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ABSTRACT
Concrete has high compressive strength, stiffness, low thermal and electrical conductivity, but it lacks in tensile, flexural strength as well as structure formed is brittle and not tough. When fibers with small staple length are added to concrete, fibers precipitates through the cement slurry to create a second phase at the interface making the structure tough and flexible. The flexibility leads to energy absorption which leads to tougher system. This second phase makes the structure tougher which will act as shock absorbing zone and avoid cracking.
1. INTRODUCTION
Concrete is the combination of dissimilar material. As in concrete, a single component is not used, so this total structure can be called as composite. When fibers with small staple length are reinforced in concrete matrix structure, the fibers precipitate through the cement slurry solution to create a second phase at the interface. This second phase makes the structure tougher. In case of vibration or heavy shocks, this second phase will act as shock absorbing zone and avoid any reflective cracking [1]. The fiber reinforcement in building materials dates from centuries ago, and it all started with natural fibers. In ancient Egypt some 3000 years ago, clay was reinforced by straw to build walls [2].
The concrete has a high compressive strength but relatively low tensile strength. The combined use of regular concrete and steel reinforcing bars improves the tensile strength but unfortunately steel reinforced concrete has high permeability that allows water and other aggressive elements to enter, leading to carbonation and chloride ion attack resulting in corrosion problems. Steel bar corrosion is in fact the main reason for infrastructure deterioration [3, 4]. The concrete structure formed is brittle; fibers reinforcement makes concrete tough and flexible. The flexibility leads to energy absorption which leads to tougher system. [5].
According to American Concrete Institute (ACI) Committee 544, Fiber Reinforced Concrete (FRC) is classified in four categories on the fiber material type. These are SFRC for steel fiber reinforced concrete, GFRC for glass fiber reinforced concrete, SNFRC for synthetic fiber reinforced concrete including carbon fibers, and NFRC for natural fiber reinforced concrete [6].
2. FIBERS AS REINFORCED COMPONENT IN CONCRETE
Though concrete lends itself to a variety of innovative design, it can be enhanced further with the effective use of fiber reinforcement. The development in fiber reinforced concrete has now provided a reliable solution to many problems faced by conventional concrete technology.
Hossein Sarbaz [7] et al studied the properties of sand–natural fiber mixtures for road Construction and stated that during the last decade there has been a renewed interest in the natural fibre as a substitute for glass, motivated by potential advantages of weight saving, lower raw material price, and ‘thermal recycling’ or the ecological advantages of using resources which are renewable. On the other hand natural fibers have their shortcomings, and these have to be solved in order to be competitive with glass. Natural fibers have lower durability and lower strength than glass fibers. However, recently developed fibre treatments have improved these properties considerably. To understand how fibers should be treated, a closer look into the fibre is required.
D. Almeida [8] et al studied the use of natural fibers as reinforcement in cement composites and stated that the vegetable world is full of examples where cells or groups of cells are ‘designed’ for strength and stiffness. A sparing use of resources has resulted in optimization of the cell functions. Cellulose is a natural polymer with high strength and stiffness per weight, and it is the building material of long fibrous cells. These cells can be found in the stem, the leaves or the seeds of plants. Hereunder a few successful results of evolution are described, so if proper fiber extraction technique is used, one could get valuable fibers as well as the farmers too will earn some revenue from waste.
Natural fibers have many advantages such as low specific weight, which results in a higher specific strength, it is a renewable resource; the production requires little energy, natural fibers have good thermal and insulating properties. Producible with low investment at low cost
Disadvantages of natural fibers are lower strength properties, variable quality, moisture absorption causes swelling of the fibers, Lower durability, fibre treatments can improve this, Poor fire resistance, Price fluctuate by harvest results
M.Y. Gudiyawar [9] et al studied the application of manmade fibers in concrete reinforcement and concluded that the choice of manmade fibers varies from synthetic organic materials such as polypropylene or carbon, synthetic inorganic such as steel or glass. Currently the commercial products are reinforced with steel, glass, polyester and polypropylene fibers. The selection of the type of fibers is guided by the properties of the fibers such as diameter, specific gravity, young’s modulus, tensile strength etc and the extent these fibers affect the properties of the cement matrix.
