Compressive strength of low cement/high fly ash concrete

Abstract

The effect of replacing 35 to 50 percent of cement by fly ash, on the compressive strength of lean concrete mixtures (350 lbs/yd 3; 210 kg/m 3) was investigated, using two ASTM Class F and two ASTM Class C fly ashes. The test results showed that, particularly in lean concrete, fly ash has significant effect on the properties of fresh and hardened concrete. It was presented in the previous paper (1) that the workability of the fly ash concretes was found to be much better, and the water requirement lower; the rate and volume of the bleeding water were either higher or about the same, and setting was slower, depending on the type and properties of the fly ash and on the mix proportions. This paper evaluates the effect of fly ash on the compressive strength development of the hardened concrete. Test results show that, compared with the Reference Mix which reached the required strength (2000 psi; 14 MPa) at 28 days, the concretes with the above fly ash replacement levels reached the strength at 35 to 170 days, depending on the replacement percentage and the properties of fly ash. From the standpoint of concrete behavior, it was concluded that in lean concrete, the use of a large amount of fly ash, as replacement for both cement and sand, is highly beneficial.