Mechanical and bond-slip behaviour of ultra-high performance concrete exposed to high temperature

The residual mechanical behaviour, including the steel-concrete bond, was investigated in this research project for Ultra-high performance concrete (UHPC) with compressive strength of 101–134 MPa and maximum aggregate size below 0.04 mm. Specimens were heated to 200, 400, 600, 800 and 1000°C, reinforced with steel, polypropylene (PP), polyvinyl-alcohol (PVA) or hybrid fibres, for a total of six different mixes. Compressive, 4-point and pull-out tests were carried out to evaluate the compressive strength (on cylinders), the tensile strength (in bending) and bond strength, both at ordinary temperature (25°C) and after heating to high temperature and cooling down to room temperature. The test results show that (a) steel fibres guarantee a higher residual strength but fail to prevent UHPC’s thermal spalling; (b) PP fibres are more effective than PVA fibres in controlling thermal spalling; and (c) hybrid fibres (PP + steel fibres) are the most effective in terms of spalling control and residual strength. In general, other hybrid fibres are shown to be unable to control concrete spalling at temperatures around 400°C. As for the tensile strength in bending, long steel fibres have an edge over mixed steel fibres (with different lengths). Contrary to the compressive strength (that starts decreasing at 600°C), the tensile strength continues to increase above 600°C and starts decreasing at 800°C. As for bond performance, in the thermal range 25–1000°C, pullout tests were performed by using 12-mm deformed bars and three different values for the bonded length/bar diameter ratio (L/D = 2, 4 and 6). On the whole, the decay of bond strength with the temperature is similar to that of the compressive strength. Long steel fibres show higher bond strength, whereas mixed steel fibres offer advantages in other stages such as higher initial stiffness. Based on test data, empirical equations are proposed for the bond-slip law in a high-temperature environment.