Concrete admixtures, also known as concrete chemicals, are substances added to concrete during or before its mixing to modify its characteristics, such as consistency, curing temperature range, setting time, or appearance. Unlike cement, aggregate, and water, admixtures are not essential components of concrete but can significantly enhance its properties.
One of the most common uses of concrete admixtures is to make cold-weather setting concrete. Calcium chloride is an example of an admixture that has been used for this purpose for a long time. By reducing the freezing point of water in concrete, calcium chloride enables concrete to set and harden even in cold temperatures, which would otherwise be detrimental to the concrete’s properties.
Admixtures can also be used to adjust the workability of concrete, which refers to its ease of mixing, placing, and compacting. For instance, water-reducing admixtures can increase the slump or flow of concrete without adding water, making it more manageable and reducing the amount of water required. On the other hand, retarding admixtures can delay the setting of concrete, allowing more time for placement and finishing.
Another application of concrete admixtures is to improve the durability and performance of concrete in harsh environments. For example, air-entraining admixtures can create tiny air bubbles in concrete, making it more resistant to freeze-thaw cycles, while corrosion inhibitors can protect steel reinforcement from corrosion caused by exposure to chemicals or saltwater.
In summary, concrete admixtures are materials added to concrete to enhance its properties and performance in various applications. They have been used in concrete construction for a long time and continue to be developed to meet new challenges and requirements.
Types of Concrete Chemicals (Admixtures) and Applications
Concrete admixtures can be classified into five major categories based on their functions. These categories include retarding admixtures, accelerating admixtures, superplasticizers, water reducing admixtures, and air-entraining admixtures. However, there are other important admixtures that do not fit into these categories but are still essential. These admixtures perform functions such as bonding, shrinkage reduction, damp proofing, and coloring.
Retarding admixtures are used to delay the setting of concrete. Accelerating admixtures, on the other hand, speed up the rate of concrete hardening. Superplasticizers are used to increase the workability of concrete without increasing the water content. Water reducing admixtures reduce the water content needed to produce concrete of a particular consistency. Air-entraining admixtures are used to incorporate tiny air bubbles in concrete to improve its durability in freezing and thawing conditions.
Apart from these categories, there are other important admixtures with specific functions. Bonding admixtures are used to improve the bond between existing and new concrete surfaces. Shrinkage-reducing admixtures help to reduce the amount of shrinkage in concrete. Damp proofing admixtures reduce the permeability of concrete to water and other liquids. Coloring admixtures are used to give concrete a particular color or to enhance its appearance.
1. Retarding Admixtures
Retarding admixtures are substances that can prolong the setting time of cement by slowing down its hydration process. They are particularly useful in hot weather conditions, where high temperatures and large amounts of concrete can accelerate setting time. Retarders that also act as water reducers are called water-reducing retarders and are classified as type D according to ASTM C 494. This type of admixture can result in concrete with greater compressive strength due to its lower water-cement ratio.
Retarding admixtures are composed of both organic and inorganic agents. Organic retardants consist of unrefined calcium, sodium, and NH4 salts of lignosulfonic acids, hydroxycarboxylic acids, and carbohydrates. Inorganic retardants include oxides of lead and zinc, phosphates, magnesium salts, fluorates, and borates.
Different types of retardants have varying effects on concrete properties. For instance, lignosulfonate and hydroxylated carboxylic acids can delay the initial setting time of concrete by at least an hour but no more than three hours when used at temperatures ranging from 65 to 100 degrees Fahrenheit.
Concrete contractors need not memorize these chemical-specific results. Instead, they can rely on the expertise of concrete suppliers to provide appropriate admixtures and concrete mixes based on the specific job requirements and goals.
2. Accelerating admixtures
Accelerators play a vital role in shortening the set time of concrete, which is especially useful when pouring concrete in cold weather. They can help to speed up the process of form removal, surface finishing, and even early load application. However, it is essential to exercise caution while selecting the type and proportion of accelerators, as commonly used accelerators can result in increased drying shrinkage of concrete.
Calcium chloride is a commonly used accelerator that can significantly reduce the time of set and increase the rate of strength gain in concrete. However, it is crucial to ensure that the calcium chloride used meets the standards set by ASTM D 98. The addition of excessive amounts of calcium chloride to a concrete mix can cause problems such as rapid stiffening, increased drying shrinkage, and even corrosion of the reinforcement.
