14. Hot and Cold Weather Concreting – Fresh State Behavior

Issues:

• Rapid evaporation leads to plastic shrinkage cracks.
• Shortened setting time causes handling difficulties.
• Increased water demand reduces strength.

In hot weather, several challenges arise when working with fresh concrete. First, high temperatures can cause the water in the mix to evaporate quickly, which might lead to cracks on the surface known as plastic shrinkage cracks. Secondly, when the concrete sets too quickly, it may become difficult to work with, resulting in complications during the placement and finishing processes. Lastly, the demand for water in the mix increases in hot conditions, yet if this is not carefully managed, it can weaken the concrete’s overall strength.

Imagine making a custard on a hot day. If the mixture heats up too quickly, the custard could set before you're ready to pour it into the mold, resulting in lumps. Similarly, hot weather can cause fresh concrete to start setting too fast for a smooth finish.

Measures:

• Use chilled water or ice in mix.
• Schedule pours during cooler hours.
• Use retarders and hydration stabilizers.
• Shade aggregates and formwork.

To mitigate the problems associated with hot weather concreting, several effective strategies can be employed. First, using chilled water or adding ice to the concrete mix helps keep the temperature down. It's also beneficial to plan concrete pours during the cooler parts of the day, such as early morning or late afternoon. Furthermore, incorporating retarders into the mix can extend the setting time, allowing for easier handling. Finally, shading the aggregates and formwork from direct sunlight can prevent them from heating up, thus reducing the chances of rapid evaporation.

Consider how you might keep ice for a picnic. To keep the ice from melting too quickly in the sun, you place it in a cooler, use ice packs, and avoid opening the cooler constantly. Similarly, in concrete work, managing temperature with chilled materials and proper planning can ensure better results.

Issues:

• Freezing water expands and damages the paste structure.
• Very slow hydration rate.

Cold weather presents its own set of challenges for fresh concrete. When the temperature drops, there is a risk that any water in the concrete mix may freeze. This freezing can cause the water to expand, which can ultimately damage the structure of the concrete paste, leading to weaknesses. Additionally, the process of hydration—the chemical reaction that allows concrete to harden—occurs at a much slower rate in colder temperatures. This means the concrete takes longer to reach its desired strength, which can delay construction activities.

Think of how a glass of water becomes ice in the freezer. If you fill a glass too full, the water will expand as it freezes, potentially breaking the glass. Similarly, water in concrete can expand when it freezes, causing damage. The slower process of ice forming relates to how hydration slows down in cold conditions.

Measures:

• Heat mixing water and aggregates.
• Use accelerators (e.g., calcium chloride).
• Protect freshly placed concrete with insulated blankets.
• Avoid frozen subgrade placement.

To combat the issues related to cold weather, several measures can be undertaken to ensure fresh concrete is adequately protected. One effective approach is to heat the mixing water and aggregates before adding them to the mix, which helps maintain a suitable temperature. Additionally, using chemical accelerators, like calcium chloride, speeds up setting and hydration, making it easier for the concrete to gain strength in cold conditions. Covering the freshly poured concrete with insulated blankets helps retain heat and moisture, while also preventing freezing from occurring. Lastly, it is crucial to avoid placing concrete on frozen ground, as this can adversely affect bonding and stability.

Consider bundling up in layers of warm clothing on a chilly day to stay warm. Similarly, using heated materials and insulated coverings helps keep the concrete warm and allows it to cure properly, much like staying warm in cold weather helps prevent frostbite.

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