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Placing and Finishing Concrete |
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Mixing, transporting, and handling of concrete should be carefully coordinated with placing and finishing operations. Concrete should not be deposited more rapidly than it can be spread, struck off, consolidated, and bullfloated. Concrete should be deposited continuously as near as possible to its final position. In slab construction, placing should be started along the perimeter at one end of the work with each batch placed against previously dispatched concrete. Concrete should not be dumped in separate piles and then leveled and worked together; nor should the concrete be deposited in large piles and moved horizontally into final position. Concrete that will be visible, such as slabs like driveways, highways, or patios, often needs finishing. Finishing is the operation of consolidating, leveling, and creating a concrete surface of desired texture and hardness. Finishing makes the concrete attractive and serviceable. Concrete slabs can be finished in many ways, depending on the intended service use. Options include various colors and textures, such as exposed aggregate or a patterned-stamped surface. Some surfaces may require only strikeoff and screeding to proper contour and elevation, while for other surfaces a broomed, floated, or troweled finish may be specified. In slab construction, screeding or strikeoff is the process of cutting off excess concrete to bring the top surface of the slab to proper grade. A straight edge is moved across the concrete with a sawing motion and advanced forward a short distance with each movement. Bullfloating eliminates high and low spots and embeds large aggregate particles immediately after strikeoff. This looks like a long-handled straight edge pulled across the concrete. Screeding and bullfloating should be done before the bleed water rises to the surface. All other finishing procedures must wait until bleeding has stopped, generally when all the sheen has left the surface. Early finishing can cause crazing, dusting, blistering and scaling. Quality Steps for Finishing • Do not begin finishing until all bleed water has evaporated and concrete is firm enough to leave only ¼ inch deep footprints Waiting time is affected by the weather conditions and the amount of water, cement, and chemical admixtures in the concrete. • Outside concrete should use air entrained concrete (5 to 8%) for freeze-thaw protection and should not be steel troweled. Bull float and broom finish is recommended. • Never sprinkle water or cement on concrete while finishing it. This may cause dusting or scaling. • Never use a Jitterbug. Jointing is required to eliminate unsightly random cracks. Contraction joints are made with a hand groover or by inserting strips of plastic, wood, metal, or preformed joint material into the unhardened concrete. Sawcut joints can be made after the concrete is sufficiently hard or strong enough to prevent raveling. After the concrete has been jointed, it should be floated with a wood or metal hand float or with a finishing machine using float blades. This embeds aggregate particles just beneath the surface; removes slight imperfections, humps, and voids; and compacts the mortar at the surface in preparation for additional finishing operations. Where a smooth, hard, dense surface is desired, such as a floor, floating should be followed by steel troweling. Exterior slabs like driveways, patios and sidewalks are usually not troweled. Do not trowel air-entrained concrete. Troweling should not be done on a surface that has not been floated; troweling after only bullfloating is not an adequate finish procedure. A slip-resistant surface can be produced by brooming before the concrete has thoroughly hardened, but it should be sufficiently hard to retain the scoring impression.
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