Concrete Admixtures
The use of concrete admixtures is fairly standard for both residential and commercial builds. While admixtures provide benefits and solve problems when properly utilized, they may also contribute to defects if applied improperly. For example, if an accelerating admixture is used incorrectly or contains too much calcium chloride, there is evidence that drying shrinkage can be increased and lead to cracking.
While it may be difficult to determine whether the improper use of an admixture is the culprit behind a defect found during an inspection, knowledge about the different types of admixtures and their functions can be beneficial for inspectors who seek a greater understanding of the complex and dynamic elements of the concrete materials of a building’s components.
Types of Admixtures
Concrete admixtures are classified by their functions and the properties they lend to concrete. They include:
- retarding;
- accelerating;
- water-reducing;
- air-entraining; and
- super-plasticizers.
Each of these types of admixtures is used to achieve different effects and solve specific problems. There are several other common admixtures used for purposes that do not fit within these categories, such as bonding and waterproofing mixtures.
- Retarding admixtures are used to increase the set time of concrete by slowing hydration. This is especially useful for counteracting the effects of high temperatures, which cause concrete to harden at a faster rate. Since most retarders also act as water reducers, they are often called water-reducing retarders.
This type of admixture works by forming a film around the compounds, which slows their reaction with water. The thickness of the film determines how much the rate of hydration is retarded and, by extension, how long the concrete will take to set. Both organic and inorganic agents may be used in retarding admixtures. Unrefined calcium, sodium, NH4, carbohydrates, salts of lignosulfonic acids, and hydrocarboxylic acids are all examples of organic retardants. Inorganic retardants include zinc and lead oxides, phosphates, magnesium salts and borates.
- Accelerating admixtures work in the opposite manner from retarding admixtures, decreasing the set time of concrete. This helps counteract the effects of cold weather, which causes hardening at a slower rate. The use of these admixtures may also allow early removal of forms and early surface finishing. In certain cases, they may also allow for early load application.
Chemical compositions of accelerators may include organic compounds, such as triethanolamine, as well as inorganic compounds, such as soluble chlorides, carbonates, silicates and fluorosilicates. Calcium chloride is the most commonly used accelerator for concrete, but chloride-free accelerators are becoming more popular due to the problems associated with calcium chloride, which can include corrosion of steel reinforcement, rapid stiffening, and an increase in drying shrinkage.
- Water-reducing admixtures can be used to increase set time in hot weather, similar to retarding admixtures, but their main role is to allow for a more fluid mix with a lower water content. Commonly, 5% to 10% less water may be used in the mixture when water-reducing admixtures are utilized.
Active ingredients in water-reducing admixtures fall into three categories:
- salts and modifications of hydroxylzed carboxylic acids (HC type);
- salts and modifications of lignosulfonic acids (lignins); and
- polymeric materials (PS type).
In addition to improving workability by increasing the fluidity, water-reducing admixtures also increase the strength of concrete, as well as its freeze-thaw resistance.
- Air-entraining admixtures are another additive used to enhance concrete’s durability in relation to freeze-thaw cycles, which is especially important in colder climates. These admixtures disperse small air bubbles in the mix. The bubbles then become part of the matrix that binds the aggregate together in the hardened concrete. Air-entrained concrete also has the added benefit of being more workable, and the use of air-entraining admixtures reduces bleeding and segregation of fresh concrete, as well.
The chemicals most commonly used for air-entrainment fall into four categories:
- synthetic detergents;
- salts of petroleum acids;
- salts of wood resins; and
- fatty resinous acids and their salts.
- Super-plasticizers are also known as high-range water reducers. These admixtures can be used to reduce the water content of concrete by up to 30%, creating a highly fluid and workable mix. This flowing mix is used in heavily reinforced structures and placements in which adequate consolidation by vibration cannot otherwise be easily achieved.
Most commercial formulations of super-plasticizers fit into one of four families:
- sulfonated melamine-formaldehyde condensates (SMF);
- sulfonated naphthalene-formaldehyde condensates (SNF);
- modified lignosulfonates (MLS); and
- polycarboxylate derivatives.
- Other Types of Admixtures
There are several types of admixtures that fall outside the realm of the main five categories above.
- Bonding admixtures are sometimes used to facilitate bonding new or fresh concrete with old or set concrete. These may include the addition of compounds such as acrylics, polyvinyl chlorides, acetates and butadiene-styrene co-polymers.
- Coloring agents, either integral or surface-applied, are another form of admixture that is commonly used. Some types have the additional effect of accelerated surface hardening, which can be advantageous.
- Waterproofing and damp-proofing admixtures include soaps, butyl stearate, mineral oil and asphalt emulsions. These additives can help reduce water penetration into the larger pores of the concrete.
- “Anti-freeze” admixtures have a very high amount of an accelerating agent that produce an extremely fast set time. These are generally used in commercial applications.
Concrete mix can be compromised if directions are not followed explicitly. It is often necessary to add supplemental materials in order to balance out any negative side effects that the use of an admixture may have on the finished concrete. An in-depth knowledge of the interrelated effects of different admixtures is generally required in order to utilize them successfully and avoid compromising the finished concrete. This makes choosing the appropriate admixture the responsibility of experts.
The process or finished product may be weakened further if several parties are involved at different stages of the manufacture. The final quality of the finished concrete can be the result of individual decisions on the part of the producer, placing contractor and the builder. This makes communication an important factor.
Some admixtures may have a questionable impact on the environment. Super-plasticizers, for example, may pollute groundwater and surface waters. Ongoing research is being conducted to determine how different admixtures impact the environment.
