Concrete Admixtures

Admixtures are ingredients other than water, aggregate and Portland cement that are added to a concrete mixture immediately prior to or during mixing. These ingredients can be used to modify the properties of concrete by affecting its workability and set time, compensating for the effects of temperature during curing, ensuring the quality of the concrete during mixing, transport and placing, and even for adding pigment.  

The use of concrete admixtures is fairly standard for both residential and commercial builds. Concrete admixtures are used to improve the behavior of concrete under a variety of conditions. They can reduce the cost of construction, modify properties of hardened concrete, ensure quality of concrete during mixing/transporting/placing/curing, and overcome certain emergencies during concrete operations. They are used to improve the quality of concrete during mixing, transporting, placement and curing.

Concrete admixtures are classified by their functions and the properties they lend to concrete. They include:retarding,accelerating,water-reducing,air-entraining,superplasticizers.

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.

· 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.

· 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.

· 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.

· Superplasticizers 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 superplasticizers fit into one of three families:

· sulfonated melamine formaldehyde condensates (SMF);

· sulfonated naphthalene formaldehyde condensates (SNF);

· polycarboxylate derivatives(PCE).

Concrete admixtures can be useful for addressing problems related to concrete set times, and can improve the quality of concrete when used correctly by experts. Understanding the functions and purposes of different admixtures is helpful when considering options for a new build, and Jiankai Admixtures who know more about the benefits and uses of admixtures may be better able to answer their clients’ questions.