Key takeaways
- Plastic shrinkage cracking occurs when the rate of evaporation from the concrete surface exceeds the rate at which bleed water rises to replace it, which is why the same mix can perform differently on two days on the same site.
- The evaporation measure is a combined reading of air temperature, relative humidity, concrete surface temperature and wind speed, and it is the combination rather than any single reading that triggers action.
- Wind is the most underrated input in the measure because it strips the saturated layer above the surface, so a mild day with a strong dry breeze can be more damaging than a still hot day.
- Curing must begin while the surface is still saturated, because once the surface has dried the curing membrane or water is protecting a surface that has already begun to shrink.
- Finishing operations that run past the point where bleed water has stopped rising cause more surface damage than the curing regime applied afterwards can correct.
Hot weather does not crack concrete. It removes moisture from the surface faster than the mix can replace it, and the surface shrinks against a core that is still close to its original volume. The restraint comes from the concrete underneath, and the crack opens to relieve the tension. That is plastic shrinkage cracking, and it happens within hours of placing, often before anyone has left the site.
The practical consequence is that the curing method matters less than the curing decision. A builder who soaks a slab at midday has often started too late, because the surface has already dried and the shrinkage has already begun. A builder who covers and windscreens at the right moment, using a method that is adequate rather than elaborate, will usually outperform the first.
The right moment is not a feeling. It is a reading, and it can be taken on site with equipment most residential builders already own or can borrow. This article sets out how to take that reading, what to do with it, and where the commercial risk sits when it is ignored.
Why hot weather is a moisture problem, not a temperature problem
Concrete placed in hot conditions has two competing processes running at the surface. Bleed water is rising from the body of the mix as the solids settle. Evaporation is removing water from the surface into the air. As long as bleed water arrives at least as fast as evaporation removes it, the surface stays saturated, and a saturated surface cannot shrink.
When evaporation outruns bleed, the surface layer dries. It then loses volume while the concrete beneath it does not, and the resulting tension is relieved by cracking. The cracks are usually short, parallel and irregular, and they appear in the first hours. They are not the same as long-term drying shrinkage cracks, which form much later and are driven by moisture loss through the full depth of the slab.
This is why air temperature alone tells you very little. A mild day with a strong dry wind can strip a surface faster than a hot still day. A humid day slows evaporation even when the temperature is high. The mix design also shifts the balance: a mix with a high bleed rate is more tolerant of evaporation than a lean mix with low bleed, and a mix with a retarder behaves differently again. The point is that the risk on any given day is a property of the weather and the mix together, and only one of those is on the delivery docket.
The evaporation measure: four readings, one decision
The measure that matters combines air temperature, relative humidity, concrete surface temperature and wind speed. Each of these drives the rate at which water leaves the surface, and the effect of the four together is greater than the sum of the parts. A single reading in isolation will mislead you.
Air temperature and humidity determine how much moisture the air can still take up. Concrete surface temperature determines how vigorously the water at the surface is trying to leave; a slab in direct sun can be substantially hotter than the surrounding air, which is why the surface reading is taken at the concrete and not from a weather station. Wind speed determines how quickly the saturated layer just above the surface is replaced with drier air; without wind, that layer becomes humid and slows further evaporation, and with wind it is stripped away continuously.
Combining the four gives an evaporation indicator that can be compared against a threshold. The threshold itself is not something to improvise from a blog article. It should come from the project specification, the concrete supplier’s recommendations, or the guidance published by Cement Concrete and Aggregates Australia, whose technical material is the standard local reference for this calculation. If the specification is silent, ask the supplier in writing before the pour, and keep the reply.
Taking the readings on a residential site
None of this requires a laboratory. What it requires is that the readings are taken at the right place and at the right time, and that the person taking them knows what they are for.
Take air temperature in the shade at the pour location. On a large site the difference between the street frontage and a courtyard behind the building can be several degrees, and the concrete only cares about the second one. Take relative humidity from a handheld meter or from the current observation for the locality, understanding that a locality observation is an approximation and a meter at the slab is better.
Take the concrete surface temperature by placing a contact thermometer or an infrared meter on the actual surface, in the sun if the slab is in the sun. This reading is the one most often skipped, and it is the one that most often explains a failure on a day that did not feel extreme.
Estimate wind speed at the surface. A handheld anemometer gives a number; a flag, a length of light tape or the behaviour of dust will give you a category. What matters is whether there is meaningful air movement across the slab, because even a moderate breeze on a dry day can push the evaporation indicator up sharply.
Applying the evaporation measure on site
- 1Record air temperatureTake the air temperature in the shade at the pour location, not at the site office, because the two can differ across a large site.
- 2Record relative humidityRead humidity from a portable meter or the current bureau observation for the locality; low humidity widens the gap the surface is trying to close.
