unit – 3 special concretes · reported that dispersion of fibre reinforcement would improve the...
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UNIT – 3 SPECIAL CONCRETES
P.Muthuraman
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Syllabus
Polymer concrete, Sulphur infiltrated concrete, Fibre reinforced concrete, High strength concrete, High performance concrete, Vacuum concrete, Self compacting concrete, Geopolymer concrete, Reactive powder concrete, Concrete made with industrial wastes.
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Polymer Concrete
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Polymer Concrete● Concrete is porous
● The porosity is due to air-voids, water voids or due to the inherent porosity of gel structure itself.
● Reduces strength.
● Reduction of porosity results in increase of strength of Concrete.
● Vibration, pressure application spinning ,none of these methods could really help to reduce the water voids and the inherent porosity of gel, which is estimated to be about 28%.
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● The impregnation of monomer and subsequent polymerisation is the latest technique adopted to reduce the inherent porosity of the concrete, and to improve the strength and other properties of concrete.
Type of Polymer Concrete
– (a) Polymer Impregnated Concrete (PIC).
– (b) Polymer Cement Concrete (PCC).
– (c) Polymer Concrete (PC).
– (d) Partially Impregnated and surface coated polymer concrete.
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Polymer ImpregnatedConcrete (PIC)
● A precast conventional concrete, cured and dried in oven, or by dielectric heating from which the air in the open cell is removed by vacuum.
● Then a low viscosity monomer is diffused through the open cell and polymerised by using radiation, application of heat or by chemical initiation.
● It is necessary to know the concentration of water and air void in the system to determine the rate of monomer penetration.
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● Obtaining a maximum monomer loading in concrete by the removal of water and air from the concrete by vacuum or thermal drying, the latter being more practicable for water removal because of its rapidity evacuation of the specimen prior to soaking in monomer.
● This eliminates the entrapment of air towards the centre of the specimen during soaking which might otherwise prevent total or maximum monomer loading.
● The application of pressure is another technique to reduce monomer loading time.
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Mainly the following types of monomer are used:
● (a) Methylmethacrylate (MMA),
● (b) Styrene,
● (c) Acrylonitrile,
● (d) t-butyl styrene,
● (e) Other thermoplastic monomers.
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Polymer Cement Concrete (PCC)
● Polymer cement concrete is made by mixing cement, aggregates, water and monomer such plastic mixture is cast in moulds, cured, dried and polymerized
Monomers used:
● (a) Polyster-styrene.
● (b) Epoxy-styrene.
● (c) Furans.
● (d) Vinylidene Chloride.
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● This material is claimed to be specially dense and non shrinking and to have high corrosion resistance, low permeability and high resistance to vibrations and axial extension.
● However, such polymer concretes tend to be brittle it is reported that dispersion of fibre reinforcement would improve the toughness and tensile strength of the material.
● The use of fibrous polyester concrete (FPC) in the compressive region of reinforced concrete beams provides a high strength, ductile concrete at reasonable cost.
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Polymer Concrete (PC)● Polymer concrete is an aggregate bound with a polymer binder
instead of Portland cement as in conventional concrete.
● The graded aggregates are prepacked and vibrated in a mould.
● Monomer is then diffused up through the aggregates and polymerisation is initiated by radiation or chemical means.
● A silane coupling agent is added to the monomer to improve the bond strength between the polymer and the aggregate.
● In case polyester resins are used no polymerisation is required.
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Partially Impregnated (or Coated in Depth CID) and Surface Coated (SC) Concrete
● Partial impregnation may be sufficient in situations where the major requirement is surface resistance against chemical and mechanical attack in addition to strength increase.
● Even with only partial impregnation, significant increase in the strength of original concrete has been obtained.
● The partially impregnated concrete could be produced by initially soaking the dried specimens in liquid monomer like methyl methacrylate, then sealing them by keeping them under hot water at 70°C to prevent or minimise loss due to evaporation.
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● The polymerisation can be done by using thermal catalytic method in which three per cent by weight of benzoyl peroxide is added to the monomer as a catalyst.
● It is seen that the depth of monomer penetration is dependent upon following:
– (a) Pore structure of hardened and dried concrete.
– (b) The duration of soaking, and
– (c) The viscosity of the monomer.
