Feb 10, 2009 - Despite the economical and environmental benefits of concrete involving ... long-term properties compared with conventional con- crete made entirely with ... a concrete mix involving RCA, the volumetric ratio and relevant ...
Magazine of Concrete Research, 2009, 61, No. 7, September, 477–490 doi: 10.1680/macr.2008.61.7.477
Shear strength of reinforced recycled concrete beams without stirrups G. Fathifazl*, A. G. Razaqpury, O. Burkan Isgor{, A. Abbas}, B. Fournier} and S. Foo** Adjeleian Allen Rubeli Consulting Structural Engineers; McMaster University; Carleton University; Amec Americas; Universite´ Laval; Public Works and Government Services Canada
A new method of mixture proportioning is used for investigating the shear performance of reinforced concrete (RC) beams made with coarse recycled concrete aggregate (RCA). In this method, RCA is treated as a two-phase material comprising mortar and natural aggregate therefore to proportion the concrete mixture with RCA, the relative amount and properties of each phase are considered separately. Using the new mix proportioning method, several beams were designed and tested to study the effect of a number of parameters including the shear span-todepth ratio and beam size on the serviceability and strength of RCA concrete beams without shear reinforcement. For each beam its load–deflection curve, shear deformations, diagonal cracking load, crack pattern, ultimate shear strength and failure mode were determined. The results showed that the shear performance of RC beams made with RCA can be comparable, or even superior, to that of beams made entirely with natural aggregates at both serviceability and ultimate limit states, provided the proposed mixture proportioning method is used. Furthermore, the simplified methods of ACI and CSA standards as well as Eurocode 2 were found applicable to all reinforced RCA-concrete beams.
Introduction Despite the economical and environmental benefits of concrete involving recycled concrete aggregates (RCA),1 the construction industry has not embraced it, especially for structural applications. This is partly a consequence of previous findings reported in the literature and of the prevailing belief that concrete made with coarse RCA has inherently inferior short- and long-term properties compared with conventional concrete made entirely with natural aggregates.
* Adjeleian Allen Rubeli Consulting Structural Engineers, Ottawa, ON, Canada y Department of Civil Engineering, McMaster University, Hamilton, Ontario, Canada { Department of Civil and Environmental Engineering, Carleton University, Ottawa, Ontario, Canada } Amec Americas, Calgary, Alberta, Canada } Department of Geology and Engineering Geology, Universite´ Laval, Que´bec, Quebec, Canada ** Public Works and Government Services Canada, Gatineau, Quebec, Canada (MACR 800203) Paper received 14 December 2008; accepted 10 February 2009
In the past, the majority of investigations have reported similar shear cracking pattern and failure modes in conventional reinforced concrete (RC) and reinforced recycled concrete (RRC) beams, but lower diagonal cracking load and ultimate shear strength have always been presented.2–4 Furthermore, smoother crack interface, less effective aggregate interlock mechanism and consequently less ductile shear behaviour in RRC beams compared with conventional RC beams have been reported.2 Consequently, questions have been raised with respect to the applicability of the existing empirical relations for calculating the concrete contribution to the shear resistance of conventional RC members, commonly denoted as vc , to RRC beams, especially at larger shear span-to-depth ratios.2,4 This has been mainly attributed to the less effective aggregate interlock mechanism in RRC beams compared with conventional RC beams. It is shown in the current study that the previously reported lower shear strength of RCA-concrete – that is, concrete made with RCA – is not an intrinsic property of RCA-concrete; rather, it is the consequence of an improper method of mixture proportioning. Until now the conventional mix proportioning method for normal concrete has been commonly used 477
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G. Fathifazl et al. for RCA-concrete, with some adjustments, such as increase in the cement content, but with no special consideration given to the residual mortar quantity in RCA.5–10 However, RCA is a two-phase material comprising residual mortar and original virgin aggregate; thus concrete made with coarse RCA, if proportioned in accordance by conventional methods, would contain less natural aggregate and more mortar compared with the concrete containing an equal volume of natural aggregate only. The reason is that in the conventional mix design, the residual mortar in RCA is treated as part of the aggregate rather than mortar. In the current authors’ opinion, the lower total natural aggregate content of RCA-concrete is generally responsible for the observed inferiority of both plain and RRC concrete compared with conventional concrete. From the shear resistance perspective, the smaller natural aggregate content of RCA-concrete results in fewer coarse aggregate particles crossing the cracked shear plane, which results in reduction of the roughness or asperity of the crack faces and consequently reduction in the effectiveness of aggregate interlock mechanism to resist shear in members made of RCA-concrete. It is suggested here that the observed inferior properties of RCAconcrete are not intrinsic, but instead are the consequence of its composition, and the inferiority can be eschewed by adjusting its composition through application of a proper method of mix proportioning. To test this hypothesis, a new mix proportioning procedure was developed by the authors of the current paper,11 based on the fundamental premise that RCA is a two-phase material comprising residual mortar and original virgin aggregate; therefore, when proportioning a concrete mix involving RCA, the volumetric ratio and relevant properties of each phase must be accounted for separately. In other words, it cannot be assumed, as is currently customary, that RCA simply replaces natural aggregate in the mix because it also modifies the overall mortar content of the mix owing to the presence of residual mortar in RCA. The main feature of the proposed method, termed equivalent mortar volume (EMV), is the treatment of residual mortar in RCA as part of the total mortar volume of concrete. The total mortar volume is considered as the sum of the residual and fresh mortar volumes in RCAconcrete. Concrete proportioned based on this method has been found to have the same or superior fresh and hardened properties compared with an equivalent conventional concrete with the same volume of fresh mortar as the total volume of mortar in the companion RCA-concrete.11 Because the RCA-concrete mixes proportioned by the EMV method do not suffer from the inferiorities of similar concrete proportioned by the conventional method, it is expected that RC beams made of RCAconcrete proportioned by the EMV method will not experience lower ultimate shear strength compared with conventional RC beams made of natural-aggregate478
concrete. To verify this hypothesis, an extensive experimental study was carried out. Concrete mixes proportioned by the EMV method were used to cast a large number of beams. As the key parameters that are known to affect the concrete contribution to the shear resistance of RC member, vc , are the shear span-todepth ratio and the beam size,12 the effects of these parameters on vc in reinforced RCA-concrete beams are investigated. RCA from two different sources are used in the study, and are designated as RCA-M and RCA-V, which were obtained from demolition concrete recycling plants in Montreal (M) and Vancouver (V) respectively. The original virgin aggregate in RCA-M is limestone while that in RCA-V is river-bed gravel.
