#CW0316-08 Concrete Washouts

The solution of concrete and water in concrete washouts has an extremely high pH, often above 12.0 pH units, and will drastically change the pH of the soil and run off water if spilled. The tolerable NAL (Numeric Action Level) for pH ranges from 6.5 to 8.5 pH units. Concrete spills and runoff will create conditions of exceedance.

High pH levels can kill adult fish and damage developing juvenile fish. When freshwater pH becomes highly alkaline (9.6 or greater) the effects on fish may include: death, damage to outer surfaces such as gills, eyes, and skin, and an inability to dispose of metabolic wastes. High pH strips a fish of its slime coat and ‘chaps’ the skin of the fish.

Concrete washouts must be emptied while below 75% capacity to ensure no spills will occur. These washouts should be inspected often to ensure containment is not breached. Plastic secondary containment is advised in case primary containment fails, spills occur near the washout, or the primary plastic is torn by concrete.

Concrete washouts should be constructed 50 ft. away (if applicable) from any storm drains to avoid washout water discharge and clearly labeled with signage to inform trades where concrete washing is permitted.

As concrete dries and is exposed to the carbon dioxide in the atmosphere, the concrete goes through carbonation and is reduced in pH units by generating carbonic acid. After the concrete has been reduced, the stabilized pH is often between 6.0 pH and 7.0 pH. Due to this process, concrete should not be poured within 48 hours of a rain event because the concrete will not be cured or within the acceptable pH range.