TMT stands for Thermo-Mechanically Treated — high-strength steel reinforcement bars made by rapidly quenching the hot rolled bar's surface with water while the core stays hot, then letting the core heat re-temper the surface. This gives a bar with a hard, strong outer layer and a soft, ductile core: strong yet able to bend without snapping.
TMT bars are the standard reinforcement for RCC in Indian construction, having largely replaced older mild-steel and twisted bars. They are graded by yield strength — Fe 415, Fe 500, Fe 550 and Fe 500D — with Fe 500 the most common for housing. The ribs (deformations) on their surface grip the concrete and prevent slipping.
Bars are sold by diameter (8, 10, 12, 16, 20, 25 mm) and priced by weight. The unit weight of a round bar follows the d²/162 rule — a 12 mm bar weighs 0.889 kg/m. Estimating steel for an element means preparing a bar bending schedule: the cutting length of each bar, its unit weight and the total.