Slab moisture-emission testing is commonly required before contractors can install an impermeable floor topping on concrete slabs on grade. Several tests are available to define whether an excessive slab-moisture condition exists, which can cause debonding of the topping. As the following case history illustrates, however, a slab can pass moisture-emission tests yet still contain too much moisture for adequate performance of an impermeable topping.


The project involved installation of an impermeable epoxy-terrazzo floor topping on a concrete slab that appeared to have been constructed according to project specifications and commonly accepted ACI 302 practices. Although the slab tested "dry" using accepted slab moisture-emission testing procedures, the floor topping later debonded.


Why did the terrazzo topping debond from the concrete slab when accepted and widely used construction, design, and testing procedures were followed? When reconstructing the case history for this project, investigators discovered that the slab moisture-emission test performed by the terrazzo contractor occurred during a 24-hour period that broke long-standing temperature records. Since the building was well-ventilated to prepare for installation of the epoxy-terrazzo topping, it's likely that the exterior and interior air temperatures closely matched when the topping was placed. Because little or no differential in pressure existed between the two environments, no vapor transmission occurred. This resulted in a favorable slab moisture-emission test. After the building's air conditioning was turned on several weeks later, a vapor-pressure differential resulted. The moist under-slab air migrated to the above-slab environment. Compounding the problem was the slab's relatively high water-cement ratio, which was indicated in the petrographic analysis.