Tensile Strznoth

Normal Consistency.

The amount of water necessary to make the strongest mortar varies with each cement. It is commonly expressed in per cents by weight, although in part at least it depends upon volume. The variation in the amount of water required de pends upon the degree of fineness, the specific gravity, and the chemi cal composition. If the cement is coarsely ground, the voids are less, and consequently the volume of water required is less. If the specific gravity of one cement is greater than that of another, equal volumes of cement will require different volumes of water. The chemical composition has the greatest influence upon the amount of water necessary. Part of the water is required to combine chem ically with the cement, and part acts physically in reducing the cement to a plastic mass; and the portion required for each of these effects differs with different cements. The dryness and porosity of the sand may also appreciably affect the quantity of water required. The finer the sand, the greater the amount of water required. Again, the same consistency may be arrived at in two ways—by using a small quantity of water and working thoroughly, or by using a larger quantity and working less.

Various methods have been used for identifying a particular plasticity, and different standards of consistency have been pro posed; * but none are without objection. The attempt is to adopt a consistency which shall be a compromise between that which will give the greatest strength and that which will give the most uniform results. Two methods of obtaining the same degree of plasticity will be described, viz.: 1, the one proposed by the Committee of the American Society for Testing Materials, which has been generally adopted in this country, and which for convenience will here be called the penetration method; and 2, a method somewhat like one frequently employed in France, and which will here be called the ball method.

Penetration

Method. A paste of neat cement has the proper plasticity when a rod or "piston" of a certain diameter and weight will penetrate the mass to a certain depth. The apparatus required is known as a Vicat penetration apparatus, and consists of a base supporting an arm through which a bar weighing 300 grams (10.57 oz.) slides freely. The lower end of the bar is a cylinder 1 centimeter (0..39 in.) in diameter. To the bar is attached an index which moves over a graduated scale. The paste is placed in a conical hard-rubber ring 7 centimeters (2.76 in.) in diameter at the base and 4 centi meters (1.57 in.) deep. This is the same apparatus as used in making the activity test (§ 153), except that the needle is replaced by the rod, and a weight on the top of the sliding bar has been changed to compensate for the difference in weight between the rod and the needle.

The paste must be mixed as follows: "The material is weighed and placed on the mixing table and a crater formed in the center, into which the proper percentage of clean water is poured; the material on the outer edge is turned into the crater by the aid of a trowel. As soon as the water has been absorbed, which should not require more than one minute, the operation is completed by vigor ously kneading with the hands for an additional one and a half minutes, the process being similar to that used in kneading dough.

The paste should then be quickly formed into a ball with the hands, completing the operation by tossing it six times from one hand to the other, maintained 6 inches apart; the ball is then pressed into the hard-rubber ring, through the larger opening, and smoothed off. The ring is then placed on its large end on a glass plate, and the smaller end is smoothed off with a trowel. The paste, confined in the ring resting on the plate, is placed under the rod bearing the cylinder, which is brought into contact with the surface and quickly released."* The paste is of normal consistency, i.e., of proper plasticity, if the rod penetrates to a depth of 10 millimeters. If the penetration is not the correct amount, a new portion of cement should be weighed out and mixed with more or less water as the case may require, and a new trial made. For any particular cement the exact amount of water required to produce the standard degree of plasticity can be determined only by experiment; but portland cements require from 18 to 24 per cent, usually 19 to 21, and natural cements from 30 to 40 per cent, usually from 34 to 37 per cent.

The consistency recommended above is wetter than has fre quently been employed in the past; but is believed to give more uniform results than a dryer mixture. Some specifications, particu larly those of the U. S. Army Engineers, require that all cements be mixed with the same quantity of water; but this is not generally considered good practice, since the action of different cements is more nearly the same when mixed to a uniform consistency than when mixed with a uniform quantity of water.

With the usual portland cement only about 12 to 14 per cent of water is required for chemical combination, and consequently the water required to produce normal plasticity is considerably more than is required for the hydration..