The behaviour of high-strength concrete beams

© 1997 Dr. Raghu S. PendyalaThis thesis reports on experimental and analytical studies on high-strength concrete (50 MPa <f' c < 100 MPa) beams. The literature review consistently shows that the extrapolation of the existing design procedures meant for normal strength concrete is not appropriate for high-strength concrete. In particular, this thesis reports on: • Ductility of high-strength concrete beams • A full range stress-strain model for high-strength concrete beams • An equivalent rectangular stress block for high-strength concrete • An amended formula for the shear strength of high-strength concrete Results from experimental and analytical studies on ductility of beams reveal that high-strength concrete beams have significantly greater curvature ductility than normal strength concrete beams. This study also reveals that significantly more reinforcement can be used with high-strength concrete beams as compared to normal strength beams, and still maintain adequate ductility. The full range stress-strain model for high-strength concrete (referred to in this thesis as the Modified Scott Model) has been so derived that the resulting moment-curvature envelopes closely match the experimentally derived moment-curvature relationships of the beams tested in this project. The Modified Scott Model was derived from an existing stress-strain model by suitably modifying the parameters which define the descending and the residual portions of the stress-strain relationship. Various studies show that existing rectangular stress blocks are unconservative for high-strength concrete members. Especially, the unconservativeness is magnified in the design of high-strength concrete columns. The proposed rectangular stress block is a two parameter model, one parameter to define the intensity of stress and the other parameter to define the depth of the stress block. Though this thesis primarily looks at high-strength concrete beams, a limited validation with experimental data from literature on columns has been done. It was found from the experimental and analytical study reported in this thesis, that the shear strength of concrete beams diminish with increasing strength. One of the main findings was the shear resistance of concrete is minimum in the concrete strength range of 50 MPa - 70 MPa. Hence the shear provisions for concrete contained in AS3600 have been modified to reflect this deficiency. The proposed shear design equations have been validated with data from literature. This thesis also critically reviews the various recommendations for minimum shear reinforcement for high-strength concrete beams in literature. Finally, recommendations for further research have been made