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The demand for ready-mix concrete (RMC) is increasing day by day in the big cities across the country. The primary reasons behind this are convenience of using RMC in high-rise structures, shortage of space at construction site, saving of time related to the preparation of concrete on-site, and better quality of RMC compared to the conventional concrete. Nevertheless, because of heavy traffic congestions in the cities, especially during weekdays, the time required to haul RMC from plant to construction site is very high. Therefore, keeping concrete workable for such a long time without compromising the required strength has become one of the most challenging tasks in the RMC industry. Moreover, high ambient temperature in summer makes the situation worse, since high temperature adversely affects the workability of fresh concrete. While in winters it takes time to set and cure the concrete. Selection of appropriate type and dosage of chemical admixture can play an important role in this regard.

Superplasticizers

The main purpose of using superplasticizers is to produce flowing concrete with a high slump in the range of seven to nine inches to be used in heavily reinforced structures and in placements where adequate consolidation by vibration cannot be readily achieved. The other major application is the production of high-strength concrete at w/c's ranging from 0.3 to 0.4.

Superplasticizers, also known as plasticizers or high-range water reducers (HRWR), reduce water content by 12 to 30 percent and can be added to concrete with a low-to-normal slump and water-cement ratio to make high-slump flowing concrete. Flowing concrete is a highly fluid but workable concrete that can be placed with little or no vibration or compaction. The effect of superplasticizers lasts only 30 to 60 minutes, depending on the brand and dosage rate, and is followed by a rapid loss in workability. As a result of the slump loss, superplasticizers are usually added to concrete at the jobsite.

Composed of more advanced, higher-value chemicals than standard and mid-range water reducers, superplasticizers decrease the water-cement ratio while providing other benefits as: increased density, improved bond strength, greater volume stability and reduced shrinkage cracking, increased abrasion resistance, decreased permeability, reduced segregation and bleeding, and higher early and ultimate strength.

Superplasticizers are used for the production of flowing concrete, self-compacting concrete, or self-leveling concrete and for the production of high strength and high-performance concrete.

Accelerator

Curing concrete during cold weather can result in an inferior product with substandard properties. Curing also takes much longer, adding to job costs and extending the time before the concrete surface can be used.

Chemical admixture called ‘accelerator’ are used to increase the rate of concrete strength development or to reduce concrete setting time. Accelerating admixtures increase the rate of early strength development, reduce the time required for proper curing and protection, and speed up the start of finishing operations. Accelerating admixtures are especially useful for modifying the properties of concrete in cold weather.

Although there are many types of accelerators, calcium chloride continues to be one of the most preferred. No other accelerator has been used so successfully for so long.

Using Calcium Chloride in concrete produces an accelerated rate of hydration in cement. It warms the concrete mixture and allows workability of placement in cold weather. By using Calcium Chloride in ready mix, it develops high early strength. Three day strength is seen in one day and seven day strength is seen in three days. Calcium Chloride allows the ready mix to maintain its workability. Calcium Chloride use decreases water usage. Increased strength is achieved at lower material and labour costs with Calcium Chloride. For quality, economy and reliability, Calcium Chloride is the best choice for cold weather ready mix.

In case of ready mix concrete, calcium chloride should be added at the plant site if the concrete is to be discharged within one hour after the start of mixing. Otherwise, it should be added at the job site, mixing for at least three minutes or 30 revolutions of the mixer, whichever is longer.

In addition to the important contributions of cold weather protection and early strength of concrete, calcium chloride provides other benefits as well: It improves workability - regardless of mixture design, less water is required to produce a given slump when calcium chloride is used. Improves strength of air-entrained concrete - calcium chloride compensates for the reduction in strength with a higher cement factor concrete. Reduces bleeding - this is due to the early stiffening produced by acceleration and allows earlier final finishing. These advantages combine to produce better quality concrete faster. Concrete acceleration with calcium chloride greatly facilitates completing jobs as quickly and economically as possible.

Retarders

In some cases more time is needed to place concrete because of difficult placing condition or delay may occur in transportation. In case of ready mix concrete, concrete is manufactured in the central batching plant and transported over a long distance and often to the work sites which may take significant time depending on the mode of transportation. In that case, if the concrete is desired in a plastic state for perfect final placing and compaction, then the setting time of concrete will have to be increased or simply delayed by retarding its hydration process.

Retarding admixtures are added in the concrete to delay the setting time of the concrete. When retarders are used in the concrete, it temporarily slows down the chemical reaction (hydration). Hence the concrete remains in plastic stage and remains workable for a longer time. The retarding admixtures form a film around the cement compounds (e.g., by absorption). This film prevents or slows the reaction with water. How much the rate of hydration will be retarded is governed by thickness of this film. When this film breaks down, then the normal hydration starts.

The factors influencing the degree of retardation are water-cement ratio, cement content, C3A and alkali contents present in the cement, type and dosage of the concrete retarder, and the stage at which the retarder is added to the concrete mix itself.

Retarding admixtures, which slow the setting rate of concrete, are also used to counteract the accelerating effect of hot weather on concrete setting. High temperatures often cause an increased rate of hardening which makes placing and finishing difficult. Retarders keep concrete workable during placement and delay the initial set of concrete. Most retarders also function as water reducers and may entrain some air in concrete.