Performance Evaluation of Different Repair Concretes ...

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Farnam Ghasemzadeh, S.M.ASCE1; Siavash Sajedi2; Mohammad Shekarchi3;. Hamed Layssi ...... Quarterly building bulletin, London, 5–7. Vaysburd, A. M. ...
Performance Evaluation of Different Repair Concretes Proposed for an Existing Deteriorated Jetty Structure

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Farnam Ghasemzadeh, S.M.ASCE 1; Siavash Sajedi 2; Mohammad Shekarchi 3; Hamed Layssi, Ph.D., Aff.M.ASCE 4; and Milad Hallaji 5 Abstract: Performance of different cementitious repair materials is studied as part of an ongoing repair project on an existing reinforced concrete jetty in harsh environment of Persian Gulf. Concrete repair materials with different cementitious binders including ordinary portland cement, silica fume, and ground granulated blast furnace slag were tested. Different performance criteria such as mechanical properties, durability characteristics, and dimensional compatibility with substrate concrete were studied according to a proposed rational approach. It is concluded that a repair technique containing ternary pozzolanic materials has the best performance and could be proposed for marine environment. DOI: 10.1061/(ASCE)CF.1943-5509.0000496. © 2014 American Society of Civil Engineers. Author keywords: Compatibility; Ground granulated blast furnace slag; Performance; Repair; Concrete; Silica fume.

Introduction Degradation of RC structures in harsh environments has become a great concern. The annual cost of repair and maintenance of RC structures and bridges is a growing crisis in many regions such as U.K. and North America (Swiss 1989; Lachemi et al. 2007). Persian Gulf region is among the most severe environments for RC structures and infrastructures, with high risk of deterioration of concrete due to corrosion of steel in splash and tidal zones (Shekarchi et al. 2011; Parhizkar et al. 2006). This is responsible for severe deteriorations of RC structures and reduced of service life of structures. Poor repair techniques as well as inappropriate repair materials might spread the deterioration process to the adjacent parts and accelerate the rate of deteriorations. Therefore, selecting appropriate repair techniques and materials is crucial in increasing the service life of a structure and eliminating the need for repair of the repairs (Soleimani et al. 2010; Wood 2009; Vaysburd et al. 2000, 2004b). A wide range of cementitious repair materials with varying performance characteristics are used for repair of RC structures. The common practice in selecting repair technique involves 1

Ph.D. Student, Dept. of Civil, Construction, and Environmental Engineering, North Carolina State Univ., Centennial Campus, Constructed Facilities Laboratory, 2414 Campus Shore Dr., Campus Box 7533, Raleigh, NC 27695-7533 (corresponding author). E-mail: [email protected] 2 Ph.D. Student, Dept. of Civil Engineering, Univ. of Akron, Akron, OH 44325. 3 Associate Professor, Director of Construction Materials Institute, School of Civil Engineering, Univ. of Tehran, No.8, Behnam Alley, Vessal St., Enghelab Ave., Tehran, Iran. 4 Postdoctoral Research Fellow, Giatec Scientific, Inc., 301 Moodie Dr., Suite 302, Ottawa, ON, Canada K2H 9C4; formerly, Dept. of Civil Engineering and Applied Mechanics, McGill Univ., 817 Sherbrooke St. W., Montreal, QC, Canada. 5 Ph.D. Student, Dept. of Civil, Construction, and Environmental Engineering, North Carolina State Univ., Centennial Campus, Constructed Facilities Laboratory, 2414 Campus Shore Dr., Campus Box 7533, Raleigh, NC 27695-7533. Note. This manuscript was submitted on January 8, 2013; approved on July 2, 2013; published online on July 4, 2013. Discussion period open until October 21, 2014; separate discussions must be submitted for individual papers. This paper is part of the Journal of Performance of Constructed Facilities, © ASCE, ISSN 0887-3828/04014013(10)/$25.00. © ASCE

