Q. On an upcoming job, the owner is thinking about using paper stencils. I've seen the coloring and texturing part of this operation done before, so I think I'll be OK there. But is there anything special I need to do when placing the slab?
A. Forming and subgrade preparation procedures are the same for stencil-patterned concrete as for other concrete installations—with the exception of rectilinear forming. Greater care must be taken to ensure that corners are square and straight, and form lines are parallel. Patterns can make forming problems very noticeable. Any quality concrete mix design is acceptable. "An advantage of stencil-patterned concrete is that any standard mix design can be used," says Mike Murray, Murray Decorative Concrete Supply, Shawanee, Kan. "I stress to everyone the importance of good mix designs using normal aggregate sizes with minimal deleterious aggregate. Special mix designs for other decorative products often involve smaller aggregates, higher cement counts, more water, and therefore more shrinkage."
As with preparation, there is nothing unusual about placing the concrete. It's placed, struck-off, bull floated, and edged. Workers then place the stencils. Two workers on opposite sides of the slab manipulate the stencil so that the center touches the concrete first. Then they carefully lower the stencil so that it all makes contact with the concrete and is properly aligned with the preceding stencil. If this operation doesn't go right, the stencil must be lifted and set down again. Pulling on either end or trying to shift it from side to side rather than fixing a poor alignment will distort and possibly tear the pattern.