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P.K.Mallick General Sponsor
Joined: 14 Dec 2008 Posts: 1098 Location: Kanpur. p.k.mallick1962@gmail.com

Posted: Sat Mar 24, 2012 2:01 am Post subject: 


sspawar wrote:  Dear Mallik Saab,
Can you paste calculation/steps of acceptance criteria for your case, how it is fit with the 1978?
Regards 
Respected Pawar Sir
Let us look into acceptance criteria of IS:4561978.
The concrete shall be deemed to comply with strength requirement if:
a}every sample has a test strength not less than the characteristic value;
OR
b)the strength of one or more samples though less than the characteristic value,is in each case not less greater of :
1)the characteristic strength minus 1.35 times the standard deviation ;and
2)0.8times the characteristic strength and the average strength of all samples is not less than the characteristic strength plus (1.651.65/root of number of samples)standard deviation.
Hence of acceptance criteria of IS:4561978 is quite simple if every sample has a test strength not less than characteristic value. It becomes complicated only when sample strength is less than characteristic value.
In our case individual sample strength was more than 25N/sqmm ,in fact it was consistently between 25N/sqmm to 28 N/sqmm and rarely going above 28N/sqmm but never below 25N/sqmm.
Hence it was outrightly acceptable as M25 concrete as per acceptance criteria of IS:4561978.
But it was not acceptable as per IS:4562000 because the mean strength determined from any group of four consecutive test results was less than 28 N/sqmm .
Warm Regards.
_________________ P.K.Mallick
p.k.mallick1962@gmail.com


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rajmane ...
Joined: 26 Jan 2003 Posts: 160 Location: Chennai

Posted: Sat Mar 24, 2012 7:18 am Post subject: ACCEPTANCE CRITERIA OF CONCRETE 


(1) Individual Cube test result should be never considered as per IS:4562000.
(2) We have to consider always a concrete sample test which is made of 3 cube specimens. We should never any one cube test for any decision separately and it has to be a part of a concrete sample.
(3) The mean strength determined from any group of four consecutive test results means each test result is an average of 3 cubes. If this is not case, we can not decide or should not decide.
(4) IS code requires actual standard deviation to be always considered, not from the table of IS:456. Without this number/information of SD, we can not and we must not declare 28 N/sqmm or 25 N/sqmm as belonging M25, as per code.
(5) Let us remember that we are talking about samples drawn from concrete which is highly heterogeneous and hence we have to respect its variability of strength and apply statistical principles.
N P Rajamane
(ExSERC) Head, Centre for Advanced Concrete Research (CACR), SRMU
On Thu, 22/3/12, P.K.Mallick <forum@sefindia.org> wrote:
Quote: 
From: P.K.Mallick <forum@sefindia.org>
Subject: [SEFI] Re: ACCEPTANCE CRITERIA OF CONCRETE
To: general@sefindia.org
Date: Thursday, 22 March, 2012, 10:22 PM
P.K.Mallick wrote: Respected all
Concrete Grade M25
1)Individual Cube test result is more than 25 N/sqmm.
2) The mean strength determined from any group of four consecutive test results is less than 28 N/sqmm but more than 25 N/sqmm.
The concrete is to be rejected or accepted ?
Well,few days back Er. Pawar made an inquiry about the case.Well,the case was resolved.But I am not sure whether the solution arrived at was technically correct one? ok,let me tell you what happened.The case was like this 1)individual cube test result was more than 25 N/sqmm and 2)the mean strength determined from any group of four consecutive test results is less than 28 N/sqmm but more than 25 N/sqmm.
The Grade of concrete was M25.
So as per the acceptance criteria of IS:4562000,the concrete did not comply the strength requirement.
Hence the next step was to go for Core test as per clause 17.4 of IS:4562000 and the core tests complied the requirement of clause 17.4.3 of IS:4562000 and the concrete was accepted.
Let me reproduce the clause 17.4.3 here and that says:
Concrete in the member represented by a core test shall be considered acceptable if the average equivalent cube strength of cores is equal to atleast 85percent of the cube strength of concrete specified for corresponding age and no individual core has a strength less than 75 percent.
In this case the cube strength of concrete specified for corresponding age was taken as 25N/sqmm and core tests satisfied the requirement.
This result did not surprise me at all as earlier all cubes are found to be above 25N/sqmm and the mean strength determined from any group of four consecutive test results was less than 28 N/sqmm but more than 25 N/sqmm.
But I would like to bring to the notice of all of you another aspect of this problem here.
Though acceptance criteria of concrete as per IS:4561978 is totally different from that of IS:4562000,the core strength criteria of IS:4561978 and IS:4562000 ARE IDENTICAL.(Refer clause 16.3.3 of IS:4561978 and clause 17.4.3 of IS:4562000)
THE ABOVE MENTIONED CONCRETE WOULD HAVE BEEN ACCEPTED STRAIGHT AS PER ACCEPTANCE CRITERIA OF IS:4561978. But it was not acceptable as per IS:4562000.But ultimately we accepted the concrete as per core test criteria of IS:4562000 WHICH IS IDENTICAL TO THAT OF IS:4561978.
Should say that writer of code forget to revise the core test criteria of IS:4562000?

