Concrete, wood, brick, stone and mortar are porous materials that contain salts. The ground in which these materials can come into contact also contain salts. Capillary action can literally suck water and transport it through porous building materials.

Capillary Action

Salts dissolved by groundwater can be transported by capillary action through porous soil. Building materials in contact with soil will naturally wick the water inward and upward. Take concrete footings — they are typically poured directly onto soil without any capillary break. Sometimes this is called rising damp. This is the beginning of how water can wick upward into a structure.

Destructive Pressures

When the capillary flow of water reaches the surface of a building material, evaporation occurs. As the water evaporates, salt is left behind. As this evaporation of capillary flow continues, the salt concentration increases, which creates an imbalance, and nature abhors imbalance and always wants to put things back into equilibrium. This is process is called osmosis. To re-establish equilibrium through osmosis, water rushes toward the salt deposit to dilute the concentration. This rush of water creates massive hydrostatic pressures within the porous material, and these pressures are destructive.

• diffusion vapor pressure: 0.3 to 0.5 psi
• capillary pressure: 300 to 500 psi
• osmotic pressure: 3,000 to 5,000 psi

As you can see from the list above, osmosis can create pressure that is greater than the structural strength of concrete, which can be from 2,000 psi to 3,000 psi. The action of water rushing to the surface due to capillary action creates incredible forces that can cause materials to crack, flake and break apart.

Spalling

When efflorescence leads to strong osmotic pressures—greater than the strength of the building material—and the material literally breaks apart, the resulting damage is called spalling. Hydrostatic pressure can cause spalling, but spalling can also be caused by freeze-thaw cycles in building materials that have a high moisture content.

Both efflorescence and spalling can be prevented with capillary breaks, such as by installing a polyethylene sheeting under a concrete slab.

Identifying Efflorescence

• fungi that rot wood;
• water damage to sheetrock; and
• reduced effectiveness of insulation.
  

Prevention

• An impregnating hydrophobic sealant can be applied to a surface to prevent the intrusion of water. It will also prevent water from traveling to the surface from within. In cold climates, this sealant can cause material to break during freeze/thaw cycles.
• During home construction, bricks left out overnight should be kept on pallets and be covered. Moisture from damp soil and rain can be absorbed into the brick.
• Install capillary breaks, including polyethelene sheeting between the soil and the building material, such as concrete.

Removal