SPECIAL CONCRETE

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Special Concrete means the concrete used or made for specialcases, such as:

concreting in hot weatherconcreting in cold weather

Frequently, concrete may be used for some special purpose forwhich special properties are more important than those commonlyconsidered.

Sometimes, it may be of great importance to enhance one of theordinary properties. These special applications often becomeapparent as new developments using new materials or asimprovements using the basic materials.

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Some utilize special aggregates (lightweight aggregate, steelfiber, plastic fiber, glass fiber, and special heavy aggregate).

Some special properties - increased compressive and tensilestrength, water proofing, and improved chemical resistance areachieved with polymers, either as admixtures or surfacetreatment of hardened concrete.

Admixtures for coloring concrete are available in all colors.The oldest and cheapest is perhaps carbon black. Admixturescausing expansion for use in sealing cracks or under machinebases, etc., include powdered aluminum and finely groundiron.

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Special admixtures are available for use where the natural aggregate is alkalireactive, to neutralize this reaction. Proprietary admixtures are available thatincrease the tensile strength or bond strength of concrete. They are useful formaking repairs to concrete surfaces.

“Hot Weather Concreting” defines hot weathers as any combination of thefollowing conditions that tends to impair the quality of the freshly mixed orhardened concrete:

• High ambient temperature• High concrete temperature• Low relative humidity• Wind speed• Solar radiation

• The success of many hot-weather concreting operations depends on the stepstaken to slow the cement hydration reactions within the concrete and tominimize the rate of evaporation of moisture from the freshly mixed concrete.

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Potential concrete problems in hot weather are likely toinclude:

• Increased water demand• Increased rate of slump loss• Increased rate of setting• Increased tendency for plastic-shrinkage cracking• Increased difficulty in controlling entrained air content• Decreased 28-day and later strengths• Increased tendency for differential thermal cracking• Greater variability in surface appearance• Increased permeability

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“Cold Weather Concreting” defines cold weather concretingas a period when for more than three (3) consecutive days, thefollowing conditions exist:

• The average daily air temperature is less than 5°C (40°F) and,

• The air temperature is not greater than 10°C (50°F) for more thanone-half of any 24 hour period.

• Concrete placed during cold weather will develop sufficientstrength and durability to satisfy intended service requirements onlyif it is properly produced, placed and protected.

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THE OBJECTIVES OF COLD WEATHER CONCRETING ARE TO:

• Prevent damage to concrete due to freezing at early ages

• Assure that concrete develops the required strength for the saferemoval of forms

• Maintain curing conditions that foster normal strengthdevelopment without using excessive heat

• Limit rapid temperature changes in the concrete to preventthermal cracking

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REACTIVE POWDER CONCRETE

• A new generation concrete• Compressive strength- 200 to 800 mpa• Tensile strength- 25 to 150 mpa

• To improve homogeneity of concrete by reducing maximum size of the particle

• To create optimum packing density

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• To reduce amount of water in concrete• To add short steel fibers• To apply hardening under pressure and in

temperature• Formulated from

– Portland cement– Silica fume– Super plasticizers– Water & steel fibers

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Bacterial concrete

• Ability of living organism to form minerals• Microbial biominerology• Metabolically diverse form of life• Microbial life- -7deg cel to 120 deg cel• Mineral growth

» Passive growth» Metabolic activity

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• Nucleation• Growth of crystal from over saturated solution• Cell wall &external sheaths

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Bacteria used

• Bacillus pasteruic• Pseudomonas aeruginosaIt improves

stiffnessstrengthimpermeability

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Expansive cement

• Cause expansion• To fill grouts• Light weightChemicals used

hydrogen peroxidemetalic aluminiumactivated carbon

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Chemical admixtures

• To modify setting properties• To protect surface• Applied on surface of moulds

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Fiber reinforced concrete

Fiber-reinforced concrete (FRC) is concrete containing fibrous materialwhich increases its structural integrity. It contains short discrete fibers thatare uniformly distributed and randomly oriented.

Fibers include steel fibers, glass fibers, synthetic fibers and natural fibers.Within these different fibers that character of fiber-reinforced concretechanges with varying concretes, fiber materials, geometries, distribution,orientation and densities.

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• horsehair was used in mortar and straw in mud bricks. In the early 1900s, asbestos fibers were used in concrete, and in the 1950s the concept of composite materials came into being and fiber-reinforced concrete was one of the topics of interest.

• There was a need to find a replacement for the asbestos used in concrete and other building materials once the health risks associated with the substance were discovered. By the 1960s, steel, glass (GFRC), and synthetic fibers such as polypropylene fibers were used in concrete, and research into new fiber-reinforced concretes continues today.

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Effects of fibers

• Fibers are usually used in concrete to control cracking due to both plasticshrinkage and drying shrinkage. They also reduce the permeability of concreteand thus reduce bleeding of water.

• Some types of fibers produce greater impact, abrasion and shatter resistance inconcrete. Generally fibers do not increase the flexural strength of concrete, and socannot replace moment resisting or structural steel reinforcement. Indeed, somefibers actually reduce the strength of concrete.

• The amount of fibers added to a concrete mix is expressed as a percentage of thetotal volume of the composite (concrete and fibers), termed volume fraction (Vf).Vf typically ranges from 0.1 to 3%. Aspect ratio (l/d) is calculated by dividingfiber length (l) by its diameter (d). Fibers with a non-circular cross section use anequivalent diameter for the calculation of aspect ratio.

