Structural fire performance of bonded post-tensioned concrete bridges

Abstract

Post-tensioned concrete bridges are widely adopted in practical engineering. However, the fire hazards of such bridges are often ignored, although they may cause degradation of material properties, debonding between strands and surrounding material, concrete spalling, etc. possibly leading to potential damage or even collapse. Performance-based structural analysis is useful in predicting their structural-fire behaviour. The bonding of prestressing strands is essential to the strength of bonded post-tensioned concrete members. Failure to consider the bond degradation may overestimate the structural strength during fire. To investigate the bond behaviour of grouted strand at ambient temperature, elevated temperatures and after cooling down from elevated temperatures, pull-out tests were carried out taking into account the influence of temperature, eccentricity of strands, number of prestressing strands in a duct, cooling method and additional reinforcement. An analytical model was proposed to predict the bond behaviour of strand in grout at elevated temperatures. To investigate the response of bonded post-tensioned concrete bridge beams under hydrocarbon fire, furnace fire tests accompanied by numerical simulation were carried out. Two common types of concrete bridge beam, i.e. box-section and tee-section, were considered. Other factors considered included load level and fire exposure time. The surface temperature, strand temperature as well as mid-span deflection of specimens were measured throughout the fire tests. Residual load-carrying capacity was also tested after specimens cooled down to ambient temperature. The behaviour of specimens was modelled with a three-step simulating approach comprising fire simulation using the software FDS and sequentially coupled thermo-mechanical analysis with the software ABAQUS. The construction process of the specimens was modelled to capture its actual stress state when exposed to fire. The bond behaviour between prestressing strand and surrounding grout was considered. The numerical models developed could predict the fire and structural behaviour of post-tensioned concrete bridge beams covering the pre-fire, fire and post-fire periods. With the validated numerical approach, the potential hazards of tanker truck fire on real-life segmental bonded post-tensioned concrete bridges were studied. A localized fire model was developed to predict the distribution of adiabatic surface temperature of a bridge under different fire scenarios. Fire models with various parameters related to the bridge dimensions and the fire intensity were built up in FDS so that the profile of surface temperature could be obtained to correlate with the parameters of localized fire model. The surface temperature generated from the localized fire model could then be applied as thermal boundary condition to a three-dimensional finite element model of the bridge. The response of the bridge was then studied through a sequentially coupled process of thermal and mechanical analyses. Simulation results showed that the global structural responses such as deflection and post-fire load-carrying capacity were seldom influenced by the tanker truck fire. However, localized damage deep in concrete at the location of tendons could be induced. Fire induced tension or separation between adjacent segments would do harm to the durability of the tendons and post-fire retrofitting should be carried out in such cases. (490 words)