Leaching of alkali from freshly cast concrete in contact with water
Creators: name: Adamson, Robert
Publisher: name: RMIT University
Year: 2024

description: There has been very little work done on the effect of leaching of alkali from concrete in to water which can cause environmental pollution. After a recent construction of culverts, a road authority in Australia observed that the pH of water had increased causing adverse effects on wildlife. Work presented in this thesis therefore was aimed at understanding the leaching of alkali from concrete in contact with waterways and also understanding what would be an effective barrier to minimise the pollution of water.<br><br>Preliminary experiments conducted on stagnant water indicated that the pH of water can rise to values as high as 11.5 after exposure of concrete to water. A literature review showed that though there had been some experiments using a supply of recycled water and more using stagnant water, there had been little to no research on the effects of concrete on a constant supply of flowing water. Eight major variables were identified as contributing to the pH change in water when in contact with concrete and five were short listed for the experimental study. <br><br>In order to optimise the laboratory work, the theory of Design of Experiments (DOE) was adopted to plan a series of experiments and analyse the effect of major variables. Five parameters: surface area to volume ratio of concrete, flow rate, the exposure to air, presence of pozzolanic additives in concrete and the age of concrete at exposure were varied in the investigation. Custom-designed testing equipment was built in the laboratory for observing the change in pH of flowing water when exposed to freshly cast concrete at various ages. Thirty four experiments were conducted using the testing apparatus and the results were analysed.<br><br>Experimental results indicated that the change of pH of water vs time follows a parabolic curve with peak pH being reached within 20 to 180 minutes from exposure depending on the variables selected. Major variables affecting peak pH were identified as the surface area to volume ratio of concrete and the flow rate. The major variable affecting the time to reach peak pH from initial exposure was observed to be the flow rate. Two models were derived using the theory of Design of Experiments (DOE) to predict the peak pH and the time to reach peak. Whilst the relationship for peak pH had a poor correlation with the results of the validation tests, the relationship for time to reach peak pH appeared to be sound. Poor correlation of the peak pH, flow rate and surface area to volume ratio relationship was attributed to the fact that the major variable appears to be the total surface area of the specimen and not the surface area to volume ratio as initially hypothesised.<br><br>Experiments conducted on a number of potential barriers identified that a water borne acrylic based emulsion was most effective in reducing the leaching of alkali from concrete. However, it has been concluded that this barrier requires further investigation since it appears to contain ammonia which in itself is harmful to wildlife.<br>