improving the impermeability of the repair materials, the compressive strength, and accelerating the strength gain; however, several other properties are also important for a successful repair (Momayez et al. 2005). Improper selection of repair materials, poor workmanship, and inadequate characterization of substrate concrete are believed to be responsible for failure of a repair technique (Vaysburd et al. 2004a; Emmons and Vaysburd 1994). Repair material should be compatible with the substrate concrete because the composite section of the repair material and the substrate concrete has to withstand all stresses induced by the applied loads under different environmental conditions during the designed service life (Emmons et al. 1993; Morgan 1996). Compatibility therefore is a not only a function of material properties but also depends on how the repair system would respond to the exposure condition. To date, a generally accepted method for selecting an appropriate repair mix for a particular repair purpose of RC structures is not available. While appropriate repair can improve the performance of the structure to a desirable level, poor repair materials and techniques may cause serious harms not only to the existing structure but also to the repaired parts in a relatively short period of time (Raupach 2013). Choosing the proper repair technique is complicated and mostly depends on the state of the damaged structure, desired performance level in service life period, and financial issues. This paper presents results of an experimental study on compatibility of different cementitious repair materials with substrate concrete. The concrete mixture for the substrate was chosen to resemble the concrete that was originally utilized in a RC jetty structure located at Persian Gulf in south of Iran. A rational method is proposed in order to evaluate the performance of repair materials as well as choosing the most compatible repair material with the substrate concrete. This method is applicable for any other projects aimed at selecting suitable repair materials.

Case Study The jetty structure under study is located near Bandar-Abbas port, in the Persian Gulf, south of Iran. Detailed information regarding the structure, environmental condition of the site, diagnostic investigation of corrosion damage, and previously applied repair system are provided in Moradi-Marani et al. (2010) and

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Materials

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Ghassemzadeh (2010). Inadequate cover thickness and severe environmental condition resulted in premature corrosion of reinforcing bars and triggered deterioration of structural elements. On the other hand, application of incompatible repair materials with the substrate concrete accelerated deterioration phase and extended deteriorations to the intact adjacent parts of the elements. Premature failure of the repair technique was attributed to early age cracking of the repaired areas due to shrinkage (dimensional incompatibility) and/or reinforcement corrosion (electrochemical incompatibility).

The cementitious materials used in the research reported in this paper were portland cement (PC) equivalent to ASTM Type I and Type II, silica fume, and slag. Table 1 shows the chemical composition of these materials. Crushed limestone aggregate and river sand were used as coarse and fine aggregates, respectively. Coarse aggregate had maximum size of 16 mm and fine aggregate had fineness modulus of 3.2. Two superplasticizers were used for the mixes in order to improve the workability of the fresh concrete. The first superplasticizer was based on the blend of melamin sulphonate and organic agent (Sika R-4), and the second superplasticizer was based on an optimized carboxylate (Glenium).

Research Methodology Different repair mixtures were tested for several performance criteria. The performance has been studied in three different categories, as follows: (1) mechanical properties, (2) durability characteristics, and (3) dimensional stability. Appropriate test procedures were utilized to investigate each parameter. For each category, specimen preparation, curing, and casting were in accordance with standard test procedures. Fig. 1 shows the important parameters investigated to achieve a compatible repair material.

Mixture Proportions Four different repair mixtures were designed using different cementitious materials including cement Type II and pozzolanic materials. Table 2 summarizes the mix designs of repair materials. The BASE-C mixture represents the original concrete used in the jetty structure. This mix design had a total cement (Type I) content of 440 kg=m3 with no additional pozzolanic materials. The water to cement ratio W=cm was 0.34.

Compatible Repair

Dimensional Stability Requirements

Durability Requirements

Mechanical Strength Requirements

Protection against attack of corrosive materials

Compressive strength Flexural strength

Relevant transport properties

Crack resistance

Bond strength Modulus of elasticity

Sorptivity

Shrinkage

Creep Tensile strength

Permeability Resistivity

Fig. 1. Important parameters to achieving the durable concrete repair

Table 1. Chemical Compositions and Properties of Binding Materials Binder

SiO2

Al2 O3

CaO

MgO

Fe2 O3

SO3

Na2 O

K2 O

Specific gravity

Cement Type I Cement Type II Ground granulated blast furnace slag (GGBS) SF

21.60 21.00 35.50 93.16

5.40 5.00 10.00 1.13

62.60 63.00 36.50 —

3.600 1.800 9.500 1.60

3.40 3.50 0.70 0.72

1.90 1.60 1.86 0.05

0.78 0.50 0.50 —

0.44 0.60 0.53 —

3.10 3.15 2.86 2.11

Table 2. Mix Proportioning of Repair Concretes and Substrate Mix proportioning Concrete

C (kg)

Slag (kg)

SF (kg)

SP (%)

Coarse aggregate (kg)

Fine aggregate (kg)

W=cm

Slump (mm)

R R-SF R-S R-SFS BASE-C

420.0 390.5 315.0 285.5 440.0

— — 105 105 —

— 29.5 — 29.5 —

0.5 0.6 0.5 0.5 0.7

793 793 793 793 720

1,000 988 992 980 1,161

0.38 0.38 0.38 0.38 0.34

145 155 160 150 145

© ASCE

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All of the repair mixtures were proportioned to have the same total cementitious materials of 420 kg=m3 and water to cementitious materials ratio of 0.38. Mixture R is plain repair concrete without any pozzolanic admixtures. In the R-SF and R-S mixtures, SF and S have replaced cement by 7.5 and 25.0% (of cement weight), respectively. For mixture R-SFS, 32.5% of cement was replaced by a combination of SF (7.5%) and slag (25%). The use of higher w/cm and lower cementitious materials content in these repair materials, compare to BASE-C mix, is justified by practical and economical concerns. Pozzolanic materials in repair mixes could compensate the higher w/cm ratio and lower cementitious contents.

