TESTS ON CEMENT 



217 



construction, it varies from 25 to 35, and must be determined 

 for each particular sand. 



EXAMPLE. How much water is required in a mixture of 

 1 part of cement and 3 parts of crushed-quartz sand? The 

 neat cement requires 19% of water to give normal consis- 

 tency. 



SOLUTION. Here, N = 19, 5 = 30, and n = 3. Substituting 

 these values in the formula, 



3X19 + 30X3 + 1 

 4X(3 + 1) 



Sand for Mortar Tests. The size, gradation* and shape 

 of the particles of sand with which cement mortars are made 

 have great influence on the resulting strength. There are two 

 varieties of standard sand for cement testing, one an artificial 

 sand of crushed quartz, the particles of which are angular in 

 shape, and the other a natural sand from Ottawa, Illinois, the 

 particles of which are almost 

 spherical. Both sands are 

 sifted to a size that will pass 

 a sieve of 20 meshes to the 

 inch and be retained on a 

 sieve of 30 meshes, the dia- 

 meters of the sieve wires be- 

 ing .0112 and .0165 in., re- 

 spectively. The Ottawa sand 

 will develop strengths in 1-3 

 mortars about 20 to 30% 

 greater than those obtained 

 with crushed quartz, and it is 

 theoretically the better sand 

 for testing, but, at present, , 



crushed quartz is more exten- 

 sively employed. On most FIG. 4 

 important works, tests for purposes of comparison are also 

 made of the actual sand- entering the construction. 



Form of Briquet. The form of tensile briquet, adopted as 

 standard in the United States, is shown in Fig. 4. Its 

 cross-section is exactly 1 sq. in. 



