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CEMENT AND MIX DESIGN.
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P.K.Mallick
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PostPosted: Sun Feb 01, 2009 9:51 am    Post subject: Reply with quote

Respected Mithilesh Sir,
Thanks a lot for your observation. I will just put across two points for your observation.

1) IS:10262-1982 was only reaffirmed on 2004. There is no change from the 1982 code and that Graph was also available before reaffirmation.
2)Yes,there is lot of improvement in cement quality and at the same time you will appreciate the fact that there is lot deterioration in the environment too.
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PostPosted: Sun Feb 01, 2009 11:45 am    Post subject: CEMENT AND MIX DESIGN. Reply with quote

Dear sefian,

i think, lot of debate is going on about cement and concrete strength. i feel that in last 20 years, lot of improvement has been observed in cement quality, but i don't see any improvement in concrete quality at construction sites keeping aside mega infrastructure projects. i wish, that we should have debate on what as a professional/ institution, role we have to play in improving construction practise rather than debating on cement strength which is a only branded ingredient being used as on date in India in making concrete.

Regards,

J.D. Gandhi
--- On Sun, 1/2/09, mks <forum@sefindia.org> wrote:

Quote:
From: mks <forum@sefindia.org>
Subject: [SEFI] Re: CEMENT AND MIX DESIGN.
To: general@sefindia.org
Date: Sunday, 1 February, 2009, 2:17 PM

Repected Mallick Sir and Experienced Sifians

Thank u Mallick Sir for your nice elaboration on the issue. I do agree 100% with your comment that, we shall use the designated compressive strength of the cement in general.

But on the other hand a lot of improvements has been done in the quality and strength of Indian cement since 1982 to 2003 (before reaffirmation of IS 10262). I think Kipping in view of these improvements, the graph in form of Fig-02 has been included in IS: 10262 – 1982 (Reaffirmed 2004).
It is also the fact that the test results of different samples of cement taken from same batch of cement produced in the same manufacturing unit may varies within some range (minimum to maximum) In my opinion, to generalized the representation of the sample to the bulk quantity and to take the advantage of the improvement of the quality of the cement, the graph of Fig – 2 IS 10262 have 6 nos of curve namely A,B,C,D,E & F having range of 28 – Day strength of cement tested according to IS: 4031- 1968 as 31.9 – 36.8 Mpa, 36.8 – 41.7 Mpa, 41.7 - 46.6 Mpa, 46.6 – 51.5 Mpa, 51.5 – 56.4 Mpa, 56.4 – 61.3 Mpa respectively instead of Grade of cement like 33, 43, 53 Grade of cement.

If interpretation of these curves is limited to minimum strength of designated grade of cement then we have only three grades of cement like 33,43,53 grade as per respective IS code. In this case the curve B, D and F has no use.

There fore I am in opinion to take the proper advantage of the strength of cement of concerned batch of supply while doing the Mix design based on test results as per relevant IS and this result should also be compared with the manufacturers test certificates for the concerned batch of cement.

comments from Seniors are highly appreciated

Regards
mithilesh

P.K.Mallick wrote:  Respected Mithilesh Sir,

Thanks for your response.Let us look into Indian cement Market. OPC has almost vanished from the market. It is now PSC and PPC. The former is abundant in the Eastern Part of the country.
Now let us look into respective IS Codes for those cement.
IS 1489(Part-1):1991(Reaffirmed 2000) PORTLAND-POZZOLANA CEMENT-SPECIFICATION(FLY ASH BASED)

IS 1489(Part-2):1991(Reaffirmed 2000) PORTLAND-POZZOLANA CEMENT-SPECIFICATION(CALCINED CLAY BASED)

Clause 7.4.1 the average compressive strength at 672+=4h is 33 Mpa,Min
Hence strength wise it is equivalent to Grade-33 OPC.

IS:455:1989(Reaffirmed 2005) PORTLAND SLAG CEMENT-SPECIFICATION

Clause 6.4 the average compressive strength at 672+=4h is 33 Mpa,Min
Hence strength wise it is also equivalent to Grade-33 OPC.

It is my observation of last six months that particular brand of PPC gives a compressive strength on an average 53 Mpa. Now the question comes while designing concrete mix should we take the observed compressive strength of 53 Mpa or 33 Mpa.
Again it is of my opinion (conservative of course) that while designing mix we should restrict to 33Mpa because fly ash used in PPC is obtained from the exhaust gases of coal fired power station and the quality of fly ash depends on the ash content of the coal used. See the variables associated Fly-ash. So depending upon type of Fly-ash used, the strength of PPC shall vary and we can not question the manufacturer as long as the minimum requirement of 33Mpa is satisfied. Hence is it not risky proposition to depend on undependable observed compressive strength? Of course the consolation is that cement manufactures usually get the fly ash from one source.

