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FLOOR COVERINGS AND MOISTURE

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Floor-Covering Materials and

Moisture in Concrete

 

by Steven H. Kosmatka

Concrete Engineer

ã Portland Cement Association

Moisture in concrete slabs, while essential for curing, can be detrimental to the adhesion of floor-covering materials such as resilient tile or sheet flooring,  paints, coatings, and sealers. Flooring materials sometimes fail because the alkaline moisture in the concrete interferes with the bonding properties of the flooring or its adhesive. Therefore it is wise to test the moisture condition of the slab before flooring is applied. The test should show that the slab is dry enough to accept the particular floor covering.

The drying period required will vary with environmental conditions, type and thickness of concrete, and location of the slab. For example, slabs on the ground require longer drying periods than suspended slabs. Usually, several months of drying are required after the moist cure. (Several manufacturers recommend that concrete be at least 60 days old before their floor covering is installed.) Lightweight concrete may require a longer drying period than normal-weight concrete.

Concrete will take longer to dry out in an environment with a natural high humidity than in one with a low humidity. In some regions of high humidity, special measures may be required, such as dehumidifying and heating the air to dry the concrete sufficiently.

 

Moisture Tests

The best way to determine if a slab is ready for flooring material is to test-rather than guess-the moisture condition of the slab. Before using one of the several methods discussed below, the floor-covering manufacturer should be consulted as to which test or what degree of dryness is required for their particular product.

Polyethylene sheet test.

The moisture condition of a concrete floor can be checked by taping a 4-ft (1.2-m) square sheet of polyethylene or other clear plastic film to the slab surface. If no moisture accumulates on the underside of the plastic film after 24 hours(or before the flooring material or its adhesive would cure) the slab may by considered dry enough for placement of the flooring material. If visible moisture collects under the plastic film during the time required for the bonding material to cure, a poor bond is likely.

Relative humidity test.

This test is intended for use when moisture-sensitive floor-covering materials, such as linoleum, cork, and rag -felt-backed vinyl are to be bonded to concrete slabs. A relative humidity meter is placed under an 18-in. (450-mm) square sheet of polyethylene film, and the edges of the film are sealed to the slab with tape.The meter and film should be left undisturbed until the meter reading stabilizes, usually within 24 to 72 hours. Reference 1 recommends a relative humidity of 80% or less for adequate bonding of moisture sensitive materials. If the reading is more than 80%, the floor should be dried longer and retested. Caution: This method may not be applicable in high humidity regions of the country.

Mat moisture bonding test.

The moisture and surface-bonding characteristics of concrete slabs that are to receive floor coverings of rubber tile, solid vinyl tile, and vinyl sheet may be checked with the mat moisture and bonding test. The test may also be used before placing any resilient flooring on slabs from which paint. oil, adhesive, curing compound, or other coatings have been removed. The test is performed by placing 24-in. (600-mm) square linoleum or vinyl sheet mats on two adhesive bands-one a water-soluble adhesive, the other a. water-resistant latex adhesive. The edges of the mat are taped to the floor. After 72 hours, the mats are removed and the adhesives examined. The presence of too much moisture will partially or completely dissolve the water-soluble adhesive , while the water-resistant adhesive will be stringy with little bond. If after further drying and retesting, moisture in the slab still affects the adhesives, more moisture-resistant alternate flooring materials should be used.

A similar test is recommended by Armstrong World Industries, Inc. In the Armstrong Bond and Moisture Test, 3x 3-ft (0.9x0.9-m) panels of the project floor-covering material are bonded to the slab with the specified adhesive. If the panels, spaced about 50 ft (15m) apart, are still securely bonded after 72 hours, the floor is considered sufficiently dry and clean to install the floor covering. "Securely bonded" means that an unusual amount of force would be required to lift a panel and the adhesive would cling to both the slab and the flooring panel.

