How to Identify and Fix Concrete Moisture

Identifying and addressing moisture issues will help guarantee installation success.

Introduction – Concrete moisture vapor emissions

Concrete slabs on grade that have not been poured over an effective vapor retarder are subject to high levels of moisture vapor emissions that can cause failures in flooring adhered to the concrete. For this to occur, two additional conditions must be met – the concrete must have a well developed pore structure to transport the moisture and there must be a continuous source of moisture under the slab. Concrete poured with a high water to cement ratio results in an unusually permeable slab that readily allows moisture to move through it. Sources of moisture are usually plentiful – seasonal rains, landscape irrigation, snowbanking, a broken pipe or soil that stays continuously wet through capillary rise from the water table.

As moisture vapor moves through the concrete, it carries soluble alkaline materials always found in concrete such as sodium, potassium and calcium hydroxides. As these corrosive materials become concentrated in the upper portions of the concrete, they can damage the flooring systems or the concrete itself. This damage may be evidenced by disbonding, adhesive breakdown, osmotic blisters, mold or staining.

Moisture Vapor Measurement

There are several methods of moisture detection that are subjective, qualitative tests – they give a “wet-not wet” result. Included in these would be the plastic sheet test and the electronic moisture meter . These tests are only indicators and are not recognized by the flooring industry as valid, stand-alone moisture tests.

The two tests currently accepted by the flooring industry for measuring moisture in concrete are the calcium chloride test and the relative humidity probe test. In the United States, the calcium chloride test is the most commonly used and is governed by ASTM F-1869. In this test a dish of anhydrous calcium chloride is pre-weighed and placed on clean concrete. The dish is covered with a plastic dome and sealed to the floor. After 60-72 hours, the dome is removed, the dish weight and weight gain is calculated. The results are expressed in pounds of moisture emitted from 1,000 sq. ft. in a 24 hour period. The safe levels of moisture varies from 3-5 pounds depending upon the flooring system being installed. Calcium chloride test results are significantly affected by ambient air humidity and slab temperature. Therefore it is critically important to have the area climatized to the anticipated service conditions for at least 48 hours prior to testing. In practice this may not always be easy to accomplish.

Although calcium chloride testing is the accepted standard for moisture testing in the United States, the test does have its limitations. It has always been believed that the test measured the dynamic of moisture vapor moving through the slab. However, new scientific evidence suggests that the test actually measures static moisture in the top inch of the concrete and is not a good indicator of moisture deep in the slab. With this in mind, a high calcium chloride reading would be a good indication that the concrete is too wet to receive flooring. However, a low reading may only indicate that the upper portion of the concrete is dry enough to receive flooring, but moisture deeper within the slab has not been measured and may be at unacceptably high levels. Once low permeability flooring is installed, the total moisture in the slab redistributes itself to reach equilibrium and the upper portion of the concrete may become wetter than when the calcium chloride test was taken.

In several European countries, the preferred method of measuring moisture in concrete is the relative humidity probe. This test is governed by ASTM 2170. In this test, holes are drilled into the concrete at specified depths and fitted liners are inserted to the bottom of the hole. The liners are capped and allowed to equilibrate for 72 hours before inserting the probe and taking the relative humidity reading. If this reading is taken at close to mid-slab depth, it will be representative of the level of moisture that the flooring will be exposed to after the slab is covered and a new equilibrium reached. Acceptable levels of relative humidity within the slab range from 75-85% depending upon the flooring installed. Humidity probe testing is much less sensitive to short term fluctuations in ambient air humidity and slab temperature than the calcium chloride test. This is a definite advantage when testing areas that cannot be climate controlled such as aircraft hangars and automotive repair facilities.

The humidity probe test is relatively easy to do and the test kit including the electronic meter can be purchased for around $500. We recommend adding the humidity probe test to the calcium chloride test and using both sets of data to reach a decision about moisture content in the concrete.

In conjunction with moisture testing, conducting pH testing on the concrete is also commonly done. This testing is always very subjective and the results are often misleading or meaningless. To effectively measure concrete alkalinity, the surface must be abraded to break through the carbonated layer that always shows a low pH. Once into the concrete interior, pH readings will necessarily be high because soluble alkalinity in concrete is abundant. We believe that high alkalinity in the concrete is a given and that the critical factor is the amount of moisture available to transport the alkalis to the concrete/flooring interface.

