CONCRETE 209 lime should a little more than fill the voids in the pebbles and sand. Beton, or concrete made with hydraulic lime or cement, was em- ployed in ancient times in some of the most renowned works of history. The factitious stones used by the Babylonians, Egyptians, and Phoanicians, as well as by the Greeks and Eomans, were all a species of beton, or what is now called concrete. The Romans used it in large quantities in the construction of har- bors 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 rup- turing 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 tritu- rating it in a mill until it is brought to a pe- culiar 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 compac- tion is an important part of the process, and is effected by ramming the mixture with iron rammers weighing from 10 to 20 Ibs. Blocks are made in moulds whose parts are separable. Layer after layer is thrown in and well con- densed 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 there- fore may be of almost any shape, so that fig- ures 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 sprink- lings of water. In time they become as im- pervious 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 erect- ed, particularly in France. Of the Vanne aqueduct for supplying water to Paris, 37 m. have been executed in beton agglome>6. In the forest of Fontamebleau there are about 3m. 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 ter- minus of the Suez canal, is also built of beton of Teil lime and Port Said sand, and is a mono- lith 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 propor- tions were : lime in powder, 548 Ibs. ; desert sand, one cubic yard, mixed with sea water and poured into moulds, and hardened by ex- posure 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 ex- posed 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 em- bankment 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 em- bankment 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 re- quired size would be about 353 cubic feet. The expense of quarrying and transporting such enormous blocks of stone led to the em- ployment of artificial stone, or beton. Two kinds of blocks were manufactured ; the first
