Concrete is the cornerstone of modern construction. However, pouring concrete in hot weather introduces a series of technical challenges at every stage—from mix preparation to placement and curing. Elevated temperatures accelerate cement hydration, shortening setting times, which can adversely affect both fresh and hardened concrete performance. Additionally, rapid evaporation of water from fresh concrete reduces workability, leading to shrinkage cracks and strength loss. These factors pose significant risks to the long-term durability and service life of concrete structures. Nevertheless, with appropriate precautions, concrete poured in hot conditions can perform as well as that placed under ideal circumstances. In this article, prepared in accordance with the ACI 305R-20 Guide to Hot Weather Concreting, we will explore all the essential technical details, practical applications, and critical considerations for successful concrete placement in hot weather conditions.
Understanding Hot Weather Concreting
Hot weather concreting refers to the process of placing concrete under conditions that accelerate moisture loss and cement hydration. This typically includes high ambient temperatures, low relative humidity, and high wind speeds. When the rate of evaporation surpasses the rate at which bleed water rises to the surface, issues like cracking and strength reduction can occur.
According to the American Concrete Institute’s ACI 305R-20 guide, precautions should be taken when the evaporation rate exceeds 0.2 lb/ft²/hour (1.0 kg/m²/hour). Research indicates that curing fresh concrete at temperatures around 100°F (38°C) during the first 24 hours can reduce its 28-day compressive strength by 10–15% compared to concrete cured at standard temperatures. Therefore, it’s crucial to protect both in-place concrete and test specimens from rapid moisture loss and to cure them according to standards.
It’s important to note that hot weather effects aren’t confined to summer months. Windy and dry conditions or days with low relative humidity can pose similar risks, even if ambient temperatures aren’t particularly high. For instance, precautions taken on a calm, humid day differ from those required on a sunny, windy day. Even with constant air temperatures, increased evaporation rates can lead to surface issues like plastic shrinkage cracks. If the evaporation rate approaches 0.2 lb/ft²/hour (1 kg/m²/hour), measures to maintain surface moisture become essential.
Impact of Hot Weather on Concrete Performance
High temperatures directly affect both fresh and hardened concrete, posing serious risks to quality, strength, and longevity. Key issues include:
- Rapid Setting and Loss of Workability: Elevated temperatures increase cement hydration rates, significantly shortening setting times and reducing workability. In environments above 35°C, concrete can become unworkable within minutes, necessitating rushed finishing operations that may compromise surface quality.
- Slump Loss (Workability Issues): Slump, or concrete’s flowability, is highly sensitive to environmental factors. In hot weather, even a 1% change in aggregate moisture content can alter slump by 1-2 inches (25–50 mm). Additionally, concrete with reduced workability experiences increased friction within the mixer during transport, raising its temperature and further accelerating hydration, leading to performance losses.
- Plastic Shrinkage and Surface Crusting: Exposure to sun and wind causes concrete to lose moisture before setting, resulting in surface shrinkage cracks, crusting, and delamination. If the surface hardens while underlying layers remain plastic, blistering and peeling can occur.
- Long-Term Strength Reduction: While initial strength gain may seem faster in hot conditions, long-term strength often suffers. ASTM standards recommend curing test specimens at 68–78°F (20–26°C). Curing at 100°F (38°C) or higher can reduce 28-day strength by 10–15%.
- Air Content and Abrasion Resistance Issues: High temperatures complicate control of air entrainment in concrete. Unevenly distributed air bubbles can lead to faster surface wear, especially in structures subjected to freeze-thaw cycles, compromising durability.
- Delivery Timing Challenges: In hot weather, concrete loses workability over time. Extended transport or on-site delays can increase the mix’s temperature, reduce slump, and severely diminish workability, potentially leading to rejected batches or structural issues like cold joints.
- Inaccurate Test Results: Field specimens must be protected from sun, wind, and rapid evaporation per ASTM C31 standards. Failure to do so can yield test results that underestimate actual concrete performance, jeopardizing quality control and project safety.
Preparations for Hot Weather Concreting
Successful hot weather concreting depends not only on measures taken during placement but also on thorough pre-planning. Unpredictable weather effects can cause significant on-site issues, making a systematic approach essential. Key preparation steps include:
- Pre-Placement Coordination Meeting: At least 30 days before pouring, convene all stakeholders—contractors, concrete suppliers, site engineers, and lab personnel—to discuss:
- Concrete mix design and target slump
- Concrete placement schedules
- Curing methods and equipment
- Availability of backup equipment
- Material Selection and Mix Design:
- Opt for cements with low heat of hydration.
