TMT Reinforcement Bars In RCC Construction Basic Guidelines

The use of RCC construction is increasing day by day. In fact, concrete is the globally largest consumed material only next to the water. With the increasing height of structures, buildings, chimneys, cooling towers, there is a substantial increase in steel reinforcement per cum of concrete.

Unfortunately, in India, many particularly developers use steel rebar produced from the non-integrated steel process, as such steel is available at lower rates. Such steel should never be permitted in any structure other than temporary.

The higher density of steel per cum of concrete in tall structures which will result in lower bond and high possibility of corrosion of steel. 

Introduction

There are basically two distinct process of manufacturing steel rebars.

One is the sophisticated, mechanized process that ensures, high-quality steel rebars and is the process adopted by all developed countries. This process for simplicity is designated as the “Primary” process.

The other is a primitive process adopted in developing countries like India. In this process quality control is highly limited or poor. This process for simplicity is termed herein as “Secondary”. By the way, these terminologies are no more used in Indian Codes. But these terminologies are used herein for simplicity.

The table indicates the limitation of secondary rebars. Hence, from quality and durability consideration, such secondary steel Rebars should never be used. Fortunately, practically all government agencies prohibit the use of such reinforcement bars. Since developers do not come under the government or any autonomous body these bars are used y many developers including for high rise buildings. It is given to understand the use of Secondary Steel Bars in India is quite high and close to 50%.

