abrasion.—in this country it is not customary to test bricks and stone by means of the abrasion process, though many english materials have been dealt with in this manner on the continent.
abrasion tests are of special value in regard to paving bricks, and this mode of experiment is largely carried out in the united states. as mr. h. ries remarks,21 the abrasion test approximates closely the conditions under which the paving brick is used, and is, therefore, an important one. the usual method of conducting this test is to put the bricks in an ordinary “foundry rattler,” filling it about one-third full. it is then rotated at the rate of about 30 revolutions per minute, and about 1,000 turns are sufficient. the bricks are weighed before and after to determine loss by abrasion.
a more recent modification is to line the “rattler” with the bricks to be tested and then put in loose scrap iron. this is claimed to give more accurate results, and avoids loss by chipping due to the bricks knocking against each other, as in the previous method, although that has been somewhat obviated by professor orton, jun., by the introduction of a few billets of wood into the rattler.
the abrasion test may also be made by putting the weighed bricks on a grinding table covered with sand147 and water, and noting the weight before and after grinding. this last method seems to us to be decidedly the best, provided the bricks be weighted, that the weight is constant, that the feed of sand and water is uniform, and that the bricks to be tested are placed equidistant from the centre of the turning table. if this last point be not attended to, it will be obvious that in course of the revolutions the sand will tend to accumulate towards the centre of the table, and the bricks placed in that vicinity would receive more than their fair share of abrasion, as compared with those bricks situated near the edge of the table. conversely, those bricks near the periphery would be subjected to greater grinding action, from the circumstance that the table would move faster underneath them than under those bricks nearer the centre of the table.
the bricks should certainly be weighted in such abrasion tests, and it seems desirable that the weights should be so adjusted that the weight of the brick is also taken into account. it is obvious that the abrading action of, say, street traffic, will be the same on a brick, no matter what the latter weighs, depending on the area of surface exposed to traffic. and if we experiment with one brick, weighing say 7 lbs., and another weighing 14 lbs., the greater weight of the latter, will (c?teris paribus), by the abrasion tests as usually adopted, give a much higher result than would the lighter brick. on the other hand, if the 7 lbs. brick be weighted another 7 lbs., then the results would be strictly comparable, provided always that the area exposed to abrasion in each case be the same, and that the other conditions we have laid down are strictly observed.
knowing as we do that the rough and ready method of “rattling” cannot possibly give truly comparative148 results, we do not intend to enlarge much on the results of the american tests; but the following are suggestive as shewing the connexion between the tests for absorption, rattling, and strength combined.
some valuable and interesting tests were recently made by the ohio geological survey, to determine the relative merits of fire-clays and shales for the manufacture of paving bricks, as well as the influence, if any, of the method of manufacture adopted. twenty-two varieties of shale bricks, or bricks the largest constituent of which is shale, were grouped together: fifteen varieties of fire-clay brick; four varieties composed of shale and fire-clay mixed in equal proportion; and three varieties made from ohio river sedimentary clays. the averages of these four classes of results were as follow:—
tests of fire-clay and shales.—paving bricks.
absorption. rattling. crushing.
square
inches. cubic
inches.
shales 1.17 17.61 7,307 1,764
fire-clay 1.62 17.32 6,876 1,678
mixture 1.44 18.72 5,788 1,400
river clay 1.36 19.02 4,605 1,176
from a series of tests recently made by mr. fickes,22 the following factors were educed:—
1. a brick which stands the “rattling” test well, has ample crushing strength and rarely chips under less than 5,000 lbs. per square inch, or crushes under less than 10,000 lbs. the crushing strength tends to vary with the resistance to abrasion, however, but more slowly and irregularly.
149 2. the transverse strength also tends to vary with the resistance to abrasion, but more slowly and irregularly.
3. the toughest bricks usually absorb the least water.
specific gravity.—the practical value of knowing the specific gravity of a brick has, perhaps, been a little over-rated by writers on the subject. at the same time we do not deny that there is some use in ascertaining this property. foremost, we have to mention its value in conjunction with absorption in arriving at a rough and ready means of gauging the strength of a brick, without having actual recourse to the crushing machine. it appears to us, however, that the specific gravity of bricks is rarely quoted in a proper manner, and until there is one uniform method, the results will always be at a discount. we allude to the fact that some experimenters take the specific gravity of a porous brick, without stating whether the amount of water absorbed, during the process, was taken into account in arriving at the specific gravity or not. theoretically, of course, the substance to be dealt with is non-porous, and experimenters, worthy the name, either render the brick waterproof, or, ascertaining the amount of water the brick has absorbed, take that into consideration in calculating results.
the writer is in the habit of quoting the specific gravity in two ways, viz.: (a) the true specific gravity, and (b) the specific gravity of the particles. in an elementary treatise like the present, however, it is not desirable to enlarge on this subject.
