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TESTS FOR SOUNDNESS. ART. 52 ORDINARY TESTS. 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 differently in different material. Cement in which the 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 visi- ble change of form, but after weeks or months gradually lose coherence, and soften until entirely disintegrated. The method in common use for testing unsoundness is to make small cakes or pats of neat cement, usually about 3 or 4 inches in diameter and 1/2 inch thick, upon a plate of glass, and keep them in air or water for a few days, carefully watching them to see if they show any signs of distortion or surface cracks, which may indicate a tendency to disintegration. The German standard specifications require that the 161 162 HYDRAULIC CEMENT. cakes for this test shall be 1.5 centimeters thick at the centre and have thin edges. These cakes are placed in water 24 hours after they are made, or at least not until they are firmly set, and observations are con- tinued over a period of 28 days, when, if no cracks or distortions appear, the cement is considered sound. " The cakes, especially those of slow-setting cement, must be protected against draughts and sunshine until their final setting. This is best accomplished by keep- ing them in a covered box lined with zinc or under wet cloths. In this manner the formation of heat cracks is avoided, which are generally formed in the
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centre of the cake, and may be taken by an inexperi- enced person for cracks formed by blowing." The Committee of the American Society of Civil Engineers recommend the following: "Make two cakes of neat cement two or three inches in diameter, about one half inch thick, with thin edges. Note the time in minutes that these cakes, when mixed with water to the consistency of a stiff plastic mortar, take to set hard enough to stand the wire test recommended by General Gillmore, i/12-inch diam- eter wire loaded with one fourth of a pound, and i/24-inch loaded with one pound. " One of these cakes, when hard enough, should be put in water and examined from day to day to see if it becomes contorted, or if cracks show themselves at the edges, such contortions or cracks indicating that the cement is unfit for use at that time. In some cases the tendency to crack, if due to free lime, will disappear with age. The remaining cake should be kept in the air and its color observed, which for a TESTS FOR SOUNDNESS. 163 good cement should be uniform throughout, yellowish blotches indicating a poor quality the Portland cements being of a bluish gray and the natural cements being dark or light, according to the character of the rock of which they are made. The color of the
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