Concrete is a mixture of gravel, broken stone, slag, or brick, called agglomerate or aggregate, with either ground lias lime, Portland or Selenitic cements, known as the matrix when mixed with water. It is used in bad or uncertain soils to form a sound foundation for the superstructure, also for fireproof arches, and coverings to the ground, under floors, to keep out damp and receive layers of hard floors.
The proportions in which the agglomerate and matrix are mixed vary considerably, according to the weight of the proposed superincumbent mass, and the materials of which it is made. The usual admixtures are : 1 of hydraulic lias lime to from 3 to 5 parts of ballast or agglomerate, containing sand, gravel, slag, brickbats, etc.; 1 part of Portland cement to from 4 to 7 of ballast, etc.; or 1 part of Selenitic cement to from 6 to 8 of ballast, etc.
The matrix, consisting of the sand and cement, or lime, is often mixed into a mortar first, and then poured on to the agglomerate on a wooden platform, after which the whole is turned over and thoroughly incorporated. Otherwise, the lime or cement is thoroughly mixed with the agglomerate when dry, after which the water is added, and it is again turned over several times for the second mixing.
This mixing being done, the resulting concrete is either wheeled and thrown into the trenches from a height, or laid in gently from buckets, each stage of the process being followed by careful ramming together and levelling up.
No large stones should be allowed in the agglomerate or aggregate. Those for concrete foundations should be small enough to pass through a ring 2 1/2 inches in diameter; while those for arches, lintels, and other special work must pass through a ring 1 1/2 inches in diameter.
Marbles are pure limestones, of very hard and compact natures, varying in colour, and capable of taking a high polish. They are found in Devonshire, Dorsetshire, Scotland, and Ireland; while considerable quantities come from the continent of Europe, and elsewhere. As a building material marble is chiefly used for ornamental purposes, in internal work only, such as columns, staircases, chimneypieces, and the like.
Serpentine is a similar kind of material, used for similar purposes, but much softer in its nature, though compact, and capable of receiving a first-class polish. It is of various colours, in streaky forms, from which it derives its name; and it belongs to the igneous rock formation, partaking of the nature of a granite or porphyry, and is found near the Lizard, on the south coast of England. When free from white streaks it is suitable for external ornament, though better adapted for internal work.
Trap or Whinstone is a hard mixture of feldspar and hornblende, of a crystalline nature, very tough, and of a green or black colour, used for setts, paving, road-metal, etc. The best quality is found in Northumberland; at Penmaenmawr, in Wales; and in Scotland.
Terra-cotta, a substitute for stone, for dressings to buildings, is the product of a rich clay found in several parts of the country, especially at Ruabon, Rowley Regis, and Tamworth. It is made by mixing several kinds of clays with ground glass or pottery to prevent shrinkage. The mass is ground to a powder, mixed in water, finely strained, kneaded, and then cast into moulds of the required patterns, etc., then dried and baked in domed kilns, at a very high temperature, and gradually cooled.
It may be either of a rich red, pink, or buff colour, according to the presence of iron, the light varieties indicating insufficient burning, and a green tint a bad material, which will be very absorbent. The blocks in which terra-cotta is made are cast hollow, soaked in water, and filled in with fine concrete before being set in position in a similar manner to stonework. Terra-cotta has the advantages of great durability and hardness, combined with cheapness and light weight; but against these must be set the fact that the unequal shrinkage, caused in the burning by the nature of the mixture, is apt to prevent the work, especially mouldings, carrying good lines.
Artificial Stones are made, generally speaking, of finely ground stone, ballast, and sand, mixed with Portland cement or other setting materials.
Victoria or silicated stone consists of granite dust and Portland cement, mixed to a paste, cast into moulds, dried and immersed in a silicate of an alkali to give it a hard, durable, and stronger weathering face. It is principally used for pavings, as a substitute for the Yorkshire stones, for sills, copings, landings, steps, etc., because it is a cheaper article.
Apoenite and Ransomts Patent Stone are similar materials; the former being made, in the first instance, from a mixture of sand, silicate, and Portland cement, while "Ransome's" variety is made from sand, silicate, and chalk, mixed to a paste, cast into moulds, and immersed in chloride of calcium.
Both kinds are greatly used for steps, landings, pavings, and all other purposes where natural stones are used. They have the advantages of being cheap, requiring little labour, are non-absorbent, of great tensile and compressive strength, and light weight.
Peters9 Patent Artificial Sandstone can be made of any colour and of grain according to the sand used in its preparation. It can be made in any district, and not necessarily at the manufactory, the quality and strength of the stone as at present ascertained being everything that can be desired.
Pozzuolana is a substitute for sand, of a clayey and ashy nature, found in Italy, in the neighbourhood of volcanoes, and is mixed with lime to make mortars and water cements.