Concrete, in common with all known materials, deforms somewhat when it is subjected to a load. The visible sagging of a beam or slab, especially under conditions of extreme loading, is perhaps the simplest example, but whether they deform visibly or not, all structural members regardless of composition undergo some deformation which takes place immediately under load is called elastic deformation. This deformation would disappear at once if the load were immediately removed. Creep, however, is the gradual yielding under load, as contrasted with the immediate yielding which we call elastic deformation. It should not be confused with another phenomenon called plastic flow which results form overstressing and is generally considered to be a type of incipient failure. Where creep is at least partly recoverable and occurs at very low stress, plastic flow is nonrecoverable an occurs only at very high stress. Possibly the most acceptable explanation of creep involves two separate approaches: the viscous flow theory, which embraces the notion that when concrete is under load the cement paste flows gradually in transferring stress to the aggregate; and the seepage theory which endeavors to explain at least a part of creep as being due to the loss of water from the cement gel. The viscous flow theory seems to explain adequately the fact that the rate of creep reduces steadily with time. As stresses are transferred from the cement, which has marked ability to flow, and the aggregate which is relatively inert, it is reasonable to expect that the rate of creep would gradually decline as equilibrium is approached.