- 3Measure surface temperatureMeasure the concrete surface temperature directly, because a slab in direct sun can sit well above air temperature and drives evaporation harder.
- 4Estimate wind speedEstimate wind speed at the surface using a handheld anemometer or the current observation, since wind is the input most often ignored.
- 5Combine the readingsCombine the four readings into a single evaporation indicator and compare it against the threshold your specification nominates.
- 6Decide and recordIf the indicator is at or above the threshold, start curing and wind protection immediately and record the decision in the site diary.
Record all four readings, the time, and the decision you took. The diary entry is not paperwork for its own sake. If a crack pattern is disputed later, the readings are the evidence that the curing decision was made deliberately rather than by habit, and that is a very different position to defend.
What to do when the indicator is high
When the combined indicator reaches the threshold in your specification or supplier advice, the response is not to pick a curing method from a list. It is to change the conditions.
Wind is the input you can most easily remove. A temporary screen or shade cloth positioned to break the airflow across the slab can drop the evaporation rate substantially without touching the concrete. Shade also reduces the surface temperature, which attacks the problem from a second direction. On an exposed site, the cost of a screen is trivial compared with the cost of a failed surface.
Start curing earlier than you would in mild weather. In practice this means the curing regime begins while the surface is still saturated, which in hot conditions is often while finishing is still under way. Water curing, covers and membranes all work on the same principle: they keep the surface saturated so it cannot shrink. A membrane applied to a surface that has already dried is protecting a surface that has already cracked.
Consider the timing of the pour itself. Where the programme allows, placing earlier in the day or waiting for a milder window reduces the evaporation rate the concrete will experience during its most vulnerable hours. That is a programme decision, not a concrete decision, and it is properly made the day before with the readings in front of you.
Where finishing goes wrong in the heat
The most common and most expensive hot weather error on residential work is finishing past the point where bleed water has stopped rising. When the surface is still bleeding, working it pushes water and fines around and produces a weak, dusty, sometimes crazed surface. When the surface has dried but the concrete underneath has not stiffened, working it tears the surface and closes the cracks that have already begun to form, only for them to reappear.
The window between these two states is short in hot weather. It is shorter still in wind. The finisher needs to know the mix, the bleed behaviour and the conditions, and needs the authority to wait rather than to keep working a surface that is not ready. On a hot windy day, waiting for the right moment and then finishing quickly is almost always better than starting early and working the surface repeatedly.
If a surface has already dried before finishing is complete, the correct response is not more trowelling. It is to accept the surface as it is, start curing immediately, and let the slab cure. Additional passes on a dry, plastic surface make the outcome worse, not better.
How the weather risk interacts with safety
Hot weather concreting is also a work health and safety matter, and the two conversations should not be held separately. Safe Work Australia publishes guidance on working in heat that covers hydration, rest breaks, acclimatisation and the recognition of heat illness. Concrete work in hot conditions combines heavy manual effort, radiant heat from the surface and often direct sun, which is a combination that produces heat stress quickly.
The practical overlap is that the same conditions that raise the evaporation rate also raise the risk to the crew. A pour that is pushed through a hot windy afternoon to avoid a programme delay exposes both the concrete and the people to conditions that neither handles well. Safe Work Australia also publishes guidance on concrete pumping, which is relevant where placing is done by pump and where the pump crew is working to a different set of constraints than the finishing crew.
Planning the pour around the conditions, rather than reacting to them, addresses both problems at once. That means a decision about start time made with the forecast and the readings in hand, and a nominated person on site with the authority to stop if the conditions move outside the plan.
What changes the evaporation rate and what does not
| Raises the rate | Does not raise the rate |
|---|---|
| Wind across the surface | Concrete colour or pigment |
| Low relative humidity | Aggregate nominal size on its own |
| High concrete surface temperature | Slump class on its own |
| High air temperature | Time of day as a standalone factor |
| Direct sun on an exposed surface | Distance from the batching plant |
Worked example: a slab in western Sydney
A builder in western Sydney is pouring a residential ground slab in late summer. The forecast for the pour day is hot with a dry north-westerly and low humidity. The builder takes readings at the pour location in the morning: air temperature is high, humidity is low, the surface in the sun is reading above air temperature, and there is a steady breeze across the slab. The combined indicator is well above the threshold in the specification.
The builder does three things. First, the start time moves earlier, with the pump booked for first light and the concrete supplier notified. Second, a shade screen is erected on the windward side of the slab before the first truck arrives, and shade cloth is positioned over the area that will be finished last. Third, the finisher is told that the window will be short and that waiting is expected, and the curing materials are staged at the slab edge rather than in the compound.
