Concrete

Concrete (Lat. concrescere, to grow together), a name given by architects and engineers to a mixture of common lime, hydraulic lime, or hydraulic cement, with sand and gravel or broken stones. A mixture of sand and hydraulic lime is also called concrete. It is used in the construction of the foundations and walls of buildings, of fortifications and of piers in harbors or in the sea, of aqueducts, sewers, and other structures much exposed to the action of the elements, or subjected to great pressure, being, well adapted to these purposes in consequence of the hardness and insolubility it attains in solidifying. Formerly the name concrete was only applied when common lime was used, but that kind of concrete is now rarely made (except under extraordinary circumstances and by a newly applied process which will be noticed at the end of this article) when hydraulic lime or cement can be conveniently obtained. In the preparation of concrete with common lime, the London architects formerly employed one part of rich lime to six parts of unscreened Thames ballast, which usually consists of six parts of coarse pebbles to one of sand. The rule is that the slaked lime should a little more than fill the voids in the pebbles and sand.

Beton, or concrete made with hydraulic lime or cement, was employed in ancient times in some of the most renowned works of history. The factitious stones used by the Babylonians, Egyptians, and Phoenicians, as well as by the Greeks and Romans, were all a species of beton, or what is now called concrete. The Romans used it in large quantities in the construction of harbors and piers in the Mediterranean, and for aqueducts and roads, many portions of which exist at the present day. Their matrix was made of pozzuolana or trass, mixed with fat lime. The best modern concrete is made by using Portland cement, but a good article is also made with Teil lime and other hydraulic limes. An important quality that should be possessed by a lime or cement is that of setting slowly, to secure its more perfect crystallization and a firmer binding of the gravelly material. It is also desirable as allowing any portions of the material which may not have been perfectly slaked to expand in the process without rupturing the partially hardened structure; and this shows the importance of having the matrix well slaked and thoroughly incorporated before hardening begins. - Our advanced knowledge of the preparation of concrete is much owing to the experiments of M. Francois Coignet of Paris, whose manufacture of beton agglomere has a world-wide celebrity.

In this process, the matrix or paste is first prepared by mixing about three parts of hydraulic lime or cement with one part of water, and thoroughly triturating it in a mill until it is brought to a peculiar state of sticky consistency, which is best known by experience. Clean, sharp sand is then added in the proportion of about two parts to one of lime or cement, or one and a half to one of the paste. This is then well ground in a powerful mill and incorporated with an equal bulk, or a little more, of broken stone or coarse gravel. This may be formed into blocks of any required size, or the whole structure to be erected may be formed as a monolith. In either case, mechanical compaction is an important part of the process, and is effected by ramming the mixture with iron rammers weighing from 10 to 20 lbs. Blocks are made in moulds whose parts are separable. Layer after layer is thrown in and well condensed by ramming, and that this may be effected is one of the reasons why the amount of water should be carefully proportioned. The moulds are taken off in sections, and therefore may be of almost any shape, so that figures and traceries of any desired form may be copied.

The blocks are then left exposed to the air and weather for several weeks or months, sometimes receiving occasional sprinklings of water. In time they become as impervious to water as many natural stones, and capable of resisting the influences of frost in the open air, or the action of sea water. When monolithic walls are made, a form is used open at the top and bottom, in which the concrete is rammed till it is filled, when it is raised as successive layers are added. In this way many fine structures have been erected, particularly in France. Of the Vanne aqueduct for supplying water to Paris, 37 m. have been executed in beton agglomere. In the forest of Fontainebleau there are about 3 m. of arches, some of which are 50 ft. high. The whole structure, including arches and pipe, is one mass of solid masonry, without joints. A Gothic church at Vezinet,' near Paris, having a spire 130 ft. high, is also a monolith of concrete, and exhibits, it is said, every evidence of durability. The lighthouse at Port Said, the northern terminus of the Suez canal, is also built of beton of Teil lime and Port Said sand, and is a monolith 180 ft. high. The jetties which form the harbor at Port Said are built of huge blocks of concrete formed of the same material.

In their construction about 120,000 tons of Teil hydraulic lime were used. There were 25,000 blocks, each weighing 25 tons. The proportions were: lime in powder, 548 lbs.; desert sand, one cubic yard, mixed with sea water and poured into moulds, and hardened by exposure to the air for two or three months. M. Pascal, the chief engineer in the construction of docks at Marseilles, made a mortar in the proportion of Teil lime three parts, sand five parts, which was formed into a concrete by adding to one volume of mortar two volumes of broken stone. - One of the most extensive applications of hydraulic concrete in modern times was made by the French in their works at the harbor of Algiers, commencing in 1831. The mole which shelters the harbor is so exposed to winds that breaches were constantly being made in it by the force of the sea, and to such an extent that in former times the Moors were compelled to keep employed a large number of workmen to repair it, at an annual expense of more than $60,000. When the French commenced the reconstruction of the mole in 1833, after the failure of partial repairs, the first operation was to raise an outer embankment of large stones, under whose cover the foundation might be partially restored.

