The absorption test is capable of giving data of value in regard to the degree of vitrification, hut to be generally useful as a standard for grading brick the limiting per cent of absorption should be determined for each variety of brick, a task which is impracticable, except where there are only a few brands of brick upon any particular market.
Hence this test is valuable to a user of brick only as a heck upon the uniformity of a particular brand. It could be used to exclude any soft and porous brick, but such material would be un hesitatingly rejected on account of its general appearance.
The National Brick Manufacturers' Association recommend that when this test is employed, it be made according to the follow ing method: " 1. The number of bricks constituting a sample for an official test shall be five.
2. The bricks selected for conducting this test shall be such as have been previously exposed to the rattler test. if such are not available, then each whole brick must be broken in halves before the test begins.
" 3. The bricks shall be dried for forty-eight hours continuously, at a temperature of 230 to 250 degrees Fahr., before the absorption test begins.
" 4. The bricks shall be weighed before wetting, and shall then be completely immersed for forty-eight hours.
" 5. After soaking, and before re-weighing, the bricks must be wiped till free from surplus water and practically dry on the surface.
" 6. The samples must then be re-weighed at once. The scales must be sensitive to one gram.
"7. The increase in weight, due to absorption, shall be calcu lated in per cents of the dry weight of the original bricks." Shale paving bricks when tested as above usually show less than 2 per cent of absorption, some of the best ranging from 0.75 to 1.50; less than 0.5 probably indicates an over-burned and brittle brick. Good paving brick made of impure fire clay rarely absorb less than 2.5 per cent, and often over 5 per cent.
To apply this test to determine uniformity of quality, it will be necessary to establish limits of the absorption. This may be done by inserting in the specifications a clause somewhat like the following:* "At the time of submitting bids, the bidder shall furnish twenty five bricks of the make specified in his bid, on which a rattler test and an absorption test shall be made. A requirement of the bricks furnished for the pavement will be that their absorption tests shall fall within the following limits: When the result of the absorption test of the sample bricks is below 2 per cent, the limits shall be per cent less and 1 per cent more than the sample; when between 2 per cent and 5 per cent. the limits shall be 1 per cent less and 11
per cent more than the sample." Transverse Strength. This is determined by resting the ' brick upon two knife-edges and applying a steady pressure on the upper side of the brick through a third knife-edge placed midway between the other two. The results are expressed in terms of the modulus of rupture, which is computed by the following formula: in which R represents the modulus of rupture in pounds per square inch, W the breaking load in pounds, 1 the distance between sup ports in inches, b the breadth of the brick in inches, and d the depth of the brick in inches. The brick may be tested edgewise or flat wise, although the former is usually the better method, since then W is larger. The knife-edges should be rounded transversely to a radius of about one sixteenth of an inch and longitudinally to a radius of about 12 inches, to secure better contact and to prevent the brick from being crushed at the edges. Some authorities recommend grinding opposite edges of the brick to parallel planes, but this is a useless expense. If the brick is warped, the contact between the brick and the knife-edges can easily be made entirely satisfactory by placing pieces of metal under the blocks carrying the lower knife-edges, or by shifting the brick longitudinally, or by turning it.
The modulus of rupture of bricks that have given excellent service in a pavement varies from 1,500 to 3,500 pounds per square inch, usually from 2,000 to 3,000. Owing to apparently unavoid able variations in the structure of the brick it is not possible to attain closely concordant results in making this test; and with the utmost care in selecting the brick and in making the tests, the variation from the mean ranged from 8 to 30 per cent, and on the average was about 20 per cent.* The cross-breaking test furnishes a means of comparing the toughness of various kinds of paving brick. The uniformity of the results for any particular kind of brick indicates its structural soundness, freedom from air checks, etc., and shows whether the material has been properly treated in the various stages of manu facture. The transverse strength indicates the resistance of the brick to cross breaking when laid in the pavement on an unyield ing and uneven surface; but this element is not entitled to much consideration, since brick are seldom thus broken in the pave ment, at least not until nearly worn out.