Concrete is probably the most widely used construction material in most parts of the world. The power of concrete draws from its ability to set and cure quickly, take on different forms and shapes, be pumped to high vertical distances, and bond with rebar to create a powerful structural material (reinforced concrete) excellent in both tension and compression.
Due to various applications of concrete, different properties such as workability, durability and final cure strength may be desired. For example, concrete that is to be used in heated silos is required to not only be waterproof, but durable under high temperatures. A practical example of admixture use in large-scale construction projects is in the Burj Khalifa, the tallest building in the world. High-performance concrete was used with an added slurry plasticizer, due to the concrete being pumped to such great vertical distances that it would have inevitably set, bled, or segregated before placement. In the case of the Burj Khalifa, it would have been impractical, slow, and expensive to place the concrete using a crane.
An admixture is simply an additive that improves the behavior of concrete under certain conditions. They can be used to improve the quality of concrete during mixing, transportation, placement, or curing. These can be chemical admixtures, such as superplasticizers, or mineral admixtures that contain fly ash, pozzolana, hydraulic or volcanic ash and silica fumes.
1. Water Proofing Admixtures
Waterproofing is one of the most important functions of admixtures, particularly for the construction of water retaining structures such as swimming pools and water tanks. Waterproofing is also important for concrete structures that have either a part of or their whole structure immersed in or exposed to water, for example, basements and lift pits.
2. Retarding Admixtures
For contractors delivering ready-mix concrete over long distances or in hot climates, retardants may be added to the concrete to delay its setting. This may be crucial where it is not possible to have a concrete batching plant on site.
3. Accelerating Admixtures and Air-Entraining Admixtures
Concrete that is placed in very cold climates may take too long to set. While not particularly a major issue in Africa, contractors in European countries have to be aware of accelerating admixtures since the quality and performance of their concrete projects depend on it. Likewise, air-entraining admixtures reduce the likelihood of degradation due to freeze-thaw action in cold climates.
4. Plasticizers and Super-Plasticizers
These reduce the possibility of concrete segregating or bleeding, and are important where concrete is being pumped to significant vertical distances, or if it is being placed or poured from a distance.
5. Water-Reducing Plasticizers
Depending on the concrete mix design performed, the addition of water beyond a certain limit reduces the strength and durability of concrete. Less water is always better, although workability is traded. Water-reducing plasticizers reduce the amount of water required for workability to produce a higher strength class of concrete. In regions where fresh, non-saline water for mixing is difficult or expensive to come by, water-reducing plasticizers may be used.
Do I Need Concrete Admixtures for a Regular Building Project?
This wholly depends on what, where, and why you are building. For simple projects especially in Kenya, the use of admixtures is fairly reduced, apart from the construction of water tanks and swimming pools. Their use is more pervasive in commercial and large-scale projects where top-notch properties of concrete are desirable.
