Fins and offsets are abrupt irregularities that often appear on formed concrete surfaces. They can be caused by damaged, displaced, mismatched, or misplaced forms and liners.
Fins are narrow ridges on vertical or horizontal surfaces caused by mortar leakage between adjacent form elements or between the form and a previously completed lift of concrete. They are most likely to occur when high-slump concrete is used. Offsets are abrupt changes in alignment or dimension, either horizontally or vertically. They can result from inadequacies in form design and construction or unanticipated jobsite conditions.
To reduce or prevent fins, give special attention to maintaining accurate, tight joints between form panels and to supporting panel and form-sheathing edges. Careful workmanship in form erection is essential to prevent troublesome offsets.