 (𝑇1) 𝑚 𝜆 Where: 

Sd (T1) is the ordinate of the design spectrum at period T1.



m is the total mass of the building.



λ is the correction factor.

From EC8 Clause 4.3.3.2.2, the building has more than two storeys, or λ = 1,0

𝐹𝑏 = 0.93 ∗ 1323 ∗ 1 = 1230 𝑘𝑁

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3.4

Numerical Model & Pushover Analysis

In order to evaluate building based on the displacement, the nonlinear static analysis referred to as “push over” analysis, is used. Using this method, the displacement capacities of a structure can be obtained as it reaches its limit of structural stability. This method is in favor because it can also provide useful information about the nonlinear behavior of the structures, the location of formation of plastic joint and the way of redistribution of the loads, etc. which cannot be obtained by the linear static analysis. It is an incremental analysis, where each increment pushes the frame laterally, passing all of the possible stages, until the potential mechanism collapse. The steps that are followed in this method are: First the load pattern is assumed. This is one of the most important issues in the pushover analysis. It is suggested by EC8 § 4.3.3.4.2.2, that two lateral load patterns are recommended: 

“Uniform” pattern, based on lateral forces that are proportional to mass regardless of elevation (uniform response acceleration);



“Modal” pattern, proportional to lateral forces consistent with the lateral force distribution in the direction under consideration determined in elastic analysis (in accordance with 4.3.3.2 or 4.3.3.3).

Then, application of the load pattern on the structure until the displacement of a certain point of the structure reaches a certain value, the target value. The target value can be predicted by generating the capacity curve, which represents the total base shear-roof displacement relationship can be generated. The values of internal displacements and the forces are then determined and the different failures of structural elements can be seen during the process. The above process continues until the structure displacement exceeds the target displacement or collapse of the structure occur.

3.4.1 Distribution of the horizontal seismic forces for the Pushover analysis The simplified pushover analysis takes into account the vertical forces acting on the frame in the earthquake combination with a steadily increasing lateral force. “Modal” pattern will be used with the lateral forces determined in accordance to EC8 § 4.3.3.2. Triangular lateral load pattern was used to determine the horizontal forces. According to Eurocode 8 § 4.3.3.2.3, the horizontal forces Fi should be taken as being given by:

80


Fi = Fb(

𝑧𝑖 𝑚𝑖 ) 𝛴 𝑧𝑗 𝑚𝑗

Where: 

zi, zj are the heights of the masses mi mj above the level of application of the seismic action (foundation or top of a rigid basement).



Fi , horizontal force acting on storey i;



Fb, seismic base shear in accordance with expression (4.5);



mi,mj , storey masses

To analyse the case study for the worst case scenario, the loads were applied in the weaker direction (y-direction) which had the lower stiffness. Due to similarity of the bays, the forces are distributed by diving the total force in each floor by the numbers of bays. Table 3-2: Horizontal force distribution on each frame

Level Floor 1 Floor 2 Floor 3 Floor 4 Floor 5 Floor 6

h (m) 3 6 9 12 15 18 Sum

Mfloor (t) 229 229 223 216 216 210 1323

φ 0.11 0.25 0.39 0.67 0.88 1.00

Fi (kN) F/Frame 9 62 18 124 26 180 33 234 42 292 48 339

3.4.2 Elastic Response Spectra The building is constructed on ground type A, in seismic zone II with PGA = 0.24 g. The elastic acceleration and displacement response spectra derived are described in equations below and illustrated in Figure 3-7. 0 ≤ T ≤ 𝑇𝐵 ∶ 𝑆𝑒 (𝑇) = 𝑎𝑔 ∗ 𝑆 ∗ [1 +

𝑇 ∗ (2.5𝜂 − 1)] 𝑇𝐵

𝑇𝐵 ≤ T ≤ 𝑇𝑐 ∶ 𝑆𝑒 (𝑇) = 𝑎𝑔 ∗ 𝑆 ∗ 𝜂 ∗ 2.5 𝑇𝑐 𝑇𝑐 ≤ T ≤ 𝑇𝐷 ∶ 𝑆𝑒 (𝑇) = 𝑎𝑔 ∗ 𝑆 ∗ 𝜂 ∗ 2.5 ∗ [ ] 𝑇 𝑇𝑐 𝑇𝐷 𝑇𝐷 ≤ T ≤ 4s ∶ 𝑆𝑒 (𝑇) = 𝑎𝑔 ∗ 𝑆 ∗ 𝜂 ∗ 2.5 ∗ [ 2 ] 𝑇 In this case T1 = 0.655 s so, 𝑇𝑐 ≤ T ≤ 𝑇𝐷 = 0.4 ≤ 0.655 ≤ 2

81


Elastic Responce Spectrum 0.8

Sa (g)

0.6

0.4

0.2

0 0

0.5

1

1.5

2

2.5

3

3.5

4

T(s) Figure 3-7: Elastic Acceleration Response Spectrum

3.5Static Pushover analysis in SeismoStruct When the load pattern is defined, the magnitude of lateral force increases progressively maintaining the relevant load proportions unaltered during the entire development of pushover analysis. The applied load is an incremental load in the beam nodes and the way which the load factor is incremented throughout the analysis is a response control with a specific target displacement. The load factor that is calculated at each step, is the numerical coefficient that is necessary to apply to the load pattern in order to obtain the lateral displacement [51]. Performance-based engineering sets great attention in the identification of the exact instance in which the different performance limit states are reached. Different performance criteria are introduced in SeismoStruct in order to evaluate and record the non-structural damages, structural damages and collapse states. The following collapse mechanisms are adopted: 

Spalling of the concrete core: is initiated at extreme fibre compression strains between 𝜀𝑠 = 0.006 and 0.01 (Calvi, 1999). A limit of 𝜺𝒔 = 𝟎. 𝟎𝟎𝟐 is assumed as a conservative estimate of the onset of structural damage.



