Joined: 26 Jan 2003
|Posted: Wed Mar 07, 2012 4:09 pm Post subject: Summary of E-Conf -Days 9 & 10
|Concrete Durability - - - - -- demands sincere EFFORTS from all in chain.
For last 10 days we are discussing about the ways & means for improvements of Concrete
Durability as also the factors / impediments to durable and sustainable concrete.. Experts and
engineers have echoed their views providing sufficient information and sane advice in this
regard touching the conventional concreting as well as RMC .Source ingredient materials
& their testing , their gradation, proper design mix ,w/c ratio, cements and blending, Fly Ash ,
admixtures , mixing , handling and placing , Curing - - - all issues came for discussions.
It may be appreciated that concrete technology is highly expertise field requiring life long
association with either research, or site execution or Testing. Experts have demonstrated
their valuable practical knowledge and experience . I feel that Structural & Civil Designers &
members of the structural Engineering forum of India comprising of about 15000 in numbers
who mostly work in air-conditioned office environments (due to planning/design related
jobs) and have less insight into concrete technology matters of highly practical nature
will be benefited most from the discussions in the e- conference, though I further
admit that everyone in any capacity related to construction world will have tremendous
gains from the deliberations.
Postings on days 9 & 10 - - - - - - - -
Dr Narayan Nayak replied the queries of Mr Bhowmick stating that blending at site by FA may
be preferred upto 50%.There is no problem if blending is done at Cement Plant but that may benefit
cement manufacturer.Due to Indian Codal restriction of 35% FA, cement plants are using less than it.
Knowledge in this area is adequate and NPC Ltd is using FA replacement upto 50%.in some NPP.
Concrete mixing shall be done using batching plant and Pan mixer. De-shuttering time needs to be
increased. Quality of fly Ash must meet requirements of IS 3812.
Er Jignesh thanked Dr Nayak on detailed explanation on pile cut-off level posted yesterday and will
study his BOOK on Foundations.
Conference Moderator and highly acknowledged Concrete Technologist Mr Vijay Kulkarni provides yet
another posting about chemistry of cement production.The major factors responsible for durability
problems is due to changing characteristic of Portland cement.Over years the C3S content and
fineness has increasing trends causing speedy reaction and high early strength but accompanied
with high HOH. In view ofthe changing chracter of cenets , the design strength are achieved with
low cement contents and higher w/c ratio . Higher speed of construction is need of present times.
Use of SCM's as modern day cement reduces HOH as well as peak temp and its early occurance.
Researchers feel that resulting microstructure with SCM is far superior to pure OPC concrete. There
is no doubt that a careful choice of SCM must be needed. Under HVFA concrete large portion of
OPC is relaced by FA , Low w/c ratio is must to have early strength.Also low w/c ratio and puzzolonic
reaction of (OPC + Fly Ash ) can guarantee lower permeability, which in turn is Durability.
Dr N Subramanian, the conf -Moderator, provided download -able links for many papers on HVFA
concrete and also provided information and links of papers by Prof Mehta for benefit of construction
community.He also posted information on chemistry of cement which is really interesting. The C3S
content has been increased from 21%(1920) to 56% (1990) whereas C2S content has been reduced
from 48% to 17% during this long decades of cement production.He requested Mr Vijay Kiulkarni
to substantiate on these changing trends towards 53 grades etc. Fineness of cement always
increases HOH .Regarding Curing ,Internal curing is done on 16 highways decks in Newyork
as part of Federal study , and similar studies /research is needed in India to encorage Internal
Curing for large projects.
Er Chintan P states that 90% of civil construction jobs are carried by unskilled/illiterate
manpower . Mindset in present construction is same as 50 yrs back.Improvements are
Mr Dipak Bhattacharya feels that discussion on Fly Ash role will only be blind unless details
of its composition is not addressed and questioned whether it has been standardized.FA is
naturally available material coal, fully burnt , half burnt and is as varying as naturally available coal in its
variable mineral content. He feels that Fly Ash whose mineral content is neither standardized nor in
anybody's control , how can it be used in cement .
