Hot Weather Challenges in Concrete Construction

Hot weather can significantly impact various stages of concrete production and placement, influencing hydration rates, moisture movement, long-term strength, and durability. This article explores the effects of hot weather on concrete, with a focus on different production and placement stages.

Effect of Hot Weather on Concrete Production and Placement

1. Increased Water Demand

• In hot weather, moisture loss reduces the workability of fresh concrete, necessitating higher water demand to maintain the required workability.

2. Rapid Loss of Workability

• High temperatures accelerate both hydration and moisture loss, leading to a rapid decline in concrete workability.

3. Decrease in Concrete Setting Time

• Elevated hydration rates result in a considerable reduction in the initial setting time of concrete, limiting the available time for transit, placement, and finishing.

4. Plastic Shrinkage Cracks

• Evaporation rates rise with temperature and wind speed, causing rapid drying of freshly placed concrete. This can lead to the development of plastic shrinkage cracks on the surface.

5. Strength of Concrete

• Higher temperatures boost early strength due to increased hydration rates. However, long-term strength diminishes due to moisture loss and decreased workability affecting compaction.

6. Durability of Concrete

• Reduced workability prompts the addition of more water, creating pores as moisture dries quickly. This porosity impacts concrete durability.

Placing Concrete in Hot Weather

At temperatures between 75ºF and 100ºF, concrete faces challenges, exacerbated by low relative humidity and high wind velocity. Cooling methods include sprinkling aggregates with water for evaporation, though ice or liquid nitrogen can also be used, albeit at additional cost. Precautions such as sunshades, windbreaks, and communication with the concrete provider are essential to prevent problems.

Basic Guides for Hot Weather Concreting

Plan in Advance:

• Ensure equipment and materials are ready before hot weather sets in.

Keep Subgrade and Forms Moist:

• Prevent absorption of water from the mix by maintaining moisture in the subgrade and forms.

Use Sunshades and Windbreaks:

• Deploy protective measures like sunshades and windbreaks to counteract rapid drying.

Preparation Before Ready-Mix Arrival:

• Have everything ready for concrete placement to avoid delays.

Communication with Concrete Provider:

• Maintain constant communication with the ready-mix concrete provider for coordination.

Immediate Placement and Finishing:

• Place, strike off, and darby concrete immediately upon arrival.

Use Evaporation Retardants and Covers:

• Employ evaporation retardants, misting with water, or vapor-proof sheet covers to prevent rapid drying and other issues.

Temporary Covers:

• Place continuously moistened burlap over fresh concrete, removing it in small sections ahead of finishing.

Finishing Techniques:

• Substitute high-risk finishing practices with alternatives like a burlap drag or broom finish.

Timely Curing and Sealing:

• Initiate curing when surfaces resist marring and seal with a quality sealer for at least 30 days after placing and curing.

Crew Protection:

• Ensure crew protection in high temperatures, emphasizing hydration and sun safety practices.