Q: When using a fog spray to prevent plastic shrinkage cracking during a concrete flatwork placement, aren't you taking a chance that the water-cement ratio at the surface will increase, reducing concrete quality?

A: With the proper nozzle and positioning, fog spraying shouldn't leave standing water on the concrete surface. It simply increases the relative humidity above the slab, reducing the possibility of plastic shrinkage cracking. The standing water might be from several causes:

  • Overlap of the nozzle spray (nozzles too close together). Nozzles pointed down instead of horizontally
  • Too high a discharge rate in gallons per minute (try reducing the water flow or changing nozzles) The mist not having fine enough droplets (usually caused by using an incorrect nozzle). You can find fog-spray nozzles in the catalogs of some finishing-tool manufacturers. If you are using the wrong nozzle, try switching to one that delivers fewer gallons per minute or puts out a finer spray.