Required Average Compressive Strength

The Required Average Compressive Strength is defined in ACI 301-96 (References 1), where it is designated f_cr'.

If there are enough Test Strengths ( see Minimum Number of Test Strengths ) to obtain a Standard Deviation, f_cr' is the greater of:

• f_c' + 1.34 x k x s or
• f_c' + 2.33 x k x s - 500 (psi)

Where:

f_c' = the Specified Compressive Strength

k = the k-factor specified in ACI 301-96

s = the Standard Deviation of the Test Strengths

If there are not enough Test Strengths to obtain a Standard Deviation, f_cr' is obtained from f_c' according to the following table.

fc' (psi)	fcr' (psi)
less than 3000	fc' + 1000
3000 to 5000	fc' + 1200
5000 to 10,000	fc' + 1400
10,000 to 15,000	fc' + 1800

These values are obtained from table 4.2.3.3.b of ACI 301-96.