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P.K.Mallick General Sponsor
Joined: 14 Dec 2008 Posts: 1098 Location: Kanpur. p.k.mallick1962@gmail.com

Posted: Sat Mar 24, 2012 2:16 pm Post subject: Re: ACCEPTANCE CRITERIA OF CONCRETE 


In the cited example "individual sample" is considered not "individual cube". In the explanation ,it is mentioned as "individual sample" not "individual cube".
For M25 grade of concrete,the mean of the group of 4 nonoverlapping consecutive test result should be more than 28N/sq mm(minimum). If established standard deviation (rounded off to nearest 0.5N/sq mm) is more than 3.636,the allowable limit for M25 Grade concrete as far as mean of 4 nonoverlapping is concerned shall be more than 28N/sq mm.But in any case 28N/sq mm is the minimum allowable for M25 grade concrete.
_________________ P.K.Mallick
p.k.mallick1962@gmail.com


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rajmane ...
Joined: 26 Jan 2003 Posts: 160 Location: Chennai

Posted: Sat Mar 24, 2012 4:54 pm Post subject: ACCEPTANCE CRITERIA OF CONCRETE 


Mr P.K.Mallick
Can you look into observations and comment?
I disagree that we can decide anything based on mere compressive strength of cube alone We must accept distributive nature of compressive strength of concrete and use this.
N P Rajamane
On Sat, 24/3/12, P.K.Mallick <forum@sefindia.org> wrote:
Quote: 
From: P.K.Mallick <forum@sefindia.org>
Subject: [SEFI] Re: ACCEPTANCE CRITERIA OF CONCRETE
To: general@sefindia.org
Date: Saturday, 24 March, 2012, 7:31 AM
sspawar wrote: Dear Mallik Saab,
Can you paste calculation/steps of acceptance criteria for your case, how it is fit with the 1978?
Regards
Respected Pawar Sir
Let us look into acceptance criteria of IS:4561978.
The concrete shall be deemed to comply with strength requirement if:
a}every sample has a test strength not less than the characteristic value;
OR
b)the strength of one or more samples though less than the characteristic value,is in each case not less greater of :
1)the characteristic strength minus 1.35 times the standard deviation ;and
2)0.8times the characteristic strength and the average strength of all samples is not less than the characteristic strength plus (1.651.65/root of number of samples)standard deviation.
Hence of acceptance criteria of IS:4561978 is quite simple if every sample has a test strength not less than characteristic value. It becomes complicated only when sample strength is less than characteristic value.
In our case individual sample strength was more than 25N/sqmm ,in fact it was consistently between 25N/sqmm to 28 N/sqmm and rarely going above 28N/sqmm but never below 25N/sqmm.
Hence it was outrightly acceptable as M25 concrete as per acceptance criteria of IS:4561978.
But it was not acceptable as per IS:4562000 because the mean strength determined from any group of four consecutive test results was less than 28 N/sqmm .[/color:2f6fc4d69f]
Warm Regards.
P.K.Mallick
p.k.mallick1962@gmail.com

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sspawar ...
Joined: 05 Jun 2009 Posts: 1171

Posted: Sun Mar 25, 2012 2:03 am Post subject: 


