Placing & Finishing
Placing & Finishing
Concrete delivery should be closely coordinated with placing and finishing operations. Concrete should not be poured faster than it can be worked – spreading, consolidating, bullfloating. Instead, concrete should be poured continuously as near as possible to its final position. You should never dump concrete into separate piles and work together.
Placing concrete in forms and then consolidating is popular for some types of construction. The consolidation process uses vibration and gravity to compact fresh concrete within the forms and around embedded items and reinforcement. This process eliminates unwanted pockets of trapped air, honeycomb and stone while maintaining significant amounts of entrained air. The vibration temporarily suspends the natural friction between the aggregate particles, causing the concrete to behave as a liquid. Internal friction resumes as the vibration is stopped.
Visible concrete – slabs used for driveways, patios, etc. – will often need finishing. You can finish your concrete in many ways, including color tinting and texturing. Patterned-stamped finishes are very popular on projects of all sizes. Some surfaces may require just strikeoff and screeding to proper contour and elevation. Others may be better off with a broomed, troweled or floated finish. Strikeoff and screeding 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 progresses 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. 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, floating should be followed by steel troweling. 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.
Curing & Drying
Curing is one of the most important steps in concrete construction, because proper curing greatly increases concrete strength and durability. Concrete hardens as a result of hydration: the chemical reaction between cement and water. However, hydration occurs only if water is available and if the concrete’s temperature stays within a suitable range. During the curing period-from five to seven days after placement for conventional concrete – the concrete surface needs to be kept moist to permit the hydration process. The most common method of curing (and one of the simplest) is to utilize a liquid membrane which is sprayed or rolled on the surface of a slab after finishing to prevent premature drying of the surface. Water may be used when you are able to insure constant wetting for at least 3 days.
Curing in extreme weather
Temperature extremes make it difficult to properly cure concrete. On hot days, too much water is lost by evaporation from newly placed concrete. If the temperature drops too close to freezing, hydration slows to nearly a standstill. Under these conditions, concrete ceases to gain strength and other desirable properties. In general, the temperature of new concrete should not be allowed to fall below 50°F during the curing period. Membrane curing does not keep the concrete warm enough in freezing temperature, nor does calcium chloride accelerator prevent concrete from freezing. Only adequate insulation or heating will maintain proper curing temperatures during freezing weather.
Curing shortcuts to avoid
Avoid any curing method that lets the surface dry in a short time. Quick drying stops the hardening process, thus making a weak surface that is likely to scale.
After curing, newly placed outdoor concrete needs time to dry in warm air. For best results, plan to place your concrete early enough in the season so that it has one month of temperatures above 40°F for curing and still another month for drying out before hard freezes are expected.
Water-repellent coatings or sealers can help prevent damage from freeze/thaw cycles and salting. They deter water from getting into the surface pores. Newly cured concrete should have a period of air-drying before being sealed.
Do not use salt or other deicers during the first winter. Use sand instead to improve traction. Even light applications of salt, or salt carried on cars may cause severe scaling of newly placed concrete. Fertilizers are not an acceptable deicer at any time.
Proper drainage should be maintained to avoid saturation of concrete.