Experimental investigation Mixture proportions Two mix types were prepared for each RCA source using ordinary Portland cement: (a) a control concrete mix made with coarse natural aggregate of the same kind as that in the companion RCA and proportioned according to the American Concrete Institue (ACI) method for normal concrete;13 and (b) a companion RCA-concrete mix involving replacement of the coarse natural aggregate by coarse RCA and proportioned according to the EMV method.11 The fine aggregate in all the mixes was natural sand. Note that using the EMV method ensured equal total mortar volume in the two mix types. To compensate for the deficiency in the total natural aggregate volume of RCA-concrete mix compared with its companion natural-aggregate-concrete mix, in the former, which contained both RCA and fresh coarse natural aggregate, the fresh natural aggregate volume, V RCA-concrete , was set equal to the total residual mortar NA volume in RCA-concrete, V RCA-concrete . The fresh naturRM al aggregate and the natural aggregate contained in the RCA in each mix were of the same kind; depending on the RCA source, that is Vancouver against Montreal, they were either river gravel or crushed limestone. The specific gravity and absorption capacity of the aggregates were determined using the standard testing procedures of the American Society for Testing and Materials (ASTM).14 The residual mortar content of each RCA type was determined based on a new method that involved the immersion of RCA in sodium sulphate solution and exposure to several freeze–thaw cycles.15 Table 1 shows the weighted average properties for RCA-M, RCA-V, natural limestone, natural gravel and river sand. Both the natural and the recycled concrete aggregates had a nominal maximum size of 19 mm. These aggregates were presoaked while the fine aggregate was kept moist for 24 h before mixing. For each mix, six 100 3 200 mm cylinders were prepared and cured in a moist room for 28 days to determine the compressive and splitting tensile strengths of Magazine of Concrete Research, 2009, 61, No. 7
Shear strength of reinforced recycled concrete beams without stirrups Table 1. Average physical properties of coarse and fine aggregates Aggregate
RCA-M RCA-V Limestone River gravel River sandy
Moisture content: %
1.1 1.3 0.2 0.2 4.0
Absorption content: %
Specific gravity
RMC*: %
Bulk
SSD
Apparent
2.31 2.42 2.70 2.72 2.70
2.42 2.50 2.71 2.74 2.72
2.64 2.64 2.73 2.79 2.76
5.40 3.30 0.34 0.89 0.54
41 23 — — —
* Residual mortar content ¼ oven-dry weight of residual mortar/oven-dry weight of RCA Fineness modulus of 2.60
y
the concrete, using three specimens for each test. Another three 150 3 300 mm cylinders were similarly prepared to determine the 28-day elastic modulus of each concrete mix. An additional nine concrete cylinders were prepared and cured adjacent to and in the same manner as the test beams to evaluate the compressive strength (three 100 3 200 cylinders), splitting tensile strength (three 100 3 200 cylinders) and elastic modulus (three 150 3 300 cylinders) of the pertinent concrete at the time of the testing of the beams. Table 2 presents the proportions of the mixes for the beams, with the mix designations defined in the last row of the table. Note that the proportions of the fine and coarse
aggregate are based on oven-dried and air-dried states respectively. The air-dried coarse aggregate was prepared in individual size fractions (35%, 25% and 40% for 4.75 mm, 9.5 mm and 12.5 mm fraction sizes respectively) and subsequently combined to produce the desired grading. All the beams for each RCA source were cast simultaneously using a truck mixer, but as the number of beams made of the control concrete mixes was small, a pan mixer was used to prepare the mixes for the control beams. Table 3 shows a summary of the fresh and hardened properties of the mixes used. All of the steel used as flexural reinforcement was
Table 2. Mix proportions of reinforced recycled concrete and control beams Beam ID
Mix proportions: kg/m3
RCA content: % Water
63.5 0 74.3 0
EM CL EV CG Mix designation nomenclature
151 193 161 191
Cement
335 430 358 424
Sand
Coarse aggregate
630 808 645 763
RCA
Natural aggregate
720 0 813 0
414 835 281 900
WRA*: ml
AEy : ml
1055 None 1132 None
35 92 38 91
E or C: mix proportioned based on EMV (E) or conventional method (C); and 2) M, V, L or G: mix made with RCA-MO (M), RCA-VA (V), natural limestone (L) or natural gravel (G)
* WRA: Water reducing agent, y AE: Air entraining
Table 3. Fresh and hardened properties of investigated concrete mixes Mix ID
Fresh properties
Hardened properties f c9 : MPa
Ec : GPa
Hardened ªc : kg/m3 f t : MPa
Slump: mm
Air content: %
Fresh ªc : kg/m3
28 days
Test date
28 days
Test date
28 days
Test date
EM
96
6.4
2341
41.6
36.9
29.8
24.6
3.4
2.8
2333
EV
60
4.5
2398
49.1
43.5
31.8
27.1
3.7
3.4
2364
185 160 220 200
5.9 6.4 6.4 6.2
2333 2330 2347 2358
37.1 38.8 33.8 34.4
38 38.3 35.9 32.8
30.3 31.7 30.5 31.3
24.5 25.2 27.9 27.1
3.2 3.8 3.3 3.3
3.0 2.7 3.2 3.2
2308 2324 2308 2322
CL CG
Batch-1 Batch-2 Batch-1 Batch-2
Magazine of Concrete Research, 2009, 61, No. 7
