STEEL REINFORCEMENT- An Overview

• Steel reinforcement is very important constituent of Reinforced Concrete Structure.

• Steel is the time proven match for reinforced concrete structures which is designed with the principal that steel and concrete act together to withstand induced force on RC Structure.

• The properties of thermal expansion for steel and concrete are approximately same, at least upto a considerably high temperature, along with this steel having its excellent bendability property and very strong to withstand tensile load to makeup the deficiency of concrete to cater tensile load, makes the best material as reinforcement in concrete structures.

• In normal reinforced concrete structures, the steel bars employed as reinforcement are known as passive reinforcement, also termed as rebars. In prestressed concrete structures, the reinforcement (steel wires) are stressed prior to subjecting the structures to loading, which may be viewed as active reinforcement.

Brief History of rebars in India • In India, the history of R.C.C. structures is more than

100 years old.

• Plain Mild Steel (MS) rebars of grade Fe-250 were used widely till about 1967 in India.

• But for economic reasons, the need for reduction in the steel used in R.C. structures was felt which consequently led to increase the yield strength of rebars.

• The high yield strength was first imported to rebars by raising Carbon as well as manganese contents and to a great extent by a mechanical process involving stretching and twisting of mild steel beyond yield plateau and subsequently releasing the load, which is called as cold twisting.

• These are called cold twisted deformed (CTD) rebars having yield strength in the range of 405 MPa (Grade-40). The trade name of these bars are Tor-Steel. Tata Steel also produced the same grade rebars named as Tiscon.

• Tor Steel, called Tor-40 with its characteristic(yield) strength of 415 N/mm2 (or 415 MPa) proved to be much economical than conventional mild steel as the weight of steel reinforcement could be reduced to a great extent for same design loads, due to almost 65% higher strength.

• Tor steel, thus, took over the scene in seventies and maintained its supremacy till nineties in India. Though in Europe, where CTD process was developed, gave up its use in 70s, because of the inherent technical deficiencies of CTD rebars.

• Thermomechanically treated (TMT) rebars were introduced in India during 1980-85. It is a better technology which could produce higher strength rebars up to 500, 550 or even up to 600 MPa, side by side the disadvantages of CTD technology could also be overcome. Currently TMT rebars are overruling the Indian market.

• In several advanced countries, stainless steel rebars (with higher chromium content) have been introduced which is anti corrosive but costly.

• In countries like Germany, France, Holland and Japan, Fiber reinforced plastic (FRP) rebars have been started being used.

Typical Cross Section of MS, CTD and TMT Rebars

Important Characteristics of rebars.The characteristics which determines the criteria of a good rebar are :-

• Good strength for economic design of R.C.C. structure.

• Good gripping with concrete i.e. good bonding.

• Thermal expansion characteristics compatible with concrete.

• Good ductility to withstand cyclic loading (for example earthquake loading) or impact.

• Good bendability which is essential for giving

requisite shape to rebars to suit the demand of the structures.

• Good weldability for the ease of good reinforcement cage binding and saving of lap length.

• Good resistance against corrosion for higher durability of R.C. Structure.

The pros and cons of Plain Mild Steel rebars :

• M.S. bars were very good in ductility, bendability, weldability and corrosion resistance.

• But its bond with concrete was poor because of its smooth surface. The minimum bond length was more.

• The yield strength of M.S. bars was 250 MPa, for which much more steel would be required in R.C. Structure making it heavier and costlier.

• The main disadvantage of M.S. bars was its low strength, thus uneconomical. This is the main reason why it was out of the market.

• Because of its low Carbon Content, M.S. rebars had excellent in possessing good ductility property, had defined yield strength and good weldability property.

• Corrosion resistance property of M.S. bars is in fact best out of all steel rebars developed so far.

Introduction of Cold twisted deformed bars since 70s.

The manufacturing process of CTD rebars are essentially comprised of three stages :1. Production of quality billets : In this stage chemical composition of steel is modified by increasing % of Carbon and Manganese.