USED EXTENSIVELY IN BRIDGE DECKS
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APPLICATIONS OF POLYMER IMPREGNATED CONCRETE:
● Prefabricated structural elements
● Prestressed concrete
● Marine works
● Desalination plants
● Nuclear power plants
● Sewage works-pipe and disposal works
● Ferrocement products
● For water proofing of structure
● Industrial applications
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SULPHUR-INFILTERATED CONCRETE● Sulphur, sand and coarse aggregate are the ingredients of
this concrete.
● Molten sulphur is added to the preheated aggregates in a mixture. The hot mix is immediately transferred into the moulds to fill them completely.
● The products manufactured with sulphur concrete need no curing and the moulds can be stripped immediately as the sulphur solidifies rapidly under normal temperatures.
● One of the major advantages of these products is that they can be remoulded and concrete can be reused with minimum or no wastage.
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● These products have very low absorption and less permeability.
● Strength upto 44 MPa have been reported when 30 % of sulphur, 50% of sand and 20% of coarse aggregate are mixed.
● These are therefore versatile for use as precast slab elements of canal and tunnel linings.
● Two procedures are adapted “A” after 24hours of moist curing, the specimen is dried in heating cabinet for 24hours at 1210C.
● Then the dried specimens are placed in a container of molten sulphur at 1210C for 3 hours.
● Specimens are removed from the container, wiped clean of sulphur and cooled to room temperature for 1hour and weighed to determine the weight of sulphur in filtrated concrete.
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● In procedure “B”, the dried concrete specimen is placed in an airtight container and subjected to vacuum pressure of 2mm mercury for 2hours.
● After removing the vacuum, the specimens are soaked in the molten sulphur at atmospheric pressure for another half an hour.
● The specimen is taken out, wiped clean and cooled to room temperature in about 1hour.The specimen is weighed and the weight of sulphur-impregnated concrete is determined.
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Fibre Reinforced ConcreteFibre reinforced concrete can be defined as a composite material consisting of mixtures of cement, mortar or concrete and discontinuous, discrete, uniformly dispersed suitable fibres.
Some of the fibres that could be used are
● steel fibres,
● polypropylene,
● nylons,
● asbestos,
● coir,
● glass
● and carbon
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● Fibre is a small piece of reinforcing material possessing certain characteristic properties.
● They can be circular or flat. The fibre is often described by a convenient parameter called “aspect ratio”.
● The aspect ratio of the fibre is the ratio of its length to its diameter. Typical aspect ratio ranges from 30 to 150.
● Steel fibre is one of the most commonly used fibre.
● Generally, round fibres are used.
● The diameter may vary from 0.25 to 0.75 mm. The steel fibre is likely to get rusted and lose some of its strengths.
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● Polypropylene and nylon fibres are found to be suitable to increase the impact strength.
● They possess very high tensile strength, but their low modulus of elasticity and higher elongation do not contribute to the flexural strength.
● Asbestos is a mineral fibre and has proved to be most successful of all fibres as it can be mixed with Portland cement.
● Tensile strength of asbestos varies between 560 to 980 N/mm2.
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● Glass fibre is a recent introduction in making fibre concrete.
● It has very high tensile strength 1020 to 4080 N/mm2.
● Glass fibre which is originally used in conjunction with cement was found to be effected by alkaline condition of cement.
● Therefore, alkali-resistant glass fibreby trade name “CEMFIL” has been developed and used.
● Carbon fibres perhaps posses very high tensile strength 2110 to 2815 N/mm2 and Young’s modulus.
● It has been reported that cement composite made with carbon fibre as reinforcement will have very high modulus of elasticity and flexural strength.
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Factors Effecting Properties of Fibre Reinforced Concrete
● Type of fibre,
● Fibre geometry,
● Fibre content,
● Orientation
● Distribution of the fibres,
● Mixing and compaction techniques of concrete,
● Size and shape of the aggregate
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Aspect Ratio of the Fibre● It has been reported that upto aspect ratio of 75, increase in the
aspect ratio increases the ultimate strength of the concrete linearly.
● Beyond 75, relative strength and toughness is reduced.
●
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Orientation of Fibres● One of the differences between conventional reinforcement and
fibre reinforcement is that in conventional reinforcement, bars are oriented in the direction desired while fibres are randomly oriented.
