Most structural concrete is made by placing or "casting" plastic concrete into spaces enclosed by previously constructed forms. The plastic concrete hardens into the shape outlined by the forms. The size and shape of the formwork are always based on the project plans and specifications.
Forms for all concrete structures must be tight, rigid, and strong. If the forms are not tight, there will be excessive leakage at the time the concrete is placed. This leakage can result in unsightly surface ridges, honeycombing, and sand streaks after the concrete has set. The forms must be able to safely withstand the pressure of the concrete at the time of placement. No shortcuts should be taken. Proper form construction material and adequate bracing in place prevent the forms from collapsing or shifting during the placement of the concrete.
Forms or form parts are often omitted when a firm earth surface exists that is capable of supporting or molding the concrete. In most footings, the bottom of
the footing is cast directly against the earth and only the sides are molded informs. Many footings are cast with both the bottom and the sides against the natural earth. In these cases, however, the specifications usually call for larger footings. A foundation wall is often cast between a form on the inner side and the natural earth surface on the outer side.
Forms are generally constructed from either earth, metal, wood, fiber, or fabric.
Earthen forms are used in subsurface construction where the soil is stable enough to retain the desired shape of the concrete. The advantages of earthen forms are that less excavation is required and there is better settling resistance. The obvious disadvantage is a rough surface finish, so the use of earthen forms is generally restricted to footings and foundations. Precautions must be taken to avoid collapse of the sides of trenches.
Metal forms are used where high strength is required or where the construction is duplicated at more than one location. They are initially more expensive than wood forms, but may be more economical if they can be reused repeatedly. Originally, all prefabricated metal forms were made of steel. These forms were heavy and hard to handle. Currently, aluminum forms, which are lightweight and easier to handle, are replacing steel.
Prefabricated metal forms are easy to erect and strip. The frame on each panel is designed so that the panels can be easily and quickly fastened and unfastened. Metal forms provide a smooth surface finish so that little concrete finishing is required after the forms are stripped. They are easily cleaned, and maintenance is minimal.
Metal-wood forms are just like metal forms except for the face. It is made with a sheet of B-grade exterior plywood with waterproof glue.
Wooden forms are by far the most common type used in building construction. They have the advantage of economy, ease in handling, ease of production, and adaptability to many desired shapes. Added economy may result from reusing form lumber later for roofing, bracing, or similar purposes. Lumber should be straight, structurally sound, strong, and only partially seasoned. Kiln-dried timber has a tendency to swell when soaked with water from the concrete. If the boards are tight-jointed, the swelling will cause bulging and distortion. When green lumber is used, an allowance should be made for shrinkage, or the forms should be kept wet until the concrete is in place. Soft woods, such as pine, fir, and spruce, make the best and most economical form lumber since they are light, easy to work with, and available in almost every region.
Lumber that comes in contact with concrete should be surfaced at least on one side and both edges. The surfaced side is turned toward the concrete. The edges of the lumber may be square, shiplap, or tongue and groove. The latter makes a more watertight joint and tends to prevent warping.
Plywood can be used economically for wall and floor forms if it is made with waterproof glue and is identified for use in concrete forms. Plywood is more warp resistant and can be reused more often than lumber. Plywood is made in 1/4-, 3/8-, 1/2-, 9/16-, 5/8- and 3/4-inch thicknesses and in widths up to 48 inches. Although longer lengths are manufactured, 8-foot lengths are the most common. The 5/8- and 3/4-inch thicknesses are most economical; thinner sections require additional solid backing to prevent bulging. However, the 1/4-inch thickness is useful for forming curved surfaces.
Fiber forms are prefabricated from impregnated waterproofed cardboard and other fiber materials. Successive layers of fiber are first glued together and then molded in the desired shape. Fiber forms are ideal for round concrete columns and other applications where preformed shapes are feasible since they require no form fabrication at the job site. This saves considerable time and money.
