Behavior of Reinforced Concrete Deep Beams with Large Openings Strengthened by External Post ? Tensioning Strands

In this research, nine simply supported reinforced concrete deep beams with large web openings strengthened by using external post tensioning strands have been cast and tested up to failure under one-point load. These beams were divided into three groups according to strengthening schemes by using external strands. Each of these groups consisted of three beams having different opening ratios (0.4, 0.6 and 0.8). Where the opening ratio is defined as the opening length divided by the overall depth of the section. All beams having the same dimensions of 150 mm width, 400 mm overall depth and 1600 mm total length with clear span equal to 1280 mm. All deep beams have been designed with same interior reinforcement (same design). The main variables in the present study were the external strands configurations (strengthening schemes) and the openings ratio. For all deep beams the height of opening was kept constant of 160 mm (0.4H). Since the pre-stressing force in strands was not a variable parameter in this study, so that the jacking stress was intended to be constant of 600 MPa for all strengthening beams. The results show that increasing openings ratio caused a decreasing in first cracking and ultimate loads and increasing in mid span deflection for all beams. While, for same openings ratio, the strengthening using horizontal post tensioning strands scheme was more effective than using vertical strengthening scheme in increasing the first cracking and ultimate load capacities and reducing the deflection. The maximum percentages increasing in first cracking and ultimate loads were (57.1 and 53.3) % respectively for opening ratio equal to 0.8 when using horizontal strengthening scheme. Also, the results show insignificant effect of increasing openings ratio on strands stress for both strengthening schemes. In other hand, there is a significant decrease in bottom (tension) reinforcement when using horizontal strengthening scheme as compared with beams without strengthening at same load level and for same openings ratio. The maximum percentage decreasing in main steel bar stress was about 88 %.