ACCEPTANCE CRITERIA FOR STRENGTH OF CONCRETE

Acceptance criteria for strength of concrete.

1st) variation in percentage of stregth of single cube not more than and not less than  15% of average strength percentage of sample.

Let start  with one example:
For M 35 grade of concrete the following results are arrives of a sample which is given below:

weight           load           strength         
kg                  kn               N/mm 2                       

8.720  -     1020.1   -        45.33
8.700  -     980.4     -        43.57         
8.724  -     965.1     -        42.89

Average strength  of sample: 43.93 N/mm2


If we convert individual strength in to percentage than we
N/mm 2             PERCENTAGE
45.33     -             129.51%                                                       
43.57     -              124.48%                                   calculation for % (45.33/35)=129.51%                                     
42.89     -             122.54%

And for average strength it will be come - 43.93- 125.51%

If we calculate variation of single strength percentage of cube to average strength percentage of sample than it will be following:

129.51     -      125.51    =     4%
124.48     -      125.51    =   -1.03%
122.54     -      125.51    =   -2.97%

So it is satisfy first criteria.



2nd) Strength of four non overlapping consecutive test result not less than

fck+3 or f ck +0.825* Standard deviation
( which ever greater)
and for individual it is f ck -3.


where f ck - characteristic of compressive strength of concrete.

if the sampling of concrete is not done by IS 456

Than it is

                         f ck+4  or   f ck+ 0.825*standard deviation (which is more)
                   
                         and for individual it is   f ck-2


WHAT IS THE MEANING OF FOUR CONSECUTIVE NON OVERLAPPING TEST RESULT IS GIVEN BELOW:


LET START WITH  EXAMPLE .

Here we describe it with M 35 grade of concrete strength, result of samples are given  below:

wt.           Load        strength        Average
Kg              KN          N/ mm2        N/mm2
8.510       980            43.55
8.512       921            40.93             41.81
8.628       922            40.97                         


8.610        898             39.11             
8.598        878             39.02           39.22                     
8.602        890             39.55

8.600       1010            44.88     
8.624         990            44.00           43.55           
8.630         940            41.77             


8.600       900.1           40.00             
8.498       878.4           39.04           39.38 
8.598       880.1           39.11


8.604       940.1            41.78         
8.502       840.4            37.35          39.34 
8.558       875.4            38.90


8.648      899.4             39.97
8.548     1000.1            44.44            42.21 
8.595     950.4              42.24

For calculaing non overlapping consecutive test result: we must calculate results of strength of  four consecutive samples which is non overlap i.e in above six samples results we add average strength  of 1 2 3 4 sample for non overlapping consecutive result but we can't add 1 2 3 5 because in this case overlapping occure and non consecutive result.

If we need four consecutive test result for non overlapping cube from the above given example it must be

41.81+39.22+43.55+39.38 (average result of samples) = 40.99 N/mm 2

40.99 N/mm 2 is greater than f ck +3 i.e (35+3= 38 N/mm 2 ) than it is satisfy the 2nd criteria.

And for individual result it is come :35 N/mm 2 - 3 N/ mm 2 =  32 N/ mm 2  which is more than above any single result of sample.


HERE GRADE OF CONCRETE M 35...