Part I: How effective are our reinforcements?
What you need to know about steel rebarsAmongst the vast numbers of man-made invented composites, the reinforced concrete comprising of cement, water, sand, gravel and steel, perhaps is a composite material that had maximum impact in revolutionizing the quality of life of human being. After curing, the concrete attains appreciably high strength in compression but remains very poor in tension, flexure, shear and torsion. It was an accidental discovery when a French gardener named Joseph Monier in 1867 observed that a phenomenal improvement in durability and strength of his flower pots took place after iron nails were embedded in castings made by Portland cement, sand and water. This discovery led to a series of developments in designing of rebars to achieve stronger and longer lasting concrete structures. It was a long journey for the currently popular TMT steel rebars to reach to its optimum properties imparting improved life to concrete structures. Now TMT rebars are used all over the world for erection of bridges, flyovers, high and low rising buildings, pavements, concrete roads, canals, dams etc. These rebars are produced by quenching and tempering the process having composite microstructure with ductile core area and harder outer rim. This dual structure imparts strength as well as ductility to TMT rebars. They are free from any distortion of crystals of steels and therefore are more resistant to corrosion with desired ratio of tensile and yield strengths. The absolute carbon content is also kept at lower side in such types of rebars which improves their weldability.
Figure 1 : Cross section of TMT bars is composite microstructure. The core area is soft pearlite – ferrite and the outer rim is harder tempered martensite microstructure
Unfortunately production process of TMT type of rebars needs special mills with very strict control of quenching and tempering parameters. This increases their cost of production. Many companies still produce rebars without quenching and tempering treatments and market them in many countries. It is not possible to differentiate between TMT and non TMT bars just by looking from naked eye. The users unaware about the quality of such rebars use them in their construction work. As shown in the following figure 2, both TMT and normal bars (non TMT) have ribs and it is not possible for consumers to differentiate between them. It can be differentiated only after looking in to the rim formed on the cross section of rebars. TMT bars develop a circular rim around the cross section but non TMTs do not exhibit such a rim.
Figure 2 : Digital photographs of CTD, TMT and non TMT bars
How to find out whether the rebars are TMT or non TMT
It is very simple. Cut a small piece of the bar. Grind its cross section area on emery paper to remove scratches. Dip the cross section in acidified solution of alcohol for 30 seconds (98% alcohol + 2% nitric acid). Remove the bar from the solution and let them get dried. If a circular ring appears along the periphery of its cross section, it is TMT bar. The photograph of cross section of TMT bar after testing is shown in the above figure 2.
Part II of this series of write up will describe the “Caribbean vis-à-vis international scenario on use of rebars for reinforcement of concrete structures”