Evaluation of Thenoz Methodology to Measure Concrete Air Permeability

Abstract

This paper describes how permeability measurements in cement-based materials are one of the important ways to estimate their durability. Several methodologies have been applied for permeability measurements in this kind of materials but large discrepancies in results due to different assumptions of each methodology have prevented strong conclusions about a definite measurement method. Considering that a better understanding of flow mechanisms in permeability measurements can provide an improvement in present methods, the authors propose to apply the Thenoz methodology in permeability analysis of cement-based materials. Thenoz methodology was developed to measure the rock air permeability and has shown good results for cement-based materials. It consists in measurements of air permeability in porous media applying a pressure gradient on both transversals sides of samples. This pressure gradient forces air to flow through the concrete resulting in a reliable analysis of the permeability coefficients. A major question in this method is the right model definition in function of fluid compressibility and flow characteristic, laminar or turbulent. This paper applies Thenoz methodology to describe air permeability in concrete made with granulated blast-furnace slag cement. Concrete specimens were made with constant water/cement (w/c) = 0.42 and submitted to curing in air during 28 days (named conventional concrete). The results were compared with high-strength concrete specimens tested in accordance with the same methodology. At the age of the tests, specimens were oven-dried for 24 hours at 80°C before air permeability tests. Four different pressure gradients were used in permeability tests to describe the flow versus pressure gradient behavior. The degree of accordance with Darcy’s law is shown by the results. Both Mach number and Reynolds number were determined. The results did not show air compressibility during the test, in accordance with Mach number, and results show a laminar flow during tests, in accordance with Reynolds number results. The results show a laminar flow during tests and a good agreement with Darcy’s law, suggesting that Thenoz methodology is an interesting alternative to measure air permeability in cement-based materials.