12-10-2016, 10:26 AM
COMPARISION OF STRENGTH CHARACTERISTICS OF POLYPROPYLENE FIBRE REINFORCED CONCRETE WITH CONVENTIONAL CONCRETE
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INTRODUCTION
CONCRETE
Concrete is a mixture of cement, sand, stone aggregates and water. Cement, usually in powder form, acts as a binding agent when mixed with water and aggregates. This combination, or concrete mix, will be poured and harden into the durable material with which we are all familiar. There are three basic ingredients in the concrete mix:
1. Portland cement.
2. Water.
3. Aggregates (coarse and sand).
Portland cement – The cement and water form a paste that coats the aggregate and sand in the mix. The paste hardens and binds the aggregates and sand together.
Water – Water is needed to chemically react with the cement (hydration) and too provide workability with the concrete. The amount of water in the mix in pound compared with the amount of cement is called the water cement ratio. The lower the w/c ratio, the stronger the concrete. (Higher strength, less permeability).
Aggregates – Sand is the fined aggregate. Gravel or crushed stone is the coarse aggregate in most mixes.
PLAIN CEMENT CONCRETE
Plain cement concrete (PCC) is used to provide rigid impervious bed to RCC in foundation where the earth is soft and yielding. PCC can be used over brick flat soling or without brick flat soling. Plain cement concrete can also called only “cement concrete (CC)” or “binding concrete”.
General Specification of Plain Cement Concrete
• In plain cement concrete coarse aggregate should be hard durable and free from impurities.
• Fine aggregate should contain sharp, angular, grain.
• Cement should be fresh Portland cement.
• Mixing should be done by hand mixing or by machine mixing.
• Laying and compaction should be done before setting of concrete i.e. within 30minutes.
REINFORCED CONCRETE
It is a composite material in which concrete’s relatively low tensile strength and ductility are counteracted by the inclusion of reinforcement having higher tensile strength and/or ductility. The reinforcement is usually, through not necessarily, steel reinforcing bars (rebar) and is usually embedded passively in the concrete before the concrete sets. Reinforcing schemes are generally designed to resist tensile stresses in particular regions of the concrete that might cause unacceptable cracking and/or structural failure. Modern reinforced concrete can contain varied reinforcing materials made of steel, polymers or alternate composite materials in conjunction with rebar or not. Reinforced concrete may also be permanently stressed (in compression), so as to improve the behavior of the final structure under working loads.
In the United States the most common methods of doing this are known as pre-tensioning and post-tensioning.
For strong, ductile and durable construction the reinforcement needs to have the following properties at least:
• High relative strength.
• High toleration of tensile strain.
• Good bond to the concrete, irrespective of pH, moisture, and similar factors.
• Thermal compatibility, not causing unacceptable stresses in response to changing temperature.
• Durability in the concrete environment, irrespective of corrosion or sustained stress of example.
TYPES OF CONCRETE
1. Normal Concrete.
2. High Strength Concrete.
3. High performance Concrete.
4. Pervious Concrete.
5. Self Compacting Concrete.
6. Roller Compacted Concrete.
7. Fiber Reinforced Concrete.
8. High Volume Fly Ash Concrete.
9. Foam Concrete.
10. Geo-Polymer Concrete.
NORMAL CONCRETE
The concrete in which common ingredients i.e. aggregate, water, cement are used is known as normal concrete. it is also called normal weight concrete or normal strength concrete. it has a setting time of 30-90 minutes depending upon moisture in atmosphere, fineness of cement etc. The development of the strength starts after 7 days the common strength values is 10MPa (1450 psi) to 40MPa (5800 psi). At about 28days 75-80% of the total strength is attained. Almost at 90 days 95% of the strength is achieved.
HIGH STRENGTH CONCRETE
Compressive strength of high strength concrete mix is usually greater than 6,000 pounds per square inch. High strength concrete is made by lowering the water cement (W/C) ratio to 0.35 or lower. Often silica fume is added to prevent the formation of free calcium hydroxide crystals in the cement, which might reduce the strength at the cement aggregate bond. Low w/c ratio and use of silica fume make concrete mixes significantly less workability, which is particularly likely to be a problem in high-strength concrete application where dense rebar cages are likely to be used . To compensate for the reduced workability in high strength concrete mix, super plasticizers are commonly added to high strength mixtures. Aggregates must be selected carefully for high strength mixes, as weaker aggregates may not be strong enough to resist the loads imposed on the concrete and cause failure to start in the aggregate.
HIGH PERFORMANCE CONCRETE (HPC)
HPC is used for concrete mixture which possesses high workability, high modulus, high strength, high density, high dimensional stability, low permeability and resistance to chemical attack. HPC STARTED as strength Concrete in 1970's and made possible by advance of super plasticizer. HPC a better version of high Strength Concrete, Low w/c (0.3 to 0.35), Finer cements, more C3S in cement.
