Here as just a few words which are frequently used incorrectly in concrete construction parlance. The most frequent confusion is between contraction, expansion, and isolation joints. When improperly used, there is confusion regarding the function of the joint in question. Contraction and expansion joints control cracking resulting form volume changes. Isolation joints are placed between component parts of concrete structures and appurtenances that are expected to experience differential movement due to unlike function, mass or shape. Another mix up occurs when people are talking about air entraining verses air entrained. Materials used to entrain air are properly referred to as air entraining. If you are speaking of the grout or concrete in which air has been entrained, the proper term to apply is air entrained. Another are of difficulty is between poured versus placed concrete. In reality this is not a matter of incorrectness but more a matter of preferred usage. Despite the fact that much concrete actually is "poured" in place, it is agreed by all authorities that concrete should be "placed." Self-leveling concrete should be a thing of the past in view of our present knowledge of its many disadvantages. Another source of confusion is the difference between admixture, additive and addition. Admixtures are materials added to concrete at the time that its other ingredients (cement, aggregates and water) are mixed. Additives and additions are materials used in the manufacture of portland cement. Prestressed concrete is now a commonplace on the construction scene, but there is still a great deal of confusion regarding the use of the words prestressed and post-tensioned. "Prestressing" describes the entire concept of introducing compressive forces in concrete to counteract the tensile stresses to be expected when a load is applied on the member. "Post-tensioning," together with "pretensioning," are terms which indicate the time at which the prestressing steel is stressed in reference to the placing and hardening of concrete in the member.