We have placed some beams on an architectural concrete job with an attractive rustication pattern. However, cracks forming in the bottoms of the beams have detracted seriously from their appearance. What can be done to minimize their size when we do another such job?
In the regions of flexural tension in the bottom of an architectural beam the depth of the rustication should be kept to a minimum. Where a rustication is used on the bottom surface of a beam, additional concrete must be added below the steel equivalent to the depth of the rustication. The result is that any cracks that occur in the tension zone will be wider at the surface. If the surface happens to be sandblasted the width will be further accentuated by rounding of the edges of cracks. For these reasons the depth of the rustication should be made more shallow in the tension zone, thus reducing the amount of excess cover on the steel and making the bottom widths of the cracks narrower.