Pull-out behaviour of steel fibres in cracked concrete under wet–dry cycles – deterioration phenomena

This paper presents observations and results of an experimental investigation on the mechanisms altering the pull-out behaviour of partially pulled hooked-end steel fibres inside an artificial crack exposed to wet-dry cycles for six months. Mechanical and corrosion damage at the surface of the steel fibre were investigated by 3D optical interferometric profiling and petrographic analyses to describe mechanical damage and healing processes at the matrix surrounding the fibre.

Mechanical damage observed in the cementitious matrix and at the fibre confirmed that, after debonding, the pull-out process was governed by yielding of the fibre hook, fracture of the adjacent matrix and friction between the hook and the matrix. The partial restoration of the adhesive bond after exposure was attributed to a combination of autogenous healing around the fractured fibrematrix interface and accumulation of corrosion products around the intersection of the fibre with the crack.

The increase of the mechanical bond strength was ascribed to autogenous healing processes at the mechanically damaged matrix around the fibre hook. Results of this study support the hypothesis that the increase of the pull-out force transferred by hooked-end steel fibres bridging cracks in concrete exposed to wet-dry cycles is mainly related to autogenous healing and carbonation of the damaged matrix around the fibre than corrosion damage of the steel.