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Knowledge Tree

Volume2 Issue6

June2010

Re-inforce the RebarsDear Readers,

We are very thankful toreaders for your continuedsupport in making thiseffort more meaningful bygiving the valuable sug-gestions.

The importance of rein-forcement bars in con-struction is not less thanconcrete , but many atimes they get misused or

mis-handled.Low quality of raw materi-als , procurement fromunapproved rerollers /Manufacturers , improperstorage , cutting , bending& placing methods leadsto its woes.

For the correct function ofrebar's as tensile forcecarriers , good materialwith excellent bond withconcrete is necessary.

We have highlighted theimportance of these issuesin achieving good & dura-ble construction work.

We hope this would createawareness among us for abetter construction experi-ence.

With Best wishes

KSV & TSRBR

Developed by KSV & TSRBR , for Internal circulation of L&TECC DivisionE&GP OC.

For any Suggestions or Queries Please write to ksv@lntecc.com or bangartsr@lntecc.com

What is rebar?

Rebar is another name for reinforcement bars i.e. steel bars used in concrete construction.

Why rebar's are used in Concrete construction ?

As we are aware that concrete is good in Compression & week in in Tension. As the con-crete member is subjected to many types of loads other than only compression, it becomesnecessary to reinforce / strengthen the concrete for taking the additional Tensile loads.

Steel being stronger in tensile , Bend properties & economical than other metals, steel inthe form of round bars are preferred as reinforce media.

How do the steel serve the purpose & why shape of rebar is round than square ?

Steel bars are not connected directly to the load. The load coming on the concretemember is transferred by the bond between the concrete & steel.

Steel expands when in tension & with in its yield point, steel returns to normal shapeup on load removal. Hence the original shape of the concrete member is preserved.

Apart from taking the tensile loads , steel bars transfer the shear force developed inconcrete members like beams , slabs Etc. safely.

As the round shape has more surface area than square / or any other shape for thegiven volume , Round shape is preferred.

What kinds of steels are available in market & what is suitable for use in the concreteconstruction ?

In the earlier days mild steel , cold twisted deformed bars ( CTD ) were widely used.

Latter with advancement in the technology, apart from the one mentioned Hot processedTMT bars are available in various sizes , properties.

In general Mild steel , CTD , TMT , Corrosion resistant steels ( CRS ) are widely used inconstruction.

What is TMT process how does it helped the construction industry ?

TMT broadly stands for Thermo Mechanical Treatment .

Till the invention of this process the biggest challenge for the reinforcement bar makers isachieving high strength from the steel without loosing the ductile properties it is famous

for.

This was not so easy , as one property or other was lost during heating or

treatments. So cold treatments were carried liketwisting the rebar's which gave additional strength.This is not enough for the slim , lean structures

which required higher strengths.

TMT is a manufacturing process in which thebars are subjected to mechanical forces like pres-

sure & thermal forces like Heating & Cooling.

The steel bars are heated to 950oC & rolled to required size &shapes.

This is followed by sudden cooling of the surface by using coldwater at high pressure for fraction of a Minute.

This process creates hard surface on the outer layer around 20 %of the total volume due to quenching process but leaves the inner

From Editors

Contents :

Rebar & its importance

Page -1-3

Summary , Puzzle , Your

Voice

Page

4

TMT bars

are widely

used in Con-

crete Con-

struction.

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core soft & malleable.

Thus the good properties we are looking at i.e. improved strength , bendability , elongation & Corro-sion resistance are achieved.

What are the chemical & physical properties of these rebar's ?

All rebar's made in India shall conform to IS 17862008 revision.

With further advancement in TMT process, 7 classes of rebar's are produced. They are designatedas Fe415 , Fe415D . Fe500 , Fe500 D , Fe550 , Fe550 D , Fe600.

All these bars shall satisfy the chemical & physical properties as mention in Table1 & 2

Sizes varying from 6 , 8 , 10 , 12 ,16 , 20 , 25, 28 , 32 , 36 mm are available with fixed standard length of 12 m.

Non standardsizes or lengthscan also be ob-tained on spe-cial order.

How do I know whether the steel is good or bad or rejected ?

The rebar's shall be sourced only from approved manufacturers. Rerolled steel / steel from direct market / from Trad-ers shall be avoided.

All bars when received shall bear the identification mark & Tag.

Manufacturer identification & grade is embossed on the bar itselfat every 1 m interval. The batch Number , Heat Number , Quantity

Etc. details are attached to each bundle of bars through Tags.

The information on these tags shall be verified with Delivery challan & any discrepancy shallbe informed to manufacturer.

Samples shall be taken from the bundle after leaving 300mm from the ends. The length ofsample shall be min.1m & shall be verified for unit weight , physical & chemical properties.

TMT bar with hardouter & soft inner cores

Table1

Table2

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The unit weight of rebar's shall be as perTable3. the bars shall weigh with in toler-

ances as stated in Table-4

The first batch received shall be allowed forconsumption in work Up on confirmationfrom approved lab only. For routine receipts,the materials may be allowed to be usedwhile the test is in progress if sufficient reli-ability is achieved in the previous supplies. How ever proper traceability

on the batch received & where it is used shall be maintained.

