The durability of concrete using concrete plant wash water
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Hundreds of ready-mix concrete trucks are dispatched daily from ready-mix concrete plants. On average, a concrete truck has a carrying capacity ranging from 7 to 9 cubic metres of concrete, requiring about 1500 litres of water. In addition, 500 to 1300 litres of water are used to wash out the excess concrete when the truck returns. Based on these figures, it is clear that the ready-mixed concrete industry consumes large amounts of fresh water. The purpose of this study is to determine the feasibility of using the wash water as mixing water in new concrete. The specific goal of this project is to determine the influence of the high pH and dissolved solids content of the wash water on the durability of concrete, particularly with respect to de-icing salt induced corrosion of steel reinforcement. Two types of mix designs were used in this research: a standard class N and a higher strength, structural C2 concrete. Two sets of concretes specimens were made with both mix designs: one with wash water and one with clean water. The project consists of a multi-component experimental program, beginning with wash water characterization, then pore solution and cement chemistry, followed by an evaluation of the effect of wash water on concrete workability and mechanical properties, resistance to de-icing salt scaling, and corrosion of reinforcing steel embedded in the concrete. In addition, the effect of the wash water on the effectiveness of air entraining agents (AEA) is being determined using air void analysis of specimens with different AEA contents. On the basis of the results obtained from the comparative study using wash water versus tap water, it appears that wash water can be used as mixing water for the production of concrete without compromising the durability properties related to corrosion and salt scaling performance. In addition, the mechanical and plastic properties of wash water concrete meet all standards pertaining to the use of wash water concrete and are similar to those of tap water concrete. Finally, the pore solution, thermal analyses, and water analyses of wash water and tap water showed comparable results. Overall, with regards to the tests conducted thus far, the use of wash water as mixing water in concrete poses no durability concerns.