Damage mechanism of ultra-high performance fibre reinforced concrete at different stages of direct tensile test based on acoustic emission analysis.

• Damage mechanism of UHPFRC is connected with different tensile stages of its stress–strain curve. • Tensile cracking mode and in-plane shear cracking mode in UHPFRC are identified via AE average frequency. • Fibre-matrix debonding is distinguished from fibre pullout in UHPFRC via AE energy. • Contributions of matrix cracking, fibre–matrix debonding and fibre pullout at different tensile stages are quantified. The two strain-hardening stages for ultra-high performance fibre reinforced concrete (UHPFRC) under direct tensile loading are the swift strain-hardening (stage II) and widely held strain-hardening (stage III) stages. A direct tensile test with acoustic emission (AE) registration and air permeability measurement was conducted on three UHPFRC types. Stage II displays more superior crack-width control, higher tensile stiffness, and better impermeability compared to stage III. Based on the analysis of three AE parameters, the damage mechanism of UHPFRC at different tensile stages was identified. The AE source location provides strong evidence of multiple randomly distributed micro-defects (<0.01 mm) at stage II. The AE average frequency and energy can distinguish three UHPFRC damage patterns (matrix cracking, fibre–matrix debonding, and fibre pullout) and evaluate their contributions at different tensile stages. Matrix cracking is dominant at stage II. Matrix cracking and fibre–matrix debonding is roughly balanced at stage III. The large-scale fibre–matrix debonding is generated accompanied by marginal fibre pullout at the strain-softening stage. The stress state of steel fibres at UHPFRC crack sections at stages II and III was also analysed. The configuration of the strain-hardening stage is determined by the relationship between the fibre–matrix bond strength and the tensile stress of the steel fibres at the crack sections. [ABSTRACT FROM AUTHOR]