One obstacle to the wider use of decorative concrete is cost: it's simply more expensive than plain concrete. Owners, therefore, naturally conclude they are getting a higher quality product. Those expectations begin with good form lines. Although good formwork should be part of plain concrete work, too, here are some of the issues that concern owners of decorative work.
A form line that is supposed to be straight but isn't draws attention away from the rest of the job and can cause owner dissatisfaction (see photo 1). For straight lines, form boards should be set to a dry line in order to keep them straight and at the proper elevation. The forms should also be kicked or braced at least every 4 feet so that they don't move during concrete placement.
Photo 2 shows a new concrete slab that butts up against an asphalt driveway. The problem is that the slab is an inch above the driveway, causing a tripping hazard. Care must be taken to meet existing elevations.
When you set elevations, you also determine pitch, which regulates how water will flow off a slab. The best pitch is ¼ inch per foot (2%). Never set less than 1/8 inch per foot (1%) as water will remain stationary on a slab. You also need to provide yourself with a safety factor in the event of a striking error. Owners of decorative flatwork will be dissatisfied with birdbaths. Check out ASCC Position Statement #7, which states that birdbaths may be an unavoidable consequence of a flatness tolerance.
Maintaining right angles
Many stamped concrete patterns and engraved lines are rectilinear. If the corners of a slab aren't perpendicular and the forms aren't parallel, the patterned concrete will highlight the error.
Providing nicely shaped curving lines is perhaps the most difficult forming operation. Setting elevation is more difficult, too, with curving lines, but it can be done quickly by setting a dry line over the curved forms at the proper elevation. Compare the plane of the line to the forms and adjust them so that they are the same. It's an easy way to maintain proper elevation.
Forming small and large radii requires different forming materials, and sometimes they must be joined. Care must be taken to ensure good transitions at these points. Photo 3 shows the wrong way to bring different materials together. Photo 4 shows a much better transition method.
Curving shapes have to look good. Photo 5 is an example of a curve that started out well but flattened out near the end. When you are forming a curve, it helps to join the form boards first. Then when you start pushing the form into position, the form boards work together to provide a natural looking shape.
An easier way to form curving shapes is to use flexible extruded high-density polyethylene forms (photo 7). Mike Lane, president of Plastiform, suggests pushing a piece of PVC pipe through the extrusion holes in their forms to help carry the shape of a curve from one length of form to the next. Metal Forms Corporation's PolyMeta forms come with either rigid aluminum end connections for straight splices or flexible polyethylene end connectors for curving splices. Four-inch-wide forms can form radii as tight as 3 feet. There is a keyway on the backside of both of these forms to insert cam-lock clamps for attaching different types of stakes anywhere along the form. Both forms come in 4-inch and 6-inch widths.
Good forming is an art that requires good craftsmanship. Learning good skills helps to ensure that owners are satisfied.
For more information:
Metal Forms Corp., call 414-964-4550, visitwww.metalforms.com.
Plastiform, call 800-358-3007, visitwww.plastiform.com.