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G. Fathifazl et al. grade 400 bars in accordance with the requirements of CAN/CSA G30.18.16 Nominal bar diameters varied from j8 mm to j30 mm; the j8 mm bar was smooth while all the other bar sizes were deformed. In the coupon tests, the deformed bars exhibited a clear yield plateau with their yield strength varying between 407 and 473 MPa and their ultimate strength varying between 572 and 733 MPa. The round smooth bar had a yield strength of 530 MPa and ultimate strength of 596 MPa. The elastic modulus for all the bars was approximately 180 GPa, which seems smaller than the expected value of 200 GPa. Details of test beams The test programme comprised 20 longitudinally reinforced beams without shear reinforcement. In addition to the type of concrete, the other test parameters included beam shear span/depth ratio, a/d, and beam size.12 Four a/d ratios (1.5, 2, 2.7 and 4) were selected to cover the shear behaviour of short, intermediate and slender beams, which are known to exhibit different shear strength even if they are otherwise identical.17 For each RCA-concrete type proportioned by the EMV method, four beams were tested – that is, one beam for each a/d ratio. All the beams were prismatic with
200 mm wide rectangular cross-section, and with an overall depth ranging from 350 to 375 mm (effective depth of 305 5 mm). For each RCA type, a control beam with a/d ratio of 2.70 was made of the companion natural-aggregate-concrete. Therefore, in total, ten beams were tested to investigate the effect of a/d ratio on the shear behaviour and strength of RRC beams without shear reinforcement. All the beams were longitudinally reinforced, with their reinforcement ratios given in Table 4. To facilitate tracking of the shear deformations and diagonal crack movements, the west shear span of each beam was instrumented with a rosette of linear variable differential transducers (LVDTs) as described below; consequently, to ensure shear failure occurrence in the instrumented half of the span, the other half was reinforced with j10 mm closed steel stirrups as shown in Figure 1(a). There was only one exception, in which the whole beam was inadvertently reinforced with stirrups, and this case will be further discussed in a subsequent section. To study the size effect, four 200 mm wide rectangular beams with depths of 250, 375, 450 or 550 mm, and with a constant a/d ratio of 2.70 were built for each RCA type. Once again, for each RCA type a 200 mm wide by 375 mm deep control beam was made of
Table 4. Test beams details Effect of a/d ratio Beam ID
a/d
Dimensions D
EM-1.5N EM-2N EM-2.7N CL-2.7N EM-4N EV-1.5N EV-2N EV-2.7N CG-2.7N EV-4N
1.50 2.00 2.59 2.59 3.93 1.50 2.00 2.59 2.59 3.93
300 300 309 309 305 300 300 309 309 305
Stirrup spacing(s)
Longitudinal bottom bars (r, %)
L 1900 2200 2600 2600 3400 1900 2200 2600 2600 3400
150 200 200 200 200 150 200 200 200 200
2 3 3 3 3 2 3 3 3 3
No. 20 (1.00) No. 20 (1.5) No. 15 + 2 No. 15 No. 15 + 2 No. 15 No. 20 + 2 No. 20 No. 20 (1.00) No. 20 (1.5) No. 15 + 2 No. 15 No. 15 + 2 No. 15 No. 20 + 2 No. 20
(1.62) (1.62) (2.46)
(1.62) (1.62) (2.46)
Size effect Beam ID
EM-L EM-M CL-M EM-H EM-HH EV-L EV-M CG-M EV-H EV-HH
480
a/d
2.69 2.59 2.59 2.73 2.73 2.69 2.59 2.59 2.73 2.73
Dimensions: mm A
L
201 309 309 381 476 201 309 309 381 476
2080 2600 2600 3180 3700 2080 2600 2600 3180 3700
Stirrup spacing(s)
135 200 200 200 200 135 200 200 200 200
Longitudinal bottom bars (r, %)
2 3 3 2 2 2 3 3 2 2
No. 20 + 1 No. 15 No. 15 + 2 No. 15 No. 15 + 2 No. 15 No. 25 + 2 No. 15 No. 25 + 2 No. 20 No. 20 + 1 No. 15 No. 15 + 2 No. 15 No. 15 + 2 No. 15 No. 25 + 2 No. 15 No. 25 + 2 No. 20
(1.99) (1.62) (1.62) (1.83) (1.68) (1.99) (1.62) (1.62) (1.83) (1.68)
Magazine of Concrete Research, 2009, 61, No. 7
Shear strength of reinforced recycled concrete beams without stirrups West
East
a
600 L
beam depth as 250 mm (low-L), 375 mm (medium-M), 450 mm (high-H) and 550 mm (very high-HH), respectively. These beams were similarly reinforced as those previously described.
a
h d
2 No. 10 No. stirrup (west) No. 10 @ S (east)
Instrumentation and test set-up
As 200 (a) sx West
ST3 ST1 ST2
Sy
East
h/2 x (b)
EM-4·0N
CL-2·7N EM-2·7N
EM-2·0N
EM-1·5N
Figure 1. Beam details: (a) details of shear beams without shear reinforcement; (b) arrangement of LVDT rosette for sheared deformation measurements
EV-2·0N
EV-1·5N
(a)
Electrical resistance strain gauges were used to measure the strain in the longitudinal reinforcement and the concrete. Beam deflection was measured using linear potentiometers placed along the beam. All the beams were simply supported and tested in four-point bending. The load was applied by a 1000 kN servo-controlled hydraulic actuator, attached to a rigid frame. The actuator applied the load by stroke control to a steel spreader beam supported by two heavy-duty rocker-and-roller assemblies symmetrically located 300 mm from the midspan of the beam. The load was applied using a displacement rate of 0.01 mm/s, and the automatic data acquisition system recorded data every 10 s. As the majority of previous research has reported wider cracks, smoother crack interface and less ductile behaviour for RRC beams compared with conventional concrete beams,2–4 to verify the validity of these findings with regard to the beams in this investigation, the beams were outfitted with a rosette of three LVDTs (ST1, ST2 and ST3), arranged as illustrated in Figure 1(b). The length of the LVDTs and the location of the rosette were chosen based on the expected location of the major diagonal crack. The LVDTs were placed to measure both diagonal tension and diagonal compression deformations in the instrument part of the beam, with the former giving an indication of crack width and the latter providing information about the extent of deformation sustained by the concrete in the diagonal strut.
