Behavior of unbonded prestressed continuous concrete slabs with the middle and edge span subjected to fire in sequence

This paper presents the results from four three-span unbonded prestressed continuous concrete slabs that were 12.6 m×1.5 m×0.12 m in size. The tests were designed to show the effect of the negative reinforcement length in the zone of negative moment and the influence of the sequence of the middle span and the edge span exposure to fire. Based on the standard temperature–time curve (ISO 834) used in the test, the processes involving water and cracks in the slabs were described. After the analysis, the deformation, the mechanism of the failure mode, the regularity of the prestressing and the force redistribution of the support were investigated. The study demonstrated that the failure mode of a slab under fire is mainly due to tensile fracture of the concrete and tendons. The sequence of spans under fire and the length of negative reinforcement have important effect on the deformation and the failure mode. Different lengths of negative reinforcement lead to destructive hinge presence at the different position. The presence of cracks ① and ② is attributed to the thermal bowing action and stiffness redistribution. The crack ④ extending to the top surface along the tendons is the effect of tensile splitting due to the lower temperature in the tendon. The reinforcment perpendicular to the trajectory of the tendon should be laid out and the minimum steel ratio 0.2% is recommended. The support force redistribution is attributed to the structural system change, which is caused by the fracture of the concrete and tendon under fire.

► Four full scale slabs are used to carry out the experimental study.
► Different spans under fire and the length of negative reinforcement are considered.
► Visual observations such as change of water and failure mode are presented.
► Structural behaviors are given in detail.
► The top layer reinforcement should be laid out in the slab design is suggested.