How Hot Weather Conditions Affect Concrete Curing:
Curing concrete involves keeping it moist and at a stable temperature soon after placement. This ensures proper hydration — the chemical reaction that generates the concrete’s strength and durability. In hot weather, concrete can gain strength quickly early on, but high temperatures may ultimately reduce its long-term strength. Additionally, elevated heat can cause rapid surface drying, raising the risk of plastic shrinkage cracks before the concrete sets.
Temperature plays a vital role in the curing process. Higher temperatures accelerate hydration, and consequently, strength development. Concrete temperature should be maintained above 50°F to ensure an adequate rate of strength gain. Additionally, a uniform temperature throughout the concrete section during the strength development phase helps prevent thermal cracking.
In the case of exposed concrete, factors like relative humidity and wind conditions become significant as they influence the rate of moisture evaporation from the concrete surface. Failure to address these factors properly could lead to issues such as cracking, compromised surface quality, and reduced durability.
Why Hot-Weather Curing Matters:
1. Predictable Strength Gain
Ensuring predictable strength gain is essential in concrete construction. Studies reveal that concrete exposed to dry conditions may experience up to a 50 percent reduction in its potential strength compared to adequately moist-cured concrete.
High-temperature placements might result in rapid early strength, but it could lead to diminished later strengths. Conversely, concrete placed in cold weather will require more time to gain strength, potentially delaying form removal and subsequent construction.
2. Improved Durability
Proper curing significantly enhances concrete’s durability. Well-cured concrete exhibits improved surface hardness, making it more resistant to wear and abrasion. Additionally, curing renders concrete more impervious to water, safeguarding it against the infiltration of moisture and water-borne chemicals. This increased durability and protection contribute to a longer service life.
3. Better Serviceablility and Appearance
Optimal curing practices not only impact strength and durability, but also affect the serviceability and appearance of concrete. Inadequately cured concrete can result in a soft surface with diminished resistance to wear and abrasion. However, with proper curing, undesirable concrete surface effects like crazing, dusting, and scaling are significantly reduced.
Best Practices for Curing in High Heat
Moisture Retention Strategies:
- Wind breaks, foggers, and misters — shield the concrete and mist it before finishing.
- Wet coverings like burlap or cotton mats kept damp with hoses offer consistent moisture.
- Straw topped with tarps can work, but requires daily watering, wind protection, and a thickness of about 6 inches.
- Damp earth, sand, or sawdust are effective for slabs — just avoid materials that stain.
- Continuous sprinkling or ponding ensures a wet surface, but watch that water temperature isn’t more than 20 °F cooler than the concrete and that pond edges contain the water.
Using Moisture-Retaining Materials:
- Liquid membrane-forming compounds (ASTM C309 compliant): sprayed about an hour after finishing, ideally in two perpendicular coats. For bright floors or tile surfaces, choose non-reactive or wash-off types.
- Plastic sheeting (ASTM C171, ≥4 mil thick): lay flat and overlap edges; use dark plastic if under 60 °F or reflective plastic when above 85 °F. Secure the edges to avoid wind lift. Beware of wrinkled sheets creating discoloration.
- Waterproof paper: two-ply Kraft paper meets ASTM C171 standards — good for surfaces where plastic might mar the finish.
*Note that evaporation retardants help only before setting; they don’t replace proper curing.
Temperature Management:
Hot concrete generates heat from hydration. In hot weather, keep temperatures from dropping or rising more than 5 °F per hour in the first 24 hours to avoid cracking. Techniques like water curing or sprinkling can help regulate temperatures. Shield the concrete at night to prevent rapid chilling that could weaken early strength development.
Using Chilled Mix Water
One effective way to control temperature from the very beginning is by using chilled mix water. When ambient temperatures are high, the concrete mix itself can heat up before it’s even placed. Substituting part or all of the mixing water with chilled water — or even ice — can reduce the initial concrete temperature significantly.
Why It Works: Lowering the mix temperature slows the rate of hydration, helping to reduce thermal stress and giving workers more time for placement and finishing.
When to Use: On hot days (especially when air temps exceed 85°F), in large pours, or when delays in placement are expected.
Figure 1: Chiller unit at the Triangle Ready Mix concrete facility in Morrisville, NC.
How It Helps: Keeps internal temperatures within acceptable limits, reducing the risk of early-age cracking & improving final strength.
Be aware that products marketed as evaporation retardants are specifically intended to decrease the rate of evaporation before the concrete sets, helping to prevent plastic shrinkage cracking. These materials should not be employed for the final curing process.
Conclusion
- Hot weather speeds hydration and early strength gain—but uncontrolled heat can reduce final strength.
- Keeping concrete moist and gently cooled for several days is essential to reduce cracking and preserve strength.
- Use well-chosen materials and methods like wet coverings, membrane compounds, or plastics suited to job-site conditions.
- Monitor temperature changes carefully—limit fluctuations to under 5°F per hour during the critical first day.
By carefully managing moisture and temperature during hot weather conditions, you help to ensure that concrete develops a durable, strong, and defect-free finish.
