Investigating the Behaviour and Strength of Unbonded Pre-tensioned RC Slabs Subject to Flexural Loads

Unbonded pre-tensioned systems, in which tendons are greased and wrapped in plastic sheathing, have several benefits in comparison to bonded systems. However, analytical evaluation of their capacity is taxing due to the iterative compatibility conditions necessary to determine the system’s behavior using the empirical equations available in the literature. To investigate the behaviour and strength of unbonded pre-tensioned RC slabs subject to flexural loads, four simply-supported post-tensioned slabs with unbonded tendons were tested in flexure. Experimental failure loads and tendon strains were compared to the ACI 318-19 provisions. While the provisions for unbonded specimens were found to be accurate, the strain in the unbonded tendons exceeded the yield stress in all specimens, suggesting that ACI 318-19’s stress limitations on unbonded post-tensioned concrete are unwarranted. The unbonded system also showed better crack control at the failure stage when non-prestressing steel reinforcement is used when compared to system with no non-prestressing steel.