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Asphalt Blocks

pavement, cent, surface, sheet and inches

ASPHALT BLOCKS.

Asphalt paving blocks are frequently formed of a mixture of asphalt cement and crushed stone. The stone used is mainly trap, or granite, broken so as to pass a inch screen. In early work, limestone was used; this was found to lack durability on account of the softness of the stone. The mixture is similar to that used for the surface of a sheet pavement, containing about 8 per cent to z z per cent of asphalt cement, 7 per cent to to per cent limestone dust, and crushed stone 8o per cent to 85 per cent.

The materials are heated to a temperature of about 300° F., and mixed while hot in an apparatus arranged to secure the even distribution of the ingredients through the mass. The thorough incorporation of the various materials in the mixture is of first importance in producing homogeneous and uniform blocks, while the quality of the materials used needs as careful inspection as in the case of the surface material for sheet pavements.

When the mixing is complete, the material is placed in moulds and subjected to heavy pressure, after which the blocks are cooled suddenly by plunging into cold water.

These blocks have usually been made larger than paving-bricks, the common size being 12 inches long, 3 or 4 inches wide, and 4 or 5 inches deep. They are laid in the same manner as brick, as closely in contact as possible, and driven together. Under the action of the sun and the traffic, the asphalt blocks soon become cemented together through the medium of the asphaltic cement, and form, like the sheet asphalt pavements, a practically impervious surface. They are often laid

upon gravel base, although in the best work a light concrete foundation is employed.

In forming the asphalt block pavement the road-bed is brought to subgrade in the ordinary manner and rolled, leaving room for the pavement of uniform thick ness to be placed upon it. A layer of gravel 4 or 5 inches deep is then placed and rolled, or a base of con crete is formed, with a cushion coat of sand I to 2 inches, and then the paving blocks. The blocks are pressed together in the courses by the use of a and the courses driven against each other with a maul to reduce the joints as much as possible. A coating of sand is given to the surface of the pavement, and it is rammed to a firm and uniform surface, as in the case of brick.

These blocks have the advantage over sheet asphalt for the smaller cities, that the blocks may be formed at a central point and shipped ready for use to the site of the proposed pavement, and that no special plant need be erected in each town where they are to be constructed. They have given satisfaction in use, and have frequently shown good durability in wear under moderate traffic. It is claimed that they are less slippery and may be used upon steeper slopes than sheet asphalt. The cost of transportation of the blocks makes this pavement expensive in many locali ties not in close proximity to the place of and prevents them from competing successfully with other pavements.