The Concrete Construction Process (Part III)
Floating and troweling
Floating and troweling is a process of compacting and smoothing the surface which is performed as the concrete is starting to harden. This would be standard procedure for driveways and sidewalks. After concrete has hardened, mechanical finishing can be used to roughen the surface to make it less slippery or to polish the surface as a decorative step to bring out the beauty of a special aggregate such as marble chips.
A recently developed process, which is growing in popularity, involves the use of concrete dyes and surface molds to emulate the appearance of bricks, decorative pavers, or even ceramic tile. When done properly this type of decorative concrete is almost indistinguishable from the real thing.
Once concrete has been placed and consolidated it must be allowed to cure properly to develop good final properties. As the concrete hardens and gains strength it becomes less and less vulnerable, so the critical time period is the first hours and days after it is placed. Proper curing of concrete generally comes down to two factors, keeping it moist and keeping it supported. Hydration of cement, as the word itself implies, involves reaction with water. To cure properly, the cement paste must be fully saturated with water. If the relative humidity level inside the concrete drops to near 90%, the hydration reactions will slow, and by 80% they will stop altogether. This will not only prevent the concrete from gaining its full strength, but it will also generate internal stresses that can cause cracking.
To keep concrete moist, it can be covered with plastic or damp fabric to prevent evaporation, or sprayed periodically with water. Spraying is particularly helpful, because the original mix water is not enough for the cement to hydrate fully. The additional water will not penetrate through a thick concrete structure, but it will help create a stronger surface layer. Pools of water should not be allowed to form on the surface, however, as this will leach and degrade the concrete underneath.
When concrete is placed using formwork, there is generally a desire to remove the formwork as quickly as possible to continue the construction process. However, if this is done too soon, the fresh concrete will deform under its own weight. This will lead to a loss of dimensional tolerances, cracking, or even a complete collapse. Similar problems occur if loads are applied to the surface of a floor or slab too early.
The weather plays an important role in the curing process. Hot windy weather leads to rapid evaporation and thus particular care must be taken to keep the concrete moist. Cold weather causes the concrete to harden much more slowly than hot weather. This delays the construction process, but leads to better concrete in the long run, because the hydration products develop differently at different temperatures. If fresh concrete freezes, however, it will likely be destroyed beyond repair.
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