Mechanical characteristics of Quenched and Self-Tempered (QST or TMT) steel reinforcing bars used in concrete structures

Quenched and Self-Tempered (QST) steel reinforcing bar (rebar herein) is widely used owing to its high strength and ductility over conventional hot-rolled or cold deformed bars. A typical QST rebar constitutes a composite microstructure of a hard 'tempered martensite' (TM) peripheral ring and a ductile 'ferrite-pearlite' (FP) core. TM and FP are predominantly responsible for the strength and ductility, respectively exhibited by a QST rebar. A good quality QST rebar cross-section shows a uniform, continuous, and concentric TM-ring around the FP core. However, recent studies reported that poor quality QST rebars with inadequate/defective cross-sections could influence mechanical and durability properties. This study evaluates the effects of inadequate cross-sectional phase distribution (CSPD) on the mechanical properties of QST rebars. In stage 1, tensile test on TM and FP extracted from a QST rebar clearly showed that FP is ductile with low strength, while TM had a brittle behaviour with high yield and ultimate strengths. In stage 2, the composite response of QST rebars collected from different manufactures were evaluated using tensile and bend tests to correlate the effect of CSPD. Tensile test results show a significant variability (up to 15%) in the stress-strain behavior and mechanical properties exhibited by rebars with inadequate CSPD while the failure pattern shows a composite response. Poor-quality rebar specimens in bend tests showed cracks which could induce crevice corrosion. Hence, CSPD has a significant influence on the mechanical and corrosion properties of a QST rebar. Acceptance criteria based on TM-ring percentage area and distribution in the CSPD is proposed to check the quality of the QST rebars.