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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 (Ref 1). Although the slab tested "dry" using accepted slab moisture-emission testing procedures, the floor topping later debonded.

The project, located in the Dallas/Fort Worth area, required installation of more than 10,000 square feet of epoxy-terrazzo topping on the ground floor of a new building. The slab design specified installing a 6-mil vapor barrier on top of a prepared soil sub-grade and covering the barrier

with a 4-inch-thick layer of select clayey sand at or slightly above optimum moisture content. Crews wetted and compacted the sand, then placed and finished the concrete slab in late December 1993. The concrete was allowed to harden but no curing compounds were used, since they were not required in the project specs.

Several forensic investigations were also performed soon after the first blisters were noticed. The tests included:

· A petrographic analysis of two concrete cores taken from areas of contrasting epoxy-terrazzo topping performance

· Moisture-content testing of the sand layer and subgrade materials

· Slab moisture-emission testing The petrographic analysis indicated that the concrete was well-proportioned and had a water-cement ratio of 0.51 to 0.57. The concrete floor slab also met or exceeded the design thickness of 4 inches.

The moisture content of the clayey-sand material was found to be 8% to 11% approximately 15 months after slab construction. The layer also appeared to be relatively dense, arid investigators estimated that the sand contained sufficient moisture to be at or slightly less than the optimum moisture content necessary for compaction. The sand seemed to be damp or moist, but not saturated.

During the sand-layer investigations, the position of the vapor barrier on top of the soil subgrade was also assessed. The barrier appeared to be placed somewhat higher than the exterior finish grade, as required by ACl 302.

Site grading around the building was also examined arid seemed to be adequate to prevent ponded water However, a sprinkler system to maintain the lawn area watered up to the building’s edge.

After investigators observed the condition of the epoxy-terrazzo floor topping and compared the design detail to the constructed floor, the question still remained: Why did the terrazzo topping debond from the concrete slab when accepted and widely used construction, design, and testing procedures were followed?

An article on avoiding arid repairing slab moisture problems provided the answer (Ref. 2). The article describes the driving force for moisture movement through a slab as the differential in vapor pressure between the above- and below-slab environments. Figure 1 shows the vapor pressure of moisture in air at different temperatures and relative humidities.

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 according  to Figure 1, a temperature of 91⁰F with a relative humidity of 60% indicates an above-slab air-vapor pressure of 0.44 psi. The under slab temperature at the time of the mat test was estimated to be about 75⁰F with a relative humidity of about 100%, which also results in a vapor pressure of 0.44 psi. 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, indicating that the epoxy-terrazzo topping could be installed.

After the building’s air conditioner was turned on seven weeks later, the interior temperature dropped below 80⁰F while the relative humidity remained at about 60%. The resulting above-slab vapor pressure was ten estimated at 0.27 psi. The climate-control system, therefore, caused a vapor-pressure differential between the above- and below-slab environments of about 0.17 psi. Because pressure differentials are balanced by migration from a high-pressure environment to a low-pressure environment, the moist under-slab air migrated to the above-slab environment. This moist air-vapor migration transmitted the damage-causing moisture. More than a year after climate-control of the building interior, the vapor pressure differential is still greater than 0.1 psi.

Compounding the problem is the slab’s relatively hi water-cement ratio, which was indicated in the petrographic analysis. As Figure 2 shows, significant wet-curing time is needed to obtain a relatively impermeable paste for concrete mixes with water-cement ratios of 0.51 to 0.57 (Ref. 3). Approximately 25 to 135 days of curing is required to obtain a relatively impermeable cement paste. Unfortunately, the contractor did not facilitate curing by applying a curing compound.

The impact of the vapor barrier on the slab’s moisture content is unclear Although the barrier was installed according to ACI 302 recommendations, ACI is vague about the ground moisture conditions requiring vapor-barrier use. Section 302.1W subsection 2.4.1 states: "Vapor bafflers aggravate the problems of plastic and drying shrinkage cracking. Their use should be avoided if ground moisture conditions permit. If ground conditions require their use, a 3-inch layer of approved granular selfdraining, compactible fill over the vapor barrier (and under the concrete) reduces these problems."

It further states: "Where floor coverings, household goods, or equipment must be protected from damage by moist floor conditions, vapor barriers are frequently used under the slab."

It seems that the primary reason for installing a granular fill over a vapor barrier is to minimize plastic shrinkage cracking and to act as a bleedwater blotter.

What can floor contractors learn from this case study?  It seems clear that they should take the following precautions when constructing slabs on grade to be covered by impermeable toppings: