How to assess acceptability of concrete structure based on cube strength

1) Clause 9.2.2 of IS 456 – 2000 :
a) “The mix shall be designed to produce the grade of concrete having the required workability and a characteristic compressive strength not less than appropriate values given in table 2. The target mean strength of concrete mix should be equal to the characteristic strength plus 1.65 times the standard deviation.”
b) Above clearly explains that the mix designed for use when made in lab condition should be able to give a strength of characteristic strength plus 1.65 times the standard deviation. Based on binomial theorem the meaning is that atleast 5% can still fall below the characteristic strength.

2) Clause 15.1.1, last line :
a) “In all cases, the 28 days compressive strength specified in table 2 shall alone be the criterion for acceptance or rejection of the concrete.”
b) Above clearly explains that characteristic strength is the limit for acceptance / rejection / review.

3) Different cases as guideline for interpretation :
a) Step 1 :
i) Check if the variance in cube results exceed 15% as given in clause 15.4. of IS 456 – 2000.
ii) If yes, remove that set of data from further statistical analysis and declare it as faulty sample preparation. Corrective action will be to tighten the sampling and moulding procedure as well as check the mould dimensions.
iii) If no, move to next step given below.
b) Step 2 :
i) If 28 days cube strength (average of 3 samples) is equal to or above characteristic strength
(1) Check if any of the individual value in the three results taken for averaging falls below Characteristic strength minus 4 Mpa.
(a) If yes, concrete is accepted. Corrective action will be to tighten the sampling and moulding procedure as well as check the mould dimensions.
(b) If no, concrete is accepted. It is wrong to say that they should have strength of Characteristic strength plus 4 Mpa.
c) Step 3 :
i) If 28 days cube strength (average of 3 samples) is below characteristic strength,
(1) Check if any of the individual value in the three results taken for averaging falls below Characteristic strength minus 4 Mpa.
(a) If yes, the structure has to be evaluated using NDT equipment.
(b) If no, move to the next step below,
(2) Check if for the same concrete proportion and similar material used earlier from the same plant, the mean of atleast four consecutive results at any time earlier from your plant cube test data (average of three cubes) is atleast Characteristic strength plus 4 Mpa.
(a) If yes, the mix has proved itself that it is capable of giving a higher strength under similar conditions and hence it is a problem with the material used or any batching error.
(b) If no, the concrete is bad and structure needs to be reviewed for the lower strength. The mix has not been designed for the bad degree of control in material quality and batching. The assumed standard deviation during initial design stage has to be increased so that the mix is capable of giving a higher strength with the existing material and plant