Proper wetting of manmade fibers is difficult. High temperatures can also cause unwanted changes of the fibre surface or even destroy the fibers. Nevertheless, a low price, reasonable processing temperatures and recyclability are the reason for a growing interest in polypropylene. Unmodified PP however, will not have a proper adhesion with the fibers by applying consolidation forces alone. Mechanical properties are hardly improved; the fibers simply act like filler.
Synthetic polymer fibers are derived from organic polymers and include acrylic, aramid, nylon, polyester and polypropylene. The most common forms of these fibers are smooth monofilament, twisted, fibrillated, and tridimensional material. They have low modulus and are chemically more inert than the other types of fibers and hardly corrode in the alkaline cementation matrix. Polypropylene and polyester fibers have hydrophobic surface.
Indrajit Patel [10, 11] et al studied basic properties of fibers reinforced concrete and stated that the use of these fibers as reinforcement reduces permeability, seepage of water, holds small aggregates together, improves both compressional and tensile strength of concrete, improves abrasion resistance by 40% thereby making for improved life of walkways, roads, and industrial flooring,
Manmade fibers have poor bonding with concrete and to improve this special treatment is required. The cost of manmade fibers specially designed for reinforcement would be higher. Polyester is unstable at PH level above 10 when the concrete temperature rises above 60o Celsius. During hydration and setting alkali is generated which degrades the few manmade fiber like polyester. Manmade fibers are not eco-friendly; recycling is a tedious work.
3. ROLE OF FIBERS AS REINFORCED COMPONENTS
Concrete is brittle, Concrete lacks in toughness and ductility, Concrete has limited flexural/Split tensile strength. Concrete have lower abrasion and wear resistance. Concrete does not adequately safe guard steel reinforcement from permeability. Concrete posses inherently micro cracks and its propagation. Concrete have limited impact resistance [12].
Both natural and manmade fibers used as reinforcement can overcome the problems associated with concrete. Natural fibers are available abundantly in nature and can be used to reinforce polymers to obtain light and strong materials. Fibers such as Banana fiber, Coir, Kenaf, hemp, flax, bamboo, jute, Polyester, Polypropylene, Glass etc are used in reinforcement of concrete and found that this enhance the ductility and toughness of the cement matrix, and increase significantly its tensile, flexural, and impact strengths [13].
Integral benefits of reinforcement fibers embedded in concrete are reduction in crack formation, reduction in water penetration, improvement in abrasion resistance, improvement in impact strength, improvement in flexural & compressive strength and there are many advantages as follows.
Sanjay Kala [14] studied the effect of secondary reinforcement synthetic fibers for concrete, shotcrete & mortars and stated that; tendency of the concrete to crack has been accepted as its natural characteristic. Cracks occur in concrete when stress within the concrete exceeds the strength of the concrete at that specific time. Providing higher structural strength to the concrete can cater to stress from the external forces. Fibers act as an internal support system, facilitating the retention of a homogenous concrete mix. Fibers randomly oriented in the concrete matrix provide a unique bridging mechanism by virtue of which intrinsic cracks formed are intercepted and bridges by the fiber right at the micro level. Higher probability of Fiber- Crack encounters contributes to the development of concrete’s optimum long-term integrity throughout its life. Fiber parameters which govern the crack control and failure inhibition action include high fiber area, high bond strength, balanced fiber pull-out & rupture strengths, high fiber aspect ratio (L/D).
Fibers also act as an internal support system retaining a more homogeneous concrete mix. Fibers discourage the natural segregation and settlement of concrete ingredients. The internal support system provided by the fiber results in a more uniform bleeding because the mix water is not displaced and rapidly forced to the surface by downward movement of concrete ingredients.
Conventional Concrete under application of continuous loading is found to undergo brittle failure. FRC on the other hand exhibits better ductile characteristics and is found to sustain more load after peak before brittle failure. Thus permeability of concrete is lowered by reduction of plastic crack formation which further reduces water percolation.
Abrasion is resisted when the surface of concrete has uniform quality paste. Fibers contribute to the development of this quality paste by contribution of plastic settlement and plastic shrinkage crack control. Fiber reinforced concrete pavements can sustain greater wear and continual pounding than non-fiber reinforced concrete pavements, extending their service life.
Fiber imparts to the concrete much needed modulus of elasticity during the freeze thaw cycles and hence mitigating the damages. When exposed to freezing and thawing action, the durability of concrete is found to decrease concomitant with losses in its strength but its toughness is affected by the presence of fibers. A high content of long fibers produces a toughness-retaining effect.
