Concrete Cube Failure and Acceptance Criteria

Dear Sir,
It is clear that we have to follow table 11 for accept concrete. But in the same IS 456: 2000, Refer clause 15.1.1 which quotes " in all cases, the 28 days comprehensive strength specified in table 2 shall alone the criterion for acceptance or rejection of concrete."
some times, concrete will achieve strength as per Table 2 but if is not greater than table 11, can we reject it?
 

Dnyan Deshmukh

Staff member
Dear Sir,
It is clear that we have to follow table 11 for accept concrete. But in the same IS 456: 2000, Refer clause 15.1.1 which quotes " in all cases, the 28 days comprehensive strength specified in table 2 shall alone the criterion for acceptance or rejection of concrete."
some times, concrete will achieve strength as per Table 2 but if is not greater than table 11, can we reject it?


For acceptance and rejection of the concrete, the strength at 28 days given in table is only the criteria, but when you achieve it, really does matter, it may be you can get that in 7 days or you might required 60 days.

The concrete technology is changed over time and advanced in a way that, you can modify the each property of concrete you prepare and the desired strength or acceptance criteria get changed as per the mix designing of that concrete.

Concrete mix designing is done base on the basic properties and strength requirement, optimization is often done to save on money and also to reduce national wastage of cement. if there is no requirement of early strength you can plan for late strength gaining designs to make it more economic.

What matter in all the cases is the value given in table shown in IS code is achieved at the design age of the concrete.


It do happens that, concrete sometimes does not achieve the desired strength or do show behavior in strength curve which looks abnormal. such cases need to be investigated and decision need to be taken base on investigation. in some cases (where you know, the concrete is not going to meet the desired criteria) you need to take opinion and suggestions from structural consultant for acceptance or for doing strengthening of it.
 
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Concrete Cube Failure and Acceptance Criteria
More panic situation in any project comes after failure in cube strength and most of the Engineers or not aware about acceptance criteria of it.

I asked many about what is acceptance criteria, most of the people mixed the things and not able to tell the correct way.

So decided to write article on it.

View attachment 1660

Before going to acceptance criteria directly, we will go step by step towards it, so that we can also able to identify the root causes and does concrete really failed in compressive strength test.

First we should be well aware about how to sampling and cube casting activity, if this goes wrong your result might indicate data which can’t be concluded.

You can learn this from below thread link.

How to do concrete sampling and cube casting

Now you did correct sampling and cube casting activity, now let’s move on to testing part.

You know we did numbering on the cube and cube number 1,2 &3 represents sample number 1, cube number 4,5&6 represents sample no 2 and so on.

Why we numbered and we should take cube for testing according to sample.

In statistical analysis and finding root causes of failure it helps a lot.

As per IS 456, the individual cube strength should not have variation more than + / - 15% of average of that three cubes.

What if you get more variation in any test result, how you will find the root cause of it.

Say you are testing 3 cubes and the numbers written on it is in serial and represents a sample.

Say you have cube numbered 4,5 & 6 – by numbers I can say it’s a sample no 2 and taken after x cum of concrete.

If you have cube numbers like 3,5&7 taken for testing – it won’t represent a single sample, because it’s a mixture of multiple sample.

Always do verify cubes taken for testing at any age are from same sample.

Now you have cubes which represent single sample and same workmen who filled it.



Suppose you got results like 15, 20 and 25 mpa

What it will indicate – it will clearly indicate that, there is issue in cube casting and you can discard those results easily.

So this will give you a power to have better Quality control because you know the reason of failure and where to take action.

One single sample of 3 cubes casted by a single person and numbered correctly, will never show variation in results and you will always get results with lesser variation than +/- 15%.

When it’s more than this, it indicates cube casting done improperly or you took sample cubes incorrectly for testing.

Hope you will never see a fancy results like above example after implementing it and can caught root causes in failure easily by eliminating possibility of cube sampling and casting issue.

Will write another article on other causes of failure in cubes sometime in future when I get time.



So move on to acceptance criteria

IS 456, specifies the acceptance criteria for compressive as well as flexure strength.

As flexure is 0.7 of square root of compressive strength, it is important to get passed in compressive strength results.



When we do cube testing, we write its results in cube test register with serial numbers, say 1 is xyz date concrete from abc location, 2 is def date concrete from ghi location and so on.

When we compare acceptance criteria, IS say average of 4 non overlapping consecutive test results should not be less than fck + 0.85 x standard deviation or fck + 3mpa (whichever is higher) for grades of concrete M20 and above.

What is mean by 4 non overlapping consecutive test – it means when you average out 4 test results for acceptance criteria, they should not overlap in calculations.

Acceptable averages

1,2,3&4

5,6,7&8 and so on serial number wise results should be average out.

