How to cure concrete after the pour

Curing concrete means keeping it moist, and at a workable temperature, long enough for the cement to keep reacting with water and gain strength. CCAA's Concrete Basics says keeping concrete moist strengthens the bond between the paste and the aggregates, and that concrete does not harden properly if it is left to dry out quickly. Holcim describes curing as protecting fresh concrete from evaporation and temperature extremes that might harm cement hydration.

In practice, curing a slab or driveway comes down to four steps:

1. Start straight after the surface is finished, the same day, not the next morning.
2. Choose one method: add water (fine mist or ponding), hold water in (plastic sheeting or wet covers), or seal it in (a curing compound).
3. Keep the whole surface evenly moist the entire time. Letting it dry out between waterings defeats the purpose.
4. Keep going for at least 3 days, and 7 days where you can, unless your plans or concreter set a longer period.

Curing matters because cement needs water for days and even weeks to reach its potential strength and durability. CCAA's Guide to Concrete Flatwork Finishes says cement hydration takes days and even weeks rather than hours, so curing must run for a reasonable period. It warns that without any curing the concrete surface strength achieved could be as much as 30 per cent below its potential. Holcim adds that if concrete dries out during curing, as can happen in hot weather, the chemical reaction stops at the point where the concrete loses its moisture.

CCAA's flatwork guide lists what good curing does for a slab:

• Improves compressive strength.
• Reduces drying shrinkage cracking.
• Reduces surface dusting.
• Improves durability and the protection of reinforcement.
• Hardens the surface, so it resists abrasion and wears better.
• Improves how well aggregates bond into the surface on exposed aggregate finishes.

Every curing method either adds water to the concrete or slows the water leaving it, and CCAA's Concrete Basics names those as the two most common approaches. Holcim says choosing between them is generally a matter of cost, and of which method gets in the way of other site work least. Each has trade-offs on a tropical site:

Holcim says flooding, ponding or mist spraying are theoretically more effective than retention methods, but they are not always practical because of job conditions. For driveways on a fall, our guide to pouring concrete on a slope covers that kind of site.

Plastic sheeting works when it is held down, sealed at the edges and checked. CCAA's Concrete Basics says to weigh the sheets down with sand, timber or bricks, look under them from time to time, and sprinkle water and replace them if the concrete feels dry; condensation on the underside is a good sign. Holcim suggests overlapping the edges and sealing them with sand, tape, mastic or planks, and misting the surface just before the sheeting goes down.

The colour of the plastic is where Australian advice differs. Cement Australia's FAQ mentions covering the slab with black plastic, while CCAA's flatwork guide says sheeting used for hot weather curing should be clear or light coloured, not black. A curing compound should go on soon after finishing, following the manufacturer's instructions, according to CCAA. Holcim notes that white pigmented compounds reflect the sun and greatly reduce the concrete's surface temperature, and their colour shows where they have been applied, while with clear types it can be hard to tell whether the whole area is covered.

Curing should start as soon as the concrete surface is finished, on the day of the pour. CCAA's Concrete Basics says curing is commenced just after finishing, and Cement Australia says protection should start as soon as the final surface finish is produced and not the next day. CCAA's flatwork guide puts a limit on the delay: curing should begin as soon as possible but no more than 3 hours after finishing, or the benefits will be markedly reduced.

The one catch is water. CCAA's hot weather data sheet says the surface needs to be hard enough before water curing can start without damaging it, which can leave the surface drying in the gap between finishing and curing. That gap is where a covering or a compound, applied once it will not mark the surface, protects the slab. The same data sheet says curing is particularly important in the first day after placement. Where formwork comes off, CCAA says the newly exposed edges need curing too.

Keep concrete moist for at least 3 days, and for 7 days where you can. CCAA's Concrete Basics says household concrete jobs must be cured for at least 3 days, that 7 days gives better strength and durability, and that the longer concrete is cured, the closer it gets to its potential strength and durability. Holcim's points to keep in mind include curing concrete for at least 7 days. Longer periods apply to some finishes and exposures:

Cement Australia's slab guide suggests a gentle hose spray on the surface and edges twice daily for 7 days. CCAA's Concrete Basics is stricter on one point: hosing in the morning and again at night and letting the concrete dry out in between is no good. On a hot FNQ day, follow the stricter line: if twice daily watering leaves the surface dry in between, add a cover or mist more often. Our exposed aggregate concrete guide covers that finish in more detail.

