Shear Capacity of Self-Compacting Concrete Petra Van Itterbeeck 1, Niki Cauberg 2, Benoit Parmentier 3, Ann Van Gysel 4 and Lucie Vandewalle 5

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1 Shear Capacity of Self-Compacting Concrete Petra Van Itterbeeck 1, Niki Cauberg 2, Benoit Parmentier 3, Ann Van Gysel 4 and Lucie Vandewalle 5 1 Project leader, Laboratory Structures, Belgian Building Research Institute (BBRI), Avenue P. Holoffe 21, 1342 Limelette, Belgium, pvi@bbri.be 2 Head of Laboratory Structures, BBRI, Avenue P. Holoffe 21, 1342 Limelette, Belgium, nca@bbri.be 3 Head of Division, BBRI, Avenue P. Holoffe 21, 1342 Limelette, Belgium, bp@bbri.be 4 Senior Lecturer, Lessius - Campus De Nayer, Jan De Nayerlaan 5, 2860 Sint-Katelijne-Waver, Belgium, Ann.VanGysel@lessius.eu 5 Professor, KULeuven, Kasteelpark Arenberg 40, 3001 Heverlee, Belgium, Lucie.vandewalle@bwk.kuleuven.be ABSTRACT: Since its appearance on the Japanese market in the eighties Self-Compacting Concrete has really thrived all over the world. Much research has already been devoted to the fresh properties of the concrete. There nevertheless still exist some questions with regard to the mechanical properties of this innovative material. Shear capacity is one of the topics of concern. The high paste volume and associated lower aggregate volume necessary to achieve the desired fluidity and self-compacting ability of the concrete may reduce its shear capacity, since this might for instance reduce aggregate interlock. Therefore in this study full scale beam specimens were tested under four point-bending load, measuring not only force but also deflections and crack development. The shear capacity of several selfcompacting concrete mixtures is examined and compared to that of traditional, vibrated concrete. The influence of different parameters such as reinforcement ratio and geometry of the beams on the shear capacity is investigated. The experimental results were compared with those obtained for traditional, vibrated concrete and with Eurocode 2 predictions. The occurring failure modes were also examined. The results of this research tend to indicate that selfcompacting concrete is characterized by a similar to even slightly better shear capacity than traditional vibrated concrete. Eurocode 2 still provides a safe prediction of the shear capacity of self-compacting concrete. Keywords: Shear, Eurocode 2, self-compacting concrete 1

2 INTRODUCTION Due to the very brittle nature of shear failure in concrete elements, great attention is dedicated to the prevention of this failure mode. In the past a lot of research has already been conducted with regard to the shear resistance of concrete beams without shear reinforcement. These studies have shown that different mechanisms contribute to the shear resistance of the concrete element at the level of the crack. Following three mechanisms are generally recognised: (a) Aggregate interlock across the crack (b) Shear resistance of the compression area (c) Dowel action of the tension reinforcement Taylor [1] estimated the contribution of each of these mechanisms to the overall shear resistance of the concrete section and found that aggregate interlock represents 35 to 50% of the total shear resistance, while shear resistance of the compression area accounts for another 20 to 40% of the total shear resistance. Dowel action of the tension reinforcement, plays only a minor role and is only responsible for 15 to 25%. A large fraction of the shear resistance can thus be attributed to the aggregate interlocking mechanism at the crack face. Due to the lower aggregate volume fraction usually adopted in self-compacting concrete (SCC) compared to traditional vibrated concrete (VC) some engineers assume that the shear resistance might be lower than that of traditional vibrated concrete with a similar compressive strength. The available literature with regard to the shear resistance of SCC is quite limited and often contradictory on this topic. In the work of Hassan et al [2] a large number of SCC beams with different depths and longitudinal reinforcement ratios were tested in shear. The results of this study revealed a lower shear resistance for SCC than VC. This finding was also confirmed by the study of Bugueňo et al [3] in which not only beams were tested but also smaller push-off specimens. Experiments on pushoff specimens conducted by Desnerck et al [4]-[5] showed on the other hand a slightly higher shear resistance for SCC than VC. Schiessl et al [6] developed a specialised test, to isolate the aggregate interlock from the other occurring mechanisms. The results of this study indicated a lower aggregate interlock for SCC. In literature, different models can be found to predict the shear resistance of concrete sections [7]. It is however still not clear whether these equations can be adopted with sufficient safety for SCC. Eurocode 2 (EC2) being the basis for the calculation of concrete structures in Europe, within this paper the EC2 expression (cfr. Equation 1) for the shear resistance of concrete sections without shear reinforcement will be closely analysed for SCC. V 1 3 C k f k b d v k b d Rd, c Rd, c 100 l ck 1 cp w min 1 cp w (1) Where: is the partial factor for concrete (= 1.5) f ck is the characteristic compressive strength of the concrete [MPa] 200 k d d is the effective depth of a cross-section [mm] ρ l is the reinforcement ratio for the tensile longitudinal reinforcement b w is the smallest width of the cross-section in the tensile area [mm] σ cp is the compressive stress in the concrete from axial load or prestressing [MPa] k 1 is a coefficient (= 0.15) The Eurocode 2 equation, valid for traditional vibrated concrete, expresses the shear resistance of the concrete section without shear reinforcement in function of the compressive strength of the concrete, the longitudinal tensile reinforcement ratio, the geometry of the section and the potential presence of an axial load on the section (e.g. prestressed concrete). The aggregate interlock mechanism is not directly included in the shear model utilised by EC2. As a result a SCC section will theoretically exhibit a similar resistance to shear loading than a VC section if a concrete of similar compressive strength is utilised. 2