If you do like averaging below

1,2,3&4

If you have low results at 6 and 8 and if you try to make averaging like 3,4,5&6 – then its overlapped to make you benefit by tweaking, this is not allowed and meaning of 4 non overlapping consecutive test results.

If you have good quality control at concrete, then your standard deviation will be less than 2 or slightly more.

In those cases, or in maximum cases applicable acceptance criteria will be always like fck + 3 mpa.



Sometimes there is a possibility that, in one of the test sample, you might get lower results and which makes that group of 4 non overlapping consecutive test result’s to not able to meet acceptance criteria.

In that situation, IS 456 also specifies, Individual test sample results should not be less than

fck – 3 mpa for M20 and above grade concretes.

So you should also check these criteria when single sample have lower results.

Each test results should meet fck + 3mpa criteria in order to ensure no failure and detect the issues at earliest.

Also it is important to observe failure patterns to get clues on what might be wrong or OK.

View attachment 1661
good one
 
+ / - 15%
Concrete Cube Failure and Acceptance Criteria
More panic situation in any project comes after failure in cube strength and most of the Engineers or not aware about acceptance criteria of it.

I asked many about what is acceptance criteria, most of the people mixed the things and not able to tell the correct way.

So decided to write article on it.

View attachment 1660

Before going to acceptance criteria directly, we will go step by step towards it, so that we can also able to identify the root causes and does concrete really failed in compressive strength test.

First we should be well aware about how to sampling and cube casting activity, if this goes wrong your result might indicate data which can’t be concluded.

You can learn this from below thread link.

How to do concrete sampling and cube casting

Now you did correct sampling and cube casting activity, now let’s move on to testing part.

You know we did numbering on the cube and cube number 1,2 &3 represents sample number 1, cube number 4,5&6 represents sample no 2 and so on.

Why we numbered and we should take cube for testing according to sample.

In statistical analysis and finding root causes of failure it helps a lot.

As per IS 456, the individual cube strength should not have variation more than + / - 15% of average of that three cubes.

What if you get more variation in any test result, how you will find the root cause of it.

Say you are testing 3 cubes and the numbers written on it is in serial and represents a sample.

Say you have cube numbered 4,5 & 6 – by numbers I can say it’s a sample no 2 and taken after x cum of concrete.

If you have cube numbers like 3,5&7 taken for testing – it won’t represent a single sample, because it’s a mixture of multiple sample.

Always do verify cubes taken for testing at any age are from same sample.

Now you have cubes which represent single sample and same workmen who filled it.



Suppose you got results like 15, 20 and 25 mpa

What it will indicate – it will clearly indicate that, there is issue in cube casting and you can discard those results easily.

So this will give you a power to have better Quality control because you know the reason of failure and where to take action.

One single sample of 3 cubes casted by a single person and numbered correctly, will never show variation in results and you will always get results with lesser variation than +/- 15%.

When it’s more than this, it indicates cube casting done improperly or you took sample cubes incorrectly for testing.

Hope you will never see a fancy results like above example after implementing it and can caught root causes in failure easily by eliminating possibility of cube sampling and casting issue.

Will write another article on other causes of failure in cubes sometime in future when I get time.



So move on to acceptance criteria

IS 456, specifies the acceptance criteria for compressive as well as flexure strength.

As flexure is 0.7 of square root of compressive strength, it is important to get passed in compressive strength results.



When we do cube testing, we write its results in cube test register with serial numbers, say 1 is xyz date concrete from abc location, 2 is def date concrete from ghi location and so on.

When we compare acceptance criteria, IS say average of 4 non overlapping consecutive test results should not be less than fck + 0.85 x standard deviation or fck + 3mpa (whichever is higher) for grades of concrete M20 and above.

What is mean by 4 non overlapping consecutive test – it means when you average out 4 test results for acceptance criteria, they should not overlap in calculations.

Acceptable averages

1,2,3&4

5,6,7&8 and so on serial number wise results should be average out.

If you do like averaging below

1,2,3&4

If you have low results at 6 and 8 and if you try to make averaging like 3,4,5&6 – then its overlapped to make you benefit by tweaking, this is not allowed and meaning of 4 non overlapping consecutive test results.

If you have good quality control at concrete, then your standard deviation will be less than 2 or slightly more.

In those cases, or in maximum cases applicable acceptance criteria will be always like fck + 3 mpa.



Sometimes there is a possibility that, in one of the test sample, you might get lower results and which makes that group of 4 non overlapping consecutive test result’s to not able to meet acceptance criteria.

In that situation, IS 456 also specifies, Individual test sample results should not be less than

fck – 3 mpa for M20 and above grade concretes.

So you should also check these criteria when single sample have lower results.

Each test results should meet fck + 3mpa criteria in order to ensure no failure and detect the issues at earliest.

Also it is important to observe failure patterns to get clues on what might be wrong or OK.