- Incorporate supplementary cementitious materials like fly ash or slag to moderate heat generation.
- Use retarders, water reducers, and extended set admixtures to enhance workability. Test admixture combinations under site conditions to ensure compatibility.
- Concrete Cooling Techniques:
- Store fine and coarse aggregates in shaded areas. Since coarse aggregate temperature significantly influences overall mix temperature, keeping them cool is critical.
- Depending on the temperature conditions, a portion of the mixing water can optionally be replaced with crushed ice or liquid nitrogen to help lower the concrete’s temperature.
For example, reducing:
- Cement temperature by 8°F (4.4°C)
- Water temperature by 5°F (2.7°C)
- Aggregate temperature by 1.5°F (0.8°C)
can each lower concrete temperature by approximately 1°F (0.5°C). - Delivery and Placement Planning:
- Schedule pours during cooler periods, such as early morning or nighttime.
- Plan transport routes and on-site access to minimize delays.
- Predefine the order in which mixers will discharge to ensure timely placement.
On-Site Best Practices for Hot Weather Concreting
Effective hot weather concreting requires vigilant on-site practices:
- Coordination Between Deliveries: Maintain constant communication between the batching plant and site to synchronize mixer arrivals, site access, and unloading times. Delays can lead to premature setting, affecting workability and the concrete’s ability to adhere to surfaces.
- Layer Placement Timing: In formwork pours, place concrete in successive layers promptly. Delays between layers can cause cold joints. Use retarders if necessary to extend setting times.
- Vibration Equipment and Backup Vibrators: Proper consolidation is vital for strength and surface quality. Ensure sufficient internal and external vibrators are available, along with backup equipment and generators, to prevent delays due to equipment failure.
- Availability of Curing Equipment: Begin curing immediately after finishing to prevent rapid moisture loss. Equipment may also be available during application:
- Wet covers (e.g., burlap),
- Fogging systems,
- Anti-evaporation curing chemicals,
- Surface coating protective materials.
Initiate curing as soon as finishing is complete to avoid surface cracking, dusting, or strength loss.
Best Practices for Hot Weather Concreting
Achieving high-quality, durable concrete in hot weather involves strategic material choices and application methods:
- Pour During Cooler Hours: Schedule pours for early morning or nighttime to minimize exposure to heat and evaporation.
- Protect Materials from Heat:
- Store aggregates in shaded areas.
- Insulate water tanks to prevent heat gain.
- Paint cement silos with reflective (white) coatings to reduce solar absorption.
- Use Light-Colored Mixer Drums: White or light-colored mixer drums reflect sunlight, reducing heat buildup during transport and pumping.
- Apply Evaporation Retardants: When wet curing or fogging isn’t feasible, use sprayable moisture-absorbing (monomolecular) films to slow evaporation and reduce plastic shrinkage cracking, especially on large surface areas.
Testing and Inspection Recommendations
Ensuring concrete quality in hot weather requires meticulous testing and monitoring:
- Accurate Field Testing: Conduct on-site tests per ASTM standards to assess:
- Concrete temperature (ASTM C1064/C1064M)
- Slump
- Air content (ASTM C231)
- Density (ASTM C138)
- Proper Curing of Test Specimens: Store cylinders or cubes at 73 ± 3.5°F (23 ± 2°C) in a moist environment, shielded from sun and wind.
- Monitor Evaporation Rates: Regularly record:
- Air temperature
- Relative humidity
- Wind speed
- Evaporation rate
If evaporation exceeds 1.0 kg/m²/hour, enhance surface protection using wet curing, fogging, wet coverings, or monomolecular films.
- Maintain Detailed Records: Document:
- Concrete production and delivery times
- Water addition amounts
- Types and dosages of chemical admixtures
- Concrete placement procedures
These records support quality assurance and facilitate transparent project management.
Conclusion: Turning Hot Weather Concreting Challenges into Success
Hot weather conditions present significant challenges for concrete placement and curing. However, with careful material selection, precise timing, strategic workforce planning, and rigorous testing protocols, these risks can be effectively managed. Adhering to ACI guidelines and leveraging field experience demonstrate that high-quality concrete is achievable even under hot conditions. Success begins with proactive planning and attention to detail.
