Ready mix producers have the know-how, equipment and materials to produce concrete that is as specified, batch after batch. You get concrete capable of delivering the performance characteristics ordered. It all boils down to the question, "What is needed from the concrete?" To determine the concrete performance needed for the placing, and structural and exposure conditions of the job, several factors must be considered. Is the concrete for a sidewalk or a heavy-duty industrial floor? Will it be protected from weathering, as are foundations and interior columns, or will it be exposed to the weather, like driveways and exterior walls? All these factors, and more, have entered into the decision by the specifications writer in determining the qualities needed for the job in question. Usually four specified factors in a ready mix order govern the performance of the concrete: (1) compressive strength; (2) maximum size coarse aggregate; (3) slump; and (4) air content. The strength concrete will need to develop hinges mainly on (1) required structural capabilities- what stresses it must resist; and (2) the environmental conditions to which it will be exposed. The first of these considerations is usually best determined by a structural engineer. Exceptions to this might be home driveways, sidewalks, patios and the like, but in these cases it is the exposure which often determines the needed strength. The maximum size coarse aggregate you order should be determined by such factors as (1) spacing of reinforcing steel; (2) inside form dimensions; and (3) available aggregate. In general, maximum aggregate size should not exceed one-fifth the narrowest inside form dimension and should not exceed three-fourths the minimum clear spacing between reinforcing bars. Slump should be specified as low as is consistent with efficient placing and thorough consolidation. This will vary with such matters reinforcement concentration, configurations of formwork, slope of surface on which it is to be placed, and availability of vibrators.