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.
- 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
- 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.
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