Pull-out behaviour of chloride-degraded waste tire steel fibers in mortar: influence of surface morphology and corrosion products

Abstract

Single-side pull-out tests were conducted on crimped recycled steel fibres from waste tire wires and on Dramix 3D industrial steel fibres, both embedded in mortar and exposed for 30 and 60 days to a 7% (by mass) sodium chloride (NaCl) solution. The main objective of this research was to clearly evaluate how chloride-induced degradation influences the interfacial bond strength and mechanical performance of recycled crimped steel fibres embedded in cement mortar. Fibre tensile properties were also measured, and surface characterisation was performed via digital microscopy, Scanning Electron Microscopy (SEM), Energy-Dispersive X-ray Spectroscopy (EDS) and X-ray Diffraction (XRD). The results show that tire steel fibres exhibited lower corrosion resistance (mass loss 4.50% vs. 0.46% for Dramix) but a 438% roughness increase, leading to a temporary improvement in pull-out performance at 30 days (maximum load +9.94%; toughness +4.40%; average bond strength +14.56%), which declined at 60 days due to detachment of ferrihydrite and lepidocrocite layers. In Dramix fibres, goethite and magnetite formation concentrated damage in the hook region, causing pull-out reductions of −7.2% at 30 days and −9.6% at 60 days. Tensile tests revealed greater strength losses in tire fibres (−11%) than in Dramix (−5%); however, it was shown that the pull-out resistance depends more on the fibre adhesion and the surface characteristics than on its intrinsic strength.