Rectangular Blocks on Concrete. The use of a concrete base under a surface of the Nicholson type effected a marked improvement in the wear of the pavement.
Round block pavements were also sometimes placed upon a concrete foundation.
In using a concrete foundation a cushion coat of sand is commonly employed on top of the concrete in which to bed the blocks in order that they may be brought to an even surface. Sometimes a thin layer of cement mortar is used in place of the sand upon the concrete; and in London some pavements have been constructed with a thin layer, about inch, of asphalt mastic over the concrete, the blocks resting upon the mastic.
A pavement of this type is shown in Fig. 26.
In laying a pavement of this kind a course of blocks is first set across the street, and then a strip of wood of the thickness of the joint is set against the row of blocks and left until the next course is placed, or sometimes spuds with heads of the thickness of the joints are driven to the head in the side of each block, and the next row of blocks are set against the spuds.
In some pavements of this kind hydraulic cement is employed in filling the joints, and in some instances the lower half of the joint is filled with coal tar paving cement and the upper half with hydraulic cement mortar. The cement mortar gives a harder wearing
surface, and protects the pitch from the softening action of the sun in warm weather. In later practice the width of joint has been gradually reduced until the blocks are set in contact with each other, occasional expansion joints being provided.
These pavements have been extensively used in England, and to a smaller extent in the United States. They have been fairly satisfactory in use but have been, for the most part, superseded by treated blocks.
Treated Block Pavement. Wood blocks treated by some preservative process for the purpose of preventing decay and of hardening the block so as to give better resistance to wear have come into use somewhat extensively since about 1900. These pavements have given good service in use. They are rather expensive in first cost, but have in some instances been found durable under heavy traffic and are fairly economical.