TESTS ON CEMENT 215 



wire screen, however, is raised so that it is an inch above 

 the surface of the water; also, there must be provided a cover 

 that is close enough to retain the steam without creating 

 pressure. The steam test is less severe than the boiling test 

 and is somewhat less accurate. 



Results of Tests for Soundness. The result of the normal 

 tests, if properly made and interpreted, may be considered 

 reliable guides to the soundness of the material, and cement 

 failing in these tests should always be rejected. The acceler- 

 ated tests, on the other hand, furnish merely indications, and 

 are by no means infallible. A cement passing the boiling 

 test can generally be assumed sound and safe for use, but, 

 if failure occurs, it simply means that other tests should be 

 performed with greater care and watchfulness. It often is 

 advisable to hold for a few weeks cement that fails in boiling, 

 so that the expansive elements may have an opportunity to 

 hydrate and become inert; but if the material fulfils all the 

 conditions except the boiling test, and is sound in the normal 

 tests up to 28 da. , it is generally safe for use. All things being 

 equal, however, a cement that will pass the boiling test is 

 to be preferred. 



TESTS FOR TENSILE STRENGTH 



The tensile-strength test is for the purpose of ascertaining a 

 measure of the ability of the material to withstand the loads 

 that the structure must carry. This test is made by form- 

 ing specimens, called briquets, of cement and cement mortar, 

 and determining the force necessary to rupture them in ten- 

 sion at the expiration of fixed intervals of time. Cement 

 constructions are rarely called on to withstand tensile stresses, 

 but if the tensile strength is known, the resistance to other 

 forms of stress may be computed with a fair degree of accur- 

 acy. The tensile-strength test is the most convenient for 

 laboratory determinations, on account of the small size of the 

 specimens and the comparatively low stress required to cause 

 rupture. 



Cement is tested both neat or pure and in a mortar com- 

 monly composed of 1 part of cement and 3 parts of sand. 

 The period at which the briquets are broken have been fixed 



