Construction · Concreting
Prepare the weather questions before the pour.
Use the forecast to prepare for a conversation with the supplier and the person responsible for placement. A general weather app cannot approve a mix or a pour.
Build a weather brief for the whole operation
- Identify the placement location and planned delivery, placement and finishing times.
- Review hourly weather over the operation and the protection period specified for the job.
- Record forecast rain timing, air temperature, humidity and wind, with the source time.
- Confirm how actual concrete temperature and local site conditions will be measured.
Why air temperature is not enough
Queensland TMR’s concrete specification considers air temperature, relative humidity, concrete temperature and wind when assessing evaporation. It also addresses protection from rain. That specification applies to the work governed by it; use your project’s applicable specification and supplier instructions.
Atmosphr has no concrete-temperature sensor or mix data. It cannot calculate a reliable project-specific evaporation rate from the daily maximum alone, and it does not label a forecast as “safe to pour”.
How weather affects fresh concrete
Hot, dry and windy. Cement Concrete & Aggregates Australia (CCAA) defines hot weather for concreting as any combination of high air temperature, low relative humidity and high wind speed, so a mild but dry, windy day can count. When water evaporates from the surface faster than bleed water rises to replace it, the surface dries and shrinks, and plastic shrinkage cracks can form while the concrete is still plastic. CCAA also warns of thermal cracking when a hot day is followed by a cool night, especially in thick elements.
Cold. CCAA’s cold weather data sheet explains that low temperatures slow cement hydration, so setting takes longer and strength develops more slowly. Freshly placed concrete is vulnerable to freezing both before and after it sets; as a general rule it must be protected from freezing for at least 24 hours after placement.
Published temperature and evaporation limits
The limits below are quoted from the named documents. Use the specification that actually governs your job; these are not general rules for every pour.
| Source | What it says |
|---|---|
| AS 1379 (as summarised by CCAA) | Concrete temperature at the point of delivery must be within 5 °C to 35 °C. |
| CCAA Hot Weather Concreting | Precautions are usually recommended when air temperature rises above 30 °C, particularly with a hot, dry wind. As a general rule, take precautions against premature drying if evaporation exceeds 1 kg/m²/h. |
| CCAA Cold Weather Concreting | Precautions are usually recommended when air temperature falls below 10 °C. |
| TMR MRTS70 (July 2022), clause 13.1 | No placement if concrete temperature is below 10 °C or above 35 °C; if, based on three days of site records and the Bureau forecast, air temperature is likely to exceed 45 °C during placement or within two hours after; or if formwork or reinforcement exceeds 55 °C. If air temperature is likely to exceed 35 °C, a hot weather procedure is required. |
| TMR MRTS70, clause 13.2 | If predicted evaporation during placement and finishing exceeds 0.75 kg/m²/h, measures must reduce it below that value, using the latest Bureau data on the day. An evaporation retarder is mandatory, applied within 10 minutes of placement and screeding. |
| TMR MRTS70, table 10.5 | Place and compact within 75 minutes of charging the mixer at a concrete temperature up to 32 °C, 60 minutes from 32 to 35 °C, and reject the load above 35 °C. |
MRTS70 estimates evaporation from air temperature, relative humidity, concrete temperature and wind velocity. Examples of control measures it gives are chilled water or ice in the mix, barriers against wind and sun, and pouring in cooler parts of the day.
Rain on fresh concrete
MRTS70 clause 13.3 says concrete must not be poured in the rain, or when rain is imminent, unless a waterproof covering protects all exposed surfaces; tarpaulins or roofing are its examples. Water must be removed from forms before pouring. Concrete exposed to significant rain between the start of placement and the start of curing is rejected, and the specification lists the effects it is guarding against: increased workability, ponding and cratering of the surface.
That is why the hourly rain timing matters more than the daily total. A shower forecast for the finishing window is a bigger problem than heavier rain the next day once curing has started, and covers need to be on site before they are needed.
Questions for the supplier and engineer
- What concrete temperature can you guarantee at delivery on this day, and how will it be measured?
- If it is hot, can the mix use chilled water, ice or a set-retarding admixture, and has that been trialled?
- How long is the haul, and does it fit the specified placement time at the expected concrete temperature?
- Is an evaporation retarder on site, and who applies it?
- If it is cold, is a higher strength or high-early-strength cement appropriate, and how long must the concrete be protected?
- Who calculates the evaporation rate on the day, from which data, and who can stop the pour?
- What protection is ready for rain, sun and wind, and how long must it stay in place?
CCAA offers a Smart Concreting app that combines air temperature, humidity, wind speed and estimated concrete temperature to estimate evaporation. Atmosphr’s hourly temperature, humidity and wind can help you see a risky afternoon coming days ahead, but the calculation on the day should use the data your specification names.
Questions to resolve before booking
Which forecast source and site measurements will be used? What conditions trigger a review? Who makes the decision? What protection is available, and how long must it remain effective? Write the answers into the job’s plan rather than treating a green weather icon as acceptance.
Use the construction planning checklist to capture the review, and the rain guide to separate forecast timing from a daily total.