Concrete Surface Damage

Concrete is a mixture of stone and sand (or “aggregates) that is combined with water and cementing materials. As concrete is placed and finished, the aggregate begins to settle into the paste and a thin layer of the paste rises above the aggregate mixture.

This paste layer can separate from the main body of the concrete and begin to peel away (also known as “mortar flaking”). This separation can occur as circular or oval pieces above several pieces of the aggregate (known as “spalling”) or as small holes (known as “pitting”).

The most common causes of flaking, spalling and pitting are impacts, weathering, and freeze/thaw cycles. Salts and de-icers applied intentionally for ice melting or unintentionally melting off a parked vehicle can stress concrete surfaces and cause spalling and pitting. In some cases improperly cured concrete and corrosion of steel can also cause separation. In rare cases contamination of the aggregates by expansive material such as coal, ironstone or organic matter can lead to pitting and areas where the stone has popped out.

Most of the measures used to reduce surface defects are related to good concrete mixture design, proper placement and proper curing – this is often in the hands of the builder. These measures include the use of air- entrained concrete mixes with a low water to cementing material ratio; delayed finishing until the water sheen on the surface has dissipated gone; properly edging concrete near forms; and the use of isolation and construction joints. Concrete subject to freeze/thaw cycles and de-icing chemicals should not be power trowelled.

Reducing the Effect of De-icing Salts

Homeowners can reduce the effect of de-icing and road salt by applying a breathable surface sealer. The two most common concrete surface sealers are silane and siloxane. These sealers penetrate the concrete to about 3 mm but allow the concrete to breath, preventing a build-up of vapour pressure between the concrete and sealer that can occur with some film-forming materials. The sealer becomes embedded within the concrete, making it more durable to abrasive forces and ultraviolet deterioration. It can also provide longer protection than fill-forming sealers.

Sealer treatment and re-treatment should be carried out according to the manufacturerʼs directions, but certain general guidelines apply to all:

  • Application of any sealer should only be done on concrete that is clean and allowed to dry for at least 24 hours at temperatures above 16°C (sealers cannot penetrate the surface if the concrete is moist and voids are filled with water).
  • At least 28 days should be allowed to elapse before applying sealers to new concrete.
  • Other surface preparation may be necessary if the concrete is old and dirty.
  • Concrete placed in the late fall should not be sealed until spring because the sealer may cause the concrete to retain water that may only add to freeze-thaw damage during the colder winter months.