FROST DURABILITY OF CONCRETE CONTAINING VISCOSITY-MODIFYING ADMIXTURES

The use of viscosity-modifying admixtures in special concrete intended for underwater placement and repair has increased in the last decade. Such admixtures have also been employed in the production of highly f lowable yet cohesive concrete that can be used in a variety of land-ba sed applications. Limited information exists about the frost susceptib ility of such concrete. The effect of an adding sequence of the air-en training admixture, in relation to the viscosity-modifying admixture a nd the high-range water reducer on an air-void system is evaluated. Th e impact of two commonly used viscosity-modifying admixtures, welan gu m and hydroxypropyl methylcellulose, on frost durability (ASTM C 666) and scaling resistance (ASTM C 672) is determined for fluid mixes made with w/c ratios of 0.32, 0.40 and 0.45. Test results indicate that th e air-entraining admixture is most effective when added after the visc osity-modifying admixture and high-range water reducer: Providing that an adequate air-void system is secured, concrete made with welan gum or hydroxypropyl methylcellulose can exhibit adequate frost durability similar to that of conventional concrete. Regardless of the presence of a viscosity-modifying admixture, concrete with a 0.45 w/c ratio and a spacing factor less than 400 mu m can develop a frost durability co efficient greater than 75 percent. An excellent durability coefficient can be obtained with concrete made with a w/c ratio of 0.32 or 0.40, again, independent of the viscosity-modifying admixture. Regardless of the spacing factor and the presence of a viscosity modifier, highly f luid concretes can develop poor scaling resistance that can be partial ly attributed to the relatively high porosity of the ''skin'' concrete .