EV-2·7N
Experimental results
EV-4·0N
GG-2·7N
The results are presented with focus on the effects of the a/d ratio and beam size on the shear behaviour and strength of RCA-concrete beams without shear reinforcement. (b)
Figure 2. Typical crack patterns of test beam with different a/d ratios at failure: (a) EM and CL beams; (b) EV and CG beams
natural aggregate concrete to compare its shear strength with that of an otherwise identical beam made of RCAconcrete. Therefore, a total of ten beams were tested to investigate the size effect on the shear strength of reinforced RCA-concrete beams; Table 4 gives the details of these beams. The following notation is used to designate them: symbols 1.5N, 2N, 2.7N or 4N refer to the nominal a/d ratio of 1.5, 2, 2.7 or 4 and N signifies no shear reinforcement; L, M, H or HH characterises the Magazine of Concrete Research, 2009, 61, No. 7
Effect of a/d ratio Failure modes. Figure 2 illustrates the cracking patterns of the beams with different a/d ratios. In this figure, the dark lines represent the major diagonal cracks leading to shear failure, the grey lines the minor shear and flexure-related cracks, and the dark zones the crushed concrete areas. All the beams failed in shear, except beams EV-1.5N and EV-2.7N, which failed in flexure prior to shear failure. The flexural failure of beam EV-2.7N was caused by a fabrication mistake that resulted in the reinforcement of this beam with stirrups throughout its length and thus to an inadvertent increase in its shear strength. After inclined cracking, the RCA-concrete beams 481
G. Fathifazl et al. with a/d ratio of 1.5 or 2 behaved akin to a tied arch carrying the load by direct compression by way of struts extending from the loading plates to the supports and by the longitudinal tension reinforcement acting as tie. Consequently, they exhibited considerable shear capacity. On the other hand, the beams with a/d ratio of 2.7 or 4.0 did not develop the same shear resistance mechanism; therefore, they failed shortly after the formation of the major diagonal crack. These observations are consistent with the known behaviour of conventional concrete beams with similar a/d ratios.17 Considering the ratio of the failure load to the inclined cracking load as an indicator of ductility, or ductility index, beam EM-1.5N had the highest ductility index of 2.59 compared with 2.07, 2.00, and 1.0 for beams EM-2.0N, EM-2.7N and EM-4.0N. Similarly, beam EV-1.5N had a ductility index of at least 3.06 compared with 2.89 and 1.56 for beams EM-2.0N and EM-4.0N. Beam EM-2.7N had a ductility index of 2.00, which is much higher than the value of 1.38 for the companion control beam CL-2.7N made of conventional concrete. The latter ductility index values contradict the reported findings of other researchers with regard to the ductility of RCA-concrete members. Figures 3(a) and (b) illustrate the effect of a/d ratio on the longitudinal steel strain variation at a distance d from the west support face with shear load for both EM 210 EM-1·5N
180
and EV beams. It can be seen that the longitudinal steel reinforcement near the west support in beams EM-1.5N and EV-1.5N yielded. This is because of the high shear resistance of these beams that was sustained by the arch mechanism, the maintenance of which requires the development of a substantial tensile force in the tension reinforcement acting as the arch tie. Consequently, the resistance of these beams was limited by the tensile capacity of the longitudinal reinforcement rather than the shear capacity of the RCA-concrete. Ultimate shear strength. Figure pffiffiffiffiffi 4 illustrates the normalised shear resistance (vc = f c9 ) of the EM and EV test beams with different a/d ratios. For convenience, the vc value for each beam is normalised by the square root of the compressive strength of its concrete. pffiffiffiffiffi Notice that generally the vc = f c9 value increased as the a/d ratio decreased. As stated earlier, this is mainly owing to the arch mechanism resistance which depends on the magnitude of the diagonal compression and on the inclination of the thrust line of the arch, which is a function pffiffiffiffiffi of the a/d ratio. According to Figure 4(a), the vc = f c9 values of EM-1.50N, EM-2.0N and EM-2.7N beams were, respectively, 128%, 102% and p 9%, ffiffiffiffiffi higher than that of EM-4.0N. Furthermore, the vc = f c9 values of EV-1.50N and EV-2.0N beams were 88% and 68% . higher thanpthat ffiffiffiffiffi of EV-4 0N (Figure 4(b)). Note that the actual vc = f c9 value for EV-1.5N may be higher than that given in Figure 4(b) because it failed in flexure
EM-2·0N 0·6
120
0·5
EM-4N
CL-2·7N
90
0
1000
2000
3000
4000
0·1
5000
Microstrain (a) 210
0·0 (a) EV-1·5N
180
0·6
EV-2·0N CG-2·7N
150
0·5
120
0·3 0·2
EV-4N
30
0·1
EV-1·5N
60
CG-2·7N
90
EV-4·0N
EV-2·7N
EV-2·0N
0·4
Vc /f ⬘c
Shear: kN
EM-1·5N
0·2
0
EM-2·7N
30
0·3
CL-2·7N
EM-2·7N
EM-4·0N
60
EM-2N
0·4
Vc /f ⬘c
Shear: kN
150
0 0
3000
6000
9000
Microstrain (b)
Figure 3. Effect of a/d ratio on longitudinal steel strain near the support: (a) EM and CL beams; (b) EV and CG beams
482
0·0 (b)
Figure 4. Effect of a/d ratio on the ultimate shear resistance of RCA-concrete beams: (a) EM and control CL beams; (b) EV and CG beams Magazine of Concrete Research, 2009, 61, No. 7
Shear strength of reinforced recycled concrete beams without stirrups
200
30
160
120 20 80
Vc: kN
10
40 0
30
120 20 80 10
40
0 EV-1·5N
40
0
0
EM-1·5N
EV-2N (b)
27
80
18
Vc: kN
Vc: kips
120
40
Vc: kN
(a)
EM-2N
160
36
120
27
80
18
40
9
Vc: kips
Vc: kN
160
40
Vc: kips
200
estimating the concrete contribution to the shear resistance of concrete beams can be applied to RCAconcrete members with their concrete mixes proportioned by the EMV method, the vc values for the tested beams are compared with the values calculated using three well-known concrete design codes. The bar graphs in Figure 5 show that none of the tested beams had smaller shear strength than the calculated values according to the simplified methods of the Canadian Standard CSA A23.3-0418 and the American Code ACI-31819 (equation (11.3)), and Eurocode 2,20 regardless of the a/d ratio or the RCA source. It can be observed that in many cases the calculated values are rather conservative, particularly for the beams with a/d ratio of 2 or less, mainly due to the higher contribution of the arch mechanism to the shear resistance at lower a/d ratios. Consequently, for RCA-concrete designed by
