FORM DESIGN - Construction Knowledge.net

1 Forms play a major role in concrete Construction . They give the plastic concrete its shape and hold it until ithardens. Forms protect the concrete , assist in curing it, and support any reinforcing rods or conduitsembedded in the concrete . This chapter familiarizes the carpenter with the DESIGN and Construction of varioustypes of DESIGNF orms for concrete must be tight, rigid, and forms are not tight, loss of concrete may cause ahoneycomb effect, or loss of water may cause sand-streaking. The forms must be braced enough to stayin alignment.

2 Special care is needed when bracingand tying down forms used in applications such asretainer walls, where the mass of concrete is largeat the bottom and tapers toward the top. In thistype of Construction and in the first pour for wallsand columns, the concrete tends to lift the formabove its proper elevation. (Field Manual 5-742gives formulas and tables for designing forms ofproper strength.)FORM CONSTRUCTIONA lthough forms are generally constructed fromwood, fiber, earth, or metal, the TO carpenterusually constructs wood or fiber forms are the most common in buildingconstruction.

3 They are economical, easy to handle,and easy to produce, and they adapt to manyshapes. Form lumber can be reused for roofing,bracing, or similar should be straight, strong, and onlypartially seasoned. Kiln-dried lumber tends to swellwhen soaked with water. Swelling may causebulging and distortion. If green lumber is used,allow for shrinkage, or keep it wet until theconcrete is in place. Softwoods (pine, fir, andspruce) are the most economical; they are light,easy to work with, and generally coming in contact with concrete should besurfaced (smooth) on the side towards the concreteand on both edges.

4 The edges may be square,shiplap, or tongue-and-groove. Tongue-and-groovelumber makes a more watertight joint, whichreduces is economical to use for wall and floorforms; however, plywood used for this purposeshould be made with waterproof glue and markedfor use in concrete forms. Plywood is warp-resistantand can be used more often than other advantage of using plywood for forms is thegreat number of sizes available. It is made inthicknessesCAUTION Watch for protruding nails. They are theprincipal cause of accidents on form work. Inspect tools frequently.

5 Place mud sills under shoring that rests onthe ground. Protect all men on scaffolds and on theground. Do not raise large form panels in heavy gustsof wind. Brace all shoring securely to preventcollapse of form 1/4, 3/8, 1/2, 5/8, and 3/4 inch, and in widths upto 48 inches. The 8-foot lengths are mostcommonly used. The 6/8- and 3/4-inch thicknessesare most economical. Thinner plywood requiressolid backing to prevent deflection. The 1/4-inchthickness is useful for curved cardboard and other fiber materialsare used for round concrete columns and otherpreformed shapes.

6 Forms are made by gluinglayers of fiber together and molding them to theright shape. The advantage is that fabrication atthe job site is not AND FOOTING FORMSWhen possible, earth is excavated to form a moldfor concrete -wall footings. If wood forms areneeded, the four sides are built in for two opposite sides are made at exactfooting width; the other pair has two end cleats onthe inside spaced the length of the footing plustwice the sheathing thickness. One-inch-thicksheathing is nailed to vertical cleats spaced on 2-foot centers. Two-inch dressed lumber should beused for the : Panels are held in place with formnails until the tie wire is installed.

7 Nailsshould be driven only part way from theoutside so that they can be easily wires are wrapped around the center holes on each side of the cleat should be lessthan 1 inch in diameter to prevent mortar bars must be placed before the wire forms 4 feet square or larger, stakes are drivenas shown in Figure 5-1. These stakes, and 1 x 6boards nailed across the top of the form, preventspreading. Panels may be higher than the requireddepth of footing, since they can be marked on theinside to show the top of the footing. If the footingsare less than 1 foot deep and 2 feet square, formscan be constructed of 1-inch sheathing withoutcleats (Figure 5-2).

8 When placing a footing and a small pier at thesame time, the form is built as shown in Figure ensure that support for the upper form does notinterfere with the placement of concrete in thelower form, 2 x 4 or 4 x 4 piecesFigure 5-1. Typical large footing formare nailed to the top of the lower form (asshown). The top form is then nailed to and bracing of forms for wallfootings are shown in Figure 5-4. The sides are2-inch lumber held in place by stakes and heldapart by spreaders. The short brace shown ateach stake holds the form in FORMSWall forms are made of wall "panels" and otherparts shown in Figure 5-5 page 5-4.

9 TheseDarts are described as follows:Wall PanelsWall panels are made by nailing thesheathing to the studs and can be built inplace or prefabricated wall panels should be nomore than 10 feet long so that they canbe easily handled. Figure 5-6, page 5-4,shows how wall panels are connected andhow wall corners are forms the surface of theconcrete. It should be smooth, especiallyif the finished surface is to be exposed. Itis normally 1-inch (3/4-inch dressed)tongue- and-groove lumber or 3/4-inchplywood. concrete is plastic when placedin the form, so sheathing should bewatertight.

10 Tongue-and-groove lumber orplywood gives a watertight sheathing to prevent bulgingfrom the weight of the studs make the sheathing rigid. These studsare generally made from 2 x 4 lumber. Studs alsorequire reinforcing when they extend more than wales reinforce the wall form. They also tiewall panels together and keep them in a straightline. They run horizontally and are lapped at give the forms stability. Themost common brace uses ahorizontal member and a diagonalmember nailed to a stake and to thestud or wale. The diagonal memberof the brace should make a 30 anglewith the horizontal bracing may be providedby strongbacks (vertical members)behind the wales or in the cornerformed by intersecting wales.