REINFORCING STEEL

REINFORCING STEEL

1. General

This portion includes the quality aspects of tension, compression, and temperaturereinforcing steel, including welded wire fabric. The work includes furnishing,fabrication, and placement of reinforcement for cast-in-place concrete, including bars,welded wire fabric, ties, and supports.

2. Requirements

1. All reinforcement shall be free from loose mill scales, loose rust and coats of paints, oil, mud or any other substances which may destroy or reduce bond. Sand blasting or other treatment is recommended to clean the reinforcement.
2. Special precautions like coating of reinforcement may be required for reinforced concrete elements in exceptional cases and for rehabilitation of structures.
3. The supplier shall submit the manufacturer’s records of chemical and physical properties of each batch of billet steel bars and a certificate that the respective material furnished meets the requirements for the steel reinforcement specified. The manufacturer’s records shall include certificates of chemical analysis, tensile and bend tests of the reinforcement.

2.1 Reinforcing bars

Reinforcing steel shall consist of Mild Steel (MS), Cold Twisted Deformed (CTD) orThermo Mechanically Treated (TMT) bars.

1. Mild steel bars: Mild steels are hot-rolled steels having a yield region on their stress-strain curves and exhibit considerable elongation at failure.
2. Cold twisted deformed bars: Hot rolled steel is artificially strengthened by working on it in a cold state, adopting methods such as extrusion, flattening, drawing, twisting etc. The use of steel hardened by extrusion and cold-worked by flattening as reinforcement in members subjected to repeated loads (Such as bridges) is not recommended. As cold-worked steels have no clearly marked yield point, exhibit brittle failure (Elongation less than 3 to 4%).
3. Thermo mechanically treated bars: Thermo mechanical treatment is a process of manufacturing high strength deformed bars in which the bars are intensively cooled immediately after rolling. Sudden reduction in temperature creates an extremely brittle and strong surface layer (Martensite) with a relatively more ductile and less strong core (Ferrite and perlite). This process improves the strength and ductility (Elongation as high as 20%) compared to CTD.
4. The typical cross sections of the bars shall be as shown in figure.
5. The various grades and definitive properties of reinforcing bars as adopted by the relevant BIS are given in table .

       a) Mild steel               b) CTD                      c) TMT

Typical cross sections of MS, CTD and TMT bars.

6. If the steel has excessive surface rust, dust or other deleterious material then it shall be sand blasted. Sand for blasting shall not contain materials deleteriousto the durability of the reinforcement or concrete.
7. All reinforcement shall be pressure washed with fresh water after erection andimmediately before placing concrete.

2.2 Welded steel wire fabric

1. Steel fabric reinforcement shall comply with the requirements of IS: 1566 andshall be delivered to Site in flat mats.
2. Welded intersections shall not be spaced more than:

1. 300 mm for plain round bars.
2. 400 mm apart for deformed high yield bars in direction of calculated stressexcept when used as stirrups.

2.3 Tie wire

1. Tie wire shall conform to the requirements of IS: 280.
2. Tie wires shall be black annealed mild steel having diameter not less than1mm.

3. Storage and handling

1. On delivery, bars in each lot shall be legibly tagged by the manufacturer.
2. Storage of reinforcement shall be on suitable structures, built at minimum 450mm above the ground surface, to prevent damage and accumulation of dirt,rust and other deleterious matter.

 

Proper storage of reinforcement steel

3. Storage facilities shall be such as to permit easy access for inspection andidentification.
4. Reinforcement bundles shall be clearly tagged with bar schedule and bar markreference.
5. The reinforcement shall not be roughly handled, dropped from a height orsubjected to shock loading or mechanical damage.
6. Steel reinforcing bars shall be kept clean and shall be free from pitting, looserust, mill scale, oil, grease, earth, paint, or any other material which mayimpair the bond between the concrete and the reinforcement.
7. The reinforcement shall be covered to ensure protection from condensation andother deleterious materials.

4. Placing of reinforcement

1. Rough handling, shock loading (prior to embedment) and the dropping ofreinforcement from a height shall be avoided. Reinforcement shall be securedagainst displacement outside the specified limits.
2. Spacers, chairs and other supports detailed on drawings, together with suchother supports as may be necessary, should be used to maintain the specifiednominal cover to the steel reinforcement.
3. Spacers or chairs should be placed at a maximum spacing of l.0 m and closerspacing may sometimes be necessary. Spacers, cover blocks should be ofconcrete of same strength or PVC.

5. Welded joints or mechanical connections

1. Welding of high strength deformed bars and wires shall be permitted if theCarbon Equivalent (CE) is more than 0.53 when micro alloys are used and 0.42when micro alloys are not used. The value of CE is determined as per IS: 1786.
2. Welding of reinforcements shall be done in accordance with therecommendations of IS: 2751and IS: 9417.
3. When welded joints or mechanical connections in reinforcementare used, quality tests as per IS: 1786 shall be carried to prove that the jointsare of the full strength of bars connected. The frequency of tensile and bendingtests is one for every 100 joints or as decided by the Engineer.                                                                                

Couplers for reinforcing bars

6. Tolerances

1. Unless otherwise specified in the Drawing, the reinforcement shall be placedwithin the following tolerances:

1. For effective depth equal to 200 mm or less = ±10 mm
2. For effective depth more than 200 mm= ±15 mm

2. Unless specified otherwise, actual concrete cover shall not be deviated from therequired nominal cover +10 mm. No tolerance is allowed for clear cover.

