Ductility of Concrete Members Partially Prestressed with Unbonded and External Tendons

Abstract

Partial prestressing is basically a prestressed concrete approach in which prestressed reinforcement or prestressed and non prestressed reinforcement is used such that tension and cracking in concrete is allowed under service loads. Although there were studies on ductility in partial prestressing available in literature, it is necessary to take up further studies on ductility by applying different types of experiment data for better understanding. Curvature ductility is chosen for the present study and the same is evaluated using the data published in literature. Curvature ductility μ is defined as the ratio of curvature at ultimate ϕ_u to that of yield ϕ_y. Totally the data of 45 specimens were used for the study. Shape of the specimens consists of rectangular section and T-beam section. There is a strong opinion among researchers and practicing engineers that compressive reinforcing steel would enormously improve the ductility of concrete members. In view of this, data of specimens with and without compressive reinforcement was also studied. Parametric studies by taking into account of different parameters on ductility namely, reinforcement index, yield curvature, deflection curvature, M_ult/M_yld and Partial Prestressing Ratio (PPR) in partially prestressed concrete members of unbonded post tensioning tendons have been carried out. There are many criteria reported in literature regarding curvature at yield. However, criteria namely: (i) yielding of reinforcing steel; (ii) 0.75 M_n, where M_n corresponds to 0.003 strain; and (iii) intersection point of two lines of moment-curvature curve have been studied. It is observed that Reinforcement index is the influencing parameter in evaluating ductility for partially prestressed concrete beams with unbonded and external tendons. Partial Prestressing Ratio (PPR) is not an influencing parameter for evaluating ductility. There is a proportional decrease in M_ult/M_yld and a proportional increase in the yield moment and ultimate moment according to the ascending order of reinforcement index. Curvature ductility is linearly proportional to the deflection ductility, if the reinforcement index is in ascending order. Curvature ductility is the better index for evaluating ductility of unbonded partially prestressed concrete members rather than displacement ductility.