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E-conference on Concrete Durability

 
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narayan_nayak
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PostPosted: Fri Mar 09, 2012 10:08 am    Post subject: E-conference on Concrete Durability Reply with quote

Dear All,

I am very happy at the response generated by this conference, views expressed, concern / apprehensions raised and clarifications provided etc.

This is probably the last opportunity to express my views as the closing date is approaching fast. Hence my views / clarifications on some of the issues raised by experts are as follows :-

1. Cover Blocks - Mr. Jayant Kulkarni has rightly pointed out importance of the cover blocks and definitely at majority of the sites quality of cover blocks provided is far from satisfactory i.e they are very poor. In fact even the importance good cover blocks and good croncrete in cover zone is not understood by many. As a result as pointed out by one of the experts people even provide Gravel Piece, Mozac tile piece as cover blocks which definitely fall down during the concreting operation. There are cases where even wooden block has been used as the cover block!

Hence to overcome all these difficulties and to ensure that cover blocks are of good quality, as pointed out by me in the one of the first views, in Gammon Realty Ltd. we provide prefabricated concrete cover blocks of M60 grade concrete. These are excellent and can be used for all grades of concrete as long as grade of concrete is M60 and below. For higher grade we manufacture specially at site where we provide vibrating table for compaction of the concrete of the cover blocks and this concrete is of the same grade as that in the structure in consideration.

Yes, some prefer PVC cover blocks as they are impervious and generally sturdy. I have used PVC cover blocks at some of our important projects earlier but now I avoid them and prefer concrete blocks of M60 grade. The reason for this is that there will be compatibility issue between PVC and Concrete and there has to be micro crack / opening between PVC and concrete. This can be verified by the microscopic observations, of course I have not done it.

2. Fly Ash - Many have reservations / apprehensions for using fly ash in concrete. Many feel that fly ash is "ash" and should not be used in concrete !

When we say we should use fly ash, we mean we should use processed fly ash, processed to meet IS codal requirement. I have found that whenever we use processed fly ash, the results are very satisfactory. Further, when we use such fly ash, we at regular interval get it tested and there are agencies available for such testing. In majority of the cases we have found they meet the codal requirements.

Unlike this we accept micro silica with the certification given by the supplier and use it in cocnrete without any test !

It may be pertinent to point out here to all concerned that Indian produced fly ash (processed fly ash) is exported to other countries for use in concrete. Dirk India Pvt. Ltd. alone exports 40% of their product mainly to Middle East, East Africa and Asian countries. In terms of volume it is approximately 400 t/day.

Many have expressed concern about quality of fly ash and hence would like to refrain from its use in concrete. What is the difficulty when it meets IS codal requirement? Why nobody has raised quality issues regarding aggregate particularly fine aggregate? Quality of natural sand in Mumbai is very poor, quality varying from season to season, some even use creek sand!

Majority use coarse in concrete aggregate produced from uncontrolled industries. How many are bothered about presence of mica or alkali silica activity or even deleterious substances!

My sincere advice is please use processed fly ash meeting codal requirement to make concrete more durable and greener. Use of fly ash improves concrete resistance to alkali-silica reaction, to sulphate attack, better resistant than OPC concrete in subsoil conditions containing industrial waste. This is based on hardcore experience at sites where I have carried out large number of tests including strength tests for 180 days.

Regarding the pond ash, my advice is to use this pond ash as fine aggregate only in concrete that too on carrying out experiments. Further I have used the same for producing fly ash bricks and it is found to be quite good with respect to strength and permeability.

Some people have reservation of adding fly ash at site, as they are concerned about quality of the concrete produced. I prefer mixing at site as long as one has computerised batching plant. In such a case we can use more fly ash than presently used by manufacturer and it is found to be cost effective. It may be realised that processed fly ash is still chapter than cement at many locations in India.

For those who have apprehensions about quality of concrete produced at site with fly ash then these apprehensions need not be restricted to fly ash only. It could be applicable for water, chemical admixture etc. It is to be noted that normally people do not raise questions on adding micro silica at site though some have reservations for adding fly ash at site batching plant. As pointed out by Dr. N. Subramanian, fly ash is finer than cement but only about 50% more finer but silica fume is 80 times finer!!

3. Fineness of cement - Some experts rightly said that OPC 53 grade may be supplied as OPC 43 grade or even OPC 33 grade, as relevant Indian codes specify only minimum fineness requirement. It is high time that we modify relevant BIS codes to give range of fineness for different grades of cement i.e. 33, 43 & 53.

