The effect of drying preconditioning on the South African durability index tests

This study investigated the effect of different drying regimes in the preconditioning stage on Durability Index (DI) test results. The moisture condition of specimens needs to be stable and uniform for the tests to be accurate and reliable. Three drying regimes were used: (a) standard oven drying method of 50 °C for 7 days, (b) oven drying at 50 °C to constant mass, and (c) drying using a solvent replacement method with isopropanol. Concrete mixes were designed using three w/c ratios (0.40, 0.50 and 0.65) and four binders. The isopropanol and oven drying to constant mass methods were found to remove different amounts of moisture compared with the standard drying method. Most (about 80%) of the moisture was removed within 7 days when oven dried. Statistical analysis suggested that, for certain mixes, the drying method had an effect on the DI results, with the chloride conductivity test being the most sensitive. For quality control purposes, it is impractical to wait until specimens are completely dry, which in some cases took up to 17 days, before performing the DI tests. The practical solution is for specimens to be tested at not less than 7 days and not more than 8 days of drying.