View attachment 1661

How to take mean of 4 non overlapping results when we have 5 samples in total (Say in the case of 60 cu.m. of concrete)?

We can take avg. of 1,2,3 and 4.
What are we supposed to do for the balance 1 no. sample i.e. No. 5?
 

Dnyan Deshmukh

Staff member
I think you are getting confused with samples, samples do represent the days concrete, 5 samples do represent 5 concrete pours either done on single day or done 1 each day.

1 sample do represent the achieved strength of the concrete for particular pour at particular age.

As the acceptance criteria is applicable for 28 days test results and in most of the cases 28 days strength is termed as final.

there is much more to do than playing theory.

the low results directly indicate the failure in concrete.

thought results showing failure consist of many factors which need to be studied.

so for 5th sample you can check for individual criteria or you can consider 5th result is set of 4 which has same value and do assessment for its acceptance.

In short, for concrete with grade up to M80, you should always have basic acceptance criteria of Fck + 3 Mpa, hard wired in your mind - this helps in assuring, you always do pass the criteria, even when you really not checking it.

To ensure Fck + 3 or beyond is achieved all the time and to reduce the chances of failure further more, mixes are targeted to higher values base of acceptability of failure or low results ration.

I hope now your doubt is clear.
 
We have 6 samples for Day 1 (say for slab) namely sample 1,2,3,4,5,6 and 2 samples for Day 2 (column) namely 7,8

We'll now have total 8 samples and mean of group of 4 non overlapping samples comes out to be,
X1 =Mean (1,2,3,4) and X2 = Mean (5,6,7,8)

Suppose X2 does not satisfy the acceptance criteria, which implies sample 5,6 for slab on Day 1 and 7,8 for column for Day 2 are deemed to fail.

What to do in such a situation, does this imply that the batch of concrete of slab and column are rejected (in case it fails in NDT and core cutting too).
 

Dnyan Deshmukh

Staff member
Such huge sample size do shows, you must be pouring 1,000 + cum of concrete on every day. and most probably from multiple RMC production unit to suffice such huge requirement of the concrete.

Suppose, you are getting a bad concrete from RMC producers unit two represented by sample 2.

For such a big structure and question and that two after doing all NDT test and getting failed results. You need to consult your structural consultant who did designing of your such structure as there is no TOM DICK and Hary on internet could solve your problem or advice you on the same.

In practical reality, people do work to achieve something by referring acceptance criteria to set their own standard to ensure they never create a catastrophic event.

Even if such event do happen like Corona Outbreak, there is no consultant or expert on this earth available to say, nothing will happen just go and enjoy building structure, you will not get affected.

I hope you must have understood now the concept of acceptance criteria for concrete strength.

When there is failure in achieving basics, you need to do following things first;
1. Grab all the data of concrete, production, placing, curing, testing etc.
2. Do analysis of it and try to make some conclusion base on the data and make some rough predictions.
3. Meet structural consultant, show him all report and your analysis, also discuss what you could predict from such event.

Now wait for what your structural consultant decides, remember, for the structural stability, structural consultant is responsible.
and by sharing such event with him, you are ensuring that, you haven't done anything wrong to structure and if structural consultant thinks that, you did wrong, he / she will give the decision for it.

Kindly understand the principle and do forget about theory, this is for doing control, to assure best, you need to do mix design of it so that your control never goes below this control line and you take appropriate actions before occurrence of failure or Outbreak of Virus.
 

ARN

Newbie
Acceptance criteria wont talk about cubes of concrete taken for days concrete or say for columns or slab casted.

Acceptance criteria is same like how we derive standard deviation for perticular grade of concrete.


It has nothing to do with number of cubes you cast for perticular concrete.

It talks about whole concrete you do at your project and not about indivisual.

If you want to check for indivisual sample (cube results of 1 concrete, refere indivisual criteria).


15/12/18 - sample test result of 28 days - 1 test.

16/12/18 - sample trst results of 28 days - 2nd test.

17/12 / 18 - 3rd test.

18/12 /18 - 4th test and so on.

Non overlapping consecutive test reaults average of 4 means

Average of cube results of 15, 16, 17, 18 etc.

2nd set will be 19, 20, 21, 22 etc and so on.

It will never be like
Set of 15, 18 , 20, 21 etc.

In above example concreting is done everyday and their sample results of 28 days are available.
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Dear Dnyan Sir,
your write-up on cube results acceptance criteria is informative and wonderful. Still I request, Please reconfirm the below non-overlapping consecutive concrete cube results write-up. As I think it would be casting from consecutive non overlapping batches from RMC Plant or different Transit Mixer Truck on same day in RMC plant or on site during concreting of structure. (Concrete must be supplied from single batching plant).

Sir, giving some below examples, request please review it at your end.

