The Function of Concrete Retarding Admixtures

a worker finishing concrete on the job site

Concrete is a widely used construction material composed of water, cement, and rock and sand aggregates. Water is the key ingredient in concrete, and when mixed with cement, creates a paste that binds the aggregates together.

The water causes concrete to harden through the process of hydration. Water must be pure to prevent reactions that may affect the hydration process. Likewise, water-to-cement ratio is a critical factor as too much or too little water affects the concrete’s strength and workability.

Aside from water, adding other chemicals to concrete may affect its properties. These chemicals are called admixtures. Chemical admixtures may be added before or during the mixing of the ingredients for concrete. They affect concrete’s curing temperature range, setting time, colour, and workability.

One of the most common types of chemical admixtures added in concrete is retarding admixture. Concrete retarders slow down hydration, hence, lengthening the time for concrete to set. Retarding admixtures may be composed of the following:

    • Organic agents such as unrefined calcium, sodium, NH4, salts of lignosulfonic acids, hydroxycarboxylic acids, and carbohydrates.
    • Inorganic agents such as oxides of lead and zinc, phosphates, magnesium salts, fluorates and borates.

Why Use Concrete Retarding Admixtures?

Adding retarding admixtures while concrete is being mixed extends the transition time of the mixture from its paste-like or plastic texture to a rigid state. Below are situations where retarding admixtures can be used:

When Working during Hot Weather

Heat accelerates the stiffening of concrete. Since the majority of construction projects like road, building, and bridge construction are done under the heat of the sun, retarding admixtures can help decrease curing time and prevent concrete from early stiffening. Concrete retarding admixtures delay the hydration process, allowing more time for texturing or grooving concrete.

When Placing Several Layers of Concrete

a worker layering concrete

Retarding admixtures can be used in projects where multiple layers of concrete are required. Together with other considerations such as temperature, cement type, and mix proportions, adding retarding mixtures will prevent the formation of cold joints. A cold joint is a plane created when there is a delay in the application of a subsequent layer of concrete on the first layer. This happens when the first layer has begun to set or harden, and when the next layer is applied, the two batches fail to mix, forming a gap or space between the two layers.

When Long Transport Time is Involved

In some construction projects, concrete is not mixed onsite but instead need to be transported via trucks or dumpers. Adding retarding admixtures to the concrete mixture will extend the time between mixing and placing, especially if the construction site is far from where the concrete was mixed. Retarders also help prevent concrete from sticking or setting in the truck in case there is a delay in transport.

As explained above, retarding admixtures delay the hydration process in concrete, allowing more time before it hardens or becomes rigid. Retarding admixtures counters the effect of high temperature, eliminates cold joint formation, and enables farther hauling of concrete.

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