Concrete diminishes: slightly in volume in setting in air, and in monolithic construction this contraction is sufficient to produce cracks throughout the walls and floors. Some method should always be employed, therefore, to allow for expansion and contraction and to make the cracks follow false joints in the work. One method adopted for accomplishing this result in walls is shown by Fig. 246. Through joints are formed at intervals by means of an iron plate, shown at P and on the plan.. This plate is pulled up as the wall increases in height, leaving an open joint through the wall. Wherever these joints occur, recesses, are left in every alternate course, as shown at A A. These recesses are afterward filled with concrete blocks, formed separately and set in mortar like a stone. If the concrete contracts or settles the break will take place in the joints thus formed, and will not show on the face of the wall.
jointed on the surface, as shown in Figs. 242 or 245, the window heads should be made in the form of a flat arch and a recess left for the key, which should be put in afterward. It is also advisable to have a through joint over the centres of all windows.
When first attempting a concrete building, the architect will do well to consult with some person who has had experience with concrete building as to the best arrangement of overcoming the effects of contraction and expansion.
Concrete walls, with iron ties imbedded, when cracked, however, are not in the bad condition of stone or brick walls without such bond, as the iron ties may be depended upon to prevent spreading or falling.
One of the rooms in the Leland Stanford, Jr., Museum was designed to be the receptacle of many valuables, and to render it burglar-proof, the floor, walls and ceiling had copper wires imbedded in the concrete not over 3 inches apart, forming a continuous circuit, and designed to strike an alarm bell at the University if any wire should be cut.
This device has also been in use in the U. S. Sub-Treasury in San Francisco for several years; it would seem to be very effective for prison walls and cells.
Iron rods, old iron or steel, may be imbedded in the walls, floor and ceiling to as great an extent as may be deemed necessary; these, being firmly held by the concrete, will be very difficult to cut or remove. Such vaults would also be thoroughly fireproof, and, if made of sufficient thickness, would keep their contents unharmed, even should the building be completely destroyed.
"On one occasion, while building a concrete bank vault in an interior town, several tons of worn-out plowshares were placed in the concrete in such positions as would be most likely to discourage burglars in attempting to cut through the wall." *