- D. Jothi [15] studied the application of fiber reinforcement concrete in constructions and stated that though concrete has strength but it is britt The first-crack strength characterizes the behavior of the fiber-reinforced concrete up to the onset of cracking in the matrix, while the toughness indices characterize the toughness there after up to specified end-point deflections. Residual strength factors, which are derived directly from toughness indices, characterize the level of strength retained after first crack simply by expressing the average post-crack load over a specific deflection interval as a percentage of the load at first crack. The importance of each depends on the nature of the proposed application and the level of serviceability required in terms of cracking and deflection. When a propagating crack front encounters a polymer fiber array, the homogenous growth will be disrupted as the front penetrates between the fibers, and additional fracture work is required to overcome the barrier effect as the penetration depth increases, the bridging force rises rapidly and eventually the fibers will fail, either through fiber pull-out or breakage. Due to the decrease in fracture resistance, the crack front will jump forward until the crack growth driving force is reduced to the critical value to arrest the advancing crack.
Fibers improve the inter particle cohesion on account of enhanced surface area on account of fiber length and dimension. This cohesion reduces heterogeneity of concrete mix thus promotes the concrete fluidity. Hence the user gains on account of enhanced adhesion and lesser rebound loss of mix.
Long term durability of concrete is enhanced with the use of quality mix designs workmanship and fiber reinforcement. The unique advantage with fiber reinforcement of reduced shrinkage cracks, plastic settlement, uniform bleeding, reduced plastic crack formation, increased abrasion resistance, reduced water migration added toughness and post crack residual strength synergistically combine to allow the concrete to develop its optimum long term durability and integrity.
One of the important attributes of FRC is the enhancement of fatigue strength compared to plain concrete. Flexural strength is defined as the maximum flexural fatigue stress at which the beam can withstand two million cycles of non-reversed fatigue loading. In many applications, particularly in pavements and bridge deck overlays, full depth pavements and industrial floors, and offshore structures, flexural fatigue strength and endurance limit are important design parameters mainly because these structures are subjected to fatigue load cycles. The endurance limit of concrete is defined as the flexural fatigue stress at which the beam could withstand two million cycles of non-reversed fatigue loading, expressed as a percentage of the modulus of rupture of plain concrete. Strength using the same basic mixture proportions, the flexural fatigue strength when determined with fibers shows that the endurance limit for two million cycles had increases by 15 to 18 percent.
Steel fibers only functions after the concrete has cracked, its function being to slow down the propagation of the shrinkage cracks from the surface into the slab. Fibers prevent the initiation of the cracks at an early age and thus entirely prevent the problem of crack propagation and fracture from arising. High tensile strength fibers create a tighter aggregate interlock at cracks and contraction joints, which increases load carrying capacity and provides more stable stress transfer.
4. METHODS OF PREPARING FIBER REINFORCED CONCRETE
M.K. Goyal [16] studied the fibers in concrete and explained the methods of using fibers as reinforcement. Depending upon the quantity of work or nature of the structure to be built it can be used. These fibers can be used for various applications such as fine plastering, rough plastering, concrete, pavements and flooring. Only variation done is in amount of fibers used as well as the length of fibers used. Fibers can be used in various ways such as manual mixing, Site mixing using auto mixer. For Water proofing 3 mm fiber is used with proportion of 40 gm per liter of water proof emulsion. For fine plaster 4.8 mm fiber with proportion of 100 Gms /50 kg of cement. For rough plaster 6 mm fiber with proportion of 125 Gms /50 kg of cement is used.
For concrete site mix 12 mm fiber with proportion of 900 Gms / m3 is used. For Flooring,
Pavement 24 mm fiber with proportion of 600-1800 Gms / m3 of is used. The methods of using
fibers as reinforcement are simple and easy. Depending upon the quantity of work or nature of the structure to be built it can be used.
4.1 Manual Mixing
In manual mixing initially the required quantity of fibers are mixed in water, continuous stirring will help to disperse the fibers from each other. Before pouring this water in concrete it should be seen that fiber get separated from each other, now immediately use this water for mixing in the dry mortar. The water in which fibers are dispersed are used to prepare the concrete depot.
For best results it is recommended that twin depot method should be used instead of using single depot method, it is because when small concrete depots are made fiber used for reinforcement gets separated easily and gives best results.
