Fiber Reinforced Concrete

FIBER REINFORCED CONCRETE

FIBER REINFORCED CONCRETE

ABSTRACT

Concrete is weak in tension and strong in compression .Even though reinforcement is provided in tension zone micro cracks are developed in the tension and compression zone. The propagation of these cracks can be arrested by using fiber reinforcement in concrete. The fiber reinforcement is provided using different materials like steel carbon, glass fibers and polypropylene fibers. The fibers 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.

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 paper describes the general properties and application of fiber-reinforced concrete used in construction. The promise of thinner and stronger elements reduced weight and controlled cracking by simply adding a small amount of fibers is an attractive feature of fiber-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 fiber, namely steel, carbon, asbestos, jute, glass, polythene, nylon, polypropylene, fly ash, polymer, epoxy, superplasticiser, etc.

ROLES OF FIBRE

Fibers are usually used in concrete to control cracking due to both plastic shrinkage and drying shrinkage. They also reduce the permeability of concrete and thus reduce bleeding of water. Some types of fibers produce greater impact, abrasion and shatter resistance in concrete. The amount of fibers added to a concrete mix is expressed as a percentage of the total 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 dividing fiber length (l) by its diameter (d). Fibers with a non-circular cross section use an equivalent diameter for the calculation of aspect ratio. If the modulus of elasticity of the fiber is higher than the matrix (concrete or mortar binder), they help to carry the load by increasing the tensile strength of the material. Increase in the aspect ratio of the fiber usually segments the flexural strength and toughness of the matrix. However, fibers which are too long tend to "ball" in the mix and create workability problems

WHY FIBER REINFORCED CONCRETE IS USED?

Plain and reinforced concrete stuctures are full of flaws such as pores, air voids, shrinkage cracks, etc., even before mechanically loaded.These flaws, especially small in size (micro cracks), grow stably under external loading and unite with existing or newly formed micro cracks until large fracture is formed which causes the collapse of the structure.Concrete is a material weak in tension and its tensile strength approximately ranges from 8 to 15 percent of its compressive strength.The initiation and propogation of these initial cracks and flaws during loading govern the mechanical behaviour of concrete subjected to different loading conditions.For a concrete structure subjected to tension, the cracks propogate in a direction perpendicular to the applied load.On the other hand, for a concrete structure subjected to purely uniaxial copression, the cracks propagate in the same direction as the applied compressive load. Since different mechanical responses of concrete structures under different loading conditions can be explained by fracture process, it is essential to understand when the cracks initiate and how they propagate with increasing load.The presence of micro cracks at the mortar aggregate interface is responsible for the weakness of plain concrete.The weakness can be removed to some extent byinclusion of fibers in the mix.

CRACK ARRESTING MECHANISM OF FIBERS

When the loads imposed on concrete approach

• that for failure, cracks will propagate, sometimes rapidly fibers in concrete provide a means of arresting the crack

• growth. After the concrete cracks in tension, the fibers continue to take the load, provided the bond is good. When the fiber strain reaches its breaking strain, the fibers fracture resulting in load transfer to the fibers of adjacent layers. This process continues which results in shifting of neutral axis. Failure occurs when the concrete in compression reaches ultimate strain. Reinforcing steel bars in concrete have: the same
• beneficial effect because they act as long continuous fibers.
• Short discontinuous fibers have the advantage, however, of beinguniformly mixed and dispersed throughout the
• concrete. Fibers are added to a concrete mix which normally

contains cement, water and fine and coarse aggregate .

• Among the more common fibers used are steel, glass,
• asbestos and polypropylene

TYPES OF FIBER REINFORCED CONCRETE

Natural fiber reinforced concrete

Steel fiber reinforced concrete

Polypropylene fiber reinforced concrete

Nylon fiber reinforced concrete

Asbestosfiber reinforced concrete

Glassfiber reinforced concrete

Carbon fiber reinforced concrete

NATURAL FIBER REINFORCED CONCRETE

Naturally available reinforcing materials can be used effectively as reinforcement in Portland cement concrete. Natural fiber reinforced concrete is suitable for low-cost construction, which is very desirable for developing countries. It is important for researchers, design engineers, and the construction industry to vigorously pursue the use of local materials. For economical engineering solutions to a variety of problems, natural fiber reinforced concrete offers a viable alternative that needs to be fully investigated and exploited. Wood fibers derived from the Kraft process possess highly desirable performance-to-cost ratios, and have been successfully substituted for asbestos in the production of thin-sheet cement products, such as flat and corrugated panels and non-pressure pipes. Straw-reinforced, sun-dried mud bricks for wall construction, and horse hair in mortar, are typical examples of how natural fibers were used long ago.

APPLICATIONS

In Africa, sisal fiber reinforced concrete has been used extensively for making roof tiles, corrugated sheets, pipes, silos, and gas and water tanks. Elephant grass fiber reinforced mortar and cement sheets are being used in Zambia for low-cost house construction, while wood and sisal fibers are being used for making cement composite panel lining, eaves, soffits, and for sound and fire insulation. Kraft pulp fiber reinforced cement has found major commercial applications in the manufacture of flat and corrugated sheet, non-pressure pipes, cable pit, and outdoor fiber reinforced cement paste or mortar products for gardening. The durability of these products in outdoor exposure has been demonstrated with nearly 10 years of commercial use of these materials.