While in Storage, the Tags shall be kept intact.

How to store the rebar's materials ?

As proper storage increases shelf life prevents rust , contamination &gives good return per investment , it is necessary to preserve the rebar's.

All rebar's shall be stored in gated open yards. The yard shall be developed by leveling ,spreading the quarry dust / stone dust. Minimum slopes shall be maintained for gooddrainage during rains. Elevated platforms shall be constructed using concrete or brick or

wooden sleepers & the bars are stored above ground.

In all the cases stop rolling arrangement shall be made at the sides to prevent rolling of the

bars.

For increased protection from aggressive environments like sea side or rain prone areas ,or in summer the rebar's shall be protected by tarpaulin cover.

In case of rejection the board shall be displayed & No materials shall be consumed fromthe lot.

What are the cutting & bending methods to be followed?

Bars shall not be cut with gas / with welding gun. The bars shall be cut using

Shear cutting machine

Hack saw cutter

Disc cutter

Manually

Bar bending schedule shall be made for every structure for determining the cut-ting length & minimizing wastage.

Bars shall be cut to size & bent using bending table up to 16 mm bars & bend-ing machine for big dia bars.

What care shall be taken during cutting & bending of bars ?

Min. bending radius of 4d shall be maintained for all bars up to 25mm & 6d forabove 25mm.

While bending stirrups, a min. length of 8d shall be left beyond bent portion.

For laps in any member , more than 50% of the bars shall not be lapped in onelocation. The laps shall be staggered by min. of 1.3 times the lap length .

Bars in crossing shall be suitably joggled to avoid level difference & Less cover.

I don't have one diameter as given in drawing , can I use another dia ?

Yes , it is possible to change the dia of the bar , However the area of reinforce-ment shall remain same. This can be achieved by adjusting the spacing of the new dia. But this shall be carried with

the consent & approval of the customer & designer.

What can I do for reducing wastage & how to deal with scrap ?

BBS preparation with careful planning of laps is necessary for the reduction of wastages

Alternatively for similar type of structures bars of required length can be obtained from manufacturer apart fromstandard length. This reduces the wastage to Zero.

The scrap shall be segregated as usable & Non usable scrap.

Bar Dia Per Batch Individual

Sample

10-

16mm

5% -6 %

Above

16mm

3% -4 %

Table3

Table-4

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All usable scrap above 2m lengthcan be reused by giving laps.

How to improve corrosion resis-tance properties of reinforcedbars ?

While working in corrosion proneareas , it becomes necessary to protect the

rebar's from atmospheric action.

Though the cover given to concrete isenough , additional precautions are beingtaken either by using corrosion resistantsteel bars or by providing corrosion resistant coating on the

bars.

The process called as fusion bonded epoxy coating reduces thechances of bond failure between coating & rebar , concrete &also increases the corrosion properties. This is factory appliedprocess & needs touchup at site if any damages are found dur-

ing cutting , bending & installations.

I want to weld the rebar's for fixing the inserts / in lap area ,can I do ?

TMT bars being made through thermo mechanical process ,any application of heat at any place lowers the Tensile proper-

ties.

No welding is recommended for any installation to main bars.You may provide additional bar near the main bar for welding

Rebar's are used to provideTensile properties to the concrete

member.

For the forces are transferred to steel onlyby the bond between concrete & bars. Hence therebar's shall be kept free from all adulterantslike soil , oil , grease , cement , concrete debrisEtc.

Check the material before receipt for manufac-turer , batch & Test Certificates.

Conduct the routine tests on the bars for confir-mation.

Store the material above the ground to preventcontamination.

Cover the rebar's with tarpaulin during summer/ rainy seasons to decelerate corrosion.

Use cutting machines for cutting the bars.

Don't use gas cutters / welding rods , thesemethods alter the physical & chemical propertiesbeyond repair.

Don't carry the welding unless the process &procedure & operators are qualified.

Summary

A.RevathiE&GP OC wrote :

Dear sir, Knowledge Tree regardingEarth work was really very Good , though I

am in electrical part I could get to know about

civil also thru this..

Mr.Bala BrahmamaHyderabad Cluster Qual-ity HeadE&GP OC wrote :

Dear Sir, The details you have provided inApril2010 Volume-2, Issue-4 aboutConcreting in Summer is really helpful tous. The Concrete crossword in this issueis really interesting. We are grateful to youfor providing this information to us and ex-pecting more and more to learn through

KNOWLEDGE TREE.

Your

Voice:

ContactCivilQ

ualityDept

foranyqueries

/Comments

Keep writing.

Your suggestions will

improve this effort.

Disclaimer :

No part of this publication shall be reproduced without the written permission from the Editors. The views expressed here need not be the views of the L&TECCD. L&T doesn't take any responsibility of what so ever for the views expressed by the Editors / Authors. This is for the internal circulation of L&T -ECCD only.

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Concrete Qual-ity depends onRebar Work-

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