4. RMC supply - Some have raised doubts regarding quality of concrete supplied by Ready-mix concrete plant supplier (RMC). Yes, I have also encountered such problems. Such problems do occur even when RMC supplier takes due precautions to produce quality concrete at plant site but when it comes to pouring point, the client / contractor may be unhappy with the quality of concrete. There are some situations where it may be difficult for the RMC supplier to foresee such problems. For example -
a. Lead time - In Mumbai based on traffic situation lead time for given location from RMC plant to given construction site may vary widely say from half an hour to 4 hours. Admixture effect may go haywire. In such situation admixture may be put in stages at different locations i.e. plant site and pouring site, but rarely done.

b. Temperature variation during the day and night could be substantial and this adds the quality problem when the lead time is significant. It is good practice to cover the transit mixture with hessain cloth and keep it moist to overcome such problems to a certain extent.

c. Waiting at site - Contractor may order the supply considering he will be ready at site considering reliastic lead time. It is often observed that for some reason or other because of delay on the contractor side or delay by the consultant / supervisory staff concreting gets postponed by considerable time but by that time transit mixer with concrete is at the site. The supplier and contractor would like to see that concrete is not wasted and as a result shortcuts are adopted resulting in quality problem.

5. Performance based specifications - I would agree with Mr. Vijay Kulkarni that we should aim to adopt performance based specification in future and may start with certain iconic project as suggested by Ms. Alpa Sheth. In fact what is more important is if the contractor, consultant and the owner are open for this as a team, better product will be produced. What is needed is the training of our mind set for accepting such a change.

It is indeed a very good suggestion from Ms. Alpa Sheth to involve researchers and professors (who are inclined for innovations) on the codal committees along with cement manufacturers and contractor's representatives. But one of the difficulties, it is observed from some of the codal committees that because of the funding problem from the colleges concern, some professors / researchers workers fail to attend codal committee meetings even after they have been nominated.

6. Contractors view point - Mr. Alok Bhowmick is right when he said majority of the comments are from academicians and consultants but I can tell you my views are that of hardcore contractor, as I have got more than 35 years actual construction experience and I have got only limited teaching and consultancy experience (each of 8 years) and I always look for innovations, improvements and field applications on conducting relevant field experiments.

7. Repair and Maintenance - Points raised by Mr. Vinay Mehta are very relevant. Yes, we need guidelines or codes for maintenance of concrete structures and a book highlighting maintenance practices. Many experts from various disciplines including contractors, cement manufacturers, chemical admixture manufacturers, consultants etc. met together for the last 2 years and recently (February 2012) published a Handbook on Advanced Concrete Technology (edited by Dr. N.V. Nayak and Mr. A.K. Jain) which also contains one chapter on "Repair and Maintenance of Concrete."

Finally I once again compliment Moderators Mr. Vijay Kulkarni and Dr. N. Subramanian, brain behind the conference Ms. Alpa Sheth and Mr. Vikramjeet for excellent summary of views expressed in the E-conference.

With warmest regards to all,


N.V. Nayak  



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PostPosted: Sat Mar 10, 2012 3:24 am    Post subject: E-conference on Concrete Durability Reply with quote

Dear Members,

I profusely thank Mr Nayak for his posting on 9th March, wherein he has summed up the discussions held so far and covered all aspects of concrete durability in an excellent manner. I rate this posting as one of the best in this E-Conf. and yes, it does reflect the view point of a hard core Contractor.  Thanks once again.


Some of the issues that is still not covered / debated in the conference are :


a) Client's perspective on Quality / Durability.


I am of the view that ultimately durability (or quality of work) is Client's driven. For any major project, Quality assurance plans needs to be prepared by Client (or their Consultants) covering planning, design, detailing, including construction and maintenance aspects. In case Client is not conscious about durability, situation will not improve on ground. 
 
b) Concrete Properties with Blended Cement.


Lot of discussion took place about use of blended cement. I would like experts to also comment on the effect of blending on the relationship between compressive strength and stress-strain properties of concrete. The limiting strain figure of 0.0035 in concrete for limit state design has not changed since time immemorial. I believe this figure was derived with OPC and ought to be different for blended cements. Is there any conclusive research material on this aspect ? 


c) Prestressed Concrete Structures


Prestressed concrete structures deserves much greater attention than a reinforced concrete structure in terms of durability. Corrosion in prestressing steel can have disastrous consequence. There are several case studies of failure of old bridges (Thane Creek Bridge, Mandovi Bridge), which is essentially due to corrosion of prestressing steel and lack of grouting. 


