The essential ingredients of concrete are cement, aggregate (sand, gravel) and water. When mixed in carefully prescribed proportions, they produce a workable mass, which can take the shape of any formwork into which it is placed and allowed to harden in.
Concrete technology is one that requires a great deal of know-how and experience. Therefore, only very general aspects can be dealt with here. If detailed information is required, specialized literature should be consulted, or professional advice sought.
Preparation of concrete mix
• Depending on the use and desired performance of the concrete, careful selection of the type and proportion of cement, aggregates and water is necessary, which is best done by a series of tests (if the qualities of the materials are not standardized or well-known from experience).
• In most cases, a good grain size distribution of fine and coarse aggregate (sand and gravel) is necessary, in order to leave no voids, which weaken the concrete. The more voids, the more cement and water are needed.
• Aggregate particles with rough surfaces and angular shapes create more friction than smooth, rounded particles, which are easier to compact. Silt, clay and dust should be removed, as they interfere with the bond between cement and aggregate, and require more water.
• The water should be as clean as possible, as salts and other impurities can adversely affect the setting, hardening and durability of the concrete. Seawater should be avoided as far as possible, especially in reinforced concrete, in which the steel easily corrodes.
• In special cases, a variety of admixtures can be used, depending on whether the setting should be accelerated or retarded, waterproofing and chemical resistance should be improved, and so on. Correct dosage and quality control are vital to achieve satisfactory results and save costs.
• The aggregate and cement should be well mixed in the dry state. Just before the concrete is used, water is added gradually while the mixing continues. As the water: cement ratio determines the strength and durability of the concrete (excess water produces air voids! ), the addition of water requires special care.
• In ready-mixed concrete, supplied from a central batching/mixing plant, by truck mixers (which are still rare in developing countries) principally the same criteria apply. However, a study by the Cement Research Institute, India, recommends the transportation of "semidry" mixes in small non-agitating vehicles (cheaper! ) and completion of mixing prior to final placing.
• The uniformity of fresh concrete is usually measured by the slump test: filling a conical mould in four layers of equal volume and rodding each layer 25 times, smoothing the top, lifting off the mould and measuring the difference in heights of the mould and the fresh concrete specimen. Slumps between 25 and 100 mm are most suitable.
• Mixes are specified primarily by grade designations, eg C7, C10, C25, etc., which refer to their compressive (C) strengths in N/mm2 (MPa).
• Formwork, which can be reused many times, is usually made of timber boards or steel panels, with joints sufficiently tight to withstand the pressure of compacted concrete, and without having any gaps through which the cement paste can leak.
• The texture of the hardened concrete surface can be predetermined by the type of formwork. If smooth surfaces are needed, concrete remnants from previous castings should be scraped off the forms.
• In order to facilitate removal, the inner surfaces of the formwork should be oiled with a brush or spray.
• If reinforcement is required, it is placed in the formwork after oiling, and spacers (pieces of stone or broken concrete) are placed between the steel and the oiled surface, such that the formwork and steel do not come into contact with each other. This is needed to prevent the steel from remaining exposed on the concrete surface, where it can easily rust.
• The choice of formwork must take into account ease of assembly and removal. In some cases, the formwork can be designed to remain in place (permanent shuttering); for example, where an insulating layer or special facing is needed, these can constitute the formwork (or part of it).
Placing and curing
• The concrete is transported from the mixer to the formwork by cranes, dumpers, barrows, buckets, pipes, or other means, depending on the available facilities. In many developing countries, long chains of workers pass the concrete in small metal pans from one to another. If the concrete is not produced on the site, ready-mixed concrete is brought in a special truck.
• The concrete must be placed without interruption to fill complete sections each time, since joints between concrete placed at different times are weak points.
• After a certain amount of concrete is in the formwork it needs to be compacted to fill up all voids. This is most effectively done by means of a vibrator (either attached to the formwork, or immersed in the concrete) which releases the trapped air. However, for most low cost constructions, which do not need high strengths, hand compaction with a suitable rod can be quite sufficient.
• It is important to immediately wash all the equipment that has been in contact with the concrete, as it will be difficult to remove after hardening.
• The formwork is removed after a few days when the concrete it hard enough. But strength development (curing) takes place over several weeks and a vital prerequisite is that the concrete is kept wet for at least 14 days, eg by covering it with wet jute bags which are regularly watered.
• All the above points, from preparation of concrete mix to curing, apply likewise to in situ construction (at the building site) and to prefabrication.
• Plain mass concrete, with graded or predominantly small sized aggregate, for foundations, floors, paving, monolithic walls (in some cases), bricks, tiles, hollow blocks, pipes.
• No-fines concrete, a lightweight concrete with only single size coarse aggregate (dense or lightweight) leaving voids between them, suitable for loadbearing and non-loadbearing walls, in-fill walls in framed structures or base coarse for floor slabs. No-fines concrete provides an excellent key for rendering, good thermal insulation (due to air gaps), and low drying shrinkage. The large voids also prevent capillary action.
