The transverse strength is tested by supporting the brick upon two knife-edges near its ends and bringing a load through a third knife-edge upon the middle of the brick. The test may be made upon any ordinary testing-machine by providing the necessary knife edges, but, like the compression test, requires care in manipulation to get good results. It is specially im portant that the brick have a perfectly even bearing upon the supports before the application of the load, in order that it may not be subjected to a twist under the load. The method adopted by the commission for this test is as follows: I. Support the brick on edge, or as laid in the pavement, on hardened steel knife-edges rounded longitudinally to a radius of 12 inches and trans versely to a radius of one-eighth inch, and bolted in position so as to secure a span of six inches.
II. Apply the load to the middle of the top face through a hardened steel knife-edge, straight longi tudinally and rounded transversely to a radius of one sixteenth. inch.
III. Apply the load at a uniform rate of increase till fracture ensues.
IV. Compute the modulus of rupture by the for mula in which f = modulus of rupture in pounds per square inch; w = total breaking load in pounds; l = length of span in inches = 6; b = breadth of brick in inches; d = depth of brick in inches.
V. Samples for test must be free from all visible irregularities of surface or deformities of shape, and their upper and lower faces must be practically parallel.
VI. Not less than io brick shall be broken and the average of all be taken for a standard test.
The commission included this test with the crush ing test in the recommendation that the test was to be considered optional and "not necessary as a proof of excellence. " This test is easier to conduct satis factorily, and probably gives, in general, a more reli able indication of the value of the material than the crushing test. It calls into play the tensile as well as compressive strength of the brick. The interior structure is shown by the break, and an opportunity is given to judge of the uniformity and homogeneous character of the material.
The fracture of a tough and homogeneous speci men under a transverse load should be a clean break through the middle of the brick, and a close observa tion of the breaks may frequently be of considerable assistance in forming an idea of these qualities, al though they may not be directly represented by the load required to break the specimen. The shattering of the brick in breaking, or an irregular break extend ing from the point of application of the load to one of the points of support, usually indicates brittleness of the material.
The modulus of rupture of good paving-bricks commonly ranges between z000 and 300o pounds per square inch, sometimes reaching 3500 or even 4000 pounds. It is usually somewhat greater for brick laid flat than for brick on edge.