Improved Construction

IMPROVED CONSTRUCTION. The cost of labor and material required for forms is an important part of the cost of any concrete work, and particularly of thin walls or parts of buildings; and con sequently there have been many attempts to reduce the cost of forms. However, none of the improvements has made any great reduction in the cost of forms, nor are any of the improvements without accompanying objections; and therefore there is still room for im provements in reducing the cost of the materials and the labor for forms.

Only a few of the many attempts to improve the forms for mass concrete work will be referred to here.

Sectional Forms.

For structures having large areas of flat surfaces, such as a retaining wall, the forms are sometimes made in sections, which, after the concrete has set, may be taken down in one piece and be set up again, thus facilitating the placing and the removal of the forms. The sections are made of planks fastened together with battens, and are placed one above the other or end to end as may be required, and are held in place by being lightly nailed to the upright posts.

Sectional forms are also made to be used without continuous upright posts, the two opposite sides being held in position by bolts and separators, and successive sections being held together by pro jetting metal lugs on the outside. Fig. 18 shows one of the several varieties of sectional or panel forms for wall construction. The rods run through pipes which act as spacers and also facilitate the removal of the rods. The bottoms of the panels overlap the part of the wall already in place, the forms being supported by the pipe and rod.

Fig. 19 shows another method of making sectional forms.* In using the frame, a section of wall is built up one foot high and allowed to set; and then the frame is removed by unbolting, and is raised one foot higher, the rear face of the form being moved inward sufficiently to allow for the corresponding batter of the wall. Fig. 20 shows a somewhat similar arrangement of the forms for a coping.* The frame may be taken down by removing the bolts B and C. Notice the beveled strips m, m, m.

For

m. Sometimes instead of using posts continuous from top to bottom of the wall, slotted frames 5 or 6 feet long are so arranged that the short posts may be raised as the work pro gresses without removing the bolts which hold the sides of the form to gether, thus permitting the removal of the lower plank for use again.

Fig. 21, page 166, shows this form of construction, the slotted frames being ready to be moved up.

Plank Holders.

Upon the market are several patented metal plank-holders that permit the addi tion of one plank at a time and do not require either nails or uprights; but their value has not yet been established by experience.

Metal Forms.

Metal forms have been tried; but, up to date, wood has proved to be the most economical material for forms for concrete work, except possibly for small sewers and conduits. If sheet metal is placed on a wood back or on a metal stiffening frame, there is danger of its becoming dented, bent, or otherwise defaced so as to give an imperfect surface to the concrete; and if the metal covering is sufficiently thick to resist damage, it is too heavy and too expensive.

Be fore beginning to deposit concrete, all debris, such as sawdust, shavings, blocks of wood, etc., should be re moved from the inside of the forms. This precaution is particularly im portant with forms for columns, gir ders, beams, and floors.

All dry mortar left on the forms during the previous day's work should be removed before beginning to deposit concrete, as otherwise an imperfect face will be obtained.

As a rule the forms should not be removed within 48 hours after the concrete is deposited; and in cold weather they should be allowed to remain longer than in warm weather to give ample time for the cement to set. The forms can be removed from the back of a wall sooner than from the face. With mass concrete it is usually safe to remove the forms from the face when the concrete has set so it can not be indented with the thumb nail; but with arches, columns, and girders, more time should be allowed. In concrete building-work it is usually desirable to remove the forms as soon as possible in order to use them elsewhere, but removing forms too soon has frequently been the cause of serious accidents; and hence it is wise when placing the concrete for columns, girders, and floors, to mould some cubes or beams which are later broken in a testing machine to determine whether or not it is then safe to remove the forms.

In this connection it should not be forgotten that concrete sets faster during a hot dry day than during a damp muggy one, and also that occasional batches will set abnormally slow either because of slower-setting cement or of impurities in the sand.

For data on the cost of forms, see fi 417 and Table 46, page 258, and also "Forms, Cost of," in index.