All the bricks used in the same pavement should also be of the same degree of hardness and toughness in order that the pavement may wear evenly, and to this end careful inspection should always be given to the bricks proposed for use, and all of those which are defective, soft from imperfect burning, brittle from overburning or quick cooling, cracked or distorted by unequal shrinkage, should be rejected. An examina tion of the color and size of the bricks may frequently be useful in determining for any particular material whether individual bricks have received the proper degree of burning, after the engineer has become familiar with the make of brick under examination. The amount of shrinkage in burning is often a quite reliable index of the degree of burning to which the material has been subjected, and specifying within somewhat narrow limits the variation in size of bricks to be used together may often conduce to greater u'hiformity in the material employed. Some makes of brick vary quite appreciably in size for small differ ences in extent of burning, and without materially affecting the value of the product, but it is desirable to sort them closely and use those of each size by themselves.
The mistake is frequently made of placing too high value upon the element of hardness, which when car ried to an extreme is sometimes attained at the ex pense of toughness, the brick becoming brittle and easily shattered. The author (under his guaranty on a pavement) has, on one occasion, been obliged to replace a small number of hard bricks, which at the time of laying were supposed to be among the best of the lot, on account of their becoming shattered under traffic, while somewhat softer brick from the same kilns, the use of which was questioned by the inspector, proved quite satisfactory in the same work.
The sizes of paving-bricks vary considerably as made by different manufacturers, the most common sizes approximating to those of building brick, varying from about 2f to 28 inches in width, 8 to 81 inches long, and 4 inches deep, about 56 to 63 bricks being required for a square yard of pavement. A few makers also produce a brick, of about the same size but only 31 inches deep. A larger size, usually about 3 X 9 X 4 inches, and known as a paving-block to distinguish it from the smaller brick, is quite largely empj.oyed. These usually require from 43 to 47 blocks per square yard.
Larger blocks have been tried, but have not come into general use, while some manufacturers make smaller sizes, requiring 70 to 75 bricks to the square yard of pavement.
Opinions differ as to the best sizes for use in pave ments, some engineers specifying the smaller bricks, others the larger blocks. A good pavement can be built of either if proper attention be given to selecting the material. The sizes preferred by the various manufacturers depend largely upon the character of the clay or shale with which they have to deal. With some materials the size is limited by the distortion of large blocks in burning, and the smaller bricks are preferaLle; with others, larger blocks may be made at less cost in proportion to area of pavement, and perhaps with better and smoother work resulting. There seems to be no necessity for any increase in the usual depth of 4 inches as is sometimes proposed, and it may be possible that the adoption of the depth, 31 inches, now frequently used may in many instances somewhat lessen the cost of the pavement without affecting its length of service.
Much difference of opinion has been developed among engineers as to the advisability of rounding the corners of the brick, some requiring that the blocks be repressed with corners rounded to a radius of 'k of of an inch, while others specify square-edge brick, and in some instances that they shall not be repressed.
On the one hand, it is maintained that in service the sharp corners will soon be knocked off and worn down more roughly and unevenly than if originally rounded, while, on the other hand, it is _claimed that if a rigid filler like Portland cement be used, the joint may be filled level with the surface of the brick, and be much less likely to chip out than if the joint be widened at the top so as to cause the filler to pre sent a thin edge at the sides. Both contentions seem reasonable under certain conditions, and the method of construction and character of filler used will ordi narily determine the proper form for the brick.
The desirability of repressing the brick is also a much discussed question, it being argued by some that the repressing of the material forms a more dense and compact block and increases its probable wear in use, and by others that the pressure applied to the material after it comes from the brick-machine disturbs . the structure and injures the fiber of the brick, often forming laminations which are elements of weakness. With some materials this last contention seems to have some basis in fact, but in other and probably most materials no such condition can be found on examination of the structure of the brick. The, views of individual manufacturers upon the question seem to depend mainly upon the kind of material they have to work with, and it would be difficult from existing data to say that either method necessarily gives the best results. Possibly those materials which approach most nearly to actual vitrification and are subject to considerable shrinkage .during the burning are but little affected in final density by the compression of the block before burning. It should be observed in this connection that whatever may be the value of repressing as to its effect upon the quality of the brick and wear of the pavement, it undoubtedly has the effect with some kinds of brick of giving a smoother and better surface to the pavement by producing a more regular and uniformly shaped In order to give sufficient space between the bricks for the joint-filling, some manufacturers make re pressed bricks with lugs on one side to hold them a given distance apart when laid in the pavement. The wisdom of this under ordinary circumstances -seems doubtful, as small joint-space is usually desirable, and experience shows that bricks laid close, even if care fully driven up, will usually give plenty of space for filling. Spacing-lugs are seldom required in specifica tions, but engineers have sometimes required their use for work on steep grades with the idea of giving better foothold to horses than the thin joints would afford. There may be some advantage in this, and some pavements so constructed on grades of above eight per cent have given satisfactory results, the lugs usually projecting about half an inch from the face of the brick, but in the author's own experience he has been unable to notice any difference in the use of the pavement with or without the wide joints, all being laid with bevel-edge brick.
Repressed bricks and blocks are frequently made with a groove or two, extending lengthwise of the brick on each side and sometimes across the ends, for the purpose of keying the blocks together when filled with the joint-filler. This may be an aid to rigid construction, though not necessary to good work.
It is usually limited to the larger blocks, which fre quently have thin lugs, as well as the grooves, and are known as groove- and lug-blocks. They are well calculated to give a very firm construction.