Great care should be taken to center the briquette properly in the clips, as cross strains lower the observed breaking strength. An eccentricity of -- inch may reduce the strength 20 per cent.
The Speed. The more rapidly the load is applied the greater will be the results obtained. For a number of years the standard rate in this country was 400 pounds per minute, but the recently adopted standard specifications (see Appendix I) require a rate of 600 pounds per minute.
The French and German standard specifications require 660 pounds per minute.
The specifications adopted by the American Society for Testing Materials in 1904, and adopted also by various other national engineering societies, require that "the minimum tensile strength of briquettes 1 inch square in cross section shall be within the limits shown in Table 13, and there shall be no retrogression in strength within the periods specified. For example, the minimum requirement for the twenty-four hour neat-cement test should be some specified value within the limits of 15Q and 200 pounds; and so on for each period stated." Specifications for strength should be fixed, within the limits in Table 13, to suit the conditions under which the cement is to be used, and also in accordance with the personal equation of the one who is to make the tests. If the one who is to test the cement is not already an expert, he should get a number of standard brands and test them to determine his personal equation in comparison with the data in Table 13, and then write the specifications accordingly.
Table 14, page 82, shows the results of tests of 7,000 bri quettes of 783 samples of 16 brands of portland cement tested at a single laboratory in 1905, and of many thousand tests of twelve brands of natural cements made at two laboratories in 1905. Com pare these results with those in Table 13 to see the relation between the standard requirements and the ability of the manufacturers to meet them.
For additional data on the strength of mortars composed of different proportions of cement and sand see Fig. 4 (page 112), Fig. 7 (page 122), and Fig. 8 (page 123).
It not infrequently occurs that several samples of cement are submitted, and it is required to deter mine which is the most economical. One may be high-priced and have great strength; another may show great strength neat and be coarsely ground. If the cement is tested neat, then strength, fineness, and cost should be considered; but if the cement is tested with the proportion of sand usually employed in practice, then only strength and cost need to be considered.
Table 15 shows the method of deducing the relative economy when the cement is tested neat; and Table 16 shows the method when the cement is tested with sand. The data are from actual practice in 1892, and the cements are the same in both tables. Results similar to the above could be deduced for any other age; the circumstances under which the cement is to be used should deter mine the age for which the comparison should be made.
The above method of equating the results gives the advantage to a cement which gains its strength rapidly and which is liable to be unsound; and therefore this method should be used with discre tion, particularly with short-time tests.
Cement is so variable in quality and intrinsic value that no considerable quantity should be accepted without testing it to see that it conforms to a specified standard. A careful study of Art. 2 preceding, will enable any one to prepare such speci fications as will suit the special requirements, and also to give the instructions necessary for applying the tests.
For specifications that may properly be called the American standard specifications for cement, see Appendix I.
For additional data on matters relating to the manufacture, the chemical composition, and the method of testing lime and cement. see the following: