Evaluation of Shear Strength of High Strength Concrete Beams

Abstract

In this thesis, the shear properties of High Strength Reinforced Concrete (HSRC) beams have been investigated on the basis of available research data and experimental work at Structural Laboratories of University of Engineering and Technology Taxila-Pakistan. The shear capacity of High Strength Reinforced Concrete (HSRC) beams is relatively less investigated in the contemporary research, as most of the research data available is based on the results from normal strength reinforced concrete with compressive strength of 40MPa or less. There is a general consensus amongst the researchers in the field of Structural Engineering and Concrete Technology that the shear strength of HSRC beams, unlike the Normal Strength Reinforced Concrete (NSRC) does not increase, in the same proportion as the increase in the compressive strength of concrete, due to brittle behaviour of the High Strength Concrete. Hence the current empirical equations proposed by most of the building and bridges codes for shear strength of HSRC beams are less conservative as compared to the Normal Strength Reinforced Concrete (NSRC) beams. This major observation by the researchers is the main focus of this research. An extensive literature review of the shear properties of Normal Strength Reinforced Concrete (NSRC) beams and High Strength Reinforced Concrete (HSRC) beams was undertaken. Additionally the shear strength of disturbed region (D-Region) was also studied. In disturbed region the ordinary beams theory based on Bernoulli’s theorem is not applicable. In the literature review of disturbed regions special emphasis was laid over Strut and Tie Model (STM), which is an emerging analysis and design tool in the current research in reinforced concrete. The literature review was followed by the experimental work, which comprised of 70 high strength reinforced concrete beams and 9 two ways high strength concrete cobles. Beams were cast in two sets of 35 beams each, one set without web reinforcement and other with web reinforcement. For each set of 35 beams 3five values of longitudinal reinforcement and seven values of shear span to depth ratio were selected to mainly study the behaviour of slender beams, where typical shear failure can be anticipated. These beams were tested under monotonic load at the mid span to examine the contribution of various parameters like longitudinal steel, shear span to depth ratio, and web reinforcement, on the shear capacity of HSRC beams. It has been observed that the shear strength of beams has been increased with the increase in longitudinal steel and shear reinforcement but it has reduced with the increase in the shear span to depth ratio. The beams with low longitudinal steel ratio and no web reinforcement failed mainly due to shear flexure cracks. However the beams with longitudinal steel ratio of 1% and more failed mainly due to beam action in shear tension failure. The beams with small shear span to depth ratio and large values of longitudinal steel ratio however failed due to shear compression failure. The shear failure of HSC beams with large values of longitudinal steel and shear span to depth ratio was however more sudden and brittle, giving no sufficient warning before failure, which has been observed as serious phenomena in the shear failure of HSC beams. The addition of web reinforcement increased the shear strength of all beams tested. The failure mode was also affected. The obvious contribution of the minimum web reinforcement was avoiding the sudden failure of the HSC beams. These test results were also compared with the equations of some international building and bridges codes and methods for shear strength of HSRC beams. It has been noticed that these equations do not provide equal level of safety in the shear design of HSRC beams. Some of the codes are over conservative, while few others are less conservative for the shear design of HSRC beams. Comparison of the observed shear strength of tested HSRC beams with the results of the codes equations used, reveal that most of these equations are less conservative for shear design of HSRC beams at lower values of longitudinal steel for both cases of beams with and without web reinforcement, particularly for 4longitudinal steel ratio less than1%. Hence additional care may be required for shear design of HSRC beams at large values of shear span to depth ratios. To analyze the behaviour of typical disturbed region in concrete structures, the basic rationale of Strut and Tie Model (STM) was used for the analysis and design of two way corbels. These corbels were tested under monotonic loads applied at the overhanging portion of the corbels. The actual shear capacities of these corbels were compared with the theoretical shear capacities of the corbels worked out with the STM. The actual and theoretical values of the shear were falling close to each other. Their comparison reveals that STM can be further tested as more simple and reliable tool for analysis and design of disturbed region (D-Region) in concrete structures, through more experimental research. Further research work on shear properties of HSRC beams with higher values of compressive strength of concrete in the beam region and more experimental research on the disturbed region including pile caps, deep beams, dapped ended beams and corbels has been recommended at Engineering University-Taxila Pakistan.