Concrete sets during the pour day but keeps curing for days afterwards, and the two are easy to confuse. CCAA's Concrete Basics describes setting as the stiffening of concrete once it is no longer soft, and says setting takes place after compaction and during finishing; a worker still leaves footprints in setting concrete. After concrete has set, CCAA says, it begins to gain strength and harden. Curing is what keeps that strength gain going.

A slab that looks hard by the afternoon has set, not cured. That is why curing has to continue after the concrete feels firm, and why the time a pour takes on the day is a separate question from how long the slab needs looking after. Our guide to how long a concrete pour takes covers the pour day itself. Cement type can change the picture too: Cement Australia says builders (GB) cement may gain strength more slowly early on than GP cement, and curing may be more critical for full strength development.

Far North Queensland's warm, windy and, on the Tablelands, dry afternoons make curing harder, not easier. CCAA's Concrete Basics says that in hot weather above 30 degrees, or in high winds and low humidity, concrete can dry out quickly, so extra care is needed with curing. It also says warmer weather gives concrete a higher early strength, but CCAA's hot weather data sheet adds that concrete cured at lower temperatures will generally reach higher ultimate strength. Holcim considers 23 degrees the ideal temperature for hydration.

Holcim treats evaporation above 0.5 kg of water per square metre per hour as potential trouble with plastic cracking, and above 1.0 as the point where precautions are almost mandatory. Its worked example, 25 degrees air, 40 per cent humidity and a 20 km/h wind, comes out at 1.1. Bureau of Meteorology averages show Mareeba's afternoons in the dry build-up are close to those conditions:

Mareeba's October mean maximum is 30.8 degrees, so a Tablelands slab poured in the build-up faces heat, wind and dry air at once. For pour-day precautions in the heat, see pouring concrete in hot weather, and for wet season pours, pouring concrete in the rain.

Most curing faults come from letting the surface dry, or from treating it unevenly. Each of these traces to the Australian guidance cited in this article:

• Waiting until the next day. CCAA's flatwork guide says benefits are markedly reduced if curing starts more than 3 hours after finishing.
• Wetting and drying cycles. CCAA's Concrete Basics links crazing, a network of fine surface cracks, to rapid drying or alternate wetting and drying, and Holcim says alternate drying and wetting of a cover may cause cracking.
• Cold water on a hot slab. CCAA warns that cold curing water on concrete on a hot day can crack it through sudden contraction, and suggests warm water or another method.
• A hard jet from the hose. CCAA says the spray must be a very fine mist or it will damage the surface.
• Letting covers dry out. A dry hessian or sand cover draws water out of the concrete, according to Holcim.
• Wrinkled plastic on coloured concrete. CCAA's flatwork guide says plastic is not generally recommended where a uniform colour is wanted, and Cement Australia says hydration staining on coloured slabs cannot be corrected later.
• The wrong compound under a later finish. CCAA says some curing compounds make painting or sticking down floor coverings harder or impossible, so check with the supplier first.
• Under-curing overall. CCAA lists poor curing, or a surface drying too quickly, among the causes of dusting, a fine powder that comes off on your fingers.

Who cures the slab is a question to settle with your concreter or builder before pour day, because curing has to start within hours of finishing. Ask whether the concreter will apply a compound or sheeting as part of finishing, whether you are expected to keep a cover damp or a mist running afterwards, which method they will use, how long they expect it to run, and what you need to do each day. If a decorative finish or a sealer is planned, CCAA advises checking that the curing products are compatible with it.

Our service is pumping and placing the concrete, so curing is arranged between you and your concreter, following the plans.

Curing starts once the slab is finished, so a pour that runs to time leaves the concreter daylight to finish and cover it. To plan the day, read how long a concrete pour takes, then talk to us about the pump. For driveways, house slabs and pool surrounds, see residential concrete pumping, or visit concrete pumping for our 31m, 28m and 20m boom pumps and line pump. We work across Cairns and the Tablelands, and we are open 24 hours, Monday to Sunday, so an early start is possible. When you have a date, contact us or call 0413 345 977.