Specimen Preparation and Testing Mechanical Tests Samples for mechanical testing were prepared according to ASTM C192 (ASTM 2003a). Three specimens were used and tested to determine different mechanical properties at different ages through a set of well-known experimental procedures, including the following: (1) cylindrical compressive strength [according to ASTM C39 (ASTM 2003b)], (2) Brazilian splitting tensile strength [according to ASTM C496 (ASTM 2003c)], (3) flexural strength [according to ASTM C78 (ASTM 2003d)], and (4) modulus of elasticity [according to ASTM C469 (ASTM 2003e)]. Performance of the bond between the repair and substrate concretes was studied using bisurface shear test method proposed by Momayez et al. (2005). In this method, cubic specimens of 150 × 150 × 150 mm were casted in two steps, as follows: (1) one-third of molds was filled with styrofoam and two-third of the molds was filled with the substrate concrete, BASE-C [Fig. 2(a)]; and (2) after 24 h, the substrate specimens were removed from the molds and their surfaces were roughened using a steel wire brush [Fig. 2(b)]. The specimens were cured in water for 28 days. Specimens were then kept in dry condition (temperature 23°C and relative humidity 45%) for 150 days to

simulate the conditions of existing substrate concrete (Shin 2000). Afterwards, the surface of the substrate concrete was cleaned with a wire brush and water-jet a few hours before placing the repair concrete to ensure that the bond surface is clean and saturated [Fig. 2(c)]. The repair concrete was cast in the remaining one-third of the cubic molds. After curing the repaired samples, the bisurface shear tests were conducted [Fig. 2(d)]. In order to evaluate the performance of bond in repaired specimens, the bisurface shear test was also done on the integrated cubic samples with BASE-C mix design. Durability Tests Four different test methods were used to investigate durability of the repair materials, as follows: 1. Water absorption. Water absorption test was conducted according to BS 1881, part 122 (British Standards Institution 1983). For every concrete mix, after 28 days of moist curing, three cylindrical cores of 75-mm diameter and 130-mm height were extracted from 200-mm cubes. These cores were immersed in water for 30 min. Water absorption for each specimen was measured as the increase in mass and was expressed as percentage of the mass of the dry specimen. 2. Electrical resistivity. Three 100-mm cubes were used for determining the bulk electrical resistivity of each mix over a period of 180 days. The specimens were kept in water during this period. The bulk electrical resistivity was measured over a frequency of 1 kHz based on alternating current impedance spectrometry (ACIS) method by using a concrete resistance meter (Ping et al. 1992). Fig. 3 shows used electrical resistivity meter in this test. 3. Oxygen permeability. Four cylindrical discs of 150-mm diameter and 50-mm height were extracted from cylindrical specimen (150-mm diameter × 300-mm height) for each concrete. Then oxygen gas permeability test was carried out in accordance with the Cembureau-method (regime B) according to RILEM (1999).

Fig. 2. Representation of bisurface test method: (a) casting of substrate concrete; (b) preparing the rough surface in substrate concrete using a steel wire brush; (c) finished substrate surface before casting repair concrete; (d) loading of bond specimens [images reprinted from Ghassemzadeh (2010), with permission] © ASCE

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every 2 days through the curing period. Length change measurement was further recorded for a period of 500 days after curing. • Flexural creep. Two concrete beams of 100 × 100 × 1,250 mm were used for each mix to determine the variation of time dependent flexural deflection over a 400-day period. Free span of simply supported beam was 1,200 mm and two constant weights of 35 kg were applied at 375 mm from each free end of the beams, making a constant flexural moment at the center of the beam. Midspan deflection of the beams was measured using a semionline LVDT data acquisition system. Fig. 4 shows a view of the creep and shrinkage testing room.

