As concrete buildings go ever higher and concrete spans become longer and longer, the use of lightweight aggregate structural concrete is growing. Lightweight concrete, as far as this article is concerned, is a concrete having a 28 day compressive strength is excess of 2,000 psi, and an air dry unit weight less than 115 pounds per cubic foot. The problems most frequently encountered in designing a lightweight concrete mix are maintaining workability while minimizing unit water content, and establishing availability of mix water for hydration. High absorption rates and rough texture prohibit the accurate determination of the bulk specific gravity of lightweight aggregates. This problem is solved by application of a "specific gravity factor" which takes into consideration the total water requirement, including that amount needed to satisfy aggregate absorption characteristics. This specific gravity factor is established by means of trial mixes. Another mix design method is the volumetric method. The volumetric method is probably the one most often used when dealing with familiar materials. After the initial design has been derived using the known volumes of moisture free aggregates, trial mixes are prepared to determine the amount of water to be added to secure the desired slump. When a satisfactory design has been diveloped, the known moisture free volumes of aggregates are converted to batch weights form previous unit weight determinations made on the separate aggregate sizes. Batch weights are then adjusted in th efield for any moisture contained in the aggregates at th etime of batching. Although the trial mixes require some intuitive feeling on the part of the mix designer, coupled with characteristic data that can be provided by the aggregate producer, several general principles should be kept in mind. If fine and coarse aggregate are considered separtely, the total lightweight aggregate for one cubic yard of concrete will usually be form 30 to 32 cubic feet on a dry, loose basis. Total waer requirement for the particular aggregate is best obtained form the aggregate supplier. It can range from 350 pounds per cubic yard for a well-shaped, coated type to 500 pounds or more for one that is poorly shaped and highly absorptive. Air entrainmentis highly desirablein in lightweight concrete and will usually be at least 3 percnet with up to 8 percent commonly used.