4.2 Auto mixer
When using the fiber for big sites usually for preparation of concrete auto mixers are used. In this procedure, take 5-10 liters of water in mixer vessel and put the required quantity of fibers in the roatating drum, then all the other ingriedients such as sand, gravels and cement are added and roatated till a homogeneus concrete is prepared which is ready to used.
4.3 Ready Mix Concrete Plant
These type of plants are used for mass construction, here initially fibers are dispersed in water but the batch size is quite huge. This water is poured automatically through water pump in mixer chamber for preparation of concrete. Once this concrete is preapared at RMC plant this fiber reinforced homogeneus concrete is transported to the site using heavy trucks with rotating drum.
5. CHARACTERIZATION OF FIBER REINFORCED CONCRETE
Ellen Lackey [17] identified and selected of composites test standards. The development of design standards is a critical need for the expanded use of concrete composites. One aspect of the development of design standard is the need for the identification, selection and development of appropriate testing standards. Several characteristics of fiber reinforced concrete have to test. Some of them are Slump Test, Setting time of concrete, soundness of Concrete, Tensile strength, Compressive strength, Flexural strength of concrete, Moisture absorption these test methods are as follows.
5.1 Slump Test
- M. Shetty [18] in his book titled, ‘concrete technology explained related testing method. As per ASTM C 143 standard test method for slump of concrete will be carried out. Slump test will be performed to check the workability and consistency of the fresh concrete with fibers. The test specimen shall be formed in a mould made of metal adhere readily attached by the cement paste and in the form of the lateral surface of the frustum of a cone with the base 20cm in diameter, the top 10cm diameter, and the height 30cm. The base and the top shall be open and parallel to each other and at right angles to the axis of the cone. The mould shall be provided with foot pieces and handles. A tamping rod of round, straight steel 5/8 inches in diameter and approximately 24 inches in length is needed. The mould will be removed immediately from the concrete by raising it carefully in a vertical direction. The slump will be measured immediately by determining the difference between the height of the mould and the height over the original centre.
5.2 Setting Time
As per ASTM 403 standard test method for setting time of concrete will be carried. This test will be done using Vicat’s needle apparatus. In this test a needle will be tried to penetrate on a concrete mould. The time at which needle ceases to penetrate gives the initial setting time. Setting time is not less than 30 minutes & final setting time not more than 10 hours [18].
5.3 Soundness
As per ASTM C 666 standard test method for soundness of concrete will be carried. This test will be done using Le Chartelier apparatus. The purpose of this test will be to find out advances whether a cement concrete structure is in danger of disintegrating or expanding or contracting so as to cause distortion or cracking in structure. Unaerated ordinary, rapid hardening and low heat cement shall not have an expansion more than 10 mm or after aeration it should be 5 mm [18].
5.4 Tensile Splitting Strength
As per ASTM C 496 standard tensile splitting strength test will be performed using cylindrical specimen. In determining the modulus of rupture by a beam specimen the upper, part of the specimen is in compression while the lower part is under tension. In the split tension test the specimen is under tension, therefore it gives a better idea about the tensile strength of concrete. Apparatus Besides the conventional testing machine, supplementary bearing bar or plate and bearing strips made of plywood are necessary to perform the test. The specimens are exactly the same as that used in compression tests. While testing diametrical lines shall be drawn on each of the specimen and the diameter shall be measured accurately. One of the plywood strips shall be centered along the centre of the lower bearing block. The specimen shall be placed on the plywood strip. A second plywood strip is placed lengthwise on the cylinder, centered on the lines marked on the ends of the cylinder. The load will be applied continuously and without shock, at a constant rate within the range 100 to 200 psi/min splitting tensile stress until failure of the specimen. The maximum applied load indicated by the machine at failure is recorded together with the type of failure and appearance of the concrete. The proportion used is 1:2:4 The briquettes will be tested for breaking strength at the 28th day after forming the briquette. As per standard three moulds are prepared and the average value is taken [18].
5.5 Compressive Strength
As per TS EN 12390-3 Standard compressive strength test will be performed. The most important property of concrete is the compressive strength is determined by loading the properly molded and cured specimens as dictated by the standards. The testing machine may be of any type of sufficient capacity which will provide the required rate of loading.. As per standards 15x15x15 cm cubic moulds will be prepared. Three specimens should be prepared for each different mix or intended age.. At the end of 28th day the cubic specimens is ready for the compression test [18].
