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Alternatives to early strength cements for controlling <E

 
This forum is locked: you cannot post, reply to, or edit topics.This topic is locked: you cannot edit posts or make replies. Thank Post    www.sefindia.org Forum Index -> E-conference on Durability of Concrete - Feb 27 to March 11 - 2012
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alpa_sheth
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Joined: 26 Jan 2003
Posts: 254
Location: Mumbai

PostPosted: Tue Mar 06, 2012 12:00 pm    Post subject: Alternatives to early strength cements for controlling <E Reply with quote

Thanks very much, Mr Kulkarni, for some very clear thoughts on the chronology of events in the History of Concrete.  
I cannot agree with you more re. the speed required in current projects. On one of our projects, the contractor had cast 40 slabs within one calendar year. So while we want that speed, we refuse to understand the price we sometimes seem to pay for this.  

Thanks Dr Nayak for your input on the subject as also to Mr Bhowmick, Mr Chokshi, Mr Bhattacharya and others for a lively discussion on this topic. Clearly the opinion is split down the middle. A more fractured verdict than the recent poll results.  

Let me try to bullet what I have understood from the posts of our esteemed participants and some reading-

<![if !supportLists]>a) <![endif]>Use of fines in cements has increased over the decades, in our search for early strength. This has made the cements more crack prone.  
<![if !supportLists]>b) <![endif]>Fly Ash is an environment friendly SCM and can address some of the cracking problem but is not a cure all for all problems and can sometimes make matters worse (thanks Mr. Maruf for that paper of Mr. Burrows)  
<![if !supportLists]>c) <![endif]>Judicious sourcing of Fly Ash is required. There is Fly Ash and there is Fly Ash. You may not use Pond Ash or Bottom Ash. IS 3812 Parts 1 & 2 specify Fly Ash requirements for its use as a pozzolona in cement or concrete admixture. Most of the Fly Ash produced at our Thermal plants fails to meet these standards.  
<![if !supportLists]>d) <![endif]>To make fly ash usable in concrete, there is a need to provide fly ash in graded form and grind the fly ash as required to meet the requirement of the construction industry. Such packaged and graded flyash is available and should only be used. However, it costs almost same as cement (ours not reason why!) so it again defeats the purpose  
<![if !supportLists]>e) <![endif]>Fly Ash performs better in moist environment than in arid regions.  
<![if !supportLists]>f) <![endif]>Fly Ash should not be used with high alkali cements

My question-  
Does Fly Ash performs better as a SCM in concrete as a cement replacement than as a pozzolona in manufacture of cement? Given the problems in the fly ash grading, I would think the latter is better option (I’m putting cost aside for now) but I would like the experts to respond.  


Best regards,
Alpa

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RKBhola
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PostPosted: Tue Mar 06, 2012 1:44 pm    Post subject: Alternatives to early strength cements for controlling <E Reply with quote

Dear Ms.Sheth,

Thank You for the very well bulleted points of this discussion.  As regards the question of using Fly Ash at the site as an SCM for cement replacement v/s use as pozzolana in cement manufacture, I think it is a matter of the Quality Controls that we are able to practically implement at the sites.  Even if we ignore the matter of availability of the desired quality of Fly Ash for use as SCM, I would presume that the controls at the cement manufacturers end would anyday be more stringent than that which can be implemented at the sites. 
With the pressure from the contractors upon the 'Mix Designers' to reduce the cement content to 'barely pass' status (in order to maximise profits), and coupled with the lax quality controls at the site, it is desirable to opt for use of Fly Ash as pozzolana in manufacture of cement.


Regards


RK Bhola


 

On Tue, Mar 6, 2012 at 6:00 PM, alpa_sheth <forum@sefindia.org (forum@sefindia.org)> wrote:
Quote:
           Thanks very much, Mr Kulkarni, for some very clear thoughts on the chronology of events in the History of Concrete.
I cannot agree with you more re. the speed required in current projects. On one of our projects, the contractor had cast 40 slabs within one calendar year. So while we want that speed, we refuse to understand the price we sometimes seem to pay for this.

