Strengthening of RC beams with Ultra High Performance Concrete

Abstract

This study investigates the structural performance of retrofitted RC-UHPC (Ultra-High Performance Concrete) composite members through measuring the effectiveness of strengthened RC beams with different regions and thickness of added UHPC. Due to its high compressive and tensile strength, thin UHPC over layers are applied to increase tensile strength in flexural strength in tension region with reinforcement and the compressive resistance in flexural compression region. For adhesion between existing concrete and UHPC, sandblasting is applied to give enough roughness. Two series of tests were performed under four-point loads. First test program consists of 12 specimens strengthened with change in thickness, regions, fiber volume and additional reinforcement except one control beam of rectangular section. For the second program, 14 ordinary reinforced concrete T-beams were prepared to develop shear failure modes. Because strengthened beams with UHPC are supposed to enhance both positive and negative moment capacity, jacketing methods applied to this experimental program have three different schemes: UHPC U shaped-jacketing, UHPC casting on the flange, and UHPC casting on the flange and Aramid FRP U shaped-wrapping. The experimental results show that strengthening beam with UHPC is effective to enhance strength and deflection. Especially, UHPC U shaped-jacketing method showed better performance than that of other methods by the improvement of strength, stiffness and ductility for both positive and negative moment zone. On the other hand, UHPC overlay method showed the additional treatment to improve adhesion performance between old concrete and UHPC. Strengthening effects are proportion to the strengthened thickness without shear failure. For additional reinforcement with UHPC, the increase in strength and ductility were shown. AFRP with UHPC and additional steel fiber in UHPC showed a little increase in strength. Wire mesh showed no contribution to performance. Based on the test result of this study, theoretical analysis determined by a cross-sectional analytical model based on plane section showed 15% error range about rectangular sectional beams, 21% error range for T-shaped sectional beams. In case of T-shaped RC beams showing debonding failure, average interfacial shear stress were calculated and compared with current design code and slant-shear test results. It showed that for positive moment, the highest shear stress occurred at overlaid UHPC panel