TESTS FOR QUALITY.
The main advantage of physical tests of paving brick lies in giving definite evidence having a bearing upon the properties and qualities of the brick. To make this evidence useful, the relation of the method of making the tests and their results to the qualities thereby determined must be understood. In several of the tests numerical standards may be set for general use. However, in many cases and especially for some of the tests which may be made, it is best to consider that the results are advisory in nature and that hard and fast limits may not be set. In subsidiary tests the results may give evidence which confirms find ings otherwise made or which throws light upon unsettled questions and aids in interpretation of data obtained by other tests.
In tests of materials it is not essential that. the material shall be subjected to the same action in the process of testing as it will receive in the structure in which it is to be placed. The cold bend test of steel is one of the most useful and instructive of tests, but it differs radically from any condition of service in which the steel will be placed. The value of a test will depend upon the properties determined, and the criterion will be, does the test establish definitely certain properties of the material, or does it give definite evidence concerning specific qual ities, and does not the method give results similar to those found in service. Thus the ordinary rattler test is quite unlike the action of traffic on a street, but if it determines the toughness and hardness of a brick sufficiently well it serves its purpose. Because high grade
paving brick do not crush in service is not conclusive evidence that the results of crushing tests do not give important information concerning the qualities of a given lot of brick. Of course, a test which approx imates the conditions of wear and stress in the street pavement has a distinct advantage in that it appeals to the lay mind and gives the muni cipal officer and the tax payer confidence in the findings which would not be possible in a test of seemingly less direct applicability. What ever the test, its purpose and the bearing of the results on the qualities desired in the brick should be understood and accepted by all.
The tests which have been used, some of them very commonly, others only occasionally, are: 1. the rattler test (called also the impact and abrasion test) ; 2. the absorption test; 3. the crushing test; 4. the cross-breaking test; 5. the specific gravity test. The rattler test is commonly considered to determine toughness and hardness, or resist ance to impact and abrasion. The absorption test gives information bearing upon the degree of hardness to which the brick has been burned. The cross-breaking test and the crushing test determine strength and incidentally give evidence of the hardness and toughness of the brick. The specific gravity test must be classed among those tests which are of value in giving general information. The manner of making these tests will now be described and some discussion given of the meaning of the results found by the various tests.