119. Activity

A mortar is said to have set when it has attained such a degree of hardness that it cannot be altered without causing a fracture, i. e., when it has entirely lost its plasticity. Some cements set quickly, while others are comparatively slow. A quick-setting cement is especially valuable in constructions under water.

Test of Activity. - To test hydraulic activity mix cement with just enough clean water, at a temperature of from 650 to 700 F., to make a stiff paste and make one or two cakes or pats 2 or 3 inches in diameter and about inch thick. As soon as the cakes are prepared, immerse in water at 65° F. and note the time required for them to set hard enough to bear a 1/12-inch wire loaded to weigh pound and 4 pounds, respectively. "When the cement bears the light weight it is said to have begun to set; when it bears the heavy weight it is said to have entirely set. Cements, however, will increase in hardness long after they can just bear the heavy wire. The activity of the cement is measured by the time which elapses between the time when the first weight is supported and that when the second is just borne." An increase of temperature will cause the cement to set quicker while cold retards it. As a rule Portland cements should support the heavy wire in from two to five hours.

120. Soundness

Tests for the soundness of Portland cement should be made in the same way as described in Section 113 for tests of Rosendale cement. The color of the cake dried in the air should be uniform bluish gray throughout, yellowish blotches indicating poor cement. The cake left in the water should be made with thin edges, and, if at the end of twenty-four hours it shows fine cracks around the edges, it is unsafe to use in damp places, but if there are no cracks it may be considered safe. This is a very simple test to make, and should be made in all cases where the cement is to be used under water.

121. Fineness

There is no doubt that properly burnt cement, when ground extremely fine, is, as compared with one coarsely ground, much stronger when used with sand, as the finer the particles the better they can surround the sand and aggregates, thus more strongly cementing them together. The finely ground cement is also the safest to use. The hard-burnt cements, finely ground, make the strongest mortars.

Measuring Fineness. - "The degree of fineness of a cement is determined by measuring the per cent, which will not pass through sieves of a certain number of meshes per square inch." A cement that will pass through a sieve of 2,500 meshes (No. 35 wire gauge) with only 5 to 10 per cent, residue is sufficiently fine for any building construction.

122. Strength

The most important test of cement is that of its strength. This is generally made by testing the tensile strength of the cement either neat or mixed with sand. Although cement mortar is generally subject only to a compressive strain, its resistance to compression is so much greater than to tension that in most cases of the failure of mortar it is broken by tensile stress.

Briquettes. - The method of testing the tensile strength of cement or mortar is to form a cake or brick of the cement or mortar in a mould, and after a certain limit of time it is pulled apart and the force required for producing fracture carefully noted. Figs. 57 and 58 show the shape of the briquette and the clamps for holding, as recommended by the Committee of the American Society of Civil Engineers.

The Machine. - There are many machines for sale, made especially for testing the strength of cement. Fig. 59, from Baker's "Treatise on Masonry Construction," represents a cement-testing machine that can be made by an ordinary mechanic at small expense. It is not as convenient nor quite as accurate as the more elaborate machines, but it is sufficiently accurate for all practical purposes. "The machine consists essentially of a counterpoised wooden lever, 10 feet long, working on a horizontal pin, between two broad uprights, 20 inches from one end. Along the top of the long arm runs a grooved wheel carrying a weight, W. The distances from the fulcrum in feet and inches are marked on the surface of the lever, and also the corresponding effect of the weight at each point. The clamp, C, for holding the briquette is suspended from the short arm, 18 inches from the fulcrum. The clamps are of wood and are fastened by clevis joints to the lever arm and bed plate respectively. The pin is iron and the pin holes are reinforced by iron washers. When great stresses are required extra weights are hung on the end of the long arm. Pressures of 3,000 pounds have been developed with this machine."

122 Strength 10058

Fig. 57.

122 Strength 10059

Fig. 58.

122 Strength 10060

Fig. 59.

In applying the load on the briquette it is recommended that it start at o and be increased regularly at the rate of 400 pounds per minute for neat Portland cement, and 200 pounds per minute for natural cements and mortar.

A rough test may be made by suspending the clamps from a beam or trestle and hanging a bucket or box from the lower clamp, into which sand may be run until the briquette breaks, and the weight then weighed.