Er Vinay Mehta acknowledged the gains from e conference and requests delegates to throw
light on guidelines for repairs and maintenance scheduling of concrete structures. In India and
Third World countries , maintenance is forgotten entity and engineers do not know the
maintenance budgets.The is urgent need for evolution of Code in this regard. He also requested
for information on standards ,if presently exist in this respect.
Madam Alpa Sheth after glancing report referred by Er Jignesh states that the report results seem
doubtful since compressive strength results at room temp are significantly lower for 56 days
than 28 days and therefore advised to stick to reports from reputed journals.
Er Jignesh , very active participating delegate , referred to two documents on use of FA in
concrete work and found the reports to be conflicting in the sense that one advocates use of
FA with caution while other guides for extensive use of FA.He feels that FA based concrete
can be certainly used in moist environment like water conveying/retaining structures ,foundations
but shall be used with lot of caution in dry areas. I admire Er Jignesh for his interest and
logical discussion/conclusions .
Madam Alpa Sheth expressed her thoughts in a transparent manner while thanking Dr Kulkarni
Dr Nayak, Mr Bhowmick, Mr Bhattacharya , Mr Jignesh and others for lively discussions.
The gist of discussions is very nicely summarised. The use of fines has increased in quest
for early gain of strength, though cements are more crack prone.FA address some cracking
problems but not cure for all.FA must confirm to standards and pond ash/bottom ash cannot
be used . Most of FA produced at TPS is not meeting desired standards.To make FA useable
it must be in graded form and grinded to make packaged FA for use, but may cost about cement
and may defeat the purpose.FA performs better in moist environment than arid.FA shall not be
used with high Alkali cements .
Er RK Bola thanked Madam Alpa for well bulleted points and stated that use of FA at Cement
plant is more preferable since QC is stringent at plants whereas at site the lax quality control
and maximization of profit may be the concerns not favoring its use at site.Er LK Jain
responded to it stating that available FA must be of consistent quality and where batching
mixing plant is not there , PPC is option.
Dr N Subramaninan thanked Er HS Harsoda for proving link for CPWD Handbook. He aslo
suggested Handbook of concrete repairs and maintenance by Peter H Emmons and
Mr Vijay Kulkarni, the Moderator , posted his views based on discussions taken so far in
in the e-conf. while stating that discussions are hovering around issues like role of cements
with changing characteristic, Use of SCM's , QA/QC practices, BIS provisions, special concretes
such as HPC, SCC, HVFAC & UHPC etc. Time is there to change from conventional approach
of defining durability thru max cement content, grade of conc, max w/binder ratio,This change
is happening in advanced countries by Performance Specifications referred as P2P.
The P2P addresses the clearly defined End result requirements, conc mix optimization and
field operations conducted to ensure owner's expectations, focus on conc behavior and
characteristics that matter, encouragement to concrete manufacturers to obtain benefits
from unique materials and combinations, technological expertise and knowledge of local
materials. Risks and responsibilities are clearly defined and penalties and incentives are
innovatively incorporated . He feels that P2P has lot of potential in India and referred to the
Dr N Subramanian feels that presently in India where quality of concrete produced even at
RMC is lacking and in absence of specific prescriptive requirements aided with rampant
corruption the concept of P2P may be good for researchers at present. Mr Vijay Kulkarni
responded to the observation of Dr NS and states that it is not his contention that India go
whole hog on P2P since even in USA the performance based concrete examples are
few. However future may lie in P2P and this could be applied in some pilot projects.
Er L K Jain feels that view's of Prof Mehta are valid.Engineers resist even 20% FA addition
due to lack of understanding ,imaginary quality concerns . HVFA has become distant dream.
