Concrete Retarders are the mixture that slows down the chemical process of hydration so that the concrete remains plastic and workable for a long time.
Retarders are used to overcome the accelerating effect of high temperatures on establishing the properties of concrete in hot climates.
It is used for pouring consolidation castings without the formation of cold joints, also used in grouting oil wells.
Oil wells are sometimes taken up to a depth of about 6000 meters deep where the temperature may be about 200°C.
The annular spacing between the steel tube and well must be sealed with cement grout.
Sometimes at that depth stratified or porous rock strata may also require to be grouted to prevent the entry of gas or oil into some other strata.
For all these works cement grout is required to be in mobile condition for about 3 to 4 hours, even at that high temperature without getting set.
Use of concrete retarders is often used for such requirements, sometimes concrete may have to be placed in difficult conditions and delay may in transporting and placing.
In ready mixed concrete practices, concrete is manufactured in a central batching plant and transported over a long distance to the job sites which may take considerable time.
In the above cases the setting of concrete will have to be retarded, so that concrete when finally placed and compacted is in the perfect plastic state.
Types of Concrete Retarders:
Retarders are divided into 2 categories, depending on the nature of the retarders.
- Hydroxycarboxylic acids and their salts.
Inorganic or Chemical Retarders:
- Salts of Pb, Zn, Cu, As, Sb.
Effects of Retarding Admixtures on Properties of Concrete:
Initial compressive strength of concrete is less than uniform concrete’s compressive strength
2. Practicality and rational value:
Retarders have a small effect on the practicality of concrete; they cause initial deceleration to increase by 60–100 mm.
3. Slump Loss:
To reduce slump loss and increase initial functionality, retarding admixtures has been shown to be very effective.
Concrete Retarders is very efficient in decreasing slump loss and on this way rising the preliminary workability.
4. Air Entrainment:
Inhibitors don’t usually enter the air however there are some types of retarders.
In particular, depending on hydroxycarboxylic acids, these can actually reduce air content.
5. Freeze-thaw cycle:
Mixtures that enter the air are typically used to increase the freeze-melting resistance of concrete.6. Bleeding:
Since the setting process is delayed, the retarder components are always prone to bleeding.
7. The heat of Hydration:
Penetration does not reduce the heat production of concrete, although the peak temperature increases, causing the concrete to turn into concrete.
8. Volume Distortion:
Creep and drying shrinkage should not be affected by the addition of retarders; however plastic shrinkage may increase slightly.
If the concrete is properly cured, the dim concrete should be as durable as flat concrete.
Advantages of Concrete Retarders:
- Retarders extend the setting time of the concrete and maintain functionality and consistency for extended periods of time.
- In large construction projects, concrete retarder prevents the formation of cold joints in gradual lifts and make the concrete workable.
- With the use of concrete retarders, you can compensate for the accelerated impact of high temperatures towards the initial setting time and reduce the risks of long-distance delivery in hot weather.
- The use of retarder improves the permeability of concrete by improving the delayed setting period and the workability of the concrete.
- Concrete retarder help to reduce isolation and bleeding in concrete in which poor sand grading is unavoidable.
Disadvantages of Concrete Retarders:
- Concrete retarders may be used with water to avoid cracks and bleeding.
- Depending on its water-cement ratio, cement content, and the amount of C3a in the cement, the dose of retiring additives should be within the limit.
- Concrete retarder can undergo variable action on different types of cement when used in varying quantities.
Uses of Concrete Retarders:
Most commonly used in concrete in the summer season.
The retarders sprayed on the surface of the formwork, prevent hardening of the matrix at the interface of the concrete and formwork, while hardens the rest of the concrete.
Retarders are used for large or large scale construction work & gritting oil wells.
Frequently Asked Questions:
What is concrete retarder?
It is a chemical agent that slows down the chemical process.
How do retarders work in concrete?
concrete retarder is used to slow the rate of setting of concrete.
By slowing down the initial setting time, the concrete mixture can remain in a fresh mixing state for a long time before coming to its rigid form.
How much retarder is in a yard of concrete?
In liquid form, the volume of the retarder concrete is measured as the liquid ounce per hundreds of cement in the mixture.
Does retarder weaken concrete?
No, retarders are primarily intended to counter the harmful consequences of heat on concrete mixtures.
Concrete Retarder delayed hydration of cement.
A large number of pours and castings are consolidated without forming cold joints.