Results and Discussions Fig. 3. Representation of bulk electrical resistivity measurement test set up [reprinted from Ghassemzadeh (2010), with permission]

4. Chloride diffusion. Bulk chloride diffusion samples were prepared according to ASTM C1556 (ASTM 2003f). Three cylindrical specimens (150-mm diameter × 300-mm height) were cast and cured for 28 days. Cylinders were later cut into 100-mm thick slices. The concrete slices were sealed with epoxy polyurethane coating on all sides except the sawed surface to simulate one-dimensional diffusion. These specimens were immersed in a sodium chloride solution with a concentration of 165 g=L and a temperature of 23°C. Powder samples were provided from these slices. Detail of sampling can be found in Shekarchi et al. (2009). Powder samples were analyzed for acid-soluble chloride content according to ASTM C1152 (ASTM 2003g) and ASTM C114 (ASTM 2003h). Dimensional Stability Total shrinkage and flexural creep tests were carried out to evaluate the dimensional stability of repair concrete mix designs (specimens were kept in controlled environment with humidity of 50  2% and temperature of 25  1°C during the testing period), as follows: • Total shrinkage. Total shrinkage test was performed on three 100 × 100 × 500-mm prism specimens for each mixture according to ASTM C157 (ASTM 2003i). After casting, specimens were covered with wet burlap for 24 h. After removing the specimens from the molds, initial length was measured using a comparator in accordance with ASTM C490 (ASTM 2003j). Specimens were later cured in water for 6 days and the length change was measured

Mechanical Properties Compressive Strength Table 3 shows mechanical properties of repair concrete material, substrate, and coefficient of variation (COV) related to each mixture at any age. The silica fume repair material (R-SF) and slag repair material (R-S) have the highest and lowest early age (3 and 7 days) compressive strength, respectively. This can be attributed to the large specific surface of silica fume (with high amorphous silica particles) compared to that of slag, which can accelerate the hydration reactions at early ages. On the other hand, the maximum compressive strength gain was observed in R-SFS and R-S mix designs which contained slag. In specimens R-SFS in which SF and slag are used, compressive strength gain begins from early ages and continues through late ages. Similar observations are reported by Jianyong and Yan (2001). The compressive strength of the concrete mixtures containing slag increases dramatically over the time as a matter of long-term hydration of slag. Modulus of Elasticity Modulus of elasticity E is one of the most important factors affecting the repairing system (Emberson and Mays 1990). The difference between the modulus of elasticity of repair material and the substrate concrete can result in nonuniform stress distribution and deformation at their interface, which can cause certain problems under specific loading conditions (Czarnecki et al. 2000). In this paper, the ratio of elastic modulus of repair concrete to the elastic modulus of the substrate concrete has been studied. As long as this ratio is close to 1, a uniform stress distribution in the repair system is expected. From the results, the ratio values were 1.03, 1.06, 1.03, and 1.08 for

Fig. 4. Representation of underloading creep and shrinkage specimens in controlled testing room with humidity of 50  2% and temperature of 25  1°C [reprinted from Ghassemzadeh (2010), with permission] © ASCE

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R-SF

R-S

R-SFS

BASE-C

33=4.7 44=5.0 70=3.5 74=3.3

40=3.6 44=3.3 61=4.0 63=3.6

3.83=4.6 5.16=3.9

3.72=5.3 4.47=4.3

3.50=3.8 3.80=5.0 4.20=4.2

2.60=4.2 2.90=4.7 3.60=4.4

0.48=4.9 0.90=3.8 0.93=4.4

0.48=4.4 0.46=3.6 0.51=4.2

39=4.00

36=3.5

the R, R-SF, R-S, and R-SFS mixes, respectively, which indicates the uniform stress distribution for evaluated repair concretes. Tensile Strength External loading, volume change, and incompatibility between repair and substrate materials can induce tensile stresses in repair system. Since concrete is weak in tension it is expected to be cracked under relatively low tensile loads. Any cracks can create a direct path for penetration of the corrosive ions into the concrete. This phenomenon can initiate the corrosion process and accelerate the deterioration of the structure. It is therefore critical for the repair material to have adequate tensile strength. Table 3 shows the results of tensile strength for investigated mix designs. The repair mixture containing a combination of SF and slag achieved the highest tensile strength at the age of 28 days. The R-S mixture (repair concrete with slag) had the second highest tensile strength at 28 days. This indicates effectiveness of slag in increasing the tensile strength of concrete compared to silica fume. These observations are consistent with findings of Jianyong and Yan (2001).

