Pore Structure and Thermal Conductivity of Cryogenic Concrete

Abstract

The pore structure, which controls the main properties of concrete, evolves due to phase changes in the pore network during cryogenic freezing of concrete. This study investigates the influence of such pore structure evolution on the thermal conductivity of different concrete mixtures. Such information would be useful in the design of low thermally conductive concrete for use in liquefied natural gas (LNG) containment structures. Five concrete mixtures including hardened cement paste were prepared using different aggregates and admixtures. The mixtures incorporated river sand as fine aggregate, and traprock and limestone as coarse aggregates. Mixtures without aggregates incorporated different amounts of blast furnace slag (BFS) and granulated polyurethane foam (PUF) and sawdust. The porosity and pore size distribution of concrete specimens were monitored at ambient and freezing temperatures using proton nuclear magnetic resonance (NMR). Thermocouples inserted into concrete specimens at different radial locations monitored the temperature history during cryogenic freezing and thawing. A new inverse analysis technique that simultaneously fits the temperature profile at two different locations during thawing of frozen specimens was used for thermal conductivity determination. The results indicate that among the different mixtures, the total porosity shows a stronger correlation (R = 0.88) with thermal conductivity than the mean pore size (R = 0.52) at freezing temperatures. The total porosity (R = 0.75) was also more influential at ambient temperature. The thermal conductivity results so far suggest the possibility of designing a low thermally conductive concrete by improving on concrete mixture designs incorporating some of the aforementioned admixtures. 