FIBRE REINFORECD CONCRETE

 ABSTRACT

Concrete  is weak in tension and strong in compression .Even though reinforcement is provided in tension zone microcracks are developed in the tension and compression zone.the propogation of these cracks can be arrested by using fibre reinforcement in concrete.The fibre reinforcement is provided using diffrent materials like steel carbon,glass fibres,polypropylene fibres.The fibres are very small which are distributed over the whole area of concrete .because of this we can  not only arrest crack formation but also we can increase flexural ,shear ,torsion,strength,freezing &thawing resistance.


1 INTRODUCTION

In all countries, the construction industry is rapidly developing based on the invention of different materials and products in engineering fields. Engineers have attempted various types of materials in order to make the task more efficient reducing time, cost, improving durability, quality and performance of structures during their lifetime. Sophisticated analyses on structural Idealization have made a tremendous impact on the development of construction materials.This digest describes the general properties and application of fibre-reinforced concrete used in construction. The promise of thinner and stronger elements, reduced weight and controlled cracking by simply adding a small amount of fibres is an attractive feature of fibre-reinforced concrete.The quality of good and durable concrete does not depend only on the quality of raw materials but also on proper mix-design, use of admixtures, placement, vibration and efficient curing. A number of additives are being used with concrete to enhance structural properties. Such additives are different types of fibre, namely steel, carbon, asbestos, jute, glass, polythene, nylon, polypropylene, fly ash, polymer, epoxy, superplasticiser, etc.

 ROLES OF FIBRE

                When the loads imposed on concrete  approach that for failure, cracks will propagate, sometimes rapidly ;fibres in concrete provide a means of arresting the crack growth. Reinforcing steel bars in concrete have: the same beneficial effect because they act as long continuous fibres. Short discontinuous fibres have the.advantage, however, of being uniformly mixed and dispersed throughout the concrete. Fibres are added to a concrete mix which normally contains cement, water and fine and coarse aggregate . Among the more common fibres used are steel, glass, ·asbestos and polypropylene.

3 .  FIBERS USED
Some of the fibers that could be used are
  • Steel fibers
  • Carbon
  • Glass
  • Polypropylene
  • Nylons
  • Asbestos
  • Coir

4.STEEL FIBRE REINFORCED CONCRETE  (SFRC)

From early 70's, extensive research work has been carried out in the field of concrete by using the steel fibre reinforcement with normal concrete using hydraulic cement. Most of the research work was performed to study the improvement of the structural behavior of the beams and slabs in bending, shear, torsion, freeze thaw, durability, shrinkage and impact/fatigue properties.           
·The design procedure of steel fibre reinforced concrete (SFRC) members have been developed based on various research works by complimenting procedure for the fibre contribution. It has been observed that the main parameters which influence the behavior of SFRC beams and slabs would depend on 'Aspect Ratio' (length / diameter), shape and orientation of fibres, size of aggregates and the size of specimen. The volume of fibres generally lies between 0.5 to 1.5% of the total concrete mass. The quality of steel fibres should conform to  ASTMA 820.
  


Fig..4.1 Dramix glued steel fibers


    Fig.4.2 Steel ribers






4.1  MIXING

Mixing of fiber reinforced concrete  needs careful conditions to avoid spalling of fibers,segregation and in general the difficulty of mixing the materials uniformly.

4.3  THE TYPICAL PROPORTIONS FOR FIBER  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
Air –content              : 6 to 9%
Fiber content            :0.5 to 2.5% by vol of mix
                                 :Steel-1%78 kg/m3
                                 :Glass-1%25 kg/m3
                                 :Nylon-1%11 kg/m3



3.1 Advantages

The following advantages can be obtained for SFRC Structural Components :
Increased flexural, shear, torsion strength and freezing – thawing resistances, Increased durability, fatigue strength, cracking and thermal resistance and reduce    shrinkage.

3.2 Uses

SFRC elements are suitable to use in the following areas:

1 Slabs and Bridge Decks, Airport Pavements,Parking areas, Fence Posts.
2  Embankment protection, Machine foundation, Manhole covers and Dams.
3 Storage tanks, Precast Concrete Members, Slab- Column connections, shotcreting    
4 Repair of cavitations. 