IS code / IRC code do not cover the durability aspect for prestressing steel adequately. This is nicely covered in Eurocodes / FIB bulletins, which recommends different level of protection for different environmental class in prestressing steel also.


d) Detailing of Reinforcement & Use of Mechanical Couplers


This is one area where not much discussion took place. Any expert opinion on this issue ?


I once again take this opportunity to thank Mr Nayak, Dr Subramanian, Ms Alpa, Mr Choksi, Mr Kulkarni and all others who have taken part actively in the E-Conf.


With best wishes


Alok Bhowmick 
 

On Fri, Mar 9, 2012 at 11:57 PM, narayan_nayak <forum@sefindia.org (forum@sefindia.org)> wrote:
Quote:
           Dear All,

I am very happy at the response generated by this conference, views expressed, concern / apprehensions raised and clarifications provided etc.

This is probably the last opportunity to express my views as the closing date is approaching fast. Hence my views / clarifications on some of the issues raised by experts are as follows :-

1.  Cover Blocks - Mr. Jayant Kulkarni has rightly pointed out importance of the cover blocks and definitely at majority of the sites quality of cover blocks provided is far from satisfactory i.e they are very poor. In fact even the importance good cover blocks and good croncrete in cover zone is not understood by many. As a result as pointed out by one of the experts people even provide Gravel Piece, Mozac tile piece as cover blocks which definitely fall down during the concreting operation. There are cases where even wooden block has been used as the cover block!

Hence to overcome all these difficulties and to ensure that cover blocks are of good quality, as pointed out by me in the one of the first views, in Gammon Realty Ltd. we provide prefabricated concrete cover blocks of M60 grade concrete. These are excellent and can be used for all grades of concrete as long as grade of concrete is M60 and below. For higher grade we manufacture specially at site where we provide vibrating table for compaction of the concrete of the cover blocks and this concrete is of the same grade as that in the structure in consideration.

Yes, some prefer PVC cover blocks as they are impervious and generally sturdy. I have used PVC cover blocks at some of our important projects earlier but now I avoid them and prefer concrete blocks of M60 grade. The reason for this is that there will be compatibility issue between PVC and Concrete and there has to be micro crack / opening between PVC and concrete. This can be verified by the microscopic observations, of course I have not done it.

2. Fly Ash - Many have reservations / apprehensions for using fly ash in concrete. Many feel that fly ash is "ash" and should not be used in concrete !

When we say we should use fly ash, we mean we should use processed fly ash, processed to meet IS codal requirement. I have found that whenever we use processed fly ash, the results are very satisfactory. Further, when we use such fly ash, we at regular interval get it tested and there are agencies available for such testing. In majority of the cases we have found they meet the codal requirements.

Unlike this we accept micro silica with the certification given by the supplier and use it in cocnrete without any test !

It may be pertinent to point out here to all concerned that Indian produced fly ash (processed fly ash) is exported to other countries for use in concrete. Dirk India Pvt. Ltd. alone exports 40% of their product mainly to Middle East, East Africa and Asian countries. In terms of volume it is approximately 400 t/day.

Many have expressed concern about quality of fly ash and hence would like to refrain from its use in concrete. What is the difficulty when it meets IS codal requirement? Why nobody has raised quality issues regarding aggregate particularly fine aggregate? Quality of natural sand in Mumbai is very poor, quality varying from season to season, some even use creek sand!

Majority use coarse in concrete aggregate produced from uncontrolled industries. How many are bothered about presence of mica or alkali silica activity or even deleterious substances!

My sincere advice is please use processed fly ash meeting codal requirement to make concrete more durable and greener. Use of fly ash improves concrete resistance to alkali-silica reaction, to sulphate attack, better resistant than OPC concrete in subsoil conditions containing industrial waste. This is based on hardcore experience at sites where I have carried out large number of tests including strength tests for 180 days.

Regarding the pond ash, my advice is to use this pond ash as fine aggregate only in concrete that too on carrying out experiments. Further I have used the same for producing fly ash bricks and it is found to be quite good with respect to strength and permeability.

Some people have reservation of adding fly ash at site, as they are concerned about quality of the concrete produced. I prefer mixing at site as long as one has computerised batching plant. In such a case we can use more fly ash than presently used by manufacturer and it is found to be cost effective. It may be realised that processed fly ash is still chapter than cement at many locations in India.

