=Concrete Materials.=--The coarse aggregate for the concrete may be broken stone or pebbles screened from natural gravel. Durability is necessary, but it is also important to have uniformity in the concrete so that the road surface will wear uniformly and consequently keep smooth. Supplies of broken stone are likely to contain a small percentage of soft pieces and such of these as are at the surface when the concrete is finished will crush under traffic, leaving a pit in the surface. Pebbles screened from gravel are also likely to be variable in durability and should be carefully inspected if they are to be used as aggregate for concrete roads. The harder limestones, some sandstones, pebbles from many of the gravel deposits and practically all of the igneous rocks make satisfactory aggregates for the concrete road. Sometimes none of the coarse aggregates readily available are sufficiently durable or uniform for the wearing surface of the concrete road, but a suitable aggregate may be obtained at relatively high price by shipping considerable distances. In such cases what is known as the two course type of concrete road is employed. The wearing course usually is about 2 inches thick and is constructed with selected aggregates of good quality shipped in for the purpose. The lower course is constructed of aggregates which do not possess the desired qualities for a wearing course, but which are satisfactory for concrete not subjected to abrasion. The aggregates for the wearing course will be selected with the same regard for uniformity and durability that would be the case if they were for the one course pavement. Bank run gravel, or run of the crusher stone, is generally not sufficiently uniform as regards proportion of fine and coarse material to produce uniformity in the concrete, and the use of aggregates of that character is not permissible for the wearing course, but under proper inspection they may be used for the lower course of two course pavements. =Fine Aggregate.=--The fine aggregate is generally natural sand, but a mixture of natural sand and stone screenings is sometimes employed. The fine aggregate of whatever character must be clean, free from organic matter and sand, must contain no appreciable amount of mica, feldspar, alkali, shale or similar deleterious substances and not exceed two and one-half per cent of clay and silt. The sand is of such a range of sizes that all will pass the one-fourth-inch sieve and that not exceeding about five per cent will pass the 100-mesh sieve.