General Concrete Moisture Facts

Capillary Action

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. It 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.


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

These act 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.


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.

  • While compressive strength has long been considered the most important quality control characteristic of concrete, where a floor topping is to be installed, permeability may be of equal importance.
  • Water, whether liquid or vapor, always seeks the path of least resistance.
  • Moisture travels twice as far horizontally as vertically.
  • There is a significant difference between moisture content and moisture movement (emission).
  • Moisture usually travels through a concrete slab more readily as vapor than as a liquid.
  • 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 with a pH of 12.5 or more. Once cured and dried, surface alkalinity through carbonation drops to normal range.


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