WHEREAS, From the experimental work done so far by this commission, or by others so far as is known to us, in the application of the cross-breaking and crushing tests to paving-bricks, it is not possible to show any close relationship between the qualities necessary for a good paving material and high struc tural strength as indicated by either of these tests, "Resolved, That for this reason the commission rec ommends that these tests shall be considered as purely optional in the examination of paving material, and not necessary as a proof of excellence.'' It is to be observed that the actual crushing strength of a brick is not a matter of special importance in so far as any danger of the crushing of the material in the pavement is concerned, as no stress can there come upon it under ordinary circumstances which would endanger even a very weak specimen from direct crushing. It is thought, however, that to some extent the value of the brick is indicated by its resistance to crushing, coupled, of course, with a proper examina tion of its other necessary attributes. A brick which
possesses a high crushing strength is not necessarily a good paving-brick, as it may at the same time be brittle or of such composition as to easily disintegrate under the action of the weather; but one that yields to a low crushing strength is usually weak in wearing qualities and not fit for the purpose. For this reason this test is commonly included in specifications pre scribing tests, although it is recognized that the rela tive wearing qualities of various makes of brick can not be graded by its results. A good paving-brick, in the form of a 2-inch cube, will usually show a re sistance to crushing of not less than 10,000 pounds per square inch. Much higher values are sometimes used in specifications, but their advantage is at least doubtful.