Admixtures and the Contractor
A great deal has been said about the advantages concrete admixtures hold for concrete producers, but relatively little has been said regarding their value to the contractor. And they do have value to the contractor since he is the one who benefits most from their use. High quality admixtures can help the contractor improve his concrete work by making concrete easier to place and finish and by helping the concrete meet the designer's strength and durability requirements. The contractor, to achieve required strength, watertightness, and durability characteristics in the hardened concrete was often forced to work with a stiff, difficult to place mix. Today, however, it is possible to control concrete so that it meets the contractor's needs both in the plastic and hardened state. A number of things have rendered this possible. One of these is the development of water reducing admixtures. A water reducing admixture can be used to achieve consistent high strengths without altering slump by reducing water requirement and thereby lowering the water/cement ratio. One way to increase early strength is through the use of an accelerator, the most common of which is calcium chloride. Calcium chloride can also accelerate the rate of set. This, depending on job conditions, can either be an advantage or a drawback. Calcium chloride, however, should not be used if there is a possibility of alkali-aggregate reactivity. Neither should chloride be employed in prestressed concrete because of the danger of corrosion. There is a need for concrete with a retarded rate of hardening during hot weather. Initial set retarders are now available which provide the desired degree of set retardation but also provide a rapid rate of strength gain. These retarders are especially valuable where extensive areas, intricate forms and /or dense reinforcement make placing operations time consuming. The most commonly used retarders characteristically act as water reducing agents, which helps account for the good rate of strength gain.