The Runyon Surface Prep 4.5″ Non-Threaded 18 Segment Premium Concrete Grinding Cup Wheel is designed to give you a perfectly smooth finish. The premium 18 segment gives a smoother finish than the premium 9 segment. It also provides a longer life on concrete and masonry surfaces. With twice as many segments these cup wheels are designed to be a little less aggressive and have a longer life than the 9 or 12 seg version. This makes them a popular option for softer concrete floors. They are also popular for general concrete prep.