It was intended to form this embankment of pierre perdue, and to employ natural stones of 100 to 140 cubic feet, and 212,000 cubic feet were thus used at a great expense; but during the next winter the embankment was completely destroyed by the waves, one block of 141 cubic feet having been carried completely across the mouth of the harbor. It thus became necessary to rebuild the entire embankment of blocks so large that no action of the sea could move them. Under the circumstances it was found that the required size would be about 353 cubic feet. The expense of quarrying and transporting such enormous blocks of stone led to the employment of artificial stone, or beton. Two kinds of blocks were manufactured; the first in the water in the place they were intended to occupy, and the second on shore, to be afterward thrown into the sea. The first were made by immersing beton in caissons which were constructed of timber and plank, and lined on the inside with tarred cloth, the four sides being connected by hinged angle irons so as to be readily unshipped. The second kind of blocks were prepared on shore in moulds of timber and plank, resting upon an inclined plane, ending at the point where the block was to be sunk.

After the beton had properly set, the sides were removed and the block launched into the sea. In making the mortar for the first description of blocks, one part of rich lime was slaked and made into a paste and then mixed with two parts of Italian poz-zuolana, while for" the blocks made on shore pozzuolana was mixed with an equal quantity of sand. The lime employed was from a hard, granular limestone, weighing about 156 lbs. per cubic foot. Slaked and reduced to the consistence of a thick pulp, it was found to absorb once and a half its weight of water and to increase in bulk 75 per cent. One volume of this with two volumes of pozzuolana made two volumes of mortar. To form the concrete, one part of mortar was incorporated with two parts of broken stone of about 1 1/2 cubic feet. The entire work was performed in five years at a cost of less than $420,000, notwithstanding that the mole at the time of the occupation of Algiers by the French army in 1830 was in a state of complete dilapidation, in spite of the extensive repairs which had been annually made by the Moors for two centuries.

The success met with in this work established two important facts: 1, that blocks of beton can be made sufficiently strong to resist the action of the heaviest waves, and form indestructible masses; 2, that these blocks are immovable by the waves when constructed above a certain size, determined by experiment to be about 353 cubic feet. One great advantage in the use of blocks of concrete over those of stone is in the diminished cost of handling and transportation, because the concrete blocks can be made at the place where they are used. The beton used by Vicat for the bridge of Souillac, upon the Dordogne, was composed of 26 parts of hydraulic lime in paste, 39 of granitic sand, and 66 of gravel. It was found to diminish in volume in the proportion of 1.31 to 1. - The immense masses of concrete which form the foundations of the East River bridge between New York and Brooklyn are composed of one part of Rosendale cement, two of sand, and four of coarse beach gravel from an inch to 2 1/2 inches in diameter. The cement and sand were first mixed with water in a mill, and afterward incorporated with the gravel by means of shovels used by hand, the latter operation being performed for the most in the caissons, which were 172 ft. in length by 102 ft. in width.

As this concrete lies below the bed of the river, and is therefore confined, there is little doubt that it will endure for centuries. The concrete blocks which form the foundation of the piers that are now (June, 1873) under construction at the Battery in New York harbor, are composed of one part of Portland cement, two of sand, and five of broken trap rock from the Palisades on the Hudson. The matrix of cement and sand was first formed by hand, and afterward incorporated with the broken stone by the same process. It was then shovelled into forms and compressed by ramming. After the beton had set, the forms were removed, and the blocks were exposed to the weather for two months. The concrete with which the natural stone masonry above the water floor is filled in is mixed in the same way, and rammed down in the spaces left by the walls and arches. Concrete is used as a foundation for roads and for cement walks in parks. A good article for this purpose may be made by using equal parts of Rosendale and Portland cements with sand and broken stone. - The artificial stone of Mr. Frederick Ransome, patented in 1856, made by the patent concrete stone company at East Greenwich, near London, England, is one of the remarkable productions of the day, not only furnishing an excellent building stone, but also a material for making a superior quality of grindstones.

