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compact shell and the poorer kernel should be made in one operation. Where this is not possible, and the shell is added subsequently, numerous iron ties should be used.

"2. From the chemical point of view, cements or hydraulic limes, rich in silica and as poor as possible in alumina and ferric oxide, should be used, for aluminate and ferrate of lime are not only decomposed and softened rapidly by sea-water, but they also give rise to the formation of double compounds, which in their turn destroy the cohesion of the mass by producing cracks, fissures, and bulges. The salts contained in sea-water, especially the sulphates, are the most dangerous enemies of hydraulic cements. The lime is either dissolved and carried off by the salts, and the mortar thus loosened, or the sulphuric acid forms with it crystalline compounds as basic sulphate of lime, alumino-sulphate and ferro-sulphate of lime, which are segregated forcibly in the mortar, together with a large quantity of water of crystallisation, and a consequent increase in volume results. The separation of hydrate of magnesia is only the visible but completely innocuous sign of these processes. The magnesia does not in any way enter into an injurious reaction with silica, alumina, or ferric oxide, it is only displaced by the lime from its solution in the shape of a flocculent, slimy hydrate, which may be rather useful in stopping the pores, but can never cause any strain or expansion, even if it subsequently absorbed carbonic acid. The carbonic acid, whether derived from air or water, assists the hydraulic cement as a preservative wherever it comes into contact with the solid mortar. It could only loosen the latter if present in such an excess that bicarbonate of lime might be formed.

"3. The use of substances which render the mortar, at any rate in its external layers, denser and more capable of resistance. Such substances

are

"(a) Sesquicarbonate of Ammonia (from gas liquor) in all cases where long exposure to the air is impossible. Such a solution applied with the brush, or as a spray, and then allowed to dry, converts the hydrate of lime into carbonate of lime. The latter is not acted upon by the neutral sulphates present in sea-water. It must be repeated that it is a decidedly erroneous opinion that the texture of otherwise sound cements is injured by the action of carbonic acid; on the contrary, it renders them more capable of resistance, except in the above-mentioned case, which is extremely rare, when bicarbonate of lime is formed and goes into solution.

"(B) Fluosilicates, among which magnesium fluosilicate is most to be recommended. The free lime is converted into calcium fluoride and silicate of lime, and, in conjunction with the liberated hydrate of magnesia, these new products close the pores of the mortar. Both salts are sufficiently cheap to be used on a large scale.

"(2) Last, not least, Barium Chloride. Two or three per cent. of the weight of the cement is dissolved in the water with which the concrete is mixed. This forms perfectly insoluble barium sulphate with the sulphates.

of the sea-water, while the magnesia remains in solution as magnesium chloride. Although in this case there can be no further closing of the pores, yet the insoluble barium sulphate, which is formed, affords some protection and does not give rise to any increase of volume (swelling). From 2 to 3 per cent. of barium chloride does not in any way diminish the strength, as has been proved by the comparative tests of English and German cements. Frequently the strength of the mortar is increased by this addition. This substance is only to be used in the external, perfectly watertight skin of concrete; in other words, in the 4 to 8-inch coating, composed of 1 cement, 1 to 2 sand, and 3 to 4 coarse gravel, flint, broken stone, &c."

Practical Notes on Mixing Concrete for Marine Work.

1. A heavy aggregate is desirable. If mixed by hand, the materials should be laid out on a platform of deals, in order to secure freedom from dirt and impurities, and covered by the cement in a thin layer. The whole should be turned over thrice dry, and as many times wet, before depositing.

2. The concrete should not be tipped from a height greater than 6 feet, or there will be a tendency for the heavier portions of the aggregate to separate from the lighter. For great depths, shoots may be employed with men stationed at the foot to shovel the mass immediately into position. The work should be well rammed and consolidated.

3. As many rubble burrs, or stone plums, should be imbedded as the fluid concrete can adequately enclose. No two burrs should be in contact, and none should be set within 12 inches of the face of the wall. If the burrs are porous, they should be wetted before insertion.

4. The concrete should be deposited without delay after mixing, and should remain entirely undisturbed during setting. After the setting of each layer, the surface should be prepared for the reception of the next layer by picking, washing, and sweeping. In mass work, layers should not exceed 2 to 4 feet in height.

5. Concrete blocks should not be used under 14 days after mixing, and preferably the period will be extended to three or four weeks.

6. Concrete bags have a tendency to break away at the ends. Consequently, they should be slightly longer than the nett length required.

7. Wind screens should be provided in windy weather, otherwise the cement will be largely wasted, even if the concrete be not allowed to suffer thereby.

8. Concrete mixing should be avoided as far as possible during keen frost, except in situations where the concrete is deposited directly under water, or is soon afterwards covered by the tide. Where continuous operations are essential on shore, artificial warmth from braziers and fires may be utilised to raise the surrounding temperature, and salt-water may be employed in mixing on account of its lower freezing point. An American

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practice is to dissolve 1 lb. of salt in 18 gallons of water when the temperature is 32° F., and to add 3 ounces for every 3° of lower temperature. The surface of such work, left for the night, must be protected by boards, tarpaulins, sacking, gravel, or littered straw.