3 EXPERIMENTAL WORK In order to evaluate the shear resistance of self-compacting concrete (SCC) a test program was initiated at the Belgian Building Research Institute (BBRI). Full scale beam experiments were conducted for SCC and VC. The results and crack patterns were analyzed and compared. The ability of Eurocode 2 (EC2) to safely predict the shear behavior was also evaluated for both concrete types. Materials and mix proportions In this study powder type SCC mixtures with limestone filler were investigated. Two SCC mixtures were tested in the scope of this paper. The mix proportions of the two selected SCC mixtures are given in Table 1. These two SCC mixtures were chosen due to their (1) distinctive higher paste volume (35 to 40 vol%) and (2) distinctive lower coarse aggregate content (25 to 28 vol%) in comparison to VC. Additionally, the two SCC mixtures differ from each other in: (1) compressive strength (2) W/C ratio (3) powder content and (4) fresh concrete properties. For both mixtures a river sand 0/4 and rounded aggregates 4/14 were used. Table 1 Mix proportions and characteristics of the SCC mixtures SCC1 SCC2 Cement CEM I 42.5R HES [kg/m³] Limestone filler [kg/m³] Sand 0/4 [kg/m³] Aggregate 4/14 [kg/m³] W/C Average Slump-flow ± stdev [mm] ( * ) 600 ± ± 50 Average cube compressive strength [MPa] ( * ) values rounded to 10mm For comparative reasons in addition to the above mentioned two SCC mixtures also a VC mixture was tested with more or less the same compressive strength as the SCC 1 mixture. The VC mixture was a ready mixed concrete delivered at the BBRI laboratory with following concrete specifications: C30/37, d max 16mm and S3. (The concrete was conform the European Standard EN 206-1) Test scheme and specimens Four point bending experiments were conducted on beams with a span of 2000mm and a section of 160mm x 250mm (width x depth). Primarily, no shear reinforcement was present in the section allowing as such to evaluate the V Rd,c parameter of the Eurocode 2 for SCC. The test scheme and specimen identification can be found in Table 2. The research parameters included in this study are: (1) the concrete type (SCC versus VC), (2) compressive strength class (SCC1 versus SCC2) and (3) amount of longitudinal reinforcement ( l ) (B1 versus B2). Since Eurocode 2 also stipulates that a minimum shear reinforcement ( w,min ) should be present in the beam section, this parameter was also added to the test scheme (B2 versus B3). (the minimum shear reinforcement did not include placement rules for the stirrups of Eurocode 2) For the traditional vibrated concrete additionally the influence of the effective depth of the section (d) (B1 versus B4) was investigated. The beams were designed in such a manner that shear failure would prevail, avoiding bending failure an d/or anchorage failure. An optimal anchorage of the tensile longitudinal reinforcement was also provided to correctly activate the reinforcement within the shear failure mechanism. 3

4 Table 2 Test scheme Configuration B1 B2 B3 B4 Width [mm] Height [mm] Longitudinal reinforcement Longitudinal reinforcement ratio [%] Minimal shear reinforcement No No yes No Figure 2 shows the typical details and reinforcement arrangement for the tested beams. For beam type B2 and B3 at mid-span 4 longitudinal tensile reinforcement bars with a diameter of 12mm are present to achieve sufficient bending resistance (see Figure 2). However, only 2 of these rebars (the 2 lowest) are present (and sufficiently anchored) at the supports, bridge the shear crack and thus contribute to the shear resistance of the beams. Table 3 Tested beam specimens B1 B2 B3 B4 SCC SCC VC A total of 27 beams were tested in the scope of this study (see Table 3). As mentioned previously four point bending experiments were conducted with a loading rate of 10kN/min (see Figure 1). The shear span to depth ratio a/d a being the distance from the point load to the support and d the effective depth of the beam adopted during tests was kept constant at 3. Three was chosen in order to prevent direct transfer of the load (or a part of the load) to the support. To account for the occurrence of this phenomenon, Eurocode 2 actually prescribes that a reduction factor should be applied to the shear load for values of a/d 2.5. Figure 1 The test setup 4

