介绍
Concrete Admixtures
Concrete admixtures are materials in the form of powder or fluids that are added to the concrete to give it certain characteristics which is not obtainable with plain concrete mixes. Admixtures are added to concrete batch immediately before or during mixing to modify the properties of the wet mix or the hardened concrete. Depending on their functions, concrete admixtures can be classed under the following categories:
• Workability Admixtures
Admixtures that improve the workability, texture and pourability of the concrete mix without altering water requirements are known as workability admixtures. Such admixtures reduce the viscosity of the concrete mix which in turn makes the concrete easier to handle, spread and mold. Ductilizing agents and fluid crack preventers are examples of widely used workability admixtures.
• Accelerators
Accelerators are special kinds of admixtures that are used to speed up the process of hardening and setting of concrete. This category includes calcium chloride and other chlorides. Such admixtures are especially useful when frost or weather conditions are threatening to delay the curing process.
• Retarders
Retarders are quite the opposite of accelerators. These admixtures slow down the process of hardening and setting, which gives the finishers enough time to ensure that all the required adjustments are made to the concrete mix.
• Air Entraining
Such admixtures are to produce concrete that has improved workability and finish. The presence of air entrapped between the particles of the concrete mix provides better pliability with reduced surface cracking and segregation. Such admixtures also reduce the plastic shrinkage and protect against the harmful effects of freeze-thaw cycles.
• Corrosion Inhibitors
Corrosion inhibitors are used to protect the steel reinforcement from corrosion. Such admixtures are mostly used in structures above or near ground level.
• Waterproofing Admixtures
Waterproofing admixtures are designed to reduce the permeability of the concrete, thereby making it resistant to water absorption and less prone to environmental damage.
• Shrinkage Reducing Admixtures
Shrinkage reducing admixtures are used to minimize the volume change and drying shrinkage cracks. Such admixtures usually contain plasticizers and superplasticizers that reduce particle segregation and densification of the mix when it begins to harden.
• Bonding Admixtures
Bonding admixtures should be used for improving the bond between original concrete and repair materials or between separate concrete members. Such admixtures form a thin film on the aggregate and concrete surfaces which halts the curing process long enough to ensure a strong bond between the different surfaces.
Concrete admixtures are extremely useful in improving the workability and end-use performance of the concrete. Such admixtures are also preferred for facilitating on-site preparation and reducing job time and labor costs. Furthermore, admixtures have the potential to reduce the cost of concrete by replacing traditional admixtures. Overall, it can be safely said that concrete admixtures are an integral part of modern concrete production and will continue to play an important role in the production of concrete in future.
