· by Michael Blair, Nashville's Concrete
A concrete driveway or patio that looks fine the week it is poured can show its problems by the second season. Three surface complaints account for most of them: a surface that dusts off as fine powder, a surface that flakes away in thin patches, and a white deposit that appears and reappears. They look related and they are not. Each comes from a different mistake in placing, finishing or curing, and they range from harmless to a slab that will not last.
Dusting: powder on your hands
A slab that dusts has a weak surface layer. The usual cause is finishing while bleed water was still on the surface, which works water back into the top of the slab and leaves a thin, weak paste at the top. Too much water in the mix, a pour in cold weather when the slab was slow to set, or troweling too early all produce the same layer.
Dusting is mostly a surface problem. The slab underneath is usually sound, and a penetrating hardener or a densifier applied to the surface stops the powdering on a driveway or garage floor. It will not turn a weak surface into a strong one; where the top is soft enough to scratch with a key, that layer is going to wear through.
Scaling: patches that flake away
Scaling is the surface breaking off in thin flakes, exposing the aggregate below, usually after the first winter. It means the surface froze while it still held water, or was never air-entrained for a climate that freezes. Middle Tennessee is on the edge of that: our winters bring enough freeze-thaw cycles to scale a poorly finished slab, and deicing salt makes it much worse by increasing the number of cycles the surface goes through.
The finishing mistakes behind scaling are the same as dusting, with overworking the surface added: a hard trowel finish on an exterior slab seals the top and traps water. Exterior flatwork here should be broom finished, air-entrained and cured, and it should see no deicing salt in its first winter. Scaling cannot be reversed; a resurfacing overlay is the repair for a slab that is otherwise sound.
- Bleed water finished into the surface: dusting and scaling
- Hard trowel finish outdoors: scaling
- No air entrainment, or salt in the first winter: scaling
- No curing, in sun and wind: surface cracking and dusting
Efflorescence: the white bloom
The white film that appears on new concrete, and often on the retaining wall or block beside it, is efflorescence: soluble salts carried to the surface by water moving through the concrete, left behind as the water evaporates. New concrete has plenty of both. It is not a defect in the concrete and it does no structural harm.
It is a moisture indicator, though. Efflorescence that keeps returning in one area of a slab means water is coming up through it from below, which on a Nashville lot usually means poor drainage under the slab or a downspout discharging next to it. Brush it off dry; if it persists, wash with a dilute acid-based efflorescence cleaner and rinse well. Sealing a slab that is still blooming traps the salts under the sealer and turns them white permanently.
What to watch on the day of the pour
All three defects are decided in the first day, and most of the prevention is visible if you are there. The crew should wait for the sheen of bleed water to disappear before floating and troweling. An exterior slab should get a broom finish, not a slick one. Curing compound should be sprayed on, or the slab covered and kept wet, as soon as finishing is done; a new slab left bare in sun and wind loses its surface water before it has hydrated. In summer the pour should be early; in cold weather it should be blanketed.
If you are reading this after the fact, the sensible order is to test the surface, hold off on sealer until a full drying season has passed, and keep salt off it. A slab with a dusting surface can be hardened. One that is scaling in patches is a conversation with the contractor about an overlay, and about what was done on the day.