ACI 212.3 reminds us that chemical admixtures are designed to enhance the properties of concrete in the plastic and hardened states, increase efficiency of cementitious material, and improve the economy of concrete mixtures. While concrete contractors are not required to have the same level of understanding of admixtures as concrete producers, a refresher in some of the basic types of admixtures and their benefits should simplify the contractor's job.
It is important to remember admixtures don't include secondary cementitious materials—such as silica fume or other pozzolans—though they can bring similar benefits to concrete. The primary function of silica fume and other pozzolans is not their abilities to function as admixtures. And, for the purpose of this article, pigments for coloring concretes have been omitted, even though ACI defines them as admixtures. The following is a list of the major types of chemical admixtures and a brief description of their use and possible compositions.
Air-entraining admixtures are used to introduce microscopic air bubbles in concrete to improve the durability of concrete exposed to freeze/thaw cycles. According to ACI 212.3, there are two main types of air-entraining admixtures, which must meet the requirements of ASTM C 260. Water-soluble air-entraining admixtures are formulated from a variety of different organic substances, including among other things wood resin salts, synthetic detergents, and salts of petroleum acids. Solid materials with high internal porosity and a suitable pore size—such as hollow plastic spheres, crushed brick, expanded clay or shale, and diatomaceous earth spheres—can be added and seem to act like air voids.
Accelerating admixtures increase concrete's rate of hydration and strength development at an early stage. Calcium chloride is the cheapest and most common of the soluble inorganic salts, though bromides, fluorides, carbonates, and other salts also will work. Soluble organic compounds—such as triethanolamine and calcium formate—offset the retarding effects of water-reducing admixtures and provide noncorrosive acceleration. Quick-setting admixtures made from materials—such as tricalcium aluminate, ferric salts, and sodium fluoride—promote flash setting for shotcrete and seal leaks against hydrostatic pressure. Miscellaneous solid admixtures—such as calcium-aluminate cements, finely divided silica gels, soluble quaternary ammonium silicates, even silica fume—can accelerate strength gain or hydration.
Water-reducing and set-controlling admixtures have wide-ranging effects in fresh and hardened concrete and represent the largest offering of available products. Water reduction improves strength and can improve finishing characteristics. These admixtures also can lower hydration temperatures, delay setting, and reduce shrinkage. As shown in the table, if a company manufactures a water-reducing admixture, it more than likely produces a set-controlling admixture and a combination of the two.
Admixtures for flowing concrete are an outgrowth of the expanding role of water-reducing and set-controlling admixtures in concrete construction. High-range water reducers (superplasticizers) produce high-slump concrete that maintains cohesiveness without segregation, excessive bleeding, or abnormal retardation. They are uniquely suited for heavily reinforced applications that require no vibration. Superplasticizers often consist of sulfonated naphthalene or melamine condensates, though polycarboxylates are a new class, introduced in the past decade, that is expanding the use of self-consolidating concrete. Viscosity modifying admixtures also are commonly used in flowing concrete and self-consolidating concrete.
Other miscellaneous admixtures are available for a variety of applications.
Gas-forming admixtures help maintain concrete's initial volume, counteracting settlement and bleeding, by generating or liberating bubbles in the mix. At higher volumes, these admixtures—generally consisting of hydrogen peroxide, aluminum powder, or activated carbon—can be used to make lightweight concrete.
Grouting admixtures include many of the admixtures in concrete to impart the corresponding properties to grouts.
Extended set-control admixtures, including carboxylic acids and phosphorus-containing organic acids and salts, are used to stop or severely retard the cement hydration process in fresh concrete. They differ from conventional set-controlling admixtures because they stop the hydration process of both the silicate and aluminate phases of portland cement.
Bonding admixtures—usually some kind of organic polymer or latex—are used to create the higher tensile strength and greater durability of polymer-modified concrete, though their use also results in a loss of compressive strength.
Pumping aids for concrete are admixtures that help in concrete placement, whether they are lubricants and fine fillers for overcoming friction and segregation or thickeners for increasing the concrete's cohesiveness. They frequently include natural and synthetic organic polymers, organic flocculants, emulsions, high-surface-area inorganic materials, and finely divided inorganic materials.
Flocculating admixtures, though not mentioned on the table, are often synthetic polyelectrolytes, and increase bleeding rate, reduce flow, and increase early strength.
Fungicidal, germicidal, and insecticidal admixtures commonly include polyhalogenated phenols, dieldrin emulsion, and copper compounds.
Permeability-reducing admixtures are used to make the concrete hydrophobic. Consisting of soaps, butyl stearate, and certain petroleum products-they delay the penetration of moisture into concrete.
Pozzolans have been documented to mitigate alkali-aggregate reaction, though other soluble salts of barium and lithium and other admixtures work as well.
Corrosion-inhibiting admixtures are a class of chemicals that help delay or control the deterioration of concrete reinforcing steel. Many chemicals have been evaluated for this purpose, including chromates, phosphates, hypophosphorites, alkalis, nitrites, and fluorites.
Antiwashout admixtures increase the cohesiveness of concrete that is pumped under water. Based on natural or synthetic gums and cellulose-based thickeners, these admixtures can reduce the loss of cementitious materials due to washout by as much as 50%.
Freeze-resistant admixtures are used to lower the freezing point of concrete, permitting placement and curing of concrete in very cold conditions. Nonchloride set accelerators are often the basic ingredient, though at much higher doses than in above-freezing temperatures.