Casting Of Concrete 6o3557

Placing Once the concrete has been adequately mixed, it must be placed into the formwork that defines its final position and shape. If the concrete is to be reinforced, the rebar must already be in place so the concrete can flow around it (see Figure 22). If the concrete mixing truck can be located close to (and higher than) the site, then the concrete can be poured directly into the forms. In cases where this is not possible, the concrete can be transferred in buckets by a crane or by wheelbarrow. When this is impractical due to the distance required or the size of the job, the fresh concrete can be pumped through a system of pipes or hoses to the site by special concrete pumps. Concrete that is to be pumped has more stringent requirements for workability. If the concrete is too dry, it will not pump well, while if it is too wet it will tend to segregate. Segregation can also occur if the concrete falls into the formwork too quickly, as larger aggregate particles will tend to be driven downward. Consolidation Once the concrete is in place, it should be consolidated to remove large air voids developed during placement and to make sure that the concrete has flowed into all of the corners and nooks of the formwork. This process is also called compacting. Overconsolidation can lead to segregation and bleeding, but underconsolidation is more common, resulting in less-than optimal properties. The two most common methods of consolidation is are vibration and roller compacting. Vibration is a mechanical process that transfers pulses of shear energy to the concrete, usually by a probe that is inserted several inches into the concrete. Each pulse of shear energy momentarily liquefies the concrete, allowing it to flow very freely. This is the standard consolidating method for general construction projects with the exception of roads. The shear energy will only travel through a limited thickness of concrete, so when a thick concrete structures is being placed the fresh concrete is poured in layers, with each layer consolidated before the next is poured over it. Vibration is a noisy and labor-intensive step, requiring expensive and specialized equipment. For this reason, there is growing use of self-consolidating concrete which flows so freely (through the use of chemical ixtures) that mechanical consolidation is not needed (this is discussed in Chapter 13). Roller compaction is a simpler and more cost-effective technique that is suitable for roads and very large mass concrete structures such as dams. A specialized vehicle with a heavy roller on the front (a familiar sight in roadside construction zones) is driven over the fresh concrete to drive it into place and remove excess air. The fresh concrete used is very stiff so that it can the weight of the machine as it es over. Finishing For concrete floors and pavements, the appearance, smoothness, and durability of the surface is particularly important. Finishing refers to any final treatment of the concrete surface after it has been consolidated to achieve the desired properties. This can be as simple as pushing a wide blade over the fresh concrete surface to make it flat (screeding). 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 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.

Curing 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 ed. 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. Not only will this prevent the concrete from gaining its full strength, but it will also generate internal stresses that can cause cracking. To keep fresh and young concrete moist, it can be covered with plastic or damp fabric to prevent evaporation, or sprayed periodically with water. Spraying is particularly helpful when the w/c of the concrete is low, 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.