HVFA cannot be compared with SCC. Also SCC can be developed with FA content in large
proportions. SCC does not mean less durable conc. Responding to the posting of Madam
Alpa , Mr Jain states that in case FA quality is not upto mark, it needs to be processed. Still
approach shall be to utilize all available FA. Parameters which are not important can be
ignored by relaxing the standards.Next step is to utilise pond Ash but uniformity of parameters
is needed. For use of graded and packaged FA, efficient mixers are required and Pan or
double shaft mixers at batching plant are OK but non of inclined rotating drum(including TM)
provide satisfactory mixing of FA. For high Alkali cement high volume FA may be adopted
since FA react with alkali to form a gel ,good for strength and Alkali if remain in cement is a
weakness. Performance of FA mixed in cement during its grinding is marginally better than
mixing FA in concrete mixer. For higher performance and high durability concrete , multiple
blending in form of (OPC+FA+GGBS+silica flume) would be wiser. Er LK Jain's views
expressed in posting are note worthy for high appreciation.
Er V. Veena wants to know about partial replacement of cement by coconut husk ash, Corn
cob ash and peanut shell ash.
Er Jayant , a consulting Er , has posted very valid points regarding Cover (to reinforcement)
in his posting titled COVER STORY. At sites cover is not adequate, cover blocks are of poor
quality .Cover blocks must be of one strength higher than parent concrete. With deficiencies in
cover at places , enemies of concrete durability like moisture, chemicals etc penetrate and
attack reinf leading to corrosion and all efforts of durable concrete get null and void.
PVC cover blocks have better strength and impermeability but bonding with parent concrete
may not be good. He wants his Cover story to be given due attention in the conference
and rightly so, I also think. Er LK Jain stated that cover block shall be strong enough for
100 kg Line load which requires M40 strength and w/c=0.40 .
Dr N Subramanian acknowledged the importance of cover since cover is the real defence
against corrosion. Plastic cover blocks have no bad effects .In USA the concrete surfaces
are not plastered and quality of cover blocks is really good.In India provision of cover blocks
is pathetic , they even use broken tiles /or aggregates .Also increasing cover beyond
certain limit may lead to higher crack width problems. Proper curing strengthens cover
region tremendously .
Er Jignesh states that Mr Dipak Bhattacharya raised valid points regarding quality of FA.
Ash content in overseas coal is only 4 to 6% as against 35 to 42% in India.This is the reason
in overseas TP P that ash storage area is not there whereas Indian TPP have large
areas for ash storage.He also provided comparison data of constituents of FA of
India and overseas. Seeks advice from experts in reference to this data for
consideration of FA usage in concrete .Dr N Subramanian also agrees that not all the
FA produced by TPP are useful for concrete applications. FA are of two categories
namely Class F (Anthracite and Bituminous coal produces) and Class C(lignite and
sub bituminous coals). Class C FA is preferable and used by ready mix suppliers.
FA for use in portland cement concretes shall conform to ASTM C 618 requirements.
FA shall not be mixed at site ,instead use of PPC is preferable.
Er Jignesh feels that FA can be mixed at site also.Specifications indicate FA can be
added to OPC in batching plant with max 20-25% .
Er Jayant Kulkarni discussing relation between FA and chemistry of cement feels that
deliberations have been taken place in e conf without referring to related chemistry
of cement ie basic composition on percentage of C2S & C3S in cement and amount of
Ca(OH)2 getting free after hydration reaction. The use of FA as SCM,will depend
upon the availability of free Ca (OH)2 . He expects experts to share their thoughts
based on chemistry of cement.
Er Alok Bhowmick expressed satisfaction over deliberations of e-conf and feels that opinions
are mostly from experts with academic or research background.Expect s participation
of contractor's representatives and Consultant's perspective since views of those executing
the jobs at site are important.
Madam Alpa Sheth responded to Mr Kulkarni stating that performance based option are favored
by more and more countries.Performance based options is needed for situations not envisaged
by Codes especially for Iconic buildings or precision projects. Thus it is to enhance the codal
requirements and in no way circumvent them.Madam Alpa also agrees with Mr Kulkarni that
option of performance based must be contained in the Codal stipulations. The other important
issue is that in India conc code committee's are controlled by cement industry ,be them there,
but the representation must also be from independent conc technologists, academicians,
researchers and designers.
With much appreciation , expecting to learn more in remaining few days of e- conf
and with best regards
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