The test for normal consistency is made to determine the proper quantity of water to be used in the paste or mortar for tests of time of setting or strength. In the preparation of paste or mortar for these tests, variations in the quantity of water used, or in the methods of mixing and molding the specimens, may produce considerable differences in results. A standard method is therefore prescribed.
The time of setting is tested for the purpose of determining whether the cement is suitable for a given use, rather than as a measure of the quality of the cement. Testing for time of setting consists in arbitrarily fixing two points in the process of solidification called the initial set and the final set. This is accomplished by noting the penetration of a standard needle carrying a given weight into the mass of cement.
The test for fineness is to determine whether the cement is properly ground. Only the extremely fine powder is of value as cement. The coarse parts, while having some cementing value, are practically inert when used in sand mortar.
The test for tensile strength of cement pastes and mortars is made for the purpose of demonstrating that the cement contains the active elements necessary to cause it to set and harden properly. Cement is not usually subjected to tensile stresses in use, but the tensile test has commonly been employed because it offers the easiest way to determine strength, and seems to give a satisfactory means of judging the desired qualities.
The proper conduct of any test for strength is a matter requiring care and experience. There are a number of points connected with the conditions and manipulation of the tests which have important effects upon the results. These are—the form of the briquette, the method of mixing and molding, the amount of water used in tempering the mortar, the surroundings in which the mortar is kept during hardening, the rate and manner of applying the stress, the temperatures at which all the operations are performed. In order to secure uniform results, it is essential that the tests be standard ized in all these particulars.
Soundness is the most important quality of a cement, as it means the power of the cement to resist the disintegrating influences of the atmosphere or water in which it may be placed. Unsoundness in cement may vary greatly in degree, and show itself quite dif ferently in different material. Cement in which unsoundness is
very pronounced is apt to become distorted and cracked after a few days, when small cakes are placed in water. Those in which the disintegrating action is slower may not show any change of form, but after weeks or months gradually lose coherence and soften until entirely disintegrated.
The object in the tests is to accelerate the actions which tend to destroy the strength and durability of the cement. As the tests must be made in a short time, it is necessary to handle the cement in such manner as to cause these qualities to show quickly.
Normal method which has been commonly employed is to make small cakes, or pats, of cement paste about 3 inches in diameter and z inch thick at the center, with thin edges, upon a plate of glass about 4 inches square. These pats are kept twenty four hours in moist air and then allowed to stand for twenty-eight days in water, or in the air. The pat during this period should show no signs of cracking, checking, distortion, or disintegration. This is known as the normal test, and has been relied upon as the final test for soundness. This test is defective in requiring too much time and also, in some instances, fails to discover defective material in which the action is very slow.
Accelerated tests have been proposed for the purpose of hastening the hardening of the cement and causing unsoundness to show more quickly. In most of these tests, heat is employed to accelerate the changes taking place in the cement, and they are known as accelerated tests.
These tests have usually been made by subjecting small pats of the cement to the action of hot water or steam and observing whether cracking or disintegration takes place. Sometimes small bars of cement are used and the increase in length of the bar meas ured after exposure to the hot water or steam. The expansion of unsound cement should be much greater than that of sound cement. The tensile strengths of briquettes of cement which have been exposed to hot water or steam are sometimes measured and compared with the strengths of similar briquettes kept at normal temperatures. The heat should cause a considerable increase in strength of sound cement.