CONCORDIA. 253 CONCRETE. works, etc. The city contains Nazareth Acad- emy, Saint Aloysius's School, and library asso- ciation and high school libraries. Population, in 1800, 3184; in 1900, 3401. CONCORDIA (Lat., from con-, together + cor, lieart) . A Roman divinity, the goddess of harmony. Jlauy temples were l)uiU. in her honor, the oldest of them by Camillus in B.C. 3G7, after the passing of the Licinian Laws. It stood on the Forum between the Temple of Saturn and the Mamertine Prison, and was built of white marble with a richly carved cornice. The interior was Hanked with rows of columns against the walls, with niches which held masterpieces of Greek sculpture. It con- tained, besides, paintings and a collection of precious stones. It was twice rebuilt, in B.C. 121 and under Augustus, but only scanty ruins now remain. The temple sometimes served as a meeting-place for the Senate, and Cicero there delivered his last oration against Catiline. The goddess was represented as a matron, holding in her right hand a saucer-like vessel {patera), or an olive branch, and in her left the horn of plenty. Her symbols were two liands ela.sped together, and two serpents entwined about a wand. CONCRESCENCE (Lat. concresccntia, grow- ing together, from concrescere, to grow together, from GO»i-, together -1- crescere, to grow; con- nected with Lat. creare, to create). The grow- ing together of j'oung plant organs by reason of mutual contact or pressure when forming. The term has also been applied to the apparent union of neighboring organs, such as the flower parts. In these cases, however, each organ begins its development independently, and those which apjiear to be united are lifted by the growth, underneath the separate rudiments, of a region of tissue which belongs equally to all. See Teratology; Flower. CONCRETE (from Lat. concretus, p.p. of concrescere, to grow together) . Opposed to ab- stract. See Abstr.vction. CONCRETE. An artificial stone composed of hydraulic cement (q.v.), sand, and broken stone or gravel, or other hard material in small frag- ments. The mixture of sand and cement is commonly called the matrix, and the broken stone or other material is similarly called the aggregate. The matrix may be either lime and sand or cement and sand mortar, but is more usually the latter. The aggregate may be peb- bles, gravel, broken stone, broken bricks, shells, slag, coke, etc., but the most commonly used aggregates are broken stone and gravel. Broken stone gives a stronger concrete than gravel, other things being equal. The proper proportions of the several constituents composing concrete is considered to be attained when the cement paste exactly fills the voids in the sand, and the matrix exactly fills the voids in the aggregate; less than enough mortar to fill the voids in the aggregate results in a weaker and more porous concrete, and more than enough adds to the cost of the concrete without increasing its strength. It is evident from this statement that the relative proportions will vary with the char- acter of the sand and aggregate employed. A fair range of proportions for most engineering works is cement, one part; sand, one to three parts; aggregate, four to six parts. There is considerable diversity of opinion as to the amount of water to be used in making concrete. According to one extreme view, llie amount of water should be such that the cunercle will quake when tamped; according to the other extreme, the mixture should be mad<' so dry that water will barely flusli to the surface when the concrete is tamped. Current practice varies all the way between these two e.xtremes. The manufacture of concrete consists simply in mix- ing the water, cement, sand, and aggregate of which it is composed. To obtain the best results this mixture should be exceedingly thorough ; the ideal mixture is attained when every grain of sand is covered with a film of cement paste and evei-y fragment of aggregate is covered with a coating of mortar. Both h<and mixing and ma- chine mixing are employed in practice. In hand mixing the proper proportions of cement and sand are deposited on a timber platform and mixed dry by repeated turnings with a shovel. The proper quantity of water is then added, preferably with a spray, and the mixture then turned and returned with shovels until the water is thoroughly and evenly ineorporat-ed with the cement and sand. The aggregate is then thor- oughly wetted and is mixed with the mortar by similarly repeated turnings with shovels. A variety of concrete-mixing m.ichines are em- ployed, some being intermittent and some being continuous in operation, the hatter sometimes automatically measuring the proportions of cement, sand, aggregate, and water. Perhaps the most common form of intermittent mixer is a cubical iron box hung on trunnions at diagonally opposite corners; the cement, sand, and aggregate in the proper proportions are placed in the box through a suitible door in one side which can be closed and fastened: the water is admitted through the hollow trunnions, and the box is put in revolution by an engine or other motive power. After from ten to twenty turns the box is brought to rest, its contents of thoroughly mixed concrete dumped out into barrows or cars and a new charge of cement, sand, and aggregate introduced for mixing. A common form of con- tinuous concrete-mixer consists of a trough or cylinder in which a spiral or bladed screw shaft revolves; the raw materials are introduced con- tinuously at one end. and a continuous discharge of mixed concrete takes place at the opposite end. There are numerous other forms of con- crete-mixing machines. The value of concrete as a structural material consists in its property of changing from a plas- tic condition into a hard, rigid, artificial stone by the setting and hardening of the cement paste. (See Cement.) Concrete composed of one part cement, two parts sand, and six parts broken stone has a compressive strength of from ten to twenty tons per square foot at the age n{ one year. The method of laying the concrete after mixing depends upon the position in which it is placed and upon the form in which it is to be used in the structure. Yhen used in the form of blocks, the blocks are made by placing the plastic mix- ture in suitable molds in thin layers and tamp- ing each layer thoroughly with woo<len or iron rammers before placing the succeeding layers. The mixture remains in the molds until it is hard, when the block is removed and laid in the struc- ture just as a corresponding block of stone would be laid. The more common method of using con-
