Effect of surface moisture on air-permeability kT and its correction

It is a well-known fact that moisture in the pores hinders the flow of gas through them, thus having a strong effect on the gas-permeability of concrete. Contrary to laboratory tests, where specimens can be preconditioned by drying, site air-permeability tests are normally conducted under the natural moisture conditions prevailing at the moment of the measurement. Swiss Standard SIA262/1-E: 2019 prescribes that the site air-permeability test (double vacuum cell or Torrent method) is applicable to measure the air-permeability coefficient, kT, only when the surface moisture content m does not exceed 5.5%, m measured with an electrical impedance-based instrument. This paper analyses 50 sets of parallel data of kT and m, recorded during different drying processes, originated from five independent investigations. The analysis confirms that a relation of the type $$kT = kT_{0} \cdot e^{ - \delta \cdot m}$$ can be fitted to the large majority of the cases ( $$\overline{R} = 0.95)$$ , with δ falling within 1.0–2.0 in 84% of the 50 cases analysed, with a median value of 1.45. This analysis allows the authors to propose a practical method to correct the effect of m on kT, the robustness of which is verified by a sensitive analysis. The correction is of little practical relevance for surface moistures between 4.5% and 5.5%. It is expected that this correction may be included in future versions of standards.