CHLORIDE INGRESS IN HIGH-VOLUME FLY ASH CONCRETE

Through contact with de-icing salts or as a result of sea salt spray or direct seawater wetting, chlorides can enter a concrete structure causing a break-down of the passivation layer around steel and subsequently corrosion. In high-volume fly ash concrete, fly ash reacts with calcium hydroxide leading to a lower degree of alkalinity of the pore solution and in turn a higher capacity to bind chloride ions. The pozzolanic reaction products will refine the concrete microstructure. However this reaction proceeds at a slower pace than the hydration of portland cement making high-volume fly ash concrete at young age much more permeable than traditional concrete with no replacement of cement by fly ash. The resistance against chloride ingress of concrete mixtures with 0%, 35%, 50% and 67% of the cement content replaced by low calcium fly ash was measured with a non-steady state migration test (NT BUILD 492). At 28 days, the chloride migration coefficient increased with increasing fly ash content. Due to the proceeding pozzolanic reaction, the chloride migration coefficients at three, six and twelve months were lower for all concrete mixes with fly ash compared to traditional concrete. However, chloride ingress can occur not only by diffusion, but also by capillary absorption. With equipment for accelerated degradation test, developed in-house, the four concrete mixtures were subjected to a cyclical submersion test into 3.5% NaCl solution for 14 weeks. This test resulted in a decreased resistance of the mixtures with fly ash compared to the reference concrete. To model the chloride ingress, different types of transport behaviour of chloride ions should be taken into account