High Performance Concrete Admixtures for Improving the Properties of Concrete
Admixtures are the ingredients in concrete which are other than the hydraulic cementitious material, water, aggregates or fiber reinforcement that are used as ingredients of a cementitious mixture to modify its freshly mixed, setting or hardened properties and that are added to the batch before or during mixing. Admixtures are usually further defined as a non–pozzolanic (does not require calcium hydroxide to react) admixture in the form of a liquid, suspension or water-soluble solid. Some admixtures have been in use for a very long time, such as calcium chloride to provide a cold-weather setting concrete. Others are more recent and represent an area of expanding possibilities for increased performance. Not all admixtures are economical to employ on a particular project.
Water-reducing admixtures improve concrete’s plastic (wet) and hardened properties, while set-controlling admixtures are used in concrete being placed and finished in other than optimum temperatures. Both, when used appropriately, contribute to good concreting practices. Also, both admixtures should meet the requirements of ASTM C 494, (Table 1).
Reducing the w/c ratio of concrete has been identified as the most important factor to make durable, high-quality concrete. On the other hand, sometimes the cement content may be lowered while maintaining the original w/c ratio to reduce costs or the heat of hydration for mass concrete pours.
Water-reducing admixtures also reduce segregation and improve the flow ability of the concrete. Therefore, they are commonly used for concrete pumping applications as well.
Water-reducing admixtures typically fall into three groups: low-, medium- and high-range. These groups are based on the range of water reduction for the admixture. The percent of water reduction is relative to the original mix water required to obtain a given slump (Table 2). While all water reducers have similarities, each has an appropriate application for which it is best suited. Table 3 presents a summary of the three types of water-reducing admixtures, their ranges of water reduction and their primary uses. Their effect on air entrainment will vary depending on the chemistry.
How They Work?When cement comes in contact with water, dissimilar electrical charges at the surface of the cement particles attract one another, which results in flocculation or grouping of the particles. A good portion of the water is absorbed in this process, thereby leading to a cohesive mix and reduced slump.
Water-reducing admixtures essentially neutralize surface charges on solid particles and cause all surfaces to carry like charges. Since particles with like charges repel each other, they reduce locculation of the cement particles and allow for better dispersion. They also reduce the viscosity of the paste, resulting in a greater slump.
Effects on ConcreteWater-reducing admixtures are primarily used to reduce the water-cementitious content of concrete, thus increasing strength. In some cases, they can be used to increase the workability or slump of the concrete providing for easier placement. Mid-range water-reducing admixtures were developed to increase the slump beyond the range available with regular water reducers without the excessive retardation that had been known to occur. High-range water reducers, commonly called superplasticizers, were developed for high-strength and high performance concrete applications.
Superplasticizers, e.g., Multiplast Super can take a 3- inch slump concrete to a 9-inch slump without risk of segregation and without compromising its strength. Many precasters can benefit from the use of a superplasticizer, especially because of its improved high early strength development.
All water-reducing admixtures increase strength development as a result of better dispersion of the cement. This increases the exposed surface area of the cement particles, allowing for more complete hydration of the cement.
Set-Controlling AdmixturesSet-controlling admixtures alter the rate of the cement’s hydration and, therefore, the rate of setting (stiffening) of the paste. Coincidentally, they also may affect the hardening or strength gain after the paste has set. Setcontrolling admixtures include retarding and accelerating admixtures.
Retarding AdmixturesThese admixtures, Multiplast R slow down the hydration process. They may also reduce the setting time of cement. Retarding admixtures fall into two categories: regular and extended-set. Regular, most commonly referred to as just “retarders,” are used to place concrete in hot climates when long travel times are expected or, in case of emergency, when placement is delayed. They are also commonly used for mass concrete pours to prevent cold joints.
Extended-set control admixtures are those used to delay hydration for many hours or even days. These are usually a twocomponent admixture system. The first component is a retarder (stabilizer) which delays the setting of concrete. The second component is an accelerator (activator) which overcomes the retarder. The concrete typically reaches initial set in a few hours after the activator is applied.
How they work Retarders essentially slow early hydration by reducing the rate at which tricalcium silicate (C3S) reacts with water. Furthermore, retarders slow the growth of calcium hydroxide crystals. Both reactions develop the early setting and strength gain characteristics of paste. The effect remains until the admixture is incorporated into the hydrated material, thereby removing it from the solution and allowing for initial set to occur. The duration of retardation is based on the dose and chemistry of the retarder, cement composition, temperature and the time it was added to the mix.
AcceleratorsThese admixtures increase the cement’s rate of hydration. Multiplast ACC are designed to increase the rate of hydration of C3S, thereby increasing early strength. There are two types of accelerators: rapid and normal.
Rapid accelerators can set concrete in minutes and are used in shotcreting applications, to make repairs against hydrostatic pressure or when very rapid setting is required. These are typically not used in precast concrete applications.
Standard or normal accelerators are used to speed up construction in cold-weather concreting conditions; however, it is important to note that they are not antifreezing admixtures.
Effect on concrete: Both retarders and accelerators seem to have negligible effects on air entrainment. However, when water-reducing agents are included, such as lignosulfonates, some air may be entrained.
Retarders tend to reduce one-day strengths and usually increase later-age strengths . Retarders may also increase slump loss and cause an early stiffening of the mixture, even though the strength gain has been delayed. Retarders tend to lose their effectiveness as concrete temperature increases. They also tend to increase the plastic shrinkage.
Accelerators typically increase early strengths. However, laterage strengths may be reduced relative to the same concrete without the accelerator. They also tend to increase early-age shrinkage and creep rates, but tests have shown that ultimate values seem to be unaffected.