Copyright Goodheart-Willcox Co., Inc. 406 Section 5 Concrete Summary • To receive concrete, forms should be prepared, leveled, and adequately braced. Subgrades should be smooth, compacted, and moist. • If a form is deep, the concrete should be compacted or vibrated after each layer is placed. • After concrete is placed, it is screeded. Excess concrete is struck off to bring the top surface to the correct grade or elevation. • Floating of concrete should follow immedi- ately after screeding to prevent bleeding. • A jointing tool is used to cut a control joint or contraction joint in the concrete. Cracking due to shrinkage is intended to occur at the joint. • Floating embeds aggregate particles just beneath the surface of the concrete, removes imperfections, and compacts the concrete at the surface in preparation for other finishing operations. • When a smooth, dense, hard surface is desired, steel troweling is performed after floating. Brooming produces a slightly roughened surface. • The temperature of concrete at the time of placing should generally be 50°F to 70°F (10°C to 21°C). • Air-entrained concrete is not as susceptible to damage caused by freezing and thawing as standard concrete. • Three basic joints used in concrete construc- tion are isolation joints, control joints, and construction joints. • Decorative and special concrete finishes include colored concrete, exposed aggregate, textured finishes, geometric patterns, nonslip and sparkling finishes, and combination finishes. • Exposed aggregate finishes can be created by using the seeding method (keeping the level of the concrete surface about 3/8″ to 1/2″ lower than the forms, then filling the space with the extra aggregate), exposing the stone in conven- tional concrete, or placing a thin topping course of concrete containing special aggre- gates over a base of regular concrete. • Popular types of cast-in-place (CIP) concrete walls include rustic, grid-patterned, colored aggregate, window walls, and sculptured. • Four basic cast-in-place concrete roof and floor structural systems are pan joists, waffles, flat plates, and flat slabs. • In tilt-up construction, concrete walls are cast in a horizontal position on-site or in another location and lifted from the casting position or truck to their final location in the building. Review Questions Answer the following questions using the information in this chapter. 1. Why should the subgrade be moistened before placing concrete? 2. Why are forms treated with oil or other preparation? 3. When placing concrete, the concrete should not be allowed to drop freely more than ______′. 4. Air pockets can be worked out of plastic concrete by puddling, spading, ______, and ______. 5. Too much vibration when removing air pockets by mechanical vibration can cause ______ of particles. 6. To ensure a good bond when placing fresh concrete on hardened concrete, the hardened concrete should be level, clean, ______, and ______. 7. Concrete that is shot into place by means of compressed air is called ______. 8. The fi rst fi nishing operation performed on concrete is usually ______. 9. A radius may be put on the edge of a concrete slab with a tool called (a)n ______. 10. What problems may result from performing fi nishing operations on a concrete slab while the bleed water is present? 11. List three reasons why concrete is fl oated. 12. Sidewalks and driveways are frequently ______ to produce a slightly roughened surface. A. broomed B. troweled C. fl oated D. None of the above.