Flexural behavior of post-tensioned normal-strength lightweight concrete one-way slabs

Twelve partially post-tensioned lightweight concrete (LWC) one-way slabs were tested under a symmetrical top one-point loading and a symmetrical top two-point loading. For each loading type, unbonded prestressing three-wire strands were arranged with a straight profile of constant eccentricity and a harped profile of variable eccentricity. The span-to-depth ratio also varied among 15, 35, and 55. The ultimate moment and the corresponding stress increase in unbonded tendons measured from the present LWC specimens were compared with those of post-tensioned normal-weight (NWC) concrete beams and ACI 318-11 predictions. Additionally, to generalize strain distribution-based equations to predict the stress increase in unbonded tendons at ultimate strength of flexural members, an equivalent plastic hinge length-to-tendon length ratio (to explain the reduced stress of unbonded tendons) and a coefficient (to straightforwardly obtain the neutral axis depth) were formulated from a total of 200 test data and a comprehensive parametric study, respectively. From comparisons between predictions and experimental results, the ACI 318-11 design equations for the stress increase in unbonded tendons were found to be fairly conservative in both the NWC and the LWC beams, though an unsafe condition is occasionally observed in those specimens with a reinforcing index between 0.27 and 0.35 under one-point top loading. Conversely, the predictions obtained from the currently proposed equations are in good agreement with test results.