In colder climates, calcium chloride should not be used as an anti-freeze. This is because a large amount of calcium chloride is required to lower the freezing point of the concrete, which can end up ruining the concrete. It is, therefore, crucial to be mindful of the potential consequences of using calcium chloride and to use it only in appropriate amounts and situations.
3. Superplasticizers
Superplasticizers, also known as plasticizers, are a type of admixture used in concrete that fall under the category of water-reducing admixtures. These admixtures are commonly used in the construction industry to enhance the performance of concrete. Compared to mid-range water reducers, superplasticizers are high-range water reducers that allow for greater flowability and larger water reduction without significantly affecting the setting time or increasing air entrainment.
Superplasticizers are available in different types, each with specific ranges of required quantities of concrete mix ingredients and corresponding effects. They are capable of maintaining a specific consistency and workability of the concrete at a significantly reduced amount of water. The dosage of superplasticizer required depends on the particular concrete mix and the type of superplasticizer used. Furthermore, the use of superplasticizers can produce high-strength concrete, but as with other admixtures, they can also affect other properties of the concrete.
To determine the specific effects of superplasticizers on concrete properties, one should consult the manufacturer or concrete supplier. It is essential to note that the effects of superplasticizers may vary depending on the type of superplasticizer and the specific concrete mix being used. Therefore, it is crucial to ensure that the correct type and dosage of superplasticizer are used to achieve the desired concrete properties.
4. Water reducing admixtures
Water reducing admixtures are additives used in the production of concrete that can help reduce the amount of water required to achieve a desired consistency, known as the slump. These admixtures can also increase the slump of the concrete while using the same amount of water. However, one side effect of using water reducers is that they may affect the initial set time of the concrete.
In hot weather conditions, water reducers are particularly useful for concrete placement and can aid in pumping. They are able to help maintain the desired consistency of the concrete without requiring excess water, which can be difficult to manage in hot weather. By using water reducers, concrete producers can reduce the amount of water needed for production while maintaining quality.
Another type of admixture, known as a water-reducer plasticizer, is a hygroscopic powder. This type of admixture is able to entrain air into the concrete mix by affecting the surface tension of water, which can provide some of the benefits of air-entrainment. Air-entrainment is a process where small air bubbles are introduced into the concrete to improve its durability, particularly in freezing and thawing conditions. Using a water-reducer plasticizer can provide similar benefits without the need for additional air-entrainment additives.
5. Air-entraining admixtures
Air-entraining agents are used in concrete to produce small air bubbles. The primary advantage of using these agents is that they improve the durability of the concrete in freeze-thaw cycles, which is particularly important in colder regions. Although adding air to the concrete can result in a reduction in strength, this can usually be mitigated by decreasing the water-cement ratio. This can be achieved through the use of better workability, which the air-entraining agent provides, or by incorporating other suitable admixtures. However, it is crucial to note that the use of admixtures in concrete should only be carried out by an expert, as some of them can have negative interactions when combined.
6. Bonding admixtures
Bonding admixtures are a type of compound or material that can be added to concrete to aid in the bonding of fresh concrete with old or set concrete. Examples of these admixtures include polyvinyl chlorides, acetates, acrylics, and butadiene-styrene co-polymers. These materials help to ensure that the new concrete adheres properly to the old, creating a stronger bond overall.
Coloring agents are another type of admixture that has become increasingly popular in recent years. These agents are often applied to the surface of concrete, particularly in the case of patios and walkways, and can also serve to harden the surface of the concrete. It’s important to note, however, that surface applied coloring admixtures should generally not be used on air-entrained concrete.
In addition to surface applied coloring admixtures, integrally colored concrete is also an option. This involves adding the coloring agent directly to the concrete mix itself, resulting in a uniform color throughout the entire structure. Overall, the use of admixtures can provide a number of benefits when it comes to concrete construction, including improved bonding and enhanced aesthetic appeal.
7. Waterproofing and damp proofing admixtures
Admixtures such as soaps, butyl stearate, mineral oil, and asphalt emulsions are commonly used to reduce the amount of water that penetrates the larger pores of concrete, thus providing waterproofing and damp proofing. On the other hand, “antifreeze” admixtures, which are often expensive, serve as accelerators that enable a very rapid set-time. However, they do not possess the properties to protect against freezing independently. These antifreeze admixtures are not typically used for residential projects.