Grant's experiments show that with 19 per cent of water, making the briquettes into a stiff paste, they stood from 28 to 40 per cent more tensile stress than when 25 per cent of water was used, making the cement of the consistency of stiff grout.2
9 oz. of water to 40 oz. of cement, or about 22 per cent, is recommended by Messrs. Gibbs and Co.
With hot or quick-setting cements neat more water will be required than with cool or slow-setting cements.2
With mixtures of 1 cement and 3 sand, about 11 to 12 per cent may be used for those which set in less than thirty minutes, and 10 per cent for those that take longer.
Briquettes mixed with salt water are rather stronger than those with fresh water, but salt water should not be used in cement intended for building or rendering the walls of houses to be inhabited, because it tends to keep them damp. Dirty water would of course injure the cement by introducing impurities which would prevent proper adhesion, and hot water should not be used except for experiments to make the cement set more quickly.
The cement to be tested is formed into a briquette shaped in one of the forms shown in section in Figs. 83 to 86.
1 Grant, M.P.I.C.E., vol. lxii. p. 124. 2 Grant, M.P.I.C.E. 1880, vol. lxii. p. 158.
The transition from the thicker parts of the briquette to the minimum or breaking section should be gradual - all angles avoided - the shoulders should be so shaped that the bearing of the clips upon them is uniform - the clips hung so that the stress shall pass through their central points.
The form first used in this country is shown in Fig. 83; the principal angles were afterwards rounded off as shown in Fig. 84, which is not a good form, for it generally breaks as shown by the dotted line, and not at the minimum section.
Whenever the clips bear upon a considerable part of the surface of the briquette, as in Figs. 83, 84, it is very difficult to prevent them from pressing more at one point than another, and thus causing want of uniformity in the stress.
To avoid this the clips are sometimes done away with, and the briquette is suspended by pins with knife-edges passed through holes in its ends, as in Figs. 85, 86, which represent one of the forms used by Mr. Grant in his experiments.
The last form adopted by the Board of Works is shown in Figs. 86, 86a. The change of form in the briquette is very gradual; the clips are rounded so as to bear on it at only four points, are hung on knife-edges kk, and have loose joints at BB, so that the stress may pass through their centre points. This form of briquette seems to be the best that has been introduced.
Fig. 86. Curve of jaws.
It will be understood that the briquettes shown in the figures are all l1/2 inch x l1/2 inch =21/4 square inches, at the waist or part intended to be broken. This is clearly seen in Figs. 85, 85a. In many cases the weakest section of the briquette is made only lxl square inch (see p. 183), and briquettes of this size are said to give a higher resistance per square inch than the larger ones. In testing, the mean of six briquettes should be taken.
In some cases cement which appears perfectly good in every way has a tendency to crack and swell when placed under water. This action, which is commonly known as "blowing," is caused by the cement being under-burnt, by its containing an excess of lime, or by its not being properly "cool," that is, free from unslaked particles.
In order to detect this tendency to blow, the briquette placed under water should be carefully watched.
If it is inclined to blow, it will show signs of expansion after a day or two under water; in extreme cases the samples will entirely break up, but a few cracks about the edges are the commonest indications.
One is placed under water and watched; if twenty-four hours after its immersion there are no fine cracks round the edges, the cement may be considered safe.
1. A bottle is filled with paste made from the neat cement. If, after the cement has been set some days, the bottle remains uncracked, it may be considered that the cement is not too hot.
If the cement has shrunk within the bottle it is probably under-burnt; the shrinkage can be detected by pouring in a little coloured water.
2. Another test is to fill a piece of glass tubing with neat cement paste, and to note whether there is any shrinkage.
3. A rough method of ascertaining whether the cement is cool enough for use, is by plunging the bare arm into the cement.
If it feels hot the cement has not been sufficiently weathered, and requires further turning over.