The description of asphalte most approved for this purpose, and used by the patentees with the greatest success, is that known as the ‘Pyrimont,’ and supplied by the Seyssel Asphalte Company.
The formation of vaults or domes, particularly those of an ornamental character, is one of the most advantageous applications of the concrete. As no expense is involved in the cutting of groins, coffers, ribs, &c., it is in itself less costly than brick or stone, besides saving considerable expense in the strength of the outer walls, which would be required to withstand the lateral thrust of ordinary vaulting. It is moreover better adapted for decorative treatment in colour or relief.
Large vaults have recently been formed over the principal apartments of the Foreign Office. The ceiling of the principal staircase-hall, 70 feet long by 26 feet wide, is divided into three compartments, two of which are semi-cylindrical coffered vaults, and the centre one is formed as a dome, with solid pendentives. The ceiling over the Cabinet-room has a span of 36 feet; it is semi-circular in form, with groined openings to the side-windows, and is divided into compartments by plain arched rims formed in the concrete. The vault itself is only 9 inches in thickness. A section of this ceiling is shown (diagram E).
The largest work upon which the Dennett system of fireproof flooring has yet been adopted is the new St. Thomas’s Hospital. It is here applied in the ordinary flat-arched form to the corridors, wards, and other rooms as the foundation for the asphalte covering of the flat roofs, and as coffered vaulting in large spans to the chapel, Governor’s hall, &c. Some idea of the extent of this building may be formed when it is stated that the fireproof arching covers an area of more than eight acres.
The strength of the arches has been frequently tested by actual experiments, both as to their capacity for bearing dead pressure and with regard to their resistance to impact from falling weights, moving loads, &c. These experiments have been generally instituted by the architects upon whose works the construction has been adopted, and they have invariably produced the most satisfactory results.
The cost of the construction varies somewhat, according to the distance from the gypsum quarries, which are almost entirely confined to the counties of Derby and Nottingham. The cost of the arching in London, in ordinary spans, as shown by diagram A, including centreing, is about 75s. per square of 100 superficial feet. A finished upper surface, where such is required, involves an additional expense of from 15s. to 25s. per square. These prices do not, of course, include iron girders; but as so few of these are required, as compared with other methods of fireproof construction in which concrete is used, this system will be found to possess, besides the acknowledged merits of strength, rigidity, and highly fireproof character, the advantage of economy, being from 25 to 50 per cent. cheaper than other methods.