steel reinforcement for home builders

1.      What is steel reinforcement? Why is it required in a concrete structure?

 Concrete is a material that is very strong in compression, but weak in tension. To compensate for this imbalance in a concrete slab behavior, steel reinforcement bar is cast into it to carry the tensile loads. The surface of the reinforcement bar may be patterned to form a better bond with the concrete. Steel should necessarily be strong in tension and, at the same time, be ductile enough to be shaped or bent. Steel rebar is most commonly used as a tensioning devise to reinforce concrete to help hold the concrete in a compressed state.

The aim of the reinforced concrete designer is to combine the reinforcement with the concrete in such a manner that sufficient of the relatively expensive reinforcement is incorporated to resist tensile and shear forces, whilst utilizing the comparatively inexpensive concrete to resist the compressive forces.
To achieve this aim, the designer needs to determine, not only the amount of reinforcement to be used, but how it is to be distributed and where it is to be positioned. These decisions of the designer are critical to the successful performance of reinforced concrete and it is imperative that, during construction, reinforcement be positioned exactly as specified by the designer.
Steel rebars can take both tensile as well as compressive loads. Steel & concrete have got similar temperature coefficients, making them expand/contract together and help the bonding.
The welding of reinforcement is generally not permitted for high tensile steel, since heating of hot rolled bars causes brittle fracture in the reinforcement. In the case of cold worked deformed (CTD) steel bars, heating causes the reinforcement to revert to mild steel as it loses the effects of strain hardening. Welding is normally permitted on mild steel and in some cases quenched and self tempered steel rebars.

2.      What are the different grades of reinforcement steel available?

Mild steel bars conforming to IS: 432 (Part I) and Cold-worked steel high strength deformed bars conforming to IS: 1786 (grade Fe 415 and grade Fe 500, where 415 and 500 indicate yield stresses 415 N/mm2 and 500 N/mm2 respectively) are commonly used. Grade Fe 415 is being used most commonly nowadays. This has limited the use of plain mild steel bars because of higher yield stress and bond strength resulting in saving of steel quantity.
Fe 500 D, Fe 550D (where D denotes ductile) and Fe600 grade in normal, EQR (Earthquake Resistant) & CRS (Corrosion Resistant) quality.

3. What is TMT bar? What is the difference between TMT, Tempcore and Thermex reinforcement bars?

Thermo Mechanical Processing, also known as Thermo-Mechanical Treatment (TMT), is a metallurgical process that integrates work hardening and heat treatment into a single process.

Thermex and Tempcore are both reputed trade names registered by the two major providers of TMT technology. When properly operated both Thermex and Tempcore can produce good quality TMT rebars. Both are highly complex technologies involving specialized computerized equipments to control and produce desired grade and quality of steel.
Manufacture of high strength reinforcing steel is usually carried out through one of three alternative process routes:
1. Hot rolling after micro-alloying
2. Hot rolling followed by cold work and
3. Hot rolling followed by heat treatment (TMT).

The TMT process is actually a heat treatment imparted to steel on-line just after rolling.

4.      What are the different sizes of TMT available?

Normally TMT is available in 6-50 (mm) (6,8,10,12,16,20,25,28,32,36,40,45,50), where 6-12 mm is available in coil as well as straight form .Cold-worked steel high strength deformed bars start from 8 mm diameter. For general house constructions, bars of diameter 6 to 20 mm are used.

5.      Quality factors to be cheked during purchase of steel reinforcement:

• Weight/unit length: TMT Rebars should have standard weights, lengths as mentioned in the standards (IS 1786).
• Bond Strength: TMT Rebars should have uniform and precise parallel rib pattern, which ensures excellent bonding with concrete, and provides superior strength to the buildings.

• Dimensions: Rebars should have close dimensional tolerance and superior surface finish,
• Bendability: Rebars must have high elongation so that they are capable of absorbing large amount of energy by plastic deformation before undergoing failure by fracture. This facilitates easy bending, making work easier and faster at construction sites.
• Weld-ability: TMT Rebars should have low carbon equivalent which is directly related to hydrogen induced cold cracking (the most common weld defect for steel). Higher concentration of carbon and other alloying element tend to increase hardness and decrease weld-ability. TMT process can produce the desired yield strength with steel of relatively lower carbon equivalent by adjusting the on-line heat treatment parameters. Low level of carbon content leading to excellent weldability.
• Corrosion Resistant: TMT Rebars should have minimum or negligible residual stresses in order to prevent any chance of rusting.
• Earthquake Resistance: TMT Rebars having high UTS/YS ratio signifies that steel is capable of elongating to a large extent when loaded beyond yield point as in the situation of an earthquake. It is with this view that some of the international specifications stipulate fairly high values for the UTS / YS ratio
• Durability & Strength: TMT Rebars should have very low level of Sulphur (S) and Phosphorous (P) as they reduce the strength of the material and make them brittle. Excess level of S can lead to hot shortness wherein the melting point of steel gets lowered, thus making steel more prone to failure under extreme high temperature conditions. Excess level of P can lead to cold shortness wherein the steel undergoes brittle fracture while working in extremely cold condition and thus becomes more prone to cracking.
• Cost Savings: TMT Rebars having high UTS/YS ratio and high elongation results in lesser consumption of the similar TMT of the same sectional properties but with low UTS/YS ratio and lesser elongation. Usually TMT with high strength saves about 10-12%  steel than corresponding lesser strength TMT.
• Manufacturing technology: TMT Rebars should be produced using the superior and clean steel billets with highly controlled steel chemistry with very low levels of sulphur and phosphorus (less than 0.035% Lower the level of sulphur and phosphorous, more is the strength of steel.), and very low levels of inclusion and tramp elements. To ensure this Check the manufacturers website to identify manufacturing process.
• Steel test certificates are issued from NABL (National Accreditation Board of Laboratories) certified Laboratory.