● To see the effect of randomness, mortar specimens reinforced with 0.5 per cent volume of fibres were tested.
● In one set specimens, fibres were aligned in the direction of the load, in another in the direction perpendicular to that of the load, and in the third randomly distributed.
● It was observed that the fibres aligned parallel to the applied load offered more tensile strength and toughness than randomly distributed or perpendicular fibres.
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Workability and Compaction of Concrete● Incorporation of steel fibre decreases the workability
considerably. This situation adversely affects the consolidation of fresh mix.
● Even prolonged external vibration fails to compact the concrete. The fibre volume at which this situation is reached depends on the length and diameter of the fibre.
● Another consequence of poor workability is non-uniform distribution of the fibres.
● Generally, the workability and compaction standard of the mix is improved through increased water/cement ratio or by the use of some kind of water reducing admixtures.
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Size of Coarse Aggregate● The maximum size of the coarse aggregate should be restricted
to 10 mm, to avoid appreciable reduction in strength of the composite.
● Fibres also in effect, act as aggregate. Although they have a simple geometry, their influence on the properties of fresh concrete is complex.
● The inter-particle friction between fibres, and between fibres and aggregates controls the orientation and distribution of the fibres and consequently the properties of the composite.
● Friction reducing admixtures and admixtures that improve the cohesiveness of the mix can significantly improve the mix.
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Mixing● Mixing of fibre reinforced concrete needs careful conditions to avoid
balling of fibres, segregation, and in general the difficulty of mixing the materials uniformly.
● Increase in the aspect ratio, volume percentage and size and quantity of coarse aggregate intensify the difficulties and balling tendencies.
● A steel fibre content in excess of 2 per cent by volume and an aspect ratio of more than 100 are difficult to mix.
The typical proportions for fibre reinforced concrete is given below:
● Cement content : 325 to 550 kg/m3● W/C Ratio : 0.4 to 0.6● Percentage of sand to total aggregate : 50 to 100 per cent● Maximum Aggregate Size : 10 mm● Air-content : 6 to 9 per cent
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Applications● Overlays of air-field,● Road pavements,● Industrial flooring,● Bridge decks,● Canal lining,● Explosive resistant structures,● Refractory linings the fabrication of precast products like
pipes, boats, beams, stair case steps, wall panels, roof panels, manhole covers,
● Fibre reinforced concrete sometimes called fibrous concrete, is manufactured under the trade name “Wirand Concrete”(U.S.A).
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Fibre content :
● 0.5 to 2.5 per cent by volume of mix
● Steel —1 per cent 78 kg/m3
● Glass —1 per cent 25 kg/m3
● Nylon —1 per cent 11 kg/m3
Current Development in FRC
● " High fibre volume micro-fibre systems.
● " Slurry infiltrated fibre Concrete (SIFCON).
● " Compact reinforced composites.
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High Strength Concrete● High-strength concrete has a compressive strength greater
than 40 MPa.
● High strength concrete is made by lowering the water cement (W/C) ratio to 0.35 or lower.
● Due to low w/c ratio it causes problem of placing ,to
overcome from this superplasticizer used.
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Materials for High-Strength Concrete:
Cement:● Almost any ASTM portland cement type can be used to obtain concrete
with compresive strength up to 60 MPa.
● In order to obtain higher strength mixtures while maintaining good workability, it is necessary to study carefully the cement composition and fineness.
Aggregate:
● In high-strength concrete, the aggregate plays an important role on the strength of concrete.
● The low-water to cement ratio used in high strength concrete causes densification in both the matrix and interfacial transition zone, and the aggregate may become the weak link in the development of the mechanical strength.
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Guidelines for the selection of materials:
● The higher the targeted compressive strength, the smaller the maximum size of coarse aggregate.
● Up to 70 MPa compressive strength can be produced with a good coarse aggregate of a maximum size ranging from 20 to 28 mm.
● To produce 100 MPa compressive strength aggregate with a maximum size of 10 to 20 mm should be used.
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Special methods of making high strength concrete
● Seeding: This involves adding a small percentage of finely ground, fully hydrated Portland cement to the fresh concrete mix. This method may not hold much promise.
