What causes concrete pop-outs in New Brunswick driveways?

Concrete pop-outs in NB driveways are almost always caused by reactive aggregate — small stones in the concrete mix that absorb water, freeze, and literally explode out of the surface, leaving a shallow conical crater. They're one of the most frustrating cosmetic problems NB homeowners face, because they keep recurring and there's not much you can do to stop them once the concrete is placed.

Here's the mechanism: certain aggregate types — particularly shale, chert, or porous limestone particles — are naturally absorbent. When these particles sit near the surface of a concrete slab, they absorb moisture from rain, snowmelt, and de-icing brine. When temperatures drop below freezing, that water expands by 9% inside the particle. The expanding particle pushes against the surrounding concrete and eventually pops out, leaving a pit typically 1/2 to 2 inches in diameter and 1/2 to 1 inch deep. NB's 150+ annual freeze-thaw cycles mean this process happens repeatedly throughout the winter.

Salt makes it significantly worse. Sodium chloride and calcium chloride de-icers drive water deeper into the concrete surface and create more freeze-thaw events at the concrete surface than the ambient air temperature alone would cause. Driveways in Moncton, Dieppe, and Saint John where road salt splash is constant tend to develop pop-outs faster than protected walkways.

The second cause of pop-out-like craters is freeze-thaw scaling at aggregate level — where the paste (cement and fine sand) surrounding larger stones spalls away, exposing the aggregate rather than launching it. This looks similar to a pop-out but happens because the concrete lacked adequate air entrainment. All exterior concrete in NB should be air-entrained to 4–7% — this creates microscopic air bubbles that give freezing water room to expand without fracturing the paste.

Can Pop-Outs Be Repaired?

Individual pop-outs can be patched with a polymer-modified concrete repair mortar. Clean the crater thoroughly, apply bonding agent, and fill. The challenge is that patched pop-outs are usually visible as discoloured spots and new ones will appear elsewhere in the slab over time if the aggregate quality was poor throughout the mix. Full surface resurfacing can hide existing damage, but new pop-outs may push through the resurfacer too if the underlying aggregate continues to react.

Prevention for new concrete: Specify aggregate from a reputable local ready-mix supplier who tests their aggregate for soundness per CSA A23.1 requirements. Ask explicitly for air-entrained concrete (4–7% air content), keep water-cement ratio low, and apply a silane/siloxane penetrating sealer before the first winter and every 2–3 years thereafter. Switch from road salt to sand for traction — this alone dramatically slows pop-out development on sealed driveways.

If your driveway is developing widespread pop-outs and is over 20 years old, replacement with a properly specified air-entrained mix is worth considering. New Brunswick Concrete can connect you with local contractors who can assess whether repair or replacement is the right call for your situation.