Since long it has been claimed that the departure from linearity of the σ-ε relation for concrete, up to failure under compression, is the result of cracks formation, first along the aggregate-paste interface, which later bridge the aggregates until, at around 75-80% of the maximum load (critical stress level), the matrix cracks become unstable, such that if that load is sustained, fracture will happen after a certain period (due to tertiary creep). This fact has been verified measuring volumetric strains, acoustic emission and ultrasound. The presence, growth and opening of such cracks must have an effect on the permeability of concrete, which was confirmed by water- and gas-permeability tests. The phenomenon was also studied applying a standard NDT method for air-permeability, based on a double-chamber vacuum cell, same as applied in the here reported research. This paper reports results of air-permeability kTof concrete cubes and half-cubes subjected to the following loading conditions: unloaded, sustained loads of 30, 60 and 90% of the ultimate load (measuring kT under the applied static load and the residual value after removing it) and dynamically during a test with monotonically increasing load till failure. The results indicate that a certain degree of damage, reflected in an increase of kT, is noticeably already at a 30% load level, which increases with the level of applied load. The dynamic kTtests were somewhat erratic, possibly, depending on whether the damage happened near or away from the surface where kT was measured.

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