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THEORY ANI) PRACTICE OF ENGINEERING.

Book II.

The lime is sometimes placed in the middle of a heap of sand, and after water has beenthrown on it, it is amalgamated together by hoes ; after remaining in this state a few hours,it sets and is tit for use : by this means the lime takes up a larger quantity of sand ; 72bushels of good stone lime, and 18 yards of sand, when formed into mortar will have acubical content of 315 feet.

Sand is the produce of the spontaneous disintegration of the granitic, schistose, or cal-careous rocks, and their specific gravity is the same as that of quartz, a cubic foot of whichwould weigh 162.^ pounds, but when in the state of sand it does not weigh more than 75pounds, so that the interstitial parts are more than equal to the sand itself: the proportionof lime to fill up these spaces between the particles of sand must be ascertained before wecan have a solid mass, and the difference between the weight in its loose and compactstate ought to give us the quantity of lime requisite to bind the particles firmly together.

Concrete is of very ancient use, and formed the foundations as well as hearting of thewalls in the remotest ages, and among all nations : it is made by mixing lime, coarse graveland sand together, with a moderate quantity of water. The lime should be reduced to thestate of a fine powder, which is done immediately it is brought from the kiln, by grindingit in a mill, or by pounding it; when used it is slaked and not before, the ordinarymethod employed is to mix it with the other ingredients as quickly as possible, and thenadd the water immediately before it is put into the barrow and wheeled to its destination,or where it is to be thrown down.

Neither gravel nor sand alone will form a perfect concrete, for when large pebbles aremixed with quicklime and water, they are not in any way held or cemented together, butwhen fine sand is used in the ordinary proportions of common mortar a concrete is formed;and a mixture of coarse with the fine sand renders the mass still more compact: when allthe materials are properly mixed together, the lime combines with the fine sand only, andcements the pebbles or larger stones together, forming a rubble, so that the proportions ofsand ought to be precisely those required to make mortar of the best quality.

In the sea wall at Brighton the proportions were six parts of large shingle and well-siftedsand, and one part of grey chalk lime fresh from the kiln ; these were not mixed togetherat one time, but three parts of sand and one part of lime were first made into mortar ofthe ordinary kind, which was thrown with an equal quantity of beacli shingle into a pug-mill, where they were well mixed together before being used.

Artificial stone has been formed of concrete, and the process patented by Mr. Ranger ;the moulds have at the bottom a flat board, made larger than the size of the intendedstone to be cast ; the four sides are held together by iron cramps, tightened up by meansof wedges : when the moulds are prepared they are laid out to receive the mixture ofgravel, sand, quicklime, and boiling water, which is thrown into them as rapidly aspossible, and kept constantly rammed down until the mould is full ; the surface is thenfloated over with mortar and rendered quite smooth.

Boiling water has the advantage of causing the lime to slack more rapidly, and thereforefewer moulds are required ; in half an hour these artificial stones are sufficiently set toallow of the moulds being taken to pieces, after the trenails are drawn which kept the sidestogether ; two holes are left in the concrete block to attach any tackle that may be requiredto hoist them to their destined position : when the boards which form the mould are allwithdrawn, the blocks are put up to acquire hardness, which they do in two or three months.

Whenever concrete is used for the foundation of a building, it should be thrown from asgreat a height as possible, which compresses it into a more solid mass : its depth whenlaid in trenches, or spread over an entire surface, should never be less than 4 or 5 feet, andwhere great weights are to be borne not less than 6 feet.

The hydraulic mortar in the construction of the Menai Bridge in North Wales , madeof Aberthaw lime and sand, was found to answer admirably well ; it was composed ofone measure of lime to two of sand; that used by Perronet at the bridge of Neuilly inFrance had equal measures of lime and pounded tile from the Neuilly kilns, groundvery fine.

The artificial hydraulic lime made by Guyton de Morveau at Lena, in Sweden , consisted,according to Bergman, of ninety parts of carbonate of lime, six of clay, and four of oxideof manganese.

General Treussart’s system for making artificial puzzolana is, first to reduce soft redbricks to powder and mix them with common lime which has been some time slaked, in theproportion of two measures of brick-dust to one of lime paste, with the addition of as muchwater as is necessary to incorporate them thoroughly. Common lime, sand, and brick-dustin equal quantities he also found to make a good hydraulic mortar : he observes that allhydraulic limes set much quicker in summer than in winter, and may be considered goodwhen either as hydrates, or mixed with sand, they set in the course of eight or ten days inthe summer, and .acquire a hardness sufficient to resist the pressure of the finger.