PERVIOUS CONCRETE
Pervious concrete contains network of holes or voids, to allow air or water to move through the water drainage infrastructure, and allow replacement of ground water when conventional concrete does not. It is formed by leaving out some or the entire fine aggregate (fines), the remaining large aggregate then is bound by the relatively small amount of Portland cement. When set, typically between 15% and 25% of the concrete volumes are voids, allowing water to drain. The majority of pervious concrete pavement function well with little or no maintenance. Maintenance of pervious concrete consists primarily of prevention of clogging of the void structure. In preparing the site prior to construction, drainage of surrounding landscaping should be designed to prevent flow of materials onto pavement surface. Soil, rock, leaves, and other debris may infiltrate the voids and hinder the flow of water, decreasing the utility of the pervious concrete pavement
SELF COMPACTING CONCRETE
The concrete where no vibration is required. The concrete is compacted due to its own weight. It is also called self consolidated concrete or flowing. It can also categorize as high performance concrete as the ingredient is the same, but in this type of concrete workability is increased.
This self compacting concrete is characterized by:
• Extreme fluidity as measured by the flow. Typically between 650-750 mm on a flow table, rather than slump (height).
• No need for vibration to compact the concrete.
• Placement being easier.
• No bleed water, or aggregate segregation.
ROLLER COMPACTED CONCRETE
Roller compacted concrete, sometime called roll concrete, and is low-cement content stiff concrete placed using techniques borrowed from earthmoving and paving work. The concrete is placed on the surface to be covered, and is compacted in place using large heavy rollers typically used in earthwork. The concrete mix achieves a high density and curse over time into a strong monolithic block. Roller compacted concrete is typically used for concrete pavement. Roller generated while curing than typical for conventionally placed massive concrete pours.
FIBER REINFORCED CONCRETE (FRC)
The process of micro cracks at the mortar – aggregates interface is responsible for inherent weakness of plain concrete. The weakness can be removed by inclusion of fiber in mix. FRC can be defined as a composite material consisting of mixtures of cement, mortar or concrete and discontinuous, discrete, uniformly dispersed fibers. It may also contain pozzolans and other admixtures commonly used with conventional concrete.
HIGH VOLUME FLY ASH CONCRETE (HVFA)
Fly ash (pulverized fuel ash) is a by- product of an electricity generating plant using coal as fuel. It used to be disposed of as industrial waste. Because a large amount of fly ash can be produced by power plants ever year, fly ash disposal became a major environment issue in many countries. To solve the disposal problem, since the early1960’s many countries have started to incorporate fly ash into concrete. In recent years, attempts have been made in many countries to utilize a large amount of fly ash in concrete without losing its early age compressive strength. The HVFA concrete that has developed recently uses fly ash of 60% or more by weight of cement. Desirable mechanical and durability properties of HVFA concrete have been achieved by careful selection of mix preparations and the use of chemical super plasticizers.
FOAM CONCRETE
Foamed concrete also known as cellular concrete is a specialized product that contains more than 20% by volume of air. This is achieved by the incorporation of pre-formed foam or foaming surfactant in to a cementitious base mix.
The greatest advantages of foamed concrete are its flow ability, self-compacting and self-leveling nature, light weight and low dimensional change. In addition, the material exhibits controlled low strength, excellent thermal insulation properties, and good load-bearing capacity and can be easily re-excavated, If necessary.
GEO-POLYMER CONCRETE (GPC)
The production of one ton of cement emits approximately one ton of carbon dioxide to the atmosphere which leads to global warming conditions. A need of present status is, should be built additional cement manufacturing plants or finds alternative binder systems to make concrete. On the other scenario huge quantity of fly ash are generated around the globe from thermal power plants and generally used as a filter material in low level areas. Alternative binder system with fly ash to produce concrete eliminating cement is called “GEO-POLYMER CONCRETE”. Alkali activation of alumina and silica containing blast furnace slag powder, known as GGBS (Ground granulated blast furnace slag) and fly ash typically used in Geo-polymer concrete.
POLYMER IMPREGNATED CONCRETE (PIC)
PIC is the widely used polymer concrete. The precast conventional concrete is cured and dried in oven, and then a low viscosity monometer is diffused through the open cell and polymerized by using radiation, application of heat or by chemical initiation. The monomer used is Methylmethacrylate, Styrene, Acrylonitrile, etc.
Application of PIC: Prefabricated structural elements, prestressed concrete, marine works, Nuclear power plants, Sewage works, for water proofing structures, Industrial applications.