For those who have apprehensions about quality of concrete produced at site with fly ash then these apprehensions need not be restricted to fly ash only. It could be applicable for water, chemical admixture etc. It is to be noted that normally people do not raise questions on adding micro silica at site though some have reservations for adding fly ash at site batching plant. As pointed out by Dr. N. Subramanian, fly ash is finer than cement but only about 50% more finer but silica fume is 80 times finer!!

3. Fineness of cement - Some experts rightly said that OPC 53 grade may be supplied as OPC 43 grade or even OPC 33 grade, as relevant Indian codes specify only minimum fineness requirement. It is high time that we modify relevant BIS codes to give range of fineness for different grades of cement i.e. 33, 43 & 53.

4. RMC supply - Some have raised doubts regarding quality of concrete supplied by Ready-mix concrete plant supplier (RMC). Yes, I have also encountered such problems. Such problems do occur even when RMC supplier takes due precautions to produce quality concrete at plant site but when it comes to pouring point, the client / contractor may be unhappy with the quality of concrete. There are some situations where it may be difficult for the RMC supplier to foresee such problems. For example -
a. Lead time - In Mumbai based on traffic situation lead time for given location from RMC plant to given construction site may vary widely say from half an hour to 4 hours. Admixture effect may go haywire. In such situation admixture may be put in stages at different locations i.e. plant site and pouring site, but rarely done.

b. Temperature variation during the day and night could be substantial and this adds the quality problem when the lead time is significant. It is good practice to cover the transit mixture with hessain cloth and keep it moist to overcome such problems to a certain extent.

c. Waiting at site - Contractor may order the supply considering he will be ready at site considering reliastic lead time. It is often observed that for some reason or other because of delay on the contractor side or delay by the consultant / supervisory staff concreting gets postponed by considerable time but by that time transit mixer with concrete is at the site. The supplier and contractor would like to see that concrete is not wasted and as a result shortcuts are adopted resulting in quality problem.

5. Performance based specifications - I would agree with Mr. Vijay Kulkarni that we should aim to adopt performance based specification in future and may start with certain iconic project as suggested by Ms. Alpa Sheth. In fact what is more important is if the contractor, consultant and the owner are open for this as a team, better product will be produced. What is needed is the training of our mind set for accepting such a change.

It is indeed a very good suggestion from Ms. Alpa Sheth to involve researchers and professors (who are inclined for innovations) on the codal committees along with cement manufacturers and contractor's representatives. But one of the difficulties, it is observed from some of the codal committees that because of the funding problem from the colleges concern, some professors / researchers workers fail to attend codal committee meetings even after they have been nominated.

6. Contractors view point - Mr. Alok Bhowmick is right when he said majority of the comments are from academicians and consultants but I can tell you my views are that of hardcore contractor, as I have got more than 35 years actual construction experience and I have got only limited teaching and consultancy experience (each of 8 years) and I always look for innovations, improvements and field applications on conducting relevant field experiments.

7. Repair and Maintenance - Points raised by Mr. Vinay Mehta are very relevant. Yes, we need guidelines or codes for maintenance of concrete structures and a book highlighting maintenance practices. Many experts from various disciplines including contractors, cement manufacturers, chemical admixture manufacturers, consultants etc. met together for the last 2 years and recently (February 2012) published a Handbook on Advanced Concrete Technology (edited by Dr. N.V. Nayak and Mr. A.K. Jain) which also contains one chapter on "Repair and Maintenance of Concrete."

Finally I once again compliment Moderators Mr. Vijay Kulkarni and Dr. N. Subramanian, brain behind the conference Ms. Alpa Sheth and Mr. Vikramjeet for excellent summary of views expressed in the E-conference.

With warmest regards to all,


N.V. Nayak



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This e-mail is confidential and it is intended only for the addressees. Any review, dissemination, distribution, or copying of this message by persons or entities other than the intended recipient is prohibited. If you have received this e-mail in error, kindly notify us immediately by telephone or e-mail and delete the message from your system. The sender does not accept liability for any errors or omissions in the contents of this message which may arise as a result of the e-mail transmission.""
     



     



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PostPosted: Sat Mar 10, 2012 6:07 am    Post subject: Re: E-conference on Concrete Durability Reply with quote

Dear Dr Nayak,

Thank you very much for your postings. They are excellent and provide insights to practical problems.

I am happy to note that you along with Mr. A.K. Jain have authored a Handbook on Advanced Concrete Technology. I am sure it will be quite useful to all of us.

