The proportions of the ingredients should always be determined by weight rather than by measure. It will be found more convenient to use metric weights for the dry ingredients. The water should then be measured in cubic centimeters, which is equivalent to weighing it in grams. The proportion of sand to be used for mortar briquets will depend upon circumstances, but for short time (seven day) tests good results are not usually obtained with natural cement if more than two parts of sand by weight are added to one part of cement. Portland cement may be tested at seven days with three parts of sand to one of cement. If too large a proportion of sand is used, the briquets are liable to be injured in handling, and very low strengths are not as accurately recorded by the testing machine.
The consistency of the mortar has such a marked influence on the strength obtained that its importance can hardly be overestimated. The difficulties attendant upon specifying the consistency of a given mortar have already been touched upon in § 116. The Committee of the American Society of Civil Engineers of 1885 recommended the use of a "stiff plastic" mortar, but this phrase has had various interpretations.
The present Committee in its progress report1 recommended the use of the Vicat apparatus: "In making the determination, 500 gr. (17.64 oz.) of cement are kneaded into a paste, and quickly formed into a ball with the hands, completing the operation by tossing it six times from one hand to the other, maintained six inches apart; the ball is then pressed into the rubber ring (§ 98) through the larger opening, smoothed off, and placed (on its large end) on a glass plate, and the smaller end 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 in contact with the surface and quickly released. The paste is of normal consistency when the cylinder (1 cm. in diameter and loaded to weigh 300 grams) penetrates to a point in the mass 10 mm. (0.39 in.) below the top of the ring. Great care must be taken to fill the ring exactly to the top".
The following simple test taken from French specifications will determine a good consistency of mortar to use for briquets. It should be capable of being easily molded into a ball in the hands, and when dropped from a height of one and a half feet on a hard slab, this ball should retain its rounded form without cracking. The mortar should also leave the trowel clean when allowed to drop from it. Were a smaller quantity of water used, the mortar would be crumbly and the ball would crack when dropped on the slab, while a larger amount of water would cause the mortar to adhere to the trowel and the ball would be flattened by striking the slab.
165. Another method of determining the proper consistency, which the author believes will prove very satisfactory, is to make several batches of mortar containing the same weights of cement and sand, but having different percentages of water. As each batch is mixed, the volume of the resulting mortar is measured by pressing it lightly into a metal cylinder (a small tin pail will answer the purpose), taking pains to fill the cylinder in the same manner each time. That batch of mortar which occupies the least volume, when thus lightly packed, is the one in which the amount of water used is most nearly correct. Should either the mortar which contained the least water or that which contained the most water chance to have the least measured volume, then more trials must be made until such a consistency is obtained that either more or less water will increase the bulk of the mortar. This method will give a consistency somewhat more moist than that which gives the highest results on short time cohesive tests, but it is believed that where briquets are made by hand, more uniform results will be obtained when the mortar is a trifle moist. This method is not suited to daily use, as it requires too much time, but is valuable as a check on one's ideas of proper consistency.
1 Proc. Amer. Soc. C. E., Jan. 1903; also Engineering News, Jan. 29, 1903, and Engineering Record, Jan. 31, 1903.
Tables 28 and 29 give a few of the results obtained by the author in tests to determine the effect of consistency on the tensile strength of natural cement mortars. All of the briquets were made in the usual manner and stored in fresh water until time of breaking. Each result given is the mean of from two to ten briquets. The letters affixed to each result indicate the degree of moisture which the mortar appeared to have when mixed, varying from "a," barely damp, to "i," so wet that the mortar could not hold its shape when laid on a glass slab.
a — barely damp.
b — very dry; no moisture shown on surface briquets. c — dry; slight moisture shown on surface briquets. d — trifle dry.
e — about right consistency. f — trifle moist.
g — moist.
h — very moist; would just hold shape.
i — extremely moist; would not hold shape.
The results in Table 28 were obtained with neat cement mortars of several brands of natural cement. The first point to be noted is the variation in the amount of water required by different samples to give the same consistency; thus, Brand An, sample N, when mixed with 35 per cent, water, appeared to have about the same consistency as did sample G of the same brand mixed with 30 per cent. It is also apparent that the strength of all samples is not affected alike by given variations in the amount of water used in mixing; comparing the results obtained when 45 per cent, water is used with that given when 25 per cent, water is used, it is seen that at seven days the wet mortar gives 42 per cent, of the strength obtained with dry mortar for sample 84 R, Brand Gn, while with the sample of Brand Hn the strength of the wet mortar briquet is but 16 per cent, of that given by the dry mortar. Of the six samples tested at seven days and twenty-eight days, three gave the highest strength at seven days when mixed with 25 per cent, water, and five gave the highest strength at twenty-eight days when 30 per cent, water was used. The results on Brand Ln show the greater proportionate gain with age of the wet briquets.