The 1999 edition of the ACI 530.1/ASCE 6/TMS 602 Specification for Masonry Structures (MSJC Specification) now requires that when the mean daily temperature* exceeds 100° F or exceeds 90° F with a wind velocity greater than 8 mph, new masonry construction must be fog-sprayed until damp-and this must be repeated at least three times a day until the wall is 3 days old.
A substantial amount of test data on fog-spraying has been collected and incorporated into a report titled "Research Evaluation of the Flexural Tensile Strength of Concrete Masonry." One conclusion of this research is that fog-spraying concrete masonry construction can increase bond strength in very hot and arid conditions. In addition, fog-spraying concrete masonry under these conditions can reduce shrinkage cracks in mortar.
No such supportive data currently exists for spraying clay masonry. 70-year-old research concludes that wet-curing merely saturates masonry, decreasing adhesion between mortar and brick, and it appears there is little to gain by wet-curing. Although the Brick Industry Association Tech Note 8 states that "wet-curing of masonry generally produces higher bond strength than dry curing," there is no test data to back this up.
One potential problem with fog-spraying is that introducing excessive moisture may contribute to efflorescence.
In addition to the limited test data, the MSJC ignored certain vital physical properties of the clay masonry, such as unit absorption or initial rate of absorption (IRA). Clay masonry units with very low absorption will extract very little moisture from the mortar. Introducing additional moisture into a very low absorption unit could further decrease bond.
The committee did not take into account the humidity that the assemblage is exposed to. Although the mean daily temperature might exceed 90° F, the humidity might also exceed 90%. Humidity this high will reduce moisture loss from the masonry system.
Possibly the most significant problem with the MSJC Specification is the failure to recognize that these are minimum requirements for masonry construction. Given the traditional over-conservative approach of designers, fog-spraying could become standard practice on masonry construction in the summer.
Two major problems could occur as a result of requiring clay masonry assemblages to be fog-sprayed. First, at any temperature, introducing excessive moisture into low-absorption clay masonry units and moderate-to-high water-retentive humidity can cause lack of bond and separation cracks due to planes of water that develop between the mortar and the unit. These cracks are often misinterpreted by the designer as workmanship flaws.
Second, most masonry contractors are not familiar with fog-spraying or equipped to fog- spray masonry. So fog-spraying can easily be overlooked or improperly applied by the contractor, especially if the designer tightens the requirements. Also, fog-spraying is not well defined. No criteria have been established for how wet the units should be. As a consequence, the contractor (even if not at fault) could be required to tear down the masonry and reconstruct it. However, the Code is the Code, even if it contains major oversights for contractors. So, "gentlemen, start your foggers."