Thanks Dr Nayak for your input on the subject as also to Mr Bhowmick, Mr Chokshi, Mr Bhattacharya and others for a lively discussion on this topic. Clearly the opinion is split down the middle. A more fractured verdict than the recent poll results.

Let me try to bullet what I have understood from the posts of our esteemed participants and some reading-

a) Use of fines in cements has increased over the decades, in our search for early strength. This has made the cements more crack prone.
b) Fly Ash is an environment friendly SCM and can address some of the cracking problem but is not a cure all for all problems and can sometimes make matters worse (thanks Mr. Maruf for that paper of Mr. Burrows)
c) Judicious sourcing of Fly Ash is required. There is Fly Ash and there is Fly Ash. You may not use Pond Ash or Bottom Ash. IS 3812 Parts 1 & 2 specify Fly Ash requirements for its use as a pozzolona in cement or concrete admixture. Most of the Fly Ash produced at our Thermal plants fails to meet these standards.
d) To make fly ash usable in concrete, there is a need to provide fly ash in graded form and grind the fly ash as required to meet the requirement of the construction industry. Such packaged and graded flyash is available and should only be used. However, it costs almost same as cement (ours not reason why!) so it again defeats the purpose
e) Fly Ash performs better in moist environment than in arid regions.
f) Fly Ash should not be used with high alkali cements

My question-
Does Fly Ash performs better as a SCM in concrete as a cement replacement than as a pozzolona in manufacture of cement? Given the problems in the fly ash grading, I would think the latter is better option (I’m putting cost aside for now) but I would like the experts to respond.


Best regards,
Alpa
     



     



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lkjain.ngp
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PostPosted: Tue Mar 06, 2012 7:36 pm    Post subject: Alternatives to early strength cements for controlling <E Reply with quote

Dear sirs,
It is not the question of desired quality of flyash, but the available flyash. The fhyash available should be consistent quality, i.e. variations in each parameters should be within an acceptable range.
For smaller jobs, say where batch mixing plant is not operating, PPC is an option.
L. K. JAIN

On 6 March 2012 21:35, RKBhola <forum@sefindia.org (forum@sefindia.org)> wrote:
[quote]  Dear Ms.Sheth,

Thank You for the very well bulleted points of this discussion.  As regards the question of using Fly Ash at the site as an SCM for cement replacement v/s use as pozzolana in cement manufacture, I think it is a matter of the Quality Controls that we are able to practically implement at the sites.  Even if we ignore the matter of availability of the desired quality of Fly Ash for use as SCM, I would presume that the controls at the cement manufacturers end would anyday be more stringent than that which can be implemented at the sites. 
With the pressure from the contractors upon the 'Mix Designers' to reduce the cement content to 'barely pass' status (in order to maximise profits), and coupled with the lax quality controls at the site, it is desirable to opt for use of Fly Ash as pozzolana in manufacture of cement.


Regards


RK Bhola


 

On Tue, Mar 6, 2012 at 6:00 PM, alpa_sheth forum@sefindia.org (forum@sefindia.org))> wrote:
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lkjain.ngp
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PostPosted: Tue Mar 06, 2012 9:06 pm    Post subject: Alternatives to early strength cements for controlling <E Reply with quote

This is with reference to the notes of Ms Alpa dated 6/3/12 6pm.
 