5  CARBON FIBRE REINFORCED CONCRETE 

Apart from the steel fibres, asbestos, glass carbon and other synthetic and natural fibres have also been used for reinforcing the concrete. The asbestos fibres may give pollution problem. The steel  fibres sometimes give deterioration at an early stage in the highly alkaline environment. The carbon fibres are found suitable from these points. In general for any practical case, the percentage of fibres is in the range of 2 to 4% by volume of concrete mass. The length and diameter of the fibres vary between 3 to 10 mm and 15 to 20 micrometers respectively. Due to the use of carbon fibres, the high temperature and high pressure curing.

5.1 Advantages

The advantages of using CFRC are as follows:

1.High strength lightweight concrete cabe achieved.

2.More durable in hot weather & less shrinkage value.

3.Increased freezing - thawing resistances.

5.2  Uses of Carbon Fibre Reinforced Concrete


The uses of CFRC are in different fields where the Light weight concreting is required.  The major areas are as follows:

1 Precast thin sections with lightweight concreting (up to Specific Gravity 1.0)
·.
2 Suitable for high temperature and low humidity areas. 


6  GLASS  Fibre-Reinforced  CONCRETE 

Glass fibre-reinforced polyester (GRP) composites are the most popular reinforced plastic materials used in the construction industry1. Depending on formulation and use, they may be fabricated into products that are light in weight, transparent, translucent or opaque, colorless or colored, flat or shaped sheets, with no limit to the size of object that can be made. This Digest will describe the nature, general properties, application in construction and related fields, and durability of GRP composite materials.

 6.1 General Nature and Fabrication


The two components of a GRP composite are the matrix (the continuous phase) reinforcing glass. Of itself, the matrix does not provide strength, but it serves to bond the reinforcing glass fibres and to transfer the load to the reinforcing phase. The matrix is based on cured thermosetting polyester resin. The raw material is supplied in the form of a viscous, syrupy liquid comprising the following basic ingredients: a linear unsaturated polyester; a cross-linking monomer (curing agent), usually styrene; and an inhibitor to retard cross-linking until the resin is to be used by the Fabricator. . During fabrication the monomer reacts with the polyester, resulting in cross-linking of the polyester chain and final cure

Table1  Physical and Mechanical Properties of Fibres






Fig. 6.1  Electron micrograph of partial cross section of glass CFRP composite





7  POLYPROPYLENE FIBRE REINFORCED CONCRETE:
An experimental research investigation of the fresh and hardened material properties of the fibrillated polypropylene fibre reinforced concrete is reported. Fiber lengths were 1/2 and 1/4 inch, and volume fractions were 0.1, 0.3, and 0.5%. Fibre effects on concrete properties were assessed. Properties studied were slump, inverted slump cone time, air content, compressive and flexural behaviors, impact resistance and rapid chloride permeability, and volume percent of permeable voids. An innovative method of characterizing the flexural behavior of fibrillated polypropylene fibre concrete was proposed. The new method was dependent on the post-peak flexural resistance of concrete. For impact resistance and flexural behavior, it was concluded that 1/4-inch-long fibres were more effective than 1/2-inch-long fibers for volumes of 0.3% or less, while 1/2-inch-long fibres were more effective for 0.5% volume.

7.1 Mechanical Properties and Durability of Polypropolyene Fibre reinforced High Volume FlyAsh Concrete for Shotcrete Application

Investigations at CANMET have led to the development of polypropylene fiber reinforced, high-volume fly ash concrete for shotcreteing rock outcrops. This type of concrete has a very low water-to-cementitious material ratio, fly ash content greater than 50 percent of the cementitious material, and contents of fibers up to 5 kg/m3 of concrete. The workability of the concrete is maintained by the use of high dosages of superplasticizers. This paper presents the results of CANMET investigations dealing with the development of this type of concrete. The test results show that polypropylene fiber reinforced, high-volume fly ash concrete has satisfactory workability and strength characteristics. It also has very low permeability, low drying shrinkage, adequate ductility, and toughness characteristics.

8 FABRICATION

Before mixing the concrete, the fibre length, amount and design mix variables are adjusted to prevent the fibres from balling. Satisfactory  reinforced mixes usually contain a mortar volume of about 70 per cent compared with a mortar volume of about 50 percent for typical unreinforced concrete mixes.  Fibre-reinforced cement boards contain no coarse aggregate. These products are usually made by spraying mortar and chopped fibre simultaneously.

