2.3 The Concrete Construction Process
The focus of this monograph is the chemistry, microstructure, and properties of cement and concrete, not the nuts and bolts of how to mix and pour the stuff. However, the majority of concrete is mixed and put into its final form in one continuous process, in contrast to materials like steel which are manufactured first and then assembled later (some concrete, called "precast concrete" is also made this way). For this reason it is not a good idea to completely separate the science of concrete from the more mundane process of making it. No one expects contractors and construction workers to manufacture plate glass, extrude pipe, or press drywall, but they are regularly expected to make good quality concrete.
What follows is a very brief summary of the most common process of making a concrete structure. Much more information is available from other sources.
Designing the concrete mix
The first and most important step in the process is to determine the ingredients that will make up the concrete and their proportions. As should be apparent from the previous section, there are many variables to consider including cement type, aggregate size and type, amount of water, and mineral and chemical admixtures. While a good mix design can still result in inadequate or poor quality concrete if it is not executed correctly, a bad mix design will of course always give poor results. Who is responsible for designing the mix depends on the type of project. For large, publicly funded projects the responsibility for the final design falls to a licensed civil engineer. For residential projects such as foundations and driveways it is the private contractor, whose professional reputation is generally his only credential. For do-it-yourself projects it is of course the homeowner who must design his own mix.
How does one go about generating an appropriate mix design? This is done by first determining the properties that the fresh and hardened concrete must have and then working backwards to find the most economical mix design that gives these properties. Here are some of the main things to consider:
Loads that must be supported: Concrete can be made with a wide variety of strengths, so this is often the starting place for the mix design. Since the cost of concrete scales rather closely with its strength, one does not want to make the concrete stronger than it needs to be. However, if the application will only be supporting relatively small loads, it is usually not a good idea to specify weak concrete, because weak concrete almost always lacks durability. For low load applications the quality of the concrete is determined by other factors such as resistance to freezing or wear resistance.
Workability: The workability that is required depends primarily on how the concrete is to be placed. Concrete can be poured, pumped, and even sprayed into place, and this will affect the workability that is needed. Other factors such as the shape of the molds, the rebar spacing, and the equipment available at the site for consolidating the fresh concrete after it is placed must also be considered. Workability is usually defined by the slump, which is the tendency for the fresh concrete tends to spread out under its own weight when placed onto a flat surface.
Environmental conditions: If the concrete will be exposed to severe conditions, then this may well determine the necessary concrete quality rather than the applied loads. In cold-weather locations the concrete must be able to resist freezing. Roads and must be able to withstand the corrosive effects of salt. Underground applications must be able to resist the ingress of moisture and aggressive species from the soil. For almost any type of conditions or mode of attack, the most effective defense is to keep the w/c low.
Surface wear: For some applications the physical loads tend to wear away the concrete instead of breaking it. For roads, parking garages, driveways, and industrial floors the hardness and wear resistance of the top layer of concrete will determine how long the structure lasts.
The ready-mix (batching) plant
Most concrete is batched and mixed in a central location called a ready-mix plant and then trucked to the desired location (see figure 2.1). This is often the best solution even for fairly small jobs. Ready-mix plants have a wide variety of aggregate and cement that are stored under controlled conditions, as well as good equipment for weighing and mixing. As a result, the quality of the concrete should be high and consistent. Concrete mixing trucks can be used to transport already-mixed concrete, or the mixing can actually be performed by the truck as it is traveling to the site. One potential disadvantage of ready-mixed concrete is that the time required to transport the concrete to the site may use up too much of the early period of good workability. This can generally be handled through the use of retarding admixtures.
Figure 2-1: Left: A ready-mix plant surrounded by piles of stored aggregate (40-4A). Right: A concrete truck being filled with fresh premixed concrete. (Images courtesy of the Portland Cement Association).
Mixing of concrete is a very important step for achieving good final properties, and one that can be quite difficult without the right equipment. This is one of the best reasons for using ready-mixed concrete. Mixing distributes the aggregate evenly throughout the cement paste, ensures that all of the cement has been fully saturated in water, and removes large air voids. In addition, mixing breaks up agglomerated clusters of cement particles and allows air entraining admixtures to generate the correct air void system. Undermixing leaves large flaws and thus results in inferior strength, while overmixing wastes time and energy and can destroy entrained air voids. The lower the workability, the more mixing energy and mixing time is required.
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 2-2).
Figure 2-2: Fresh concrete being poured into a framework containing steel rebar. (Image courtesy of the Portland Cement Association).
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.
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 admixtures) 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 support the weight of the machine as it passes over.
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.
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. 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.
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