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CAEMENTUM (more frequently in the plural, Caementa; Gk. λατύπη, σκύρος), rubble, or small undressed stones, used together with mortar to form the caementiciae structurae or concrete walls with which the remains of Roman buildings abound. Vitruvius, who tells us that this form of structure was not used by Greek architects, describes (2.8) two chief varieties,--the opus reticulatum, in general use in his day, the more handsome kind of work, but less durable, because it did not bind well together; and the ancient opus incertum, which was less sightly but more lasting, on account of the way in which the stones were bonded together. Schneider in his edition of Vitruvius,. Rich, and others have taken this opus incertum to be the ancient Cyclopean masonry, which was formed by laying together large irregular masses of stone without mortar and filling up the interstices with small stone-chippings; but Vitruvius expressly says, “utraque (i. e. incertum et reticulatum) ex minutissimis sunt instruenda.” A third method of wall-building which he mentions was a variation from the Greek ἔμπλεκτον, in which two parallel faces of dressed stone were erected, and the interval between them filled up with rubble. He also speaks of concrete walls lined with marble.

Mr. J. H. Middleton (Ancient Rome in 1885, pp. 28-33) has been led by an examination of the existing concrete structures of Rome to distinguish the following varieties:--

1. Unfaced concrete, generally used for foundations, but carried higher; for instance, in the substructures of Caligula's palace. The method of construction was to erect two lines of upright posts from ten to fifteen feet high, and to nail boards against them and pour the concrete in a liquid state into the bed thus constructed, and remove the framework when the concrete had set. The frame was then set on the top of the mass and the process repeated. The stones were usually of the size of a man's fist, and the mortar was composed of lime and the volcanic sand called pozzolana (pulvis Puteolanus, Vitr. 2.6). Its remarkable properties [p. 1.328]are the chief cause of the durability of Roman concrete.

2. Faced concrete walls, in which the core is always composed in the same way as the unfaced concrete; and we are thus led to the conclusion, which would not have been inferred from Vitruvius alone, that the opus incertum and opus reticulatum were only facings. (aOpus incertum, in constructing which “irregularly shaped bits of tufa were cut smooth on one face, and roughly pointed behind; the whole face of the concrete wall was then studded with these stones, the points sticking into the wall, and the smooth surfaces appearing on the face. The pieces, square, triangular or polygonal, range from two to five inches across.” This structure is found in Republican buildings, e. g. at the foot of the Scala Caci. (b) The opus reticulatum differs from the incertum in having the facing stones cut into squares and closely fitted together in a reticulate pattern. It is found in buildings erected from the reign of Augustus to that of Hadrian, as in the Mausoleum of Augustus, and (a later variety with brick-faced angles and bands of brick at intervals of two or three feet) in Hadrian's villa. (c) Concrete faced with burnt brick, the earliest dated examples of which are to be found in the Rostra and the Pantheon. This structure was largely used in many of the most substantial buildings of Rome, while solid walls of brick are not found. Vitruvius (2.8, 17) accounts for this by legal restrictions and a desire to economise space. (d) The so-called opus mixtum, a wall-facing of alternate courses of brick and small blocks of tufa found in the circus of Maxentius (circa A.D.

Caementum. (From Middleton.)

310) and until the 6th century.

Concrete was extensively used at Baiae in the Augustan age, as a foundation for edifices built out into the sea (Tib. 2, 3, 45; Hor. Od. 3.1. 33, 24. 3), the Romans having discovered that pozzolana and lime formed a hydraulic cement (Vitruvius, 2.6, 1).

The most massive relic of Roman times in this island, the great military wall which extended from the mouth of the Tyne to that of the Solway, is a structure of faced concrete, formed by erecting two faces of large stones and filling up the intervening space with alternate courses of rubble, one foot deep, and mortar, four inches deep. (Archaeologia, vol. 11.1794, p. 64.)

Many of the great Roman achievements in building, especially in distant provinces, are to be attributed to this method of construction, which enabled them to raise, with comparatively unskilled hands and from materials which are accessible in most regions or easily imported, structures which were in a short time bonded together into solid homogeneous masses.


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