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Binder Course

surface, base and layer

BINDER COURSE.

An intermediate layer known as the binder course is now commonly placed between the base and surface layer. This layer is ordinarily about I inches thick and consists of broken stone, which passes through a inch screen, mixed with sufficient bitumen to thor oughly coat the pieces of stone. The paving cement used in making the binder course should be of softer consistency than that used in the surface material, about 3 per cent of bitumen being usually required.

The materials are mixed hot, laid and rolled in the same manner as the surface layer. This binder becomes consolidated with and gives added depth and strength to the surface, thus preventing the cracks and wave surfaces which may otherwise appear. The binder, as commonly formed of broken stone, is open and porous, but in some instances stone of graded sizes and sand are employed to make a dense bituminous concrete. This is desirable practice, adding materially to the strength of the pavement under heavy traffic. It

requires a larger amount of bitumen (about 5 per cent to 6 per cent) on account of the larger surface area of grains to be coated.

The binder course has, in some instances, been replaced by a coating of asphalt paint, consisting of asphalt cement dissolved in benzene. The surface of the hydraulic base is painted with this mixture, which serves to cement the base to the surface layer.

After the completion of the hydraulic base and when it has stood a sufficient length of time to harden and dry out, the binder course is placed and compacted. The binder is spread to uniform thickness over the base by use of shovels, all of the material being shoveled over in order to secure uniform compactness. It is then smoothed with rakes having long tines, and after partially cooling rolled with a 5 or 6 ton roller.