EFFECT OF CURING ON DURABILITY OF FLY ASH CONCRETE

Findings of a 5-year laboratory investigation on the effect of curing on the durability of fly ash concrete are summarized. Three series of concrete mixes were cast such that concretes within a given series were of similar strength grade (nominally 25, 35, or 45 MPa) but varied in fly ash replacement level (0 to 50%). Following casting, concrete specimens were subjected to various curing treatments and subsequently were stored at a range of temperatures and relative humidities before testing. To assess the influence of the various curing and storage regimes on concrete performance, concrete specimens were tested for (a) compressive strength, (b) oxygen permeability, (c) carbonation in both internal and external storage, and (d) resistance to chloride ingress and steel corrosion. Results indicated that, whereas all concretes required adequate curing to achieve their potential properties, concretes containing fly ash were more sensitive to poor curing. The difference was especially marked at higher levels of replacement (50% fly ash), where curtailing the moist-curing period resulted in a large increase in the rate of carbonation and permeability and a marked reduction in compressive strength. Concretes with lower levels of ash (15 to 30%) carbonated at slightly higher rates than the control concretes but generally offered lower permeability. Resistance to the penetration of chloride ions increased as fly ash content increased, irrespective of the level of curing applied to the concrete. Poorly cured, low strength grade (C25) concretes with 50% fly ash provided greater resistance to chloride than higher grade (C45) concretes without ash. Concretes with moderate levels of fly ash (15 to 30%) require no additional curing to provide equal, if not improved, durability compared with opc concrete of the same strength grade. Concretes with higher levels of ash may require extended curing compared with opc concrete, especially if the design of the structure and conditions are conducive to carbonation. Adequately cured, high fly ash content concretes are likely to offer substantially improved durability because of their low permeability and high resistance to chloride ion ingress compared with opc concrete.