The standard steam test consists in observing the effect of steam at about 100° C. upon small pats of the cement. This test was recommended by a committee of the American Society of Civil Engineers in 1904. It has since been included in the specifications of the American Society for Testing Materials in conjunction with the normal pat test, which was the deciding test. In the modified specifications for Portland cement adopted in 1916, the normal test is discontinued and the steam test becomes the standard.
The methods for making the standard tests are described in detail, with the specifications, in the Book of Standards of the American Society for Testing Materials, and in the reprint pub lished by the Portland Cement Association.
21. Tests of Compressive Strength.—Tests of compressive strength are seldom used in specifications for cement, on account of the greater ease of making the tensile test and the lighter machines that may be employed for the purpose. These tests have frequently been made for purposes of comparison or to determine special qualities of the material. The standard test piece has usually been a 2-inch cube, prepared in the same manner as the tension specimens. This was recommended by a committee of the American Society of Civil Engineers in 1909.
As cement mortar is usually employed in compression, some engineers prefer to use the compression test in their specifications. A new tentative specification with methods of testing was recom mended by a committee of the American Society for Testing Materi als in 1916. This has not been adopted by the society as a standard, and may be further modified before such adoption. It is probable that such a standard will be adopted, to be used in conjunction with or to replace the tension test. This proposed specification with the method of making the test is given in Volume I of the Transactions of the Society for 1909. Reprints may be had from the Secretary of the Society.
22. Special Tests.—The tests ordinarily employed in determin ing the quality of cement are enumerated in the preceding sections. Other tests are frequently made to determine special qualities or for the purpose of investigating properties of cements and mortars.
Transverse Strength.—Tests of the strength of cement mortar under transverse loading are seldom employed as a measure of the quality of the material, but a-re frequently made with a view to determining the action of the material in service. Propositions have often been made to substitute the transverse for the tensile test ill the reception of material. These suggestions have usually been based upon the simplicity of the test and of the apparatus with which it may be carried out. The specimen usually employed for this purpose is 1 inch by 1 inch and 6 inches long. It is tested by placing upon knife edges 5 inches apart and bringing the load upon the middle section. Professor Durand-Claye, from a large number of comparative tests, found the unit fiber stress under transverse load to average about 1.9 times the unit stress for tension.
Adhesire Strength.—The ability of cement mortar to adhere firmly to a surface with which it may be placed in contact is one of its most valuable properties and quite as important as the develop ment of cohesive strength. Tests for adhesive strength are not employed as a measure of quality, because of the uncertain char acter of the test and the difficulty of so conducting it as to make it a reliable indication of value. The adhesive properties of the cement are to a certain extent called into play in tests of sand mor tar, and may be inferred from comparison of neat and sand tests.
Experiments upon the adhesion of mortars to various substances are sometimes made, both for the purpose of comparing the cements or methods of use, and to study the relative adhesions to various kinds of surfaces. Such experiments are quite desirable with a view to the extension of knowledge of this very important quality.
The common method of making this test is to prepare briquettes of which one half the briquette is of cement paste or mortar and the other half a block of stone, glass, or other material to be used. The cement half is made in the ordinary form for tensile specimens. The other half is made to fit the cement mold at the middle and arranged at the end to be held by a clip in the testing machine.