COMPARISON OF TESTS.
When it is desired to compare by tests the relative merits of various makes of brick, as in the case of com petitive tests, it becomes necessary to determine the comparative values of the results of the different tests and give to each a weight in proportion to its impor tance. For this purpose various formulas have been proposed in which the results of the separate tests are combined into a single term representing the supposed value of the brick. Prof. J. B. Johnson proposed the Tr • ormula V = (25 — R) + (3 — A) + + C 1000 4000 This has been used to some extent in specifications. The formula of the St. Louis Board of Public Improvements is V = + + • 4A 2000 + 4000 While Mr. H. A. Wheeler proposed the formula In all of these V = value of brick; R = loss in rattler in centage of weight of brick; T' = the transverse strength in terms of load per inch of width; T = modulus of rupture per square inch; C = crushing strength per square inch. With the range of value commonly obtained for paving-brick, Johnson's formula gives a weight of 60 or 70 per cent to the rattler test, 10 to 15 per cent each to the absorption and 'crushing tests, and 8 or ro per cent to the transverse test; Wheeler's formula gives more weight than Johnson's to the rattler test and less in proportion to the transverse test. The St. Louis formula gives a weight of about 30 per cent to the results of the rattler test and 45 per cent to those of the crushing test.
The rattle test is, without doubt, the most impor tant of these tests, as calling into play more nearly than the others the qualities necessary to wear in the pave ment; but in using such a method of comparison the conditions which may affect the results of that test as already outlined, and the limitations within which such tests are properly comparable, should be kept in mind. The wisdom of placing such supreme confi
dence in the results of this test, and rejecting the use of the others even as aids to the judgment, as seems to be done by the Commission of the Brick Manu facturers' Association, is at least open to question.
Specifications for brick pavements commonly require that the brick reach certain limiting values on some of the tests. The abrasion test is usually employed, the ordinary requirement being that the brick shall not lose more than 25 per cent of its weight on the standard test, although.in some cases not more than 20 per cent abrasion is allowed.
The transverse test is commonly employed, requir ing about 1800 to 2000 pounds per square inch as a modulus of rupture by the standard test.
The absorption is frequently limited to from 2 to 4 per cent where shale bricks are employed, a common value being 3 per cent.
The crushing test and specific-gravity test are some times, although more rarely, used. The main reliance is usually plated upon the abrasion and transverse tests.