Concrete Pixels

Construction nowadays uses different types of “cosmetics” in producing concrete inorder to achieve concrete properties that the situation demands. These cosmetics that I am refering to are commonly called admixtures. Admixtures are actually materials other than cement or aggregate or water that are added during the production of concrete i.e. before or after mixing. Though the practice of adding admixtures to concrete is now a common practice and accepted by standards such as ASTM C 494 or ASTM C 1017 , the chemistry of admixture remains a complex process that requires indepth knowledge & experience depending on the application. the most common types of admixtures are:
Superplasticizers or commonly called as plasticizers. These type of admixtures are also high range water reducers that allow large water reduction or greater flowability without substantially slowing set time or increasing air entrainment. Their water reducing effect increase the strength due to the lower water cement ratio.
Retarders as the name implies are admixtures that slow down the hydration of cement thereby allowing for a more longer setting time. They are beneficial for hot weather concreting to overcome the effect of temperature to quickly set the concrete. Like plasticizers, retarders are also water reducers thus increasing the strength of concrete.
Accelerators are admixtures that has the opposite effect of retarders i.e they shorten the setting time of concrete, thereby beneficial for cold weather concreting, early removal of forms, early surface finishing, and in some cases, early load application. Proper care must be taken while choosing the type and proportion of accelerators as they may cause an increase in the drying shrinkage of concrete.
Air-entraining admixtures entrain small air bubbles in the concrete. This is commonly used in cold areas. The major benefit of this is enhanced durability in freeze-thaw cycles. Designing such air-entrained concrete is quite a challenge but the real challenge based on experience is on the stability of the air during production. Although some strength loss maybe there due to the presence of air in concrete, it generally can be overcome by reducing the water-cement ratio or through the use of other appropriate admixtures.
Bonding admixtures are those that improves bonding of new / fresh concrete with old / set concrete. They contain materials such as polyvinyl chlorides and acetates, acrylics and butadiene-styrene co-polymers.
Coloring admixtures are commonly used in patios and walkways. In most cases it is applied at the surface and has an additional effect of surface hardening. Such surface applied coloring admixtures generally should not be used on air-entrained concrete.
Waterproofing and damp proofing admixtures are commonly use to decrease the amount of water penetration into the larger pores of concrete.
Choosing an appropriate admixture for a specific application should be done with extreme care. An experienced expert in this field to evaluate the situation and the recommendations by the supplier is a must. Alternatives to the use of admixtures should always be explored whenever possible. In addition, the environmental impact of certain admixtures must always be taken into account. Moreover, admixtures should only be combined in a concrete mix by a competent professional because some of them can interact in undesirable ways. Finally, always remember that admixtures cannot compensate for bad practice and low quality materials.