When the weather is unusually hot, the Masonry Standard Joint Committee's Specification for Masonry Structures requires that the contractor fog spray a masonry wall three times a day for 3 days following its construction. In the August 2000 issue, we described a Masonry Construction-sponsored research project, which indicated that fog spraying dramatically increases flexural tensile strength-by nearly a factor of 3 in our tests. This, we noted, could be especially important with an unreinforced wall. Tests by Erich Axsom (a University of Colorado graduate student) and by the National Concrete Masonry Association showed similar results. We performed some tests to determine how much water sprayed onto the face of a concrete block was absorbed and how long that water stayed in the block.
We placed hollow concrete blocks in an oven, heated them, covered them with plastic so that only the faces would be wetted during fog spraying, thus simulating the way a block in the middle of a wall would be exposed to fog spraying.
A garden sprayer filled with water was used to fog spray both sides of the block until water ran down the sides. The amount of water absorbed during the fog spraying was consistent. The sprayed block was then placed inside the oven. At different times, the oven door was opened and the weight recorded. We repeated the tests.
Following a similar procedure, we grouted a hollow concrete block at 115° F, covered it with plastic so the water could evaporate from the faces only, and weighed the specimen at the specified time intervals.
We started our fog spraying the day after the prisms were built. This allowed about 16 hours initially for the block to dry before it was rewetted. The hollow block specimens retained 25% to 40% of the initial water after 16 hours of drying. As expected, the grouted block specimen dried much more slowly.
Based on these results, we suggest that fog spraying increases flexural tensile strength for two reasons. First, it helps to hydrate the mortar closest to the outer surface of the masonry, where flexural tensile failures begin. Second, it reduces the drying shrinkage of the mortar, which causes cracking at the interface between the units and the mortar. The net result is an almost tripling of the flexural tensile strength (from 43 to 117 psi in our tests), which emphasizes that the strength of specimens in flexural tension is greatly influenced by the mortar properties, including extent of hydration.
The extra moisture provided by fog spraying, however, seems to be much less beneficial for specimens in compression. This is because compressive strength is not as sensitive as flexural tensile strength to small changes in mortar or grout strength and because the water must penetrate to greater depths to affect mortar and grout strength.
We calculated how much water would be needed each time a wall is fog sprayed.