Weather is one of the biggest factors in whether a concrete pour succeeds or fails. Temperature, humidity, wind, and rain all affect how ready mix concrete sets and gains strength — and understanding each one helps you plan, adapt, and protect your work.
Why Weather Matters for Concrete
Concrete does not simply dry out. It hardens through a chemical process called hydration, where water in the mix reacts with cement to form the crystalline structure that gives it strength. Temperature, humidity, and wind speed can each speed up, slow down, or disrupt this reaction — making weather one of the most important variables to manage on any pour.
Temperature: The Most Important Factor
Temperature has the most direct effect on how quickly concrete sets and how strong it becomes.
The ideal range for working with ready mix concrete is between 10°C and 21°C. Within this window, hydration proceeds steadily, working time is predictable, and the concrete reaches its design strength reliably.
| Temperature | Setting Time | Effect on Concrete |
|---|---|---|
| Below 5°C | Very slow | Risk of freezing; delayed strength gain |
| 5–10°C | Slow | Extended working time |
| 10–21°C | Moderate | Ideal working conditions |
| 21–30°C | Fast | Reduced working time |
| Above 30°C | Very fast | Difficult to finish; risk of cracking |
Cold conditions
When temperatures drop below 5°C, hydration slows significantly. Strength development takes much longer, and if the concrete freezes before it has gained sufficient strength, ice crystals forming within the mix can cause permanent structural damage.
To protect concrete in cold weather:
- Use heated concrete mixes with warm mixing water
- Clear all ice and frost from the subgrade and formwork before pouring
- Cover freshly placed concrete with insulating blankets to retain warmth
- Add accelerating admixtures to speed up the setting process
- Pour during the warmest part of the day
For a full guide to planning winter pours, see our article on pouring concrete in winter.
Hot conditions
Temperatures above 30°C cause the opposite problem. The mix sets too quickly, leaving less time to place and finish it. More water is needed to keep the mix workable, but extra water weakens the concrete. There is also a higher risk of surface cracking as the concrete cools after the initial heat of hydration.
To manage hot weather pours:
- Schedule the pour for early morning or late afternoon, avoiding peak heat
- Ask your supplier for a mix with a retarding admixture to extend working time
- Use chilled mixing water where possible
- Shade the pour area from direct sunlight
- Aim to keep the concrete temperature below 25°C at the time of placement
- Begin curing immediately after finishing
Humidity and Evaporation
Humidity affects how quickly moisture leaves the surface of fresh concrete.
High humidity slows evaporation, helping the concrete retain moisture for thorough hydration. Low humidity does the opposite — moisture leaves the surface faster than it can be replaced from within the mix. The surface layer shrinks while the body of the concrete remains stable, creating the tensile stress that leads to cracking. A 10% drop in relative humidity can noticeably increase evaporation rates.
Apply curing compounds promptly after finishing, or cover the surface with wet hessian to maintain moisture levels.
Wind: An Underestimated Risk
Wind strips the humid air from above the fresh surface and replaces it with drier air, dramatically increasing evaporation. A 10% loss of surface moisture can reduce surface strength by as much as 20%. Strong wind also causes uneven cooling, where the surface contracts at a different rate to the concrete beneath — a leading cause of plastic shrinkage cracks that can appear within hours of placing.
Use wind barriers around the pour, apply curing compounds promptly, and weight plastic sheeting at the edges to trap surface moisture.
Hot Weather and Cracking
Cracking during warm, dry, or windy conditions is one of the most common problems on site. When the surface loses moisture faster than it can be replaced from below, the top layer shrinks while the body of the concrete stays stable. Since concrete is strong in compression but weak in tension, the surface cracks to relieve that stress. These plastic shrinkage cracks typically appear within the first few hours, while the concrete is still workable.
Incomplete hydration at the surface also creates a weaker, more porous layer that is prone to dusting and early wear.
To reduce the risk:
- Dampen the subbase before pouring so it does not draw moisture from the mix
- Close the surface quickly by striking off and bull floating promptly after placement
- Apply an evaporation retarder spray between finishing passes in very hot or windy conditions
- Start curing as soon as finishing is complete — every minute of delay counts
For a broader look at what causes concrete to crack and how to avoid it, see our guide to causes of cracks in ready mix concrete.
Curing in Any Weather
Good curing is what separates concrete that lasts decades from concrete that deteriorates quickly. The goal is to keep the surface moist and at the right temperature while hydration continues.
Water curing — covering the surface with wet hessian or cotton mats and keeping it consistently damp — is the most effective method. Continue for at least seven days, longer for structural elements.
Curing compounds are a practical alternative — spray-on liquids that form a film reducing evaporation. Apply as soon as finishing is complete.
In cold weather, insulating curing blankets serve a dual purpose: they retain both moisture and heat, keeping the concrete above 5°C while strength develops.
The Concrete Society’s hot weather concreting guidance confirms that rapid surface moisture loss is a primary cause of surface defects, reinforcing the importance of prompt curing in warm or dry conditions.
How a Professional Supplier Helps
One of the key advantages of ordering from a professional ready mix concrete supplier is access to admixtures and mix adjustments that are not available when mixing on site. Retarding admixtures extend working time in hot conditions. Accelerating admixtures speed up strength gain in the cold. Water-reducing admixtures lower the water-cement ratio without sacrificing workability, improving durability in any weather.
At Express Concrete, we have been supplying ready mix concrete across London since 1991. We monitor forecasts before deliveries, adjust mixes for the conditions on site, and advise customers on timing and protection. Our four plants in Barking, Erith, Wembley, and Chessington allow us to serve customers across the capital quickly and reliably whatever the weather brings.
Find out more about our approach to quality and service, or read our full guide on how long ready mix concrete takes to set for detail on curing timelines. Contact our team today to discuss your project and get a quick quote.