7. Inspection, sampling and testing

1. Representative samples of reinforcing steel of all sizes that proposes to use in the Works must be submitted to the Engineer along with themanufacturer’s certificate, before a work is commenced, for his writtenapproval.
2. Manufacturer's certificates shall clearly state for each sample:

1. The place and date of manufacture.
2. Quantity delivered.
3. All relevant details of chemical composition, manufacture, strengths andother qualities of steel.

3. Allow 14 days for review and approval of samples by the Engineer.
4. Steel shall be delivered at Site only after approval of samples by the Engineer.
5. After delivery at Site, shall carry out tests for reinforcing steel foreach bar size required for 1st tier quality control at least two weeks beforecommencement of concrete works.
6. Tests shall be carried out in accordance with BIS.
7. The tests include those for dimensions, weight, tensile strength, bend and/orrebend, at his own cost, for each size of bar to be used in the concreteconstruction.
8. Frequency of sampling and the method of quality control shall be in accordancewith IS: 10790(Part 2).
9. Where epoxy coated steel is used, 0.3 kg samples of the coating, material fromeach batch be supplied in an airtight container and identified by the batchnumber.
10. Tensile tests providing information on following shall be done as per frequencydetailed in table
11. Elastic limit.
12. Ultimate strength.
13. Stress/strain curve.
14. Cross-sectional area.
15. Deformation/bond characteristics of deformed bars.
16. Manufacturer’s certificate shall be sufficient for chemical analysis. TheContractor shall arrange for chemical analysis of the steel supplied, at his owncost, if the manufacturer’s certificate is not available.
17. Tests shall be repeated if the source of supply of reinforcement changes.
18. For checking nominal mass, mechanical properties, bend test and rebend test,test specimen of sufficient length shall be cut from each size of the finishedbar/wire at random at a frequency not less than that specified in table.
19. The Engineer shall instruct the Contractor to break out and remove completelyall sections of the work already constructed under any of the followingcircumstances:
20. Reinforcing steel sample under test fails to meet the specificationrequirements at any time.
21. The Engineer considers that samples which were presented to him for testwere not truly representative.
22. It becomes apparent that reinforcing steel which has not been approved hasbeen used on the Works.

Test for reinforcing Steel

7.1 Field test for TMT bars

To ensure that TMT bars have a concentric tempered martensite periphery witha softer ferrite-perlite core structure, the Etching test or Ring test as per AnnexAof IS: 1786 shall be carried out.

A small piece (about 12 mm long) can be cut and the transverse face lightlyground flat on progressively finer emery papers up to ‘0’ size.

The sample can be macro etched with nital (5% nitric acid in alcohol) atambient temperature for a few seconds which shall then reveal a darkerannular region corresponding to martensite microstructure and a lighter coreregion.

8 Acceptance criteria

Acceptance or rejection of reinforcement steel is mostly based on tensilestrength. However, other test results of the steel such as chemical composition,bend/rebend is also important.

The test results of reinforcing steel shall deemed to comply with the criteriaspecified in tables .

Chemical composition of high strength deformed bars and wires

 Nominal cross sectional area, mass and tolerances of steel bars & wires

NOTE:  For the purpose of checking the nominal mass, the density of steel shall be takenas 0.00785 kg/mm2 of the cross-sectional area per metre

Mechanical properties of high strength deformed bars and wires

NOTE: TS – Tensile strength; YS -0.2 % proof stress/yield stress

RELATED CODES:

1.IS:228-Methods for chemical analysis of steels.(Parts 1 to 24)

2.IS:226-Structural steel.

3.IS:442-Specification for mild and medium tensile steel bars and harddrawn steel wire for concrete reinforcement:

(Part-1)-Mild steel and medium tensile steel bars.

(Part-2)-Hard drawn steel wire.

4.IS:280-Mild steel wire for general engineering purposes.

5.IS:432-Specification for mild steel and medium tensile steel bars andhard-drawn steel wire for concrete reinforcement:

(Part 1)-Mild steel and medium tensile steel bars.

6.IS:1786-Specification for high strength deformed steel bars and wires forconcrete reinforcement.

7.IS:1566-Specification for hard-drawn steel wire fabric for concretereinforcement.

8.IS:13620-Fusion bonded epoxy coated reinforcing bars –specification

9.IS:10790-Methods of sampling of steel for prestressed and reinforcedconcrete:

(Part 2)-Reinforcing steel.

10.IS:1387-General requirements for the supply of metallurgical materials

11.IS:1599-Method for bend test.

12.IS:1608-Metallic materials - tensile testing at ambient temperature

13.IS:2062-Hot rolled low, medium and high tensile structural steel(6th revision).

14.IS:2502-Code of practice for bending and fixing of bars for concretereinforcement.

15.IS:2770-Methods of testing bond in reinforced concrete:

(Part 1)-Pull-out test.

16.IS:2751-Code of practice for welding of mild steel plain and deformed barsfor reinforced concrete construction.

17.IS:9417-Recommendations for welding cold-worked steel bars forreinforced concrete construction.

18.IS:11587-Structural weather resistant steels.

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