Curing concrete in heat, wind and frost

Curing is not a product. It is the decision to keep water in the concrete long enough for the paste to become itself. In Australian work that decision is taken in climates that do not cooperate: a dry northerly in Adelaide, a forty-degree western Sydney afternoon, a Darwin downpour that arrives twenty minutes after the last trowel pass, a frost in the high country that treats a Saturday pour as a dare.

The first seventy-two hours are when decorative concrete is either a finish or a future repair. After that you are maintaining. Before that you are still making the material.

Plastic shrinkage, which is a wind problem

Plastic shrinkage cracks are the fine, often parallel tears that appear while the slab is still plastic — sometimes before you have left the site. They are not settlement cracks and they are not an argument about reinforcement. They are the surface drying faster than bleed water can replace what the wind and the sun have taken.

The physics is ordinary. Evaporation from the fresh face exceeds the water arriving from below. The paste shrinks while it is still a network of weak crystals. The slab tears at the skin. On a decorative floor those tears are the architecture. On a driveway they collect dirt and then the sealer, and become a map you did not draw.

Wind is more dangerous than temperature alone. A hot, still Brisbane morning can be kinder than a cooler, dry, gusty afternoon in the Adelaide hills or on a western Sydney estate with no fence yet and a long fetch across vacant lots. Hot wind stacks the problems: evaporation, a mix that stiffens in the truck, a crew that adds water, a finish that is then attempted on a slab already losing its surface.

If you can feel the wind drying your lips while the trowel is still out, the slab is already behind. Curing starts then, not at knock-off.

Shade cloth, wind breaks, a fog or mist over the face — used with care so you are not washing out cement — and a willingness to stop are the unfashionable tools. A curing compound applied three hours late is a coat of paint on a crack.

Heat: Adelaide, western Sydney, and the inland slab

Australian heat is not one weather. Coastal humidity keeps evaporation in check even when the air is warm. Inland heat with a dry wind does not. Western Sydney in a heatwave, Adelaide when the gully wind comes down, Perth on an easterly, a Riverina pour in January — these are the days when decorative work should be poured at dawn or not at all.

Heat accelerates set. The window for finishing shortens. A mix that was polite at seven is a fight at ten. Crews then do the two things that damage a face: they add water, and they trowel a crust over a still-plastic core. The crust is decorative. It will delaminate or craze. The core is still bleeding into it.

Practical heat discipline looks like this:

• Start early, and say so in the brief, not as a hope on the morning
• Keep trucks in a short queue so slump is not recovered with a hose
• Cool the subgrade; a hot, dry sand bed is an oven under the slab
• Have the cure method on site before the first truck, not in a ute that is still at the yard
• Call off when the forecast and the site wind disagree with the brochure

A mix with slag or fly ash can be a small mercy in heat because it slows the early reaction, but it will not invent moisture. Supplementary cementitious materials change the clock. They do not replace the hessian.

Dark oxide mixes make the surface hotter once they exist. A charcoal driveway in February is a frying pan that also wants to dry. Pale mixes are slightly kinder. Neither colour is a cure.

Tropical downpours

The opposite failure is a lot of water, arriving too soon. Tropical and subtropical Australia — the Top End, far north Queensland, a Brisbane storm in the wet — can put millimetres on a fresh face before the paste has any strength. Rain pits the surface, washes out cement, and leaves a sandy skin that will never take a sealer evenly.

The remedy is timing and cover, not optimism. Watch the radar as if it were a structural drawing. Have plastic on site, enough to cover the pour, and enough people to put it down without walking a pattern into the slab. Plastic laid on a still-wet trowel finish will mark. Plastic laid late will protect a surface that is already cratered.

A pour that is caught is a pour you assess honestly. Light pitting on a broom finish may be livable. Pitting on a burnished decorative floor is a different floor. Grinding later is a change of brief, not a tidy-up. If the rain wins, stop, protect what you can, and decide whether the day is a slab or a sacrificial layer.

Humidity after the storm is not curing. A saturated air can slow evaporation and help, or it can leave a mottled, dark face that scares the client and then evens out over weeks. Do not seal a tropical slab the moment it looks dry on top. The core may still be a bath.

Alpine frost

Frost is a smaller geography in Australia and a real one: the high country, tablelands, Tasmania, a Canberra winter night after a clear afternoon. Fresh concrete that freezes while it is saturated and weak loses the water it was using to build crystals. The paste can become a chalk that never recovers.

The ordinary protections are insulation and timing. Do not pour a decorative slab if the night will take the surface below zero before it has strength. Blankets, straw, or insulated covers — described here as classes of method, not as products — keep the heat the hydration itself is making. A curing compound is not a blanket.

De-icing salts are less of an Australian municipal habit than they are in other countries, but a homeowner with a mountain driveway and a bag of salt can still burn a young decorative face. Snow dumped from a car, with grit, is abrasion. Design the finish for tyres and grit if the house is in that climate. A honed, pale, indoor-looking slab at the ski-week rental is a brief that ignored the postcode.

Methods, in principle

Water, plastic and compounds

The methods are few and old. They keep water in, or they add water back, or they stop wind.

• Water curing. Ponding, sprinklers, or wet hessian kept actually wet. Effective, labour-heavy, and capable of staining or marking a decorative face if the hessian is dirty or dries in patches.
• Plastic sheeting. Stops evaporation. Marks the face if it flaps or wrinkles. Needs taping and weighting. Good on walls and on slabs whose finish can take a slight mottling.
• Curing compounds. Sprayed films — wax, acrylic, silicate and other families — that slow evaporation. They must go on evenly, at the right moment, and they must be compatible with the later sealer or grind. A wax compound on a floor that will be polished is a contaminant.
• Leaving formwork on. For walls, the forms are a cure. Strip too early in a dry wind and the face maps every sheet.

Pick the method for the finish, not for the leftover drum in the shed. A decorative slab that will be stained or polished needs a compound the next trade can live with, or no compound and a physical cover. Write that into the brief. The cheapest compound is expensive when it becomes a bond-breaker under a coating you have already paid for.

The first seventy-two hours, and who owns them

Decorative sensitivity is a staffing problem. Someone has to wet the hessian on Sunday. Someone has to check the plastic after the northerly. Someone has to stop the client from walking the floor in dress shoes because it “looks set”. If the contract ends when the truck leaves, the cure is theatre.

Hold a point at the start of the cure and at the end of the third day. Photograph the covers. Note the wind. If the slab is mottled, wait before you panic and before you seal. Moisture maps recede. Plastic-shrinkage cracks do not.

This journal’s habit is to treat curing as part of the pour, not as an aftercare brochure. The mix, the weather and the first three days are one job. Split them and the face will record the split.