Answer / joseph

The compressive strength is calculated at 3 ,7 ,28 days.if
you want to transport a precast structure of the grade
40/20 ,it should aquire 60% of design strength ..here it is
60% of 40.In seven days we shoul attain 75% of design
strength. Then your answer ; all the test cubes must attain
their 100% of design strength in 28 days. to ensure this
one we are taking test in 28 days .if cubes fail we have to
go for field test such as smith and hammer test or pulse
velosity (BS 12504 ).

Refer BS 1881 part 116
there are some factors to asses the test result

refer BS 6089

Answer / sagar sontakke

As soon as we add water to the dry content of the concrete, process of heat of hydration starts and this process will require a very long time to complete.Almost 90% to 95% of this process is completed within 28 days and remaining takes a year or two.This means that concrete attains 90% of the strength at 28 days. So to judge whether concrete has attained the desired strength, we have to test the concrete at 28 days.

Answer / imran

Answer # 4 is the correct answer of this question.

Answer / kanchan

Its is because the maximum strength i.e. 99% of strength gained by a cube in 28 days

Answer / utsav patel

strength is depend on hydration process. after 28 days aronnd 68% strength is gained by concrete. hydration process is not confined up to certain period is longlasting process .

Answer / suresh.s

the cement strength also prepared for 28 days

Answer / silpa

Cube Size=150 mmX 150 MM
Area = 22.5 Sqm.
Stress = Load / Area E
Eg : 600/22.5=26.26 N/mm

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