Specifications include, but are not limited to: Cement concrete shall be composed of specified proportions by weight of cement, aggregates, water and approved additives to form a homogeneous composition. Cement concrete shall be designated by class according to strength, cement factor, coarse aggregate size, entrained air content, slump and by weight for lightweight concrete. The classes of concrete to be used shall be designated on the plans or in the specifications for the particular work. The Contractor will furnish to the Engineer, for approval, a specific job mix formula for the particular uniform combination of materials and sources of supply to be used on each project complete with test results from trial batches. A new job mix formula shall be supplied anytime any source of material has been changed.