FILLING THE JOINTS. After the blocks have been rammed, the joints are swept full of pebbles. For ordinary stone blocks, it is usually specified that the pebbles shall pass a sieve having a finch mesh and be retained by a finch mesh. If the pebbles are too small, they will not permit the tar or cement grout, with which the joints are to be filled, to flow freely to the bottom of the joint. After the joints have been filled with these pebbles, they are tamped with a bar having a chisel-shaped end. The joints are then again swept full of pebbles and again tamped.
Three methods are in more or less common use for com pleting the filling of the joints.
1. The filling of the joints is completed by spreading fine sand over the pavement to a depth of / to 1 inch, and allowing traffic to work it into the joints. Until recently this was the only method employed, and even yet it is by far the most common. When filled in this way, the joints are not impervious; and the filling does not aid much in keeping the blocks in position.
2. Recently it has become the custom with the better class of stone-block paving to complete the filling of the joints by pouring hot tar, or a mixture of tar and asphalt, over the pebbles. The tar is applied in substantially the same way as in the case of brick pavements—see § 775. The pebbles should be perfectly dry, for an almost inappreciable amount of water will cause the tar to foam and will prevent it from adhering to the pebbles and from forming a solid joint. It may be necessary to dry the pebbles artificially.
The tar must not be applied when the pebbles are very cold. The joints should be entirely filled with the tar, to secure which it is usually necessary to pour the joints twice. To keep the con tractor from having a financial interest in not filling the joints entirely full, it is sometimes specified that there shall be brought upon the ground not less than a stated number of gallons of paving cement for each square yard of pavement, whatever remains after the completion of the work being the property of the city.
The quantity of tar required to fill the joints varies from 1 to 3/ gallons per square yard, according to the width of the joints, which varies with the quality of the stone and the workmanship.
The tar in the joints makes the pavement impervious, and therefore more sanitary. The tar also assists in keeping the blocks in position, and therefore adds to the durability and smoothness of the pavement.
3. In a comparatively few instances, the joints have been filled with Portland-cement grout, which should be mixed and applied as described for brick pavement—see § 777. The hy draulic-cement grout makes the joint impervious, holds the blocks firmly in position, prevents the edges from chipping and the top face from wearing round, and adds materially to the smoothness and durability of the pavement.