Dear Mr. Mallick
I agree with you as per code of 1978, your results are fit to the criteria.
Second thing what I want to say regarding revision of Acceptance criteria in IS 4562000.
Concept and thought behind this revision 
While making and till the testing of concrete cubes all operations and precautions are handled at laboratory level in controlled manner.
While pouring of the same designed concrete and then till its hardening many factors involves, which makes concrete in situ weak.
Factors are eg workmanship of placing of concrete, segregation, improper or over vibration and compaction, slippage of slurry, temperature and time effect, confined or external situations curing, etc.
That's why concept of accepting cube test results at its characteristic strength is found incorrect.
Cast in situ concrete results are found too below then factor of safety.
Hence Cut off line is shifted from characteristic strength to towards higher side of MTS.
But the clause of Core cutting criteria is kept same as earlier because of its variance is considered within the factor of safety.
To think that BIS body has forgotten to revise this clause is totally incorrect view.
Regards
P.K.Mallick wrote:  sspawar wrote:  Dear Mallik Saab,
Can you paste calculation/steps of acceptance criteria for your case, how it is fit with the 1978?
Regards 
Respected Pawar Sir
Let us look into acceptance criteria of IS:4561978.
The concrete shall be deemed to comply with strength requirement if:
a}every sample has a test strength not less than the characteristic value;
OR
b)the strength of one or more samples though less than the characteristic value,is in each case not less greater of :
1)the characteristic strength minus 1.35 times the standard deviation ;and
2)0.8times the characteristic strength and the average strength of all samples is not less than the characteristic strength plus (1.651.65/root of number of samples)standard deviation.
Hence of acceptance criteria of IS:4561978 is quite simple if every sample has a test strength not less than characteristic value. It becomes complicated only when sample strength is less than characteristic value.
In our case individual sample strength was more than 25N/sqmm ,in fact it was consistently between 25N/sqmm to 28 N/sqmm and rarely going above 28N/sqmm but never below 25N/sqmm.
Hence it was outrightly acceptable as M25 concrete as per acceptance criteria of IS:4561978.
But it was not acceptable as per IS:4562000 because the mean strength determined from any group of four consecutive test results was less than 28 N/sqmm .
Warm Regards. 


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durgakind SEFI Member
Joined: 04 Apr 2009 Posts: 1

Posted: Sun Mar 25, 2012 8:19 am Post subject: ACCEPTANCE CRITERIA OF CONCRETE 


Dear sir,
The acceptance on mere compressive strength has been followed by most of the field workers till date.
Is there any other factor that is to be considered???
Though there is a clause regarding the number of samples to be cored as per IS 456: 2000, it also mentions that one has to rely upon an experts advice to decide on number of samples.
Will it be correct to recheck the structural design for the obtained compressive strength and move forward???
With best regards
DURGA PRASAD. R
On Mar 24, 2012 4:11 PM, "Dr N. Subramanian" <forum@sefindia.org (forum@sefindia.org)> wrote: Quote:  Dear Er Mallick,
Interpretation of core tests is also difficult. See the paper by Prof. Adam Neville published in Concrete Int. which is available at
http://www.sefindia.org/forum/files/ci2311neville_186.pdf
Regards
NS
P.K.Mallick wrote: P.K.Mallick wrote: Respected all
Concrete Grade M25
1)Individual Cube test result is more than 25 N/sqmm.
2) The mean strength determined from any group of four consecutive test results is less than 28 N/sqmm but more than 25 N/sqmm.
The concrete is to be rejected or accepted ?
Well,few days back Er. Pawar made an inquiry about the case.Well,the case was resolved.But I am not sure whether the solution arrived at was technically correct one? ok,let me tell you what happened.The case was like this 1)individual cube test result was more than 25 N/sqmm and 2)the mean strength determined from any group of four consecutive test results is less than 28 N/sqmm but more than 25 N/sqmm.
The Grade of concrete was M25.
So as per the acceptance criteria of IS:4562000,the concrete did not comply the strength requirement.
Hence the next step was to go for Core test as per clause 17.4 of IS:4562000 and the core tests complied the requirement of clause 17.4.3 of IS:4562000 and the concrete was accepted.
Let me reproduce the clause 17.4.3 here and that says:
Concrete in the member represented by a core test shall be considered acceptable if the average equivalent cube strength of cores is equal to atleast 85percent of the cube strength of concrete specified for corresponding age and no individual core has a strength less than 75 percent.
In this case the cube strength of concrete specified for corresponding age was taken as 25N/sqmm and core tests satisfied the requirement.
This result did not surprise me at all as earlier all cubes are found to be above 25N/sqmm and the mean strength determined from any group of four consecutive test results was less than 28 N/sqmm but more than 25 N/sqmm.
But I would like to bring to the notice of all of you another aspect of this problem here.
Though acceptance criteria of concrete as per IS:4561978 is totally different from that of IS:4562000,the core strength criteria of IS:4561978 and IS:4562000 ARE IDENTICAL.(Refer clause 16.3.3 of IS:4561978 and clause 17.4.3 of IS:4562000)
THE ABOVE MENTIONED CONCRETE WOULD HAVE BEEN ACCEPTED STRAIGHT AS PER ACCEPTANCE CRITERIA OF IS:4561978. But it was not acceptable as per IS:4562000.But ultimately we accepted the concrete as per core test criteria of IS:4562000 WHICH IS IDENTICAL TO THAT OF IS:4561978.
Should say that writer of code forget to revise the core test criteria of IS:4562000?

