SPECIAL CONCRETE & CONCRETING TECHNIQUES
SPECIAL CONCRETE & CONCRETING TECHNIQUES 6.1 Light weight concrete 6.2 Plum concrete 6.3 Fibre reinforced concrete 6.4 Polymer concrete 6.5 High density concrete 6.6 No fines concrete 6.7 Ferro cement 6.8 Fly ash concrete 6.9 Ready mix concrete 6.10 Pumped concrete Lightweight concrete Lightweight concretes can either be lightweight aggregate concrete, foamed concrete or autoclaved aerated concrete (AAC). Such lightweight concrete blocks are often used in house construction. Light weight concrete - or foamed concrete - is a versatile material which consists primarily of a cement based mortar mixed with at least 20% of volume air. The material is now being used in an ever increasing number of applications, ranging from one step house casting to low density void fills. Lightweight concrete Normal concrete has a density of 2,400 kg/m3 while densities range from 1,800, 1,700, 1,600 down to 300 kg/m3. Compressive strengths range from up to 40 mpa down to almost zero for the really low densities. Generally it has more than excellent thermal and sound insulating properties, a good fire rating, is non combustible and features cost savings through construction speed and ease of handling. Lightweight aggregate concrete Lightweight aggregate concrete can be produced using a variety of lightweight aggregates. Lightweight aggregates originate from either: Natural materials, like volcanic pumice. The thermal treatment of natural raw materials like clay, slate or shale i.e. Leca. Manufacture from industrial by-products such as fly ash, i.e. Lytag. Processing of industrial by-products like FBA or slag. Lightweight aggregates Natural lightweight aggregates 1. Pumice 2 . Scoria 3. Rice husk 4. Saw dust 5. Diatomite 6. Volcanic tuff 7. Foamed lava Artificial lightweight aggregates 1. Sintered fly ash 2. Foamed slag 3. Bloated 4. Artificial cinders 5. Expended clay, slate, shale 6. Coke breeze 7. Expanded polite 8. Exfoliated ermiculite Fibre reinforced concrete Concrete made with portland cement has certain characteristics: it is relatively strong in compression but weak in tension and tends to be brittle. The weakness in tension can be overcome by the use of conventional rod reinforcement and to some extent by the inclusion of a sufficient volume of certain fibres. The use of fibres also alters the behaviour of the fibre-matrix composite after it has cracked, thereby improving its toughness The use of fibres For the effective use of fibres in hardened concrete:ibres should be significantly stiffer than the matrix, i.e. have a higher modulu Fibre content by volume must be adequate. There must be a good fibre-matrix bond. Fibre length must be sufficient. Fibres must have a high aspect ratio, i.e. they must be long relative to their diametes of elasticity than the matrix Types of fibre 1. Glass 2. Steel 3. Synthetic fibres 4. Carbon 5. Nylon 6. Polyester 1. Glass Glass-fibre products exposed to outdoor environment have shown a loss of strength and ductility. The reasons for this are not clear and it is speculated that alkali attack or fibre embrittlement are possible causes. Because of the lack of data on long-term durability, GRC has been confined to nonstructural uses where it has wide applications. Glass fibre is available in continuous or chopped lengths. Fibre lengths of up to 35-mm are used in spray applications and 25-mm lengths are used in premix applications. 2. Steel Steel fibres have been used in concrete since the early 1900s. The early fibres were round and smooth and the wire was cut or chopped to the required lengths. The use of straight, smooth fibres has largely disappeared and modern fibres have either rough surfaces, hooked ends or are crimped or undulated through their length. Modern commercially available steel fibres are manufactured from drawn steel wire, from slit sheet steel or by the melt-extraction process which produces fibres that have a crescent-shaped cross section. Typically steel fibres have equivalent diameters (based on cross sectional area) of from 0,15 mm to 2 mm and lengths from 7 to 75 mm. 3. Synthetic fibres Synthetic fibres are man-made fibres resulting from research and development in the petrochemical and textile industries. There are two different physical fibre forms: monofilament fibres, and fibres produced from fibrillated tape 4. Carbon Carbon fibre is substantially more expensive than other fibre types. For this reason its commercial use has been limited. Carbon fibres are manufactured by carbonizing suitable organic materials in fibrous forms at high temperatures and then aligning the resultant graphite crystallites by hotstretching. The fibres are manufactured as either Type I (high modulus) or Type II (high strength) and are dependent upon material source and extent of hot stretching for their physical properties. Carbon fibres are available in a variety of forms and have a fibrillar structure similar to that of asbestos. 5. Nylon Nylon is a generic name that identifies a family of polymers. Nylon fibre’s properties are imparted by the base polymer type, addition of different levels of additive, manufacturing conditions and fibre dimensions. Currently only two types of nylon fibre are marketed for concrete. Nylon is heat stable, hydrophilic, relatively inert and resistant to a wide variety of materials 6. Polyester Polyester fibres are available in monofilament form and belong to the thermoplastic polyester group. They are temperature sensitive and above normal service temperatures their properties may be altered. Polyester fibres are somewhat hydrophobic. Polyester fibres have been used at low contents (0,1% by volume) to control plastic-shrinkage cracking in concrete POLYMER CONCRETE Low component weight, high strength, smooth surfaces and extremely high resistance are four reasons why polymer concrete is the ideal material for professional drainage channels. Compared to regular concrete, MEA polymer concrete is an impermeable material with a low porosity; because of its extremely high stability, products with thin walls and thus a lower weight can be manufactured. It is made from natural