Vc: kips
rather than shear. This is indicated by the arrow at the top of the EV-1.5N bar chart. pffiffiffiffiffi According to Figure 4(a), the vc = f c9 value of EM2.7N was 14% higher than that of control beam CL-2.7N. This finding is contrary to reported findings by others who have indicated the shear strength of RRC beams to be lower than that of comparable conventional concrete beams.2–4 The reason for the previously observed lower strength can be ascribed to the use of the conventional mix proportioning method in previous studies against the EMV method used in this study. The conventional method leads to lower coarse aggregate content in the RCAconcrete and thus fewer coarse aggregate particles are expected to cross the cracked shear plane, which would reduce crack roughness and weaken the aggregate interlock mechanism of shear resistance. To ascertain whether existing codes expressions for
9
0
0
0 CL-2·7N
0
EM-2·7N
CG-2·7N
(c)
EV-2·7N (d)
120
36
Experimental
27
18
Vc: kips
Vc: kN
80
EC-2 ACI-318 (simplified)
40 9
0
CSA (simplified)
0 EV-4N
EM-4·0N (e)
Figure 5. Experimental and predicted ultimate shear strength of RRC beams with different a/d ratios: (a) a/d ¼ 1.5; (b) a/d ¼ 2; (c) a/d ¼ 2.7; (d) a/d ¼ 2.7; (e) a/d = 4 Magazine of Concrete Research, 2009, 61, No. 7
483
G. Fathifazl et al. the EMV method, one can safely use the existing codes expressions for calculating vc .
branch corresponds to the diagonal cracking load, and its magnitude is mainly a function of the concrete strength. The length of the horizontal projection of the first descending branch, which is an indicator of the reduction of stiffness of the beam, is generally a function of the a/d ratio of the beam. The deeper beams with lower a/d ratio have a shorter descending branch than the slender beams with higher a/d ratio. This is mainly due to the moment-to-shear ratio and the ratio of the cracked to the gross moment of inertia of the beam. The slope of the ascending part of the curve after the first descending part is also a function of the a/d ratio of the beam. This slope is another indicator of the shear stiffness of the beam, which is higher for the deeper beams with lower a/d ratio. At higher a/d ratios, after the inclined crack formation, the load dropped slightly, but owing to the aggregate interlock mechanism, it increased again until diagonal tension failure occurred. Figures 7 and 8 illustrate the effect of coarse aggregate type – that is, limestone against river gravel – on pffiffiffiffiffi the vc = f c9 variation with midspan deflection and with the diagonal tensile deformation, respectively. As it can be seen, generally the type of aggregate has a negligible effect on the shear strength and stiffness, irrespective of the a/d ratio.
Shear performance. Figure 6(a) illustrates pffiffiffiffiffi the effect of a/d ratio on the variation of vc = f c9 against midspan deflection in the EM and EV series of beams. Notice that as the a/d ratio increases, the post-cracking stiffness decreases irrespective of the RCA type. This can be mainly attributed to the maximum moment to the maximum shear (M/V) ratio in the beam, viz. for larger a/d ratio, for the same shear level the moment would be larger and consequently the effective moment of inertia of the section would be smaller after the formation of the cracks, leading to a noticeable drop in beam stiffness. Figure 6(b) shows pffiffiffiffiffi for the EM and EV beams the variation of vc = f c9 with the deformation measured by the LVDT bridging across the inclined crack in the west shear span (ST1). Observe that all of the curves exhibit an initial linear elastic portion, followed by a descending branch, a longer ascending part and finally another descending part after the peak load. The first descending branch is due to the formation of the first major crack and the fact that the member is under displacement control. The load at the beginning of the first descending 0·6
0·6 EM-1·5N
EV-1·5N
0·5
0·5 EM-2·0N 0·4 EV-2·0N
0·3
Vc /f ⬘c
Vc /f ⬘c
0·4
EM-2·7N
0·3 EV-4·0N
0·2
0·2
EM-4·0N
0·1
0·1
0
0 0
4
8
12
16
20
0
10
20
30
40
50
Midspan deflection: mm EV beams
Midspan deflection: mm EM beams (a) 0·6 0·6
0·5
0·5
Vc /f ⬘c
EM-2·7N 0·3 EM-4·0N
0·2
EV-2·0N
0·4
EM-2·0N
0·4
Vc /f ⬘c
EV-1·5N
EM-1·5N
0·3 EV-4·0N 0·2 0·1
0·1 0
0 0
2
4
6
8
0
10
Side deformation: mm EM beams
2
4
6
8
10
12
14
Side deformation: mm EV beams (b)
Figure 6. Effect of a/d ratio on shear behaviour of beams: (a) normalised shear stress–midspan deflection response; (b) normalised shear stress stress–diagonal deformation response
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Shear strength of reinforced recycled concrete beams without stirrups 0·6
0·6
EV-1·5N
0·5
0·5 EM-1·5N
Vc /Öf ⬘c
Vc /Öf ⬘c
0·4 0·3 0·2
EV-2·0N
0·4 0·3
EM-2·0N
0·2
0·1
0·1
0
0 0
10
20
30
40
50
0
3
6 9 Midspan deflection: mm (b)
Midspan deflection: mm (a)
12
15
0·3
0·4
EV-4·0N CL-2·7N
EM-2·7N
0·2
Vc /Öf ⬘c
Vc /Öf ⬘c
0·3 0·2
EM-4·0N 0·1
0·1 0
0 0
3
6
9
12
15
5
0
Midspan deflection: mm (c)
10 15 Midspan deflection: mm (d)
20
Figure 7. Coarse aggregate type effect on normalised shear stress resistance–midspan deflection response of beams with different a/d ratios: (a) a/d ¼ 1.5; (b) a/d ¼ 2; (c) a/d ¼ 2.7; (d) a/d ¼ 4
EV-1·5N
0·6
EM-1·5N
0·5
0·4
0·4
Vc /Öf ⬘c
Vc /Öf ⬘c
0·6 0·5
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0·3
0·2
0·2
0·1
0·1 0
0 5
4 2 3 Side deformation: mm (a)
1
0
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6
0·5
6 9 Side deformation: mm (b)
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0·3 EV-4·0N
0·4 EM-2·7N 0·3
Vc /Öf ⬘c
Vc /Öf ⬘c
12
CL-2·7N 0·2
0·2
EM-4·0N
0·1
0·1 0
0 0
1
2
3
4
5
6
7
8
Side deformation: mm (c)
0
1
2
5 4 3 6 Side deformation: mm (d)
7
8
Figure 8. Aggregate effect on normalized shear stress resistance diagonal deformation response of beams at different a/d ratios: (a) a/d ¼ 1.5; (b) a/d ¼ 2; (c) a/d ¼ 2.7; (d) a/d ¼ 4