Fiber type / Fiber length [mm] / Fiber diameter [mm] / specific gravity / Modulus of elasticity [106 MPa] / Ultimate tensile strength [103 MPa] / Elongation at break [%] / Water absorption [%]
Coconut / 51-102 / 0.10-0.41 / 1.12-1.15 / 19-26 / 120-200 / 10-25 / 130-180
Sisal / N/A / N/A / N/A / 13-26 / 276-568 / 3-5 / 60-70
Sugar cane Bagasse / N/A / 0.20-0.41 / 1.2-1.3 / 15-19 / 184-290 / N/A / 70-75
Bamboo / N/A / 0.05-0.41 / 1.5 / 33-40 / 350-500 / N/A / 40-45
Jute / 178-305 / 0.10-0.20 / 1.02-1.04 / 26-32 / 250-350 / 1.5-1.9 / N/A
Flax / 508 / N/A / N/A / 100 / 1000 / 1.8-2.2 / N/A
Elephant grass / N/A / N/A / N/A / 4.9 / 178 / 3.6 / N/A
Water reed / N/A / N/A / N/A / 5.2 / 70 / 1.2 / N/A
Plantain / N/A / N/A / N/A / 1.4 / 92 / 5.9 / N/A
Musamba / N/A / N/A / N/A / 0.9 / 83 / 9.7 / N/A
Wood fiber / 3-5 / 0.03-0.08 / 1.5 / N/A / 700 / N/A / 50-75

STEEL FIBER REINFORCED CONCRETE

During recent years, steel fiber reinforced concrete has gradually advanced from a new, rather unproven material to one which has now attained acknowledgment in numerous engineering applications. Lately it has become more frequent to substitute steel reinforcement with steel fiber reinforced concrete. The applications of steel fiber reinforced concrete have been varied and widespread, due to which it is difficult to categorize. The most common applications are tunnel linings, slabs, and airport pavements.

Many types of steel fibers are used for concrete reinforcement. Round fibers are the most common type and their diameter ranges from 0.25 to 0.75 mm. Rectangular steel fibers are usually 0.25 mm thick, although 0.3 to 0.5 mm wires have been used in India. Deformed fibers in the form of a bundle are also used. The main advantage of deformed fibers is their ability to distribute uniformly within the matrix.

Fibers are comparatively expensive and this has limited their use to some extent.

PROPERTIES OF CONCRETE IMPROVED BY STEEL FIBERS

Below are some properties that the use of steel fibers can significantly improve:

Flexural Strength: Flexural bending strength can be increased of up to 3 times more compared to conventional concrete.

Fatigue Resistance: Almost 1 1/2 times increase in fatigue strength.

Impact Resistance: Greater resistance to damage in case of a heavy impact.

Permeability: The material is less porous.

Abrasion Resistance: More effective composition against abrasion and spalling.

Shrinkage: Shrinkage cracks can be eliminated.

Corrosion: Corrosion may affect the material but it will be limited in certain areas.

DISADVANTAGES OF STEEL FIBER REINFORCED CONCRETE

Though steel fiber reinforced concrete has numerous advantages, it has certain concerns that are yet to be resolved completely.

There are complications involved in attaining uniform dispersal of fibers and consistent concrete characteristics.

The use of SFRC requires a more precise configuration compared to normal concrete.

Another problem is that unless steel fibers are added in adequate quantity, the desired improvements cannot be obtained.

However, as the quantity of fibers is increased, the workability of the concrete is affected. Therefore, special techniques and concrete mixtures are used for steel fibers. If proper techniques and proportions are not used, the fibers may also cause a finishing problem, with the fibers coming out of the concrete.

BENEFITS OF STEEL FIBERS

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

USES

SFRC elements are suitable to use in the following areas:

Slabs and Bridge Decks, Airport Pavements,Parking Areas, Fence Posts.

Storage tanks, Precast Concrete Members, Slab-Column connections, hotcreting and Repair of cavitations.

POLYPROPYLENE FIBER REINFORCED CONCRETE

PP fibers canbe produced as monofilaments or as collated fibrillated fiber bundles;their properties are related to the degree of crystallinity. PP is a linear hydrocarbon, although in some cases methyl side groups are attached to alternate carbons to improve oxidation resistance

Polypropylene is one of the cheapest & abundantly available polymers polypropylene fibers are resistant to most chemical & it would be cementitious matrix which would deteriorate first under aggressive chemical attack. Its melting point is high (about 165 degrees centigrade). So that a working temp. As (100 degree centigrade) may be sustained for short periods without detriment to fiber properties.

Polypropylene fibers being hydrophobic can be easily mixed as they do not need lengthy contact during mixing and only need to be evenly distressed in the mix.

Polypropylene short fibers in small volume fractions between 0.5 to 15 commercially used in concrete.

