Copyright Goodheart-Willcox Co., Inc. Chapter 18 Concrete Flatwork and Formed Shapes 383 access the perimeter of the slab of the concrete first. See Figure 18-12. In normal concrete, the dry materials used are heavier than water. They begin to sink or settle to the bottom of a plastic concrete mixture shortly after placement. This settling action causes bleeding, which is excess water that rises to the surface. See Figure 18-13. Bleeding can also occur if excessive water is added during concrete mixing. This usually occurs at the jobsite in an attempt to make the mix easier to place. Adding water on the jobsite also reduces the strength of the mix. Bleeding does not usually occur with air-entrained concrete. Marshalltown Company Figure 18-10. This worker is pushing the bull float. Notice the angle at which the float is held—it is riding on the back side when being pushed. Marshalltown Company Figure 18-11. This worker is pulling the bull float back toward him. The back side of the trowel is lifted up to allow the float to smooth the cream of the concrete rather than digging into it and creating a dip. Vicki Reid/iStock/Thinkstock Figure 18-12. This worker is bull floating around the perimeter of a driveway. In some situations, it is easier to float the perimeter than to bull float between the edges. Craig Jewell/Hemera/Thinkstock Figure 18-13. This concrete has been screeded and bull floated. Bleed water has appeared on the surface.