( Case No.01)
If we are doing M20 grade concreting say of 50.00 cum of footing & complete it on same day then we need to cast cube set as per below:
1st Set- 0-5 cum
2nd Set- 6-15 cum
3rd Set- 16-30 cum
4th Set- 31-50 cum

In the above case we can cast cube as per above on same day and we can test the cubes & take the average of these non-overlapping results after 28 days.
If we would get average results of these 4 - set, less than 24 N/mm2 ( Fck+4) then we can say results is fail.

( Case No.02)
If we are doing M20 grade concreting say of 30.00 cum of footing & complete it on same day then we need to cast cube set as per below:
1st Set- 0-5 cum
2nd Set- 6-15 cum
3rd Set- 16-30 cum

In the above case we can cast cube as per above on same day and we can test the cubes & take the average of these non-overlapping results after 28 days.
If we would get average results of these 3 - set, less than 24 N/mm2 ( Fck+4) then we can say results is fail.

( Case No.03)
If we are doing M20 grade concreting say of 15.00 cum of footing & complete it on same day then we need to cast cube set as per below:
1st Set- 0-5 cum
2nd Set- 6-15 cum

In the above case we can cast cube as per above on same day and we can test the cubes & take the average of these non-overlapping results after 28 days.
If we would get average results of 2 - set, less than 24 N/mm2 ( Fck+4) then we can say results is fail.

( Case No.04)
If we are doing M20 grade concreting say of 5.00 cum of footing & complete it on same day then we need to cast cube set as per below:
1st Set- 0-5 cum
In the above case we can cast cube as per above on same day and we can test the cubes & take the average of these results after 28 days.
If we would get average results of 1 - set, less than 24 N/mm2 ( Fck+4) then we can say results is fail and ask for NDT for the RCC concrete structure which is cast before 28 days.

This is what we perceived and understood from IS code 456 and as per discussion with some others. As there is no any clarity regarding " Non-overlapping Consecutive" terminology given in code or in any bonafide documents along with any practical examples.

Sir, Our understanding might be incorrect for these cases; request please review it. we are also searching the books/ codes for more clarity where any practical example regarding the same is given for the references.

Thanks
ARN
 

Dnyan Deshmukh

Staff member
Sir, Our understanding might be incorrect for these cases; request please review it.

No your understanding about it, is correct.

we are also searching the books/ codes for more clarity where any practical example regarding the same is given for the references.
When it comes to practicality, the many things can not be found as written.

Practically due to advancement in the concrete production machinery and the accuracy provided by them, in most of the cases single set of the sample is tested at the age of the 28 days by considering the amount of the concrete produced on daily basis and poured at multiple locations. so when your doing a big project, you are getting more than required sets of cubes for testing or sometimes you wont get.

If you look closely into the table of frequency of sampling, you will also find a note stating, make at least one sample for one shift (8 hours) where continuous concrete production unit is used such as RMC plant. Now look at the IS code of RMC.

IS 4926, Clause 6.3.2, you will find it states, Unless otherwise agreed between the parties involved, the minimum testing frequency to be applied by the producer in the absence of a recognized readymixed concrete industry method of production control, should be one sample for every 50 m3 of production or every 50 batches, whichever is the greater frequency.
Three test specimens shall be made up for each sample for testing at 28 days

So when you use RMC (Production Machinery with a high precision and production rate), practically you need to cast 1 set of cube for 50 cum of concrete (it assumes all batches in 50 cum will have same quality and consistency).

it also states, In order to get a relatively quicker idea of the quality of concrete, optional test on beams for modulus of rupture at 72 + 2 h or at 7 days or compressive strength test at 7 days maybe carried out in addition to 28 days compressive strength test. For this purpose the value should be arrived at based on actual testing. In all cases 28 days compressive strength shall alone be the criteria for acceptance or rejection of the concrete.

6.3.3 The purchaser shall inform the producer if his requirements for sampling and testing are higher than one sample every 50 m3 or 50 batches, whichever is
the greater frequency.

So the matter of trust changes with kind of machinery and its precision level changes, more advance and highly precise machines can be trusted with lower level of sampling frequencies.

If you have highly precise plant and pouring 1000 cum of concrete in a shift or day, your Plant machinery might be capable of producing that quantity without errors. is the human who is going to cast number of cube samples for 1,000 cum concrete as per IS 456 sampling frequency will do same with same efficiency.

what will happen due to inefficiency of the workmen to cast required big number of sample to required standard, if it shows failure, to whom you will blame and how would you will come to know about is there is problem with cube casting or concrete.

So practically the number of samples are kept as minimal to ensure more precise casting and get correct results from them without raising false alarm about concrete quality.
 
Nice Explanation Sir.

Incase, only 5-6Cum of concrete was executed at site and casted 6nos of cubes, then how you will find "mean of group of 4 non overlapping consecutive test results"

Kindly confirm the acceptance criteria..