Repair-C

BASE-C

Flexural Strength Desirable flexural strength of the repairing concrete depends on the type of repair (structural or protective repair) and the type of deteriorated structural element. There is no sound justification for using highly expensive cementitious repair material in replacing normal concrete for structural repair of deteriorated areas in tensile

Bond Strength The bond strength between the repair concrete and the substrate has an important role on the load bearing capacity of the repaired structure, considering its role on transferring load between the interface of substrate and repair concrete (Wood et al. 1990). The bond strength should be adequate enough to prevent any failure in the bond interface and it is also important in achieving a durable repair (Soleimani et al. 2010). The roughness of the substrate surface as well as type of the repair material can affect bond properties (Momayez et al. 2005). In the research reported in this paper, substrate surface in all shear specimens was roughened to the constant depth of 4–5 mm to provide the moderate roughness conditions. Table 3 summarizes the ratio of the bond strength in the repair specimens to the bond strength in integrated specimen (BASE-C), Brepaired =Bintegrated . Perfect bonding can be achieved by approaching this ratio to 1. The obtained average bond strength for integrated BACE-C specimen Bintegrated was 1.67 MPa. From Table 3, R-SFS repair concrete provides the maximum shear bond strength at the age of 28 days. By adding silica fume to the mixture, bond strength between repair concrete and substrate improves considerably. Silica fume reduces large pores in the cement paste and eliminates growth of calcium hydroxide (CH), which results in improving interfacial properties between repair concrete and substrate. Pozzolanic reaction of silica fume and CH can decrease the Ca/Si ratio with the depth. This can in turn decrease the interfacial layer between repair concrete and substrate which can increase the shear bond strength (Shin and Wan 2010). Fig. 5(a) shows failure modes of the concrete containing slag at early ages. In these specimens cracking has only occurred at the interface zone, which is not a desirable failure mode. Fig. 5(b)

(a)

Repair-C

R

Compressive strength (MPa)/COV 3 days 35=4.4 37=3.1 29=4.9 7 days 39=3.3 51=4.0 40=3.9 28 days 51=3.5 59=3.8 58=3.9 90 days 51=5.7 59=3.5 63=4.0 Tensile strength (MPa)/COV 3.10=3.7 7 days 3.20=4.1 4.20=4.2 28 days 3.80=4.7 4.30=4.8 4.77=4.2 Flexural strength (MPa)/COV 7 days 2.70=3.6 3.00=3.9 2.80=3.6 28 days 3.20=3.5 3.70=3.2 3.70=4.4 90 days 3.60=4.0 3.90=3.8 4.10=3.9 Bond strength ratio Brepaired =Bintegrated =COV 7 days 0.45=5.1 0.66=4.8 0.48=4.4 28 days 0.58=3.9 0.87=4.1 0.51=4.5 90 days 0.60=4.2 0.81=3.9 0.67=4.3 Modulus of elasticity (GPa)/COV 38=4.2 37=3.7 28 days 37=3.3

BASE-C

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Age

zones (Al-Salloum 2007). In terms of durability, however, the writers believe that using high performance repair concretes (HPRCs) in tensile regions is inevitable in corrosive environments because any flexural cracking in the repair concrete can be critical. In the repair of compressive regions, use of HPRCs is also highly recommended as it efficiently retains the capacity of the element (Al-Salloum 2007). Since the purpose of the research reported in this paper is to choose the appropriate repair concrete for the deteriorated jetty structure in Persian Gulf region, repair materials with high flexural stiffness is desirable. Table 3 shows flexural strength of the repair mixtures. The flexural strength of concrete mix design R-SF is greater than that of concrete mix design R at the age of 28 and 90 days. This indicates the effective contribution of SF in increasing the flexural strength of concrete compared to slag. The flexural strength of R-SFS is however greater than R-S which indicates the synergic effect of the combination of SF and slag in this test too.

BASE-C

Table 3. Mechanical Properties of Repair Concretes and Substrate

(b)

Fig. 5. Failure modes in bisurface specimens: (a) representation of typical failure mode in R-SF (short and long term), R-SFS (short and long term), and R-S (long term); (b) representation of typical failure mode in R-S (short term), R-C (short and long term), and BASE-C [short and long term; reprinted from Ghassemzadeh (2010), with permission] © ASCE

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displays failure modes of mix designs containing SF (in both early and late ages) and mix designs containing slag at older ages. Fig. 5(b) indicates the fair and strong bond compared to Fig. 5(a), since cracking in Fig. 5(b) has happened in both substrate and interface between repair concrete and substrate.

