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The Blocks

inch, pavement, block, joints, depth, stone, ordinary and wide

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THE BLOCKS. The blocks should be made of sound and durable stone, free from weather marks and seams, and should be of uniform hardness, since the pavement will wear unevenly if hard and soft blocks are laid together. For the appearance of the pavement, it is desirable that blocks of only one color be laid to gether.

Dressing. The blocks should be split and dressed so as to have as nearly as possible plane rectangular faces and square corners. There is a marked difference in the regularity of paving blocks of different varieties of stone and also from different quar ries of the same variety. As a rule stone paving-blocks are more carefully dressed in Europe than in America, but recently more attention has been given in this country to the form of the blocks. The more regular the blocks the thinner the joints, and consequently the smoother and more durable the pavement. In a general way, the ordinary stone-block pavement in this country has joints from to 1 inch wide with an average of about inch, while some of the better constructed pavements recently made have joints from it to inch wide with an average of about inch. The surface of a recently finished pavement laid with ordinary blocks will show depressions of about 1 inch under a 3-foot straight edge laid par allel to the curb, while the most carefully dressed blocks will show about inch. Of course, these limits vary considerably with the variety of the stone. The two grades of paving as above are very common, the former being called ordinary stone block, and the latter specially dressed stone block.

The blocks should not taper much in any direction; and for ordinary blocks it is often specified that the length of a block shall not differ at top and bottom by more than 1 inch, the width by more than inch, and the depth by not more than inch. The faces should be free from lumps or bunches; and for ordinary blocks it is often specified that there shall be no projection greater than inch, and for specially dressed blocks inch.

Size of Blocks.

There has been much discussion as to the best dimensions for stone paving-blocks; but the proper size is a matter of judgment and does not admit of determination except within limits.

The width should be such as to give a good foothold for horses; and since the horse must depend for a foothold chiefly upon the shoe-calks' catching in the transverse joints of the pavement, the width of a block should be about equal to the distance between the toe and the heel calks of a horse's shoe. On the other hand, if the blocks are too narrow the number of transverse joints will be unduly increased and the pavement will wear rapidly and be come rough.

The block should not be so long that it will fail to conform to the surface of the pavement, nor so short as to make too many longitudinal joints.

The depth should be sufficient to keep the block in position. It is usually assumed that this is 6 inches, but the stability of the block varies greatly with the manner of filling the joints (I 832), and it is quite probable that since the substitution of tar or hy draulic cement as a joint filling material a less depth would suffice. Not infrequently the depth is made greater than is necessary for stability, to allow for wear; but before any considerable depth is worn away, the blocks become very uneven and rough, and the pavement should be re-laid. The blocks may then be taken up, and be re-dressed, and be laid again. In America the depth of adjoining blocks is usually allowed to vary as much as 1 inch, while in England the limit is inch. The wide variation in the depth of adjoining blocks the most common cause of the great roughness of ordinary stone-block pavements, the shorter block having the greater depth of sand under it settles more than its deeper neighbor.

Usually the contractor buys the blocks by the thousand, but gets paid for them by the square yard; and therefore it is to his financial advantage to use as many large blocks as possible. Again, the man who sets the blocks is usually paid by the square yard, and therefore it is to his financial advantage to make the joints as wide as he may. It is very undesirable that it should be to the financial interests of the contractor and of the paver to secure a poor pavement, i. e., one having large blocks and wide joints. An excess in the width of the block is more important than in the length, since it is proportionally a larger matter, and also since it has a more important influence upon the quality of the pavement; and therefore special care should be taken to prevent an excessive width of blocks or too thick side-joints. To identify as far as possible the interests of the contractor with those of the city, the following method of measuring a stone-block pavement has been proposed.* "The blocks must be substantially smooth and square on all their faces, and within the limits of the following dimensions: Not less than 3, inches, nor more than 41 inches wide across their upper and lower faces; not less than 7 nor more than 8 inches deep; and not less than 8 nor more than 14 inches long, except where shorter stones are necessary to fill out courses.

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