Q: How pure does curing water for concrete have to be? We want to use river water to cure concrete for an elevated structural slab at a train station. The river water normally contains 100 parts per million (ppm) of salt but occasionally tests at 1,000 ppm. At 100 ppm it's considered potable but at 1,000 ppm it's not. Can we safely use it to cure the concrete slab?

A: ASTM STP 169B, Significance of Tests and Properties of Concrete and Concrete-Making Materials, discusses requirements for curing water. Chapter 43 says to ask two questions:

  • Will the curing water stain the concrete?
  • Will the curing water contain aggressive agents that might attack the concrete?

Aggressive attack is unlikely if you cure with water that's suitable for use as mixing water. Even at 1,000 ppm, the water is still suitable for mixing water according to ASTM C 94 which gives chemical limits for wash water used as mixing water. Staining is usually caused by iron or organic matter in the curing water, not by salts. And staining from curing water is uncommon, especially where a relatively small volume of water is used.