The government should lead the private steel bar producers in the right directions by encouraging production of safer and state-of-the-art high strength reinforced bars that are there in the developed worlds, imposing laws against monopolization of mediocre technology aired as excellent to save the nation from future lack of safety
Reinforced steel bars and safety
Hossain Md Aktar
Bangladesh experienced a golden time in manufacturing reinforced steel bars in the past with Chittagong Steel Mills (CSM) Limited. CSM, the former state owned steel making company was commissioned in the mid-1960s. The company contributed to the total infrastructural development of the then East Pakistan and subsequently Bangladesh through production of ingots, billets, blooms, reinforced bars, structural sections, plates, corrugated iron sheets and other products.
It was the lighthouse of the steel sector of the country as regard variety of standard products, engineering practices, expertise of employees, research and developments and social responsibilities. But due to various unfortunate reasons, the industry shut down in 1999. As the CSM used to guide the nation towards appropriate direction regarding steel and engineering, with the shutdown of the industry, Bangladesh has been suffering from scarcity of appropriate standards, quality practices and regulatory affairs of steel, especially of reinforced bars.
The ingots, billets and bars were produced through stringent quality control practices following indigenous standard practices of CSM aligned mandatorily with the concerned National Standard Specifications and voluntarily with the USA, British, German and Japanese Standard Specifications. Products were uncontroversial in quality standards. Except CSM, no other company in the country was capable of producing standard billets, raw material of graded rebars at that time. A number of small private steel mills, which were nationalized for few years after liberation as enemy properties, were also using billets of CSM to produce Grade-40 60 reinforced bars in parallel to CSM: Grade-40 with maximum percentage of carbon, manganese and carbon-equivalent respectively 0.20, 0.90, 0.35 and that of Grade-60 0.30, 1.50, 0.55; Grade-40 with minimum Yield Strength(YS) and Ultimate Strength(US) respectively 40 ksi, 60 ksi and that of Grade-60 60 ksi, 90 ksi. The US/YS ratio of both the grades is =1.50.
As the history of steelmaking of Bangladesh is concerned; in parallel to graded bars, non-graded bars are being produced from 1960s through a numbers of small private industries. These bars with inconsistent chemical, metallurgical and mechanical properties are usually produced through hot rolling of ship-break plates and substandard ingots of unknown chemistry. The trend of production of non-graded bars parallel to graded bars was competitive in the earlier days which have been decreasing sharply from end-2000s. This is because of enhanced productivity of graded bars, rapid infrastructural development, and quality consciousness of the users.
From 1990s, gradually a number of medium and large scale (in perspective of Bangladesh) steel mills emerged in private sector that had been using imported and captive billets for graded bars. The emergence of private mills was accelerated with decline of CSM and increase in domestic demands. Products were Plain, Deformed and Cold Twisted (CTD) types of Grade-40 60. The trend was going on full swing until the introduction of Low Carbon without micro-alloyed Quenched Tempered (QT) 500W rebar by a leading company in the year 2008. Through introduction of this rebar, total scenario of production of graded bars abruptly changed as almost all the major players rushed to produce QT bar installing new units and augmenting old production lines equipped with conventional hot rolling arrangements. The QT 500W bar has YS=72.50 ksi, US=83.38 ksi and US/YS ratio=1.15(few producers achieved US/YS=1.25 with US=90.63 ksi).
Producers were inspired by lower production cost and higher Yield Strength of QT 500W rebar compared to Grade-60 and hence better profitability. Initially the product was not appreciated by the market as it could not convince the designers, architects and civil engineers as well as concerned statutory bodies like Bangladesh Standards and Testing Institution, Bangladesh National Building Code 1993 and so on. It raised a small number of serious questions among the users what to do, how to cope up with or escape from the newly emerged product.
The market was also having a demand for cheaper product with higher strength conforming to less steel requirements in concrete structures.
The producers took the chance of this demand. They were so convincing with promotional activities that ultimately it gained the acceptance of users. Within few years the product captured the whole market of graded bars overcoming barriers of acceptance of market and concerned authorities and affairs. The producers who were unwilling to make changes initially also changed the previous inclination at last. Production of traditional Grade-40 60 bars completely stopped. Finally it created a closed domestic market leaving no option for the users. Ultimately, the users became compelled to use QT 500W rebar instead of earlier deformed bars of Grade-40 60 due to persistence of closed market scenario shaped by the producers.
The processes of production of conventional and QT steel bars would make us critically analyze the benefits and drawbacks in metallurgical point of views. In case of traditional bars, the hot rolled products are cooled naturally in the air. As a result, throughout the whole cross section, the microstructures remain fully ferrite-pearlite. But for QT bars, they are forced to cool rapidly by passing them through water quench-chamber just after final roll-pass.