2. Passing the reheated billets through stands to get deformed rebars of desired diameter.

3. Cold twisting the deformed rebars by imparting further strength. This is a mechanical process.

• By cold working (stretching and twisting) process, the yield plateau (the horizontal part of typical stress-strain curve) of M.S. bars are made shortened or even eliminated completely.

• Thus, CTD rebars generally do not exhibit definite yield strength as in the case of M.S. bars.

• CTD rebars does not exhibit specific yield point and 0.2 percent proof stress is considered as yield strength.

Typical stress-strain curve for (a) mild steel rebar (b) MS rebar under repeated loading (c) CTD bar (d) TMT bar

Manufacturing process of high strength rebars :-

• In India two routes of manufacturing of steel are followed :-

• Primary steel making route : In this process the billets which are rerolled to form rebars are manufactured as follows :-

• Through this route the chemical composition and physical properties of steel are much better, the targeted property can be achieved.

• It is possible in bigger steel plants with the facilities as shown above.

• Secondary Steel Making Route :- In this process, the rebars are manufactured from rerollable pencil ingots produced out of scrap steel materials such as scrap rails, automobile scrap, defense scrap, scrap generated from ship braking or discarded structures etc.

• Mostly induction furnace is used in this route where refinement of molten scrap to control the contents of Carbon, Sulphur, Phosphorous etc. to the desired levels can not be done.

• Obviously, the rebars produced through secondary steel making route have got inherent shortcomings.

• Unfortunately, in India more than 50% rebars are manufactured through secondary steel making route.

• As per Cl.- 1.6 of IS : 1786 (2008) rebars produced through this route is not acceptable, if the metallurgical history is not fully documented or not known.

The short comings of CTD rebars :• The CTD rebars, have inherent problem of inferior

ductility, weldability and increased rate of corrosion due to presence of residual stresses and higher carbon content.

• Additionally, cold twisting being labor intensive, enhances cost of production with limitations on production rate.

• In Europe, where it was developed, gave up its use in 70s, a few years after its development.

The pictures of different types of CTD Rebars used in India

• But in India, till 2000, CTD rebars were used almost totally. Even now it is in use.

• During cold twisting process a part of residual strain is withheld in the periphery of CTD bars. This locked-in strain initiates the corrosion process faster.

• Fewer repairs were required in R.C. structures prior to use of CTD bars. After introduction of CTD rebars in 70s, the repairing of R.C structures had become a specialized industry by itself.

The picture shows the difference between the quality of CTD and TMT

Rebars

Further development : TMT technology or microalloying.

• Microalloying is suitably changing the chemical composition of steel with addition of Niobium (Nb), Vanadium (V), Boron (B) and Titanium in combination or individually (not exceeding 0.3% and the carbon equivalent not exceeding 0.53%) followed by air cooling, thus, meeting most of the requirements and producing high strength reinforcement bars having yield strength of 500 and 550 MPa.

• This process can meet the requirement of higher strength but with higher cost. The achieved ductility and weldability is also low due to higher carbon equivalency.

Picture of the Micro Alloy Rebar

• A more economical and competitive process is so called – Thermo Mechanical Treatment Process.

• Steel Rebars produced through this process can meet the requirement of high yield strength, at the same time with superior ductility, weldability, bendability and comparatively better corrosion resistant property (better than CTD rebars but not better than M.S bars).

• Comparison of features of three main processes for production of high strength rebars :

Properties Sl. No.

Production Process

Treatment Production Costs Ductility Weldability Corrosion

resistance 1. Cold

twisting Cold work hardening by means of stretching, twisting

High due to labour intensive and limitations in rate of production

Poor Good (deteriorating due to heat generated by welding)

Very Poor

2. Micro alloying

Addition of alloying elements as C, Mn, V, Nb etc.

High (due to alloying elements and equipments)

Good Poor (due to high carbon equivalent)

Moderate

3. TMT Rapid Cooling and Controlled Cooling from rolling heat

Lower Excellent Excellent Good.

-: TMT Process :-• The billets (produced preferably from primary steel

making route) are reheated around 9500C – 11000C in reheating furnace and it is hot rolled to a certain extent.

