Someone may doubt about “T” letter on rebar. What is meaning of “T” ? Why the producer input “T” letter on rebar? What is difference of SD40 and SD40 T?
In TIS standard 24-2548 : Steel bars for reinforced concrete : deformed bars are classified deformed bars by the chemical composition and mechanical properties into 3 grades denoted by the symbols SD30, SD40 and SD50 (clause 3.1)1 and request to mark the “T” letter permanently and legibly embossed on every deformed bar for the deformed bar that produced by heat treatment only like Tempcore process follow clause7.1 (4)1. The main reason this mark is to identify the tensile test piece method. Because of the tensile test piece of the heat treatment deformed bars shall be the full section of the bar as rolled (not finished mechanically) follow last line in clause 220.127.116.11
So the “T” symbol is represented the production process only. The deformed bars with or without “T” symbol shall be conformed the chemical composition and mechanical properties that given in TIS standard 24-2548 only, otherwise that product is not acceptance for this standard. For example, the cold bending test of SD40 grade shall be bent to 180? without cracking on the outside radius of the bent portion by mandrel diameter equal to 5 times the nominal diameter of that rebar, the minimum elongation of SD50 grade shall be 13%, etc.
Other properties that not require on this standard like the heat resistance, corrosion resistance, etc. Normally it isn’t testing because this deformed bar is use to reinforced concrete for construction and covered by concrete that has minimum concrete cover length follow the building code requirements for structural concrete and commentary (ACI 318M-05) (for Thai standard is follow E.I.T. Standard 1008-382) on Chapter 7.7-Concrete protection for reinforcement, clause 7.7.5-Corrosive environment or clause 7.7.7-Fire protection3.
In the research of energy based temperature profile for heat transfer analysis of concrete section exposed to fire on one side showed the properties of reinforced concrete after fire. Because of concrete is heat insulator, so the temperature at surface of concrete that exposed to fire will highest and this temperature will decrease when distance from the concrete surface is increase. If increase the concrete covering, the rebar will get less effect of temperature on fire.4 In the research of turning up the heat – full service fire safety engineering for concrete structure found the compressive strength of concrete will reduce rapidly when temperature higher than 500?C and concrete heated in excess of 600?C is of no use structurally. But the hot-rolled steel rebar will lose the residual strength when temperature higher than 600?C. 5 Because of the deformed bars with or without “T” symbol will don’t change microstructure in 200-600?C. So the rebar that has suitable concrete covering length will received less effect on fire and delay to lose the residual strength. Follow the TIS standard 24-2548, SD40 grade has specified the carbon + (manganese/6) shall be less than 0.55%. And in Iron-Carbon Equilibrium Diagram6 , if the percent of carbon in range 0.09 – 0.53%, the steel start to melt when the temperature higher than 1400?C. So the deformed bars with or without “T” symbol don’t difference on the heat resistance to use in construction significantly.
For welding deformed bar by electric, follow the department of Public Works and Town & Country Planning of Thailand standard 1103-52 : Reinforced steel standard7 Clause 5.6 Welding rebar by electric has specified the welding method, type of electrode, welding preparation, etc. And in the research of Welded Tempcore reinforcement behavior for seismic applications,8 the strength of welded rebar is similar to un-welded rebar and the welded rebar will break at the out of welding zone when do the tensile testing.
For the mechanical joint of rebar like coupler that has use in construction generally, the rebar with coupler shall be break at the outside of coupler when do the tensile testing and the strength shall be pass the specification in TIS 24-2548, too. Otherwise, that mechanical joint method isn’t suitable for use in construction
So the deformed bars that has chemical composition and mechanical properties follow TIS standard 24-2548 are qualify to use in construction follow the design and construction standard like E.I.T. Standard 1007-34, E.I.T. Standard 1008-38, etc.