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This forum is locked: you cannot post, reply to, or edit topics.This topic is locked: you cannot edit posts or make replies.    www.sefindia.org Forum Index -> E-Conference 20th June 2005
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dcrai
E-Conference Moderator


Joined: 26 Jan 2003
Posts: 9

PostPosted: Tue Jul 05, 2005 8:30 am    Post subject: Moderators' Post Reply with quote

Dear Colleagues,

It has been an interesting week on this e-Conference, though there have
been fewer postings than expected. We are aware that a few of your
postings may have been lost as our servers were somewhat erratic in the
beginning. It seems that as structural engineers we are little bothered
about the issues of steel reinforcement beyond the available yield
strength and bar diameters. We hope that you are not thinking that the
other issues related to manufacturing, marketing, transportation, bar
detailing, bar bending, handling at the site, arranging reinforcement in
formwork, etc., and quality control at each of these steps, have little to
do with you! It is true that there are people who specialize in each of
these areas and are the part of the project team, but it is the structural
engineer who has prepared the ‘blue print’ of all that goes on and their
role is no less crucial even for steel reinforcement.

An interesting design related issue that came up quite early regarding the
shear strength of rebars, especially when they are used to  ‘bridge’ a
potential shear plane. A great deal of experimental research is available
with “push-off” specimens tested either uncracked or pre-cracked shear
plane. The results indicate that in uncracked case, the diagonal cracks
which appear first, rotate with further load stretching the rebars and
subjecting them to tensile stresses rather than shearing stresses. In
initially cracked specimens, the relative slip causes the separation of
surfaces (i.e., movement normal to the crack, because the cracked surfaces
are never smooth!), as a result rebars are elongated and hence are
stressed in tension. An excellent review of these can be had in a report
of ACI-ASCE committee 426 in ASCE Structural Journal, June 1973.

A couple of references were made of IS 13920 regarding properties of
rebars for seismic applications.  Generally, for such applications there
are specific requirements in three areas which are different from normal
rebars: (a) Elongation at least in the range of 12-14%, (b) the ratio of
UTS to YS greater than 1.25 with an upper limit on YS and finally, (c) a
low YS. Meeting the elongation requirement has not been a problem with the
available rebars of all types and probably all grades. However, the
problem lies in fulfilling the requirement of UTS to YS ratio, which even
for good quality TMT bars available in Indian market are in the range of
1.11 to 1.16.  A larger ratio means significant strain hardening which is
necessary to ensure adequate plastic hinge length and plastic rotation
capacity. Further, higher than expected yield strength can lead to brittle
modes of failure in shear or bond.

Until recently (March 2002), IS 13920 did not permit the use of higher
grade rebars (Fe 500 or greater) and many believe that it was due to poor
elongation ability of available Fe 500 or greater rebars. On the contrary,
it is mostly to do with the ductility of RC sections and members, an
appreciable value of which are possible easily with low grade (YS) rebars
(along with low grades of concrete, preferably less than M35).  Higher
grades of material (steel and/or concrete) usually result in slender
member with small sections with smaller volume of concrete to confine for
energy dissipation and hence lower ductility.  Furthermore, there are
problems associated with bond and anchorage as well. As a result,
traditionally, usage of higher grades of material has been discouraged in
seismic applications. We probably need more focused experimental studies
to quantify these effects, especially now when we have decided to use
higher grades of steel.

There are many questions or rather concerns with the advent of TMT rebars.
Many question its very name and argue that all rebars, including CTD are
subjected to thermal and mechanical processes and therefore are valid
TMTs. They prefer to use the term ‘quenching and tempering’ which is more
descriptive of the manufacturing process and probably ‘exact’.
Manufacturers would probably like designers to specify the rebars by the
brand name that they are promoting and that is where the role of BIS is
very crucial in providing a level playing field, which benefits all and
promotes competition.

The importance of marking each rebar at specified interval can not be
emphasized further and it should include information about the
manufacturer (code name), type of steel (virgin, scrape or re-rolled), bar
diameter and Grade (YS) and possibly the method of manufacturing (TMT vs
CTD).  Of course, one can mark them without affecting the pattern of lugs
(or deformations), especially in the transverse direction for any loss of
bond, etc.

Inadequate quenching can lead to really poor elongation and strength
degradation in case of TMT bars. It has been pointed out that there are
many instances in which no or inadequate quenching of so called TMT bars
were found. Are there some simple methods to detect these or one has to
only rely on tension tests?  TMT bars manufactured by vendors other than
SAIL or TISCO are available in the market, and due to their lower prices,
are preferred by contractors and builders. We cannot be sure of the
frequency of testing at the manufacturers' works, and what happens to the
lot from which the tested specimen has failed to meet the needed
requirements. Such lots from which the failed specimens have been taken
may very well find their way into the market. It is therefore necessary to
conduct frequent testing of such rebars manufactured by the minor players
before they are used in the works.

TMT bars are weldable only when the chemical composition is right, say
equivalent carbon content is below say 0.55% and carbon less than
0.2-0.25%. How this can be ensured, especially where recycled steel is
used without adequate quality control?

There is still a week to go and we would like to hear from many of you who
are silently reading the proceedings of the e-Conference. On individual
level, we keep hearing about concerns that you have about steel
reinforcement and now it is time to share this with us and all in this
profession.

With best wishes for more active e-Conferencing,

Durgesh Rai and Indrajit Barua
Moderators



~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Durgesh C Rai, PhD
Dept. of Civil Engineering
IIT Kanpur 208 016
Ph. 0512 259 7717 (o) 8236 (h)
Fax 0512 259 7866

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