Copyright Goodheart-Willcox Co., Inc. 46 Section 2 Introduction to Masonry Backpack Concrete Vibrators Figure 3‑52 shows a backpack concrete vibrator. This unit is beneficial when the construction site has lim- ited electrical power or access to electrical power is not available at the construction site. The concrete vibrator’s power source is a 2.5 hp gasoline engine. Shafts can be purchased in lengths from 2′ to 21′. Heads are available in rubber or steel. The backpack weighs approximately 30 lb. 3.3.11 Hand and Power Trowels A concrete mason’s hand finishing trowel is the last tool used in the finishing process of a concrete slab. These trowels range in size from 10″ to 20″ long and 3″ to 4 3/4″ wide. See Figure 3‑53. The first troweling of a slab is generally performed with a wide trowel that is 16″ to 20″ long. The last few troweling operations are usually done with a “fanning” trowel, which is 14″ to 16″ long and 3″ to 4″ wide. Power trowels have three or four rotating steel trowel blades. Sizes vary from 36″ to 48″ in diameter. See Figure 3‑54. They may be powered by electricity or a Allen Engineering Corporation Figure 3‑52. The backpack concrete vibrator oscillates from 11,000 to 12,000 vibrations per minute. Stanley Goldblatt Figure 3‑53. Mason’s concrete finishing trowel. Marshalltown Company Figure 3‑54. Power trowels have rotating steel blades. These machines are used on large projects. A B Allen Engineering Corporation Figure 3‑51. A—This device has built-in handles to make it easier to carry and operate when moving the head into the concrete and around the jobsite. B—This high-frequency concrete vibrator has an extra-long power cord of 49′ to reach most locations on a concrete project.