How to Cure Concrete
Curing concrete means one thing: keeping water in it long enough for the cement to finish reacting. NRMCA puts the goal in one line, to "prevent loss of moisture and maintain a favorable temperature to support continued hydration." There are three ways to do it: add water, cover it, or seal the surface with a curing compound. All three work. The one that does not is the one most people actually use, which is watering it now and then and letting it dry out in between.
Why curing matters more than it looks
A slab that dries out early stops gaining strength. Penn State's civil engineering course material on curing says concrete "allowed to dry in air will gain only 50% of the strength of continuously moist-cured concrete," and NRMCA's CIP 11 on curing in-place concrete gives the same figure as a worst case. A field comparison by FHWA, covered in what happens if you skip curing, puts a more typical loss nearer 16 percent.
The bigger effect is at the surface, which dries first and hydrates least. CIP 11: "Lack of curing will result in a less durable surface with poor resistance to wear and abrasion," and "Proper curing reduces the potential for crazing, dusting, and scaling." On a driveway that sees salt, the top quarter inch is the part that matters, and it is the part curing protects.
When to start and how long to keep going
Start as soon as the surface can take it without damage. For a curing compound, that is after the water sheen has disappeared and finishing is done, the timing FHWA's curing guide gives from ACI 301. For water or covers, it is when the surface is hard enough not to be marked or washed out. Waiting until tomorrow means the most important hours are already gone.
Keep going for 7 days in normal weather. ACI 308.1 sets 7 days as the default above 50°F, and CIP 11 says to continue "as required by the specification, or for at least 3 to 7 days." High early strength concrete needs less: Arizona DOT's curing inspection guide requires 7 days for normal cement and 3 days for Type III. In cold weather the clock runs slower, and water curing becomes the wrong method entirely; see pouring concrete in cold weather.
Water curing: how often to water new concrete
Water curing is the best method and the easiest to do badly. ACI's FAQ on curing methods lists "ponding, continuous sprinkling, covering with wet burlap, cotton mats, or similar materials." What those have in common is that the surface never gets the chance to dry.
That is the answer to how often to water. It is not a number of times a day, it is often enough that the surface never dries between waterings. Penn State's curing notes: "While a constant spray is not essential, care is needed to ensure that the concrete does not dry out between applications." NRMCA is blunter in CIP 11: "Alternate wetting and drying is not an acceptable curing practice." Its CIP 3 on crazing explains the cost: "Intermittent wetting and drying the surface will increase the potential for craze cracking," the fine web of surface cracks people blame on the concrete.
On a hot, windy day a hosed slab dries fast. So in practice, wet curing a home slab means one of these:
- A lawn sprinkler or soaker hose on a timer. Penn State notes lawn sprinklers work for flat surfaces and soaker hoses for vertical ones. Set the timer short and frequent rather than long and rare.
- Wet burlap, kept wet. Burlap holds water against the surface between waterings, but only if it is rewetted before it dries. Dry burlap wicks water back out of the concrete.
- Soak it, then cover it. Wet the slab and lay plastic over it. The plastic does the work between your visits, and this is the most realistic method for anyone not home all day.
Two cautions. Use water that is not much colder than the concrete: Penn State says ponding water should be no more than 20°F cooler than the surface, and FHWA's limit is 10°C (18°F), because a cold soak on warm concrete causes thermal cracking. And do not blast a fresh surface with a hose nozzle. Penn State warns that water "could also erode the concrete surface."
Plastic sheeting
Plastic keeps water in without any watering, which makes it the default for most DIY pours. Use sheeting meeting ASTM C171, at least 4 mils thick, and weight or tape the edges so wind cannot get under it. Lay it as soon as the surface will not mark, and leave it for the full curing period.
The catch is appearance. CIP 11 warns that wrinkles "will cause dark streaks or a mottled appearance," and NRMCA's CIP 23 on discoloration explains the mechanism: where the plastic touches the surface in some places and not others, "it leads to variable hydration that causes variations in color." On a garage floor nobody minds. On colored, stamped or exposed decorative concrete, use a curing compound instead, which CIP 23 suggests "may be a better alternative."
