Form Construction

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. The following discussion on forms provide illustrations to help you understand the names of all the formwork members. Study these illustrations carefully so that you will understand the material in the next section.

Foundation Forms

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. It’s 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. A form design based on high strength and rigidity considerations is generally not necessary. Figure 15 shows a foundation form for a small structure.

Simple Foundation – Whenever possible, excavate the earth and use it as a mold for concrete footings. 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.

1. Make the first pair of opposing panels as shown in Figure 16 (a) to exact footing width.
2. 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.
3. Nail two cleats to the ends of the interior sides of the second pair of panels (Figure 16 (b)). The space between these panels should equal the footing length plus twice the sheathing thickness.
4. Nail cleats on the exterior sides of the panels (b) spaced on 2 foot centers.
5. 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.
6. 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.
7. Pass No. 8 or No. 9 black annealed iron wire through these holes and wrap it around the center cleats or 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.
8. 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.
9. 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 as shown in Figure 16. 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 16. 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 17.

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. Build a pier form as shown in Figure 18. The footing form should look like the one in Figure 18. You must provide support for the pier form while not interfering with concrete placement in the footing form.

Bearing Wall Footings – Figure 19 shows a typical footing formwork for a bearing wall, and Figure 20 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 wet concrete by placing a 2 by 2 inch board along the center of the wall footing form, shown in Figure 20. After the concrete is dry, 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 the wall. Although not discussed in this training manual, there are several commercial keyway systems available for construction projects.

Columns

Square column forms are made of wood. Round column forms are made of steel or cardboard impregnated with waterproofing compound.

Figure 21 shows an assembled column and footing form.

1. Construct the footing forms.
2. Build the column form sides.
3. Nail the yokes to them.

The column form should have a cleanout hole cut in the bottom to remove construction debris. Nail the pieces that you cut to make the cleanout hole to the form. You can replace them right before placing concrete in the column. The intention of the cleanout is to ensure that the surface which bonds with the new concrete is clear of all debris.

Wall Forms

Wall forms, as shown in Figure 23, may be built in place or prefabricated, depending on the form shape and whether the form will be reused. Some of the elements that make up wooden forms are sheathing, studs, walers, braces, shoe plates, spreaders, and tie wires.

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. You can also use plywood or hardboard, and tongue and groove sheathing 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, run studs 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. Double walers supply this reinforcing. They 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 rigidity. Walers 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, known as spreaders, are cut the same length as the thickness of the wall and are placed between the forms to maintain proper distance between forms. Spreaders are not nailed but are held in place by friction and must be removed before the concrete covers them. Attach a wire securely to each spreader so 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 forms securely against the lateral pressure of unhardened concrete. Always use a double strand of tie wire.

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 waler, as shown in Figure 24. The diagonal member should make a 30° angle with the horizontal member. You may add additional bracing to the form by placing vertical members (strongbacks behind the walers or by placing vertical members in the corner formed by intersecting walers. Braces are not part of the form design and do not provide any additional strength.

Reinforcement – Wall forms are usually reinforced against displacement using ties. Two types of simple wire ties used with wood spreaders are shown in Figure 24. The wire is passed around the studs, the walers, and through small holes bored in the sheathing. Each spreader is placed as close as possible to the studs. View A shows the tie set taut by the wedge. View B shows the tie set taut by twisting with a small toggle.

As the concrete reaches the level of each spreader, knock the spreader out and remove it.

Figure 25 shows 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.

Wire ties and wooden spreaders have been largely replaced by various manufactured devices which combine the functions of the tie and the spreader.

Figure 26 shows one of these. It is called a snap tie. These ties are made in various sizes to fit 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 walers, 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 stripping the forms, use a special wrench 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.

Determining the Load on a Snap Tie – Use these steps to determine the total load on a snap tie:

1. Figure the contributing area of form. Multiply the distance between the ties
   horizontally by the vertical distance between ties.
2. Multiply the contributing area of form by the unit pressure per square foot (PSFT) of the concrete on that area of form as shown in Table 5.

The following examples walk you through the process of checking the load on snap ties. The safe load on snap ties is 2,250 pounds per square foot (psf).

Example 1:

1. 8’ of foam panel with snap ties at 24” on center (o.c.) (8 ties per 4’ x 8’ panel) as shown in Figure 27
2. Poured at 5’ per hour (rate of pour) at a 70° temperature
3. Contributing area of form = 2’0” x 2’0” = 4 square feet
4. 5’ rate of pour at 70 = 793 pounds of pressure per square foot (PSFT)
5. 4 (area of form) x 793 (unit pressure per square foot.) = 3,172 psf pressure of load on snap tie

CAUTION

This exceeds the safe capacity of the snap tie and must be reduced either by slowing the rate of concrete poured per hour or by reducing the snap tie spacing (increasing the number of snap ties).

Example 2:

1. 8’ of foam panel with snap ties at 16” o.c. (18 ties per 4’ x 8’ panel)
2. Poured at 5’ per hour (rate of pour at a 70° temperature)
3. Contributing area of form = 1.33 x 1.33 = 1.8 square feet
4. 5’ rate of pour at 70 = 793 PSFT
5. 1.8 (area of form) x 793 (unit pressure per sq. ft.) = 1,427 psf pressure of load on snap tie

This is well within the 2,250 psf safe load on snap ties.

