Permeability and durability of high volume fly ash concrete under an applied compressive stress

The durability of concrete is one of its most important properties and has been an attractive subject for research in recent years. One of the criteria, which determine concrete durability, is permeability. Transport processes in concrete have been investigated for several decades. However, the correlation between transport coefficients and applied stress has received only little attention. The scope of this study encompassed two major research focuses. The first involved developing a test method capable of measuring the water permeability of concrete under an applied stress. The second involves investigating the permeability of high volume fly ash (HVFA) concrete at early ages. Two sets of tests were carried out. Special emphasis was placed on understanding the influence of stress application on the permeability of concrete at early ages (1-3 days). In the first set, four normal concrete mixes were investigated for effects of stress on early age concrete. In the second set, three high volume fly ash (HVFA) mixes were made to investigate the effects of fly ash on the permeability of fresh concrete. For each mixture, three 100 x 200 mm cylinders and 2 cylindrical hollow core specimens with a 50 mm diameter hollow cylindrical core at the center were cast. The cylindrical specimens were used to determine the compressive strength. The hollow core specimens were placed in specially designed cells such that water would permeate under pressure, and the collected water was drained out to a collection reservoir where its mass was measured by a computer-controlled scale. In the first set, one of the permeability cells was mounted in a testing machine to apply a certain compressive stress on the specimen during the test, but in the second set, permeability of normal concrete were compared with HVFA concrete without stress. Results indicated that the presence of a compressive stress below a certain threshold value decreased the permeability, but when the applied stress exceeded this threshold, a significant increase in the permeability occurred. Addition of fly ash as a supplementary cementing material due to the retardation and slow strength gain in concrete, increases the permeability of fly ash concrete at early ages.