Pouring Concrete in Hot Weather: Australian Guide

Expert guidance: Jason Chandler is a recognised leader in high-performance, sustainable cements and concretes, bringing more than 22 years of construction industry experience in technical, operational, project management and innovation roles.

Elevated temperatures are often associated with summer, but heat, wind and low humidity can combine to affect concrete placement and finishing in any season. Hot weather is the focus here, but high wind or low humidity can also create significant risk.

Regardless of conditions, wet down formwork and the subgrade before placement. After finishing, apply a sacrificial evaporative agent such as aliphatic alcohol to the concrete surface. Extreme weather makes these standard precautions even more important.

Temperature limit: 35°C concrete

AS 1379: Specification and supply of concrete places a limit of 35°C on concrete temperature. This is the concrete temperature, not simply the forecast air temperature.

As a rule of thumb, up to about 32°C, concrete temperature is usually 2–3°C higher than ambient temperature. If the air temperature is approaching 30°C, put hot-weather precautions in place.

Sydney Seasonal Conditions for Concrete Pouring

Sydney's climate creates distinct concrete challenges across the year. Western suburbs (Penrith, Blacktown, Campbelltown) run 3–5°C hotter than the CBD and coast.

What Hot Weather Does to Concrete

1. Accelerated setting

Cement hydration doubles in speed for every 10°C rise in concrete temperature. A mix at 32°C sets roughly twice as fast as a mix at 22°C. Your effective working window drops from 90 minutes to 45–60 minutes.

2. Increased water demand

Workers instinctively want to add water when concrete stiffens in the heat. This is the worst thing you can do. Adding 10L of water to a 1m³ load reduces 28-day strength by approximately 2–3 MPa. See the guide to adding water to concrete for more detail.

3. Plastic shrinkage cracking

Concrete is "plastic" before it sets. If the surface dries faster than the concrete mass during this state, the surface shrinks and cracks, much like a dried lake or river bed.

Evaporation depends on air and concrete temperatures, wind speed and humidity. Above 0.5 kg/m²/hr, the risk of plastic shrinkage cracking is heightened. Above 1.0 kg/m²/hr, cracking is almost certain without protection such as windbreaks, shade or aliphatic alcohol. Sydney's hot westerly winds are particularly problematic.

6 Practical Steps for Hot Weather Pours

1. Schedule the pour for early morning. Aim for 5–6am start in summer. Concrete placed before 8am benefits from cooler temperatures and finishes before peak heat.
2. Check with the supplier about chemical admixtures. Admixtures can slow setting and allow more time for placement and finishing. A delayed set can also leave the surface exposed to drying for longer, so take precautions against plastic shrinkage cracking.
3. Pre-wet everything before the pour. Pre-wet formwork, reinforcement and the subgrade. This prevents them from drawing heat and water from the mix on contact.
4. Shade the pour zone. A shade cloth (90% block) over a temporary scaffold drops surface temperature by 8–12°C.
5. Have more people than you think you need. At 35°C with a 60-minute working window, every slow moment costs. 4–5 people for a 4m² pour is not excessive.
6. Apply a sacrificial evaporative agent as soon as the final finish is complete. Don't wait for the surface to look ready. The dye shows where the agent remains on the surface; reapply it as many times as needed.

Worked Example: 4m³ Penrith Driveway, January Pour