Residual flexural behaviour comparison between composite and monolithic beams after fire exposure

Fourteen T-shaped composite beams and seven T-shaped monolithic beams were tested after fire exposure. To investigate their distinct residual flexural behaviour, the beams, which had different load ratios and thicknesses of precast slabs, were exposed to specific fire exposure times (60, 90, and 120 min) in a fire furnace and loaded to failure after natural cooling. The results showed that the residual flexural capacity of the composite beams was generally lower than that of monolithic beams. The fire exposure time was found to dominantly influence the residual flexural capacity of the composite beams. Increasing the thickness of the precast slabs can slightly enhance the re-sidual strength of the composite beams. However, the horizontal cracks at the interface between the precast and cast-in-place concrete slabs of the composite beams reduced the horizontal shear strength and further increased the horizontal slip along this interface. Accordingly, a finite element model of the composite beams was proposed. A three-dimensional spring element was introduced into the model to consider the effect of slip at the concrete-concrete interface. To predict the residual flexural capacity of the composite beams, based on the 500 degrees C-isotherm method, three modified equations suitable for the Chinese, American, and European codes were proposed. In comparing the test values, the modified equation applicable to the European code is recommended for subsequent predictions.