Water-reducing and set-retarding admixtures are chemicals that slow down the initial reaction between cement and water by reducing the rate of water penetration to the cement and slowing down the growth of the hydration products.
As a result, concrete stays workable longer than it would otherwise. The length of time during which a concrete remains workable depends on its temperature, consistence class, and water-cement ratio, and on the amount of retarder used.
Retarded concrete needs careful proportioning to minimize bleeding due to the longer period during which the concrete remains fresh. It permits placement of large volumes of concrete over extended periods, minimizing the need for forming, placing, and joining separate units.
Effects on Fresh Concrete
- Decrease water required by at least 5% for the same slump concrete.
- Reduce rate of slump loss.
- Extend working time
- Improve finishing characteristics of concrete. This is specifically advantageous where deficient aggregate properties or gradation make concrete finishing difficulties.
- Delay in setting time of concrete. The delay in setting time increases with the increase of dosages and lower temperatures.
- It has been reported to contribute to premature stiffening.
Effects on Hardened Concrete
- Strength increase due to reduction of w/cm ratio.
- The increase in flexural strength is not the same as compressive strength.
- Long-term shrinkage may be less than that of concrete without admixture.
- Creep is reduced in proportion to the increase in the strength of the concrete.
Dosage
According to ACI 212.3R, the dosage of the admixture should be determined from information provided by one or more of the following sources:
- Results from construction works where the admixture employed under good field control, preferably using the same materials and under conditions similar to those expected.
- Laboratory tests carried out to evaluate the admixture.
- Information and technical literature from the manufacturer.
Applications
- It is used mainly to offset the accelerating effect of hot weather and to keep concrete workable during the entire placing period. This is crucial for the prevention of cracking of concrete beams, bridge decks, or composite construction caused by form deflections.
- Utilized to keep concrete workable until succeeding lifts can be placed without development of cold joints in the structural unit.
- This type of admixture is used when the complexity of slip-forming demands a slow rate of rise.