Q.: Your article on decorative form liners (May 1987, page 431) failed to comment on suitable concrete mixes. If we have ribbed form liners, can we expect problems with aggregate being too large to fit into the ribs?
A.: Apparently this must not be a common problem. We consulted manufacturers' literature, books on formwork, and several American Concrete Institute (ACI) guides and standards, but found only one source that contained a specific warning. In a recently-issued jobsite guide, Greenstreak says, "For ribbed textures the aggregate should be smaller than the width of the rib. Oversize aggregate can cause honeycombing and chipping on the ribs."
Even more critical to good results with ribbed form liners is proper consolidation of the concrete by vibration. Several sources stress the need for added vibration because the textured liner has so much more surface area than a flat form. ACI recommends depositing concrete in layers 12 to 18 inches thick and vibrating one lift at a time, extending the vibrator at least 6 inches into the previous lift at each insertion point. The vibrator is held stationary 10 to 30 seconds or until consolidation is judged to be complete. Then it is withdrawn at about 3 inches per second. Symons Corporation application guides suggest that, with a deeply textured form liner, the penetration into the previous lift be 6 to 12 inches and the rate of withdrawal be reduced to 1 to 2 inches per second.