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rajmane ...
Joined: 26 Jan 2003 Posts: 160 Location: Chennai

Posted: Sun Mar 25, 2012 9:50 am Post subject: ACCEPTANCE CRITERIA OF CONCRETE 


24 March 2012
Dear Mr. Mallick and Mr Pawar, and others
Let us see the following.:
(1) If a big mass of given concrete is converted into concrete samples, then each sample (not one cube, but each sample being average of three cubesr as per IS:4562000), would have different strengths numerically, due to inherent heterogeneity (inevitable) of concrete mix.
(2) However, despite the varying strengths, all these various strengths can be shown to follow a 'normal distribution' (ND) curve meaning that the distribution curve itself (X axis  compressive strength, fc, Y axis frequency of occurrence of compressive strength, fc) can be represented by an equation containing two constants, mu and SD.
(3) The value mu on the ND curve is,
mu=average compressive strength=population mean strength=target mean strength=compressive strength
This value has the highest frequency of occurrence, since, the curve has highest ordinate value at the middle and is symmetrical about this value.
(4) SD=Standard deviation, represents the spread of the curve. Lower SD means sharper curve with lower degree of variation)
(5) Characteristic strength, fck, is a compressive strength value on the x axis of the ND curve, such that area of the curve to the left of fck value is 5% and remaining curve to the right is 95%. This means that the strength values of the concrete which are numerically more than fck, constitute 95% of the test data and thereby only 5% of the values are less than fck.
This indicates that if a strength value occurs, it is 95% chance that the value could be more than fck or it is only 5% chance that the value could be less than fck. Thus, a fck indicates a failure rate of 1 in 20 which is considered as acceptable. Therefore, fck is used in the structural design calculations.
(6) In contrast to above, on the ND curve, mu=population mean=target mean strength value, indicates a value on the xaxis, which has 50% of the ND curve on left of it and another 50% to the right of it. Thus, any strength test sample value could have 50% chance of being more than fm or 50% chance of being less than fm i.e., a failure rate of 1 in 2 which is too risky to be used for structural design. Therefore, fm value is not used in the structural design and not for any acceptance.
However, fm is used to design the concrete mix at the development of the mix stage, since, it has highest value of frequency. During mix design, we are not sampling, but, whole concrete mix mixed is converted into cube specimens and the strengths of the test samples are determined. This should be done as then the whole heterogeneity of concrete is converted into specimens and strength distribution is captured to large extent.
(Thus, in the spirit of concrete technology, I feel, it is quite inadequate to test only 3 cubes at the mix design stage)
(7) When four samples are drawn from the concrete (with known distribution of strengths characterised by mu and SD), then, statistical principles are applied to the average value, xbar, of these four samples.
(Please remember and observe, we are not talking about 4 cubes, but four samples, each sample itself is an average of three cubes, as per IS:4562000)
This average value of xbar is then tested statistically for its degree of representation of Target mean strength (mu, population) with 90% confidence as acceptable using the formula:
Estimate of population mean = mu = xbar + or  SD*z/sqrt (n)
where
xbar = Average value of 4 samples drawn from the whole population of concrete
n = 4 = No of samples drawn from the whole population of concrete
z = 1.65 = A statistical parameter (actually from the xaxis of 'Standard Normal Distribution Curve' ) representing the 90% confidence we are imposing on the sample average value of xbar
If sefians agree above, I wish to go further, otherwise, I can not proceed since I wish to respect and keep in mind that concrete made at any given quality control would have distribution of strengths.
The difference between fm and fck is not due to lack of quality control on the site. We must also note that the target mean strength also occurs in the field concrete when the large number of sample test data is considered, where 'n' is a large value in the above mentioned equation.
I request the sefians with deep understanding of statistics with regards to population mean, standard deviation, sample average, etc contribute.
I do not wish to state/agree here that 'target mean strength' is only for mix design stage and not for achievable on the site.
The term '1.65*SD' in the equation for target mean strength of IS:4562000 does not represent actually the lack of quality of concrete on site as this aspect is served appropriately by value of SD.
Let us remember that only randon errors are acceptable on site and the errors which we can estimate or anticipate are not accounted during the mix design stage.
Thanking you
Yours sincerely
N P Rajamane (ExSERC),
SRM University
On Sun, 25/3/12, sspawar <forum@sefindia.org> wrote:
Quote: 
From: sspawar <forum@sefindia.org>
Subject: [SEFI] Re: ACCEPTANCE CRITERIA OF CONCRETE
To: general@sefindia.org
Date: Sunday, 25 March, 2012, 7:33 AM
Dear Mr. Mallick
I agree with you as per code of 1978, your results are fit to the criteria.
Second thing what I want to say regarding revision of Acceptance criteria in IS 4562000.
Concept and thought behind this revision 
While making and till the testing of concrete cubes all operations and precautions are handled at laboratory level in controlled manner.
While pouring of the same designed concrete and then till its hardening many factors involves, which makes concrete in situ weak.
Factors are eg workmanship of placing of concrete, segregation, improper or over vibration and compaction, slippage of slurry, temperature and time effect, confined or external situations curing, etc.
That's why concept of accepting cube test results at its characteristic strength is found incorrect.
Cast in situ concrete results are found too below then factor of safety.
Hence Cut off line is shifted from characteristic strength to towards higher side of MTS.
But the clause of Core cutting criteria is kept same as earlier because of its variance is considered within the factor of safety.
To think that BIS body has forgotten to revise this clause is totally incorrect view.
Regards
P.K.Mallick wrote: sspawar wrote: Dear Mallik Saab,
Can you paste calculation/steps of acceptance criteria for your case, how it is fit with the 1978?
Regards
Respected Pawar Sir
Let us look into acceptance criteria of IS:4561978.
The concrete shall be deemed to comply with strength requirement if:
a}every sample has a test strength not less than the characteristic value;
OR
b)the strength of one or more samples though less than the characteristic value,is in each case not less greater of :
1)the characteristic strength minus 1.35 times the standard deviation ;and
2)0.8times the characteristic strength and the average strength of all samples is not less than the characteristic strength plus (1.651.65/root of number of samples)standard deviation.
Hence of acceptance criteria of IS:4561978 is quite simple if every sample has a test strength not less than characteristic value. It becomes complicated only when sample strength is less than characteristic value.
In our case individual sample strength was more than 25N/sqmm ,in fact it was consistently between 25N/sqmm to 28 N/sqmm and rarely going above 28N/sqmm but never below 25N/sqmm.
Hence it was outrightly acceptable as M25 concrete as per acceptance criteria of IS:4561978.
But it was not acceptable as per IS:4562000 because the mean strength determined from any group of four consecutive test results was less than 28 N/sqmm .[/color:b95c7c5a1f]
Warm Regards.

