Question

As a construction manager what would you do if a seven days concrete test results well below what is required?

Answer 1

As we know that the strength of concrete at different ages in comparison with the strength at 28 days after casting.

Age of concrete	Compressive strength Gain (%)
3 days	30 %
7 days	67.5 % (70% for safety)
14 days	90 %
28 days	99 % (70% for safety)

After understanding the 7 days concrete test is below expected strength,hence chances for not achieving target due to its characteristic strength of concrete.

sometimes below result also occurs due to the way it has been tested is also important.

we need to start with finding out the reasons for failure

# while doing concreting for site the collecting sample for test the possibilities is a cube is good but concrete is weak or vice versa & concrete, cube both are weak.

@ concrete strength is initially weak for some cases due to human errors but the eventually deciding criteria for acceptance of concrete is 28 days.

still, we can't take any chances so we can go for NDT(Non-Destructive Testing) for assurance of concrete strength.

a) Rebound Hammer or Schmidt Hammer (ASTM C805)

b) Ultrasonic Pulse Velocity (ASTM C597)

after doing the above testing if still, concrete result does not meet the essential criteria than we have to wait until 28 days.

than further discussion with RC consultant for Semi Destructive Tensing or something else for a concrete structure.