Experimental Investigation On Shear Strength Of Large Size Reinforced Concrete Beams

The safety of large size reinforced concrete beams and thick slabs have a significant impact on the security of the entire structure. Recent investigations indicated that the ACI shear design method and Chinese code for design of concrete structure were unconservative when applied to large reinforced concrete members without web reinforcement. Considering the fact that there is not sufficient experimental data of large size beams among the test data on which the ACI method was based, it is necessary to study the shear behavior of large size reinforced concrete beams. The research is supported by the Program of the National Natural Science Foundation of China (No.50778034) and the Key Program of the National Natural Science Foundation of China (No.50838001). Based on the on study on the large size reinforced concrete beams under concentrated loading, the following investigations on shear properties were further carried out:(1) The depth effect on reinforced concrete beams without web reinforcement was studied through the shear test on specimens with different depths. The experimental results revealed that the cracking shear stress and ultimate shear stress decreasing as the effective depth increasing. Compared to specimen of the depth of500mm, when the effective depth increased by112%and158%, the cracking shear stress decreases by31%and34%and the ultimate shear strength is reduced by26%and41%.(2) The effect of longitudinal reinforcement ratio on the shear capacity of reinforced concrete beams without web reinforcement under concentrated loading was studied. The results show that, the changing of longitudinal reinforcement ratio has a significant influence on shear strength in the case of the lower longitudinal reinforcement ratio. Compared to specimen with longitudinal reinforcement ratio of0.67%of the specimen, the cracking shear stress for specimen with longitudinal reinforcement ratio of1.10%increased by26%, the ultimate shear strength increased by73%.(3) The effect of longitudinally-distributed web reinforcement of large beams on the shear strength was described. Compared to beams with same depth without web reinforcement, specimens with distributed reinforcement achieved higher shear stress at failure. Rate of increment was observed to decrease with increasing depth. The increment in ultimate shear strength between the600mm and1000mm overall depth beams was51%and14%. (4) The effect of the minimum stirrup ratio on the shear strength was analyzed. That can significantly improve the ultimate shear strength of the specimen. For beam with depth600mm, the increment in ultimate shear strength was75%. For beam with depth1000mm, the increment in cracking shear stress and ultimate shear strength was17%and76%, respectively.(5) The effect of depth on cracking spacing of inclined cracks was studied. The studies show that average crack spacing at level of longitudinal steel is independence to beam depth but related to concrete cover thickness. The average crack spacing at mid-depth of specimen is related to beam depth, showing size effect to extent.(6) The crack width and cracking speed were investigated through the test, and the varied parameters including the position, beam depth, longitudinal reinforcement ratio and web reinforcement. At the same shear stress, the crack width and cracking speed in0.5h0from the top of specimen were greater than that in crack tip and level of longitudinal steel. The crack width and cracking speed increase as the depth increase and longitudinal reinforcement ratio decrease. The configuration of web reinforcement can restricted to limit the width and cracking speed of diagonal cracks effectively.(7) Shear provided by the compression zone of concrete in loading process was discussed. The results show that the ratio of shear transferred in the compression zone to total shear decrease as the depth and longitudinal reinforcement ratio decrease.(8) Shear test database of reinforced concrete beams without web reinforcement was established according to selection criteria. Based on the database, the empirical equations for predicting the ultimate shear strength of beams without web reinforcement was proposed through the regression analysis, taking into account the concrete strength, shear span ratio, beam depth and longitudinal reinforcement ratio, and the predicted value based on such equations was in good agreement with the tested value.(9) Due to section mechanical analysis, a formula was developed to predict the shear capacity of slender beams with a/h0>2.5without web reinforcement. The shear design formulae from Chinese codes GB50010-2010and American code ACI318-08, Canadian code CSA A23.3-04and the proposed equation were quantitatively evaluated and compared. It shows that both the GB50010method and the ACI318method are dangerously unconservative when applied to beams with depth greater than600mm or to beams with longitudinal reinforcement ratio lower than1%. The CSA method and the proposed equation had a better prediction with a small variation coefficient.(10) The safety of shear design method in Chinese code GB50010-2010for reinforced concrete members without web reinforcement was explored based on the analysis of the thick slab in a bridge that failed in shear, exploring the influence of size effect factor, minimum stirrup ratio and longitudinally-distributed web reinforcement on the shear capacity. It is shown that the code expressions into which a size effect factor has been incorporated are unable to characterize the influence of depth on shear strength of members. Minimum stirrups required by code provisions are found to be effective for large members to avoid brittle shear failure, while distributed longitudinal reinforcement is less effective for member with depth over1000mm without web reinforcement.