minerals such as quartz, basalt and granite which are mixed with a polymer binder. MEA polymer concrete is the basis for highly resilient, heavy-duty drainage channels. The high resistance of MEA polymer concrete to most chemical liquids is beneficial for the environment. Main components: Polymer-impregnated concrete (PIC) produced by impregnating or infiltrating a hardened concrete with a monomer and subsequently polymerizing the monomer in situ. Polymer Concrete (PC) Polymer concrete uses a polymer binder in place of Portland cement. Potassium silicate polymer concretes are more like Portland cement concrete in that they achieve compressive strengths in the 3,500-4,500 psi range and have a certain degree of absorption. Unlike Portland cement concrete, potassium silicate concrete offers exceptional resistance to non-fluoride acids including nitric, hydrochloric, sulfuric, and phosphoric. RMC Ready mix concrete that is manufactured in a factory or batching plant, according to a set recipe, and then delivered to a worksite, by truck mounted transit mixers . This results in a precise mixture, allowing specialty concrete mixtures to be developed and implemented on construction sites. It is popularly called as RMC, specially manufactured for delivery to the customers construction site in a freshly mixed and plastic or unhardened state. It is sold by volume expressed in cubic meters. ADVANTAGES A centralized concrete batching plant can serve a wide area. The plants are located in areas zoned for industrial use, and yet the delivery trucks can service residential districts or inner cities. Better quality concrete is produced. Elimination of storage space for basic materials at site. Elimination of hiring of plant and machinery. Wastage of basic materials is avoided. Labours associated with production of concrete is eliminated. Time required is greatly reduced. Noise and dust pollution at site is reduced. No fines concrete No-fines concrete (NFC) consists of coarse aggregate and cement paste. In the hardened state, aggregate particles are covered by a thin layer of cement paste and are in point-topoint contact with each other. At each contact point the paste forms a small fillet; these fillets hold the particles together and give strength to the concrete. Or No-fines concrete can be defined as a lightweight concrete composed of cement and fine aggregate. Uniformly distributed voids are formed throughout its mass. No-fines concrete usually used for both load bearing and non-load bearing for external walls and partitions. Mass Concrete Concrete in a massive structure, e.g., a beam, columns, pier, lock, or dam where its volume is of such magnitude as to require special means of coping with the generation of heat and subsequent volume change. Ferro cement Ferro cement is a type of thin wall reinforced concrete, commonly constructed of hydraulic cement mortar, reinforced with closely spaced layers of continuous and relatively small size wire mesh. The mesh may be made of metallic or other suitable materials” The term Ferro cement is most commonly applied to a mixture of Portland cement and sand applied over layers of woven or expanded steel mesh and closely spaced small-diameter steel rods rebar. It can be used to form relatively thin, compound curved sheets to make hulls for boats, shell roofs, water tanks, etc. It has been used in a wide range of other applications including sculpture and prefabricated building components. Ferro cement Ferro cement has a very high tensile strength-to-weight ratio and superior cracking behaviour in comparison to reinforced concrete. This means that Ferro cement structures can be relatively thin, light and water-tight. In India, ferro concrete is used often because the constructions made from it are more resistant to earthquakes. Earthquake resistance is dependent on good construction technique and additional reinforcement of the concrete. High density concrete High density or heavyweight concrete is concrete with a density greater than 2600kg/m3. Its primary use is in radiation shielding, either in nuclear power plants or in radiation therapy units. It can also be used as ballast in offshore locations such as pipelines. 6.2 Plum concrete It is a fluid, concrete can be pumped to where it is needed. Here, a concrete transport truck is feeding concrete to a concrete pumper, which is pumping it to where a slab is being poured A concrete pump is a machine used for transferring liquid concrete by pumping. There are two types of concrete pumps. The first type of concrete pump is attached to a truck. It is known as a trailer-mounted boom concrete pump because it uses a remotecontrolled articulating robotic arm (called a boom) to place concrete with pinpoint accuracy. Boom pumps are used on most of the larger construction projects as they are capable of pumping at very high volumes and because of the labour saving nature of the placing boom. They are a revolutionary alternative to truck-mounted concrete pumps. The second main type of concrete pump is either mounted on a truck and known as a truck-mounted concrete pump or placed on a trailer, and it is commonly referred to as a line pump or trailermounted concrete pump. This pump requires steel or flexible concrete placing hoses to be manually attached to the outlet of the machine. Those hoses are linked together and lead to wherever the concrete needs to be placed. Line pumps normally pump concrete at lower volumes Fly ash concrete Flyash is defined in Cement and Concrete Terminology (ACI Committee 116) as “the finely divided residue resulting from the combustion of ground or powdered coal, which is transported from the firebox through the boiler by flue gases.” Flyash is a by-product of coal-fired electric generating plants. In India, fly ash bricks are used for construction. Leading manufacturers use an industrial standard known as "Pulverized fuel ash for lime-Pozzolana mixture" using over 75% postindustrial recycled waste, and a compression process. This produces a strong product with good insulation properties and environmental benefits
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