EFFECT OF STRESS ON GAS-PERMEABILITY IN CONCRETE

The effect of compressive stress on the permeability of concrete was e xperimentally investigated using nitrogen gas as the flowing substance . A uniaxial compressive load was applied to a cylindrical hollow conc rete specimen and increased by steps until failure, while a constant n itrogen gas pressure was maintained between the walls of the hollow cy linder and the volumetric gas flow rate was measured in a steady state . Gas permeability was then calculated and compared at increasing stre ss levels. Structural lightweight concrete with water-cement ratios of 0.4 and 0.6 was compared with normal-weight concrete with equal water -cement ratios and at similar stress levels. The stress level at which gas permeability increased significantly (significant stress level) w as 76 to 79 percent of the ultimate strength for normal-weight concret e, whereas 82 to 89 percent of ultimate strength for the lightweight c oncrete. Structural lightweight concrete had both a higher significant stress level and a higher stress level at which dilation due to micro cracking was noted (critical stress level) compared to normal-weight c oncrete. Furthermore, normal-weight and structural lightweight concret e with different degrees of water saturation were tested. A higher deg ree of saturation resulted in a higher significant stress level.