Portland cement concrete is sometimes used as a foundation for the mixed macadam, but not often. Usually if the traffic is of a character requiring a concrete foundation, it is desirable to use a better wearing course than the mixed macadam, and the asphaltic concrete or sheet asphalt type of surface is employed. It is necessary to finish the surface of the concrete base with some device that will leave the surface rough to prevent the macadam from creeping. A knobbed tamper which leaves numerous irregular depressions about 2 inches in diameter and three-fourths inch deep is often employed.
=Sizes of Stone.=--For the wearing surface, stone ranging in size from 2 inches down to one-fourth inch is usually employed. If the stone is of good quality the maximum size may be but 1-1/2 inches, but soft or even medium stone of that size are likely to crush under traffic. The stone for the base course should preferably be from 3 inches down, but any available size will be satisfactory if the layer is well rolled and bonded. The base course is constructed in the same manner as water-bound macadam and any material satisfactory for the base course of macadam will serve for the base course of mixed macadam. Screenings having good bonding properties will also be required for the base course.
=Mixing and Wearing Surface.=--Several methods are employed in mixing the wearing surface. The simplest is to mix by hand with shovels. The aggregates are heated in improvised heaters which may consist of nothing more than a metal pipe two or three feet in diameter, around which the stone is piled. The mixing platform is usually a metal plate sometimes arranged so that it can be heated by means of a fire underneath. The bituminous material is heated in kettles. For some mixtures, the stone is not heated, but the bituminous material is always heated. The batch of stone is placed on the mixing platform, the bituminous material added and the materials mixed by hand.
Machine mixing is practiced much more extensively than hand mixing, being both more rapid and cheaper. The mixer is similar to a concrete mixer except that the drum is arranged so that it can be heated. The hot stone and the bituminous binder are put into the drum and mixed for the requisite length of time. Sometimes the stone is mixed cold, the bituminous material only being heated.