Flexural behavior of high-strength concrete beams prestressed with unbonded tendons

The objective of this investigation is to study the flexural behavior of high-strength concrete (HSC) beams prestressed with internally unbonded tendons. The research consists of both experimental and analytical studies, which emphasize the load carrying capacity, the stress increase in unbonded tendon (Δf_ps), the cracking behavior, and load versus deflection relationship.; The experimental program consists of testing nine simply-supported unbonded prestressed HSC beams in flexure, having a 5 in. x 9 in. rectangular cross section, a clear span length of 120 in, and a strength varying from 11 to 13 ksi. The beam specimens are divided into four groups. In each group, various parameters such as area of prestressing strand, effective stress, span-to-depth ratio, and area of non-prestressing steel are considered. Each beam is loaded with a two-point loading system. Forces in the prestressing tendons, strain in the non-prestressing steel, and in the concrete are measured. A comprehensive finite element model is developed to study the behavior of the unbonded tendons. The results from the experimental study are analyzed and compared to those from the finite element model. In addition, six different sources of experimental data on beams prestressed with unbonded tendons are also considered. Results from analytical model and experimental studies are compared with respect to the ultimate strength and the overall applied load versus midspan deflection behavior. After validating the model, a parametric study is performed to determine the effect of various parameters on the overall behavior of beams prestressed with unbonded tendons with emphasis on the ultimate load carrying capacity and the stress at ultimate in the tendons.; A summary of available prediction equations for f_ps suggested by various researchers as well as relevant design codes is presented. The evaluation of the predictions is achieved by comparing predicted and experimental results. Finally, a new prediction equation for f_ps at ultimate is developed by utilizing FEM and experimental results.; The experimental results demonstrates that the stress in prestressing strand at ultimate decreases when the amount of either prestressing steel or bonded non-prestressed steel increases. The span-to-depth ratio does not have a significant effect on the stress increase, Δf_ps . However, as the span-to-depth ratio increases (a range of 10–18.5 in this thesis), Δf_ps slightly decreases. The finite element model developed in this study can be used to predict the overall behavior of the member prestressed with unbonded tendons. The proposed equations developed in this thesis predict the ultimate stress in the unbonded tendon accurately considering three major parameters, c, L, and d_p, only.