"All to pass a one-inch screen and to have at least fifty and not more than seventy-five per cent retained on a quarter-inch screen; at least twenty-five and not more than seventy-five per cent of the total coarse aggregate (material over one-fourth inch in size) to be retained on a one-half-inch screen; at least sixty-five and not more than eighty-five per cent of the total fine aggregate (material under one-fourth inch in size) to be retained on a two hundred-mesh sieve." "_Screened Gravel and Sand Mixtures_.--Where it is impossible to obtain run of bank gravel containing the necessary binder in its natural state, screened gravel shall be used and the necessary sand and clay binder added as directed by the engineer. Gravel and sand shall be delivered on the work separately. Clay binder shall be obtained from approved pits and added as directed by the engineer." "_Run of Bank Gravel_.--When run of bank gravel is permitted either for one course or two course work, the size shall not exceed that specified for bottom or top course. If necessary, the contractor shall pass all the material through a two-inch screen for the bottom course, and through a one-inch screen for the top course. When the work consists of only one course, the material shall be of the sizes as specified for the top course. The necessary binder shall be contained in the material in its natural state, excepting that a small percentage of clay binder may be added as directed by the Engineer." =Wearing Properties.=--A certain amount of grinding action takes place on the road surface under the direct action of wheels, especially those with steel tires. Where rubber tired traffic predominates, this action is much less severe than where steel tired vehicles predominate, but the tendency exists on all roads. In addition, there is distortion of the layer of gravel under heavy loads which causes the pieces of stone in the surface to rub against each other and to wear away slowly. The gravel road in the very best condition is slightly uneven but there is comparatively little jar imparted to vehicles, and, consequently, little impact on the surface. When somewhat worn, the impact becomes a factor of some importance and the pounding of vehicles has a very destructive action on the surface. Soft pebbles will be reduced to dust in a comparatively short time.