The standard concrete compressive strength test is carried out at 28 days. 3-days and 7-days concrete tests are carried out only to monitor the early strength gain and are useful in construction where high early strength of concrete is required as per construction documents. But the values of 3-days and 7-days test results are not used for acceptance purpose.
Concrete 28-Day Strength Test
The 28 days concrete test results are accepted as standard compressive strength values. The compressive strength acceptance is specified by 220.127.116.11 of ACI 318-14 as :
- Average of three compressive tests results should be equal to or more than the specified strength.
- At least three 4 x 8 in. (100 x 200 mm) cylinders or at least two 6 x 12 in. (150 x 300 mm) cylinders should be used for compressive strength test as per sections 18.104.22.168 and 22.214.171.124 of ACI 318-14, ACI 301-16 “Specifications for Structural Concrete”, and ACI 311.6-18 “Specification for Testing Ready Mixed Concrete”.
- For concrete with specified strength upto 35 MPa (5000 psi), no strength test results should fall below the specified compressive strength by more than 3.5 MPa or 500 psi.
- For concrete with specified strength above 35 MPa (5000 psi), no test results should fall below the specified strength by more than 10%.