So, the outer portion of the bar gets quenched with subsequent self tempering that form tempered martensite at the outer layer. The microstructures of the core remain as ferrite-pearlite. There also exits a transition zone in between case and core with tempered martensite, bainite and ferrite-pearlite. The Tempered martensite is a very strong phase which gives high strength and the soft ferrite-pearlite at core gives required ductility of the bar. The QT is one of the recognized processes of high strength steel production. It is the easiest and cheapest process. Chemical composition of QT 500W is at par with Grade-40.
Martensite is a body-centered tetragonal form of iron in which some carbon is dissolved. Its a metastable phase and more or less ferrite supersaturated with carbon. Because of drastic quenching, formation of martensite from austenite imparts thermal stress that causes distortion and micro-cracks within the case of the bar whose adversities cannot totally be eliminated by subsequent self tempering.
The QT bar is claimed for better fire resistance, corrosion resistance, earthquake resistance, weldability and some other mechanical properties. But the bar is not better in most of these properties in true sense compared to traditional bars. As strength is concerned, QT 500W has YS and US respectively 72.50 ksi, 83.38 ksi compared to that of Grade-60 60 ksi, 90 ksi. Therefore, the QT 500W has more YS but less US compared to Grade-60 bar.
The bar is claimed for better ductility and hence better earthquake resistance; is it really better ductile? The percentage elongation and US/YS ratio are two of the measures of ductility. In the both the measures with constant gauge lengths, QT bar has been proven inferior compared to conventional grades. As US/YS ratio is concerned, the QT is having 1.15 compared to 1.50 of conventional grades. So, in any case, traditional bars ensure better ductility and hence better earthquake resistance compared to QT bar.
The bar is claimed for W, weld-able; is it better weld-able? According to metallurgical literature, Grade-40 is easily weldable, grade-60 is weldable with little difficulties, QT is weldable with careful and special welding procedures: (i) care needs to be taken to adhere to established procedures recommended by the manufacturer at all times for heat input preheat values, as there is nearly always a heat input limit for the welding and there can be problems with softening of the Heat Affected Zone (HAZ) if the cooling rate of the weld is slower than that of the quenching process; (ii) care needs to be taken in the selection of suitable welding consumables, particularly where there is a requirement to match the parent material properties; this can be very difficult to achieve, particularly if the weld metal needs to match the ultimate tensile strength of the steel, rather than the yield strength.
QT is claimed for better corrosion resistance; is it having better corrosion properties? Because of drastic quenching a lot of thermal stresses impact the bar that cause micro cracks within the case that ultimately lead to corrosion and quick rusting. Rusting may cause spalling cracks of the reinforced concrete which is really very dangerous for survival of civil installations. Moreover, with increase in corrosion of bars, strength decreases, ductility reduces and brittleness increases, which may lead to catastrophe in service conditions.
Is the QT bar going to be little coordinated with CTD bar? With a view to reduce the quantity of steel volume in civil installation with increased strength, hot rolled bar of 36-40 ksi was increased to 60 ksi by cold twisting reducing ductility and causing surface stresses and micro cracks. These caused high corrosion rate, which ultimately enhanced spalling cracks of the reinforced concrete. In Europe, where the CTD process was developed, gave up its use in mid-1970s, a few years after its development. But despite the findings in Europe, in India, it expanded a sturdy grip. In Bangladesh, the company that introduced QT technology also introduced CTD bar in the year 1984. All that was encouraged by the producers was the significant savings from use of CTD bar of 60 ksi against traditional bars of 36-40 ksi. Mr. RN Raikar, President of the India Chapter of the American Concrete Institute, at his opening remarks in the seminar on Reinforcement: Today Tomorrow held in Mumbai in June 2003, expressed grief that, Fewer repairs were required in buildings prior to the use of CTD bars. Today, the repair of buildings has become a specialized industry.
Over the last 16 years since the closure of CSM, the private reinforced bar producers have been making absolute contribution in the field of infrastructural development of the country. They are also contributing huge revenues to the government and creating jobs. So, they might be crowned by the nation in a way. But, for sustainable development they should make and supply right products to the country. The government should lead the private steel bar producers in the right directions by encouraging production of safer and state-of-the-art high strength reinforced bars that are there in the developed worlds, imposing laws against monopolization of mediocre technology aired as excellent to save the nation from future lack of safety.
Although it is not impossible to revive CSM as the machineries of the industry have been sold out as scrap and the possessed lands have been leased out for EPZ, the government should think to install another big steel plant to stop closed-market-scenario shaped by private producers, increase grips on them and hence provide the nation with alternative safer solutions.
Dont we want to be safer? This is the high time to answer.
The writer is Deputy Project Director, Skills Training Enhancement Project (STEP), Directorate of Technical Education