Spacing Snap Ties – Some alternatives for spacing snap ties on a 4’ x 8’ sheet
are shown in Figure 28.

Snap Tie Systems – There are a number of snap tie systems you can use; they are shown in Figures 29 through 32.

Another type of wall form tie is the tie rod, shown in Figure 33. This rod consists of an inner section that is threaded on both ends and two threaded outer sections. Place the inner section with the cone nuts set to the thickness of the wall between the forms, and the outer sections through the walers and sheathing and thread them into the cone nuts. Then thread the clamps on the outer sections to bring the forms to bear against the cone nuts.

After the concrete hardens, loosen the clamps and remove the outer sections of rod by threading them out of the cone nuts. After stripping the forms, remove the cone nuts from the concrete by threading them off the inner sections of the rod with a special wrench. Plug the cone shaped surface holes that remain with grout. The inner sections of the rod remain in the concrete. The outer sections and the cone nuts may be reused indefinitely.

Wall forms are usually constructed as separate panels.

1. Make the panels by first nailing sheathing to the
   studs.
2. Next, connect the panels, as shown in Figure 34.

Figure 35 shows the form details at the wall corner.

When placing concrete panel walls and columns at the same time, construct the wall form as shown in Figure 36. 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.

Stair Forms

Concrete stairway forms require accurate layout to ensure accurate finish dimensions for the stairways Always reinforce stairways with rebars (reinforcing bars) that tie into the floor and landing. Form them or after the concrete for the floor slab has set. Ge sure to anchor stairways formed after the slab has set 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 37 shows one way to construct forms for stair widths up to and including 3 feet.

1. 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.
2. Shore up the back of the platform with 4 by 4 supports, as shown in Figure 39. The post supports should rest on wedges for easy adjustment and removal.
3. Cut 2 by 12 planks for the side stringers to fit the treads and risers.
4. Bevel the bottom of the 2 by 12 risers for easy form removal and finishing.

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 38. 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 the same width as the beam and 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 38. Nail the sheathing to 2 by 4 inch studs placed on 3 foot centers. Nail a 1 by 4 inch piece along the studs.

These pieces support the joist for the floor panel, as shown in Figure 39, Detail E. Do not nail the beam sides of the form 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 are the same as beam forms except that the sides are notched to receive the beam forms. Nail temporary cleats across the beam opening when handling the girder form.

The entire method of assembling beam and girder forms is illustrated in Figure 39. 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 caused by the weight of the fresh concrete. The 4 by 4 posts shown in Detail E, used for shoring the beams and girders, should be spaced to provide support for the concrete and forms. Wedge them at the bottom to obtain proper elevation.

Figure 40 shows how the same type of forming can be done by using quick beams, scaffolding, and I-beams, if they are available. This type of system can be set up and taken down in minimum time.

Oiling and Wetting Forms

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.

Oil for Wood Forms – Before placing concrete in wood forms, treat the forms with a suitable form oil or other coating material to prevent the concrete from sticking to them. The oil should penetrate the wood and prevent water absorption. Almost any light bodied petroleum oil meets these specifications. On plywood, shellac works better than oil in preventing moisture from raising the grain and detracting from the finished concrete surface. Several commercial lacquers and similar products are also available for this purpose. If you plan to reuse wood forms repeatedly, a coat of paint or sealing compound will help preserve the wood. Sometimes lumber contains enough tannin or other organic substances to soften the concrete surface. To prevent this, treat the form surfaces with whitewash or limewater before applying the form oil or other coating.

Oil for Steel Forms – Oil wall and steel column forms before erecting them. You can oil all other steel forms when convenient, but you should oil them before placing the reinforcing steel. Use specially compounded petroleum oils, not oils intended for wood forms. Synthetic castor oil and some marine engine oils are examples of compound oils that give good results on steel forms.

Applying Oil – The successful use of form oil depends on how you apply it and the condition of the forms. They should be clean and have smooth surfaces. Because of this, do not clean forms with wire brushes, which can mar their surfaces and cause concrete to stick. Apply the oil or coating with a brush, spray, or swab.

CAUTION

Cover the form surfaces evenly, but do not allow the oil or coating to contact
construction joint surfaces or any reinforcing steel in the formwork. Remove all excess oil.

Other Coating Materials – Asphalt paint, varnish, and boiled linseed oil are  also suitable coatings for forms. Plain fuel oil is too thin to use during warm weather, but mixing one part petroleum grease to three parts fuel oil provides adequate thickness.

Form Failure

Even when all form work is adequately designed, many form failures occur because of human error, improper supervision, or use of 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:

• Inadequately tightened or secured form ties
• Inadequate diagonal bracing of shores
• Use of old, damaged, or weathered form materials
• Use of undersized form material
• Shoring not plumb
• Failure to allow for lateral pressures on form work
• Failure to inspect form work during and after concrete placement to detect abnormal deflections or other signs of imminent failure

Formwork and other work that needs to be completed before concrete is poured has to be verified before the concrete is poured. A concrete placement clearance form, shown in Figure 41, gives a thorough checklist to fill out before delivering and pouring concrete.

There are many reasons why forms fail. It is the Builder’s responsibility to ensure that the forms are correctly constructed according to design, and that proper techniques are followed.