c) Flyash quality if not acceptable requires modification by benefaction processes, or we can call it processed flyash. The quality norms are relative to the availability of flyash. as flyash use will increase the quality norms will have to be relaxed, and than even flyash use will be worthwhile technically.  Approach would to utilise all the available flyash. Next step would be to utilise the pond ash deposited by the thermal plants. However uniformity of the parameters (as a quality) is necessary in all cases.
Standards are debatable. Some of the limits as specified in present standard can be ignored in near future. In some regard our (IS) standards are restrictive comparative to trends in other countries. Parameters which are not essential for performance can be ignored.
d)  If variation in a parameter is outside a range that can give satisfactory quality (small variation) of concrete, benefaction /(processing) of flyash is necessary. Grading and packing of  flyash do make it convenient for use in smaller projects doing on job mixing. It be noted that for on job mixing of flyash in concrete production efficient mixes are requited. Non of inclined rotating drum mixers (including TM) are efficient enough for flyash. Efficient mixers like pan or double shaft mixers are usually part of batch mixing plants.
We should not be concerned about the cost at first go. Processed, packed and graded flyash will cost more. Once India will be consuming most of the flyash available, the cost will tend to match with the cement price, and may become costlier than cement because of demand-supply rule. Similar is the case today with micro-silica. Classified finner flyash can to some extent replace the present use of micro-silica. Even at cost comparable to cement it will make sense in using flyash as replacement of cement. Cement is the only material which largely affect the durability of concrete, and by all means we should reduce the consumption of cement in the concrete.
e)  Hypothesis can not be true. Issue is debatable.
f)  There appears to a confusion. Flyash consume the alkali from cement by converting it in to more of gel which adds to strength. alkali as remains in concrete is a big weakness of concrete. Flyash binding the alkali from cement is the main action which makes concrete with flyash more durable. With more alkali in cement, high volume flyash concrete should be practised.
 
Performance of concrete may be marginally better for flyash addition to cement while grinding, compared to its addition in to the concrete mixer. However this marginal difference does not qualify an approach to be a better one. Graded flyah does give better options. As fineness of SCM's (/flyash) available increases, one can design mix for higher strength and high performance (lower permeability & higher flexural to compressive strength, etc.)  For high performance particle size grading in micron range is important. Example: To get flexural  strength >4.5 MPa (say >M50 grade) concrete should have enough fines below  5 micron size, which is not available in cement. Gradually for more higher performance enough particles of 1 micron and below would be needed, which finner flyash can supply.
It should also be noted for higher performance concrete 7 higher durability, multiple blending of SCM's results in better solutions. One can use OPC +GGBS +flyash +silica fume.
For a concrete engineers, it is wiser to be using flyash & SCM's as the options are available.
I am sure that within a decade, situation will develop when flyash of good quality (as we term it at present) will be costly material, and material technologist will advocate used of pond ash and other SCM's which we at present regard as low quality materials.
 
L. K. JAIN


On 7 March 2012 01:06, Lalit K. Jain <lkjain.ngp@gmail.com (lkjain.ngp@gmail.com)> wrote:
[quote] Dear sirs,
It is not the question of desired quality of flyash, but the available flyash. The fhyash available should be consistent quality, i.e. variations in each parameters should be within an acceptable range.
For smaller jobs, say where batch mixing plant is not operating, PPC is an option.
L. K. JAIN

On 6 March 2012 21:35, RKBhola <forum@sefindia.org (forum@sefindia.org)> wrote:
[quote]  Dear Ms.Sheth,

Thank You for the very well bulleted points of this discussion.  As regards the question of using Fly Ash at the site as an SCM for cement replacement v/s use as pozzolana in cement manufacture, I think it is a matter of the Quality Controls that we are able to practically implement at the sites.  Even if we ignore the matter of availability of the desired quality of Fly Ash for use as SCM, I would presume that the controls at the cement manufacturers end would anyday be more stringent than that which can be implemented at the sites. 
With the pressure from the contractors upon the 'Mix Designers' to reduce the cement content to 'barely pass' status (in order to maximise profits), and coupled with the lax quality controls at the site, it is desirable to opt for use of Fly Ash as pozzolana in manufacture of cement.


Regards


RK Bhola


 

On Tue, Mar 6, 2012 at 6:00 PM, alpa_sheth forum@sefindia.org (forum@sefindia.org))> wrote:
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