Chemical concrete admixtures
The chemical admixture is a component of concrete other than Portland cement, water and aggregate added to the mixture before or during mixing.
Producers use admixtures primarily to reduce the cost of concrete construction; to modify the properties of hardened concrete; to ensure the quality of concrete during mixing, transport, placement and solidification; and to overcome certain emergencies in specific operations.
Successful use of admixtures depends on the use of appropriate ingredients and concrete casting methods. Most admixtures are supplied in ready-to-use liquid form and added to the concrete in the factory or on the job site. Certain mixtures, such as pigments, bulking agents, and pumping aids, are used only in very small amounts and are typically batched by hand from pre-measured containers.
The effectiveness of the mixture depends on several factors, including: type and amount of cement, water content, mixing time, slump, and temperature of concrete and air.
Five functions of chemical admixtures
Admixtures are classified according to function. There are five different types of chemical admixtures: bleed air, water reduction, deceleration, acceleration and plasticizers (superplasticizers).
All other types of admixtures are of a special class and their functions include corrosion inhibition, reduced shrinkage, reduced alkali-silicon reactivity, enhanced processability, adhesion, moisture and coloration. Aerated mixtures for purposely placing tiny bubbles into concrete are discussed more fully in air entrained concrete.
1. Water reducing admixtures typically reduce the desired water content of the concrete mixture by about 5% to 10%. Therefore, concrete containing water reducing agent requires less water to achieve the desired slump than untreated concrete. The treated concrete can have a lower water to cement ratio. This usually indicates that higher strength concrete can be produced without increasing the amount of cement. Recent advances in mixing technology have led to the development of medium-range water reducing agents. These admixtures reduce the water content by at least 8% and tend to be more stable over a wider temperature range. Medium-sized water reducers provide a more consistent set-up time than standard water reducers.
2. A retarder that slows the solidification rate of the mixture to counteract the acceleration of the solidification of the concrete by hot weather. High temperatures generally result in increased hardening speeds, making placement and finishing difficult. The reducer keeps the concrete feasible during the placement process and delays the initial concrete. Most retarders also act as water reducers and may entrain some air in the concrete.
3. Accelerating admixtures increases the speed of early strength development, reduces the time required for proper curing and protection, and speeds up the start of finishing operations. Accelerating the mixture is especially useful for changing the properties of concrete in cold weather.
4. Superplasticizers, also known as plasticizers or superplasticizers (HRWR), can reduce water content by 12% to 30% and can be added to concrete with low to normal slump and water-cement ratio In order to produce high slump concrete. Flowing concrete is a highly fluid but feasible concrete that can be placed with little or no vibration or compaction. The effect of the superplasticizer lasts only 30 to 60 minutes, depending on the brand and dosage rate, and then the processability decreases rapidly. Superplasticizers are usually added to the concrete in the construction site due to slump loss.
5. Anti-corrosion admixtures are special admixtures used to slow the corrosion of steel bars in concrete. Corrosion inhibitors can be used as a defense strategy for concrete structures, such as marine facilities, road bridges and parking lots, which will be exposed to high concentrations of chloride. Other specialty admixtures include shrinkage blends and base-silicon reactive inhibitors. Shrinkage reducers are used to control dry shrinkage and minimize cracking, while ASR inhibitors control durability issues associated with alkali-silica reactivity.
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