Q.: We're bidding a parking deck job that will be built with silica-fume concrete. The weather is likely to be hot and windy so we're worried about plastic shrinkage cracks. To prevent cracking we might use a fog spray until finishing is completed and a curing compound can be applied. For bidding purposes, however, we don't know how to estimate the cost of fog spraying. Can you tell us what the costs might be?

A.: We didn't have much luck getting an answer to this question. One floor contractor says the cost might be 5 ¢ to 10 ¢ per square foot, but that this was just a guess. Fogging nozzles cost about $5.00 each. They deliver a fine spray (l/2 gallon per minute) and require a minimum water pressure of about 30 psi. At a distance of 6 feet above the surface the nozzles produce a spray about 6 feet in diameter at the surface. For wide pours it might be necessary to build a movable pipe frame and mount the nozzles on it. For smaller pours, workers on each side of the slab could use garden hoses with fogging nozzles to keep the surface from drying.

The nozzles can be purchased from Goldblatt Tool Company, 511 Osage, Box 2334, Kansas City, Kansas 66110. If any of our readers can supply further information about the cost of fog spray we'll print it in Problem Clinic.

Reader Response:

The cost of fogging varies greatly depending on equipment on hand, placement size, project size, weather conditions (wind, temperature, humidity), and number of placements exposed to the weather (top decks). When fogging is needed, base your costs on using one person who has no other responsibilities than to operate the fogger(s). Fogging is a full-time job on days when it's necessary. On large projects one-time costs usually represent a small portion of the total cost of fogging. There also may be proportionately few placements that require fogging. One estimate for a medium-sized project (a 270,000-square-foot parking garage requiring extensive fogging for 30 percent of the placements and assuming no equipment on hand initially) puts the fogging cost at $0.011 per square foot.

For larger projects the cost is lower; for smaller jobs the cost may be considerably higher. Other options are available, however. One is the use of evaporation retarding materials as supplied by the major admixture firms. These chemicals have successfully prevented plastic shrinkage cracking with silica-fume concrete Perhaps the best option, though, is to place the concrete at night or delay placement until weather is better.We have seen mixed results with fog nozzles attached to garden hoses. The water pressure generally is not high enough to produce a fine mist. However, we have seen success with nozzles that incorporate compressed air to blow the water out. An effective nozzle assembly is shown in Figure 1 (below).

Figure 2 shows two of the assemblies in use on opposite sides of a deck. High-pressure washing equipment (gasoline engine-powered) may also provide a suitable mist. The equipment may be rented from local suppliers. It is available from KEW Cleaning Systems, 130 East St. Charles Road, Carol Stream, Illinois 60188 and from Mi-T-M Corporation, Box 50, Eoesta, Iowa 52068. R.A. Prisby and Mark Luther Elken Chemicals