Air-entrained Concrete: Relationship between Thermal Conductivity and Pore Distribution Analyzed by X-Ray Tomography

Abstract

The thermal conductivity values of ordinary concrete can be adjusted to those prescribed in constructions by entraining air bubbles to reduce the density of concrete in order to achieve good thermal insulation. This paper concerns the analysis of air bubble distribution in concrete obtained by micro X-ray µCT (computed tomography) and correlates it with its thermal conductivity (k). The samples were prepared of ordinary concrete varying the density by air-entraining additives, ranging between 2,277 kg/m 3 and 1,779 kg/m 3 , aiming to correlate the mechanical properties and k with the characteristics of the bubble distribution. The results show that air-entrainment leads to viable use of this material as sealer to achieve good thermal insulation, and it can be adjusted, but there seems to be a limit to air entraining. By analysis of the µCT images, it was possible to correlate the more quantity of bubbles of smaller diameter with the minor k, in dry or wet state, and to prove that there is a limit in the entrapped air content, and if it is exceeded, the coalescence occurs.

Exploring the curing condition and age effect on thermal conductivity of concrete

“…In addition, thermal conductivity of concrete can be influenced by the pore distribution and entrained air volume in the mix design. Pore distribution analysis using x-ray tomography demonstrated that, air entrainment can be used as a sealer for thermal insulation [4]. It was also shown that in the wet state, the k value is higher due to the exceedance of air entrainer that leads to coalescing of bubbles.…”

“…It was also shown that in the wet state, the k value is higher due to the exceedance of air entrainer that leads to coalescing of bubbles. However, higher air volume reduces the density and negatively affects mechanical performance [4]. A separate study was conducted to determine how k value changes with variables such as admixture type and content (like fly ash and slag), coarse and fine aggregate type and content, age, temperature, water-to-cement ratio, and humidity condition, etc.…”