● Revibration: Controlled revibration removes all the defects like bleeding, water accumulates , plastic shrinkage, continuous capillary channels and increases the strength of concrete.
● High speed slurry mixing: This process involves the advance preparation of cement - water mixture which is then blended with aggregate to produce concrete.
● Use of admixtures: Use of water reducing agents are known to produce increased compressive strength.
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● Inhibition of cracks: If the propagation of cracks is inhibited, the strength will be higher. Concrete cubes made this way have yielded strength up to 105MPa.
● Sulphur Impregnation: Satisfactory high strength concrete have been produced by impregnating low strength porous concrete by sulphur. The sulphur infiltrated concrete has given strength up to 58MPa.
● Use of Cementitious aggregates: Cement fondu is kind of clinker. Using Slag as aggregate, strength up to 250MPa has been obtained with water cement ratio 0.32.
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Fire resistance of High Strength Concrete
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Strength-weight ratio becomes comparable to steel:
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Differences Between NSC and HSC:● In normal strength concrete, the microcracks form when the
compressive stress reaches ~ 40% of the strength. The cracks interconnect when the stress reaches 80-90% of the strength.
● The fracture surface in NSC is rough. The fracture develops along the transition zone between the matrix and aggregates Fewer aggregate particles are broken.
● The fracture surface in HSC is smooth.
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APPLICATION OF HSC:
● Use of HSC in column section decreases the column size.
● Use of HSC in column decreases amount of steel required for same column.
● In high rise building, use of HSC increases the floor area for rental purpose.
● In bridges,use of HSC reduces the number of beams supporting the slab .
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HIGH PERFOMANCE CONCRETE:
“A high performance concrete is a concrete in which certain
characteristics are developed for a particular application and environments”
● High workability,
● High strength,
● High modulus of elasticity,
● High density,
● High dimensional stability,
● Low permeability and
● High resistance to chemical attack
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● Ease of placement
● Compaction without segregation
● Early-age strength
● Long term mechanical properties
● Permeability
● Durability
● Heat of hydration
● Toughness
● Volume stability
● Long life in severe environments
● High resistance to frost and deicer scaling damage
● Toughness and impact resistance
● Volume stability
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● A substantial reduction of quantity of mixing water is the fundamental step for making HPC.
● Reduction of w/c ratio will result in high strength concrete.
● But reduction in w/c ratio to less than 0.3 will greatly improve the qualities of transition zone to give inherent qualities expected in HPC.
● To improve the qualities of transition zone, use of silica fume is also found to be necessary.
● Silica fumes becomes a necessary ingredient for strength above to 80 MPa.
● The best quality fly ash and GGBS may be used for other nominal benefits.
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● For HPC shape and size of the aggregate becomes an important parameter.
● Regarding the shape of the aggregate, crushed aggregate can be used,
● But utmost care should be taken to see that aggregates are cubic in shape, with minimum amount of flaky or elongated particles.
● The latter would effect not only the strength but also adversely affect the workability.
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Vacuum Concrete● Vacuum concrete is the type of concrete in which the
excess water is removed for improving concrete strength
● It was first invented by Billner in United states in 1935.
● Reducing the final water-cement ratio of concrete before setting, to control strength and other properties of concrete.
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Need for vacuum concrete
● The chemical reaction of cement with water requires a water-cement ratio of less than 0.38.
● But it is always taken more than 0.38.
● Workability is also important for concrete.
● Workability and high strength don’t go together as their requirements are contradictory to each other.
● Vacuum concrete is the effective technique used to overcome this contradiction of opposite requirements of workability and high strength.
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Equipment's use in Vacuum Concrete
● Vacuum pump with hose pipe
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● Water separator
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● Filtering pad
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● Screed board vibrator
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● Power floater
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● Power trowel
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Procedure● With the help of screed vibrator the surface is vibrated.
● Vacuum pump is a small but strong pump of 5 to 10 HP.
● The mats are placed over fine filter pads.
● Water from concrete is vacuum and stored in water separator.
● About 3% reduction in concrete layer depth takes place.
● By the use of power trowel and power floater the surface is given a hard and smooth finish.