Oxygen Gas Permeability The rate of the oxygen gas flow through cylindrical slices of concrete is measured using a soap bubble flow-meter, when the flow rate is stabilized. The relationship proposed by Hagen-Poiseuille (Kollek 1989) was used to determine intrinsic permeability coefficient. The flow is assumed to be laminar and unidirectional

Water absorption, % by mass

Fig. 6. Water absorption of repair concretes and substrate (28-day curing)

Electrical Resistivity Concrete electrical resistivity is believed to be a key parameter in the corrosion protection of embedded steel bar in concrete. Fig. 7 shows changes in the electrical resistivity of different repair concretes. Results indicate that the repair material containing a combination of SF and slag (R-SFS) has the highest electrical resistivity, while concrete with no pozzolanic materials has the lowest electrical resistivity. Moreover, for assessing the repair concrete with high resistivity, it seems that the use of SF is necessary. According to ACI 222R-01 (ACI 2001), the corrosion rate of reinforcing steel is considered low when the electrical resistivity of concrete exceeds 20 kΩ · cm. R-S and BASE-C mixes have similar resistivity. The reason to this phenomenon could be referred to previously mentioned explanation in water absorption discussion.

1.4 1.43 1.2

1.28

1

1.24

1.08

1

0.8 0.6 0.4 0.2 0

R

R-SF

R-S

R-SFS

BASE-C

Ko ¼

Durability Characteristics

R

R-SF

2QPa Lη AðP2 − P2a Þ

ð1Þ

where K O is the permeability coefficient (m2 ); Q = volume flow rate (m3 s−1 ); A = cross-sectional area of specimen (m2 ); L = thickness of the specimen in the direction of flow (m); η = dynamic viscosity of the gas at test temperature (2.02 × 10−5 N · s · m−2 for oxygen at 22°C); and P and Pa = inlet and outlet pressures (N · m−2 ), respectively. Concrete cover of RC members protects the embedded rebars from corrosion. Therefore, corrosion of rebars is highly dependent on the penetration of aggressive agents through the concrete cover. Accordingly, permeability of the repair concrete has a significant effect on the performance of the repair system. However, using low permeable repair concrete regardless of situation is a fallacy (Emmons and Vaysburd 1996). Fig. 8 shows the permeability coefficient K O of all repair mixes. Results indicate that using pozzolanic materials can improve the impermeability of repair materials. While it seems that SF is more effective than slag, the combination of two materials has led to the least permeable sample.

Water Absorption The purpose of this test is to determine how much fluid will enter into the repair concrete through suction forces created by water molecules and their microconnections with pore walls. The more fluid enter the repair concrete through capillary action, the more susceptible concrete is to corrosion by aggressive materials, especially chloride ions. Repair materials R-SF and R-SFS had the lowest water absorption (Fig. 6), while concrete with no pozzolanic admixtures had the highest water absorption. Silica fume is more effective in reducing the water absorption of repair concrete compared to slag. The R-S mix has approximately similar water absorption to BASE-C. This can be attributed to the pozzolanic reaction of slag in the R-S mixture which compensates the higher w/cm ratio and lower cement content of this mixture compared to BASE-C. According to BS 1881, part 122 (British Standards Institution 1983), in the jetty structure under study the allowable water absorption should be less than 1.0% at 28 days.

R-S

R-SFS

BASE-C

90 80

Concrete resistivity (KΩ .cm)

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Mixtures Design

70 60 50 40 30 20 10 0 0

20

40

60

80

100

120

140

160

180

200

Age (days)

Fig. 7. Electrical resistivity of repair concretes and substrate © ASCE

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decreased chloride diffusion coefficient by 78 and 71%, respectively (compared to nonpozzolanic materials). Similar to oxygen gas permeability test, SF is more effective than slag in reducing the chloride diffusion coefficient. The Cs value for R-SF and R-S are greater than that of reference concrete; this can be related to higher level of chloride binding and sorptivity of these supplementary materials (Song et al. 2008).

40

Ko (x10-17m2)

35

38

30 29

25 24

20 15

17 14

10

Dimensional Stability

5 R

R-SF

R-S R-SFS Mixtures Design

BASE-C

Shrinkage Stresses due to volume changes have long been recognized as one of the main causes of early deterioration in repaired concrete structures. It is believed that repair concrete with lower shrinkage has better compatibility with the substrate concrete (Pattnaik and Rangaraju 2007; Brown et al. 2007; Poston et al. 2001; Mokarem et al. 2005; Decter and Keeley 1997). Fig. 9 shows the length variations during the curing period and later on in dry condition. Fig. 9 (a) shows the average total expansion in saturated curing period and shrinkage for unrestrained concrete specimens in drying room up to 30 days. The slight expansion in the curing period of concrete is attributed to the hydration process (Wood et al. 1990), while the rest of expansion is resulted from cementitious properties of concrete. The expansion of R-S and R-SFS are higher than the other mixtures; similar trends are reported by several researchers (Roy and Idorn 1982; Aly and Sanjayan 2008; Skalny et al. 2002; Mehta 1973). The initial swelling of slag concrete primarily induced compressive restraining forces which has a positive influence on reducing the potential cracking in slag concrete (Aly and Sanjayan 2008). All specimens subjected to drying condition after six days of saturated curing. Repair material R-SF exhibited a rapid rate of shrinkage which is similar to that of the repair material R in the early days of exposure. Expansion of mixtures R-S and R-SFS were compensated by the initial shrinkage after 3 and 2 days of exposure, respectively; however, this time for mixtures R and R-SF were less than 1 day [Fig. 9(a)]. After 28 days, shrinkage of R-SFS was lower than that of R-S. On one hand, using SF in the mixture R-SFS has caused more shrinkage compared to R-S at early ages. On the other hand, combination of SF and slag in R-SFS shows slightly less shrinkage than R-S in the long-term [Fig. 9(b)]. Due to the time consuming process of experimental programs in measuring the long-term shrinkage of concrete in the laboratory, researchers may measure the short-term

