The tensile stress that a cement will bear depends greatly upon the manner in which the test is made, the form of briquette, the method in which the cement is gauged, the amount of water used, etc. etc.

Method Of Making Briquettes

The briquettes, whether of neat cement or of cement and sand, are made in brass moulds of the form shown in Fig. 86.

The following directions are taken chiefly from Mr. Grant's papers, the circulars of Messrs. Currie and Sons, Messrs. Gibbs and Co., and Mr. Faija.

Briquettes Of Neat Cement

Supposing the briquettes are to be made of neat cement, and of the 1-inch square section, the procedure would be as follows : -

The cement from the different casks or sacks should be turned well over, samples from different parts of the heap should be mixed, and if hot should be spread out, especially in hot weather, so as to become thoroughly cool, and water should be carefully added, noting the proportion required to bring the mixture into such a condition that repeated pats with the trowel will bring the moisture up to the surface.

Two cakes of about 2 or 3 inches diameter and 1/2-inch thickness should be made, and the time noted in minutes that they take to set sufficiently to resist the finger nail.

"If after two hours the cake is soft enough to take the impression of a finger nail, it may be considered slow-setting."2

If the cement should be slow-setting, all the briquettes may be made at once, but if quick-setting, only three or four at a time, and if very quick only one should be made at a time.

The moulds should be cleaned with a greasy cloth, and a number of pieces of thin blotting-paper each rather larger than a mould are then placed upon a marble, glass, or slate slab, and on these the moulds are placed.

Then about 4 lbs. cement, or enough for ten briquettes, are weighed and placed upon the non-absorbent slab in a heap; in the centre of this a hole is made, into which from a graduated glass is gradually poured the quantity of water previously determined, the mixture being worked with a trowel until it becomes a short, harsh paste, the water is then discontinued, but the working with the trowel continued until the paste becomes pat and smooth.

1 M.P.I.C.E., vol. lxxi. p. 266.

2 Grant, M.P.I.C.E., vol. Ixii. p. 104.

With this paste the brass moulds are filled as quickly and solidly as possible, a small trowel being used, and the mortar beaten or lightly rammed and gently shaken until all the air has been driven out of it and the mortar has become elastic. The surplus should be cut off level, and the surface left smooth.

The whole operations of making the paste and filling the moulds until the briquettes are placed on one side should not take more than five minutes. The quicker it is done, provided it is done properly, the better, for it is most important that the cement should be at rest before the setting action commences.

When the moulds have been filled they should be numbered and laid aside, in some place where they will be secure against shaking or vibration in a wet damp atmosphere, or covered with a damp cloth till they have set sufficiently to be taken out of the moulds.

This will probably be in less than twenty-four hours, the time varying according to the rate of setting of the cement, but it must be done with great care to avoid flaws, and not too soon, or the briquettes will lose their shape and be difficult to fit into the clips of the machine.

The briquettes should then be placed upon "sheets of glass or on slabs, and laid in a flat box having a cover lined with several layers of linen, woollen, or cotton cloth, kept damp. In this box they are kept until they have hardened sufficiently to be put into water. This will vary from one or two hours to a day or more, but for uniformity, unless in the case of specially slow-setting cements, briquettes of neat cement may be kept for twenty-four hours before being transferred from this box to the shallow tanks in which they are to remain until the moment of testing.

"The numbers on the neat cement briquettes may be made with a sharp point or with a strong pencil.

" The water in the testing room should be kept at a temperature as nearly uniform as possible, say from 60° to 70° Fahr., but if the boxes in which the briquettes be kept are covered, moderate changes of temperature will not materially affect the results."1

Briquettes of Cement and Sand are made in a similar manner. About 1 lb. of cement and 3 lbs. carefully washed standard sand will make ten of the 1-inch briquettes. The proportions of water required will be from 8 to 10 per cent, and the mixture must be beaten into the mould with a spatula or light wooden mallet, so as to be as solid as possible.

The briquettes are treated like those made with neat cement, except that they should not be removed from the moulds until at least forty-eight hours after they are made, and should be kept in the boxes another forty-eight hours before they are numbered.