Strength of Concrete.

Compressive Strength.-The following results were obtained by Mr. Grant.* Experiments were undertaken with 12-inch cubes of compact concrete made with Portland cement, weighing 110.56 lbs. per bushel, and having a tensile stress of 427 lbs. per square inch after seven days' immersion in water. The tests took place at the end of twelve months.

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Experiments made with 9-inch cubes of the concrete (6 of gravel and broken stone to 1 of Portland cement) used in the construction of the Vyrnwy Dam gave 84-23 tons per square foot as the lowest resistance to compression in the case of a block little more than three months old, and 298.6 tons per square foot as the highest resistance in the case of a block three years old. The mean resistance to cracking, under compression, of all the blocks tested between two and three years after moulding was 215.6 Still higher results were obtained from blocks cut out of the hearting of the actual work. The mean resistance to cracking, under compression, of 19 blocks, between one and two years old, was 263 tons per square foot.

tons.

Transverse Strength. In an experiment by Mr. Colson † a beam of 9 to 1 concrete, 28 days old, 21 inches wide, 9 inches deep, and 3 feet 9 inches clear span, fractured with a weight of 1.044 ton applied centrally. The coefficient derived from this, for the unit beam, 1 foot wide, 1 foot deep, and 1 foot span, becomes 4 tons.

* Grant on "Strength of Portland Cement,” Min. Proc. Inst. C.E., vol. xxxii.
+ Min. Proc. Inst. C. E., vol. liv.,
p. 270.

In an experiment by Mr. Sutcliffe with a concrete block cut from a dock wall at Liverpool, and composed of 8 parts of gravel and broken brick to 1 of Portland cement, with rubble burrs incorporated, the size of the block being 25 inches wide by 23 inches deep, and the clear span 12 feet, fracture resulted from a central concentrated load of 3·25 tons, giving a coefficient of 5 tons for the unit beam.

Sir Benjamin Baker's experiments,* in which the weight of the beam itself was included, yielded the following unit breaking weights :

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Some Sample Compositions of Concrete.

1. At Arbroath, used by Mr. W. Dyce Cay, in 1887, for a dock entrance— 7 sand, gravel, and broken stone.

1 Portland cement,

2. At Sydney, used by Mr. C. W. Young, in 1883, for a graving dock

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3.61 bluestone, gauged through a 24-inch ring.

3. At Belfast, by Mr. W. Redfern Kelly, in 1888, for a graving dock. (a) For foundations in tideways

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4. At Newport, Mon., by Mr. G. D. Pickwell, in 1889, for a graving dock

1 Portland cement,

10 broken steel slag, weighing 26 feet per ton, in pieces not larger than
24-inch cubes for bulk and 2-inch cubes for face work-in both cases
unscreened from dust.

5. At Greenock, by Mr. W. R. Kinipple, between 1878-86, for dock walls

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6. At Ardrossan, by Mr. R. Robertson, circa 1889, for dock walls.

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IRON AND STEEL.

Cast iron, wrought iron, and steel are essentially the same substance in combination with different proportions of other constituents. The principal ingredient in this connection is carbon, and the following percentages are generally recognised as forming the distinctive compositions of the three classes of metal, viz. :—

From 0 to 1 per cent for wrought iron.

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Unfortunately, this quantitative differentiation is not susceptible of too strict interpretation, because other ingredients, besides carbon, exercise a powerful modifying influence upon the compounds. Their properties also depend upon the form in which the carbon is present-whether as specks of graphite, or free carbon, mechanically mixed and easily detected, or in such intimate chemical combination as to be indistinguishable from the metal itself.

A practical distinction is founded upon the behaviour of a bar of metal under certain treatment, as follows:

Steel attains great hardness when suddenly cooled, from a high temperature, by immersion in water or oil. This process has no effect upon wrought iron.

Steel which has been hardened in this way may be softened again, or tempered, by heating it and allowing it to cool gradually. Cast iron may

be hardened, but it cannot be tempered.

One drawback to the efficacy of these tests is that some modern steels, containing elements other than carbon and iron, are made softer, and not harder, by sudden cooling.

A third attempt at drawing a distinction relies upon the results obtained in the testing machine, but this method is too artificial to be of any practical value.

Altogether, it must be confessed that, while the differences in the physical properties of iron and steel are sufficiently marked to preclude any misconception, it is no easy matter to lay down any definite line of demarcation between the metals themselves. Steels containing less than 5 per cent. of carbon form an intermediate class insensibly shading into, and gradually acquiring the characteristics of, wrought iron. Such steels are commonly designated mild steels, and they furnish the bulk of the material used for structural purposes. Those compounds containing a higher percentage than 15 imperceptibly merge into the class of cast irons.

The influence exerted by carbon in modifying the physical characteristics of iron, while largely dependent upon the manner in which it enters into combination with it, may be stated in general terms as follows:

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