The paste is of normal consistency when the cylinder 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 trial pastes arc made with varying percentages of water until the cor rect consistency is obtained. The Committee has recommended, as normal, a paste the consistency of which is rather wet, because it believes that variations in the amount of compression to which the briquette is subjected in moulding are likely to be less with such a paste. Having determined in this manner the proper percentage of water required to produce a paste of normal consistency, the proper percentage required for the mortars is obtained from an empirical formula. The Committee hopes to devise a formula. The sub ject proves to be a very difficult one, and, although the Committee has given it much study, it is not )et prepared to make a definite recommendation.
Note. The Committee on Standard Specifications for Cement inserts the following table for temporary use to be replaced by one to be devised by the Committee of the American Society of Civil Engineers.
50. Time of Setting. The object of this test is to determine the time which elapsed from the moment water is added until the paste * From the report of a committee of the American Society for Testing Materials.
ceases to be fluid and plastic (called the "initial set"), and also the time required for it to acquire a certain degree of hardness (called the "final" or "hard set"). The former of these is the more important, since, with the commencement of setting, the process of crystalliza tion or hardening is said to begin. As a disturbance of this process may produce a loss of strength, it is desirable to complete the opera tion of mixing and moulding or incorporating the mortar into the work before the cement begins to set. It is usual to measure arbi trarily the beginning and end of the setting by the penetration of weighted wires of given diameters.
For this purpose the Vicat Needle, which has already been de scribed, should be used. In making the test, a paste of normal consist ency is moulded and placed under the rod L, Fig. 2, as described in a previous paragraph. This rod bears the cap D at one end and the needle H, 1 mm. (0.039 in.) in diameter, at the other, and weighs 300 gr. (10.5S oz.). The needle is then carefully brought in contact with the surface of the paste and quickly released. The setting is said to have commenced when the needle ceases to pass a point 5 mm. (0.20 in.) above the upper surface of the glass plate, and is said to
have terminated the moment the needle does not sink visibly into the mass.
The test pieces should be stored in moist air during the test; this is accomplished by placing them on a rack over water con tained in a pan and covered with a damp cloth, the cloth to be kept away from them by means of a wire screen; or they may be stored in a moist box or closet. Care should be taken to keep the needle clean, as the collection of cement on the sides of the needle retards the penetration, while cement on the point reduces the area and tends to increase the penetration. The determination of the time of setting is only approximate, being materially affected by the tem perature of the mixing water, the temperature and humidity of the air during the test, the percentage of water used, and the amount of moulding the paste receives.
The following approximate method, not requiring the use of apparatus, is sometimes used, although not referred to by the Com mittee. Spread cement paste of the proper consistency on a piece of glass, having the cement cake about three inches in diameter and about one inch thick at the center, thinning towards the edges. When the cake is hard enough to bear a gentle pressure of the finger nail, the cement has begun to set, and when it is not indented by a con siderable pressure of the thumb nail, it is said to have set.
51. Standard Sand. The Committee recognizes the grave objec tions to the standard quartz now generally used, especially on account of its high percentage of voids, the difficulty of compacting in the moulds, and its lack of uniformity; it has spent much time in investi gating the various natural sands which appeared to be available and suitable for use. For the present, the Committee recommends the natural sand from Ottawa, Ill., screened to pass a sieve having 20 meshes per linear inch and retained on a sieve having 30 meshes per linear inch ; the wires to have diameters of 0.0165 and 0.0112 inches, respectively, i.e., half the width of the opening in each case. Sand having passed the No. 20 sieve shall be considered standard when not more than one per cent passes a No. 30 sieve after one minute continuous sifting of a 500 -grain sample.
52. Form of Briquette. While the form of the briquette recom mended by a former Committee of the Society is not wholly satis factory, this Committee is not prepared to suggest any change, other than rounding off the corners by curves of i-inch radius, Fig. 3.