Ingredients added to concrete, other than Portland cement, aggregates, or water, are referred to as admixtures. While not required, they are sometimes used selectively to improve the concrete mix.
Admixtures are used in concrete to alter its properties in various ways. Some common uses include improving workability, increasing or decreasing cure time, and increasing concrete strength. Admixtures can also be used for aesthetic reasons, such as to change the color of the cement.
Whenever possible, concrete workability, water tightness, and strength should be achieved by selecting suitable types of aggregate, Portland cement, and maintaining a good water-cement ratio.
When this is not possible or special circumstances exist, such as freezing weather, hot temperatures, increase wear, or prolonged exposure to deicing salts or other chemicals, admixtures can be helpful.
Types of Admixtures Used in Concrete
The types of admixtures available today vary greatly in their functionality and purpose. Here’s a list of some of the more common types:
Admixtures can be added either before or during mixing. Because they change the properties of concrete, they should be used sparingly and only on the advice of a concrete specialist.
Air-entraining admixtures are used to entrain microscopic air bubbles in concrete. This improves the durability of the concrete, increasing its resistance to damage from freeze-thaw cycles as well as from deicers, which cause scaling.
In addition, air-entrainment provides improved workability during placement and has superior water tightness. It also requires less water per cubic yard than non-air-entrained concrete, therefore improving the water-cement ratio.
The active ingredients used in air-entraining admixtures include polyethylene oxide polymers, some fats and oils, sulfonated compounds, and detergents. Air-entraining admixtures are specified by ASTM C226.
Water-reducing admixtures reduce the amount of water needed to produce concrete of a given consistency. They can also be used to improve the slump of concrete without requiring additional water, resulting in a lower water-cement ratio.
By lowering the water-cement ratio, water-reducing admixtures can provide greater concrete strength while retaining the same workability. Lignin sulfonic acids and metallic salts are commonly used as water-reducing admixtures.
Superplasticizers are organic compounds that transform a stiff concrete mix into one that flows more freely. They work by coating cement particles, causing them to break away from the lumps that typically result from mixing cement and water.
Superplasticizers give each cement particle a negative charge, which causes them to repel each other providing a more thorough distribution. They are used either to facilitate placement of concrete or to reduce the water content of a concrete mix in order to increase its strength.
Accelerating admixtures, or accelerators for short, make concrete cure more quickly. In addition, they accelerate the strength development of concrete. Strength development can be further accelerated by using Type III high-early-strength Portland cement.
By curing concrete at higher temperatures or lowering the water-cement ratio, additional acceleration can be achieved. Accelerators are typically used in cold weather to develop strength faster in order to offset potential freeze damage.
Retarding admixtures are essentially the opposite of accelerators. They increase the amount of time it takes concrete to cure, in other words, they slow down the setting time of the cement paste. Retarding admixtures are typically used in hot weather.
By using retarders in hot weather, less water is required to cure the concrete mix. Without a retarder, more water would be needed to get the desired slump, which in turn would produce lower strength concrete.
A better water-cement ratio and increased ultimate strength can be achieved by using retarding admixtures.
Fly ash is a fine powder that is a waste product from coal-fired power plants. It can be used as an admixture to increase concrete strength, decrease permeability, and increase sulfate resistance.
Other advantages of fly ash are that it can help reduce the rise in temperature during curing and reduce mixing water. Additionally, it can improve the pumpability and workability of the concrete.
Silica fume, or microsilica, is a powder derived from electronic semiconductor chip manufacturing. It is many times finer than Portland cement and consists primarily of silicon dioxide.
When added to a concrete mix, it can produce concrete of extremely high strength that also has very low permeability. This makes it a good choice for areas of high moisture or regions with high water tables.
Blast Furnace Slag
Blast furnace slag is derived from the manufacturing of iron. As an aggregate, it can be used to improve workability and increase the strength of concrete.