APF Remediation Materials

The goal of moisture vapor remediation is to keep excessive moisture and soluble corrosive alkalinity isolated from the flooring system by creating a low permeability barrier on the surface of the concrete. A complimentary strategy to strengthen the top layer of concrete and reduce the alkalinity near the surface. Arizona Polymer Flooring has developed two products for use in moisture remediation – APF Moisture Vapor Stop (MVS) and APF Lithium Penetrant.

Amine cured epoxy coatings are generally the preferred material in coating concrete and generally considered very moisture and alkali resistant. However, laboratory analysis of some epoxy coatings exhibiting osmotic blistering over high moisture concrete showed that the liquid in the blisters contained organic substances not found in concrete such as amines, phenols and benzyl alcohol. It can be surmised that these compounds were not crosslinked into the coating’s polymer network and leached out after prolonged exposure to the moist, alkaline environment present at the concrete/coating interface, eventually resulting in bond failure.

APF Moisture Vapor Stop is a 100% solids epoxy material based on Bisphenol F resin and a very hydrophobic amine curing agent. It has been carefully formulated to insure that all of the component raw materials will be completely reacted, leaving nothing that could migrate out of the cured coating. The Bisphenol F resin contains more croslinking sites than standard Bisphenol A epoxy and produces a coating with very low permeability and improved chemical resistance. Curing the resin with a hydrophobic amine curing agent insures the ability to develop superior adhesion to wet concrete. Please consult our product data sheet for ASTM test results and application information.

MVS Joint Filler is a semi-rigid epoxy based on the same chemistry as the MVS material. It is used to fill non-moving joints and cracks prior to application of MVS coating.

APF Lithium Penetrant is a reactive silicate solution used to densify concrete as well as reduce the level of alkalinity near the surface. When the reactive silicate is applied to the concrete, it penetrates into the pores and finds always ava
ilable calcium hydroxide to react with. Calcium silicate hydrate is formed, an insoluble material that will occupy space in concrete pore structure making the concrete denser and less permeable. This strengthening is especially important when dealing with moisture saturated concrete that could be weakened by high levels of alkalinity near the surface. The silicate-calcium hydroxide reaction ties up a percentage of the concrete’s free alkali, making it unavailable for transport through the concrete by moisture vapor.

The penetrant is used prior to MVS when levels of moisture are very high as an added protection to help insure successful remediation.

APF Remediation Systems

Level One Protection Moisture Test Readings: Calcium Chloride – under 8 pounds
R. H. Probes – less than 85%

System Design: Surface is shotblasted to achieve a minimum 10 mil profile. Joints are treated with MVS Joint Filler. One coat of APF Moisture Vapor Stop is applied at 200 sq. ft. per gallon to yield 8 mils dry.

Material Costs: $0.35 – $0.45 per sq. ft.

Level Two Protection

Moisture Test Readings: Calcium Chloride – 8 – 12 pounds
R. H. Probes – 85-95%

System Design: Surface is shotblasted to achieve a minimum 10 mil profile. Joints are treated with MVS Joint Filler. Two coats of APF Moisture Vapor Stop are applied at 200 sq. ft. per gallon per coat to yield 16 mils dry.

Material Costs: $0.70 – $0.90 per sq. ft.

Level Three Protection

Moisture Test Readings: Calcium Chloride – over 12 pounds
R. H. Probes – over 95%

System Design: Surface is shotblasted to achieve a minimum 10 mil profile. APF Lithium Penetrant is applied at 200 sq. ft. per gallon. After first coat has dried, a second coat is applied at 300 sq. ft. per gallon. After an overnight dry, the surface is scrubbed wet or dry with a floor machine and a nylogrit brush. Joints are treated with MVS Joint Filler. Two coats of APF Moisture Vapor Stop are applied at 200 sq. ft. per gallon per coat to yield 16 mils dry.

Material Costs: $1.00 – $1.30 per sq. ft.

Warranty Information
When APF remediation systems are applied by certified, approved contractors or under the supervision of a factory technician, we warrantee the floor against damage from moisture/alkalinity. Both calcium chloride and R. H. probe testing are required to obtain this warranty. In the event of failure, we will cover both labor and materials to remove and replace both the remediation system and the finished flooring. The warranty period covers a 5 year period from the date of installation.

If APF remediation systems are applied by non-approved applicators or without factory supervision, the warranty shall be only that the product is made without defect and meets our published specifications. In the event that user proves that the materials were defective, his remedy shall be limited to return of the purchase price or product replacement at the option of the seller.