Placing the Concrete
Place concrete as close as possible to its final position. Do not try to move it horizontally over long distances-especially if it is high slump. Working the concrete in this manner can result in segregation of the aggregate from the paste; this thin watery cement paste may then be worked to the surface, causing later crazing, dusting, or scaling. Place new concrete against the face of any previously placed concrete, rather than in separate mounds. Separate mounds of concrete can partially set up before they are joined, which may cause a poor bond. Start placing concrete for walls or slabs at one end and work across toward the other end. For walls, Concrete Salt Lake City has found out that its best to work in lifts of 1 to 2 feet deep. For slabs, work in strips of 4 to 6 feet wide. Avoid collecting water at ends, in corners, and along form faces. Use a baffle on slopes to avoid separation and accumulation of aggregate at the bottom of the slope. Place concrete on the lower part of the slope first and move upward.
Remember to use only accepted concrete-working concrete rakes. Do not use yard rakes; they may increase segregation. A mason’s pointed hand trowel is useful for some concrete work; however, it should not be used to finish concrete.
Do not drop the concrete more than 4 feet to its final location to prevent aggregate segregation. If a longer drop is required in forms such as basement walls, use a tremie or chute. Do not drop the concrete so that it hits the sides of the forms or through congested reinforcement. Serious segregation of the aggregate may occur. Careful placement using chutes or “elephant trunks” are recommended. An elephant trunk is a 4 to 6-inch diameter tube of rubber or canvas that keeps the concrete from segregating because it drops the mix from a hopper to the final location. This equipment often can be supplied by the cement finisher or concrete contractor, such as Concrete Salt Lake City
Mechanical vibration tools (vibrators) can be very useful to remove air bubbles from the concrete, especially on walls and other deep placements of concrete. Large air bubbles and poor consolidation can create troublesome voids-called “honeycomb”-in the finished concrete. Skillful vibration of each 12- to 24-inch lift of concrete in a wall will help consolidate the lower lift with the upper lift and prevent honeycomb. Vibration of concrete provides good consolidation and improves the bond of concrete to reinforcement. In turn, good consolidation and bonding reduces the probability of bugholes, rock pockets, and other problems. When using a vibrator, the operator should watch the affected area while working the vibrator head in an up-and-down motion, going slowly and as deeply as possible. Always pull the vibrator out of the concrete slowly to prevent formation of a hole or bubble in the concrete. Use of a vibrator is not recommended according to concrete company–Concrete Salt Lake City for exposed-aggregate surfaces since the action of the machine can cause bare spots (also called sand spots).
The jitterbug is a tool used to depress the coarse aggregate down from the top surface of a concrete slab-leaves a thin coat of sand, cement, and water on the surface and makes finishing easier. However, it also leaves a weaker concrete surface that is subject to spalling and scaling. Therefore, the use of a jitterbug is strongly discouraged. Any greater ease in finishing is likely to be offset by the complaints of the homeowner when deterioration of the concrete slab begins in as little as 2 to 3 years. Make it a practice to strike off the top surface as soon as the concrete is placed to avoid disturbing the concrete later on.
Finishing the Concrete
Complete striking off and bull floating before bleed water appears, and do not begin finish floating and troweling until most bleed water evaporates or is removed. If the concrete is worked while bleed water is present, a water-rich mix that is sand heavy is created at the worst place-the surface. Dusting, scaling, and crazing are almost certain to result. For the same reason, do not add water to the surface for ease of finishing.
Sometimes concrete begins to set up while bleed water is still present, In that case finishing must begin, and one of the following steps can be taken:
- Go out on the concrete on kneeboards and sweep off the water with a long trowel, darby, hairbroom, or squeegee.
- With one person at each end, drag the surface with a rubber hose.
- Lay damp burlap on the concrete and sprinkle dry cement on the burlap. Later, throw away the cement; the burlap can be washed and reused. Do not add cement directly to the surface to take up the water; it is likely to cause the surface to crack, craze, or dust after it dries. If air-entrained concrete with a low water-cement ratio is used, excess bleeding should not be a problem.