Behavior of Hybrid Reinforced Concrete Deep Beams under Repeated Loading

Abstract

This research is devoted to investigate the experimental and theoretical behavior of hybrid deep beams under monotonic and repeated two points loading. The dimensions and the flexural reinforcement of the deep beams were kept constant.  In this work, the idea of hybrid beam is completely different. Two types of concrete were used but not in cross section. The first type which is fiberous concrete (FC) was used in casting beam sides (shear spans), while the second type was conventional concrete (CC) which was used in the middle portion of beam (between two shear spans). This was done to strengthen the deep beam sides (shear spans) against cracking due to shear (diagonal strut) failure. The experimental program includes casting and testing of 12 deep beams, six of them are tested as a control beams under monotonic loading and the others were tested under repeated loading at level 70% of the ultimate load of their control beams. The variables include type of load, type of beam, amount of web reinforcement and amount of steel fiber. It was found when adding steel fiber to the shear spans by ratio 1% and 2% under monotonic loading system, the percentages increase in the ultimate load are 29.73% and 50.81%, respectively as compared with beam without steel fiber. Also, it was observed that when the entire deep beam cast with fiberous concrete, the percentages increase in the ultimate load are 5.21% as compared with hybrid beam with the same steel fiber ratio of 1%, and 36.49% as compared with deep beam cast with conventional concrete for all its portions. The increase in web reinforcement ratios from 0.0 to 0.003 and 0.004 under monotonic loading system leads to increase in the ultimate load are 34.08% and 42.46%, respectively. Comparison of experimental results was made with corresponding predicted values using the Strut and Tie procedure presented in Appendix A of ACI 318M-11Code and with other procedures mentioned in the literature.  It was found that the Strut and Tie procedure presented in Appendix A of ACI 318M-11Code give conservative results as compared with the experimental tested results.

Keywords: deep beam, hybrid, repeated loading, strengthening, shear spans.