CRUSHING TEST.
Tests for crushing strength are open to the objection that the results obtained are extremely variable, especially as the method of making the test is not uniform. When the faces of the test cubes are ground accur ately to plane surfaces, the results with high-grade paving brick are very high, running up to 20,000 pounds per square inch. The use of prepared test cubes makes an expensive and slow method of testing. Whole brick or half brick are tested on edge, sometimes with the bear ing faces ground and in other cases not. If not ground, the faces may be bedded in plaster of Paris and crushed after the plaster has fully set, or the faces may be bedded in card-board or heavy paper. The last named method of testing is more readily made and if at least five speci mens are tested the average may be expected to give .representative re sults. In the tests described in this paper, half bricks were tested, several thicknesses of heavy building paper being used as bedding plates. Soft brick will give results as low as 1,000 pounds per square inch, when tested by this method. Occasionally a brick will run as high as 18,000 pounds per square inch. It may be expected that overburned or poor paving brick will stand a load up to 3,000 pounds per square inch.
Good pavers will range between 6,000 and 12,000 pounds per square inch.
Crushing strength is a desirable property in a paving brick. The argument that such heavy loads as are indicated by crushing values will not come upon the brick and that the brick will not be crushed in the street is a negative one. There is a relation between crushing strength and hardness. The stronger the brick the better it will resist wear in the pavement. This quality of strength is particularly desirable where. pavement is subject to heavy traffic. In comparing two •bricks giving about the s+ume rattler results. the one with high crushing strength will stand heavy traffic much better than the weaker one. For light traffic high crushing strength is not essential. It is further true that the crushing test throws light on other physical properties of a brick and is a source of evidence in the study of quality. Generally speak ing, however, this test is not of a character to be included in specifica tions, but it is of value in connection with the study of the properties of different bricks. It will be seen, also, that the cross breaking test gives information which may permit it to take the place of the crush ing test.