            Mortar with a high water:cement ratio is used to facilitate spraying. Other application methods include simple casting, which is less versatile than spraying, and press moulding, which results in a lower effective water:cement ratio, thus producing a stronger product. Chemical admixtures are added to fibre-reinforced concrete mixes primarily to increase the workability of the mix. In North America, air-entraining agents and water-reducing admixtures are usually added to mixes with a fine aggregate content of 50 per cent or more. Superplasticizers1, when added to fibre-reinforced concrete, can lower water cement ratios, and improve the strength, volumetric stability and       handling characteristics of the wet mix.

9 COMPOSITE PROPERTIES


1      Fibres can improve the toughness, the flexural strength, or both, and are chosen on the basis of their availability, cost and fibre properties. For example, polypropylene fibres significantly increase concrete toughness but have little effect on tensile strength. Mixtures of polypropylene and glass fibres, on the other hand, produce concrete with a high degree of  both toughness and flexural strength. Ratio of Toughness Values of Some Fibre-Reinforced Cementitious Materials with Respect to Unreinforced Materials.
2      These values are representative values only and may vary additionally due to differences in test methods and specific process and mix variables.

10  MAIN APPLICATIONS


1      Long-Term Performance of Alkali-Resistant, Glass-Fibre-Reinforced Cement Since the late 1960's, the use of alkali-resistant glass fibres for reinforcing cement has received appreciable attention because of their excellent engineering properties.


3      There has been concern, however, over possible adverse reactions between the fibres and the matrix, even though they are alkali resistant.3,4 The values for the properties of glass-fibre composites were obtained from samples in which the fibres had not undergone corrosion.


    The tensile strength and the impact strength of glass-fibre-reinforced cement products decrease with age if exposed to outside weather.  This time-dependent decrease in flexural strength, first noted in alkali-resistant glass-fibre composites, has also been observed in high alumina and supersulphated cements in which only small amounts of alkali are present.
5     Glass-fibre reinforced cement products that decrease with time in tensile and impact strength should not be used for primary structural applications. Glass fibres have been used successfully to avoid cracking problems due to shrinkage stresses in the production of thin sheet.



  Strength decreases have not been observed in specimens containing carbon and Kevlar fibres. Combining fibre types in cement composites is a new approach with high potential for improving the long-term performance of glass-fibre-reinforced cement products. Mixtures of polypropylene and glass fibres or, alternatively, mica flakes used as fibres may help prevent long-term decreases in tensile and impact strength.

11  ENVIRONMENTAL FACTORS


The resistance of fibre-reinforced concrete to environmental factors such as frost action depends on the quality of the concrete matrix material and should not differ substantially from that of conventional concrete. Fibres can be effective, however, in reducing frost damage because of their crack-arresting properties. Care should be taken to ensure that an adequate amount of entrained air is incorporated in the mix to provide additional resistance to freezing and salt corrosion.  Other environmental problems such as acid attack, sulphate attack and alkali-aggregate reaction are generally not augmented by the presence of  fibres unless there is a chemical reaction between the fibre and the concrete.

12  FUTURE TREND

In various countries the use of newly developed materials have already been started in different projects to achieve an economical and effective material for particular purpose. In Brunei Darussalam some of such materials have been used successfully and an increase of such a trend is being observed. The most important thing is to select the appropriate material for a specific purpose. It is expected that due to recent technological development of material science uses of concrete additives, such as fibre reinforcement, Ferro cement, polymer, epoxy and superplasticizers shall be increased in future.

 CONCLUDING REMARKS 

Innovations in engineering design, which often establish the need for new  building materials, have made fibre-reinforced cements very popular. The possibility of increased tensile strength and impact resistance offers potential reductions in the weight and thickness of building components and should also cut down on damage resulting from shipping and handling.  Although ASTM C440-74a describes the use of asbestos-cement and related products, there are, at this time, no general ASTM standards for fibre-reinforced cement, mortar and concrete. Until these standards become available, it will be necessary to rely on the experience and judgment of  both the designer and the fibre manufacturer. The onus is thus on the designer to be aware of the limitations presently inherent in some of these composites, particularly the durability of glass-fibre-reinforced products.
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