According to Figures 7(c) and 8(c), beam EM-2.7N is less stiff compared with beam CL-2.7N, but it is more ductile in the post-inclined cracking stages. This finding is again contrary to the results previously reported by others2 where RCA-concrete beams were reported to be less ductile. The reason for this difference may be ascribed to the concrete mix proportioning method. By using the EMV method, both crack interface roughness and aggregate interlock mechanism were enhanced. This is evident by comparing in Figure Magazine of Concrete Research, 2009, 61, No. 7
7(c) the results for beam EM-2.7N with those of the companion control beam CL-2.7N Serviceability. Assuming the service load to be 40% of the failure load, the diagonal crack width at service load level was estimated for these beams using the diagonal deformation measured by the LVDT ST1 parallel to the diagonal tension field. The LVDT measures the total deformation, which includes the crack opening and the deformation of concrete, 485
EM-M CL-M EM-H EM-HH
EV-2M
EV-L
(a)
GG-M
but the latter is generally much smaller than the former. By ignoring the concrete deformation, crack width values of 0.19, 0.05, 0.06, 0.01 and 0.03 mm were found for beams EM-1.5N, EM-2.0N, EM-2.7N, CL-2.7N and EM-4.0N, respectively. Similarly, crack width values of 0.22, 0.00, 0.04 and 0.02 mm were found for EV-1.5N, EV-2.0N, CG-2.7N and EV-4.0N beams respectively. Note that the higher crack width in beams with lower a/d ratio is mainly attributable to their higher ultimate shear strength, and correspondingly higher service load. Similarly, the higher crack width in EM-2.7N compared with control beam CL-2.7N is partially due to the higher failure load and therefore higher service load of the former beam. Practically all these crack widths are well below the Canadian Standard CSA A23.3-0418 recommended maximum crack width of 0.4 and 0.33 mm for interior and exterior exposures respectively.
EM-L
G. Fathifazl et al.
EV-HH
In conventional RC beams the size effect on vc is recognised by a number of design codes, including the Eurocode. In this section the results of beam size effect on the shear strength of RRC beams without shear reinforcement are discussed.
EV-H
Size effect
(b)
Figure 9. Typical crack patterns of the test beams with different depths at failure: (a) EM and CL beams; (b) EV and CG beams
0·4
EM-L
EM-M
EM-H
0·1
EM-HH
0·2
CL-M
0·3
Vc /Öf ⬘c
Failure mode. Figure 9 illustrates the cracking patterns of the EM and EV series of beams with different depths as well as those of the control beams CL-M and CG-M, which have depth of 375 mm. All the beams failed in shear with the mode of failure being diagonal tension, except beam EV-L and EV-L, which failed in flexure. After the formation of the major diagonal crack, it propagated towards the compression face of the beam. In the RRC beams with small and medium depths (effective depth of 200 and 300 mm), the aggregate interlock mechanism and dowel action were capable of sustaining the load in the post-inclined cracking stage. The increase in load eventually led to the failure of the beam in shear owing to diagonal tension. On the other hand, the RRC beams with larger depth values (effective depth of 400 and 500 mm) were not capable of load redistribution after the formation of inclined crack and they failed shortly thereafter.
0·0 (a) 0·5
486
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EV-H
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EV-L
CG-M
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EV-HH
Ultimate shear strength. Figure 10 illustrates the pffiffiffiffiffi normalised shear stress resistance vc = f c9 of EM and EV series of beams having p different effective depth. ffiffiffiffiffi Generally, the quantity vc = f c9 decreased as d increased. This may be attributed to less effective aggregate interlock resistance in larger size beams. The aggregate interlock contribution to the shear resistance depends on the maximum distance between the layers of distributed longitudinal reinforcement as stipulated in the Canadian Standard CSA A23.3-04.18 While the size effect on shear resistance of conventional concrete members is recognised by many de-
Vc /Öf ⬘c
0·4
0·0 (b)
Figure 10. Experimental nominal shear strength of RCC beams with different sizes: (a) EM and control beams; (b) EV and control CG beams Magazine of Concrete Research, 2009, 61, No. 7
Shear strength of reinforced recycled concrete beams without stirrups
120
27
80
18
40
9
EV-L
EM-L
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40
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0
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(a)
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(b)
36
120
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18 9
Vc : kN
160
Vc : kips
36
Vc : kN
160
80
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40
0
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EV-M
CG-M
EV-H
(c)
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(d)
36
120
27
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9
0
Experimental
Vc : kips
160
Vc : kN
EM-H
Vc : kips
0
Shear performance. Figure 12(a) shows the variapffiffiffiffiffi tion of vc = f c9 with midspan deflection in the beam series EM and EV with different sizes. Generally, the slope of the curves tends to decrease as the beam size increases. This is again attributed to the greater effectiveness of the aggregate interlock mechanism in smaller size beams. In the larger size beams – that is, EM-HH and EV-HH – after the formation of the inclined crack, the stiffness of the beam drops dramatically. pffiffiffiffiffi Figure 12(b) shows the variation of the vc = f c9 with the nominal size of the diagonal crack as measured by the diagonally oriented LVDT bridging over the inclined crack in the west shear span. These curves exhibit a response similar to that described earlier with reference to Figure 12(a). As the first descending part of each curve is due to the formation of the diagonal crack, it is clear that the larger-size beams experience significant loss of stiff-
Vc : kips
36
the aggregate interlock mechanism to the shear resistance of beams with smaller depth.
