Concrete Floors and Moisture

Subfloors & Underlayments

New and existing concrete subfloors must meet the requirements of the latest edition of ASTM F 710, “Standard Practice for Preparing Concrete Floors to Receive Resilient Flooring,” available from the American Society for Testing and Materials, 100 Barr Harbor Drive, West Conshohocken, PA 19428; 610/832-9500;

Note: Regardless of the type of concrete or other cement-like material used as a base for resilient flooring, in the event of underlayment failure, the responsibility for warranties and/or performance guarantees rests with the concrete or cement-like material manufacturer and not with the manufacturer of resilient flooring.

Concrete Floors and Moisture
Any concrete subfloor can be a source of moisture-related flooring failures, including above-grade concrete floors. By its very nature, concrete starts as a water-saturated mass which must cure and then dry sufficiently to allow the installation of flooring. Above-grade floors normally have only the mix water to contend with although rain, spills and water leaks can add more water. Roughly one-half of the mix water is consumed by hydration of the cement during the curing period, with the rest being slowly reduced by evaporation. Once dry enough for installation, there is little chance of future moisture related problems on above-grade concrete slabs. Concrete on-ground, or below-ground floors, have not only the mix water to consume and dissipate, they also have a potentially inexhaustible source of moisture from the ground. When covered with resilient flooring, a slab that is directly on sub-grade soil will become approximately as moist as the soil on which it is placed.

To reduce this ingress of moisture, a well-designed floor system will have a capillary break and an effective and intact moisture vapor retarder in place. Slabs on and below-grade can be affected by both water vapor and capillary rise. Below-grade slabs are closer to the water table, have poorer ventilation for drying, and have the added risk of hydrostatic pressure. On-ground concrete slabs and below grade slabs must have an effective and functional vapor retarder directly beneath the concrete to prevent ingress of moisture from the sub-base and sub-grade soil.

Resilient flooring products, whether sheet, plank or tile, function as moisture vapor retarders on top of the floor slab; if more moisture is rising from beneath the concrete than can be accommodated by the flooring and adhesive, then failure of the installation is inevitable.

Too much ground moisture can create problems for on-grade and below-grade areas of commercial and residential buildings over and beyond those relating to the installation and use of resilient flooring. These problems vary from merely slight, but unpleasant dampness to actual structural damage. Moisture near the surface of as concrete slab varies as the weather changes and moisture within the slab usually approximates the dampness of the subsoil.

Note: Water/cement ratio is the most important factor regarding moisture migration, permeability and rate of drying of a concrete slab. Water/cement ratios as low as 0.45 to 0.5 are practical and recommended by the concrete construction industry for slabs to receive resilient flooring. A water-cement ratio of 0.5 is an achievable and reasonable requirement for slabs on- or below-grade. Significantly higher water-cement ratios may lead to slower drying and problems with moisture movement through the slab, causing flooring failures.

Sources of Moisture

Capillary Action: A capillary is a small, hollow tube which allows moisture to travel from a lower to a higher level. The force which causes a capillary movement is the result of tension on the surface of liquid when it is confined in a very small tube or channel. Capillary action is often underestimated and can be very serious. Moisture can come from a water table as much as 20 feet below the ground surface. May occur simultaneously with vapor emissions but is not usually the primary cause of moisture related problems in on-grade flooring installations. Capillary breaks are effective in preventing this. Capillary breaks are constructed beneath a slab as follows:

  • 4-8″ of washed and graded gravel such as 1″ washed gravel – not crushed rock
  • 1-2″ of sand as a leveling bed over the gravel to prevent moisture barrier puncture
  • Gravel bed must have a positive gravity outflow or mechanical methods must be provided (drain tile, well pits, sump pump).

Capillary breaks do NOT prevent moisture migration in the form of water vapor.

Hydrostatic Pressure: Moving water that is forced up through the slab by the weight of water in the soil surrounding the foundation. Occurs when the water table is higher than the concrete slab. The column or depth of the water results in pressure and it is the weight of the water, relative to the height, that determines the pressure. Increased by rainfall, sprinkler use, broken pipes, runoff from grading, etc. The only control for hydrostatic pressure is draining the water to a collection point, then draining or pumping away.

Hydraulic Pressure: The action of at least two solid surfaces that act upon water to force it, under pressure, in a particular direction. (artesian wells, swelling soils, broken plumbing); very rarely the cause of moisture in slabs. Hydrostatic pressure only occurs below-grade.

Leakage: Water traveling from a higher to a lower level due to gravity. May be due to rain, flooding, sprinkler systems, runoff, poor landscaping, etc. In suspended floors, usually due to broken pipes or drains, or even roof leaks.