Same can be said about OPC-43 grade which (sometimes) is actually rejected OPC-53 Grade and even sometimes the OPC-53 comes in the bag of OPC-43 because it is not economically viable for a manufacturer to have different manufacturing facilities for different Grade of cement.

Hence let us use the designated compressive strength not the observed compressive strength while using the curves of Design Mix code.






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P.K.Mallick
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PostPosted: Wed Feb 11, 2009 7:53 am    Post subject: Reply with quote

Respected Subramanian Sir,
As advised by you I had a look into following codes:
IS 4031 : Part 6 : 1988Methods of physical tests for hydraulic cement.
IS 4031 : Part 7 : 1988Methods of physical tests for hydraulic cement:
I could not get anything useful pertaining to problem explained by me.
However I got certain clarification from one of the book on CONCRETE TECHNOLOGY written by one Indian author. I reproduce the same here:

THE STRENGTH OF CEMENT TO BE USED IN MIX DESIGN COMPUTATIONS IS NOT MEAN STRENGTH fm OF CERTAIN NUMBER OF TEST RESULTS (SAY N),BUT SHOULD BE THE CHARACTERISTIC STRENGTH fck GIVE BY EQUATION:
fck=fm-ks
WHERE "K" IS PROBABILITY FACTOR,A STATISTICAL PARAMETER FOR NOT MORE THAN 5 PERCENT TEST RESULTS TO FALL BELOW THE CHARACTERISTIC STRENGTH fck AND "s" IS THE STANDARD DEVIATION.
FOR EXAMPLE ,IF THE MEAN OF OF "N" COMPRESSIVE STRENGTH TEST RESULTS OF CEMENT IS 55,THE CEMENT WOULD BE APPARENTLY A GRADE-53 CEMENT.HOWEVER THE STANDARD DEVIATION OF THIS PARTICULAR CEMENT IS 4 MPa ,THE CHARACTERISTIC STRENGTH  WOULD BE
  fck=55-(1.65 x 4)=48.4 MPa.
THUS THE CEMENT ACTUALLY COMES UNDER GRADE-43.


Sir, I need your observation on entire issue.

WARM REGARDS.
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Dr. N. Subramanian
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PostPosted: Thu Feb 12, 2009 5:41 pm    Post subject: CEMENT AND MIX DESIGN. Reply with quote

Dear Mr. Mallick,

In my earlier reply dated 17th Jan 09, I replied the following:

{You wrote << For example if a Design Mix for M-25 results in cube strength of 30N/sqmm at site ,we should not say it as M-30 concrete>>. Yes. If you get a cube strength of 30 MPa, then you have to assume fck as 30-0.825x 4 or 30-4, whichever is less. Hence fck(approximately)= 26 MPa. Note that 30 MPa is assumed to represent mean of several cube strengths.}

I do not know what is the difference you found from my above reply and the thing you found in a book and stated in your present email. Only difference I could note is the value of k. The value of k of 1.65 can be taken only if you have 30 or more samples. If less number of samples are available, then k= 1.65[1-1/sq.root(n)]. In IS 456, for 4 specimens, k is taken as 0.825, with n=4.

Best wishes
Subramanian



From: P.K.Mallick <forum@sefindia.org>
To: general@sefindia.org
Sent: Wednesday, February 11, 2009 2:53:23 AM
Subject: [SEFI] Re: CEMENT AND MIX DESIGN.

     Respected Subramanian Sir,
As advised by you I had a look into following codes:
IS 4031 : Part 6 : 1988Methods of physical tests for hydraulic cement.
IS 4031 : Part 7 : 1988Methods of physical tests for hydraulic cement:
I could not get anything useful pertaining to problem explained by me.
However I got certain clarification from one of the book on CONCRETE TECHNOLOGY written by one Indian author. I reproduce the same here:

THE STRENGTH OF CEMENT TO BE USED IN MIX DESIGN COMPUTATIONS IS NOT MEAN STRENGTH fm OF CERTAIN NUMBER OF TEST RESULTS (SAY N),BUT SHOULD BE THE CHARACTERISTIC STRENGTH fck GIVE BY EQUATION:
fck=fm-ks
WHERE "K" IS PROBABILITY FACTOR,A STATISTICAL PARAMETER FOR NOT MORE THAN 5 PERCENT TEST RESULTS TO FALL BELOW THE CHARACTERISTIC STRENGTH fck AND "s" IS THE STANDARD DEVIATION.
FOR EXAMPLE ,IF THE MEAN OF OF "N" COMPRESSIVE STRENGTH TEST RESULTS OF CEMENT IS 55,THE CEMENT WOULD BE APPARENTLY A GRADE-53 CEMENT.HOWEVER THE STANDARD DEVIATION OF THIS PARTICULAR CEMENT IS 4 MPa ,THE CHARACTERISTIC STRENGTH WOULD BE
fck=55-(1.65 x 4)=48.4 MPa.
THUS THE CEMENT ACTUALLY COMES UNDER GRADE-43.

