Measuring Air Permeability of Concrete During Brazilian Tension Test

A vacuum flow system was developed to measure air permeability in conjunction with indirect (Brazilian) tension testing. The experimental system is designed to provide permeability and load-displacement data to correlate permeability and damage in quasi-brittle rocks and concrete. A flexible sleeve using PMC-770 (Polyurethane) is molded around the outer cylindrical concrete surface and connected to a vacuum system on the rear side. Atmospheric air is drawn through the front of the sample due to vacuum applied at the sample rear. The pressure drop across the sample and air flowrate is measured using flowmeters and pressure build-up in fixed volume reservoirs. The system is designed to cover a five order of magnitude range of air permeability by utilizing both steady-state and quasi-steady techniques. A 50 mm diameter and 25 mm long concrete specimen was used for the test. Displacement controlled loading was applied to 20 degree arc steel plates placed in contact with the concrete specimen. Change in concrete secant modulus was used to estimate concrete damage. The flow rate and pressure drop were measured at increasing load and were correlated with concrete damage. The testing revealed a significant rise in apparent permeability once the estimated damage reached a value of approximately 50%. The apparent air permeability shows to be in good correlation with damage evolution.