ParameterRebar from Non-integrated Steel Company Termed as “Secondary”Rebar from Non-integrated Steel Company Termed as “Secondary”Rebar from Integrated Steel Company Termed as “Primary Manufactures”
Re-rollersInduction furnace operators
Process BriefConverts scarp (input dimensions of scrap suitably selected keeping the final dimension of re-rolled rebar in mind into rebar by simply reheating and rolling to final shape. Very remote possibility of heat-wise traceability. No melting facilities, hence no control on steel making.)Converts the scrap into rebar by melting, casting and rolling. Inherently, the batch sizes are small so both compositional and properly variations within the supplied batches vary. Heat wise traceability is difficult due to too low heat size.Beginning from virgin Iron Ore and using Blast Furnace and Blast Oxygen Furnace and continuous casting route, re bars are produced. Good heat size and heat wise traceability maintained in final product.
Process control abilityPoor: Manual control, poorly maintained equipment, safety and environmental unfriendly As the input material billet/ingots/thicker size scrap traceability with respect to heat, chemistry and quality conformance is impossible. Control on Input material quality is very far below requirement.Poor: Manual control, Poorly maintained equipment, safety and environmental unfriendly. Much smaller heat size with much less automation. Due to lack of process control, steel cleanliness is also very poor.Very Good: Process models assisted production, technical tie-ups with world’s leading technology providers for keeping pace with the latest technology, highly trained staff, R&D back up. Level of process control is far above requirement as per IS standard.
Main raw material input refining capability: Steel cleanlinessScrap: Usually from non-standard sources-heterogeneous in composition. No control as any melting/refining facility.Scrap: Usually from non-standard sources-heterogeneous in composition. Poor: The refining kinetics, directly related with the temperature of the melt, suffers because the ability to attain the temperature in induction furnace does not compare with the LD or EAF (Electric arc furnace). Phosphorus removal ability of induction furnace is very poor so the entire phosphorus content of charged scrap is passed on to the final composition of rebar. Secondary refining, responsible for lowering the melt’s sulphur content and cleanliness control, usually attained in Ladle furnace, is not available.Basic minerals like Iron Ore, Coal fluxes etc. no purchased scrap, but internal scrap is generally used. Very Good: by selection of better raw material, Hot Metal can be of good quality for direct use in steel melt shop. If not, by pre-treatment HM is desulphurised before sending to steel melt shop. In steel melt shop, desiliconisation & dephosphorisation are done. Modern furnace are having eccentric tapping mechanism to ensure slag-free tapping to ladle which is transferred to secondary refining facility like LF. Extensive de-oxidation, desulphurization with inert gas purging + cored wire feeding makes very clean & homogeneous killed steel within very restricted chemical composition as per IS standard & mill requirement.
CastingNo facilities & hence not applicablePoor: Usually cast in static moulds which produce ingots/billets with severe and serious casting flaws which are simply passed onto the finished rolled rebars. Some of the manufacturers deploy continuous casting but without any modern facilities to aid good quality billet production.Very good: 100% metal cast through continuous casting route. These casters are equipped with closed casting facility and equipped with Electro-magnetic stirrer in the moulds. The cast billets are homogeneous across its cross-section and along its length which when rolled produces homogeneous rolled rebars with respect to chemistry and properties.
Thermo-mechanical treatmentPoor: deploys local cheap indigenous but unreliable arrangement for cooling the bars. As a result the uniformity of tempered martensite is absent. In many instances tempered martensitic layer is completely absent. Partial tempered martensitic layer of the rebar makes it vulnerable to inherent galvanic corrosion.Poor: Deploys local cheap, indigenous and unreliable arrangement for cooling the bars. As a result the uniformly of tempered martensite is absent. In many instances tempered martensitic layer is completely absent. Partial tempered martenstic layer of the rebar makes it vulnerable to inherent galvanic corrosion and also poor control of mechanical properties.Excellent: Patented Thermex or, Tempcore processes are deployed to ensure an uniform rim formation around the periphery of the rebar. Computer aided control of the cooling not only ensures that cooling is reliable for each of the rebar but it also comes handy in manufacturing rebars with different end property requirements such as super-ductile, higher strength etc.
Chemical composition control of final productNo Control: Input scrap’s composition is the composition of the rolled rebar’s composition. Usually, the selection of input scrap is price and size dependent which does not take into account the chemical composition.Poor: The input scrap’s quality dictates the end products compositional state thereby, compositional state thereby, making it very from batch to batch.Excellent: Post steelmaking in BOF or EAF – almost an hour long secondary refining treatment is practiced for the chemical composition control of melt. Several mid-treatment samples are collected and analyzed for assuring the precise compositional control before it is cast.
Property control: StrengthPartial or, uncontrolled tempered, artensitic layer results in unreliable and unpredictable strength and corrosion properties of the rebar.Partial or, uncontrolled tempered artensitic layer results in unreliable and unpredictable strength and corrosion properties of the rebar.Excellent: An all round process control and strict adherence to equipment maintenance schedules by qualified professionals – the MACRO properties are controlled which in turn ensures that the strength properties are always adhered to.
New Product Development (higher strength – Fe650/Fe700 & Requirement for high rise building)Not possiblePoor R&D facilities & Process control capability to produce higher strength steel.Excellent R&D facilities and few integrated steel plant have already developed this product and waiting for release of  revised standard of IS 1786 to launch these 2 products in market. Such critical application grade should be procured from only those plants, where process control is robust & reliable.
Property Control: DuctilityPoor: Excessive cooling: a result of poor process control and excessive quenching, result in rebar’s that exhibit very high strength but very low ductility.Poor: Both composition and TMT process control dictate the ductility properties of the rebar. Usually, both lack and variable controls of these have been observed which consequently reflects in big fluctuations of the rebar produced through this route.Excellent: An all round process control and strict adherence to equipment maintenance schedules by qualified professionals – the MACRO properties are controlled which in turn ensures that the ductility properties are always adhered to.
Property Control: Corrosion resistance propertyPoor: Poor refining and TMT process results in intrinsically poor corrosion properly of rebar produced through this route.Poor: Poor refining and TMT process results in intrinsically poor corrosion properly of rebar produced through this route.Excellent: By virtue of the control of both composition and MACRO constituent – he corrosion properties are observed to be very good.
Surface defectsRe rollers generally take diverted billets/ingots. None of the ingots/billets are checked for surface and internal soundness. Defects in billets/ingots are passed into finished goods.Due to raw material from different sources/scrap, raw material quality is not the same. This leads to surface defects and poor internal soundness of billets/ingots.Billets/slabs are checked for surface defects and internal soundness. Defects, if any, are salvaged/rejected as scrap. This ensures defect free TMT bars.
Mechanical testing for the finished goodsDue to lower heat size the amount of sampling required is very high. Testing charges will also be high. Variation in mechanical properties from lot to lot will also be very high.Due to lower heat size the amount of sampling required is very high. Testing charges will also be high. Variation in mechanical properties from lot to lot will also be very high.Due to bigger heat size the amount of sampling required is very less, testing charges will also be less, variation in mechanical properties will be minimum.
Use in Developed CountriesNot usedNot usedUsed
Approximate cost of production plant15 crore for 0.1 million ton production per annum130 crore for 0.1 million ton production per annum8000 crore per one million ton production per annum

Steel Rebar Congestion

More and more high rise buildings are constructed in India. Many of these buildings are higher than 300m. it is observed that in the core wall of such buildings, reinforcement can be very dense even of the order of 0.5 to 0.6 ton per cum. with such heavy reinforcement, the spacing of rebars does not meet codal requirement either of the American code ACI 318 or European code EN 1992-1-1 or Indian code IS 456. As a result, quite often, cover to reinforcement is not concrete, not even mortar, but just a slurry, thereby permitting steel rebars for early corrosion. Moreover, a bond developed will be far less which may result in unsafe buildings during heavy wind. It may be noted that wind speed is increasing year by year. Hence, it is very advisable to go for a higher and higher grade of steel rebars to reduce its congestion. At, present, we in India are able to manufacture steel of grade Upto and including Fe600. We should permit the manufacturing of Fe700 or even higher grade of rebar by early approval of relevant code.

ProductStandardsSulphur (max)(%)PhosphorusSulphur + Phosphorus (max)(%)
Rebars IndiaIS-1786-Fe 5000.0550.0550.105
Rebars IndiaIS-1786-Fe 500D0.0400.0400.075
Rebars IndiaTata Tiscon Fe 500D0.0350.0350.070
Rebars USAASTM A 7060.0450.035Not mentioned
Rebars JapanJIS-G3112 (SD-490 grade)0.0400.040Not mentioned

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