Fabric forming is made of two layers of nylon fabric. These layers are woven together, forming an envelope. Structural mortar is injected into these envelopes, forming nylon-encased concrete "pillows." These are used to protect the shorelines of waterways, lakes and reservoirs, and as drainage channel linings.
Fabric forming offers exceptional advantages in the structural restoration of bearing piles under waterfront structures. A fabric sleeve with a zipper closure is suspended around the pile to be repaired, and mortar is pumped into the sleeve. This forms a strong concrete jacket.
Forms for concrete construction must support the plastic concrete until it has hardened. Stiffness is an important feature in forms. Failure to provide form stiffness may cause unfortunate results. Forms must be designed for all the weight to which they are likely to be subjected. This includes the dead load of the forms, the plastic concrete in the forms, the weight of the workmen, the weight of equipment and materials, and the impact due to vibration. These factors vary with each project, but none should be ignored. The ease of erection and removal is also an important factor in the economical design of forms. Platform and ramp structures independent of formwork are sometimes preferred to avoid displacement of forms due to loading and impact shock from workmen and equipment.
When concrete is placed in forms, it is in a plastic state and exerts hydrostatic pressure on the forms. The basis of form design, therefore, is the maximum pressure developed by concrete during placing. The maximum pressure developed depends on the placing rate and the temperature. The rate at which concrete is placed affects the pressure because it determines how much hydrostatic head builds up in the form. The hydrostatic head continues to increase until the concrete takes its initial set, usually in about 90 minutes. At low temperatures, however, the initial set takes place much more slowly. This makes it necessary to consider the temperature at the time of placing. By knowing these two factors and the type of form material to be used, you can calculate a tentative design.
Strictly speaking, it is only those parts of the form work that directly mold the concrete that are correctly referred to as the "forms." The rest of the formwork consists of various bracing and tying members. In the following discussion on forms, illustrations are provided to help you understand the names of all the formwork members. You should study these illustrations carefully so that you will understand the material in the next section.
The portion of a structure that extends above the ground level is called the superstructure. The portion below the ground level is called the substructure. The parts of the substructure that distribute building loads to the ground are called foundations. Footings are installed at the base of foundations to spread the loads over a larger ground area. This prevents the structure from sinking into the ground. Its important to remember that the footings of any foundation system should always be placed below the frost line. Forms for large footings, such as bearing wall footings, column footings, and pier footings, are called foundation forms. Footings, or foundations, are relatively low in height since their primary function is to distribute building loads. Because the concrete in a footing is shallow, pressure on the form is relatively low. Therefore, a form design based on high strength and rigidity considerations is generally not necessary.
SIMPLE FOUNDATION. Whenever possible, excavate the earth and use it as a mold for concrete footings. You should thoroughly moisten the earth before placing the concrete. If this is not possible, you must construct a form. Because most footings are rectangular or square, you can build and erect the four sides of the form in panels.
Make the first pair of opposing panels (figure 7-1 (a)) to exact footing width. Then, nail vertical cleats to the exterior sides of the sheathing. Use at least 1-by-2-inch lumber for the cleats, and space them 2 1/2 inches from each end of the exterior sides of the panels (a), and on 2-foot centers between the ends. Next, nail two cleats to the ends of the interior sides of the second pair of panels (figure 7-1 (b)). The space between these panels should equal the footing length plus twice the sheathing thickness. Then, nail cleats on the exterior sides of the panels (b) spaced on 2-foot centers.
Figure 7-1.-Typical foundation form for a large footing.
Erect the panels into either a rectangle or square, and hold them in place with form nails. Make sure that all reinforcing bars are in place. Now, drill small holes on each side of the center cleat on each panel. These holes should be less than 1/2 inch in diameter to prevent paste leakage. Pass No. 8 or No. 9 black annealed iron wire through these holes and wrap it around the center cleats of the opposing panels to hold them together (see figure 7-1). Mark the top of the footing on the interior side of the panels with grade nails.