Sir, I need your observation on entire issue.
WARM REGARDS.
     


P.K.Mallick
munamallick@yahoo.co.in (munamallick@yahoo.co.in)
mallick.pravatkumar@gmail.com (mallick.pravatkumar@gmail.com)
http://360.yahoo.com/munamallick

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PostPosted: Fri Feb 13, 2009 5:37 am    Post subject: Reply with quote

Issues are different. That was on GRADE OF CONCRETE. Here is the case of cement. Kindly have a re-look on entire issue. That was a different question.

In this case my question was:
IS:10262-1882 Reaffirmed 2004(Recommended Guidelines for Concrete Mix Design) shows a graph in form of Fig-2 which gives co-relation between water-cement ratio, 28-day concrete strength and 28-day strength of cement tested according to IS:4031-1968.
Now, if a particular brand of 43-Grade cement shows strength of 50N/sqmm,while using the aforesaid graph should we use strength of cement as 50N/sqmm or restrict it to 43N/sqmm as the grade of cement is 43-Grade.


I do not find reference in the IS CODES that statistical quality control is also applicable while calculating compressive strength of cement.

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PostPosted: Fri Feb 13, 2009 5:43 pm    Post subject: Cement and Mix Design Reply with quote

Dear Mr. Mallick,

The acceptance criteria is based on statistics and probability theory. It is same whether it is concrete or cement!

Regards
Subramanian

P.K.Mallick wrote:
Issues are different. That was on GRADE OF CONCRETE. Here is the case of cement. Kindly have a re-look on entire issue. That was a different question.

In this case my question was:
IS:10262-1882 Reaffirmed 2004(Recommended Guidelines for Concrete Mix Design) shows a graph in form of Fig-2 which gives co-relation between water-cement ratio, 28-day concrete strength and 28-day strength of cement tested according to IS:4031-1968.
Now, if a particular brand of 43-Grade cement shows strength of 50N/sqmm,while using the aforesaid graph should we use strength of cement as 50N/sqmm or restrict it to 43N/sqmm as the grade of cement is 43-Grade.


I do not find reference in the IS CODES that statistical quality control is also applicable while calculating compressive strength of cement.
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PostPosted: Sat Feb 14, 2009 3:09 am    Post subject: Reply with quote

Yes, acceptance criteria is based on concept of statistical quality control where we accept or reject based on hypothesis testing for a certain degree of confidence level. Dispersion of test results for concrete is bound to be wide as concrete is not a factory made controlled product.But it is not so in case of cement because dispersion of result from mean value will not be large as cement is a factory made product. The standard deviation in case of cement will not be large as in the case of concrete. Secondly who will provide the value of standard deviation in case of cement ? Practically it is not possible to determine standard deviation of cement at construction site. IS Codes for cement do not recognize the concept of statistical quality control for  cement. I doubt whether that exists in any Foreign Code. When I so say ,I  humbly submit that I am aware of my limitation. If concept of statistical quality control for cement available in any Foreign code for construction I stand corrected and educated.
Now the question comes what is the genesis behind my question and what is the point I want drive in. Let me explain that.
1)Recently I got design mix for M-25 concrete done by one of the reputed Engineering College.To my surprise I found that the Engineering college has used compressive strength as 46 N/SQMM for PPC Cement which has a designated strength of 33 N/sqmm. On enquiry,it was told to me that Design mix is done as per the cement provided by us and it is also done strictly as per  IS:10262-1882 Reaffirmed 2004(Recommended Guidelines for Concrete Mix Design). You can understand my problem almost catch22 position.
2) Second problem is refusal of a RMC manufacture to fix the target strength as per degree of quality control as "Very good". The manufacture says that degree quality control in case of RMC  is much better than what has been designated as best in case of IS:10262-1882 .It is argued by the manufacturer that they would fix the target strength as Fck+4N/sqmm(acceptance criteria) and with the quality control they maintain the cube strength will satisfy the acceptance critera.

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