Question

1. A group of 45 tests on a given type of concrete had a mean strength of 4780 psi and a standard deviation of 525 psi. Does this concrete satisfy the requirements of ACI code for 4000 psi concrete

Answer 1

Answer:

Yes, because both design compressive stress is greater than 4000 psi

Explanation:

To design a concrete that satisfy the requirements of ACI code for 4000 psi concrete

Step 1: design the compressive stress, using the two equations below

$f_{c} =f_{cr}-1.34*s$ -------equation 1

where;

$f_{c}$ is the design compressive stress

$f_{cr}$ is the critical stress = 4780 psi mean strength

s is the standard deviation = 525 psi

$f_{c} = 4780 -1.34*525$ = 4076.5 psi

Step 2: design the compressive stress, using the second design equation

$f_{c} =f_{cr}-2.33*s + 500$ -------equation 2

$f_{c} =4780-2.33*525 + 500$ = 4056.75 psi

Therefore, since both compressive stress is greater than 4000 psi, the concrete satisfies the requirements of ACI code for 4000 psi concrete