Manufacturing of Concrete Products and Precast Elements: Processes and Equipment (edition beton)
Improper curing can cause scaling , reduced strength, poor abrasion resistance and cracking. During the curing period, concrete is ideally maintained at controlled temperature and humidity. To ensure full hydration during curing, concrete slabs are often sprayed with "curing compounds" that create a water-retaining film over the concrete. Typical films are made of wax or related hydrophobic compounds.
After the concrete is sufficiently cured, the film is allowed to abrade from the concrete through normal use. Traditional conditions for curing involve by spraying or ponding the concrete surface with water. The adjacent picture shows one of many ways to achieve this, ponding—submerging setting concrete in water and wrapping in plastic to prevent dehydration. Additional common curing methods include wet burlap and plastic sheeting covering the fresh concrete.
For higher-strength applications, accelerated curing techniques may be applied to the concrete. A common technique involves heating the poured concrete with steam, which serves to both keep it damp and raise the temperature, so that the hydration process proceeds more quickly and more thoroughly. Pervious concrete is a mix of specially graded coarse aggregate, cement, water and little-to-no fine aggregates. This concrete is also known as "no-fines" or porous concrete.
Mixing the ingredients in a carefully controlled process creates a paste that coats and bonds the aggregate particles. The hardened concrete contains interconnected air voids totaling approximately 15 to 25 percent. Water runs through the voids in the pavement to the soil underneath. Air entrainment admixtures are often used in freeze—thaw climates to minimize the possibility of frost damage.
Pervious concrete also permits rainwater to filter through roads and parking lots, to recharge aquifers, instead of contributing to runoff and flooding. To ensure the mixing is thorough enough to create nanoconcrete, the mixer must apply a total mixing power to the mixture of 30— watts per kilogram of the mix. This mixing must continue long enough to yield a net specific energy expended upon the mix of at least joules per kilogram of the mix. A superplasticizer is then added to the activated mixture which can later be mixed with aggregates in a conventional concrete mixer.
In the HEM process, the intense mixing of cement and water with sand provides dissipation and absorption of energy by the mixture and increases shear stresses on the surface of cement particles. As a result, the temperature of the mixture increases by 20—25 degrees Celsius.
This intense mixing serves to deepen hydration process inside the cement particles. The nano-sized colloid Calcium Silicate Hydrate C-S-H formation increased several times compared with conventional mixing. Thus, the ordinary concrete transforms to nanoconcrete.
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Bacteria such as Bacillus pasteurii , Bacillus pseudofirmus , Bacillus cohnii , Sporosarcina pasteuri , and Arthrobacter crystallopoietes increase the compression strength of concrete through their biomass. Not all bacteria increase the strength of concrete significantly with their biomass. Sporosarcina pasteurii reduces water and chloride permeability. Polymer concretes are mixtures of aggregate and any of various polymers and may be reinforced.
The cement is costlier than lime-based cements, but polymer concretes nevertheless have advantages; they have significant tensile strength even without reinforcement, and they are largely impervious to water. Polymer concretes are frequently used for repair and construction of other applications, such as drains. Grinding of concrete can produce hazardous dust. Exposure to cement dust can lead to issues such as silicosis , kidney disease, skin irritation and similar effects. The U.
National Institute for Occupational Safety and Health in the United States recommends attaching local exhaust ventilation shrouds to electric concrete grinders to control the spread of this dust. That same rule went into effect 23 June for general industry, hydraulic fracturing and maritime. That the deadline was extended to 23 June for engineering controls in the hydraulic fracturing industry. Companies which fail to meet the tightened safety regulations can face financial charges and extensive penalties.
Concrete has relatively high compressive strength , but much lower tensile strength. The elasticity of concrete is relatively constant at low stress levels but starts decreasing at higher stress levels as matrix cracking develops. Concrete has a very low coefficient of thermal expansion and shrinks as it matures. All concrete structures crack to some extent, due to shrinkage and tension. Concrete that is subjected to long-duration forces is prone to creep.