Fig. 8. Oxygen permeability K o of repair concretes and substrate

Chloride Diffusion The apparent diffusion coefficient Da and the surface concentration Cs can be calculated from the established chloride concentration profile based on Fick’s second law of diffusion and Crank’s solution [Eqs. (2a) and (2b)]

Cðx;tÞ

∂C ∂2C ¼ DC 2 ∂t ∂x    x ¼ Cs 1 − erf pffiffiffiffiffiffiffiffi 2 DC t

ð2aÞ ð2bÞ

where x = distance from the concrete surface (cm); t = time (years); Dc is the diffusion coefficient (cm2 =year); Cs = equilibrium chloride concentration on the concrete surface (%); Cðx; tÞ = chloride concentration at the depth of x from the surface after the time t (%); and erf is the error function. Results (Table 4) indicate that using silica fume and slag as supplementary cementitious materials has Table 4. Results from Chloride Diffusion Tests, 1-Year Exposure Concrete code

Dc (×10−12 m2 =s)

Cs (%)a

Coefficient of regression

29.50 6.37 10.10 8.60 23.20

0.62 0.76 0.65 0.61 0.60

0.95 0.98 0.98 0.91 0.94

R R-SF R-S R-SFS BASE-C a

Percent is in terms of concrete.

R

R-S

R-SF

R-SFS

R

BASE-C

500

R-SF

R-S

BASE-C

R-SFS

800 Curing Condition

Drying Condition

700

Shrinkage (microstrain)

400

Shrinkage (microstrain)

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0

300 200 100

600 500 400 300 200 100

0 0 -100

-100 0

(a)

5

10

15

20

Age (days)

25

30

35

40

0

(b)

50

100

150

200

250

300

Age (days)

Fig. 9. Shrinkage: (a) expansion and short-term shrinkage; (b) long-term shrinkage without swelling included © ASCE

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R

R-SF

R-S

R-SFS

BASE-C

4.5 Flexural creep coefficient (ϕ)

4.0 3.5 3.0 2.5 2.0 1.5 1.0

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0.5 0.0 0

50

100

150 200 250 Loading Period (days)

300

350

400

Fig. 10. Normalized midspan deflection of repair and substrate concrete beams (flexural creep coefficient)

shrinkage strains and predict the long-term values by inverse analysis method proposed by Shekarchi et al. (2012). Creep Creep characteristics of the repair concrete can affect the performance of repair system in different situations. As reported by several researchers (Kordina et al. 2000; Horimoto and Koyanagi 1995), tensile stress in the repair concrete is reduced by approximately 50% due to the effect of creep and relaxation. Larger values of creep seem to be more suitable for members or regions in tension. In contrast, lower values are more beneficial for members in compression (Cusson and Mailvaganam 1996). The deflection values of beams were normalized with respect to its initial elastic deflection to visualize the effect of creep on deformation of beams under constant load. Fig. 10 shows the normalized midspan deflection of beams versus time curve (flexural creep coefficient). In the first 28 days of loading, creep effect occurred by 74, 77, 68, 82, and 82% in terms of total creep until 365 days for the R, R-SF, R-S, R-SFS, and BASE-C concrete beams, respectively. The creep effect was high in the initial period and tended to remain constant over the time. The lowest and highest of flexural creep coefficient values were recorded for R-SFS and R-S mixes, respectively. Results indicate that using of SF might be suitable for compressive zones in order to increase the load carrying capacity of repaired members. This is due to the improved microstructure and high

