Alternatives for River Sand: the potential of offshore sand!

The overuse of river sand for construction has various undesirable social and ecological consequences. As a solution for this, various alternatives such as offshore sand, quarry dust (or manufactured sand), dune sand and washed soil have been considered. This research focuses on using offshore sand as the most viable alternative for river sand.

The purpose of using offshore sand is because the environmental and ecological impact from extracting the sand from 15 meters below sea level is minuscule. Using beach sand is not a viable solution due to the impacts it has, such coastal erosion, salt water intrusion into rivers and collapsing of river banks. It has been found that the costs for pumping and dredging will be considerably lower than for river sand. Transportation costs should not exceed that of river sand. With the intention of exploring this new challenge of using offshore sand as a viable replacement for river sand. 

Prof. Priyan Dias, along with his colleagues Prof. Anura Nanayakkara and Prabath Seneviratna, embarked on this research. The main objectives were to review the prevailing literature regarding the concept, to characterize the sands by observing the various properties such as the grading, the shell and chloride content while observing the draining and rain effects on the chloride content and finally to study the corrosion performance of the reinforced concrete made using offshore sands.

The literary review revealed that most of the documentation regarding the usage of offshore sand is from UK, whereas some European countries have also recorded the practice.  It should be noted that this is different from using sand deposits - which when deployed in the Middle East have resulted in accelerated corrosion due to the high chloride contents resulting from the long term exposure to salt spray. Also, resorting to methods such as using sea water for batching will increase chloride contents. Hence they are to be avoided at all costs. 

The research methodology involved building a 2 meter column of sand and observing the effects of natural drainage and simulated rain. Even as little as 320 mm of simulated rain (the highest average monthly rainfall in Colombo) has reduced the chloride levels well below the acceptable levels. Furthermore, using seawater within the derived acceptable limits has not increased the rate of natural corrosion in concrete with embedded steel. The accelerated-corrosion performance has been impressive, as it was no different to a chloride-free control mixture. On the other hand, using a mixture of seawater saturated sand (i.e. without allowing drainage or natural washing) has resulted in much higher corrosion.

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