Water Vapor or Vapor Emissions: Acts according to the physical laws of gasses or chemical equilibrium. Water travels from one area to another whenever a difference of vapor pressure exists. In a controlled climate (HVAC), the temperature in the room is 70 to 80° F and the relative humidity is 30% to 50%. Therefore, the air above the slab can and wants to hold more moisture, and satisfies itself by pulling moisture from the slab 24 hours a day, 7 days a week, 365 days a year until equilibrium exists which rarely if ever happens. The amount of water air absorbs is affected by temperature – the cooler the air, the less water absorbed while the warmer the air the more water absorbed. Water vapor is capable of penetrating where water in a liquid form cannot. By comparing the vapor pressure in differing environments, one can estimate the movement of vapor pressure through concrete. However, in reality, vapor emissions levels are complex due to variations in temperature, humidity, permeability, and flow path through the concrete. For these reasons, vapor emissions cannot be accurately calculated but may be measured. Anhydrous Calcium Chloride testing has shown that moisture may travel through a concrete slab more readily as vapor than as a liquid. Water vapor is a mixture of air and water.

Additional discussion of water and vapor in concrete: As a liquid or as a vapor, moisture is constantly in motion. As a liquid, it moves by capillarity through pores – farther in small diameter pores – until restrained by gravity. Water as a liquid moves from a warm to a cool environment. As a vapor, it moves from cool to warm spaces in a pore, from fresh to salty conditions, and from smaller pores to larger ones. Moisture present in building materials always carries soluble and insoluble salts. The most common are sodium chloride, calcium sulfate, calcium carbonate, and magnesium sulfate. These salts come from the materials themselves, from air, rainwater, salt-charged ground water, and from sodium and calcium chloride. When water in a saline (salt) solution evaporates, salt crystals are formed – a process known as efflorescence. As the salts crystallize, they expand, which does not normally cause a problem when the efflorescence is on the surface. When this occurs beneath a surface, it is known as subflorescence or subefflorescence, and the expansive forces can overcome the internal strength of the material and cause spalling. Water converts to a water vapor and moves toward areas of evaporation and higher salt concentration. Freshly salt-charged ground water will continuously move by capillary action toward surfaces and deposit fresh salt crystals through evaporation. Additional characteristics of some of these salts, notably sodium chloride, is that they are hygroscopic in nature, high humidity, they absorb moisture which they lose during periods of low humidity.

  • All concrete is permeable
  • While compressive strength has long been considered the most important quality control characteristic of concrete, where a floor is to be installed, permeability may be of equal importance.
  • Vapor emissions can be measured, but not accurately calculated
  • Water, whether liquid or vapor, always seeks the path of least resistance
  • There is a significant difference between moisture content and moisture movement
  • Moisture must be present in hardened concrete for the continued gain of strength and other desired properties; may continue for many years
  • Healthy new concrete is alkaline; pH of 12.5 or more. Once cured and dried, surface alkalinity through carbination drops to normal range.
  • Phenolphthalein is not a test for moisture – it turns red in the presence of alkali and moisture
  • Concrete should be moist cured for at least seven days; failure to do so may increase permeability four times or more
  • Restricting water/cement ratio to 0.40 in combination with a 7 day moist cure can result in concrete able to withstand significant hydrostatic pressure
  • Never install resilient flooring within 5 degrees above dewpoint
  • Common sources of excessive building moisture are rain, ground water, and condensation.
  • As a liquid or as a vapor, moisture is constantly in motion

Problems associated with moisture in concrete slabs:

  • Inadequate bond
  • Moisture prevents or retards setting of water-based adhesives
  • Weakening of the bond of previously set adhesives
  • Growth in tile resulting in peaked joints or curled tile
  • Discoloration (mold and mildew growth)
  • Salt (alkali) deposits at tile joints
  • Blisters in sheet flooring installations (When blisters form immediately after installation, they are normally associated with air. When a period of time passes before blisters form, they are usually a result of moisture in the slab.)
  • Bleeding of adhesive through the joints of tile
  • Shifting Tile

Moisture problems can be solved through expensive moisture remediation processes, however, the best method is prevention.

Moisture Tests For Concrete Slabs

  • Armstrong recommends conducting a calcium chloride test per ASTM F-1869, the relative humidity (in-situ) probe test per ASTM F-2170, or the Armstrong 72 hour bond test.· Hygrometer Test/ Relative Humidity Meter – Placed on floor surface and covered with plastic
  • Internal Relative Humidity of Concrete Slab
  • Rubber Manufacturer’s Association – Calcium Chloride Crystal Test
  • Current school of thought is that a reading of 3 pounds/ 1000 sq ft/24 hours will allow a successful installation of resilient flooring; reads vapor emissions
  • Delmhorst Electric Probe – Measures conductivity between stainless steel pins embedded in the concrete; directly related to the amount of moisture; the higher the conductivity the more moisture present; a good test for wood moisture content.
  • Rubber Mat Test – Lay a rubber mat on the concrete for at least 24 hours, lift and check for signs of water or darkening of the concrete
  • Polyethylene Film – Tape clear poly down for 24 hours and check for signs of moisture
  • Phenolphthalein – Turns red in the presence of alkali and moisture; color change begins at a pH of 9
  • 72 Hour Bond Test
Updated on May 12, 2023