The empirical expression has the rational basis of being derived from 

 the parabolic relationship for tangent modulus as follows: 



2 f , 



where E = tangent modulus 



£ = ultimate strain (experimental average was 0.0025) 



and the fitted condition of a = 0.56 f when E. = E^. 



c it 



ACI 



E: = ST.OOOs/fT 



1 ■'•^ c 



6xl2-in. control cylinder 



7,000 8,000 



Compressive Strength, f^ (psi) 



9,000 



Figure 4. Relationship between E: 

 and f '. 



11 



