Concrete retarders are admixtures used to slow down or retard the hydration of fresh concrete. This delays the setting time and early strength gain of the concrete. Concrete retarder admixtures are used where setting times need to be increase such as where there is a long period between batching and pouring due to long delivery distances or during hot weather concreting for example.
Concrete Retarder Admixtures
Used in a similar but opposite fashion to accelerators, concrete retarder admixtures are used to delay the setting and early strength of the concrete. Some concrete retarder admixtures work by forming a thin coating around the cement grains which delays or slows down the hydration reactions. They do not have a significant effect on the strength of the concrete after it has set.
Hot Weather Use of Concrete Retarder Admixtures
Hot weather can accelerate the rate of hydration which decreases the time available for finishing and saw cutting the concrete pavement following placement. Concrete retarder admixtures are often used when concrete pavements are installed in particularly hot weather to delay the set and counteract the effects of the hot weather on the finishing and saw cutting windows.
Generally concrete retarder admixtures are only used when the air temperature is expected to exceed 30°C, which does not happen regularly for more than a few days in the UK but is a common occurrence in many parts of the world. However in places where temperatures are often greater than 30°C other methods have been developed to avoid the use of retarders such as placing concrete at night or simply having greater labour available to finish and saw cut the concrete in a shorter time.
Concrete Retarder Admixtures used for Long Haulage Distances
Concrete retarder admixtures are also occasionally used when the haulage distances for a project are particularly long or where there are other reasons to delay the setting time of the concrete.
Care needs to be taken with the dosage, consistency and programming of the placement works to ensure that the concrete sets in the correct order. If the concrete sets in an unpredictable sequence there can be problems with differential setting of the concrete. The addition of concrete retarder admixtures can also increase the air content of the concrete, reduce the early concrete strength for up to 3 days and increase bleeding.