216 TESTS ON CEMENT 



by usage at 7 da. and 28 da. after making, although tests 

 covering much longer periods of time are necessary in research 

 or in investigative work. 



Normal Consistency.- The strength of cement and cement 

 mortars varies considerably with the amount of water 

 employed in making the briquets. Dry mixtures ordinarily 

 give the higher results for short-time tests, and wet mixtures 

 show stronger with a greater lapse of time. For testing 

 purposes, therefore, it is essential that all cements be mixed, 

 not with the same amount of water, but with the amount that 

 will bring all the cements to the same physical condition, or 

 to what is called normal consistency. Different cements 

 require different percentages of water because of their vary- 

 ing chemical composition, degree of burning, age, fineness, etc. 



The normal consistency of neat-cement pastes may be 

 determined by the method that follows. 



This method is to form of the paste a ball about 2 in. in 

 diameter and to drop this ball on a table from a height of 

 about 2 ft. If the cement is of the correct consistency, the 

 ball will not crack nor will it flatten to less than half its 

 original thickness. The percentage of water required 

 will vary from 16 to 25, depending on the characteristics of 

 the material, the average cement taking about 20%. 



Consistency of Sand Mortars. The consistency of sand 

 mortars, however, cannot be obtained by the foregoing 

 method, because the mixture is too incoherent. For mortars, 

 therefore, it is necessary to employ a formula by means of 

 which the sand consistency can be computed when that of the 

 neat paste is known. Several such formulas have been 

 devised, of which the following is adaptable to the greatest 

 variety of conditions. 



Let x be the per cent, of water required for the sand mix- 

 ture; N, the per cent, of water required to bring the neat 

 cement to normal consistency; n, the parts of sand to one of 

 cement; and 5, a constant depending on the character of the 

 sand. Then , 3N + Sn + 1 



For crushed-quartz sand, the constant 5 is 30, for Ottawa 

 sand, it becomes 25; and for the bar and bank sands used in 