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Vacuum concrete process
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Applications of Vacuum Concrete
● Industrials floor sheds like cold storages
● Hydro power plants
● Bridges ports and harbour
● Cooling towers
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Advantages of Vacuum Concrete● Increase of final strength of concrete by about 25%
● Sufficient decrease in permeability of concrete
● High density concrete
● Increase of about 20% bond strength of concrete
● Appreciable reduction of time for final finishing
● Early removal of wall forms
● Increase in durability.
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Disadvantages of Vacuum Concrete● High initial cost.
● Need trained labor.
● Need specific equipment.
● Need power consumption.
● The porosity of the concrete allows water, oil and grease to seep through, consequently weakening the concrete.
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Self Compacting Concrete
● Making concrete structures without vibration, have been done in the past.
● For examples, placement of concrete under water is done by the use of tremie without vibration.
● Mass concrete, and shaft concrete can be successfully placed without vibration.
● But these concretes are generally of lower strength and difficult to obtain consistent quality.
● Modern application of self-compacting concrete (SCC) is focussed on high performance, better and more reliable and uniform quality.
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Material for SCC
Cement : 43 or 53 grade
Aggregates :limited to 20 mm (Well graded cubical or rounded aggregates)
Mixing Water : Good Water quality
Chemical Admixtures
– Super plasticizers ( poly-carboxylated ethers)
Mineral Admixtures:
– Fly ash:GGBFS:Silica Fume:Stone Powder & Fibres
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Requirements for self-compacting concrete
The main characteristics of SCC are the properties in the fresh state.
● " Filling ability
● " Passing ability
● " Segregation resistance
● The workability of SCC is higher than “very high” degree of workability mentioned in 15 456 : 2000.
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TESTS 0N SCC
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Workability Requirement for the fresh SCC
● Initial Mix composition● Production and Placing● Mix Design● Air Content:● Tests on Concrete
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Slump flow Test
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● The procedure for this test is same as for slump flow test.
● When the slump cone is lifted, start the stop watch and find the time taken for the concrete to reach 500 mm mark. This time is called T50 time.
● This is an indication of rate of spread of concrete.
● A lower time indicates greater flowability. It is suggested that T50 time may be 2 to 5 secs.
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J-Ring Test
● The acceptable difference in height between inside and outside should be between 0 and 10 mm.
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V-Funnel test and V-Funnel test at T5 min.
● Open within 10 seconds the trap door and record the time taken for the concrete to flow down.
● Record the time for emptying.
● This can be judged when the light is seen when viewed from top.
● The whole test is to be performed within 5 min.
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Procedure for flow time at T5 min
● Open the trap door after 5 minutes after the second fill
● of the funnel and allow the concrete to flow.
● Calculate the time taken for complete discharge. It is called the flow time at T5 min.
● For V-funnel test the flow time should be between 8 and 12 seconds.
● For V-funnel flow time at T5 min. + 3 seconds is allowed.
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L box test method
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U Box Test
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Fill box test
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Orimet TestFor SCC a flow time of 5 seconds or less is considered appropriate
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Viscosity Modifying Agent:
● Most VMAs contain polysaccharides as active ingredient.
● Some starches could also be used for control of viscosity.
● Diutan gum and welan gum are often become part of certain viscosity modifying admixture.
● It is claimed that such VMA becomes compatible with all super plasticizers
● One must be careful about the sequence of addition of VMA and super plasticizers into SCC.
● VMA should be added after super plasticizers is added and mixed with cement particles.
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Geo Polymer Concrete● Geo polymer concrete is a new material that does not need
the presence of Portland cement as binder.
● Instead ,activating the source materials such as fly ash that are rich in Silicon(Si) and Aluminum (Al) using high alkaline liquids produces binder for manufacturing concrete.
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● The major problem that the world is facing today is the environmental pollution.
● In the construction industry mainly the production of ordinary Portland cement (OPC) will cause the emission of pollutants which results in environmental pollution.
● The emission of carbon dioxide during the production of ordinary Portland cement is tremendous because the production of one ton of Portland cement emits approximately one ton of CO2 into the atmosphere.
● In terms of global warming, the geopolymer concrete significantly reduce the CO2 emission to the atmosphere caused by the cement industries.
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Mixing
● After collecting the required properties i.e cement, fine and coarse aggregates, super plasticizer and water is required for normal preparation of concrete.
