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to the blocks, as shown by the arrows of the figure, but for the other half of the intersection the traffic remained as before, almost parallel with the blocks. Fig. 11 shows the method which is in use at the present time, and it is as good an arrangement as can be obtained, the principle being to have the traffic, wherever possible, at right angles with the blocks, both on the street proper and at intersections.

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The blocks generally should be laid in courses square with the street, stone to stone, and all blocks in the same course to be of uniform width. Too much care cannot be taken in keeping the joints close, as no matter how tight they may seem to be when laid, they will always show up more loosely after being rammed.

Some contractors purchase their blocks by the thousand, others by the yard. In the former case it is to their interest to have a

thousand blocks lay as many yards as possible, and so there is no desire to keep the joints close, or rather there is an inducement for the pavement to be made with large and open joints. In order to prevent this, the specifications of Philadelphia require that the blocks shall be set separately, according to their width, and so that the 34-inch blocks shall lay 32 per square yard, 4 rows to measure

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16 inches when laid; 4-inch blocks to lay 28 per square yard, 4 rows to measure 18 inches when laid; 4-inch blocks to lay 25 per square yard, 4 rows to measure 20 inches when laid; and they provide that when these conditions are not complied with a reduction of 25 per cent shall be made from the contract price for such portion of the street as does not conform to the above requirements. It would seem that these restrictions were proper and justifiable if applied only to the distance a certain number of rows should

measure, but where a variation of 4 inches is allowed in the length, the number per yard will vary according to the length, and it does not seem as if this variation is important.

After the blocks are laid they should be covered with a clean, sharp sand, free from pebbles, which shall be swept or raked into the joints until they are filled; each course should then be set up perpendicular to the surface of the street with proper tools, and all imperfect blocks removed and replaced with good ones, and then the entire surface should be thoroughly rammed. It should then be covered with a second course of sand, treated as before, and rammed the second time. This part of the work should be done with great care. If any soft spot or, as the rammer expresses it, "soft blocks" are found, they should be thoroughly rammed until they are solid and then taken up and the foundation brought to proper grade with added sand, and the blocks replaced and rammed as before. Upon the proper ramming of the pavement depends, in a great measure, how well it will keep its form and shape under traffic. The entire surface of the pavement should be covered with one inch of sand and allowed to remain under traffic a sufficient time to permit all of the joints to be thoroughly filled.

Concrete Foundation.

With this base the subgrade must be treated in the same way as for sand, and the concrete then laid upon it. After the concrete has been completed and set sufficiently so that working upon it will do it no harm, a cushion of sand should be spread over the entire surface. The amount of sand-cushion will depend in a great measure upon the uniformity of the depth of the blocks. If the blocks are of variable depths, the cushion must be deepened, as, on account of the irregularities of the concrete itself, at least 1 inch of sand should be allowed between the bottom of the deepest block and the concrete.

When a stone-block pavement is laid upon a rigid base, the joints between the blocks should be filled with a substance that will make the pavement, as a whole, water-proof. With a sand base this is not desirable or necessary, as, whatever the joint-filling, the blocks, being set on sand, would always have sufficient motion under traffic to permit water to soak through; but with a concrete

foundation a perfectly water-tight pavement can easily be obtained, and is desirable both from the sanitary and the physical standpoint. Joint-filling.

Portland Cement.-The first filler that naturally suggested itself, in order to make the pavement rigid, was a mixture of sand and cement. This was a mixture of one part of sand and one of Portland cement, and after the blocks were rammed the joints were poured full of a grout made as above. While making a solid and substantial piece of work at first, the chief objection to this filler is that if for any reason a joint becomes broken it always remains so, and accordingly it has never been used to any great extent in stone pavements.

Ferroid.-In 1886 a filler called "ferroid" was used in Buffalo. This was made up of 10 per cent ferroid, 30 per cent German rock asphalt, 25 per cent Trinidad pitch, 15 per cent coal-tar, and 20 per cent sand. The 10 per cent ferroid above was supposed to be composed of iron borings, sal-ammoniac, and sulphur. This mixture was never very extensively used.

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Murphy Grout.-Another joint-filler used to a considerable extent in the West is what is known as Murphy's Grout Filler." It is principally composed of iron slag and carbonate of lime, and when used on a street a certain proportion of clean, sharp sand is added. This is said to produce a mixture which is as hard as granite and which attaches itself closely to the blocks, making them solid and waterproof.

Tar and Gravel.-The general custom, however, in granite pavements of a concrete base is to fill the joints with gravel and paving-cement. This paving-cement in the vicinity of New York City is composed of 100 lbs. of commercial No. 4 paving-cement, 20 lbs. refined asphalt, and 3 lbs. of residum oil. This commercial paving-cement is made from coal-tar. When coal is distilled for the purpose of making illuminating-gas, one of the important products of the distillation is a liquid called coal-tar. This is a very complex hydrocarbon, which when further distilled produces what is generally known as pitch. Its consistency and exact composition depend upon the amount of distillation to which it has been subjected. It is known to the trade also as paving-cement and numbered according to its hardness. It is much like asphalt

in its general appearance, but more brittle. It can be readily distinguished from it by its peculiar odor. It is susceptible to heat and cold, cracking in winter and becoming soft in summer at a temperature which would not affect asphalt. For this reason it is necessary in using it on streets to flux it with a certain amount of asphalt.

Granite blocks on a concrete base are not laid with close joints. The idea is not to fill the joints entirely with paving-cement, but leave them sufficiently open so that they can be filled with gravel and then the interstices in the gravel filled with a pavingcement which forms a perfectly tight joint and one which, if broken during the cold weather, will soften and become perfect again at a higher temperature. The joints should be left just wide enough to allow them to be filled with a gravel which will permit the pitch to flow easily through the interstices and thus make a solid joint. A joint of an inch wide after the blocks are rammed is sufficient to accomplish this purpose.

Gravel for such a joint should be screened so that it will all be retained in a screen having 4-inch mesh, and will pass a screen of 1-inch mesh. If the gravel be finer and allowed to grade down to coarse sand, it will not allow a free flowing of the cement, and the lower part of the joint will not be filled. The pavement should be laid practically in the same manner as described for the sand base except as to width of joints and the ramming, there being so small an amount of sand under the blocks that much ramming is not needed on a concrete base. In all block pavements special care should be taken to break the joints with a lap of at least 3 inches, and preferably in the centre of the block. Where the blocks run of uneven length, the inspector will have to watch pretty carefully to see that this is accomplished. After the blocks have been laid, the gravel, which has been heated to a temperature that will positively insure its being perfectly dry, should be spread over the surface and into the joints in such an amount that when the blocks are rammed the joints shall be filled within 3 inches of the top. The paving-cement should be poured into the joints until they are full to the top of the gravel and until it ceases to run off. The joints should then be filled to the top with more gravel heated to a temperature of not less than 200 degrees, when the joints should be again poured with the paving-cement until they are entirely

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