6. The difference in the weight must be determined on scales sensitive to one grain.
7. The increase in weight due to water absorbed shall be ca: culated in per cents of the initial dry weight.
Cross-Breaking Test 1. Support the brick on edge, or as laid in the pavement, on hardened steel knife-edges, rounded longitudinally to a radius of twelve inches and transversely to a radius of one-eighth inch, and bolted in position so as to secure a span of six inches.
2. Apply the load to the middle of the top face through a hard ened steel knife-edge, straight longitudinally and rounded transversely to a radius of one-sixteenth inch.
3. Apply the load at a uniform rate of increase till fracture ensues.
4. Compute the modulus of rupture by the formula 2Lda' in which f = modulus of rupture, in pounds per square inch; w = total breaking load, in pounds; l - length of span, in inches - 6; = breadth of brick, in inches d = depth of brick, in inches.
5. Samples for test must he free from all visible irregularities of surface or deformities of shape, and their upper and lower faces must be practically parallel.
0. Not less than ten brick shall be broken, and the average of all shall be taken for a standard test.
Crushing Test 1. The crushing test should be made on half-bricks, loaded edgewise, or as they are laid in the street. If the machine used is unable to crush a full half-brick, the area may be reduced by chipping off, keeping the form of the piece to be tested as nearly prismatic as possible. A machine of at least 100,000 pounds' capacity should be used; and the specimen should not be reduced below four square inches of area in cross-section at right angles to direction of load.
2. The upper and lower surfaces should preferably be ground to true and parallel planes. If this is not done, they should be bedded, while in the testing machine, in plaster of Paris, which should be allowed to harden ten minutes under weight of the crushing planes only, before the load is applied.
3. The load should be applied at a uniform rate of increase to the point of rupture.
4. Not less than an average obtained from five tests on five different bricks shall constitute a standard test.
Properties of Paving Bricks. Paving bricks range in weight from 51 to 71 pounds; in specific gravity, from 1.91 to 2.70; in resist
ance to crushing, from 7,000 to 18,000 pounds per square inch; in resistance to cross-breaking, R -1,400 to 2,000 pounds; in absorption, from 0.15 to 3 per cent in 24 hours. The dimensions vary according to locality and the requirements of the specifications. The "standard" bricks are 21 X 4 X S inches, requiring 58 bricks to the square yard, and weighing 7 pounds each;"repressed", 21 X 4 X81 inches, requir ing 61 to the square yard, and weighing 61 pounds each; "Metropoli tan", 3 X 4 X 9 inches, requiring 45 to the square yard, and weighing 91 pounds each.
Advantages of Brick Pavements. These may be stated as follows: (1) Ease of traction.
(2) Good foothold for horses.
(3) Not disagreeably noisy.
(4) Yields but little dust and mud.
(5) Adapted to all grades.
(6) Easily repaired.
(7) Easily cleaned.
(8) But slightly absorbent.
(9) Pleasing to the eye.
(10) Expeditiously laid.
(I1) Durable under moderate traffic.
Defects of Brick Pavements. The principal defects of brick pavements arise from lack of uniformity in the quality of the bricks, and from the liability of incorporating in the pavement bricks of too soft or porous a structure, which crumbles under the action of traffic or frost.
Foundation. A brick pavement should have a firm foundation. As the surface is made up of small, independent blocks, each one must be adeqiiately supported, or the load coining upon it may force it downwards and cause unevenness, a condition which conduces to the rapid destruction of the pavement. Several forms of foundation have been used—such as gravel, plank, sand, broken stone, and concrete. The last mentioned is doubtless the best.
Sand Cushion. The sand cushion is a layer of sand placed on • top of the concrete to form a bed for the brick. Practice regarding the depth of this layer of sand varies considerably. In some cases it is only half an inch deep, varying from this up to three inches. The sand cushion is very desirable, as it not only forms a perfectly true and even surface upon which to place brick, but also makes the pavement less hard and rigid than would be the case were the brick laid directly on the concrete.