• It is followed by quenching in a specially designed water-cooling system for a short interval of time where the bars are kept till the outer surface of the bars becomes colder, while the core remains hot. This creates a temperature gradient in the bars.

• This quenching forms martensite on the surface and the core remains as austenite.

• After that the bars reach to the cooling bed and again exposed to air.

• By this process, a sort of (self) tempering occurs, by which the resultant rebar structure is of tempered martensite periphery and fine-grained ferrite-pearlite core.

• Generally speaking, the resultant soft core forms about 65-75% of the area (depending upon the desired minimum yield strength), and the rest is hardened periphery.

• The equalizing temperature together with the final rolling temperature is the most important parameter to achieve the required mechanical properties.

The figure illustrating typical manufacturing process of TMT rebars

The Microstructure of T.M.T. Bars showing peripheral

rim of tempered martensite and core of ferrite- pearlite

Tempered Martensite Rim Ferrite-Pearlite core

Different TMT Rebars used in India

Some critical issues regarding TMT rebars in India.

• Like secondary steel making route for CTD rebars, in case of TMT rebars markets also, many major steel manufacturing firms are selling their products with publicity of TMT bars though those are not manufactured from “Quenching and tempering” technology.

• Legally, nobody stops them from claiming this and selling their products as TMT because in every rolling mill, thermal and mechanical treatment is involved which may not essentially be quenching and self tempering (QST) process.

• The civil engineers should be very careful in buying the products blindly as TMT rebars. They should instead specifically ask for bars in terms of yield strength, tensile strength and elongation as specified in table 3 : Mechanical properties of high strength deformed bars and wires vide cl. no.- 8.1 of IS : 1786 (2008). Otherwise, the basic objective of the technological advancement in the field would be defeated.

• The term should also be changed from TMT to QST (Quenching and Self Tempering) so that it cannot be exploited by the manufactures who do not have proper technology.

How to differentiate between simple deformed M.S. bars and bars

undergone proper heat treatment ?

• It is difficult to distinguish between the two by visual inspection only.

• As per Annex ‘A’ of IS : 1786 (2008), a field test is described to differentiate.

• A small piece (about 12 mm. long) can be cut and the transverse face lightly ground flat on progressively finer emery papers up to ‘0’ size. The sample can be microetched with naital (5% nitric acid in alcohol) at ambient temperature for a few seconds which should then reveal a darker annular region corresponding to martensite microstructure and a lighter core region in case of properly thermal treated rebars.

• This field test should not be regarded as the criteria for rejection. The chemical and physical properties as per IS : 1786 (2008) shall be considered as acceptable.

What are the modifications from IS : 1786 (1985) to IS : 1786 (2008) :

• The physical and chemical compositions of Fe415, Fe500 and Fe550 almost remain unchanged, except elongation % for Fe550 has been increased to 10.00% from 8.0%.

• A new grade of higher strength rebar, Fe600 has been introduced.

• A new Superior Category of rebars called ‘D’ has been introduced in 2008 version of IS : 1786.

• Over three grades Fe415, Fe500 and Fe550, the superior category Fe415D, Fe500D and Fe550D have been introduced which may have same corresponding yield strength but increased elongation and tensile strength.

• By amendment on November’2012, a further superior category “S” have been introduced to Fe415 and Fe500 only which have same chemical property range with “D” category but have superior physical property range.

• On chemical composition side, this ‘D’ & ‘S’ Category rebars are having lesser permissible limit of Carbon, Sulphur and Phosphorus from their corresponding grades without ‘D’ category.

• These ‘D’ & ‘S’ category bars have better earthquake and dynamic load resistant properties.

• Instead of Carbon alone, the modified version of the code has specified the limit of carbon equivalency (CE) which include other elements like Mn, Cr, V, Mo, Ni, Cu etc.

• There is as such no difference in chemical composition range in between ‘D’ category and ‘S’ category rebars, however, ‘S’ category rebars have higher physical property range than ‘D’ category rebars.

