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Construction of Stone-Block Pavements

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Stone-block pavement for durable and effective service should be placed upon very firm foundations. Bases of concrete are usually employed and give the best results. These foundations are formed as described in, Art. 47, and consist of •a layer of concrete 4 to 8 inches thick, 6 inches being the most common depth.

In constructing the pavement, a cushion coat of sand, usually I to 2 inches thick, is spread upon the base of concrete for the purpose of allowing the bases of the paving blocks to be firmly bedded when the tops are brought to an even surface, the sand readily adjusting itself so as to fill all the spaces beneath the blocks and to offer a uniform resistance to downward motion in every part of the pavement, and in like manner trans mitting the loads which come upon the pavement to the foundation so as to evenly distribute them over the surface of the concrete. The sand used for this pur pose should be clean and dry, and all large particles sifted out, as they may prevent the blocks adjusting themselves properly. A thin layer of asphaltic cement is sometimes used in place of the sand with very good results.

The blocks should be laid as close together as pos sible in order to make the joints small. They are laid, like brick, with the longest dimension across the street, and arranged in courses transverse to the street, with the stone in consecutive courses breaking joints.

After the blocks are placed they are well rammed to a firm unyielding bearing and an even surface. Stones that sink too low under the ramming must be taken out and raised by putting more sand underneath.

As in the case of other block pavements, those of stone should be made as impervious to moisture as possible. The foundation should be kept dry, and moisture prevented from penetrating beneath the blocks where it has a tendency to cause unequal settle ment under loads or disruptions under the action of frost. In the better class of work, therefore, the joints are filled with an impervious material which cements the blocks together. Asphalt or coal-tar paving cement is commonly employed for this purpose, as with brick and wood, and seems the most satisfactory in use, although hydraulic cement mortar is sometimes used. The coal-tar cement is commonly made by mixing coal-tar pitch with gas-tar and oil of creosote, a pro portion sometimes employed being no pounds pitch, 4 gallons tar, and i gallon creosote.

The use of cement between the blocks binds them together and increases the strength of the pavement as well as the resistance of the blocks to being forced out of surface. It also deadens to some extent the noise from the passing of vehicles where asphaltic or coal-tar cement is used.

A method commonly used for filling the joints is to first fill them about one third full of small gravel, then pour in the paving cement until it stands above the gravel; then another third full of gravel, more cement as before; then gravel to a little below the top, and the joint filled full of cement; after which a coating of fine gravel is distributed over the surface.

Sometimes the joints are filled with gravel_before the blocks are rammed to surface, and the paving cement afterward poured into the joints. This has the advan tage of bringing the blocks to a very firm bearing, and secures complete filling of the joints.

Various modifications of the method above outlined are used in the principal cities for a pavement to with stand heaviest traffic and secure a maximum of dura bility; essentially it represents the best modern practice. The specifications used in New York City in 1908 contain the following requirements: " 29. On the concrete foundation, as designated, shall be laid a bed of clean, coarse, dry sand to such depth (in no case less than one and a half [1+] inches) as may be necessary to bring the surface of the pavement, when thoroughly rammed, to the proper grade.

"On this sand bed, and to the grade and crown specified, shall be laid the stone blocks at right angles to the line of the street or at such angle as may be directed. Each course of blocks shall be laid straight and regularly, with the end joints by a lap of at least three (3) inches, and in no case shall stone of different width be laid in the same course except on curbs. All joints shall be close joints except that when gravel filling is used, the joints between courses shall be not more than three-quarters (I) of an inch in width.

"After the blocks arc laid on a concrete foundation, they shall be covered with a clean, hard and dry gravel, which shall have been artificially heated and dried in proper appliances, placed in close proximity to the work, the gravel to be brushed in until all the joints are filled therewith to within three (3) inches of the top. The gravel must be washed white quartz and be entirely free from sand or dirt, and must have passed through a sieve of five-eighths () inch mesh and been retained by a three-eighths (I) inch mesh.

"The blocks must then be thoroughly rammed and the ramming repeated until they are brought to an unyielding bearing with a uniform surface, true to the given grade and crown. No ramming shall be done within twenty (20) feet of the face of the work that is being laid.

"The boiling paving cement, heated to a temperature of 300° F. and of the composition hereinbefore described, shall then be poured into the joints until the same are full, and remain full to the top of the gravel. Hot gravel shall then be poured along the joints until they are full flush with the top of the blocks, when they shall again be poured with the paving cement till all voids are completely filled."