Comparison of unbonded and bonded posttensioned concrete slabs under fire conditions

This paper compares the different structural response between unbonded and bonded post-tensioned concrete slabs under fire conditions. The results from 16 tests conducted at ambient and elevated temperatures on unbonded and bonded post-tensioned one-way spanning concrete slabs are presented. The ambient tests were continually loaded until failure, while the fire tests were subjected to an increasing heat under a static load equal to 50% of the capacity of the unbonded slabs in the cold condition. The tests investigated the effects of using different aggregates and different restraint conditions and for the bonded tests the effect of using either metallic or plastic ducts was also investigated. In all the unbonded and bonded fire tests, cracks directly inline and parallel to the tendons occurred due to thermal stresses at relatively low tendon temperatures, which were not observed in the ambient tests. Unbonded and bonded slabs with Thames gravel aggregates were shown to have a much higher deflection compared to slabs with limestone aggregates, with restrained slabs having a lower vertical deflection compared to the equivalent unrestrained slabs. It is shown that the use of plastic ducts in bonded slabs resulted in slightly higher tendon temperatures compared to the equivalent bonded slabs with metallic ducts due to the ease at which water migrated from the grout once the duct had melted. The data from the tests compared temperature distributions within the slab, vertical and horizontal displacements and strains in the tendons. Comparison of the ambient tests showed that the bonded slabs have higher ultimate strength due to strain compatibility and greater overall ductility compared to the equivalent unbonded slabs. The suite of fire tests has shown that the bonded post-tensioned concrete slabs are capable of achieving a designed target fire resistance of 90mins while the unbonded slabs had fire resistance generally lower than the expected 90mins. It is also shown that the fire resistance specified in Eurocode BS EN 1992-1-2 is generally conservative for bonded and unbonded post-tensioned one-way spanning concrete slabs, while the fire resistance given in the UK code BS 8110-2 is generally conservative for the bonded slabs and slightly unconservative for the unbonded slabs.