Experimental and analytical research on the flexural behaviour of steel-ECC composite beams under negative bending moments

Due to the strain-hardening and multi-cracking properties, engineered cementitious composite (ECC) is a new solution to the cracking issue in the negative bending moment region of steel-concrete composite beams. This paper presents an experimental and analytical study on the flexural performance of composite steel-ECC beams subjected to negative bending moments as well as the tension stiffening behaviour of reinforced ECC (R/ECC) flange slabs. Three beams with different slab materials and reinforcement ratios were tested under a hogging moment. Experimental results demonstrated significant enhancement in stiffness and crack resistance for the steel-ECC composite beams. A practical four-parameter fibre-bridging model was established to describe the strain-hardening behaviour of different ECC materials. Then, a modified analytical tension-stiffening model for R/ECC was formulated considering the strain-hardening behaviour of ECC and rebar-ECC bond-slip interaction based on the conventional tension-stiffening model for reinforced concrete. This modified model was verified by several direct tensile tests of R/ECC members. In addition, by applying the model to the traditional fibre beam-column element model, the mechanical performance and crack opening of general R/ECC structures were derived. The simulation results of the steel-ECC composite beams demonstrated satisfactory accuracy compared with the test results. Finally, a parametric study based on the new model was conducted to identify the influence of several important material and structural parameters on the flexural performance of steel-ECC composite beams under negative moments.