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P.K.Mallick General Sponsor
Joined: 14 Dec 2008 Posts: 1098 Location: Kanpur. p.k.mallick1962@gmail.com

Posted: Sun Mar 25, 2012 12:07 pm Post subject: Re: ACCEPTANCE CRITERIA OF CONCRETE 


[quote="rajmane"]Mr P.K.Mallick
Can you look into observations and comment?
I disagree that we can decide anything based on mere compressive strength of cube alone We must accept distributive nature of compressive strength of concrete and use this.
N P Rajamane
Respected Rajamane Sir
Sorry,I could not get you.Which observations and comment you are talking of ?
As regard to your disagreement to decide anything on compressive strength,I would like to say that a field engineer has got hardly any other choice.
Warm Regards.
_________________ P.K.Mallick
p.k.mallick1962@gmail.com


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bishwa SEFI Regulars
Joined: 26 Jan 2003 Posts: 22

Posted: Sun Mar 25, 2012 3:29 pm Post subject: ACCEPTANCE CRITERIA OF CONCRETE 


On 25032012 11:44, rajmane wrote: [quote] Mr P.K.Mallick
Can you look into observations and comment?
I disagree that we can decide anything based on mere compressive strength of cube alone We must accept distributive nature of compressive strength of concrete and use this.
N P Rajamane
On Sat, 24/3/12, P.K.Mallick wrote:
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bsec Bronze Sponsor
Joined: 26 Jan 2003 Posts: 202

Posted: Mon Mar 26, 2012 6:07 am Post subject: ACCEPTANCE CRITERIA OF CONCRETE 


Dear All,
Sincere thanks to Mr Rajmane, who has explained in great detail the genesis of the IS:456 codal provision on acceptance criteria for concrete cubes, which in fact is the universal statistical procedure for acceptance, applicable for many cases.
Difference between mean strength and characteristic strength has to be well understood by the Engineers involved in day to day construction works. Without this understanding there is always a scope for misinterpretation of code.
Best Wishes
Alok Bhowmick
From: rajmane [mailto:forum@sefindia.org]
Sent: Monday, March 26, 2012 9:14 AM
To: general@sefindia.org
Subject: [SEFI] Re: ACCEPTANCE CRITERIA OF CONCRETE
24 March 2012
Dear Mr. Mallick and Mr Pawar, and others
f 
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