Curing compound
A curing compound is a liquid sprayed or rolled onto the fresh surface that dries to a membrane and holds the water in. One coat, no watering, no plastic to blow away. The standard is ASTM C309, which FHWA's guide summarizes: "Type 1 is clear or translucent (Type 1-D may contain a fugitive dye). Type 2 is white pigmented." The dyed version shows you where you have already sprayed. The white one reflects sunlight and is the one to use in hot weather.
- When: once the sheen of bleed water has gone and finishing is done. Too early and you seal water into a surface that is still bleeding.
- How much: FHWA, citing ACI 308, gives 0.20 to 0.25 liters per square meter, about 160 to 200 square feet per gallon. W.R. Meadows' 1100-Clear data sheet says about 200, and Arizona DOT requires at least a gallon per 150 square feet on sidewalks and driveways. Thin coverage is the usual failure, so err heavy and even.
- What comes after: a curing compound can stop paint, tile adhesive or a coating from bonding. W.R. Meadows says not to apply flooring or coatings until its product "has dissipated and/or has been properly removed," and notes the membrane starts breaking down after about four weeks of sunlight. If the slab is getting a finish, check the compound first.
- When not to: Quikrete's data sheets say not to apply curing compounds if rain or temperatures below 50°F are expected within 24 hours, or in late fall on surfaces that will see de-icers.
Cure and seal products meet ASTM C1315 instead, which adds UV, alkali and adhesion requirements so the same film can stay on as a sealer. Sika's Sikagard-1315 KNS data sheet shows the difference in use: one coat at 200 to 400 square feet per gallon for curing only, and a second coat to seal. We compare the two standards in cure and seal.
Which curing method to use
| Method | Best for | Watch out for |
|---|---|---|
| Sprinkler or soaker hose | Slabs when someone is around to manage it | Drying between cycles, cold water on warm concrete |
| Wet burlap | Small slabs, steps, edges | Burlap drying out and wicking water back |
| Plastic sheeting | Most DIY slabs, unattended curing | Mottling on colored or decorative work, wind |
| Curing compound | Driveways, walks, decorative work, hot weather | Coverage too thin, bonding of later coatings |
| Forms left on | Edges, walls, footings | Only protects the formed faces |
On forms: ACI's FAQ notes that "Leaving metal or well-oiled wood forms in place may offer considerable protection against the loss of moisture." That is a reason not to rush them off. When to remove concrete forms has the minimum times.
Curing in hot and cold weather
Hot. Evaporation is the enemy, and the first hour matters most. NRMCA's CIP 12 recommends covering with wet burlap and plastic to prevent evaporation and curing for at least 3 days. A white-pigmented curing compound is the low-effort option. Plastic shrinkage cracks, the ones that appear before the concrete has even set, come from exactly this.
Cold. Switch from water to insulation. NRMCA's cold weather guidance says water curing is not recommended when freezing is imminent, because it leaves the concrete saturated right when it is most vulnerable. Use plastic under insulated blankets, and see pouring concrete in cold weather for how long to protect it.
For how long until you can walk, drive or build on it, see how long concrete takes to cure.
Frequently asked questions
How often should you water new concrete?
Often enough that the surface never dries out between waterings. NRMCA says alternate wetting and drying is not an acceptable curing practice and increases craze cracking. A sprinkler on a short, frequent timer, wet burlap kept wet, or soaking the slab and covering it with plastic all work.
How long should you cure concrete?
Seven days in normal weather is the ACI default above 50 degrees F, and NRMCA gives 3 to 7 days. High early strength concrete with Type III cement needs about 3.
Is it better to water concrete or cover it with plastic?
Both work if done properly. Continuous water is the most effective, but only if the surface never dries. Plastic is easier to get right unattended, though wrinkles can leave dark streaks on colored or decorative concrete, where a curing compound is the better choice.
When should you apply curing compound?
Once the sheen of bleed water has disappeared and finishing is complete, at about 150 to 200 square feet per gallon in an even coat. Do not apply it if rain or temperatures below 50 degrees F are expected within 24 hours.
Can you paint or tile over curing compound?
Check first. Manufacturers say not to apply flooring or coatings until the compound has dissipated or been removed, because the membrane can stop adhesives and coatings from bonding.