Analysis of debonding mechanism and concrete cover failure of steel and steel fibre reinforced concrete composite beams

To solve the construction problems of steel reinforced concrete structures, the rebar cage was replaced by steel fibres, thus an innovative structure, steel and steel fibre reinforced concrete (SSFRC) structure, was proposed. Based on the four-point bending tests and numerical modelling of 18 SSFRC beams, the debonding mechanism and concrete cover failure were investigated. The results reflected that SSFRC beams experienced shearing failure, debonding failure and bending failure. Bonding cracks existed in most specimens, and the generation of abundant bonding cracks led to the sudden load decrease. The uncoordinated deformation between concrete cover and top flange appeared during the internal force transmission, making the bonding interface between them resist strong tensile stress and resulting in the debonding failure. After the chemical bond force at the bonding interface failed, the lateral concrete cover was firstly torn at the top flange tip, forming bonding cracks. The trapezoidal failure zone would form in the pure bending zone if bonding cracks developed sufficiently. Increasing the amount of steel fibres could prevent the trapezoidal failure zone from separation. Influenced by debonding failure, the extrusion action between bottom flange and bottom concrete cover was aggravated, generating longitudinal splitting cracks on the bottom surface of specimen.