We have used your another excellent book "Foundation Design Manual" in our practice. I suggest this to all Sefians.

Regards
Subramanian
narayan_nayak wrote:
Dear All,

I am very happy at the response generated by this conference, views expressed, concern / apprehensions raised and clarifications provided etc.

This is probably the last opportunity to express my views as the closing date is approaching fast. Hence my views / clarifications on some of the issues raised by experts are as follows :-

1. Cover Blocks - Mr. Jayant Kulkarni has rightly pointed out importance of the cover blocks and definitely at majority of the sites quality of cover blocks provided is far from satisfactory i.e they are very poor. In fact even the importance good cover blocks and good croncrete in cover zone is not understood by many. As a result as pointed out by one of the experts people even provide Gravel Piece, Mozac tile piece as cover blocks which definitely fall down during the concreting operation. There are cases where even wooden block has been used as the cover block!

Hence to overcome all these difficulties and to ensure that cover blocks are of good quality, as pointed out by me in the one of the first views, in Gammon Realty Ltd. we provide prefabricated concrete cover blocks of M60 grade concrete. These are excellent and can be used for all grades of concrete as long as grade of concrete is M60 and below. For higher grade we manufacture specially at site where we provide vibrating table for compaction of the concrete of the cover blocks and this concrete is of the same grade as that in the structure in consideration.

Yes, some prefer PVC cover blocks as they are impervious and generally sturdy. I have used PVC cover blocks at some of our important projects earlier but now I avoid them and prefer concrete blocks of M60 grade. The reason for this is that there will be compatibility issue between PVC and Concrete and there has to be micro crack / opening between PVC and concrete. This can be verified by the microscopic observations, of course I have not done it.

2. Fly Ash - Many have reservations / apprehensions for using fly ash in concrete. Many feel that fly ash is "ash" and should not be used in concrete !

When we say we should use fly ash, we mean we should use processed fly ash, processed to meet IS codal requirement. I have found that whenever we use processed fly ash, the results are very satisfactory. Further, when we use such fly ash, we at regular interval get it tested and there are agencies available for such testing. In majority of the cases we have found they meet the codal requirements.

Unlike this we accept micro silica with the certification given by the supplier and use it in cocnrete without any test !

It may be pertinent to point out here to all concerned that Indian produced fly ash (processed fly ash) is exported to other countries for use in concrete. Dirk India Pvt. Ltd. alone exports 40% of their product mainly to Middle East, East Africa and Asian countries. In terms of volume it is approximately 400 t/day.

Many have expressed concern about quality of fly ash and hence would like to refrain from its use in concrete. What is the difficulty when it meets IS codal requirement? Why nobody has raised quality issues regarding aggregate particularly fine aggregate? Quality of natural sand in Mumbai is very poor, quality varying from season to season, some even use creek sand!

Majority use coarse in concrete aggregate produced from uncontrolled industries. How many are bothered about presence of mica or alkali silica activity or even deleterious substances!

My sincere advice is please use processed fly ash meeting codal requirement to make concrete more durable and greener. Use of fly ash improves concrete resistance to alkali-silica reaction, to sulphate attack, better resistant than OPC concrete in subsoil conditions containing industrial waste. This is based on hardcore experience at sites where I have carried out large number of tests including strength tests for 180 days.

Regarding the pond ash, my advice is to use this pond ash as fine aggregate only in concrete that too on carrying out experiments. Further I have used the same for producing fly ash bricks and it is found to be quite good with respect to strength and permeability.

Some people have reservation of adding fly ash at site, as they are concerned about quality of the concrete produced. I prefer mixing at site as long as one has computerised batching plant. In such a case we can use more fly ash than presently used by manufacturer and it is found to be cost effective. It may be realised that processed fly ash is still chapter than cement at many locations in India.

For those who have apprehensions about quality of concrete produced at site with fly ash then these apprehensions need not be restricted to fly ash only. It could be applicable for water, chemical admixture etc. It is to be noted that normally people do not raise questions on adding micro silica at site though some have reservations for adding fly ash at site batching plant. As pointed out by Dr. N. Subramanian, fly ash is finer than cement but only about 50% more finer but silica fume is 80 times finer!!

3. Fineness of cement - Some experts rightly said that OPC 53 grade may be supplied as OPC 43 grade or even OPC 33 grade, as relevant Indian codes specify only minimum fineness requirement. It is high time that we modify relevant BIS codes to give range of fineness for different grades of cement i.e. 33, 43 & 53.

