Nature And Proportion Of Water In Cement Mortar

No more water should be used than is necessary to make the cement fit for use, an excess produces porosity and retards the process of hardening.

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.

Shape Of Briquette

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 briquette is placed in the clips of a testing machine (see p. 182), and broken by slow tension. Each of the figures shows the briquette in the clips ready to be attached to the machines.

There is no doubt that the shape of the briquette has an important influence upon its strength.

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.

Shape Of Briquette 30079

Fig. 83.

Shape Of Briquette 30080

Fig. 84.

Shape Of Briquette 30081

Fig. 85.

Shape Of Briquette 30082

Fig. 85a.

Fig. 86. Curve of jaws.

Fig. 86. Curve of jaws.

Shape Of Briquette 30084

Fig. 86a.

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.

Tests For Coolness

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.

Pats should also be made about 3 inches in diameter and 1/2 inch thick, gauged in neat cement with thin edges, and placed upon pieces of glass or other non-porous material.

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.

With slow-setting cements surface cracks commencing at the centre are merely the result of the surface drying too rapidly.1

The other pat is left in the air, and should remain of a dark grey colour. If it is yellow or ochrey, the cement contains too much clay, and it is likely to be deficient in tensile strength.

Additional Tests For Portland Cement

Besides the ordinary tests above mentioned, the following rough tests will give an indication as to some important qualities of the cement before using it.

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.