Paraffin Wax as a Sealant in Sorptivity Testing

The durability and sustainability of concrete structures is a prime socioeconomic concern of contemporary society. The lifecycle of these structures is typically determined by the rate of moisture ingress, in which dissolved and unwanted substances are transported into the structure. Therefore finding a sufficient means to measure these rates is of utmost importance. A sorptivity testing scheme as outlined by a commonly used standard can be a useful means of measuring how quickly liquid can be transported unidirectionally through concrete samples in the laboratory. Currently the standard prescribes that disc specimens 100 mm in diameter and 50-mm thick be sealed in such a manner to ensure unidirectional flow through one, unsealed face of the sample, while all other surfaces are appropriately sealed. However the standard does not specify how this seal is to be achieved. Furthermore, differing methods have yielded a likewise array of varied results, which may not be representative of the true sorptivity. The objective of this paper is to propose a simple but effective means of sealing the specimens that will yield more consistent results, which are more representative of the actual absorption properties of the concrete. This was accomplished by studying the performance of electrical insulation tape and cellophane (as suggested in the standard) against paraffin wax using three different mixture designs, as follows: (1) involving normal strength concrete without air entrainment, (2) a normal strength concrete mixture with air entrapment, and (3) self-consolidating concrete without air entrapment. The prepared specimens were subjected to various freeze-thaw cycling periods, conditioned, and then tested for sorptivity. Batches of samples were exposed to freeze-thaw cycling periods of 0, 50, 100, 150, 200, and 300 cycles. Within each group of samples, half of the specimens were sealed with electrical tape and cellophane, and the other half were sealed with paraffin wax in order to conduct sorptivity testing. Specimens coated in wax typically exhibited lower absorption values than those wrapped in tape. Furthermore, the tape would not adhere perfectly with the concrete at the edge formed by the cylindrical wall and the exposed surface, resulting in increased sorptivity values. Statistical analysis was done on the rates of early-age (0–6 h) and late-age (1–8 days) sorptivity by using the F-test to examine variability (with an 80% confidence interval) and the student-$t$ test was used to evaluate the significance of differences in sample means. Generally, sorptivity rates were higher for taped samples than those for waxed samples. These results further reinforced visual observations. The waxed samples yielded more reliable results primarily because they did not absorb moisture artificially along the sides in the manner as observed in the taped specimens. Moreover, the paraffin wax sealant method also resulted in less variability within test results. Therefore it was believed that the wax facilitated more representative results of the true sorptivity. It was suggested that the procedures of the common standard discussed in this paper should be refined such that the sealing of test specimens should be done with the use of paraffin wax, or some other similar substance, such as silicone, which is impermeable and readily available commercially.