Concrete Dusting, Crazing and Curling, and How Curing Prevents Them

Dusting, crazing, plastic shrinkage cracks and curling all show up in the top fraction of an inch of a slab, and they share causes: water worked into the surface, finishing at the wrong time, and curing that starts late or stops early. The NRMCA curing guide lists less crazing, dusting and scaling among the payoffs of proper curing.

How do you tell dusting, crazing, plastic shrinkage cracks and curling apart?

Look at what the surface does. Dusting is powder. Crazing is a fine web of cracks. Plastic shrinkage cracks are longer, roughly parallel cracks that form before the slab sets. Curling is the slab itself bending at joints and edges. This table sums up NRMCA’s descriptions of each.

What causes concrete dusting?

A weak wearing surface. The NRMCA dusting sheet says working bleed water back into the top ¼ in. (6 mm) “produces a very high water-cement ratio and, therefore, a low strength surface layer.” Broadcasting dry cement to speed finishing, or sprinkling water while finishing, does the same.

The other causes on NRMCA’s list:

• Troweling in condensation. On cold concrete in humid air, water condenses on the surface; troweled in, it causes dusting. Cold basement floors are the classic case.
• Unvented heaters and engines in enclosed spaces. Carbon dioxide causes carbonation, “which greatly reduces the strength and hardness of the concrete surface.”
• Insufficient curing, which “often results in a soft surface skin, which will easily dust under foot traffic.”
• Rain, snow, drying wind or freezing on fresh concrete.

How do you prevent dusting on the next pour?

Finish at the right time and cure right after. NRMCA’s rules: use a moderate slump no higher than 5 in. (125 mm), do not finish while the concrete is still bleeding, never broadcast cement or sprinkle water, vent gas heaters, and keep moisture in the slab for the first 3 to 7 days.

For the curing step, NRMCA calls for liquid membrane curing compound, water or wet burlap “as soon as possible after finishing.” See when to apply curing compound for the timing, and wet curing vs curing compound, plastic and blankets for picking a method.

What is concrete crazing?

A network of fine random cracks caused by shrinkage of a surface layer rich in paste. The NRMCA crazing sheet says the cracks are rarely more than about 1/8 in. (3 mm) deep and show most on steel-troweled surfaces drying after being wet. They usually appear the day after placement, or at least by the end of the first week.

Crazing is cosmetic. NRMCA says craze cracks “do not affect the structural integrity of concrete” and are generally not a precursor to deterioration, especially on interior slabs. They get more obvious as dirt works into them.

What causes crazing and how do you prevent it?

The same mistakes that cause dusting, plus late curing. NRMCA names extra water to raise slump, jitterbugs and vibratory screeds that push stone down, finishing with bleed water on the surface, sprinkling cement or water, and finishing too early when the surface only looks dry. It adds that “lack of or delayed curing will result in crazing.”

Prevention, per NRMCA: place at a moderate 3 to 5 in. (75 to 125 mm) slump, on 3 to 4 in. (75 to 100 mm) of compactible fill, and start curing once finishing is done: keep it moist at least 3 days or spray a membrane curing compound. Avoid curing water more than 20°F (10°C) colder than the concrete.

What causes plastic shrinkage cracks?

The surface drying before the concrete sets. The NRMCA plastic shrinkage sheet puts the critical point where “the rate of evaporation of surface moisture exceeds the rate at which rising bleed water can replace it.” Wind over 5 mph, low humidity and high air or concrete temperatures all raise evaporation. Anything that delays setting adds risk.

The cracks are often parallel, 1 to 3 ft apart, shallow, and usually do not reach the slab perimeter. They rarely weaken a floor or pavement, but NRMCA notes they let in aggressive chemicals and can start later cracking.

To prevent them, NRMCA lists dampening a dry subgrade and forms, windbreaks and sunshades, fog sprays upwind, covering the slab between finishing passes, an evaporation retarder behind the screed, and curing with compound or wet burlap for at least 3 days. For the full hot day routine, see curing concrete in hot, windy weather.

What causes concrete curling?

The top and bottom of the slab changing size at different rates. The NRMCA curling sheet says the more common case is upward curl at the edges, when the top dries and shrinks against a bottom sitting on moist soil. Drying shrinkage is the main driver, and it rises with mixing water and paste content.

NRMCA’s ways to limit curling include the lowest practical mixing water content (slump itself “has no bearing on curling”), larger aggregate, a thicker slab, and joint spacing no more than 24 times the slab thickness. That is 8 ft (2.4 m) for a 4 in. slab. Curling diminishes with age as the moisture difference evens out.

Curing helps. NRMCA says “moist curing or a well applied high-solids curing compound will reduce rate of moisture loss and reduce the moisture differential.” Spec work follows the curing period set in ACI 308.1.

If your slab is already dusting, crazed or curled

If it dusts, NRMCA’s repair order is: remove the weak layer by sandblasting, shot blasting or a high-pressure washer; then apply a chemical floor hardener such as sodium silicate or a zinc or magnesium fluosilicate on clean, dry concrete. If dusting persists, use a coating such as latex, epoxy or cement paint.

In severe cases, NRMCA suggests wet-grinding down to sound concrete and placing a bonded topping. A floor covering can also work, but NRMCA warns that adhesives “will not bond to floors with a dusting problem,” so the surface needs preparation first. Read the curing compound and flooring guide before you put anything over it.

If it is crazed, there is no structural repair to make. Clean the dirt out of the cracks or cover it with a coating if looks matter.

If it is curled, NRMCA lists sawing extra contraction joints, grinding the lifted joints, and injecting grout under the curled area to restore support.