The main features: as a secondary concrete rebar materials, polypropylene fiber HDF-PP can be greatly enhanced its anti-cracking, permeability, impact resistance, earthquake resistance, antifreeze, scour, anti-burst, anti-aging properties And easy, pumping and water.

■Cracks in concrete block production

■Concrete improve the performance of the anti-infiltration

■The freeze-thaw resistance of concrete to improve performance

■To improve the impact resistance of concrete, bending, anti-fatigue, anti-seismic performance

■To improve the durability of concrete, anti-oxidation

■To improve the fire-resistant properties of concrete

BENEFITS OF POLYPROPYLENE AND NYLON FIBERS

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 curing

GLASS FIBER REINFORCED CONCRETE

Glass fiber reinforced concrete (GFRC) consists of high strength glass fiber embedded in a cementitious matrix. In this form, both fibers and matrix retain their physical and chemical identities, while offering a synergism: a combination of properties that cannot be achieved with either of the components acting alone. In general, fibers are the principal load-carrying members, while the surrounding matrix keeps them in the desired locations and orientation, acting as aload transfer medium between them, and protects them fromenvironmental damage. In fact, the fibers provide reinforcement for the matrix and other useful functions in fiber-reinforced composite materials. Glass fibers can be incorporated into a matrix either in continuous lengths or in discontinuous (chopped) lengths.

The design of GFRC panels proceeds from a knowledge of its basic properties under tensile, compressive, bending and shear forces, coupled with estimates of behavior under secondary loading effects such as creep, thermal and moisture movement.

Glass fiber reinforced concrete architectural panels have general appearance ofpre-cast concrete panels, but are different in several significant ways. For example, GFRC panels will, on the average, weigh substantially less than pre-cast concrete panels due to their reduced thickness. The low weight of GFRC panels decrease superimposed loads on the building’s structural components. The building frame becomes more economical.There are number differences between structural metal and fiber-reinforced composites. For example, metals in general exhibit yielding and plasticdeformation whereas most fiber-reinforced composites are elastic in their tensile stress-strain characteristics. Other important characteristics of many fiber-reinforced composites are their non-corroding behavior, high damping capacity and low coefficients of thermal expansion.

HEALTH HAZARDS CAUSED DUE TO GLASS FIBER

The National Toxicology Program classifies inhalable glass wool fibers as "Reasonably anticipated to be a human carcinogen”. Some fiberglass products warn of "possible cancer hazard by inhalation". The European Union and Germany classify synthetic vitreous fibers as possibly or probably carcinogenic, but fibers can be exempt from this classification if they pass specific tests. Evidence for these classifications is primarily from studies on experimental animals and mechanisms of carcinogenesis. Studies of fiberglass factory workers show significant increases in lung cancer but do not show clear exposure-response relationships and maybe confounded by the effects of smoking The Environmental Research Foundation has documented significant efforts by the fiberglass industry to prevent or remove cancer causing classifications.

Fiberglass will irritate the eyes, skin and the respiratory system. Potential symptoms include irritation of eyes, skin, nose, throat; dyspnea (breathing difficulty); sore throat, hoarseness and cough.

Fiberglass is resistant to mold but growth can occur if fiberglass becomes wet and contaminated with organic material. Fiberglass insulation that has become wet should be inspected for evidence of residual moisture and contamination. Contaminated fiberglass insulation should be promptly removed.

APPLICATIONS

1. The use of alkali-resistant glass fibers for reinforcing cement has received appreciable attention because of their excellent engineering properties.
2. Glass-fiber reinforced cement products that decrease with time in tensile and impact strength should not be used for primary structural applications.
3. Glass fibers have been used successfully to avoid cracking problems due to shrinkage stresses in the production of thin sheet.
4. Combining fiber types in cement composites is a new approach with high potential for improving the long-term performance of glass-fiber-reinforced cement products.
5. Mixtures of polypropylene and glass fibers or, alternatively, mica flakes used as fibers may help prevent long-term decreases in tensile and impact strength.

ASBESTOS REINFORCED CONCRETE

Asbestos is a naturally occurring silicate mineralsused commercially for their desirable physical properties. They all have in common their eponymous, asbestiform habit: long, (1:20) thin fibrouscrystals. Asbestos became increasingly popular among manufacturers and builders in the late 19th century because of its sound absorption, averagetensile strength, its resistance to fire, heat, electrical and chemical damage, and affordability. It was used in such applications as electrical insulation for hotplate wiring and in building insulation. When asbestos is used for its resistance to fire or heat, the fibers are often mixed withcement (resulting infiber cement) or woven into fabric or mats. Commercial asbestos mining began in the Eastern Townships of Quebec, Canadaand the world's largest asbestos mine is located in the town of Asbestos, Quebec.

HEALTH HAZARD OF ASBESTOS

Theinhalation ofasbestos fibers can cause serious illnesses, including malignant lung cancer, mesothelioma (a formerly rarecancer strongly associated with exposure to amphibole asbestos), and asbestosis (a type ofpneumoconiosis). Long exposure to high concentrations of asbestos fibers is more likely to cause health problems. This is most common among the miners of asbestos, since they have the longest exposure to it. The European Union has banned all use of asbestosand extraction, manufacture and processing of asbestos products.