4.1.3. Accelerating Admixtures

Accelerating Admixtures

• Purpose: Accelerate the setting and early strength development of concrete.

Accelerating admixtures are added to concrete to speed up the curing process. This means that the concrete hardens faster than it normally would without these additives. This is particularly useful in situations where quick-setting concrete is crucial, such as in cold weather conditions where prolonged working time may lead to freezing of the concrete before it sets.

Imagine trying to cook rice in cold water. It takes much longer to cook compared to using hot water. similarly, when the environment is cold, concrete takes longer to set, just like the rice. Adding accelerating admixtures is like preheating the water for the rice; it speeds up the process and prevents issues.

• Types:
  • Set accelerators: Reduce setting time.
  • Hardening accelerators: Increase rate of strength gain.

There are two main types of accelerating admixtures: set accelerators and hardening accelerators. Set accelerators focus on reducing the time it takes for concrete to set or initially harden. On the other hand, hardening accelerators help boost the strength of the concrete more quickly after it has set. This means that the concrete can be handled, loaded, or subjected to further construction activities sooner than would otherwise be possible.

Think of baking a cake. If you use a recipe that makes the cake rise faster (set accelerator), it’s ready sooner, and you can serve it quickly. If you add extra ingredients that enhance its firmness right after it has risen (hardening accelerator), the cake will be stable enough to be frosted and cut, making it ready for a party faster.

• Applications: Cold weather concreting, precast industry, repair works.

Accelerating admixtures are commonly used in several construction contexts. In cold weather concreting, where low temperatures can impede the curing process, these admixtures ensure that concrete sets quickly and achieves necessary strength, reducing the risk of freezing. They are also highly beneficial in the precast concrete industry, where speed is essential for producing elements like slabs and beams. Additionally, during repair works, where downtime needs to be minimized, these admixtures can help restore structural integrity more rapidly.

Consider a situation where construction needs to take place overnight to minimize disruption. By using accelerating admixtures, a team can pour concrete in the early evening, knowing it will set quickly. It’s like using an instant coffee mix; it saves time and allows you to enjoy your warm drink much sooner.

• Common accelerators: Calcium chloride (not used for reinforced concrete due to corrosion risk), calcium nitrate, triethanolamine.

Several chemicals serve as common accelerating admixtures. Calcium chloride is one of the most widely used, as it effectively speeds up setting time. However, it poses a risk of corrosion to reinforced steel in concrete and is therefore avoided in structures where steel reinforcement is crucial. Alternatives like calcium nitrate and triethanolamine provide similar benefits without the corrosion problems. Selection of the right accelerator is critical for ensuring both speed and long-term durability of concrete.

Think of how some fertilizers in gardening can help plants grow faster but might harm the soil if overused. Calcium chloride is like a fast-acting fertilizer that helps concrete set quickly but can harm the reinforcement if used inappropriately. Choosing the right one is crucial for the health of your plant—or in this case, the structure.

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