Q: If we fog spray flatwork to help prevent shrinkage cracking, is there any danger of increasing the water-cement ratio at the surface and reducing the strength of the concrete?

A: If you use the right nozzle and position it properly, fog spraying shouldn’t leave standing water on the concrete surface. If there is standing water when you fog spray, check the following possible causes:

  • Nozzles are pointed downward instead of horizontally.
  • The nozzles are too close together, causing the spray to overlap.
  • The discharge rate is too high (try reducing the water flow or changing nozzles) or the mist is not fine enough (usually due to an incorrect nozzle). If you’re using the wrong nozzle, try one that delivers fewer gallons per minute or puts out a finer spray.

Whatever the cause of the standing water, you should avoid finishing bleed water or fogging water into the slab surface because it could make for a weaker and more porous surface.