Freeze-thaw damage to concrete and masonry

Freeze-thaw damage happens when water soaked into concrete or masonry freezes, expands and breaks the material from inside, a little more with each cycle. Across much of Canada, with many cycles each spring and fall and de-icing salts on every walkway, it is one of the most common causes of deterioration. The key to preventing it is keeping water out, and using materials designed to resist it.

How the damage happens

Concrete, brick, stone and mortar are porous. Water gets into the pores and small cracks. When it freezes, it expands by roughly nine percent, creating pressure inside the material. If the material is saturated and has no room for that expansion, it cracks internally. When it thaws, more water gets into the new cracks, and the next freeze makes them bigger.

Two factors make the damage much worse:

• Saturation: dry material rarely suffers. Damage concentrates where water collects or keeps the material wet: horizontal surfaces, the bottom of walls, under leaking downspouts and at cracks.
• De-icing salts: salts increase the number of freeze-thaw cycles at the surface, add stress, and carry chlorides into concrete, where they corrode reinforcing steel.

What freeze-thaw damage looks like

In concrete

• Scaling: the surface flakes or peels off in thin layers, exposing aggregate. Common on driveways, sidewalks, steps and parkade slabs.
• Pop-outs: small cone-shaped pieces break out where a porous aggregate particle near the surface has expanded.
• D-cracking: closely spaced cracks running parallel to joints and edges, caused by aggregate that is susceptible to freezing.
• Spalling: larger pieces break away, often where corroding reinforcing steel is also involved.

In masonry

• Spalling brick: the face of the brick breaks off, often in walls that stay wet.
• Mortar erosion: joints crumble and recede.
• Efflorescence: white salt deposits show water is moving through the wall.
• Cracking and displacement: at parapets, chimneys and retaining walls where water is trapped.

Where to look

• Steps, landings and walkways, especially where salts are used
• Parking structures and garage slabs, where vehicles bring in salty slush
• Balconies and exposed slabs
• Parapets, chimneys and copings, which are wet from both sides
• The base of walls near grade and under downspouts
• Retaining walls without drainage
• Below leaking gutters, sills and flashings

Parking structures deserve special attention, because freeze-thaw and chloride corrosion work together. See parking garage inspection basics.

Prevention

For new concrete

Concrete exposed to freezing should be designed for it: an appropriate exposure class with air entrainment, a suitable water-cement ratio and strength, durable aggregates, and good finishing and curing. In Canada, these requirements are set out in CSA A23.1 and the project specification. Over-finishing, adding water at the site and poor curing all weaken the surface and make scaling more likely.

For existing concrete and masonry

• Manage water: fix gutters, downspouts, grading and flashing so surfaces aren't kept wet.
• Seal where appropriate: penetrating sealers can reduce water and chloride entry into concrete. Masonry needs care: some coatings trap moisture and make damage worse, so choose products suited to the material.
• Limit salt: use the least amount needed, consider alternatives on vulnerable surfaces, and wash salt off parkade slabs in spring.
• Maintain joints: keep sealant in joints and cracks, and keep mortar joints repointed with a compatible mortar.
• Keep drains clear: on parkades, balconies and roofs, so water doesn't sit.

Repair

Repairs depend on how deep the damage goes:

• Light scaling: clean and seal, or apply a suitable resurfacing product.
• Deeper deterioration: remove unsound concrete to sound material, clean or replace corroded steel, and patch with a compatible repair material.
• Masonry: replace spalled units, repoint with compatible mortar and fix the water source.
• Widespread damage: may need replacement of the element or a full rehabilitation program, designed by an engineer.

Every repair should include fixing the source of water. Without that, the damage returns. To find out how deep the damage goes, tests such as cores and chloride profiles help. See concrete testing methods explained.

How ehZee can help

We inspect concrete and masonry for freeze-thaw damage, trace the water sources behind it, recommend testing where the depth of damage is unclear, and set out repair priorities. See our concrete inspections and X-ray, or call 1-833-888-0091.

Common questions