Vc : kN
160
Vc : kips
Vc : kN
sign standards, it is clear from the present results that the same effect exists in RRC beams. pffiffiffiffiffi According to Figure 10(a), vc = f c9 of beams EM-H, EM-M and EM-L are 19%, 53% and 102%p higher ffiffiffiffiffi than that of beam EM-HH. Furthermore, the vc = f c9 of EVH and EV-L beams are 17% and 143% higher thanpthat ffiffiffiffiffi of EV-HH (Figure 10(b)). Note that the actual vc = f 9c value for EV-L may be higher than that given in Figure 10(b) because it failed in flexure rather than shear. This is indicated by the arrow at the top of p theffiffiffiffiffiEV-L bar chart. It can also be observed that the vc = f c9 value for beam EM-M is 14% higher than that of the control beam CL-M, which is made entirely with natural limestone. Once again, if the experimentally observed shear resistance of these beams is compared with the corresponding values predicted by the CSA A23.3-04,18 ACI-31819 (equation (11.3)), and Eurocode 2,20 as in Figure 11, it is shown that practically all the predictions are conservative. The only exception is the slightly higher predicted value by Eurocode 2 for beam EMHH. The degree of conservatism tends to increase with decreasing depth owing to the greater contribution of
EC-2 ACI-318 (simplified) CSA (simplified)
0 EV-HH
EM-HH (e)
Figure 11. Experimental and predicted ultimate shear strength of RRC beams with different sizes: (a) d ¼ 201 mm; (b) d ¼ 309 mm; (c) d ¼ 309 mm; (d) d ¼ 381mm; (e) d ¼ 476 mm Magazine of Concrete Research, 2009, 61, No. 7
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0·5
0·6
0·4
0·5
EV-L
0·4
0·3
Vc /Öf ⬘c
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G. Fathifazl et al.
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0·2
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EV-HH
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Midspan deflection: mm EV beams
Midspan deflection: mm EM beams (a) 0·5
0·6 EV-L
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Vc /Öf ⬘c
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EM-L
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EV-HH
0·1 0
0 4 6 Side deformation: mm EM beams
2
0
8
0
10
2
4 6 Side deformation: mm EV beams
8
10
(b)
Figure 12. Size effect on shear behaviour or RRC members: (a) normalised shear stress resistance–midspan deflection response: (b) normalised shear stress variance plotted against diagonal tensile deformation
ness and strength compared with the smaller-size beams after the advent of this crack. Furthermore, the magnitude of the change in the diagonal deformation between the initial and terminal points of the descending part is indicative of the size of the diagonal crack. Accordingly, the larger-size beams experience wider
diagonal cracks and they reach their maximum shear capacity just before the formation of the diagonal crack. Conversely, the smaller-size beams carry significantly higher shear than their diagonal cracking load. Figures 13 and 14, respectively, illustrate the effect pffiffiffiffiffi of aggregate type on the variation of vc = f c9 plotted 0·4
0·6 EV-L 0·5
0·3
CL-M
Vc /Öf ⬘c
Vc /Öf ⬘c
0·4 0·3 EM-L
0·2
EM-M
0·2 0·1
0·1 0 0
7
14
21
28
0
35
0
3
6 9 Midspan deflection: mm (b)
Midspan deflection: mm (a)
0·2
0·2 EV-H
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15
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EM-HH
Vc /Öf ⬘c
0·3
Vc /Öf ⬘c
0·3
12
0·1 EM-H
0
0 0
3
6 Midspan deflection: mm (c)
9
12
0
2·5
5·0
7·5
10
Midspan deflection: mm (d)
Figure 13. Aggregate type effect on normalised shear stress resistance–midspan deflection of beams with different sizes: (a) d ¼ 201 mm; (b) d ¼ 309 mm; (c) d ¼ 381 mm; (e) d ¼ 476 mm
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Shear strength of reinforced recycled concrete beams without stirrups 0·6
0·5
EV-L
0·5
0·4 EM⫺2·7N
Vc /Öf ⬘c
Vc /Öf ⬘c
0·4 0·3 EM-L
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CL⫺2·7N 0·2 0·1
0·1 0 0
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4 6 Side deformation: mm (a)
8
0
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1
2
5 4 3 6 Side deformation: mm (b)
7
8
0·3
0·3
0·2
0·2
Vc /Öf ⬘c
Vc /Öf ⬘c
EV-H
0·1
EV-HH 0·1
EM-HH
EM-H 0
0 0
4 2 Side deformation: mm (c)
6
0
3 6 Side deformation: mm (d)
9
Figure 14. Effect of aggregate type on normalised shear stress resistance–diagonal tensile deformation of RRC beams with different sizes
against midspan deflection and diagonal deformation. These figures show that the type of natural aggregate in the two kinds of coarse RCA used in this study did not have any significant effect on the shear strength of the beams made with these aggregates. Serviceability. Assuming the serviceability load to be 40% of the failure load of these beams, under service load approximate diagonal crack width values of 0.16, 0.06, 0.01, 0.02 and 0.00 mm were measured for beams EM-L, EM-M, CL-M, EM-H and EM-HH, respectively. Similarly, crack width values of 0.28, 0.04, 0.03 and 0.00 mm were measured for beams EVL, CG-M, EV-H and EV-HH, respectively. Note that the larger crack width in smaller-size beams is mainly attributable to their higher ultimate shear strength, and consequently their proportionally higher service load. These crack widths are well below the maximum crack width of 0.4 and 0.33 mm for interior and exterior exposures recommended by CSA A23.3-04.18
Conclusions In this paper, the results of an investigation into the shear strength and behaviour of RRC beams without shear reinforcement were presented. The focus of the study was the effect of the proposed EMV concrete mix design method on the shear capacity of RCAconcrete beams. Based on the results of the current investigation, provided the RCA-concrete mix is designed by the EMV method, the following conclusions are reached. Magazine of Concrete Research, 2009, 61, No. 7
(a) There is no major difference between the failure modes, cracking patterns and shear performance of RRC beams and conventional RC beams. Generally, the tested RRC beams had higher shear stress resistance (vc ) and were found to be more ductile after the formation of diagonal cracking than the conventional RC beams. (b) The shear strength of RRC beams increased as the a/d ratio decreased, irrespective of the source, mainly owing to the higher contribution of the arch mechanism to the shear resistance at lower a/d ratios. (c) Irrespective of the RCA source, the vc of RRC beams increased as the overall depth of the beam decreased, which is a result of the well-known size effect as in conventional RC beams. This increase is primarily attributed to the lower effectiveness of the aggregate interlock mechanism in larger-size beams. (d) Despite the slightly larger diagonal crack width in RRC beams compared with the companion RC beams, the observed crack widths in all the RRC beams were below the maximum crack width permitted by the Canadian Standard CSA A23.3-04 and ACI-318 codes. (e) For the same a/d ratio, concrete compressive strength and beam depth, the effect of aggregate type (RCA against natural aggregate) on the shear strength of RRC beams was found to be negligible. ( f ) The simplified methods of CSA A23.3-04 and ACI-318 as well that of Eurocode 2 for calculating vc were found to be conservative when applied 489
