Panel Forming
By: Ed Sauter Tilt-Up Concrete Association
Tilt-Up panel shapes are limited only by the imagination of the designers and the skill of the craftsman. A look at some of the award submittals of the past several years will put to rest any thoughts that site-cast Tilt-Up is limited. While rectangular panels are still the norm, panels with previously inconceivable bends, curves, and shapes are being constructed on a regular basis. Curved and unusual shaped panels cost more to construct than rectangular panels but they are still much less expensive to build than using vertical formed or other types of concrete construction. Many, in fact, would be impossible to build with other methods of construction. Tilt-Up eliminates expensive formwork and scaffolding. Proper panel formation is critical to project success and demands appropriate attention.
CONSTRUCTING THE PERIMETER FORMS
Aluminum, steel, and engineered lumber are gaining acceptance in the marketplace for forming, but dimension lumber is still the most widely used forming material. The initial step in constructing the perimeter forms is to use good, straight lumber. Using poor quality lumber is being penny wise and dollar foolish — it will cost you in the long term. Lumber should be ordered in lengths that will minimize the number of splices needed. Knots are typically not a problem since the sides of the panels are usually hidden in the joints. The side forms should not be oiled, because the form oil is not compatible with bond breakers. Standard dimension lumber can be used one to three times when engineered lumber and metal products can be used indefinitely.
The structural thickness of the panel determines the size of the lumber. If a panel is to be 5.5 inches thick, then the simplest edge form is a two-by-six or if the panel is to be 7.25 inches thick, then a two-by-eight- is the correct choice. The contractor and engineer should work together to produce the most efficient design. A thinner panel will often need more steel reinforcement so the cost of extra concrete (and a heavier panel) versus extra steel should be evaluated. Occasionally, the required structural panel thickness will be slightly thicker than the depth of standard dimension lumber. The first option in this scenario is to still use a two-by-six or two-by-eight and build up the top by nailing on a strip of plywood or a 1×2 (if an additional 3⁄4-inch is required). For example, if the panel is to be eight- inches thick, use a two-by-eight plus strips of plywood or ripped lumber. The second option is to use a wider piece of dimension lumber and rip it lengthwise to the designed height. For example, rip down a two-by-ten to get an 8.5-inch form height. One caution with this method is that it wastes more material. It is not practical to use taller sides and hold the concrete below the top as this makes screeding and finishing much more difficult.
While a variety of methods are available for securing the forms to the floor slab, the most common method is securing the forms using wood cleats. The wood cleats should be spaced between 36- and 48- inches apart and located at each corner. Attach the cleats to the slab by drilling a 3/16-inch hole and driving in three 16d double-headed nails, which just fit and are easy to pull out when the forms are stripped. Pins should not be shot into the slab with a stud gun to anchor the cleats, because they are hard to remove and can splinter the slab.
Another method for securing the forms is to use metal form brackets. This involves using a roto-hammer to drill a one-fourth-inch hole through the bracket hole into the slab. Next, drive in a 1/4-inch wood skewer, also known as a miniature dowel, and break it off flush with the bracket. Finally, install one 16d double- headed nail into the skewer to secure. This method is easy to strip and does not spall the slab.
The newest forming methods involve the use of adhesive-based form attachments. This eliminates the need for holes in the slab to anchor the form supports – an important consideration for many owners. Special tape or adhesives secure the bracket to the slab, which must be clean and free of dust. Portions of this system are reusable.
The juncture between the form and the slab should be chamfered. This is accomplished with a wood strip or plastic chamfer. Attach the standard 3/4-inch chamfer strips directly to the side form. A host of manufactured plastic chamfer strips and accessories are made to aid in the process. Double-sided extruded plastic chamfers are also available, and a two-by wood form can be placed within them.
FORMING DOORS AND WINDOWS
Overhead door openings are formed within the panel in a manner similar to the construction of perimeter forms, except that the anchoring cleats are inside the opening. Fewer supports are needed for door openings. Spreaders should be used where practical between the sides to maintain proper opening dimensions. Window openings are formed similar to overhead doors with the exception that recesses or attachments are usually built into the forms for glazing details.
Personnel door frames are formed differently than overhead doors or windows. The frames, typically hollow metal, are placed on the slab producing their own forms with the concrete cast around them. Since these types of forms are often grouted for stability, casting the concrete fills the hollow portion of the form eliminating the need for post grouting. The door frames should be checked to ensure that they are square and affixed securely. The bottom of the door frames terminate at the floor line, but the bottom of the actual formed opening in the panel is usually 5- to 6-inches below the bottom of the opening. This is so the floor can be placed over the rough sill to the exterior face of the wall. Polystyrene or some other easily removed filler should be used to make the block-outs for this and also for hardware attachment screws. This will save the metal drill when drilling through the frame. To prevent concrete leakage, use duct tape to cover hinge and strike boxes.
One of the advantages with openings in Tilt-Up construction is that if the location needs to be changed at the last minute it can be accomplished very simply. A quick check with the engineer to make sure reinforcement is adequate and an opening can be moved in a matter of minutes.
FORMING REVEALS AND RECESSES
Reveals or rustication strips are used to produce recesses and shadow lines in the face of the Tilt-Up panels. Reveals are formed by fastening wood, plastic, metal or particleboard strips to the floor or casting surface. These strips are usually 3⁄4-inch deep, using one-inch-by boards that are tapered for easy removal. The tapers are typically either 45 or 22.5 degrees. Some designers use 1-1/2- inch deep reveals but it is important to remember that the reveal strips reduce the effective structural depth of un-insulated panels. The location and depth of reveals must be accounted for in the design by the structural engineer. Reveals should be fastened with concrete nails or by a pneumatic stapling gun. When the panel is lifted, you want the strips to stay on the slab so they can be removed more easily. It is necessary to ensure that the reveals are in the right place, particularly if they are to match with adjoining panels.
The final step before the ironworkers can begin installing the reinforcing and embedments is to spray on the bond breaker. Two coats, applied at right angles to each other, should be used. When a light spray of water beads, it is an indication that the bond breaker is doing its job.