Tests can be performed to ensure that the properties of concrete correspond to specifications for the application. Different mixes of concrete ingredients produce different strengths. Concrete strength values are usually specified as the lower-bound compressive strength of either a cylindrical or cubic specimen as determined by standard test procedures.
Different strengths of concrete are used for different purposes. Higher-strength concrete is often used for larger civil projects. Bridges may use long beams of high-strength concrete to lower the number of spans required.
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If a structure must be very rigid, concrete of very high strength may be specified, even much stronger than is required to bear the service loads. Concrete is one of the most durable building materials. It provides superior fire resistance compared with wooden construction and gains strength over time.
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Structures made of concrete can have a long service life. Concrete is used more than any other artificial material in the world. Due to cement's exothermic chemical reaction while setting up, large concrete structures such as dams , navigation locks , large mat foundations, and large breakwaters generate excessive heat during hydration and associated expansion. To mitigate these effects, post-cooling  is commonly applied during construction. An early example at Hoover Dam used a network of pipes between vertical concrete placements to circulate cooling water during the curing process to avoid damaging overheating.
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Similar systems are still used; depending on volume of the pour, the concrete mix used, and ambient air temperature, the cooling process may last for many months after the concrete is placed. Various methods also are used to pre-cool the concrete mix in mass concrete structures.
Another approach to mass concrete structures that minimizes cement's thermal byproduct is the use of roller-compacted concrete , which uses a dry mix which has a much lower cooling requirement than conventional wet placement. It is deposited in thick layers as a semi-dry material then roller compacted into a dense, strong mass.
Advantage and Disadvantage of Concrete. Raw concrete surfaces tend to be porous and have a relatively uninteresting appearance.
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Many different finishes can be applied to improve the appearance and preserve the surface against staining, water penetration, and freezing. Examples of improved appearance include stamped concrete where the wet concrete has a pattern impressed on the surface, to give a paved, cobbled or brick-like effect, and may be accompanied with coloration.
Another popular effect for flooring and table tops is polished concrete where the concrete is polished optically flat with diamond abrasives and sealed with polymers or other sealants. Other finishes can be achieved with chiseling, or more conventional techniques such as painting or covering it with other materials.
The proper treatment of the surface of concrete, and therefore its characteristics, is an important stage in the construction and renovation of architectural structures.
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Prestressed concrete is a form of reinforced concrete that builds in compressive stresses during construction to oppose tensile stresses experienced in use. This can greatly reduce the weight of beams or slabs, by better distributing the stresses in the structure to make optimal use of the reinforcement. For example, a horizontal beam tends to sag. Prestressed reinforcement along the bottom of the beam counteracts this. In pre-tensioned concrete, the prestressing is achieved by using steel or polymer tendons or bars that are subjected to a tensile force prior to casting, or for post-tensioned concrete, after casting.
Reinforced concrete , prestressed concrete and precast concrete are the most widely used types of concrete functional extensions in modern days. See Brutalism. Extreme weather conditions extreme heat or cold; windy condition, and humidity variations can significantly alter the quality of concrete. Many precautions are observed in cold weather placement. Preventing freezing is the most important precaution, as formation of ice crystals can cause damage to the crystalline structure of the hydrated cement paste.
If the surface of the concrete pour is insulated from the outside temperatures, the heat of hydration will prevent freezing. In Canada, where temperatures tend to be much lower during the cold season, the following criteria are used by CSA A The minimum strength before exposing concrete to extreme cold is psi 3. CSA A Concrete roads are more fuel efficient to drive on,  more reflective and last significantly longer than other paving surfaces, yet have a much smaller market share than other paving solutions.
Modern-paving methods and design practices have changed the economics of concrete paving, so that a well-designed and placed concrete pavement will be less expensive on initial costs and significantly less expensive over the life cycle.