Answer / viki

the 28 days cube strength for M-25 grade
concrete should be 25N/mm2.
acceptance criteria are some conditions defined by IS 456
regarding characteristic strength of concrete when the mix
is prepared on site.strength tests are performed on 4
blocks of concrete.avg strength is calculated.
then fck+3 means that the avg strength of the concrete mix
at the site should be 3N/mm2 greater than fck of mix.
i.e. mean strength= (fck+3)N/mm2
(refer IS456 CLAUSE 16)

Answer / p c debnath

fck +3 is the average/mean strength of four consecutive set
of cube

Answer / ashwani kumar

AFTER 28 DAY M-25 GRADE CUBE STRENGTH IS 25 N/MM2 , ITS
ACCEPTANCE CRITERIA IN THE IS-456, FCK+3 IS AVG
CHARACTERSRICK SRENGTH OF CONCRETE.

Answer / n pandian

The quality of concrete is acceptable only if both 1) and 2) are satisfied:
1) As per column 2 in Table 11 of IS 456 – 2000, mean of the group of 4 
non-overlapping consecutive test results shall be greater than or equal to
a) (fck + 0.825s)   N/mm2

where fck is the characteristic compressive strength of concrete;
s is the established standard deviation which is to be arrived for  
atleast 30 samples as early as possible at site. 


Note: In the absence of established standard deviation value assumed standard deviation values given in Table 8 of IS: 456-2000 may be used.

b) (fck + 3)  N/mm2 for M15 or (fck + 4)for N20 and above
a) or b) whichever is greater








2) As per column 3 in Table 11 of IS: 456 – 2000, Individual test Results (ITR) shall be greater than or equal to (fck - 3)  N/mm2.

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