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Moderators’ Closing to E-Conference on Steel Reinforcement

 
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dcrai
E-Conference Moderator


Joined: 26 Jan 2003
Posts: 7

PostPosted: Tue Jul 26, 2005 5:35 am    Post subject: Moderators’ Closing to E-Conference on Steel Reinforcement Reply with quote

Moderators’ Closing  to E-Conference on Steel Reinforcement (27 June – 24
July, 2005)


Dear Colleagues,

It has indeed been a wonderful four weeks to talk about a material that we
use every day in our profession – steel reinforcement -- covering aspects
ranging from its metallurgy to its arrangement in the RCC member, and
cutting across several specialized professions, at this forum of
structural engineers, because reinforcement is critical for the structures
they design. Though the interest in the subject at the e-Conference grew
slowly and we very rightly extended its duration from the initial two
weeks, this definitely helped to cover more bases. While the discussion
has finally come to an end, we really look forward to continued
interaction on the SEFI site on this important topic.

In the following sections, we have made an attempt to briefly summarize
what has been discussed in this conference. However, at places we have
taken liberty to include our thoughts as well. We hope this helps!

1. Shear strength of reinforcing bars
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 to develop the so-called ‘dowel action’. Experiments
indicate that rebars are not likely to subjected to shear stresses and
almost always stressed in tension -- even in cases where seemingly they
are resisting shear.

2. Requirements of rebars for earthquake resisting members
For seismic applications there are specific requirements in three areas
which are different from normal rebars: (a) Total 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. Often, ductility of RC
members is directly linked to total elongation available for rebars used
in the member, which is not entirely true. Meeting elongation requirements
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 and an upper limit on YS. A larger UTS to YS ratio means
availability of 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, which is highly undesirable because such failure can be
sudden and violent. The Super Ductile rebar from Tata Steel is claimed to
have similar properties and it is time that other manufacturer also offer
similar products for earthquake resistant RC members. BIS committee CED 54
should be persuaded to include the rebars with these properties in IS
1786. As pointed out it, these efforts seem similar to what led to the
development of ASTM A706 rebars many years ago in the US.

Until 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, such
restrictions had mostly to do with the ductility of RC sections and
members, an appreciable value of which is 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 be
confined for energy dissipation, and hence lower overall ductility.
Furthermore, there are problems associated with bond and anchorage as
well, not to mention possibly aggravated problems of time-depemdent
deformations due to shrinkage and creep. 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 IS codes are permitting use of higher grades
of steel. Many participants now seem to accept Fe500 grade steel, but not
higher grades.

3. Shear capacity of strain-hardened plastic hinge
There has been a considerable debate over the factor 1.4 to arrive at the
moment capacity of the strain-hardened plastic hinge and hence the
required shear strength of the member to prevent its brittle shear
failure. Considering that at present there is no upper limit on the YS,
the factor 1.4 may not conservative enough. Though the number has been
often justified as a ratio of 1.25/0.87 (i.e., probable UTS value divided
by YS reduced with partial safety factor for design), it in fact attempts
to provide an expected upper bound moment strength of a plastic hinge due
to many factors. It is therefore necessary to keep a reasonable limit on
the maximum value of YS as it has direct bearing on the shear design of RC
framing members.

4. Weldability of rebars and mechanical connectors
Ability to weld rebars can make job at construction sites much easier. TMT
rebars are as easy to weld as CTD bars, however, one has to make that the
metal chemistryis right and carbon content is no more than 0.25% and
equivalent carbon content is below 0.55%. It is not clear how this can be
ensured, especially where recycled steel is used without adequate quality
control for manufacturing rebars. It has been pointed out that welding of
TMT bars are no different from conventional bars and IS 1786 may have some
provisions to ensure adequate welding for TMT bars. Some discussion took
place about the relative suitability of two kinds of couplers available
for TMT bars, i.e., taper threading versus cold forged parallel threading.

5. Corrosion protection of rebars
Much has been argued and debated over the efficacy of various corrosion
protection techniques for rebars. It emerges that galvanized rebars not
only offer adequate corrosion resistant but also have least adverse
effects on other important characteristics. The epoxy coated rebars (ECR)
may not be as effective and can cause premature deterioration, due to easy
breakage of the protective layer and failure to form the composite layer
due to chemical reaction with cement concrete as it occurs with bare
rebars. This not only makes such rebars susceptible to crevicecorrosion
but also causes easy loss of bond with the surrounding concrete. In
comparison, the protective zinc layer in galvanized rebars is a
continuously adherent film that does not break easily and results in
better bond. It was further pointed out that red oxide in the cement as
corrosion inhibitor is not at all effective as its presence in cement
concrete does not lead to development of an adherent protective layer over
rebars.

6. Marking rebars for identification
It has been pointed out that the marking on rebars should be such that it
is easily identified at the site and are placed in accordance with the
structural drawings. Marking brand name (or manufacturer) and/or
manufacturing process alone are not sufficient. Marking should include
information about the manufacturer (code name), type of steel used
(virgin, scrap or re-rolled -- to indicate its metallurgy), bar diameter
and Grade (YS) and 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. However,
it has been pointed out the current marking of brands etc. do not
adversely affect the bond characteristics of rebars.

7. Testing of TMT rebars for its authenticity of manufacturing process
It appears that the TMT bars can not be easily differentiated as CTD or MS
rebars. One has to perform a bit of involved macro-etching to verify the
presence of different crystal structures arising out of quenching and
tempering processes, i.e., martensite near the surface and pearlite
/ferrite inside. 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. 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 could 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.


Thanks again for being with us for four long weeks and I hope you enjoyed
it as much as we did. We know many of you have silently read the
proceedings of the e-Conference and I hope you found it useful. We would
like to thank all those who wrote and actively participated in the
discussion and supplied a great deal of information about steel
reinforcement. We still feel that there is still a lot to be communicated
to all the stakeholders about the reinforcement, which include architects,
builders, designers, labourers, dealers, goverment officials, end users,
etc.As echoed by one of the participants, all this information should be
available in a language that is easy to understand and the contents should
be reliable.

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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