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To obtain desired characteristics in concrete and to meet the requirements of the structures built today, several different types of admixtures are used:
Prior to concrete being put to use, it has to obtain a certain consistency. Plasticizers are chemical admixtures that decrease the water-cement ratio in concrete by around 5% to 12% and obtain the required consistency. Their addition to the mix increases the workability of the concrete and makes it easier to pour and place in accessible locations.
Super Plasticizers are also known as ‘high range water reducers’. They are an extremely effective type of chemical admixture, generally added to improve the flowing ability, strength and durability of concrete. It is a self compacting concrete that also achieves a reduction in water content without resulting in a loss of workability.
As their name suggests, Accelerators speed up the setting and hardening process of concrete. They increase the rate of hydration in hydraulic cement which then results in a quicker setting time and improved strength development. Most commonly, Accelerators are used in cold weather because they counteract the influence of cooler temperatures slowing down the curing and setting process.
Unlike Accelartors that speed up the setting and hardening process of concrete, Set Retarders do the opposite. Generally used when concrete has to be transported to long distances, set retarders slow down the setting and hardening process and prevent the concrete from setting too quickly. They prove to be extremely helpful when a delayed setting is required especially in hot weather.
Cementitious admixtures are most commonly used with ordinary portland cement or pozzolanic materials (explained below) to make highly durable concrete structures. A cementitious admixture has cementing properties that are typically used to improve the strength and durability of concrete. The ground granulated blast furnace slag is the best example of a cementitious admixture because it is highly cementitious in nature.
Pozzolans, commonly referred to as "cement extenders," are substances that exhibit cementitious properties when mixed with calcium hydroxide. The quality and workability of the concrete are improved as a result of the reaction between the components of concrete and pozzolans. A few examples of pozzolanic admixtures are fly ash, silica fume, rice husk ash and metakaolin.
The Ground Granulated Blast Furnace Slag (GGBF) is a byproduct of iron production. It is essentially a granular material formed when molten iron blast furnace slag is rapidly mixed with or immersed in water. Because of their superior durability and strength, GGBFs are typically used to make double concrete structures.
This is the byproduct of coal-fired power plants. Fly ash is essentially a fine residue generated as a result of the combustion of ground or powdered coal. This fine residue is captured from the chimneys of coal-fired power plants. When used as an admixture, fly ash lowers the concrete heat hydration and improves its workability and durability.
The admixture silica fume is the byproduct of the production of silicon metal and ferrosilicon alloys. It is an extremely reactive pozzolan, with its mere addition resulting in the concrete becoming highly durable and strong. Silica Fume reduces the permeability of concrete, thus increasing its resistance to the surrounding atmosphere and protecting steel from corrosion.
When risk husks are burnt Rice Husk Ash is produced. This byproduct of burning rice husks is used as a pozzolanic material that enhances the performance of self-compacting high-performance concrete. It also contains a high level of silica and improves the concrete characteristics of workability, impermeability, strength and resistance to corrosion.
A concrete admixture is a quick setting cement that can be used for several functions :
1) To improve the workability of the concrete without increasing or reducing the water content.
2) To accelerate its setting time.
3) To enhance the pumpability of the concrete mixture.
4) To enhance the bond between existing and new concrete
5) To reduce segregation and bleeding of the concrete
6) To decrease the rate of slump loss
7) To increase the bond of concrete to steel reinforcement
8) To decrease heat evolution and increase water tightness
The many different types of admixtures are designed to perform specific functions, such as improving workability, setting time, strength or the durability of the concrete. The type chosen affects how well the admixture will perform in concrete since each type comprises of different chemical and physical properties.
Since admixtures alter the properties of concrete, adding an excess or even an insufficient amount to the concrete can greatly affect its functionality. Most commonly, problems with segregation and bleeding can be caused when the dosage of the admixture is not at the optimum level.
Every type of cement will yield different results with different admixtures. The performance of the concrete is thus also determined by the compatibility between an admixture and cement. Any incompatibility between the two will have an impact on the concrete’s workability, segregation, durability and setting time.
The performance of the admixture also depends on the temperature of the surrounding environment. Any increase in the surrounding temperature would result in an increase in saturation dosage and impact the effectiveness of the concrete.
The different constituents of admixtures have an effect on its final performance. The addition of water, coarse aggregates, fine aggregates, cement, influence the performance, physical interaction and dispersion and efficiency of admixtures.
1) Do admixtures affect the strength of concrete ?
Yes, admixtures affect the strength of concrete. Through their water cement reduction, hydrophobic effect, pore reduction and blocking, and functionalities, the strength and durability of the concrete are considerably improved.
2) Why are admixtures used ?
Admixtures of both chemical and physical types can be used to improve the properties of the concrete, influence the slump of the concrete, influence the hydration and affect the acceleration and retardation time of the concrete amongst other benefits.
3) What determines the effectiveness of admixtures ?
The effectiveness of admixtures depends on several factors including type and amount of cement, water content, mixing time, slump and temperatures.
In conclusion, admixtures play a vital role in construction by modifying the properties of concrete to meet the specific needs of different construction projects. Whether it is to improve workability or increase strength, admixtures provide builders and engineers with greater control over the quality and durability of the structures they create. It is critical to make informed decisions when it comes to admixtures because the right admixtures can create structures that stand the test of time.