5 a/d=3.0 a F F LVDT 1 LVDT 5 a (a) Stirrup 8 LVDT 2 LVDT 3 LVDT mm 2300 mm a/d=3.0 a F LVDT 1 Stirrup 8 S = 400 mm (SCC 2) LVDT 5 S = 500 mm (SCC 1 and VC) F a (b) LVDT 2 LVDT 3 LVDT mm 2300 mm 160 Ø 8 (c) 250 Ø Ø 20 Ø 12 Ø 12 Ø 20 B1 B2 B3 B4 Figure 2 Details and arrangement of the reinforcement (all dimensions are in mm) (a) For beams of series B1, B2 and B4 (b) For beams of series B3 (c) Cross-section of the different beam setups at mid-span 5

6 RESULTS AND DISCUSSION The results for mixtures SCC1, SCC2 and VC can be respectively found in Table 4, 5 and 6. In each table the average cube compressive strength (side 150mm), experimental ultimate shear failure load (F), shear resistance (V R,cm ) calculated by EC2 with average material strength and without safety factors and the design shear resistance conform Eurocode 2 (V Rd,c ) are listed. In the last 4 columns of the tables the obtained experimental results are also compared with the Eurocode 2 predictions. Table 4 Summary of the test results for SCC mixture 1 f cm,cube F F average V R,cm V Rd,c F/V R,cm F/V Rd,c F average /V R,cm F average /V Rd,c [MPa] [kn] [kn] [kn] [kn] SCC1-B SCC1-B SCC1-B SCC1-B SCC1-B SCC1-B Table 5 Summary of the test results for SCC mixture 2 f cm,cube F F average V R,cm V Rd,c F/V R,cm F/V Rd,c F average /V R,cm F average /V Rd,c [MPa] [kn] [kn] [kn] [kn] SCC2-B SCC2-B SCC2-B SCC2-B SCC2-B SCC2-B SCC2-B SCC2-B SCC2-B Table 6 Summary of the testresults for the VC f cm,cube F [MPa] [kn] F average [kn] V R,cm [kn] V Rd,c [kn] F/V R,cm F/V Rd,c VC-B VC-B VC-B VC-B VC-B VC-B VC-B VC-B VC-B VC-B VC-B VC-B F average /V R,cm F average /V Rd,c

7 A very good prediction can be found when comparing the experimentally obtained results for the SCC mixtures with V R,cm. All results are within a range of ±10% of V Rd,cm. All experimental results for the SCC mixtures are well above the design shear resistance of Eurocode 2 (V Rd,c ), the safety is 43 to 79%. When comparing the results for VC almost all results lay well below V Rd,cm. With ratios F/V R,cm in the range , one cannot state that a good prediction of the experiments is obtained. All experimental results for the VC mixture are well above the design shear resistance of Eurocode 2 (V Rd,c ). The safety is however smaller than for the SCC mixtures with a safety of 22 to 67%. However, for specimens VC-B1-2 and VC-B1-3 cracking was also observed at the support (see also Figure 3), this might indicate the occurrence or onset of loss of anchorage of the tension reinforcing bars. As a result, the actual shear failure load for specimens VC-B1-2 and VC-B1-3 might be a little higher than the values mentioned in Table 6. This however does not change the overall conclusions stated in this paragraph. No cracking at the support was observed for any of the SCC1 or SCC2 specimens. Since the shear reinforcement within beam setup B3 represents the minimum shear reinforcement required by EC2, within all predictions for beam setup B3 the contribution of the stirrups was omitted. When comparing the experimentally obtained results of B2 with B3, for all concrete types, no significant difference can be seen. The reason for this being that, in this specific configuration, the crack develops in such a manner that it does not encounter a stirrup, as a result the stirrups do not contribute to the overall shear resistance of these beams. For beams VC-B3-1 and VC-B3-2 the a/d was increased to see if this would activate the stirrups, this measure did however not influence the developed shear crack, and thus did not lead to an activation of the stirrups. VC-B1-1 SCC1-B1-1 VC-B1-2 SCC1-B1-2 VC-B1-3 SCC1-B2-1 SCC1-B2-2 VC-B2-1 VC-B2-2 VC-B2-3 VC-B3-1 SCC1-B3-1 VC-B3-2 SCC1-B3-2 VC-B3-3 Figure 3 Observed crack patterns for beam setups B1, B2 and B3 of the SCC1 and VC mixtures A similar crack pattern was observed when comparing the results of the different beam setups for both concrete types (see Figure 3). The crack spacing at mid-span, in the area of constant bending moment, was also comparable for both concrete types. When closely analysing the developed shear crack it can be seen that the crack is characterised by a tri-linear evolution (see Figure 4). The slope of the crack is less important in the vicinity of the support and the point load. Table 7 summarizes the measured angles of the crack for the specimens of concrete type SCC1 and VC. An analysis of these results shows a slightly lower angle at the beginning (Zone 1 area closest to 7

