Q.: We desire to refinish overhead exposed concrete spans in concrete which is at a temperature of 220 degrees F and cannot be cooled. Spalls vary in diameter from roughly 1 inch to 1 foot and resemble ordinary popouts. Repairs may be cosmetic in nature and will the exposed to the weather. Can you recommend materials and a method of patching in this hot condition?
A.: It is not practical to cover the entire surface, and no external fastenings are to show.
Magnesium ammonium phosphate cements might do this job. There are five manufacturers of these materials in the United States:
Fox Industries 3100 Falls Cliff Road Baltimore, Maryland 21211
A.C. Horn 1555 Wrightwood Addison, Illinois 60101
Set Products Inc. 8501 Freeway Drive Macedonia, Ohio 44056
Steelcote Manufacturing Company 3418 Gratiot Street St. Louis, Missouri 61303
The Upco Company 4805 Lexington Avenue Cleveland, Ohio 44103
The magnesium ammonium phosphates will with stand the temperature required but will set extremely fast. It will undoubtedly be necessary to mix the material quickly at room temperature and apply it with no delay. Normally the materials set within about six or eight minutes at room temperature and of course this setting time is reduced further at higher temperatures. The manufacturer may have to incorporate a special retarder in the mix to prolong the setting time for this application.
To this difficulty is added the fact that these cements are not buttery and workable like masonry mortars. For this reason it would probably be necessary to pack the material into position and hold it there with some sort of supporting form for about 10 minutes until it has a chance to set and harden. The amount of time available for packing and screeding could only be determined by trial and might be only a fraction of a minute.
Because the material is not buttery and cannot be featheredged it would be best to cut ail spalled areas to produce sharp shoulders. This may present something of a problem considering the temperature of the concrete. One manufacturer recommends that the work be done by the shotcreting process with water added at the nozzle, a method which is said to work well with these cements. Shotcreting might not be easily adaptable, however, to these small repairs.
If there is any way that you can cool at least the surface of the concrete with fans before beginning the work, even though the mass of the concrete remains at a higher temperature, it would simplify the operations.