● Fly ash, fine and coarse aggregates, sodium hydroxide, sodium silicate, super plasticizer and extra water is needed for Geopolymer concrete.
● They are mixed in a concrete mixer continuously for a time period of 3 minutes.
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Curing of OPC and GPC
● •The conventional concrete cubes are placed in water in curing tank for total period of 28 days.
● The geopolymer concrete cubes are placed in the hot air over for heat curing then placed for ambient curing for the desired period.
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ADVANTAGES
● Cutting the world’s carbon.
● The price of fly ash is low.
● Better compressive strength.
● Fire proof.
● Low permeability.
● Eco-friendly.
● Excellent properties within both acid and salt environments.
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Applications of GPC
● Pre-cast concrete products like railway sleepers,electric power poles etc.
● Marine structures
● Waste containments etc
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● The reduced CO2 emissions of Geopolymer cements make them a good alternative to Ordinary Portland Cement.
● Produces a substance that is comparable to or better than traditional cements with respect to most properties.
● Geopolymer concrete has excellent properties within both acid and salt environments,
● Low-calcium fly ash-based geopolymer concrete has excellent compressive strength and is suitable for Structural applications
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Reactive Powder Concrete
RPC is an ultra high strength and high ductility cementitious composite with advanced mechanical and chemical properties.
There are concretes that leads the way to the achievement of the maximum compressive strength of the order 120-150 Mpa.
In order to increase the compressive strength of concrete even
further, the only way is to remove the coarse aggregate.
This philosophy has been employed in what is today known as Reactive Powder Concrete.
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RPC is not just a simple mixture of cement, water and aggregates.
Quite often, it contains mineral components and chemical admixtures having very specific characteristics, which impart specific properties to the concrete.
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COMPOSITION OF RPCCEMENT
Their C3A content, varies from 1% up to 8%. Their soluble alkali content is very low and is comprised between 0.16% and 0.38%.
SILICA FUMES
The main quality of a silica fume is the absence of aggregates.
SAND
Sand should be of good hardness, readily available and low cost. Its particle size ranges from 0.15mm to 0.6 mm. The type of sand generally used is natural and crushed.
QUARTZ POWDER
Its particle size ranges from 0.005mm to 0.025mm. It should be crystalline in nature.
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STEEL FIBRES
It should have good aspect ratio and should be able to improve ductility. Its length ranges from 13mm 25mm. It should be straight.
WATER
It should be clean from all the organic impurities as well as other dust particles. It should not be saline in nature.
SUPER PLASTICIZER
A copolymer of acrylic ester (CAE), a polynaphtalene Sulfonate (PNS) and a polymelamine sulfonate (PMS) are normally employed for the purpose.
These admixtures are synthetic polymers.
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Objectives of developing RPC● Elimination of coarse aggregate for enhancement of
homogeneity .
● Utilization of pozzolanic properties of silica fume.
● Optimal usage of super plasticizer to reduce W/C and at the same time improves compaction .
● Post- set heat treatment for enhancement of the microstructure.
● Addition of small sized steel fibers to improve ductility.
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PROPERTIES OF RPCCOMPRESSIVE STRENGTH
– -Higher compressive strength than normal Concrete.
– -It is a factor linked with durability of material.
– -Maximum compressive strength of RPC is approximately 200MPa.
FLEXURAL STRENGTH
-Plane RPC possess high flexural strength than regular concrete.
-By introducing steel fibers, RPC can achieve high strength.
The length and diameter of the fibres have a considerable impact on the strength.
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Advantages of RPC● It has the potential to structurally compete with steel.
● Superior strength combined with higher shear capacity result in significant dead load reduction.
● RPC can be used to resist all but direct primary tensile stress.
● Improved seismic performance by reducing inertia load with lighter member .
● Low &non-interconnected porosity diminishes mass transfer, making penetration of liquid/gas non-existent.
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Limitations of RPC
● Least costly components of conventional concrete are eliminated by more expensive elements.
● RPC is still in the intial stages,So long term properties are not yet known.
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Concrete made with industrial wastes.
● Construction and demolition Wastes● Lathe Waste● E- Wastes● Rubber Tyre Waste● Coconut Shell Waste● Egg Shell Powder● Fly Ash● GGBS● RHA● Slica Fume● Barchips● Alcofine
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Discussions ?
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