Pouring Concrete in Hot Weather: What Changes Once It Passes 30°C

Summer pours are normal work in the Hawkesbury, and so is the heat. The Bureau of Meteorology’s Richmond station averages about 65 days a year at 30°C or more. On those days concrete sets faster, dries from the top down and gives the crew less time to place and finish it. The cracks that come from this form in the first few hours and often aren’t seen until the next morning. This guide covers what heat does to fresh concrete, how a hot-weather pour should be run, when it should be moved, and what to ask before yours is booked.

What counts as hot weather for a concrete pour

Cement Concrete & Aggregates Australia (CCAA), the industry body for cement, concrete and aggregates, defines hot weather for concreting as any combination of high air temperature, low humidity and high wind speed. Its Hot-Weather Concreting datasheet, dated August 2026, says precautions are usually recommended once the air temperature rises above 30°C, particularly if there is also a hot, dry wind. The same datasheet notes that the Australian Standard for supplying concrete, AS 1379, requires concrete to arrive on site at between 5°C and 35°C.

So the thermometer isn’t the whole test. Wind and dry air pull water off the surface as well, so humidity and wind speed matter as much as the forecast maximum.

Those days are common here. Bureau of Meteorology records for Richmond RAAF, which go back to 1993, show an average of 15 days at 30°C or more in January, 13 in December and 11 or 12 in February. October averages six. About 19 days a year reach 35°C, and the highest temperature on record there is 47.4°C, on 4 January 2020.

What heat does to fresh concrete

CCAA lists the same set of problems for any hot-weather pour, from a house driveway to a warehouse floor. Each one comes with a question worth putting to whoever is doing the work.

The surface cracking has a name, plastic shrinkage cracking, and CCAA ties it to the rate of evaporation. Its hot-weather datasheet says precautions should be taken once water is evaporating at more than 1 kg from each square metre of slab per hour. Its older Plastic Shrinkage Cracking datasheet sets the trigger lower, at half that rate. CCAA notes these cracks rarely affect the strength of the slab, but ones that run the full depth can let water through.

For our own work, this is the commitment we give on every job: We use high-quality materials and proven construction methods to help ensure our work lasts for 25 years or more. We also use appropriate materials and techniques to control cracking in our slabs, helping to keep it very limited, if not eliminate it altogether. Please note that cracking can be affected by factors outside our control, including ground movement, weather and site conditions.

How a hot-weather pour should be run

None of what CCAA recommends is exotic. It is planning, done before the first truck is ordered. In CCAA’s words, improvising when the weather turns is not recommended.

• Pour in the cool of the day. CCAA’s advice is to program concreting for the cooler parts of the day, or at night if the site allows it.
• Talk to the concrete plant early. The supplier can adjust the mix for the conditions with set-retarding admixtures, slower cements or cooled mixing water. That has to be arranged in advance.
• Dampen the base and the forms. A dry, hot subgrade pulls water out of the bottom of the slab. It should be damp, with any excess water removed before the concrete goes in.
• Shade and windbreaks. Screens that cut the wind across the slab slow the drying.
• Evaporation retarder. A spray-on film, usually aliphatic alcohol, goes on straight after screeding and again after each pass. CCAA is blunt that it is not a curing compound and doesn’t replace one.
• Enough hands. CCAA calls for standby equipment and manpower at every stage, with forms, gear and crew ready before the first load arrives.

The crew has to be looked after too. SafeWork NSW’s guidance on working in extreme heat points the same way as the concrete advice: schedule outdoor work for early morning or late afternoon, provide shade, and rotate people or add workers. A new concrete driveway poured at first light is better for the slab and for the people finishing it.

When to postpone a concrete pour

CCAA says the first option to consider in hot, adverse weather is whether to postpone the pour. That call is easier when it has been talked through beforehand.

1. Watch the forecast for the pour day. Look at wind and humidity as well as the maximum temperature.
2. Agree who makes the call, and when. The afternoon before is better than six in the morning with trucks already loaded.
3. Move the start time first. An earlier start can be enough to get the slab placed and finished before the heat arrives.
4. Postpone if the precautions can’t be put in place. If the site can’t be shaded, the crew is short or the plant can’t adjust the mix, a different day is the better option.

The bigger the pour, the earlier this has to be settled. On warehouse floors and hardstand there is a pump, a run of trucks and a large crew to line up, and other trades are waiting on the slab. Put the hot-weather plan and the postponement call in writing with the pour date.

Curing concrete in summer

Curing means keeping the moisture in the concrete after it has been finished, and summer is when it gets skipped. CCAA’s Guide to Concrete Construction says concrete left to dry out straight away reaches only about 40% of the strength of the same concrete kept wet. Poor curing also shows up later as a dusty, weak surface and fine crazing.

For house driveways and paths, CCAA’s residential driveways datasheet gives a minimum of three days’ curing, and seven days within a kilometre of the coast or where the slab will carry heavier traffic. It also says what doesn’t count. Hosing the slab morning and afternoon doesn’t keep it continuously moist, and black plastic shouldn’t be used outdoors in hot weather because it adds heat. Coloured concrete has its own rules, which matter for stamped and stencilled finishes: no plastic sheeting, damp sand or wet hessian. For slabs and footings built to engineering drawings, the curing method and period should be on the drawings or in the specification.

Ask before a summer pour is booked

• What time will the pour start?
• How many people will be on the crew?
• What’s the plan if it’s over 30°C and windy?
• How will the surface be kept from drying out?
• How will the slab be cured, and for how long?
• Who decides to postpone, and by when?