Moisture Meters

Various types of moisture meters are available to measure relative humidity or moisture content, including embedded, well-cast insert, and surface types. The meter shown in Fig. 1 uses electrical resistivity to determine the moisture content of concrete at or below the surface. Two 1/4-in.-diameter (6-mm) holes are drilled ¾ in. (19-mm) apart into the concrete to the depth at which the moisture level is to be checked: nails are then driven into the holes. The two pins of the electrode are placed in contact with the nails and the subsurface moisture content is read from the meter.

When an impermeable flooring material is bonded to a concrete-slab, the surface moisture content increases as moisture deeper in the slab migrates to the surface. This can interfere with the bond of the flooring material to the slab. This condition is most likely to exist in arid climates where the surface of a slab dries out quickly relative to the deeper concrete.

 2e.gif (6448 bytes) g_79.gif (23560 bytes)
Fig 1. Electrode-type moisture meter
Photo courtesy of Delmhorst Insturment Company. Tovaco, New Jersey
Caution: Experience and skill are needed in using this instrument or any instrument that provides instant relative humidity or moisture content readings at the concrete surface. The moisture content may be significantly less at or near the surface than at lower depths as illustrated in Fig. 2.

Fig 2.

 

CONCRETE SUBFLOOR CONSTRUCTION

Experience has shown that capillary moisture from the soil can cause more flooring distress than the original moisture in the slab. Therefore, proper construction of the slab bed for ground-supported slabs is as important as making sure the concrete is dry before applying a floor-covering material. Moisture that migrates up from the soil can break down a cured floor covering or its adhesive, or it can collect at the concrete surface and physically uplift a floor-covering material. Some flooring materials, polyester resin for example, can form blisters under these conditions, or tile can break loose from the floor. Other floor coverings may expand and buckle in the presence of moisture.

The moisture may also be carrying dissolved concrete alkalis to the surface. Salt deposited at the concrete surface when the water evaporates can chemically attack the adhesive or flooring material unless alkali-resistant materials are used. The salt buildup may also physically separate the floor covering from the slab.

Following are some steps to consider when constructing a moisture-resistant concrete floor on ground:

· Slope landscaping away from the structure

· Use a 4-in. (100-mm) granular fill to form a capillary break between the soil and the slab ·

· Install foundation drain tile

· Use a vapor barrier, which is required by most floor-covering manufacturers

· Install insulation (optional) over or under the vapor barrier to help keep the slab temperature above the dew point to prevent moisture in the air from condensing on the slab surface

· Place a layer of sand over the vapor barrier or insulation

· Use a low-water-cement-ratio concrete to reduce evaporable water, the drying period, and moisture permeance ·

· Moist-cure 5 days for a more watertight concrete

· Allow a 2-or-more-month drying period

· Test the slab moisture Condition before placing a floor-covering material.

Summary

Before placing a floor-covering material, consult the manufacturer regarding the concrete slab moisture condition required. When a moisture-sensitive covering is to be placed on a concrete slab on ground, proper sub-slab construction practices-use of granular till and vapor barrier-should be followed (see box). After the concrete has dried a sufficient period of time, two or more months, the slab’s moisture condition should be tested. Once proper dryness is obtained and the surface is clean, the floor covering material can be applied with confidence.

References

1.)     Olin, Harold Bennett; Schmidt, John L.; and Lewis, Walter H., Construction Principles. Materials & Methods, United States League of Savings Institutions, 5th edition, Chicago, 1983,1300 pages.

2.)     Concrete Floors on Ground, EBQ75D, Portland Cement Association. 1983, 36 pages.

3.)    ONeil, Edward F., and McDonald, ,James E. An Evaluation of Selected Instruments Used to Measure the Moisture Content of Hardened Concrete, Technical Report C-76-1,Concrete Laboratory, U.S. Army Engineer Waterways Experiment Station,Vicksburg, Mississippi. February 1976. 69 pages.

 

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