Crushing of the concrete core: can occur in compression states when the material strains are larger than the ultimate crushing strain threshold. A value of 𝜺𝒄𝒓 = −𝟎. 𝟎𝟎𝟑𝟓 from Table 3.1 in EC2, § 3 is assumed.



Yielding of steel reinforcement: 𝜺𝒚𝒅 = 𝒇𝒚 ⁄𝑬𝒔 = 𝟎. 𝟎𝟎𝟏𝟖𝟓 from EC2 § 3.2.7

82

Chapter 3. Research Objectives & Methodology 

Fracture of steel reinforcement: occur in tensile states when the steel strains are larger than the fracture strain, a value of 𝜺𝒔 = 𝟎. 𝟎𝟒 is assumed because the steel might not have as much ductility as the current grades.

3.5.1 Identification of the Performance level design Performance-based seismic design (PBSD) defines a series of discrete levels of seismic performance of structures. Those performance levels correspond to limit states of structural damage. In this case Table 3-3 illustrates the performance levels that have been set by ATC58-2 in Vision 2000 report. Four performance levels are defined with regard to damage to the structure and non-structural components, as long as the consequences to the occupants of the facility. The performance objectives represent performance levels, or damage levels, that are expected to result from design ground motions. Table 3-3: Performance objectives, damage and damage descriptions for RC buildings

Performance Objective Fully Operational

Damage State

Damage Description

Slight

Negligible

Operational

Moderate

Life Safe

Extensive

Near Collapse

Complete

Minor hairline cracking (0.02”); limited yielding possible at a few locations; no crushing (strains below 0.003) Extensive damage to beams; spalling of cover and shear cracking for ductile columns; minor spalling in nonductile columns; joints cracked Extensive cracking and hinge formation in ductile elements; limited cracking and/or splice failure in some nonductile columns; severe damage in short columns

To verify that design meets the performance objectives a matrix of performance objectives is provided by SEAOC 1999. The matrix includes four seismic hazards levels and takes into account four performance levels. The criteria in the coloured map are explained below: 

Green: Basic design objective



Yellow: The three performance targets that are marked represent an acceptable level but minimum allowed for “important” structures (schools, hospitals, etc.)



Blue: Minimum level for 'very important' structures (nuclear power stations, etc.)



Black: Requires level of total protection against seismic hazard.



Red: Unacceptable

83

Chapter 3. Research Objectives & Methodology Table 3-4: Performance objectives matrix for new buildings (SEAOC 1999) Seismic Hazard Level Qualitative Description Frequent

Probability to be exceeded 50% in 3 years

Structural Performance Level

Mean Return Period 43 years

Occasional

50% in 50 years

72 years

Rare

10% in 50 years

475 years

Very Rare

10% in 100 years

975 years

Fully Operational Basic Objective Essential Hazardous Objective Safety Critical Objective Not Feasible

Operational

Near Collapse

Life Safe

Unacceptable Unacceptable Unacceptable Basic Objective Essential Hazardous Objective Safety Critical Objective

Unacceptable Unacceptable Basic Objective

Unacceptable

Essential Hazardous Objective

Basic Objective

In this case the structure has been designed based on ground acceleration value with 10% probability of exceedance in 50 years, so return period of 475 years. Since the structure is categorized as Importance Class III (schools, cultural institutions, etc.) according to EC8 § 4.2.5 (4), from Table 3-4, the performance objective for a Rare event should be ‘Essential Hazardous Objective’, on the Moderate damage state. The four damages states (Slight, Moderate, Extensive, Collapse) that have been identified by GEM Analytical Vulnerability Method [52], are used to define the limit states of the structural damage. This method was selected because it associates the damage states with the development of local damage through the structural and non-structural elements.

3.5.2 Level of performance improvement It might be impractical to bring all existing buildings up to the standard of new buildings. So the existing building codes, suggests various percentages and levels of structural performance with respect to the new building standards, which an existing structure can achieve. NZSEE § 2.2 [53], suggests that buildings with < 67%new building standards should be considered for improvement of structural performance. The initial target level improvement should be 100% of the new building standards, however this might be impracticable, so a reduction to an acceptable level can be established. On the other hand ASCE 41-13 § C2.4.1.3 defines that for structures designed using the 5%/50year hazard the risk of collapse should be achieved with a constant 75% demand adjustment factor.

84

Chapter 3. Research Objectives & Methodology Table 3-5: NZSEE Risk Classifications and Improvement Recommendations (NZESEE, 2006)

Description

Grade

Risk

%NBS

Low Risk Building

A or B

Low