Experimental study and analytical modelling of the load-bearing behaviour of reinforced UHPFRC-NC composite beams considering shrinkage

Ultra-high performance fibre reinforced concrete (UHPFRC) is a type of steel fibre-reinforced cementitious materials with high compressive and tensile strengths. For reasons of crack width limitation and load-bearing capacity, it can be advantageous to use UHPFRC in the tensile area of a reinforced structure. The high autogenous shrinkage that comes with UHPFRC can be large enough to induce substantial reinforcement pre-compression and/or cracking of the concrete, thus altering the deformation response. To study the effect of UHPFRC shrinkage on the structural behaviour of composite members, five beams made of a reinforced UHPFRC bottom layer with normal strength concrete (NC) on the top were tested under four-point bending. By varying the layer heights and the reinforcement area, the effects of the UHPFRC/NC ratio and reinforcement ratio on the flexural behaviour were investigated. Before casting, distributed fibre optical sensors (DFOS) were applied on the top side of the tensile reinforcement to measure the deformation of the steel rebars due to shrinkage restrained by the reinforcement itself and the different shrinkage behaviour of the two layers. After 28 days of curing, a reinforcement pre-compressive strain caused by the shrinkage of UHPFRC and NC up to 441 με was measured. During loading, measurements from four load cells, five displacement transducers and five strain gauges at the bottom of each tensile reinforcement were recorded, besides the continuous DFOS measurements. The yield load of the tensile reinforcement was determined by the DFOS strain measurements in which the shrinkage strain was considered. The test results revealed an increasing yield load and ultimate capacity of the composite beams with a higher replacement ratio of UHPFRC and reinforcement ratio. Two typical failure modes, namely, compression failure and tension failure, were observed, with compression failure being prevailing at higher UHPFRC replacement ratios and greater reinforcement ratios. Based on constitutive models for the UHPFRC and NC under compression and tension, an analytical method is proposed to calculate the load-deflection response of composite beams considering the reinforcement pre-strain due to shrinkage, from which the flexural capacity can be determined. The analytically determined response is in good agreement with the test data. • Determination of reinforcement pre-compression in composite beams due to shrinkage. • Experimental study on the load-bearing behavior of UHPFRC-NC composite beams. • Analytical model for composite beams considering bar pre-strain from shrinkage.