276 STRENGTH OF MATERIALS 



'227. Brick and brickwork. Brick is generally made by tempering 

 clay with the proper amount of water, and then molding into the 

 desired shape and burning. The tempered clay is used wet, dry, or 

 medium, depending upon the kind of brick desired, and these are classi- 

 fied as soft mud brick, pressed brick, or stiff mud brick respectively. 

 The position of the brick in the kiln may also determine its classifica- 

 tion as hard brick, taken from nearest the fire, medium brick from 

 the interior of the pile, and soft brick from the exterior of the pile. 



Paving brick is a vitrified clay brick or block somewhat L 

 than the ordinary brick. 



228. Compression tests of brick. For this test a whole or half 

 brick is tested edgewise or Hat in much the .same way as in the 

 crushing test for building stone. The faces which are to l>e in contact 

 with the heads of the testing machine are ground perfectly smooth 

 and parallel, or are bedded, or both. If plaster of Paris is used, it 

 should be placed between sheets of paper to prevent the absorption 

 of water by the hrick, as this may atVect its strength. In any case, in 

 testing brick or stone in eumpivssiun it is desirable to use a spherical 

 ('(impression block for one of the heads, so that in case the faces of tin- 

 test piece are not parallel the bearing will adjust itself to bring the 

 axis of the test piece into coincidence with the axis of the machine. 

 In this case, also, the load at first crack and the maximum load are 

 noted. The form of the fractured specimen is also noted ; it is usually 

 that of the double inverted pyramid. An imperfect bedding may cause 

 the specimen to split vertically into thin pieces. Cardboard cushions 

 and soft pine boards are also used in lidding brick for testing. 



The relative value of the kinds of bedding, as indicated by tests 

 made at the Watertown Arsenal * on half bricks, may be seen from 

 the following table. 



Ml ! II 



Set in plaster of Paris 6640 Ib./in.* 



Set in cardboard cushions 4430 " 



Set in pine wood 4540 



The strength of a single brick in compression cannot be taken as 

 a criterion of its strength in an actual structure, since its strength in 

 that case must depend somewhat upon the mortar used. If the 

 mortar is soft and flows (i.e. is squeezed out), the brick may fail in 



* Watertown Arsenal Report, 1901. 



