In this article, we will deal exclusively with the design of plastic concrete mixtures and try to set forth some hints which might prove to be of value in developing the best mix design for the specific application. However, the article does not detail mix design procedures. The objective of a mix design is to secure concrete which will provide the desired plastic concrete properties (workability, finishability, controlled rate of set) and the desired hardened concrete performance (strength, watertightness, durability) at the lowest practicable cost. Specification for concrete customarily set forth categorize the mix design as one of three types: (1) mixture proportions designed and controlled by the owner- usually by large forms such as government agencies and corporations with extensive engineering departments; (2) minimum cement content, maximum 28 day compressive strength; and (3) concrete strength specified (usually 28 day strength) and desired slump. Before a concrete mix for a specific job is designed, the requirements of the concrete and the characteristics of the material with which the ready mix producer must work must be determined. The mix design will also be affected by such factors as use of vibration, placing and transporting equipment and other jobsite practices. The first mix design made in the office will virtually never prove acceptable without adjustments as a result of trial mixes before the actual project gets under way and without adjustments as the project progresses. Trial mixes are need to adjust the generalized mix design to the characteristics of the material being used. Adjustments during the course of the job are needed due to changes in the concrete making materials and mixing, transporting, placing, and finishing practices. Mix design of concrete is far form being an exact science in which the designer can sit at his desk, manipulate his slide rule, and come up with the right mix design. An intimate knowledge of the characteristics of the materials with which he must work, familiarity with a mix design method which works best for his need and the ability to make adjustments as the job progresses are all needed to produce concrete which begins and ends the job satisfactorily.