Implications of coupled degradation mechanisms of cement based materials exposed to cold climates

Abstract

Designing durable concrete structures is becoming increasingly important with emphasis being placed on understanding implications on coupled degradation mechanism in order to more accurately estimate service life. This study focuses on the coupled effect of low temperatures on chloride binding capacity, ion-binder interactions and the effect on service life estimates using Life 365 software. In addition to a control (100% general use (GU) cement) paste, three supplementary cementitious materials (slag (GGBFS), metakaolin (MK), and silica fume (SF)), two water to binder ratios (0.3 and 0.5), and several isothermal and thermal cycling curing regimes ranging from 23 °C to −15 °C were tested. Outcomes of this research indicate that: (i) service life estimates vary markedly (as low as 2 years up to 40 years) when coupled effects occur, namely, freeze-thaw temperatures and chloride ingress. (ii) After curing for 56 days and being exposed to chlorides at 23 °C, the order of magnitude of chloride binding is GGBFS (23.78) >GU (21.57) >MK (16.16) >SF (6.14) for 0.5 M free chloride concentration but the trend is the same for 1.0 and 2.0 M. The dominance of the GGBFS samples was attributed to the importance of chemical binding related to the aluminate content of the mix components, resulting in the formation of increased amounts of Friedel's salt especially at higher free chloride concentrations of 1.0 M, 2.0 M and 3.0 M. (iii) Thermal cycling over a four month duration revealed that the value of binding capacity changed depending on the exposure temperature. In particular, service life estimates using Life-365 software revealed that chloride binding capacities determined at 23 °C may not be conservative when estimating service life in colder climates. For example, considering concrete containing 40%GGBFS, the ratio of the service life estimate based on 23 °C/0 °C exposure is 0.44.