Viscosity Modifiers to Enhance Concrete Performance

The hazard rate function for concrete structures is often portrayed as a "bathtub"-shaped curve, with a finite" ever-decreasing probability of early-age failures being followed by a life with a relatively low constant probability of failure that ultimately increases dramatically as the end of service is reached. Ideally, new concrete technologies should reduce the failures occurring at both ends of this service-life spectrum. VERDiCT (viscosity enhancers reducing diffusion in concrete technology) is one such strategy based on increasing the pore solution viscosity. This approach has the potential to reduce the propensity for early-age cracking while also reducing long-term transport coefficients of deleterious ions such as chlorides. In this paper, the performance of a typical VERDiCT admixture-a viscosity modifier/shrinkage-reducing admixture-is investigated in mortar and concrete, both as an addition to the mixing water and as a concentrated solution used to pre-wet fine lightweight aggregates. A reduction in early-age cracking is achieved by eliminating autogenous shrinkage stresses that typically develop in lower water-cementitious material ratio (w/cm) concrete. By substantially increasing the viscosity of the pore solution in the concrete, the resistance to ionic diffusion is proportionally increased relative to a control concrete without the VERDiCT admixture. Herein, chloride ion diffusion coefficients are evaluated for two types of concrete containing typical substitution levels of supplementary cementitious materials namely, either 25% fly ash or 40% slag by mass. For the eight concrete mixtures investigated, the effective diffusion coefficient was reduced by approximately 33% by adding the VERDiCT admixture which, in practice, may imply a 50% increase in their service life, while the autogenous shrinkage was virtually eliminated. However, these benefits in early-age cracking resistance and long-term durability are tempered by up to a 20% reduction in compressive strength that may need to be accounted for at the design stage.