Effect of curing conditions on the strength and durability of air entrained concrete with and without fly ash

In cold regions, air entrained concrete has been widely used against freeze–thaw deterioration. This paper studied the influence of curing conditions on the strength and durability of air entrained ordinary portland cement (OPC) and fly ash (FA) concrete. Four different curing conditions including standard curing (SC), air curing (AC), mild temperature curing (MC), and elevated temperature curing (EC) were utilized to cure the cast specimens. The compressive strength at 3, 7, 14, and 28 days was measured respectively. Also, the water absorption, sorptivity, air void system, as well as freeze–thaw durability after 28 days of curing were investigated. Test results suggested that AC condition impeded the strength gain of both concrete, but it affected FA concrete more than OPC concrete. EC condition improved the early-age strength while compromised the later-age strength of OPC concrete, whereas EC condition improved the compressive strength of FA concrete at both early ages and later ages. AC condition has more adverse effects on water absorption as well as sorptivity of FA concrete than those of OPC concrete. EC condition increased the water absorption and sorptivity of OPC concrete, whereas reduced the water absorption and sorptivity of FA concrete. Although the air content in fresh OPC and FA concrete were comparable, FA concrete had less proportion of microvoids (air voids smaller than 300 μm), larger spacing factor but smaller specific surface, regardless of curing conditions. EC condition caused the reduction in air content, the increase of spacing factor, and decrease of specific surface. For both OPC concrete and FA concrete, the four curing conditions produced different freeze–thaw durability factor (DF), but all the specimens passed the failure limit of 60 %. The freeze–thaw test results based on specimens under SC condition may overestimate the DF.