Name: retarders in concrete pouring
Uploaded: 2024-11-07T14:48:38.000Z
Duration: 1 min 19 s
Channel: JoVE

In hot weather concreting, when the normal setting time is shortened by the higher temperature, it can lead to the formation of cold joints between successive batches.
When retarders are used in concrete, the initially poured concrete remains plastic for longer to ensure cohesivity with the next batch of freshly poured concrete layer.
Cohesion between the layers can be advantageous for achieving a desired architectural finish with exposed aggregate.
Retarders are admixtures extending concrete&#39;s setting time; this delay can be quantified using the penetration test.
The delaying of concrete setting, or retardation, can be achieved by incorporating substances like sugar, carbohydrate derivatives, soluble zinc salts, and soluble borates, among others.
Adding around 0.05 percent sugar relative to the cement&#39;s mass can extend the setting time by roughly four hours.
Retarders are believed to alter the crystal growth in concrete, creating a more effective obstacle to continued hydration.
Later, the retarder becomes part of the structure without altering the final hydration products.

retarders

Retarders

Retarders are chemical admixtures designed to extend the setting time, which is especially useful when there is a delay in sequential concrete pours to prevent cold joints and to achieve a cohesive structure. Retarders, when used in appropriate amounts, can also enhance the architectural appearance of exposed aggregate finishes.
The function of retarders is to delay the setting of concrete, and this effect can be measured using a penetration test. The retardation process involves adding substances such as sugar, carbohydrate derivatives, soluble zinc salts, and borates to the concrete mix. A small proportion, like 0.05 percent of sugar by mass of cement, can prolong the setting time by approximately four hours. It is hypothesized that retarders interfere with the crystal growth within the concrete, thereby creating a more formidable barrier to the process of hydration. Although they alter the initial rate of strength development, retarders do not change the final composition or identity of the hydration products. The integration of retarders can result in initial lower strength, but it does not affect the ultimate strength achieved, which can be higher than in non-retarded concrete. Additionally, retarders have been observed to increase plastic shrinkage due to the extended plastic stage, but they do not affect drying shrinkage.

In hot weather concreting, when the normal setting time is shortened by the higher temperature, it can lead to the formation of cold joints between ...

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Admixtures

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retarders in concrete pouring

Retarders are chemical admixtures used in concrete to slow the setting time. They are especially helpful when one pour must follow another after a delay. By keeping the concrete workable for longer, they can help prevent cold joints and support a more cohesive structure.

Retarders can also improve the look of exposed aggregate finishes when they are used in the right amount. Their effect on setting time can be checked with a penetration test. Common retarding substances include sugar, carbohydrate derivatives, soluble zinc salts, and borates.

Even a small amount can make a difference. For example, sugar at 0.05 percent by mass of cement can extend setting by about four hours. Retarders are thought to work by interfering with crystal growth in the concrete, which creates a stronger barrier to hydration.

Retarders change the early rate of strength development, but they do not change the final composition or identity of the hydration products. Concrete may have lower strength at first, yet its ultimate strength is not affected and may even be higher than in concrete without retarders. They can also increase plastic shrinkage because the plastic stage lasts longer, but they do not affect drying shrinkage.

Retarders in Concrete Pouring

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Calcium chloride is a concrete admixture that speeds up hardening and helps concrete gain early strength faster. It does not always change the setting time itself, but it can reduce the time needed in practice. This makes it useful in cold weather and in rapid repair work where quick heat generation after mixing is helpful. A common amount is 1 to 2 percent of the cement mass. Its effect can be compared with raising the concrete temperature by 11 degrees Fahrenheit. To spread it evenly through...

Retarders are chemical admixtures used in concrete to slow the setting time. They are especially helpful when one pour must follow another after a delay. By keeping the concrete workable for longer, they can help prevent cold joints and support a more cohesive structure. Retarders can also improve the look of exposed aggregate finishes when they are used in the right amount. Their effect on setting time can be checked with a penetration test. Common retarding substances include sugar,...

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key terms and concepts

Learn how concrete retarders delay setting, help prevent cold joints, and affect strength, shrinkage, and exposed aggregate finishes in construction.

Retarders in Concrete Pouring | JoVE K12

JoVE Core Civil Engineering offers a foundational exploration of essential construction materials such as bricks, stones, steel, wood, and concrete, along with fluid mechanics topics like flow over immersed bodies, open channel flow, and dimensional modeling. It also covers key surveying concepts, including leveling, traverse adjustments, curve design, topographic mapping, and geospatial technologies (GPS & GIS). Science-in-action videos showcase laboratory research on materials, fluids, and surveying practices in real-world engineering systems.

civil engineering

Retarders in Concrete Pouring

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