Additional benefits of blast furnace slag include reduced permeability, reduced rise in temperature during curing, and improved sulfate resistance.
Pozzolans are a siliceous or combination aluminous and siliceous material that, when finely ground and combined with water, will react chemically with calcium hydroxide to form compounds possessing cementitious properties.
The advantages of using pozzolans in a concrete mix include reduced cost since less Portland cement is required, reduced environmental pollution because less production of Portland cement is needed, and increased durability.
Workability agents, as their name implies, are used to improve workability in a concrete mix. This is particularly helpful in situations where fresh concrete is harsh because of improper aggregate grading or incorrect mix proportions.
In situations where concrete is to have a troweled finish, workability is important, and therefore the use of workability agents can be beneficial.
Corrosion inhibitors are used primarily to reduce the rusting of reinforcement steel in a concrete mix. This can be particularly useful in structures that are in close proximity to roads or that form part of the road structure.
When deicing salts or other corrosion causing chemicals are used in roads during winter seasons, they can accelerate the rusting of reinforcement steel in structures. Corrosion inhibitors can be beneficial when mixed into concrete structures under these conditions.
Often times, there is a need to pour fresh concrete onto existing concrete. Because the existing concrete has already fully set, a strong bond between the new and existing concrete is difficult to achieve.
Bonding admixtures are used to increase the bond between fresh and set concrete. It is usually added into the new Portland cement mix, or it can also be applied to the top of the old concrete.
Hardening Admixtures are used in situations where a concrete surface is going to be subjected to heavy wear. An example might include a factory or warehouse floor. By using either a liquid or dry-powder hardener, the life of the floor can be extended.
Liquid hardeners work by producing a chemical reaction with the free lime and calcium carbonates in the Portland cement. Dry-powder hardeners can be applied as a dry shake to freshly poured concrete.
Often times, drawing specifications call for the use of colored concrete. Coloring admixtures can provide color to a concrete mix in various hues, though usually limited to earth tones and pastels.
Concrete can be colored by mixing pure, finely ground mineral oxides with Portland cement. When added to the mix, thorough mixing is required in order to create a uniform look. Mixing time may also be longer than normal.
Gas-forming agents can be added to concrete to cause a slight expansion in it before it hardens. A commonly used gas-forming admixture is aluminum powder.
It reacts with the hydroxides in hydrating cement and produces small hydrogen gas bubbles. This also helps eliminate any voids caused by settlement of the concrete.
Percentage of Admixture in Concrete
The percentage of admixture used in concrete varies depending on the type of admixture and the content of the cement. Typically the dosage is indicated by the manufacturer and is based on the cement content.
Generally speaking, admixtures make up a fairly small percentage of the concrete mix, with values of 5 percent or less being common. It is usually best to create a test batch prior to mixing the concrete or create mockups to determine the most effective ratio.
Disadvantages of Admixtures in Concrete
While admixtures have many benefits, as has been summarized so far, there are certain drawbacks to using them. For one, they tend to increase cost. They also alter the composition of concrete and should, therefore, be used sparingly.
Admixtures are not always easy to work with, at least until greater familiarity is gained with their use. For this reason, it’s usually a good idea to work with a concrete specialist who knows the product well.
Admixtures are used in concrete to enhance the performance of the mix in various ways. Generally added before or during the mixing process, admixtures can increase the strength the mix, accelerate or slow down the curing process among other benefits.
While workability, strength, and water tightness can be achieved by the use of adequate aggregate, Portland cement and by maintaining a good water-cement ratio, admixtures can often provide an added advantage. This is particularly true in extreme climates, or when the concrete is exposed to increased wear.
New types of admixtures are being experimented with today. Some provide environmental benefits and can help reduce pollution by reducing the need for Portland cement production. Other types of aggregates are being experimented with to allow concrete to self-heal or absorb pollution.
As these aggregates begin to gain more acceptance, they are likely to play a larger role in the construction of buildings, roads, and other types of concrete structures in the future.