. The strength of materials; a text-book for engineers and architects. Fig. 191.—Cement Testing Machine (Tension). and a gauge glass placed outside the can is provided with ascale graduated to enable the breaking stress to be read offdirect. In another common form of testing machine a jockey-weightis moved automatically along a lever arm by means of a weightcontrolled by an adjustable dashpot which enables the rateof loading to be varied. On the fracture of the specimen theweight becomes stationary and the breaking stress is read offon a scale attached to the lever. Compression Tests.—Compress
. The strength of materials; a text-book for engineers and architects. Fig. 191.—Cement Testing Machine (Tension). and a gauge glass placed outside the can is provided with ascale graduated to enable the breaking stress to be read offdirect. In another common form of testing machine a jockey-weightis moved automatically along a lever arm by means of a weightcontrolled by an adjustable dashpot which enables the rateof loading to be varied. On the fracture of the specimen theweight becomes stationary and the breaking stress is read offon a scale attached to the lever. Compression Tests.—Compression tests are not usually 406 THE STRENGTH OF MATERIALS specified for pure cement, although they are becoming morecommon. For concrete in reinforced concrete works, however,compression tests are nearly always required. A common specification is that cubes, the area of each sidebeing 50 sq. cm., of 3 parts sand to 1 part cement by weight. Fig. 192.—Cement Testing Machine (Tension). shall develop at 28 days at least 10 times the standard tensilestrength (^. e. 2000 lbs. per sq. in.). The following test results for the concrete cubes (with area THE TESTING OF MATERIALS 407 of each side 50 sq. cm.) are recommended by the ConcreteInstitute. Proportion by Volume. Crushing Strength in sq. in. Cement Sand CoarseMaterial 28 days aftermixing 120 days aftermixing 1 1-21-52 2 222 4444 1600180020002200 2400 260028003000
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