STAGES OF CONCRETE
Concrete has two main stages:
• Fresh Concrete
• Hardened Concrete
FRESH CONCRETE
Fresh concrete should be stable and should not segregate or bleed during transportation and placing when it is subjected to forces during handling operations of limited nature. The mix should be cohesive and mobile enough to be placed in the form around the reinforcement and should be able to cast in required shape without losing continuity or homogeneity under the available techniques of placing the concrete at a particular job. The mix minimum voids under the existing facilities of compaction at the site. A best mix from the point of view of compatibility should achieve a 99% elimination of the original voids present.
HARDENED CONCRETE
One of the most important properties of the hardened concrete is its strength which represents the ability if concrete to resist forces. If the nature of the force is to produce compression, the strength is termed compressive strength. The compressive strength of hardened concrete is generally considered to be the most important property and is often taken as the index of the overall quality of concrete. The strength can indirectly give an idea of the most of the other properties of the concrete which are directly related to the structure of hardened cement paste. A stronger concrete is dense, compact, impermeable and resistant to weathering and to some chemicals.
However, a stronger concrete may exhibit higher shrinkage with consequent cracking, due to the presence of higher cement content. Some of the other desirable properties like shear and tensile strength, modulus of elasticity, bond, impact and durability etc., are generally related to compressive strength. As the compressive strength can be measured easily on standard sized cube or cylindrical specimens, it can be specified as a criterion for studying the effect of any variable on the quality of concrete. However, the concrete gives different values of any property under different testing conditions. Hence method of testing, size of specimen and the rate of loading etc. are stipulated while testing the concrete to minimize the variations in test results. The statistical methods are commonly used for specifying the quantities value of any particular property of hardened concrete.
POLYPROPYLENE FIBRE REINFORCED CONCRETE
Polypropylene fibers are hydrophobic, that is they do not absorb water. Therefore, when placed in a concrete matrix they need only be mixed long enough to insure dispersion in the concrete mixture. The mixing time of fibrillated or tape fibers should be kept to a minimum to avoid possible shredding of the fibers .The type of polypropylene fiber recommended by manufacturers for paving applications is the collated fibrillated fiber.
Polypropylene fibers are new generation chemical fibers. They are manufactured in large scale and have fourth largest volume in production after polyesters, polyamides and acrylics. About 4 million tonnes of polypropylene fibers are produced in the world in a year. Polypropylene fibers were first suggested for use in 1965 as an admixture in concrete for construction of blast resistant buildings.
Subsequently, the polypropylene fiber has been improved further and is now used as short discontinuous fibrillated material for production of fiber reinforced concrete or as a continuous mat for production of thin sheet components.
Further, the application of these fibers in construction increased largely because addition of fibers in concrete improves the tensile strength, flexural strength, toughness, impact strength.
Advantages
Polypropylene fibers are Non-Magnetic, rust free, Alkali resistant, safe and easy to use. Polypropylene twine is cheap, abundantly available and is of consistent quality.
Polypropylene fibers are also compatible with all concrete chemical admixtures and can be handled with ease. The high molecular weight of polypropylene, gives it many useful properties.
Polypropylene fibers are chemically inert and hence, any chemical that will not attack the concrete constituents will not have any effect on the fiber also. When more aggressive chemicals come in contact, the concrete will always deteriorate first before fibers. The hydrophobic surface of fibers not being wet by cement paste, helps to prevent balling effect by chopped fibers and The water demand is nil for polypropylene fibers when used in concrete and there is no need for minimum amount of concrete cover.
Presence of fibers reduces the settlement and bleeding in concrete. The resistance to abrasion, freeze and thaw, Impact is improved.
NEED FOR POLYPROLYLENE FIBERS IN CONCRETE
Concrete develops micro cracks with curing and these cracks propagate rapidly under applied stress resulting in low tensile strength of concrete. Hence addition of fibers improves the strength of concrete and these problems can be overcome by use of Polypropylene fibers in concrete. Application of polypropylene fibers provides strength to the concrete while the matrix protects the fibers.
The primary role of fibers in cementations composite is to control cracks, increase the tensile strength, toughness and to improve the deformation characteristics of the composite. The performance of FRC depends on the type of the fibers used. Inclusion of polypropylene fibers reduces the water permeability, increases the flexural strength due to its high modulus of elasticity. In the post cracking stage, as the fibers are pulled out, energy is absorbed and cracking is reduced.
APPLICATIONS
Polypropylene fibers are versatile and widely used in many industrial applications such as ropes, furnishing products, packaging materials etc. They are also used in packaging, labeling, carpets, textile, apparel markets, stationery, plastic parts, reusable containers, laboratory equipment, automotive components, loud speakers, etc., Polypropylene Fiber reinforced Concrete is used in roads and pavements, drive ways, Overlays and toppings, ground supported slabs, Machine foundations, off shore structure, tanks and pools etc.,