CROSS-BREAKING TEST.
The cross-breaking test is for the purpose of determining the general strength of the brick; incidentally it gives evidence of the toughness and the hardness of the brick. It indicates the ability to resist cross breaking, twisting, or spalling by concentrated loads and is an index of the crushing strength of the brick.
Two objections to this test have at times been raised; (1) that the quality indicated by the test is not needed in a paving brick, and (2) that the results of the cross-breaking test are variable and even erratic. It is believed, however, that the test is helpful in judging of the quality and strength and toughness required of a good brick. It may be suffi cient to specify only a medium value for the modulus of rupture, and yet a brick with a fairly high value will be of higher grade. The brick which does not have the quality of high resistance to cross-breaking is likely to spall or break in the street and not to withstand traffic, even though the rattler test may show a low loss. Brick which have the tough ness and strength which go with a good modulus of rupture may show a somewhat higher loss by the rattler and yet give better results in the street than other brick whose rattler losses are lower. It must be ex pected that there will be a variation in the results shown in tests of individual brick, for quality varies considerably in ordinary paving brick. The rattler tests of individual brick vary widely. Much of the variation which has been reported in the results of cross-breaking tests is due to the method of making the test commonly employed. It is be lieved by the writer that the method here given reduces the variation due to the method to a reasonable amount and that the variation now found represents quite closely the lack of uniformity in the brick. With the
test made in the manner here described cross-breaking tests, if properly judged, become a valuable adjunct in the determination of the quali ties of a paving brick.
Brick should be tested as a beam on edge with a span of 6 inches and with the load applied at the middle of the span. The modulus of rup ture is determined by the usual formula : where W is the load applied, 1 is the span, b is the breadth of the brick, and d the depth.
Plate 3 gives a view of a brick being tested, and Fig. 32 shows details.
Attention is called to the use of steel bearing plates and to the use of the wooden block. The narrow soft steel plate gives a bedding on the brick which is slightly adjusting and overcomes the tendency to cutting. The knife edges are slightly curved in the direction of their length, to allow for irregularities or warping in the brick. The lower knife edges rest upon a wooden block which is curved laterally somewhat to allow a rocking movement. The main purpose of the wooden block, however, is to allow adjustment by its compression so that the load will be more evenly distributed and so that the work of applying the load and making the test will extend over a longer time. This arrangement allows a more accurate determination of the amount of the load and greater freedom in making the test. The results of the tests which are discussed later on, show that this method gives results well within the range of uniformity of the brick. Requirements for the cross-breaking test should specify that the brick be tested on edge, that the span be 6 inches, that the knife edges be slightly curved in the direction of their length, say with a radius of 20 inches, and that the test be made upon a wooden block similar to the one shown.