This concrete is composed of sand held together by silicate of lime, and is formed by an indirect process in which silicate of soda is decomposed by chloride of calcium. The silicate of soda is made by digesting flints with a solution of caustic soda of about 1.2 sp. gr. in boilers, under a pressure of 70 or 80 lbs. to the square inch. The solution of silicate of soda which results should have a specific gravity of about 1.7. Clean sharp sand, which has been dried in a current of hot air, is then mingled with a little finely ground chalk or marble, and the silicate of soda is added in the proportion of a gallon to the bushel of mixture, and the whole is thoroughly triturated in a mill. It becomes pasty during the operation, and susceptible of being moulded into any desired form. The mould blocks, formed by ramming, are then nearly saturated with a solution of chloride of calcium of about 1.4 sp. gr. Double decomposition immediately takes place, whereby silicate of lime and chloride of sodium, or common salt, are produced; and in a few minutes the blocks become hard enough to be handled.

They are then immersed in vats containing the solution of chloride of calcium at a temperature of about 212° F., in order that a thorough decomposition of the silicate of soda and formation of silicate of lime may be effected. The latter substance, being insoluble and firmly enveloping the grains of sand, forms a firm and it is believed durable block of stone. The chloride of sodium, being soluble, is then discharged by displacement with water. It might be supposed that the stone would be left in a very porous condition, but experiment has proved that this is not the case. Dr. Charles T. Jackson of Boston, after exhausting the air from the stone by means of a vacuum, found that it would absorb only about 16 per cent, of moisture. Under ordinary circumstances it may be considered as practically impervious to water, and is found to be quite as firm as most sandstones. Prof. Ansted of King's college, London, found that it wduld resist a crushing force of 4,200 lbs. to the square inch, and that its tensile strength was equal to 360 lbs. to the square inch; while natural Portland limestone, Bath stone, and Caen stones possessed respectively only 201, 145, and 140 lbs. tensile strength to the square inch.

It is said that the best material that has yet been found for grindstones is that made by this process. The grains of sharp sand are held so firmly by the matrix of silicate of lime that they are capable of being made to cut away iron or steel with great rapidity. Experiments made by the Messrs. Donkin of Ber-mondsey showed that, compared with the best Newcastle stone, the Ransome stone had a capacity of cutting away steel in the proportion of 50 to 1. The artificial stone possesses the advantage, for large grindstones, of having more uniformity of structure than can be found in large blocks of natural stone, and it can be more readily formed into grindstones of various sizes. - The American building block company produce an artificial stone with silicious sand and lime, by subjecting the mixture to great pressure. There are two processes. The first, that of Foster, is as follows: Rich lime is slaked with a small quantity of water, and mixed with moist sand in sufficient quantity to fill the voids. The mortar is then put in moulds and subjected to great pressure, which increases the tendency to formation of silicate of lime.

The pressure should be so great that the grains of sand are forced into almost actual contact, so that when the chemical action is completed nothing but silica, and silicate of lime holding it together, will remain in the block; the surplus of lime compounds being forced to the outside, where, by slowly hardening in the presence of air, they form a coating which does not detract from the value of the stone. Vanderburgh's process employs quick instead of slaked lime, which being mixed with moist sand is slaked by steam under pressure. A more complete union between the lime and sand is thus effected, principally in consequence of the greater degree of heat employed; and it is also found that the stone hardens more rapidly in Vanderburgh's than in Foster's process. The Union stone company of Boston, Mass., produce an artificial stone in imitation of several natural stones, the fabrication of which is founded upon the discovery of M. Sorel, a French chemist, that the double oxide and chloride of magnesium forms an excellent hydraulic cement. The materials used are magnesite (native carbonate of magnesia, which is found in various parts of the United States and Canada), chloride of magnesium, and sand, or powdered marble or other stone.

The magnesite, reduced to caustic magnesia by burning, is then mixed with sand or powdered stone in the desired proportions. A solution of chloride of magnesium is added, and the mixture is triturated in a mill, when it is turned into moulds and firmly rammed. Hardening commences immediately, so that the block may be removed as soon as the ramming is done. - In consequence of the great advance in the knowledge of limes and cements that hastaken place within the last few years, and the increased attention now being paid to the subject by chemists, engineers, and architects, it is probable that various kinds of artificial stone may in the future be used as materials of construction, in many cases, with considerable advantage. - Full information in regard to the manufacture of concrete may be found in the "Practical Treatise on Coignet Beton and other Artificial Stone," by Brevet Major Gen. Q. A. Gillmore, U. S. A. (New York, 1871); "Report on the Hydraulic Lime of Teil," by Leonard F. Beckwith, C. E. (New York, 1873); and Henry Reid's "Treatise on Concrete" (London, 1869).