Moisture Levels for the Safe Installation of Epoxy Terrazzo

Because epoxy terrazzo is a very resin rich and a very dense system, the passage of moisture from the bottom side cannot occur. Moisture from the substrate is involved in causing the disbondment of an epoxy terrazzo that can appear as blisters. Moisture migration through the concrete slab, dew point during installation and other causes of excess moisture, set up complex physical and chemical reactions that cause this disbondment.

It has been theorized that excess moisture during the installation process resulting from a high dew point will interfere with the penetration and cure of the epoxy at the concrete interface. In addition, the movement of moisture through the slab is ultimately responsible for carrying salts and basic ions to the surface. This will either react with the aggregates within the concrete, the epoxy bonded to the concrete or a crystal formation which may exert extremely high physical forces that will contribute to disbondment. Lightweight concrete holds moisture for a longer period of time than standard concrete. It may require several months or even years for the excess moisture trapped in the aggregate to leave.

To prevent this problem from arising a number of actions can be taken during the construction and installation process. First the sub-grade below the concrete slab must be designed to allow moisture to move away from the slab by using four inches of crushed rock followed by two inches of coarse stone. Sand should not be used, as this will allow for capillary flow of water towards the slab. Second, an under slab moisture vapor retarder which meets ASTM E-1745 Class C Standard must be installed directly under the concrete. This barrier must provide long term service and have a perm rating of 0.30 perms or less. The concrete itself must be high density (140lbs/cubic feet), high compressive (>5000 psi), low slump (<4 inches), low water/cement ratio (<0.45), well-graded aggregate that meets ASTM C33, and contain no salts.

Use a wood bull float to open the top of the slab to allow the bleed water to evaporate. Finish the slab with a steel trowel. Wet cure the slab for a minimum of three days. The curing process of concrete requires water. When the internal humidity of concrete drops below 80% the curing process will stop. Well cured concrete is denser and less porous. Concrete must cure for a minimum of 28 days @ 70F prior to applying a non-breathable membrane or an epoxy floor.

Prior to installing the epoxy flooring system, measure the Moisture Vapor Transmission using Calcium Chloride test kits as described by ASTM E 1907-97 or humidity test (see Form G-1). During the measurement process, temperature (both air and surface) and humidity must be recorded to provide meaning to the test. All measurements (and floor installation) should be done when the facility is equilibrated to use conditions with respect to temperature and humidity. Readings should not exceed 3.0 pounds/24 hours/1000 square feet. The carpet, resilient and tile flooring industries share this criteria.

If the MVT readings are higher than acceptable, the slab must be treated to remedy the situation or the owner must risk the possibility of moisture related problems after the floor is installed.

There are three different systems to reduce the MVT rate in the General Polymers product line. FasTop MVT, AquArmor AVC and AquArmor MCS. Each system has its own benefits for different project conditions. Please contact Technical Service to discuss which system is best suited for your specific application.

FasTop MVT is a 1/8 inch thick, self-leveling, urethane modified concrete. FasTop MVT is a self-priming one-coat system that is broadcasted to refusal with 30 mesh silica sand. Four hours after the application the excess silica can be removed and terrazzo strips and/or the terrazzo matrix can be installed.

AquArmor MSC is a 1/8 inch thick, self-leveling, waterborne epoxy slurry. AquArmor MCS requires a prime coat of AquArmor resin reduced 20% with water. Then the slurry is applied and broadcasted to refusal with 30 mesh silica sand. With both FasTop MVT and AquArmor MCS testing after application is not necessary. Both systems have no limitations on MVT levels that they can handle and can even be applied over green concrete.

Finally, throughout this process the details must be communicated and documented to all parties. Excess moisture in a slab is a result of the concrete work and the flooring contractor must be aware of the issue but should not assume its responsibility.