• If the modulus of elasticity of the fiber is higher than the matrix (concrete ormortar binder), they help to carry the load by increasing the tensile strength of thematerial. Increase in the aspect ratio of the fiber usually segments the flexuralstrength and toughness of the matrix. However, fibers which are too long tend to"ball" in the mix and create workability problems.

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Polypropylene and Nylon fibers can:• Improve mix cohesion, improving pumpability over long distances• Improve freeze-thaw resistance• Improve resistance to explosive spalling in case of a severe fire• Improve impact resistance• Increase resistance to plastic shrinkage during curingSteel fibers can:• Improve structural strength• Reduce steel reinforcement requirements• Improve ductility• Reduce crack widths and control the crack widths tightly thus improve

durability• Improve impact & abrasion resistance• Improve freeze-thaw resistance• Blends of both steel and polymeric fibers are often used in construction

projects in order to combine the benefits of both products; structural improvements provided by steel fibers and the resistance to explosive spalling and plastic shrinkage improvements provided by polymeric fibers.

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Ferro cement

• The term ferrocement is most commonly applied to a mixture of Portlandcement and sand applied over layers of woven or expanded steel mesh andclosely spaced small-diameter steel rods rebar.

• It can be used to form relatively thin, compound curved sheets to makehulls for boats, shell roofs, water tanks, etc. It has been used in a widerange of other applications including sculpture and prefabricated buildingcomponents.

• The term has been applied by extension to other composite materialsincluding some containing no cement and no ferrous material.

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• The desired shape may be built from a multi-layered construction of mesh,supported by an armature, or grid, built with rebar and tied with wire. Foroptimum performance, steel should be rust-treated, (galvanized) or stainlesssteel. (In early practice, in the desert, or for exterior scenery construction,"sound building practice" was not considered, or perhaps unknown as itgrew in some cases, from a folk craft tradition of masons collaborating withblacksmiths.)

• Over this finished framework, an appropriate mixture (grout or mortar) ofPortland cement, sand and water and/or admixtures is applied to penetratethe mesh. During hardening, the assembly may be kept moist, to ensure thatthe concrete is able to set and harden slowly and to avoid developing cracksthat can weaken the system.

• Steps should be taken to avoid trapped air in the internal structure duringthe wet stage of construction as this can also create cracks that will form asit dries. Trapped air will leave voids that allow water to collect and degrade(rust) the steel. Modern practice often includes spraying the mixture atpressure, (a technique called shotcrete,) or some other method of drivingout trapped air.

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Advantages

• The advantages of a well built Ferro concrete construction are the low weight, maintenance costs and long lifetime in comparison with purely steel constructions. However, meticulous building precision is considered crucial here. Especially with respect to the cementitious composition and the way in which it is applied in and on the framework, and how or if the framework has been treated to resist corrosion.

• When a Ferro concrete sheet is mechanically overloaded, it will tend to fold instead of break or crumble like stone or pottery. So it is not brittle. As a container, it may fail and leak but possibly hold together. Much depends on techniques used in the construction.

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Disadvantages

• The disadvantage of Ferro concrete construction is the labor intensivenature of it, which makes it expensive for industrial application in thewestern world. In addition, threats to degradation (rust) of the steelcomponents is a possibility if air voids are left in the original construction,due to too dry a mixture of the concrete being applied, or not forcing the airout of the structure while it is in its wet stage of construction, throughvibration, pressurized spraying techniques, or other means. These air voidscan turn to pools of water as the cured material absorbs moisture. If thevoids occur where there is untreated steel, the steel will rust and expand,causing the system to fail.

• In modern practice, the advent of liquid acrylic additives and otheradvances to the grout mixture, create slower moisture absorption over theolder formulas, and also increase bonding strength to mitigate thesefailures. Restoration steps should include treatment to the steel to arrestrust, using practices for treating old steel common in auto body repair.

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Polymer concretePolymer concrete is part of group of concretes that use polymers to

supplement or replace cement as a binder. The types include polymer-impregnated concrete, polymer concrete, and polymer-Portland-cementconcrete.

In polymer concrete, thermosetting resins are used as the principal polymercomponent due to their high thermal stability and resistance to a widevariety of chemicals. Polymer concrete is also composed of aggregatesthat include silica, quartz, granite, limestone, and other high qualitymaterial.

The aggregate must be of good quality, free of dust and other debris, and dry.Failure of these criteria can reduce the bond strength between thepolymer binder and the aggregate

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Uses

Polymer concrete may be used for new construction or repairing of oldconcrete. The adhesion properties of polymer concrete allow patching forboth polymer and cementitious concretes.

The low permeability of polymer concrete allows it to be used in swimmingpools, sewer pipes, drainage channels, electrolytic cells for base metalrecovery, and other structures that contain liquids. It can also be used as areplacement for asphalt pavement, for higher durability and higher strength.

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Advantages

• Advantages of polymer concrete include:• Rapid curing at ambient temperatures• High tensile, flexural, and compressive strengths• Good adhesion to most surfaces• Good long-term durability with respect to freeze and thaw cycles• Low permeability to water and aggressive solutions• Good chemical resistance• Good resistance against corrosion• Lightweight• May be used in regular wood and steel formwork• May be vibrated to fill voids in forms• Allows use of regular form-release agents

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Disadvantages

• Some safety issues arise out of the use of polymer concrete.• The monomers can be volatile, combustible, and toxic.• Initiators, which are used as catalysts, are combustible and harmful to

human skin.• The promoters and accelerators are also dangerous.• Polymer concretes also cost significantly more than conventional concrete

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