Bond model for prestressed strands in fire exposed concrete

For fire exposed concrete structures, no design rules exist for the detailed calculation of the bond stress of reinforcement. The effect of thermal stresses on the bond stress and the confining action of the surrounding concrete is currently neglected. This paper presents an elastic-plastic bond-slip model for the simulation of the anchorage behaviour of prestressed strands in concrete structures at both ambient and fire conditions. The model allows for a detailed simulation of the slip and prestress development during fire. The bond model is applied to interface elements between prestressed strands and the concrete structure in a 2D FE model. In this model the temperature effects on material properties of concrete and steel have been considered, according to Eurocode 2. The yield contour depends on the confinement action of the encasing concrete. An arbitrary relation has been set between the slip and the steel stress at one side and an artificial radial expansion of the strand at the other side. The response of the confining concrete against radial expansion of the strand is calculated in a second 2D FE model of the concrete cross section. This cross sectional model accounts for the effect of splitting cracks and also for temperature effects. The model is checked against tests on precast prestressed hollow core slabs at ambient and fire conditions.