8 the support) and end (Zone 3 area closest to the support) of the crack for the VC beams. Overall it can however be concluded that a very similar cracking behaviour (shear mechanism) can be observed. Zone 3 Zone 2 Zone 1 Figure 4 Tri-linear evolution of the shear crack. Table 7 Summary of the measured angles of the shear cracks Average Zone 1 Zone 2 Zone 3 Zone 1 Zone 2 Zone 3 SCC1-B SCC1-B SCC1-B SCC1-B SCC1-B SCC1-B VC-B VC-B VC-B VC-B VC-B VC-B VC-B VC-B VC-B Figure 5 gives an overview of the load at which the first bending crack was initiated and the failure shear load for mixtures SCC1 and VC, with more or less the same compressive strength. The shear mechanism was of a very brittle nature, the occurrence of the shear crack was thus instantaneously followed by the failure of the beam. These results show that for each beam configuration the first bending crack occurs at more or less the same load, whatever the concrete type adopted (SCC1 or VC). For the shear failure load for beam setup B1 however a clear difference can be observed between the mixtures, with a much higher shear resistance recorded for the SCC1 than the VC mixture. As mentioned previously, specimens VC-B1-2 and VC-B1-3 showed evidence of possible anchorage loss, which could lead to a slightly higher shear failure load. The difference between VC and SCC1 for the B1 setup is however substantial and can not solely be allocated to anchorage loss alone. 8

9 Force [kn] B1 B2 B3 Beam type SCC1 - shear failure VC - shear failure SCC1-1st Bending crack VC - 1st Bending crack Figure 5 Observed loads at initiation of bending cracking and shear failure for concrete types SCC1 and VC. CONCLUSIONS Since there still remain some concerns with regard to the shear resistance of self compacting concrete, this characteristic was investigated in more details. Full scale testing was undertaken on an extensive set of beam elements. The research parameters included in this study: (1) the concrete type (SCC versus VC), (2) compressive strength class (SCC1 versus SCC2) and (3) amount of longitudinal reinforcement ( l ) (B1 versus B2). The results of this study seem to indicate that the shear resistance of SCC is similar to slightly better than that observed for traditional vibrated concrete (VC). Eurocode 2 expression for the shear resistance of the concrete without shear reinforcement can still be adopted with sufficient safety. Due to the very brittle nature of the shear failure, the authors however still recommend the use of a minimal shear reinforcement for SCC, as mentioned in Eurocode 2, when the foreseen design shear load is lower than the shear resistance of the concrete section. The crack pattern and development (load at first bending crack, crack spacing at mid-span and shear crack angle) observed for the SCC and VC mixes studied within this paper were also very similar. ACKNOWLEDGEMENTS Funding by the Belgian Standardization Bureau (NBN) is gratefully acknowledged. The experimental work of Ann Stroeykens and Alexander Van Sanden of Lessius University College (Campus De Nayer) is also gratefully acknowledged. REFERENCES 1. Taylor, H.P.J., The fundamental behaviour of reinforced concrete beams in bending and shear, ACI SP-42, 1974, pp Hassan, A.A.A.; Hossain, K.M.A.; Lachemi, M., Behavior of full-scale self-consolidating concrete beams in shear, Cement and Concrete Composites, V 30, 2008, pp Burgueňo, R.; Bendert, D.A.; Effect of self-consolidating concrete mix design on the shear behaviour of RC beams, proceedings SCC2005: The second North American conference on the Design and Use of self-consolidating 9

10 Concrete (SCC) and the fourth International RILEM Symposium on self-consolidating Concrete, V 2, 2005, pp Desnerck, P.; De Schutter, G.; Taerwe, L.; Shear friction of reinforced concrete beams in self-consolidating concrete, Proceedings SCC2009: Second International Symposium on Design, Performance and Use of Self- Consolidating Concrete - China, 2009, pp Desnerck, P.; Compressive, Bond and Shear Behaviour of Powder-Type Self-Compacting Concrete, PhD Thesis University Ghent, 2011, p Schiessl, A.; Zilch, K.; The effects of the modified composition of SCC on shear and bond behavior, Proceedings: Second International Symposium on Self-Compacting Concrete - Japan, 2001, pp Shear and punching shear in RC and FRC elements, fib CEB-FIB, Technical Report, Bulletin 57, Workshop October 2010, Salò (Italy), 2010, p