Federal or state paving projects impose more restrictions and require more rigid inspections than are normally encountered on other types of construction. Before letting contracts, some states may wish their engineers to inspect and approve the ready mix plant facilities to determine fitness for maintaining the control of quality as well as capacity for producing the quantity required for daily operation. They prefer that batching equipment and mixers be used only for the specified paving job and that all materials be kept separate from those being used for other projects- first, because it is important to prevent delay in the even flow of concrete to the job and second, because of the danger of mix ups in the batches. Most states also have specifications limiting the rate of mixing and the rate of agitation according to the size of truck mixers and agitations. They also may specific a minimum number of revolutions of the drum. In order to insure the correct amount of mixing, most states further required that truck mixers be equipped with devices for counting or for verifying the number of revolutions of the drum. In the opinion of some engineers any failure to control the quality of truck mixed concrete has been caused by either inadequate control of water content, failure to maintain uniform size, grading and moisture content of the aggregates or the adverse effects of certain sequences of loading. Control of materials, therefore, remains important.