4. RMC supply - Some have raised doubts regarding quality of concrete supplied by Ready-mix concrete plant supplier (RMC). Yes, I have also encountered such problems. Such problems do occur even when RMC supplier takes due precautions to produce quality concrete at plant site but when it comes to pouring point, the client / contractor may be unhappy with the quality of concrete. There are some situations where it may be difficult for the RMC supplier to foresee such problems. For example -
a. Lead time - In Mumbai based on traffic situation lead time for given location from RMC plant to given construction site may vary widely say from half an hour to 4 hours. Admixture effect may go haywire. In such situation admixture may be put in stages at different locations i.e. plant site and pouring site, but rarely done.

b. Temperature variation during the day and night could be substantial and this adds the quality problem when the lead time is significant. It is good practice to cover the transit mixture with hessain cloth and keep it moist to overcome such problems to a certain extent.

c. Waiting at site - Contractor may order the supply considering he will be ready at site considering reliastic lead time. It is often observed that for some reason or other because of delay on the contractor side or delay by the consultant / supervisory staff concreting gets postponed by considerable time but by that time transit mixer with concrete is at the site. The supplier and contractor would like to see that concrete is not wasted and as a result shortcuts are adopted resulting in quality problem.

5. Performance based specifications - I would agree with Mr. Vijay Kulkarni that we should aim to adopt performance based specification in future and may start with certain iconic project as suggested by Ms. Alpa Sheth. In fact what is more important is if the contractor, consultant and the owner are open for this as a team, better product will be produced. What is needed is the training of our mind set for accepting such a change.

It is indeed a very good suggestion from Ms. Alpa Sheth to involve researchers and professors (who are inclined for innovations) on the codal committees along with cement manufacturers and contractor's representatives. But one of the difficulties, it is observed from some of the codal committees that because of the funding problem from the colleges concern, some professors / researchers workers fail to attend codal committee meetings even after they have been nominated.

6. Contractors view point - Mr. Alok Bhowmick is right when he said majority of the comments are from academicians and consultants but I can tell you my views are that of hardcore contractor, as I have got more than 35 years actual construction experience and I have got only limited teaching and consultancy experience (each of 8 years) and I always look for innovations, improvements and field applications on conducting relevant field experiments.

7. Repair and Maintenance - Points raised by Mr. Vinay Mehta are very relevant. Yes, we need guidelines or codes for maintenance of concrete structures and a book highlighting maintenance practices. Many experts from various disciplines including contractors, cement manufacturers, chemical admixture manufacturers, consultants etc. met together for the last 2 years and recently (February 2012) published a Handbook on Advanced Concrete Technology (edited by Dr. N.V. Nayak and Mr. A.K. Jain) which also contains one chapter on "Repair and Maintenance of Concrete."

Finally I once again compliment Moderators Mr. Vijay Kulkarni and Dr. N. Subramanian, brain behind the conference Ms. Alpa Sheth and Mr. Vikramjeet for excellent summary of views expressed in the E-conference.

With warmest regards to all,


N.V. Nayak  



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This e-mail is confidential and it is intended only for the addressees. Any review, dissemination, distribution, or copying of this message by persons or entities other than the intended recipient is prohibited. If you have received this e-mail in error, kindly notify us immediately by telephone or e-mail and delete the message from your system. The sender does not accept liability for any errors or omissions in the contents of this message which may arise as a result of the e-mail transmission.""

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PostPosted: Sat Mar 10, 2012 6:28 am    Post subject: Reply with quote

Dear Nayak Sir,

A great summary indeed for practical use.  

I was wondering, has any one talked about the construction water quality.  I understand that one of the sources of chloride attack is the presence of chlorides in water that is mixed with other concrete constituents.  

Construction water is usually arranged by owner at remote sites.  Though, tested initially, is it tested for every storage?

Regards,

Jignesh V Chokshi
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PostPosted: Sat Mar 10, 2012 4:15 pm    Post subject: Reply with quote

Dear Jignesh V Chokshi,

Thank you for raising this issue which will pose the biggest issue in future. In addition to fine aggregates, good quality of water is also not available, even for drinking purposes in several parts of India. Many women folks are walking considerable distances to fetch water. In this circumstance, we need to rethink about the water to be used with concrete. Generally it is thought that potable water is good for concrete.

A few engineers have thought about using recycled water in concrete. A paper by NRMCA of USA is enclosed. Another paper by Australian Engineers may be found at:
http://www.concrete.net.au/publications/pdf/RecycledWater.pdf
JVCSNL wrote:
Dear Nayak Sir,


Regards
Subramanian
A great summary indeed for practical use.  