modulus of the concrete containing SF. Moreover, application of slag concretes can be effective in reducing the tensile stress caused by the different loading conditions in tension elements. Rational Approach to Choose Optimized Repair Material Results of the laboratory tests showed that different repair materials exhibit different performance characteristics. Therefore, a rational approach is needed in order to make an overall judgment and choosing the most suitable material. A weighted performance index approach has been used for overall ranking of the investigated repair concrete. For every level of each performance criteria, numeric index Ri between 1 to 5 has been assigned (1 shows the worst scenario and 5 is the best). Each performance criteria has a unique ranking significance weight Ci . These assignments and significance weights are chosen based on engineering judgment of the researchers which comes from studying the past real case studies and laboratory investigations. These values can be varied at different conditions and for other experts. The overall performance of the repair concrete is then calculated using Pn RC PRC ¼ Pn i¼1 i i × 100 ð3Þ R i¼1 max Ci where PRC = performance of the repair concrete (%); Ri = numeric index; Ci = significance weight; Rmax = maximum value (in this

Table 5. Selection of Optimized Repair Concrete in Deteriorated Beam in Tensile Zone Ranking of repair concrete mixtures Ri Compatibility tests Mechanical strength tests Compressive strength Tensile strength Flexural strength Bonding strength Modulus of elasticity Durability tests Water absorption Electrical resistivity Oxygen permeability Chloride diffusion Dimensional compatibility tests Total shrinkage Flexural creep coefficient PRC (%) © ASCE

Desirable criteria

Significance coefficient Ci

R

R-SF

R-S

R-SFS

BASE-C

High values High values High values High values Similar to substrate concrete

C1 ¼ 0.5 C2 ¼ 1.0 C3 ¼ 0.4 C4 ¼ 1.0 C5 ¼ 0.9

1 1 3 2 4

3 2 4 4 3

2 4 4 3 4

5 5 5 5 2

4 3 2 1 5

values values values values

C6 ¼ 0.7 C7 ¼ 0.7 C8 ¼ 0.7 C9 ¼ 0.8

1 2 1 1

4 4 4 5

2 3 2 3

5 5 5 4

3 1 3 2

Low values High values

C10 ¼ 1.0 C11 ¼ 0.7

3 4 42

2 3 67

4 5 67

5 2 87

3 3 55

Low Low Low Low

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paper, equal to 5) of devoted ranking; and n = number of performance criteria. Table 5 shows the results of PRC. The R-SFS mix design has the highest overall ranking, making it the most appropriate choice for patch repair of deteriorated beam in tension regions. Similar approach has been previously used by Andrade and Izquierdo (2005) in which they introduced the repair index method (RIM) for choosing the most appropriate repair concrete. In the performance of repair concrete method, the final performance value is a percentage of the overall properties of the repair concrete (which is 100% for ideal repair concrete with full performance) while the value of RI represents an objective ranking for selecting a repair system. Therefore, the performance of repair concrete approach is more sensible since the overall performance value calculated by using it is more meaningful compared to RI value in RIM. Moreover, the performance of repair concrete method would be helpful for experts to consider various repair concrete based on degrees of performance, and the economical or technical point of view to select the best material.

Conclusion Performance of different repair mix designs was studied in three different categories, as follows: (1) mechanical properties, (2) durability, and (3) dimensional stability. The following conclusions can be drawn: • Concrete containing slag is highly influenced by curing time. Extending the curing time in concrete containing slag has a positive effect on improving the mechanical properties at later ages. A combination of pozzolanic materials, SF, and slag in the repair concrete have a positive synergistic effect on improving mechanical properties at both early and later ages. • According to the durability experiments, using SF is more effective than slag. In other words, to achieve durable repair concrete, it seems that use of SF is more suitable. By adding slag to SF concretes, durability properties of repair concrete will be enhanced at later ages. • Silica fume significantly increases the shrinkage strain at early ages. Therefore, SF concretes can increase the potential of cracking especially in repair systems. On the other hand, slag concrete showed contrary results due to the expansion during curing period. This phenomenon induces compressive prestressing force in repair concretes which can postpone the onset of cracking at early ages. • The slag concrete showed the highest flexural creep coefficient, and then reference concrete, SF concrete, substrate concrete, and ternary concrete (R-SFS). The highest and lowest creep values with respect to substrate concrete are suitable for repairing elements under tension and compression, respectively. • The selection of the optimized repair concrete should be based on technical and economical aspects. However, in the research reported in this paper, the performance of repair concrete method is used by taking into account technical comparison of different investigated repair materials. The performance of repair concrete algorithm enables the incorporation of nontechnical requirements in the decision-making process by defining indicators and scoring for each additional item.

Acknowledgments The experimental tests were carried out at the Construction Materials Institute (CMI), University of Tehran. This generous support is gratefully acknowledged. © ASCE

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© ASCE

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