For forms 4 feet square or larger, drive stakes against the sheathing, as shown in figure 7-1. Both the stakes and the 1 by 6 tie braces nailed across the top of the form keep it from spreading apart. If a footing is less than l-foot deep and 2-feet square, you can construct the form from 1-inch sheathing without cleats. Simply make the side panels higher than the footing depth, and mark the top of the footing on the interior sides of the panels with grade nails. Cut and nail the lumber for the sides of the form, as shown in figure 7-2.
Figure 7-2.-Typical small footing form.
FOUNDATION AND PIER FORMS COMBINED. You can often place a footing and a small pier at the same time. A pier is a vertical member that supports the concentrated loads of an arch or bridge superstructure. It can be either rectangular or round. You build a pier form as shown in figure 7-3. The footing form should look like the one in figure 7-1. You must provide support for the pier form while not interfering with concrete placement in the footing form. You can do this by first nailing 2-by-4s or 4-by-4s across the footing form, as shown in figure 7-3. These serve as both supports and tie braces. Then, nail the pier form to these support pieces.
Figure 7-3.-Footing and pier form.
BEARING WALL FOOTINGS. Figure 7-4 shows a typical footing formwork for a bearing wall, and figure 7-5 shows bracing methods for a bearing wall footing. A bearing wall, also called a load-bearing wall, is an exterior wall that serves as an enclosure and also transmits structural loads to the foundation. The form sides are 2-inch lumber whose width equals the footing depth. Stakes hold the sides in place while spreaders maintain the connect distance between them. The short braces at each stake hold the form in line.
A keyway is made in the wet concrete by placing a 2-by-2-inch board along the center of the wall footing form. After the concrete is thy, the board is removed. This leaves an indentation, or key, in the concrete. When you pour the foundation wall, the key provides a tie between the footing and wall. Although not discussed in this training manual, there are several commercial keyway systems available for construction projects.
Square column forms are made of wood. Round column forms are made of steel, or cardboard impregnated with waterproofing compound. Figure 7-6 shows an assembled column and footing form. After constructing the footing forms, build the column form sides, and then nail the yokes to them.
Figure 7-7 shows a column form with two styles of yokes. View A shows a commercial type, and view B shows yokes made of all-thread bolts and 2-by material. Since the rate of placing concrete in a column form is very high and the bursting pressure exerted on the form by the concrete increases directly with the rate of placing, a column form must be securely braced, as shown by the yokes in the figure. Because the bursting pressure is greater at the bottom of the form than it is at the top, yokes are placed closer together at the bottom.
The column form should have a clean-out hole cut in the bottom from which to remove construction debris. Be sure to nail the pieces that you cut to make the clean-out hole to the form. This way, you can replace them exactly before placing concrete in the column. The intention of the clean-out is to ensure that the surface which bonds with the new concrete is clear of all debris.
Wall forms (figure 7-8) may be built in place or prefabricated, depending on shape and desirability of form reuse. Some of the elements that make up wooden forms are sheathing, studs, wales, braces, shoe plates, spreaders, and tie wires.
Figure 7-8.-Form for a concrete wall.
CONSTRUCTION. Sheathing forms the surfaces of the concrete. It should be as smooth as possible, especially if the finished surfaces are to be exposed. Since the concrete is in a plastic state when placed in the form, the sheathing should be watertight. Tongue-and-groove sheathing gives a smooth, watertight surface. Plywood or hardboard can also be used and is the most widely accepted construction method.
The weight of the plastic concrete causes sheathing to bulge if it is not reinforced. As a result, studs are run vertically to add rigidity to the wall form. Studs are generally made from 2-by-4 or 3-by-6 material.
Studs also require reinforcing when they extend over 4 or 5 feet. This reinforcing is supplied by double wales. Double wales also serve to tie prefabricated panels together and keep them in a straight line. They run horizontally and are lapped at the corners of the forms to add rigidty. Wales are usually made of the same material as the studs.
