Metallurgy

Materials science

Rebar

Steel bar

Ferrite (magnet)

Pearlite

Microstructure

Austenite

Composite material

Corrosion

Martensite

Construction and Building Materials

Quenched and Self-Tempered (QST) steel reinforcing bar (rebar, herein) is widely used in the reinforced concrete (RC) systems across the world. A typical QST steel rebar cross-section consists of a ductile ‘ferrite-pearlite’ (FP) core and a hard ‘tempered-martensite’ (TM) periphery. A good quality QST steel rebar is expected to have an FP core encircled by a uniformly thick, continuous, and concentric TM ring/phase. This study assessed the cross-sectional phase distribution (CSPD) of QST steel rebars produced in various countries using macroetching. These countries include Australia, Bahrain, Germany, Hungary, India, Italy, Japan, Malaysia, Russia, Singapore, Sri Lanka and Switzerland. Unfortunately, many rebars with 8, 12, and 16 mm diameter revealed inadequate CSPD with discontinuous and/or non-uniform TM phase. Also, the adverse effects of this inadequate CSPD on the corrosion resistance were evaluated using Cyclic Potentiodynamic Polarization (CPP) tests. For this, the FP and TM plate/coupon specimens (15 × 15 × 3 mm) were extracted from a QST steel rebar and metallographic type specimens were prepared. Then, the specimens were immersed in chloride (Cl−) contaminated simulated concrete pore solution (denoted as SCPS). Then, the CPP tests were done after incremental addition of Cl− in SCPS to identify the Cl− threshold (Clth) of TM and FP. The FP exhibited about 10–15% lesser Clth than TM. An analytical case study showed that the use of such QST steel rebars with inadequate CSPD can result in about 20% reduction in service life. Then, bend tests were conducted on 8 mm good quality and poor quality QST steel rebars used for stirrups. Poor quality rebars with inadequate CSPD showed visible surface-cracks in some products, which could lead to severe crevice corrosion, even without the presence of chlorides. Hence, better quality control in the quenching and self-tempering process is required to improve the corrosion resistance of QST steel rebars.

Microstructural and corrosion characteristics of Quenched and Self-Tempered (QST) steel reinforcing bars