I was wondering, has any one talked about the construction water quality.  I understand that one of the sources of chloride attack is the presence of chlorides in water that is mixed with other concrete constituents.  

Construction water is usually arranged by owner at remote sites.  Though, tested initially, is it tested for every storage?

Regards,

Jignesh V Chokshi



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PostPosted: Sat Mar 10, 2012 6:21 pm    Post subject: E-conference on Concrete Durability Reply with quote

Dear Alok,
should cables really undergo corrosion? The cables are covered by sheaths and further grouted. Moreover, if sheaths are to be spliced we join with thermal tapes. Thus we make the sheaths watertight to our extreme effort. If the grout be not contaminated, the cables should not get exposed to external corrosive environment. Case of Thane creek bridge I know, since I was with SPAN at that time, we found to our awe that a good deal of end block was devoid of concrete. No one noticed that concrete did not pass through stirrups and other unstressed reinforcements. It exposed inadequacy of compaction of concrete. Similar situation I faced in NH 22 where a portion of end block developed cracks during stressing. Well I should mention that some manufacturer has supplied rust prone strands. One such lot I myself returned from one of my projects.
 
Gautam Chatterjee

On Sat, Mar 10, 2012 at 11:39 PM, bsec <forum@sefindia.org (forum@sefindia.org)> wrote:
[quote]            Dear Members,

I profusely thank Mr Nayak for his posting on 9th March, wherein he has summed up the discussions held so far and covered all aspects of concrete durability in an excellent manner. I rate this posting as one of the best in this E-Conf. and yes, it does reflect the view point of a hard core Contractor.  Thanks once again.


Some of the issues that is still not covered / debated in the conference are :


a) Client's perspective on Quality / Durability.


I am of the view that ultimately durability (or quality of work) is Client's driven. For any major project, Quality assurance plans needs to be prepared by Client (or their Consultants) covering planning, design, detailing, including construction and maintenance aspects. In case Client is not conscious about durability, situation will not improve on ground. 
 
b) Concrete Properties with Blended Cement.


Lot of discussion took place about use of blended cement. I would like experts to also comment on the effect of blending on the relationship between compressive strength and stress-strain properties of concrete. The limiting strain figure of 0.0035 in concrete for limit state design has not changed since time immemorial. I believe this figure was derived with OPC and ought to be different for blended cements. Is there any conclusive research material on this aspect ? 


c) Prestressed Concrete Structures


Prestressed concrete structures deserves much greater attention than a reinforced concrete structure in terms of durability. Corrosion in prestressing steel can have disastrous consequence. There are several case studies of failure of old bridges (Thane Creek Bridge, Mandovi Bridge), which is essentially due to corrosion of prestressing steel and lack of grouting. 


IS code / IRC code do not cover the durability aspect for prestressing steel adequately. This is nicely covered in Eurocodes / FIB bulletins, which recommends different level of protection for different environmental class in prestressing steel also.


d) Detailing of Reinforcement & Use of Mechanical Couplers


This is one area where not much discussion took place. Any expert opinion on this issue ?


I once again take this opportunity to thank Mr Nayak, Dr Subramanian, Ms Alpa, Mr Choksi, Mr Kulkarni and all others who have taken part actively in the E-Conf.


With best wishes


Alok Bhowmick 
 

On Fri, Mar 9, 2012 at 11:57 PM, narayan_nayak forum@sefindia.org (forum@sefindia.org))> wrote:
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PostPosted: Sun Mar 11, 2012 3:53 am    Post subject: E-conference on Concrete Durability Reply with quote

Dear Gautamda,

Prestressing cable corrosion is quite common feature, particularly in aggressive environment and one of the major problems faced in cold countries. In fact I remember that in early 90's, this led to even banning of post tensioning for a brief period in UK. Problems that have occurred with the durability of post-tensioning tendons in the past are :

 
(1) Quality of prestressing steels (brittle failure)  
 
(2) Grout voids and grout segregation (bleed, corrosion)  
 
(3) Water/Chlorides penetrating tendon (corrosion)
 
Corrosion protection is now a days much improved by using plastic ducts and special grouts worldwide. Fib bulletin 33 covers this aspect very nicely. 


In India, the problem is more in agressive coastal belt. Most of the past failures are caused due to inadequate grouting and poor workmanship, which eventually led to corrosion of prestressing cable. 


