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 inscriptions, the majority belonging to the end of the 4th and the beginning of the 5th centuries, have been discovered. It was taken and destroyed by Attila in 452. Considerable remains of the ancient town have been found—parts of the city walls, the sites of the forum and the theatre, and probably that of the arms factory. The objects found are preserved at Portogruaro, 1 m. to the N. The see of Concordia was founded at an early period, and transferred in 1339 to Portogruaro, where it still remains. The baptistery of Concordia was probably erected in 1100.

CONCRETE (Lat. concretus, participle of concrescere, to grow together), a term used in various technical senses with the general significance of combination, conjunction, solidity. Thus the building material made up of separate substances combined into one is known as concrete (see ). In mathematics and music, the adjective has been used as synonymous with “continuous” as opposed to “discrete,” i.e. “separate,” “discontinuous.” This antithesis is no doubt influenced by the idea that the two words derive from a common origin, whereas “discrete” is derived from the Latin discernere. In logic and also in common language concrete terms are those which signify persons or things as opposed to abstract terms which signify qualities, relations, attributes (so J. S. Mill). Thus the term “man” is concrete, while “manhood” and “humanity” are abstract, the names of the qualities implied. Confusions between abstract and concrete terms are frequent; thus the word “relation,” which is strictly an abstract term implying connexion between two things or persons, is often used instead of the correct term “relative” for people related to one another. Concrete terms are further subdivided as Singular, the names of things regarded as individuals, and General or Common, the names which a number of things bear in common in virtue of their possession of common characteristics. These latter terms, though concrete in so far as they denote the persons or things which are known by them (see ), have also an abstract sense when viewed connotatively, i.e. as implying the quality or qualities in isolation from the individuals. The ascription of adjectives to the class of concrete terms, upheld by J. S. Mill, has been disputed on the ground that adjectives are applied both to concrete and to abstract terms. Hence some logicians make a separate class for adjectives, as being the names neither of things nor of qualities, and describe them as Attributive terms.

 CONCRETE, the name given to a building material consisting generally of a mixture of broken stone, sand and some kind of cement. To these is added water, which combining chemically with the cement conglomerates the whole mixture into a solid mass, and forms a rough but strong artificial stone. It has thus the immense advantage over natural stone that it can be easily moulded while wet to any desired shape or size. Moreover, its constituents can be obtained in almost any part of the world, and its manufacture is extremely simple. On account of these properties, builders have come to give it a distinct preference over stone, brick, timber and other building materials. So popular has it become that besides being used for massive constructions like breakwaters, dock walls, culverts, and for foundations of buildings, lighthouses and bridges, it is also proving its usefulness to the architect and engineer in many other ways. A remarkable extension of the use of concrete has been made possible by the introduction of scientific methods of combining it with steel or iron. The floors and even the walls of important buildings are made of this combination, and long span bridges, tall factory chimneys, and large water-tanks are among the many novel uses to which it has been put. Piles made of steel concrete are driven into the ground with blows that would shatter the best of timber. A fuller description of the combination of steel and concrete will be given later.

The constituents of concrete are sometimes spoken of as the matrix and the aggregate, and these terms, though somewhat old-fashioned, are convenient. The matrix is the lime or cement, whose chemical action with the added water causes the concrete to solidify; and the aggregate is the broken stone or hard material which is embedded in the matrix. The matrix most commonly used is Portland cement, by far the best and strongest of them all. The subject of its manufacture and examination is a most important and interesting one, and the special article dealing with it should be studied (see ), Here it will only be said that before using Portland cement very careful tests should be made to ascertain its quality and condition. Moreover, it should be kept in a damp-proof store for a few weeks; and when taken out for use it should be mixed and placed in position as quickly as possible, because rain, or even moist air, spoils it by causing it to set prematurely. The oldest of all the matrices is lime, and many splendid examples of its use by the Romans still exist. It has been to a great extent superseded by Portland cement, on account of the much greater strength of the latter, though lime concrete is still used in many places for dry foundations and small structures. To be of service the lime should be what is known as “hydraulic,” that is, not pure or “fat,” but containing some argillaceous matter, and should be carefully slaked with water before being mixed with the aggregate. To ensure this being properly done, the lumps of lime should be broken up small, and enough water to slake them should be added, the lime then being allowed to rest for about forty-eight hours, when the water changes the particles of quicklime to hydrate of lime, and breaks up the hard lumps into a powder. The hydrated lime, after being passed through a fine screen to sort out any lumps unaffected by the water, is ready for concrete making, and if not required at once should be stored in a dry place. Other matrices are slag cement, a comparatively recent invention, and some other natural and artificial cements which find occasional advocates. Materials like tar and pitch are sometimes employed as a matrix; they are used hot and without water, the solidifying action being due to cooling and to evaporation of the mineral oils contained in them. Whatever matrix is used, it is almost invariably “diluted” with sand, the grains of which become coated with the finer particles of the matrix. The sand should be coarse-grained and hard. It should be free from dirt—that is to say, free from clay or soft mud, for instance, which prevents the cement adhering to its particles, or again from sewage matter or any substance which will chemically destroy the matrix. The grains should show no signs of decay, and by preference should be of an angular shape. The sand obtained by crushing granite and hard stones is excellent. When lime is used as a matrix, certain natural earths such as pozzuolana or trass, or, failing these, powdered bricks or tiles, may be used instead of sand with great advantage. They have the property of entering into chemical combination with the lime, forming a hard setting compound, and increasing the hardness of the resulting concrete.

The commonest aggregates are broken stone and natural flint gravel. Broken bricks or tiles and broken furnace slag are sometimes used, the essential points being that the aggregate should be hard, clean and sound. Generally speaking, broken stones will be rough and angular, whereas the stones in flint gravel will be comparatively smooth and round. It might be supposed, therefore, that the broken stone will necessarily be the better aggregate, but this does not always follow. Experience shows that, although spherical pebbles are to be avoided, Portland cement adheres tightly to smooth flint surfaces, and that rough stones often give a less compact concrete than smooth ones on account of the difficulty of bedding them into the matrix when laying the concrete. In mixing concrete there is always a tendency for the stones to separate themselves from the sand and cement, and to form “pockets” of honeycombed concrete which are neither water-tight nor strong. These are much more liable to occur when the stones are flat and angular than when they are round. Modern engineers favour the practice of having the stones of various sizes instead of being uniform, because if these sizes are wisely proportioned the whole mixture can be made more solid, and the rough “pockets” avoided. For first-class work, however, and especially in steel concrete, it is customary to reject very large