Copyright Goodheart-Willcox Co., Inc. 382 Section 5 Concrete When placing concrete against walls in an inte- rior space such as a basement, the concrete needs to be hand troweled or floated in order to position it to the correct height and to level the surface. See Figure 18-7. Hand floating this area will also aid when it is time to screed and bull float the concrete. Prior to the placement of concrete, snap lines are usually placed on walls as a guide to indicate the final elevation or finished concrete floor height. A laser level can also be used to determine elevations. See Figure 18-8. As the laser rotates in the horizontal plane, the light beam revolves in a 360° circle so the worker can mark locations on the walls as reference points for determining heights. In larger concrete jobs, screeding can be accom- plished with a power screed. See Figure 18-9. Power screeds have a gas or electric motor that operates a mechanism that causes the base to vibrate. As the screed is pulled over the concrete surface, the vibra- tion drives the surface stones down into the mixture and brings up the “cream” to the top of the concrete. Floating of concrete should follow immediately after screeding, before the bleed water has a chance to come to the surface of the concrete. A bull float or darby is used to eliminate high and low spots and to smooth the surface after it is leveled with a power screed. A darby float is smaller than a bull float and is used to float areas the bull float cannot access. When bull floating, make sure the float does not dig into the concrete. It should float on the top to smooth the surface and to drive exposed stones down into the cement. See Figure 18-10 and Figure 18-11. On certain slabs it may be easier to Marshalltown Company Figure 18-7. A worker is using a hand float to level the concrete next to a foundation wall prior to screeding and bull floating. The pattern found in the foundation wall is being used as a guide line for the top edge of the concrete floor. The worker is smoothing the area next to the wall because the screed and bull float cannot get that close to the wall. Pukhov Konstantin/Shutterstock.com Figure 18-8. A rotary laser level creates a light beam (laser beam) either in the horizontal or vertical plane. Some lasers automatically level themselves and can also shoot a vertical beam out of the top of the device. The color of the laser beam, red or green, depends on the type of laser. Marshalltown Company Figure 18-9. Concrete workers are leveling the concrete with a power screed. The power screed is riding on screed pipes that are embedded just under the surface of the freshly placed concrete. The screed pipes are resting on plastic or metal jacks or wood stakes to keep the pipes elevated to the correct height. When the workers complete the section of floor, the pipes are removed and the voids are filled in with concrete.