Evaluation of thermal conductivity in air permeable concrete for dynamic breathing wall construction

Air permeable concrete (APC) is potentially useful as a dynamic insulator. The dynamic function is achieved by passing air through the material in the direction of heat flow to facilitate heat recovery. An APC sample of 200 mm length with 60% cement filling of large voids (between 0.5 and 5 mm), was tested between 5 and 10 Pa differential pressures; permeabilities were 0.28–0.32 m2/Pa h, confirming its suitability as a dynamic insulator. To characterise properties it is necessary to determine the static thermal conductivity, i.e., no air flow. A one-dimensional heat flow model for predicting the effective thermal conductivity (λe) of APC is developed using as variables the fractions of voids, aggregate and cement paste comprising the material. Measured values of λe were 0.7–1.4 W/m K. A theoretical model predicts and further improves the performance and formulation of APC. The water/cement ratio (w/c) also controls the λe. Increasing w/c increases the volume of micropores, adding resistance to heat flow.