5.6 Flexural Strength
As per TS EN 12390-5 standard flexural strength test is performed. This may be determined by using simple beam with centre-point loading or third-point loading. In the centre-point loading the maximum tensile force will be concentrated at a single point. A loading machine with sufficient capacity and bearing blocks are the necessary equipment to perform the test. The cubic moulds of 15x15x75 cm will be prepared and cured [18].
5.7 Moisture Absorption
As per D 6489-99 [2000] standard moisture absorption of concrete will be performed. The concrete cube is kept in oven; so that the moisture present in the concrete cube will evaporate then the oven dried cube will be kept in water for 24 hours. Then the water absorbed by concrete will be found by weighing the block [18].
6. APPLICATION OF FIBER REINFORCEMENT
Bruce Perry [19,20] studied the effective use of various fibers in concrete engineering and suggested various applications such as concrete structure, water proofing applications such as overhead water tank, Dams, Terrace water proofing, plastering, shortcreting, and precast materials such as pipes, hallow blocks tiles and pavement blocks. Technical applications of these fibers are explained below.
6.1 Load bearing concrete structures
Fiber reinforced concrete are used in various applications such as slabs, beams, columns, roads, bridges, airport runways etc in all these application sit is necessary that the structure should be crack resistance the mechanics behind this is shown in without fibers strain exceed the strain capacity but in case of curve with fibers the strain does not exceed the strain capacity thus there is no crack formation. In load bearing structures stel prevents crack at the middle of the slabsteel reinforcemnet is never on the top 50mm of the concrete slab so it is obvious that there is always chance of initila microcrack formation. When fibers are used as a reinforcement material these helps in preventing the crack formation at the first place. The fibers are evenly disperesed in the concrete structure and prevents crack, some times even fiber reinforcement is capable of eliminating the costly steel reinforcemnet on ground structures such as pavements, paltforms, PCC, Trimix etc. Use of fibers helps to improve the ductility of concrete under cyclic loading. The fiber reinforcement increases the shear resistance in concrete especially at the beam column junction; fiber reinforcement also helps in better anchorage of reinforcing bar crossing the beam column joint. Use of fibers at beam column joint enhance ductility thereby increases security against collapse. In seismic zones fiber could add extra conventional lateral reinforcement also improves the elasticity of the concrete, reduces segregation and bleeding.
6.2 Water retaining Structures
Fiber reinforcement is successfully utilized in water retaining structures and water proofing. Use of various fibers in water proofing application makes the concrete structure, walls, surface cohesive and less porous. Fibers also reduce the shrinkage cracks and defects which reduce the depth of water penetration and save the structure. The various application of fiber reinforcement are overhead water tank, brick coba water proofing this makes the terrace of the building water proof. This structures are preferred in modern building where terrace garden are maintains in the jungle of concrete and big cities where land for plantation is not available.
6.3 Plastering and shotcrete
Fibers are also used in plastering applications. The various plastering applications are internal plaster, external plaster, plaster of Paris, decorative precast panels, ready mix plaster etc. The use of fibers in plastering gives many advantages such as reduces rebound losses which directly leads to saving in material cost. It also reduces water absorption and water penetration trough the plaster and ultimately through the walls. This will directly reduce the wall dampness, thus reducing the repainting cost, fibers also improves the shatter resistance in wall. External plaster application, once the fibers are utilized it is very much necessary that the water resistance capacity has to be checked out. The fiber reinforcement in plastering application improves the strength of the mortar. It also helps in controlling the crack propagation while cutting groves for conduit laying. The fiber reinforcement reduces the ugly shrinkage cracks in wall and also improves the contour surface. The fiber reinforcement also increases the pace of work by reducing the rebound loss. Due to fibers in the mortar it is possible to apply a thicker coat thus the work could be covered at a fast rate. Fibers also help in falling of the concrete or mortar thus reworking is no needed. Fiber also improves visual appearance of the plastered surface.
6.4 Precast material
Now a days in modern construction various precast materials, are used such as cement pipes, hallow blocks, pavement blocks, precast bridges etc. In all these applications fibers are used to a great extent to improve the mechanical properties.
Acknowledgement
Authors wish to express their sincere gratitude to DKTE’S TIFAC- CORE in Technical Textile.
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