Steel reinforcement is a key component of reinforced concrete structures. Cold twisted deformed (CTD) rebars were widely used in India from the 1970s due to their higher yield strength compared to plain mild steel, allowing for less steel to be used. However, CTD rebars have inherent problems including inferior ductility, weldability, and increased corrosion due to residual stresses and higher carbon content. Thermomechanically treated (TMT) rebars, introduced in the 1980s, provide higher strength with better ductility, weldability, bendability, and corrosion resistance compared to CTD rebars. TMT rebars have now become the dominant rebar type used in India.

1. STEEL REINFORCEMENT-
An Overview
A PRESENTATION BY
Rajib Chattaraj
Asst. Chief Engineer
National Highway Wing,P.W.(Roads)D.
December- 2011, Amended on november’2014

2. • 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.

3. • 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
prestressed concrete structures, the reinforcement
(steel wires) are stressed prior to subjecting the
structures to loading, which may be viewed as active
reinforcement.

4. 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.
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.

5. • 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.

6. • 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.

7. • 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
• 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.

8. Typical Cross Section of MS, CTD and
TMT Rebars

9. 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.

10. • 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.

11. 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
• 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.

12. • 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.
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.

13. 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.
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.

14. • 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.
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.

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

16. 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 :-

18. • 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.

19. • 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.

20. • 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
• 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.

21. 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,
• 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.

22. The pictures of different types of
CTD Rebars used in India

23. • 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.

24. The picture shows the difference
between the quality of CTD and TMT
Rebars

25. 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
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.

26. Picture of the Micro Alloy Rebar

27. • 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
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 :

28. 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 Addition of High (due to Good Poor (due to Moderate
alloying alloying
elements as
C, Mn, V, Nb
etc.
alloying
elements and
equipments)
high carbon
equivalent)
3. TMT Rapid
Cooling and
Controlled
Cooling from
rolling heat
Lower Excellent Excellent Good.

29. -: 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.

30. • 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
• 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.

31. • 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.
hardened periphery.
• The equalizing temperature together with the final
rolling temperature is the most important parameter
to achieve the required mechanical properties.

32. The figure illustrating typical manufacturing
process of TMT rebars

34. The Microstructure of T.M.T.
Bars showing peripheral rim
of tempered martensite and
core of ferrite- pearlite
Tempered Martensite Rim Ferrite-Pearlite core

35. Different TMT Rebars used in India

36. 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.
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.

37. • 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
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.

38. 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.

39. • 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
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.

40. 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.

41. • 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
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.

42. • 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.
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.

43. 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.
Phosp
horus
(%)
Max.
S+P
(%)
Max.
Yield
strength
min.
(MPa)
Elongati
on %
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 -
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.

44. Tensile strength testing of rebars
Universal Testing Machine The sample of rebar after the test

45. 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.

46. 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

47. Chemical composition testing of rebars
Testing of chemical composition of rebar by Spectrometre
machine along with computer software

48. 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, 180
0
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
• Rebend test: As per cl.9.4 of IS:1786(2008),the test piece
shall be bent to an included angle of 135
0
using a
mandrel of appropriate diameter. The bent piece shall be
aged by keeping in boiling water(100
0
C) for 30 min and
then allowed to cool. The piece shall then be bent back
to have an included angle of 157.5
0
. There should not be
any rupture or cracks on the bended/rebended portion.

49. Pictures of bend and rebend test
Bend Test Template for bend and rebend test

50. -: 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.
generally considered for characterization of rebars.
• From design point of view : strength, ductility and
corrosion resistant properties of rebars are
important.

51. • 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.

52. • 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
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.