SK Singh, Principal Scientist, Dharmendra Singh, and Chanchal Sonkar, Trainee Scientist, CSIR- Central Building Research Institute, Roorkee, India.

Introduction

Concrete, nowadays is not just mixing of cement, aggregate & water but it also comprises of chemical and mineral admixtures. It is becoming a more and more effective construction material as a result of addition of admixtures and improvements in production techniques. The chemical admixtures, in particular are frequently used to enhance the characteristics of both fresh and hardened concrete such as to extend or accelerate setting characteristics, entrain air, reduce water content, increase cohesiveness, enhance flow, introduce self leveling properties, improve durability and enhance strength parameters. Therefore, the role of chemical admixture in concrete is becoming important each year. It is often emphasized that the new admixtures play a more important role in concrete than new cement [1]. The recent development and advancement in chemical admixtures particularly organic based advanced super plasticizers are playing important role in production of a variety of innovative concretes such as ultra high strength concrete, high performance and self leveling concrete [2-8]. Chemical admixtures are usually used to reduce the cost of concrete construction by modifying plastic & hardened properties to ensure the quality of concrete during mixing, transporting, placing, and curing. However, sometimes, the cost of admixtures is comparable to that of the cement in high-performance concrete due to its high dosage. Admixtures are, however, not almighty. There are also problems associated with it including the production of stiffer concrete, variation in initial slump and slump loss using some types of cement, large variations in the flow characteristics using combination of different admixtures etc. Therefore, it is important to ensure the compatibility of admixtures with cement and additives before its application in concrete.

Chemical admixtures are inorganic or organic materials other than Portland cement, water, and aggregate and added to the mix immediately before or during mixing. These are added into the mix not normally exceeding 5% by mass of cement or cementitious materials. Admixtures interact with hydrating cement by physical, chemical or physic-chemical actions. The history of admixtures is as old as the history of concrete. First reported chapter on chemical admixture is published in ASTM STP 169 B in 1966 [9]. However, specifically developed water reducers or high range water reducers in Japan and Germany in late 1960s created a great change in the development of admixtures & attracted the concrete producers due to its great efficiency. This paper presents a state of the art report on chemical admixtures by addressing to limited depth due to length restrictions of paper.

NBMCW December 2013