The pour runs. The surface starts to dry earlier than it would in mild conditions, the finisher waits for bleed to stop and then works quickly, and curing begins while the surface is still saturated. The readings are recorded through the day, including the time the screen went up and the time curing started.
Six months later, the slab shows no plastic shrinkage cracking. The cost of the exercise was the earlier start, the screen, and about twenty minutes of extra supervision. Had the pour proceeded on the original plan, the likely outcome was a surface crack pattern that would have been raised at handover, argued about, and possibly repaired at the builder’s cost.
The commercial reality of getting this wrong
Plastic shrinkage cracks appear early, often within the first day, but they are usually noticed at handover or later. By then the concrete has cured, the cause is contested, and the builder is the party holding the defect. Repair of a cracked slab surface is rarely a simple cosmetic fix, and depending on the extent and the exposure, the rectification can involve grinding, patching or in serious cases replacement of the affected area.
The cost is not only the repair. It is the argument about whether the mix was wrong, the finishing was wrong, or the curing was wrong, and in the absence of records the argument usually defaults to the builder. Readings taken at the time and recorded in the diary change that. They show that the conditions were assessed, that the risk was recognised, and that a deliberate response was taken. That is the difference between a defect that was unforeseeable and a defect that was managed.
For builders working through their continuing professional development, the technical content here sits within the building materials and construction practice areas covered by the PointCPD course catalogue. The guide to how NSW builder CPD works explains how those hours are structured and recorded. If you are not yet licensed and are working through the requirements, the PointCPD home page is the starting point.
Before the pour, confirm on site
- Current forecast for the pour window, including wind, not just temperature.
- Measured air temperature, humidity, surface temperature and wind at the pour location.
- A nominated evaporation threshold and a nominated person empowered to delay or stop the pour.
- Curing materials on site and ready before the first truck arrives, not ordered after the surface dries.
- Windbreaks or screens available if the measure is close to the threshold.
- A plan for finishing that matches the bleed rate of the mix actually supplied.
- A diary format that records the readings and the decision, not just the outcome.
What to put in place before the next hot pour
The changes worth making are small and they are made before the day. Nominate a threshold from the specification or the supplier, and nominate a person who can act on it. Have the measuring equipment on site and know how to use it. Have curing materials and wind protection available before the first truck, not ordered once the surface starts to dry. Brief the finisher on the expected window and give them the authority to wait.
Then record what you did. The readings, the time, the decision and the action. In hot weather, the difference between a slab that performs and one that does not is usually not the method chosen but the moment it was applied, and the record is what proves the moment was deliberate.
Questions builders ask
Why does concrete crack in hot weather even when I cure it?
Cracking in hot weather is usually plastic shrinkage, which happens in the first hours after placing rather than after curing begins. If evaporation from the surface outruns the bleed water rising to replace it, the surface dries and shrinks against the concrete beneath it. Curing applied after that point protects a surface that has already begun to shrink, which is why timing matters more than the method.
What four things do I measure to work out evaporation from a slab?
Air temperature, relative humidity, concrete surface temperature and wind speed at the surface. The combination drives the rate at which water leaves the concrete, and no single reading is meaningful on its own. Surface temperature is the one most often skipped, and wind is the one most often underestimated.
Does wind really matter as much as temperature?
It often matters more. Wind replaces the humid layer of air sitting just above the slab with drier air, so evaporation continues at a high rate even when the temperature is moderate. A mild, dry, breezy day can strip a surface faster than a hot still day, which is why a windbreak or screen is one of the cheapest and most effective responses.
When should curing start on a hot day?
Curing should begin while the surface is still saturated, which in hot conditions is often before finishing is fully complete. Water curing, covers and curing membranes all work by keeping the surface saturated so it cannot shrink. Applying them after the surface has visibly dried is too late to prevent plastic shrinkage cracking.
What should I do if the surface dries before finishing is finished?
Stop working the surface and start curing immediately. Extra trowelling passes on a dry, plastic surface make the damage worse rather than closing it, and the cracks that have already formed will typically reappear. Accept the finish, protect the slab, and record what happened and when.
Where do I find the evaporation threshold I should be working to?
The threshold should come from the project specification, the concrete supplier’s written recommendations, or the technical guidance published by Cement Concrete and Aggregates Australia. If the specification is silent, ask the supplier in writing before the pour and keep the reply on file. Do not improvise a threshold from a general article, because it depends on the mix and the exposure.
Sources
- Working in heat · Safe Work Australia
- Concrete pumping · Safe Work Australia
- Cement Concrete and Aggregates Australia · CCAA
- Apply for an individual contractor licence · Service NSW
Published 2 July 2026. General information for builders, not legal, financial or engineering advice for a specific job. Check current requirements with the relevant regulator or standard before relying on them.