The shoe plate is nailed into the foundation or footing. It is carefully placed to maintain the correct wall dimension and alignment. The studs are tied into the shoe and spaced according to the correct design.
Small pieces of wood are cut the same length as the thickness of the wall and are placed between the forms to maintain proper distance between forms. These pieces are known as spreaders. The spreaders are not nailed but are held in place by friction and must be removed before the concrete covers them. A wire should be securely attached to each spreader so that the spreaders can be pulled out after the concrete has exerted enough pressure on the walls to allow them to be easily removed.
Tie wire is designed to hold the forms securely against the lateral pressure of unhardened concrete. A double strand of tie wire is always used.
BRACING. Many types of braces can be used to add stability and bracing to the forms. The most common type is a diagonal member and horizontal member nailed to a stake and to a stud or wale, as shown in figure 7-8. The diagonal member should make a 30° angle with the horizontal member.
Additional bracing may be added to the form by placing vertical members (strongbacks) behind the wiles or by placing vertical members in the corner formed by intersecting wales. Braces are not part of the form design and are not considered as providing any additional strength.
REINFORCEMENT. Wall forms are usually reinforced against displacement by the use of ties. Two types of simple wire ties, used with wood spreaders, are shown in figure 7-9. The wire is passed around the studs, the wales, and through small holes bored in the sheathing. Each spreader is placed as close as possible to the studs, and the tie is set taut by the wedge, as shown in view A of figure 7-9, or by twisting with a small toggle, as shown in view B. As the concrete reaches the level of each spreader, the spreader is knocked out and removed. Figure 7-10 shows you an easy way to remove the spreaders by drilling holes and placing a wire through them. The parts of the wire that are inside the forms remain in the concrete; the outside surplus is cut off after the forms are removed.
Figure 7-9.-Wire ties for wall forms.
Figure 7-10.-Removing wood spreaders.
Wire ties and wooden spreaders have been largely replaced by various manufactured devices in which the function of the tie and the function of the spreader are combined. Figure 7-11 shows one of these. It is called a snap tie. These ties are made in various sizes to tit various wall thicknesses. The tie holders can be removed from the tie rod. The rod goes through small holes bored in the sheathing, and also through the wales, which are usually doubled for that purpose. Tapping the tie holders down on the ends of the rod brings the sheathing to bear solidly against the spreader washers. You can prevent the tie holder from coming loose by driving a duplex nail in the provided hole. After the concrete has hardened, the tie holders can be detached to strip the forms. After the forms are stripped, a special wrench is used to break off the outer sections of rods. The rods break off at the breaking points, located about 1-inch inside the surface of the concrete. Small surface holes remain, which can be plugged with grout if necessary.
Figure 7-11.-Snap tie.
Another type of wall-form tie is the tie rod (figure 7-12). This rod consists of an inner section that is threaded on both ends and two threaded outer sections. The inner section with the cone nuts set to the thickness of the wall is placed between the forms, and the outer sections are passed through the wales and sheathing and threaded into the cone nuts. The clamps are then threaded on the outer sections to bring the forms to bear against the cone nuts. After the concrete hardens, the clamps are loosened, and the outer sections of rod are removed by threading them out of the cone nuts. After the forms are stripped, the cone nuts are removed from the concrete by threading them off the inner sections of the rod with a special wrench. The cone-shaped surface holes that remain can be plugged with grout. The inner sections of the rod remain in the concrete. The outer sections and the cone nuts may be reused indefinitely.
Figure 7-12.-Tie rod.
Wall forms are usually constructed as separate panels. Make the panels by first nailing sheathing to the studs. Next, connect the panels, as shown in figure 7-13. Figure 7-14 shows the form details at the wall corner. When placing concrete panel wails and columns at the same time, construct the wall form, as shown in figure 7-15. Make the wall form shorter than the distance between the column forms to allow for a wood strip that acts as a wedge. When stripping the forms, remove the wedge first to aid in form removal.