• Lightweight aggregate concrete, using expanded clay, foamed blast furnace slag, sintered fly ash, pumice, or other light aggregate, for thermal insulating walls and components, and for lightweight building blocks.
• Aerated concrete, made by introducing air or gas into a cement-sand mix (without coarse aggregate), for thermal insulating, non-structural uses and lightweight building blocks. Disadvantages are low resistance to abrasion, excessive shrinkage and permeability. However, it is easy to handle and can be cut with a saw and nailed like timber.
• Reinforced concrete, also known as RCC (reinforced cement concrete), which incorporates steel bars in sections of the concrete which are in tension (to supplement the low tensile strength of mass concrete and control thermal and shrinkage cracking), for floor slabs, beams, lintels, columns, stairways, frame structures, long-span elements, angular or curved shell structures, etc., all these cast in situ or precast. The high strength to weight ratio of steel, coupled with the fortunate coincidence of its coefficient of thermal expansion being about the same as concrete, make it the ideal material for reinforcement. Where deformed bars (which have ribs to inhibit longitudinal movement after casting) are available, they should be given preference, as they are far more effective than plain bars, so that up to 30 % of steel can be saved.
• Prestressed concrete, which is reinforced concrete with the steel reinforcement held under tension during production, to achieve stiffness, crack resistance and lighter constructions of components, such as beams, slabs, trusses, stairways and other large-span units. By prestressing, less steel is needed and the concrete is held under compression, enabling it to carry much higher loads before this compression is overcome. Prestressing is achieved either by pre-tensioning (in which the steel is stressed before the concrete is cast) or by post-tensioning (after the concrete has reached an adequate strength, allowing the steel to be passed through straight or curved ducts, which are filled with grout after the reinforcement has been tensioned and anchored). This is essentially a factory operation, requiring expensive, special equipment (jacks, anchorages, prestressing beds, etc.), not suitable for low-cost housing.
• However, the cold-drawn low-carbon steel wire prestressed concrete (CWPC) technology, developed in China, where about 3000 CWPC factories produce 20 million m3 of precast components annually, is a promising alternative. The tensile strengths of low-carbon steel wires (normal steel wires) of 0 6.5 to 8 mm are doubled by drawing them through a die at normal temperatures, producing 3, 4 or 5 mm 0 wires, and saving 30 to SO % of the steel. Concrete grades of C30 are used. The technology is easily understood and implemented, the equipment is simple (Bibl. 09.09).
• Concrete can take any shape and achieve compressive strengths exceeding 60 N/mm2.
• Reinforced concretes combine high compressive strengths with high tensile strengths, making them adaptable to any building design and all structural requirements. They are ideally suited for prefabrication of components and for constructions in dangerous conditions (earthquake zones, expansive soils, etc.).
• The energy requirement to produce 1 kg of plain concrete is the lowest of the manufactured building materials (1 MJ/kg, equalling timber; Bibl. 00.50), while reinforced concrete (with 1 % by volume of steel) requires about 8 MJ/kg.
• The high thermal capacity and high reflectivity (due to light colour) are especially favourable for building in hot dry or tropical highland climates.
• Properly executed concrete is extremely durable, maintenance-free, resistant to moisture penetration, chemical action, fire, insects, and fungal attack.
• Concrete has an extremely high prestige value.
• A variety of processed agricultural and industrial wastes can be profitably used to substitute cement and/or improve the quality of concrete.
• High cost of cement, steel and formwork.
• Difficult quality control on building sites, with the risk of cracking and gradual deterioration, if wrongly mixed, placed and insufficiently cured with water.
• In moist climates or coastal regions, corrosion of reinforcement (if insufficiently protected), leading to expansion cracks.
• Fire resistance only up to about 500° C, steel reinforcement begins to fail (if not well covered) and after fires, RCC structures usually have to be demolished.
• Demolishing concrete is difficult and debris cannot be recycled, other than in the form of aggregate for new concrete.
• Negative electromagnetic effects of reinforced concrete create unhealthy living conditions.
• Cement proportions can bereduced by careful mix design, grading of aggregates, testing, quality control and by substitution with cheaper pozzolanas; also, increased decentralized cement production with sufficient supplies and low wastage (by better bagging) can reduce costs.
• Saving in steel reinforcement can be achieved by good structural design and use of deformed bars or prestressing with cold-drawn low-carbon steel wire.
• Quality control is only possible with a well-trained team and continuous supervision.
• The improvement fire resistance of non-structural components is possible by using high-alumina cements with crushed Bred brick, which resist temperatures up to 1300° C (refractory concrete).
• Crushed fired brick (brick rejects) can be used to substitute gravel aggregate, where these are scarce (eg Bangladesh), resulting in a relatively lightweight concrete of slightly less strength but higher abrasion resistance. Since the brick aggregate absorbs water, more water is required in preparing the concrete mix.
• Expansion joints should be designed, if excessive thermal movement is expected.
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