Chapter XXIII

. In conclusion, the reader should study the specific examples of form construction for different purposes that are given throughout the book for hints as to special practice and details.

~UNIT CONSTRUCTION OF FORMS.~--Unit construction has a somewhat variable meaning in form work. In wall and tank work and in some other kinds of work unit construction means the use of form units which are gradually moved ahead or upward as the concreting progresses or of form units which are used one after another in continuous succession as the concreting progresses. In column, girder and floor work unit construction means the division of the form work as a whole and also of the individual forms into independent structural units; thus in forms for a building the column forms may be independent of the girder forms and also each column and girder form be made up of several separate units. In all cases unit construction has for its purpose the use of the same form or at least the same form lumber over and over for molding purposes. Every time the use of the same form is repeated, the cost of form work per cubic yard of concrete placed is reduced. The theoretical limit of economical repetition is then the limit of endurance of the form, the practical limit, however, is something quite different. Most concrete work varies in form or dimensions often enough to prevent the use of the same forms more than a few times, and even if these variations did not exist the time element would enter to prevent the same form or form lumber being used more than a certain number of times. Unit construction to give repeated use of forming has, therefore, its economic limits. The significance of this conclusion does not lie in any novelty that it possesses but in the fact that for any piece of work it determines the labor that may profitably be expended in working out and constructing form units.

~LUBRICATION OF FORMS.~--All forms for concrete require a coating of some lubricant to prevent the concrete from adhering to the wood with which it comes in contact. Incidentally this coating tends to give a smoother surface to the concrete and to preserve the wood against damage by its alternate wetting and drying. The great value of lubrication is, however, that it reduces the cost of removing forms. The requisite of a good coating material is that it shall be thin enough to spread evenly and to fill the pores and grain of the wood. Crude oil or petroline makes one of the best coatings, but various other greasy substances will serve. Where the forms are not to be removed until the concrete has set hard a thorough wetting of the wood just before the concrete is placed is all the coating necessary. Any concrete adhering to forms should be thoroughly cleaned off before they are used again and the wood underneath given a special heavy coating.

~FALSEWORKS AND BRACING.~--The falseworks which support the forms proper and stagings for workmen, runways, material hoists, etc., do not call for any striking differences in construction and arrangement from such work elsewhere. For wall forms inclined props reaching from ground to studding are used for walls of moderate height such as retaining walls, wing walls, and abutments. For building walls of some height a gallows frame arrangement or the common braced staging used by masons and carpenters is used. In building construction, however, movable forms are commonly employed for walls more than one story high and should always be employed above one story to save staging timber. Column forms are seldom braced unless erected without connecting girder or floor forms at their tops, and then only by diagonal props to the floor or ground. Girder and floor supports usually consist of uprights set under the girder form at intervals and occasionally under floor slab forms. The spacing of props and uprights will be regulated by the judgment of the foreman and boss carpenter; no general rule is applicable, except that enough lumber must be used to hold the forms rigid and true to line and level. The various illustrations of actual formwork which follow are the best guides to good practice.

~TIME FOR AND METHOD OF REMOVING FORMS.~--No exact time schedule for removing forms is wise in concrete work. Concrete which is mixed wet sets slower than dry concrete and concrete sets slower in cold weather than it does in warm weather. Again the time of removal is influenced by the risk taken by too early removal, and also by the nature of the stresses in the member to be relieved of support. In all cases the forms should be removed as soon as possible so that they can be used over again and so that the concrete may be exposed to the air to hasten hardening. The following suggestions as to time of removal are general and must be followed with judgment.

Using dry concrete in warm weather the forms for retaining walls, pedestals, isolated pillars, etc., can be removed in 12 hours; using wet or sloppy concrete the time will be increased to 24 hours. In cold weather the setting is further delayed and inspection is the only safe guide to follow. Very cold weather delays setting indefinitely. Forms for small arch work like sewers and culverts may be removed in 18 to 24 hours if dry concrete is used, and in 24 to 48 hours if wet concrete is used. The time for removing large arch centers should not be less than 14 days for spans up to 50 ft. if the arch is back-filled at once; when the center is not to be used again it is better to let it stand 28 days. For very large arches the problem becomes a special one and is considered in