9.2
Retarders
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Q1: What problem do retarders solve in hot weather concrete construction?
In hot weather, higher temperatures shorten concrete's normal setting time, causing cold joints between successive batches. Retarders extend the setting time, keeping initially poured concrete plastic longer so it bonds cohesively with the next freshly poured layer, preventing weak joints and ensuring structural integrity.
Q2: How do retarders work to delay concrete setting?
Retarders are believed to alter crystal growth in concrete, creating a more effective obstacle to hydration. Substances like sugar, carbohydrate derivatives, soluble zinc salts, and borates interfere with the hydration process. Later, the retarder becomes part of the structure without changing the final hydration products.
Q3: How much can sugar extend concrete setting time?
Adding approximately 0.05 percent sugar relative to cement's mass can extend concrete's setting time by roughly four hours. This small proportion demonstrates the potency of retarders in delaying the setting process, making them effective for managing concrete placement in challenging conditions.
Q4: How is the effectiveness of retarders measured?
The delaying effect of retarders on concrete setting can be quantified using the penetration test. This standardized test measures how long concrete remains workable, allowing engineers to verify that retarders are functioning as intended and providing adequate time for successive concrete pours.
Q5: What are the benefits of retarders for exposed aggregate finishes?
Retarders can enhance the architectural appearance of exposed aggregate finishes by keeping concrete plastic longer during placement. This extended workability allows better cohesion between layers, achieving desired architectural effects while maintaining structural quality and aesthetic appeal.
Q6: Do retarders affect the final strength of concrete?
Although retarders initially lower strength development during the extended plastic stage, they do not change the final composition of hydration products. Ultimate strength in retarded concrete can be higher than in non-retarded concrete, and retarders do not affect drying shrinkage.
Q7: What is plastic shrinkage and how do retarders influence it?
Plastic shrinkage occurs during concrete's plastic stage before hardening. Retarders increase plastic shrinkage due to the extended plastic period they create, though this does not affect drying shrinkage that occurs after concrete hardens, making it an important consideration in retarder selection.