Figure 7-15.-Form for panel wall and columns.
Concrete stairway forms require accurate layout to ensure accurate finish dimensions for the stairway. Stairways should always be reinforced with rebars (reinforcing bars) that tie into the floor and landing. They are formed monolithically or formed after the concrete for the floor slab has set. Stairways formed after the slab has set must be anchored to a wall or beam by tying the stairway rebars to rebars projecting from the walls or beams, or by providing a keyway in the beam or wall. You can use various stair forms, including prefabricated forms. For moderate-width stairs joining typical floors, a design based on strength considerations is generally not necessary. Figure 7-16 shows one way to construct forms for stair widths up to and including 3 feet. Make the sloping wood platform that serves as the form for the underside of the steps from 3/4-inch plywood. The platform should extend about 12 inches beyond each side of the stairs to support the stringer bracing blocks. Shore up the back of the platform with 4-by-4 supports, as shown in figure 7-16. The post supports should rest on wedges for easy adjustment and removal. Cut 2-by-12 planks for the side stringers to fit the treads and risers. Bevel the bottom of the 2-by-12 risers for easy form removal and finishing.
Figure 7-16.-Stairway form.
Beams and Girders
The type of construction used for beam and girder forms depends upon whether the forms are to be removed in one piece or whether the sides are to be stripped and the bottom left in place until the concrete has hardened enough to permit removal of the shoring. The latter type of form is preferred, and details for this type are shown in figure 7-17. Although beam and girder forms are subjected to very little bursting pressure, they must be shored up at frequent intervals to prevent sagging under the weight of fresh concrete.
The bottom of the form should be the same width as the beam and should be in one piece for the full width. The sides of the form should be 1-inch-thick tongue-and-groove sheathing and should lap over the bottom as shown in figure 7-17. The sheathing is nailed to 2-by-4-inch studs placed on 3-foot centers. A 1-by-4-inch piece is nailed along the studs. These pieces support the joist for the floor panel, as shown in figure 7-18, detail E. The beam sides of the form are not nailed to the bottom. They are held in position by continuous strips, as shown in detail E. The crosspieces nailed on top serve as spreaders. After erection, the slab panel joists hold the beam sides in position. Girder forms (figure 7-17) are the same as beam forms except that the sides are notched to receive the beam forms. Temporary cleats should be nailed across the beam opening when the girder form is being handled.
Figure 7-18.-Assembly of beam and floor forms.
The entire method of assembling beam and girder forms is illustrated in figure 7-18. The connection of the beam and girder is illustrated in detail D. The beam bottom butts up tightly against the side of the girder form and rests on a 2-by- 4-inch cleat nailed to the girder side. Detail C shows the joint between the beam and slab panel, and details A and B show the joint between the girder and column. The clearances given in these details are needed for stripping and also to allow for movement that occurs due to the weight of the fresh concrete. The 4-by-4 posts (detail E) used for shoring the beams and girders should be spaced to provide support for the concrete and forms. They should be wedged at the bottom to obtain proper elevation.
Figure 7-19 shows you how the same type of forming can be done by using quick beams, scaffolding, and I-beamsif they are available. This type of system can be set up and taken down in minimum time.
Figure 7-19.-Beam and floor forms.
Oiling and Wetting Forms
You should never use oils or other form coatings that may soften or stain the concrete surface, prevent the wet surfaces from water curing, or hinder the proper functioning of sealing compounds used for curing. If you cannot obtain standard form oil or other form coating, you can wet the forms to prevent sticking in an emergency.
Even when all form work is adequately designed, many form failures occur because of human error, improper supervision, or using damaged materials. The following list highlights some, but not all, of the most common construction deficiencies that supervisory personnel should consider when working with concrete:
There are many reasons why forms fail. It is the responsibility of the Builder to ensure that the forms are correctly constructed according to design, and that proper techniques are followed.
|David L. Heiserman, Editor||
Copyright © SweetHaven
Revised: June 06, 2015