By Brian Miller
An admixture is defined as a material other than water, aggregates, hydraulic cementitious material or fiber reinforcement that is used as an ingredient of a cementitious mixture to modify its freshly mixed, setting or hardened properties and that is added to the batch before or during mixing. As noted in Part 1, a chemical admixture is usually further defined as a nonpozzolanic (does not require calcium hydroxide to react) admixture in the form of a liquid, suspension or water-soluble solid.
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 (see Table 1).
Water reducers do essentially that: reduce the amount of mixing water required to obtain a given slump. This can result in a reduction of the water-cementitious ratio (w/c ratio), which leads to increased strengths and more durable concrete.
Reducing the w/c ratio of concrete has been identified as the most important factor to making 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 flowability 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 (see 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. Also see Part 1 of this series for a table describing the effects of admixtures and other factors on air entrainment.
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 flocculation of the cement particles and allow for better dispersion. They also reduce the viscosity of the paste, resulting in a greater slump.
Table 4 presents some of the most common materials used for each range of water reducer. Other components are also added depending on the product and manufacturer. Some water-reducing admixtures have secondary effects or are combined with retarders or accelerators. This will be discussed later.
Effects on concrete
Water-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 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.
Water reducers are dosed by weight of cement, usually referenced as fluid ounces per hundred pounds of cement (fl. oz./cwt). Most low- and mid-range water-reducing admixtures are dosed up front with the mix water. High-range water-reducing admixtures are usually added at the job site prior to placement. Contact your admixture supplier for assistance on dosage and applications of water-reducing admixtures.
Set-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. Set-controlling admixtures include retarding and accelerating admixtures.
These admixtures 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 cases 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 two-component 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.
In the precast industry, retarding admixtures are also used as surface retarders. Surface retarders are used in making exposed aggregate architectural precast. They are typically sprayed or rolled onto the forms. The concrete is then cast into the forms. The next day, the retarded cement is water blasted or brushed off the concrete’s surface, producing an exposed aggregate finish.
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.
Sugar is commonly used as a retarder, but beware that overdosing will “kill” the mix. Research has shown that doses of 0.2 percent to 1.0 percent of the weight of cement can prevent it from setting. Other retarding chemistries include lignosulfonic acids and their salts, hydroxycarboxylic acids and their salts, phosphates and their organic phosphonate salts, and salts of amphoteric metals such as zinc, lead and tin.
These admixtures increase the cement’s rate of hydration. Specifically, they 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. Accelerators have been shown to depress the freezing point only by approximately 3 F.
How they work. The chemistry of accelerators is typically placed into three groups: soluble inorganic salts, soluble organic compounds and miscellaneous solid materials. Calcium chloride, a soluble inorganic salt, is one the most commonly used accelerators because it is effective and inexpensive. However, use caution – concrete containing reinforcing steel may induce chloride corrosion. ACI-318 sets limits for the maximum amount of water-soluble chloride ions in concrete. Calcium chloride should not be used with prestressing operations. Calcium chloride also can discolor concrete and should be avoided when aesthetics are a concern.
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, later-age 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.
Dosage. Set-controlling admixtures are dosed as fl. oz/cwt. Many are dosed up front in the mix water. However, the effectiveness can be increased when dosed later after mixing has commenced. Again, consult with your admixture supplier for best results.
Some admixture chemistries provide for a combination of effects such as water reduction with retardation or acceleration. Advantages of this include reducing the number of admixtures that have to be stored and added to the concrete; less admixture incompatibility; and cost savings. Disadvantages include less flexibility and limited use when an accelerating or retarding effect is not desired. ASTM C 494 lists specifications for these combination admixtures (see Table 1).
All precasters should be using a water-reducing admixture to reduce their water-cementitious ratio, ease placement of the concrete and increase early-age strengths. When appropriate, use a retarding or accelerating admixture to compensate for temperatures outside the realm of 45-65 F. NPCA’s Quality Control Manual for Precast Concrete Plants provides more information on good hot- and cold-weather concreting practices. This is especially useful for outdoor precasting operations.