                Physical properties of various grades of steel reinforcement

Chemical properties of various grades of steel reinforcement

                                                

6. Corrosion resistant (CRS) Rebars:

Steel reinforcement bar corrosion is a particular problem where the concrete is exposed to salt water. Uncoated, corrosion resistant low carbon chromium alloyed, epoxy coated, galvanized or stainless steel rebars can be used in this situation at greater initial cost, but at significantly lower cost over the service life. Extra care is taken during the transport, fabrication, handling, installation, and concrete placement process when working with epoxy coated rebar, because damage reduces the long term corrosion resistance of these rebars

Advantages of using CRS rebars:

• Longer Life, suited for coastal environment.
• High yield strength coupled with good ductility and bendability.
• No extra precaution required in material handling and transportation.
• No maintenance required during fabrication.
• More suitable under poor workmanship conditions at site.
• No extra precaution during welding.
• Can be bent and re-bent around very small mandrels.
• Can be used as a reinforcement material in the seismic zones.

7.      What is the checklist for steel reinforcement before the placement/pour of concrete?

• Steel bars are clear, free from loose mil scales, dust and loose rust ,coats of paints, oil or other coatings which may destroy or reduce bond strength.
• Steel bars should be stored in such a way as to prevent corrosion.   
•  Steel bars should not be clean by oily substance to remove the rust.
• The bar is bent correctly and accurately to the size and shape as shown in drawings. 
•  If possible, bar of full length is used.
• Overlapping bars do not touch each other and these should be kept apart with concrete.
• The overlap if given should be staggered.
• The cranks in the bar at the end should be kept in position by using spots.
• The steel bars should not be disturbed while lying cements concrete,
• Reinforcements shall be placed and tied such that concrete placement is possible
• Without segregation, and compaction possible by an immersion vibrator.
• Required cover under steel bars should be given before laying the cement concrete

 

8.      Why cover blocks are required to be placed before concreting? What are their sizes?

Cover blocks are placed to prevent the steel rods from getting exposed to the atmosphere, and to place and fix the reinforcements as per the design drawings. Once the steel is exposed to the atmosphere, corrosion starts. Sometimes it is commonly seen that the cover gets misplaced during the concreting activity. To prevent this, tying of cover with steel bars using thin steel wires called binding wires (projected from cover surface and placed during making or casting of cover blocks) is recommended. Covers should be made of cement sand mortar (1:3). Ideally, cover should have strength similar to the surrounding concrete, with the least perimeter so that chances of water to penetrate through periphery will be minimized. Provision of minimum covers as per the Indian standards for durability of the whole structure should be ensured.

                                                            Cover block supporting steel

Shape of the cover blocks could be cubical or cylindrical. However, cover indicates thickness of the cover block. Normally, cubical cover blocks are used. As a thumb rule, minimum cover of 2" in footings, 1.5" in columns and 1" for other structures may be ensured.
Cover has the most significant effect on the long term durability of reinforced concrete and therefore of the structure. Excess cover should be avoided as micro cracking due to bending stress can result in the growth and development of cracks and resulting corrosion of reinforcement or member loss due to spalling. The correct cover is required to ensure that reinforced concrete members meet their specified design requirements.

Lack of cover on parapet anchors/starter bars results in loss of durability, pop outs and corrosion of steel reinforcement bars.

9.      How important are transverse reinforcements like links and stirrups? What precautions should be taken while tying them?

Transverse reinforcements are very important. They not only take care of structural requirements but also help main reinforcements to remain in desired position. They play a very significant role while abrupt changes or reversal of stresses like earthquake etc.

They should be closely spaced as per the drawing and properly tied to the main/longitudinal reinforcement.

10.  What is a lap or development length? Where and how should they be provided?

Lap length is the length overlap of bars tied to extend the reinforcement length.. Lap length about 50 times the diameter of the bar is considered safe. Laps of neighboring bar lengths should be staggered and should not be provided at one level/line. At one cross section, a maximum of 50% bars should be lapped.

In case, required lap length is not available at junction because of space and other constraints, bars can be joined with couplers or welded (with correct choice of method of welding).

11.  What is anchorage length?

This is the additional length of steel of one structure required to be inserted in other at the junction. For example, main bars of beam in column at beam column junction, column bars in footing etc. The length requirement is similar to the lap length mentioned in previous question or as per the design instructions.

12.  For a given diameter, how do we calculate the weight of steel per meter length of the steel bar and vice versa?

For any steel reinforcement bar, weight per running meter is equal to d²/162 Kg, where d is diameter of the bar in mm. For example, 10 mm diameter bar will weigh (10x10)/162 = 0.617 Kg/m

13.  What is the Stress-Strain curve? what is the importance of YS, UTS and

       Percentage Elongation?

Stress-strain curve is an extremely important graphical measure of the material’s mechanical properties
Yield stress: Is the stress a material can withstand without permanent deformation, Yield strength is usually stated as the stress at which a permanent deformation of 0.2% of the original dimension will result, known as the 0.2% yield stress.
OR
The stress at which a non-proportional elongation equals to 0.2 % of the original gauge length takes place (where gauge length is 5.65sqrt A. where ’A’ is the cross sectional area of the test piece).
Ultimate Tensile Strength: The maximum stress a material can withstand in a tensile test.
OR
The maximum load (force) reached in a tensile test divided by the effective cross sectional area of gauge length portion of the test piece.
Percentage Elongation: When a material is tested for tensile strength it elongates a certain amount before fracture takes place. The two pieces are placed together and the amount of extension is measured against marks made before starting the test which is expressed as a percentage of the original gauge length.

 contact rabishnair@yahoo.co.in