The variation of radon exhalation rates from concrete surfaces of different ages

The radon exhalation rates from four concrete blocks cast with ordinary Portland cement (OPC) and four with substitution of pulverized fuel ash (PFA) were monitored for 34 months starting from November 1992. At the end of the 28th month, two OPC blocks and two PFA blocks were immersed in water, one from each group for 5 days the other one from each group for 10 days. These were then taken out of water and the radon exhalation rates monitored as usual. It was observed that the radon exhalation rate in general decreased with the age of the concrete blocks and the rate increased after the blocks were immersed in water. An explanation for the first result is that the gradual dehydration of concrete as it ages will reduce the water content in the pores of the concrete, thus reducing the probability of retaining radon within the pores and the probability of radon emanation from these pores. The second result gives strong support to this assertion.