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.
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-beams - if they are available. This type of system can be set up and taken down in minimum time.
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.
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 substance 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 they should be oiled before the reinforcing steel is placed. 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, you should 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. 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. - Fuel oil, 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 of fuel oil provides adequate thickness.
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:
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; andContinue Reading