With best wishes


Alok Bhowmick
On Sun, Mar 11, 2012 at 12:05 AM, gautam chattopadhyay <forum@sefindia.org (forum@sefindia.org)> wrote:
Quote:
           Dear Alok,
should cables really undergo corrosion? The cables are covered by sheaths and further grouted. Moreover, if sheaths are to be spliced we join with thermal tapes. Thus we make the sheaths watertight to our extreme effort. If the grout be not contaminated, the   
     



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PostPosted: Sun Mar 11, 2012 6:39 am    Post subject: Selection of materials from durability point of view Reply with quote

Dear All,

With the scarcity of natural river sand, it is high time that use of Manufactured sand needs to be taken seriously. Even though in RMC plants and in some big projects Crushed Rock Fines (CRF) being used, use of M-sand is yet gain popularity.

Following are the main issues:

1. Availability of M-sand (in the strict sense) is limited. Not all crushing plants have VSI crushers. Even if they have VSI crushers, they are yet to achieve required shape and gradation.

2. M-sand manufacturers not always wash sand to remove fines passing through 75 micron. (This is necesary as fines exceeds 15% and may include clay which comes along with rocks from quarry).

3. In such a scenerio, use of CRF ( in actual it is quarry dust only)  and in many cases bad quality sand ( so called filtered sand) is being used for making concrete.

4. Use of flyash and GGBS is on the rise. Quality of flyash is a big question ?
    Whether it is M 20 or M 25 (extensively used grades of concrete) with lowest possible cement content in it and the quality of sand being used, durability is definitely going to be an issue in the coming years !!!

5. Even though quite a few durability tests (such as water permeability as per DIN 1048, RCPT as per ASTM C 1202, ISAT, Sorptivity etc.) are conducted in major projects, they are being conducted on laboratory prepared samples. In-situ durability tests are not at all being done and there is awareness in this regard.

6. There is a need to study and understand microstructure of concrete in the context of so many materials being put into concrete.  Compatibility of adnixtures (both mineral and chemical) with wide variety of cements.

Our Indian standards needs to be included (IS 456) with durability requirements in terms of performance under different exposure conditions.

R. Nagendra
Technical Director
Civil-Aid Technoclinic Pvt. Ltd.
Bangalore
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PostPosted: Sun Mar 11, 2012 1:31 pm    Post subject: Reply with quote

FLY ASH AND FLY ASH CONCRETE :


Basic issues :

1.  Source/quality of burnt coal.

2.  Coal is normally received at plant site at a size of 350 mm and down. This is crushed in crusher house building into size 20 mm and down.  Then this small sized coal is sent to mills in bunker building and coal gravels are milled there into dust which is then pulverised and thrown into boiler furnace. See, coal is two times crushed for making it into dust like powder and then it is burnt and ash is produced. Naturally ash particle size will be too small.

Method of burning in Boiler i.e. what type of oil/anticorrosion chemical used and at what temperature are also important issues for determining the quality of fly ash.

3.  Earlier, all the fly ashes produced in coal based thermal power plant were being released in air through chimney.

But at present, due to stringent Ministry of Environment & Forest  guidelines, fly ashes are captured in ESP electrostatically & also in chimney hopper to some extent and collected phneumatically at remotely located Silos (near boundary wall) for disposal. Bottom Ash or coarser ash is disposed in Ash Pond for future mine's filling/other use.

Quality of coal varies from country to country. Indian coal has heavy ash content in coal in the range of 30% to even 46% whereas in other developed countries  ash content is in the range of 5% to 20%. Sometimes boulders also enters along with the coal. Big boulders are removed mechanically, but small size stones enter into the mill and boilers.

Out of total ash, 80% is fly ash and 20% is bottom ash. Bottom ash contains much larger quantum of unburnt coal and also stone dust. Fly ash is just like talcom powder.


Now fly ash quality varies from country to country depending on coal qiality. High variation is also observed in Indial coal quality from mines to mines. Even if coal mines are fixed quality of ash varies depending on the coal mill functioning and boiler operation.

Though, ash quality varies, still India is a big producer of ash and in near future many other thermal power plants will be commissioned and start producing ashes.  

At present, cement producing companys are entering into the premises of such thermal power plants and getting a cement plant inside there and producing cement. We are also using fly ash in concrete in a limited way. But i feel that future is towards use of fly ash and we should all know the intricacies of fly ash to safegurd us and our future.


Some papers, i found in net are loaded to understand the basics of fly ash :



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