A picture of comparison between Fe normal grade and Fe ‘D’ grade as per IS : 1786 (2008) is shown :

Chemical Properties Physical Properties Grade of

Steel Carbon

(%) Max.

CE (C+Mn/6)

(%) Max.

Sulphur (%)

Max.

Phosphorus

(%) Max.

S+P (%)

Max.

Yield strength

min. (MPa)

Elongation % Min.

Tensile strength min. Total elongation

at Max. Force %

Min. Fe 415 0.30

0.42 0.060 0.060 0.110 415 14.5 10% more than yield strength

but not less than 485 MPa -

Fe 415 D 0.25 0.42 0.045 0.045 0.085 415 18.0 12% more than yield strength but not less than 500 MPa

5

Fe 500 0.30 0.42 0.055 0.055 0.105 500 12 8% more than yield strength but not less than 545 MPa

-

Fe 500 D 0.25 0.42 0.040 0.040 0.075 500 16 10% more than yield strength but not less than 565 MPa

5

Fe 550 0.30 0.42 0.055 0.055 0.100 550 10 6% more than yield strength but not less than 585 MPa

-

Fe 550 D 0.25 0.42 0.040 0.040 0.075 550 14.5 8% more than yield strength but not less than 600 MPa

5

Fe 600 0.30 0.42 0.040 0.040 0.075 600 10.0 6% more than yield strength but not less than 660 MPa

-

N.B: By further amendment, Fe415S has TS/YS ratio as 1.25 and minimum elongation percent 20%, Fe500S has TS/YS ratio as 1.25 and minimum elongation percent 18%, with their chemical composition range remained unchanged.

Tensile strength testing of rebars

Universal Testing Machine The sample of rebar after the test

Measurements of elongation of rebars

Elongation is measured in percentage on a gauge length of 5.65 √A where A is the cross-sectional area of the test piece.

Checking of nominal mass per unit length of the sample

By amendment, 45mm & 50 mm size have been introduced having CS area 1591.1mm2 & 1964.4mm2 and mass per metre 12.49 kg & 15.42 kg respectively

Chemical composition testing of rebars

Testing of chemical composition of rebar by Spectrometre machine along with computer software

Bend & Rebend Test• Bend test : As per cl.9.3 of IS:1786(2008), the test piece

when cold, shall be doubled over the mandrel by continuous pressure until the sides are parallel (i.e, 1800 bend) . There should not be any rupture or crack visible on the bended portion.

• Rebend test: As per cl.9.4 of IS:1786(2008),the test piece shall be bent to an included angle of 1350 using a mandrel of appropriate diameter. The bent piece shall be aged by keeping in boiling water(1000C) for 30 min and then allowed to cool. The piece shall then be bent back to have an included angle of 157.50. There should not be any rupture or cracks on the bended/rebended portion.

Pictures of bend and rebend test

Bend Test Template for bend and rebend test

-: Conclusions :-• Proper characterization of steel reinforcement is

very important aspect for sound and durable R.C. Structures.

• Chemical Composition and Mechanical Properties like yield strength, ultimate strength, elongation are generally considered for characterization of rebars.

• From design point of view : strength, ductility and corrosion resistant properties of rebars are important.

• From construction point of view : bendability and weldability of rebars are important characteristics.

• From economic and sustainability angle, recycling of metal scraps are inevitable. But a well defined system for this purpose like USA and Europe should be implemented in India for proper control of quality.

• Newly introduced ‘D’ & ‘S’ category of rebars in IS : 1786 (2008) and further amendment with high elongation value and better chemical composition (lesser % of carbon, sulphur and phosphorus) are better suited for structures in more earthquake prone zone.

• Fatigue strength of rebars can not be judged by mechanical properties like yield strength and tensile strength. Fatigue strength is not specified in Indian standard. Fatigue life in terms of number of stress cycle needs to be specified.

• Neither IS : 456 (2000) nor SP : 16 provides design stress-strain curve of TMT rebars. BIS should modify the relevant codes with design stress-strain curve and design value of yield strength of TMT bars.

• For environmental and sustainability reason, stainless steel rebars or FRP rebars are the future.

Thank You