Typically, for every litre of water used in concrete, a litre of water is used to wash out the truck on return from site.

The danger of watercourse contamination is significantly reduced by water-recycling systems, as is the quantity of mains water supply taken. Most ready-mixed plants now collect this wash water and recycle it.

Watercourse contamination can be reduced by:

• Minimising water used to wash out mixers and mixer trucks.
• Collecting wash water and run-off water from aggregate stockpiles in settlement tanks. After settling, the clear water is used to make concrete. The solids may be used as fine aggregate in non-critical concrete.
• Storing wash water and returned loads in agitating tanks as slurry, which is again incorporated into the concrete production process.

At a few plants, rainwater is collected and used as mixing water.For further information see Concrete and the Environment, published in CONCRETE in September 2001, pp39–46. Copies are available as a free download from the Members Area of the Concrete Society web site.