The materials used to make concrete are portland cement, water, fine aggregate and coarse aggregate. Each sack of portland cement contains 94 lb. of cement and is equal to 1 cubic foot in volume. Portland cement should be free of all lumps when used. Water should be clean and free of oil, acid and alkali. In general, water that is fit to drink is suitable for making concrete. Fine aggregates consists of sand or other suitable fine material. A good concrete sand will contain particles varying uniformly in size from very fine up to one-fourth an inch. Coarse aggregate consists of gravel, crushed stone or other suitable materials larger than one-fourth an inch. Coarse aggregates that are sound, hard and durable are best suited for making concrete. Aggregates may be tested for quality. The silt test is used to detect the presence of too much extremely fine material. In making the silt test, an ordinary quart milk bottle or quart canning jar is used. Fill the container to a depth of 2 inches with a representative sample of dry sand to be tested. Add water and shake vigorously. Allow the jar to sand for an hour, during which time any silt present will be deposited in a layer above the sand. If this layer is more than one-eighths an inch thick, the sand is not satisfactory. All concrete should be thoroughly mixed until it is uniform in appearance and all materials are uniformly distributed. Mixers should not be loaded above their rated capacity and should be operated at approximately the speeds for which they are designed. The mixing period should be measured form the time all solid materials are in the mixer drum, provided all of the water is added before one-fourth the mixing time has elapsed.