Adding a retarder to concrete is a common practice in modern construction and concrete engineering. A retarder is a type of chemical admixture that slows down the setting process of concrete, which in turn affects many fresh and hardened concrete properties. Retarders are especially useful when ambient conditions, project logistics, or desired finishes demand extra working time.
ARIT's ART-CS retarder used in concrete is an example of this type of admixture. It uses a modified starch technology to prolong the initial setting time without significantly affecting the difference between initial and final set. The product is formulated to reduce slump loss, meaning concrete stays workable longer, while maintaining early strength development. It is especially useful in high-fluidity, long-workability scenarios, such as pumped concrete, slipform construction, large-volume pours, layered casting, long transport distances, or during hot-weather construction.
When water is mixed with cement, certain compounds in the cement such as tricalcium silicate (C3S) and tricalcium aluminate (C3A) react quickly, initiating the hydration process. This leads to stiffening and later hardening of the concrete. When a retarder is added, it interferes with one or more of the mechanisms in this early hydration, often by forming a barrier, delaying nucleation of hydration products, or complexing with calcium ions, or even precipitating insoluble compounds around cement grains. [1]
This chemical interference gives the concrete an extended dormant period or plastic state during which it remains workable, meaning it can be mixed, transported, placed, vibrated, and finished more easily before initial setting begins. [2]
By slowing setting time, retarder in concrete mix provides several beneficial effects. First, workability is extended—slump retention is improved, so concrete remains fluid enough for longer, which is useful when the pour is large, or transportation takes time.
Second, retarders help reduce the risk of cold joints in concrete pours. Cold joints occur when a previous layer has already begun setting before the next layer is placed; retroactive bonding and uniformity are compromised. With retarder, successive pours can be more cohesive.
Third, retarders can improve the surface finish. When concrete sets too quickly, finishing operations become rushed, which can lead to surface defects, cracking, or uneven texture. A retarder gives workers more time for these tasks, leading to smoother, more uniform finishes.
However, adding a retarder also has trade-offs. Early strength development is delayed; i.e., concrete won't gain its initial compressive strength as quickly as untreated concrete. But with correct mix design and proper curing, the long-term or final strength is often comparable. [3] Also, extended setting may lead to increased bleeding or potential for plastic shrinkage, if curing and moisture control are inadequate.
In hot weather: High ambient temperatures accelerate hydration, causing concrete to stiffen too fast. Retarders help to slow down that process so the mix remains workable.
For long transportation or delays: When a ready-mixed concrete batch needs to travel far or there are delays at the site, a retarder ensures the mix doesn't set prematurely in the truck or pump. [4]
In large or complex pours, such as massive slabs, walls, or architectural elements with detailed formwork, where finishing takes time.
For special finishes or decorative concrete: to allow more time for treatments, stamping, exposed aggregate, or surface texture work.
To reduce thermal stress in mass concrete: because the hydration heat is released more gradually, reducing peak temperatures and associated cracking.
In summary, adding a retarder like ARIT's ART-CS to concrete essentially slows the hydration reactions that cause setting, extending the workable phase of concrete. This gives more time to mix, transport, place, finish, and cure effectively. The result is improved surface finish, fewer cold joints, better handling in hot weather or complex pours, but with slower early strength gain and tighter requirements for dosing, curing, and quality control. When used properly, retarders are a powerful tool in the concrete engineer's toolbox.
Reference:
[1] https://theconstructor.org/concrete/concrete-retarders-mechanism-types/26633/
[2] https://www.efca.info/admixtures/set-retarding/
[3] https://civinnovate.com/2024/11/26/retarders-in-concrete/
[4] https://www.hpdconsult.com/what-is-concrete-retarder/
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