G. Fathifazl et al. to predict the shear of practically all the RCAconcrete beams tested in this study. 10.
Acknowledgement The authors wish to express their sincere appreciation to Public Works and Government Services Canada and to Natural Sciences and Engineering Research Council of Canada for their financial support of this study.
References 1. Abbas A., Fathifazl G., Isgor O. B., Razaqpur A. G., Fouriner B. and Foo S. Environmental benefits of green concrete. Proceedings of Climate Change Conference, Ottawa, Ontario, 2006. 2. Han B. C., Yun H. D. and Chung S. Y. Shear capacity of reinforced concrete beams made with recycled-aggregate. ACI Special Publications, 2001, 200, 503–516. 3. Maruyama I., Sogo M., Sogabe T., Sato R., Kawai K. Shear behaviour of reinforced recycled concrete beams. Proceedings of the Conference on the Use of Recycled Materials in Building and Structures, Barcelona, Spain, 2004. 4. Etxeberria M., Marı´ A. R. and Va´zquez E. Recycled aggregate concrete as structural material. Materials and Structures, 2007, 40, No. 5, 529–541. 5. Dhir R. K., Limbachiya M. C. and Leelawat T. Suitability of recycled concrete aggregate for use in BS 5328 designated mixes. Proceedings of the Institution of Civil Engineers, Structures and Buildings, 1999, 134, No. 3, 257–274. 6. Limbachiya M. C., Leelawat T. and Dhir R. K. Use of recycled concrete aggregate in high-strength concrete. Materials and Structures/Materiaux et Constructions, 2000, 33, No. 233, 574–580. 7. Mandal S., Chakarborty S. and Gupta A. Some studies on durability of recycled aggregate concrete. Indian Concrete Journal, 2002, 76, No. 6, 385–388. 8. Gomez Soberon J. M. V. Shrinkage of concrete with replacement of aggregate with recycled concrete aggregate. ACI Special Publications, 2002, 209, 475–496. 9. Gomez Soberon J. M. V. Creep of concrete with substitution of
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normal aggregate by recycled concrete aggregate. ACI Special Publications, 2002, 209, 441–464. Gomez-Soberon J. M. V. Porosity of recycled concrete with substitution of recycled concrete aggregate: an experimental study. Cement and Concrete Research, 2002, 32, No. 8, 1301– 1311. Fathifazl G., Abbas A., Razaqpur A. G., Isgor O. B., Fournier B. and Foo S. The key to the design and production of high quality structural-grade recycled aggregate concrete. Proceedings of the 2008 Concrete Technology Forum: Focus on Sustainable Development, Denver, CO, USA, 2008. American Society of Civil Engineers (ASCE)–American Concrete Institute (ACI) Task Committee 445. Recent approaches to shear design of structures. Journal of Structural Engineering, ASCE, 1998, 124, No. 12, 1375–1417. American Concrete Institute. Standard Practice for Selecting Proportions for Normal, Heavyweight and Mass concrete, ACI 211.1-91. ACI, Farmington Hills, Michigan, 1998, ACI Committee 211. American Society for Testing and Materials. ASTM C 127-88. Standard Test Method for Specific Gravity and Absorption of Coarse Aggregate. ASTM, West Conshohocken, Philadelphia (reapproved 1993), 1996. Abbas A., Fathifazl G., Isgor O. B., Razaqpur A. G., Fournier B. and Foo S. Proposed method for determining the residual mortar content of recycled concrete aggregates. Journal of ASTM International, 2008, 5, No. 1. http://www.astm.org/ DIGITAL_LIBRARY/JOURNALS/JAI/TOC/512008.htm. Canadian Standards Association (CSA), Billet-Steel bars for Concrete Reinforcement. CSA Standard CAN/CSAG300.18-M92 (R2007), 2007, Rexdale, Ontario. Park R. and Paulay T. Reinforced Concrete Structures. WileyInterscience, New York, 1975, pp. 271–300. Canadian Standard Association. Design of Concrete Structures. CSA Standard A23.3-04, CSA, Rexdale, Ontario, 2000. American Concrete Institute. Building Code Requirements for Structural Concrete. ACI 318-05. ACI, Farmington Hills, 2005, ACI Committee 318. Comite´ Europe´en de Normalisation (CEN). Eurocode 2. Design of Concrete. CEN, Brussels, 2005.
Discussion contributions on this paper should reach the editor by 1 March 2010
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