Module - 3 Fiber Reinforced Concrete ppt

Fiber Reinforced Concrete

1. Module 3 Alternate Building Materials Fibre Reinforced Concrete (FRC)

2. Introduction Cement concrete can be cast to any desired shape, It also possesses many desirable properties like high compressive strength, stiffness, low thermal and electrical conductivity. Generally, plain concrete possesses a very low tensile strength, limited ductility and little resistance to cracking. Attempts have been made to impart improvement in tensile properties of concrete members by way of conventional reinforcements . This has been achieved by placing steel bars in regions where concrete is subjected to tension leading to what is called as RCC. Although steel bars are used as reinforcements, they provide tensile strength to the concrete members. However, do not increase the ‘inherent tensile strength’ of concrete itself. It still falls short of many more desirable properties like toughness, ductility, controlling of cracking.

3. Introduction In order to achieve all the above- mentioned properties it is essential to distribute the reinforcement uniformly throughout the c/s. Thus, there is a need for multidirectional and closely spaced reinforcement for concrete arises. Then it has been recognized that the addition of small, closely spaced and uniformly dispersed fibres to concrete would act as a crack arrester, substantially increasing it’s static and dynamic properties. This concrete is known as Fibre Reinforced Concrete (FIBRECON).

4. What if FRC Fibre reinforced concrete can be defined as a composite material consisting of mixtures of cement , mortar or concrete with more or less randomly distributed fibres. or Reinforcing the brittle concrete matrix is possible by adding the constituents to the concrete mix, short fibres of small diameter that are either metallic or non-metallic. This new material with improved mechanical properties is called “Fibre reinforced cement composite” (FRC).

5. What is Fibres Fibre is a small piece of reinforcing material which increases the structural integrity of concrete. These short discrete fibers that are uniformly distributed and randomly oriented provide strength and toughness The graph shows the stress-strain relation between FRC and Plain concrete.

6. Why fibres are used Main role of fibers is to bridge the cracks that develop in concrete and increase the ductility of concrete elements. Fibres are usually used in concrete to control cracking due to plastic shrinkage and drying shrinkage. They also reduce the permeability of concrete and thus reduce bleeding of water. They increase the ductility of concrete elements. Some types of fibres produce greater abrasion and shatter resistance in concrete. They impart more resistance to impact loads. Variation of crack and fracture resistance between FRC and plain concrete.

7. Reinforcing Material The reinforcement is in the form of short fibres of small diameter distributed throughout the matrix. The fibres can be broadly classified as: Metallic fibres Polymeric fibres Mineral fibres Natural fibres The fibres must have the following properties. Tensile strength significantly higher than that of concrete Bond strength with concrete matrix preferably of same order as or higher than the tensile strength of matrix. The elastic modulus in tension to be significantly higher than that of concrete. The poison’s ratio and the coefficient of thermal expansion preferably be of the same order as that of matrix.

8. Types of Fibres Steel fibres Glass fibres Carbon fibres Propropylene fibres Nylon fibres Coir Fibres Cellulose Fibres Hay Fibres

9. Metallic Fibre Metallic fibres are either made out of carbon steel or stainless steel. The tensile strength ranges from 345MPa to 1380MPa. The modulus of elasticity is about 200GPa. The fibres c/s may be circular, rectangular, crescent shaped or irregular. Most common steel fibres are round in c/s with dia ranging from 0.25mm to 0.75mm and a length ranging from 25mm to 60mm. Their aspect ratio (ratio of length to dia or equivalent dia ) is generally less than 100 with common range of 40 to 80. Reduced crack widths and control the crack widths tightly, thus improving durability. Improve impact and abrasion resistance. Used in precast and structural applications, highway and airport pavements, refractory and canal linings, industrial flooring, bridge decks, etc.

10. Round and straight steel fibre Fibres have been indented. Crimped Hooked at the ends Enlarged at the ends Fibres have a crescent shaped c/s. Fibres have irregular surface with crescent shaped c/s. Metallic Fibre

11. Benefits Provides tough and durable surfaces. Reduces surface permeability. They act as crack arrestor. Increases tensile strength and toughness. Resistance to impact and abrasion. Resistance to freezing and thawing

12. Polymeric fibres are by products of petrochemical and textile industries. Cheap, abundantly available, High chemical resistance, Low modulus of elasticity. The fibres types have been explored for use in cement based matrices are Acrylic, Aramid, Nylon, Polyester, Polyethylene and Polypropylene. These fibres have reasonably high tensile strength, the modulus of elasticity of most of them is quite low (except aramid). Nylon was one of the first of the polymer fibres to be included in cement based matrices. Compared with other polymeric fibres, Aramid fibres have higher tensile strength and modulus of elasticity and hence they can enhance the mechanical properties like tensile and bending strength of the composite. Applications in cladding panels and shotcrete. Polymeric Fibres

13. Benefits Increase resistance to plastic shrinkage during curing. Improve structural strength. Improve ductility. Reduce steel reinforcement requirements. Improve impact resistance and abrasion resistance.

14. Mineral Fibres High tensile strength, 1020 to 4080 N/mm 2 Generally, fibers of length 25mm are used. Improvement in impact strength. Increased flexural strength, ductility and resistance to thermal shock. Used in formwork, swimming pools, ducts and roofs, sewer lining etc. Glass fibres are the predominately used mineral fibre. Glass fibre is silica based glass compounds that contain several metals oxides. These fibres relatively high tensile strength and modulus of elasticity compared to polymeric fibres. These are the most commonly used fibres for structural applications. In the initial stages borosilicate glass fibres (E- glass) and soda-lime-silica glass fibres (A-glass) were employed to reinforce cement-based composites.

15. Benefits Highly durable and safe. Requires very low maintenance. Installation is quick and cost effective. Weather and fire resistant. Economical. Energy efficient.

16. Natural Fibres In many parts of world artificial fibres like steel or polymer fibres are not available, attempts have been made to incorporate naturally occurring fibres extracted from plants in cement based composites. A unique aspect of these fibres is the low energy needed for their extraction. A major problem in the use of these fibres in cement/concrete matrix is that the durability of the composite. These fibres are economical, attempts have made to overcome the problem of durability either by use of admixtures in concrete to reduce its alkalinity or by protecting fibres by some special treatment. Some of the natural fibres used in Portland cement composite are bamboo, coconut (coir), jute, sisal, sugarcane bagasse, wood and elephant grass. They may undergo organic decay.

17. Even though these fibres are sufficiently strong in tension, their modulus of elasticity is quite low. Bamboo fibres have tendency to absorb water, which adversely affects the bonding between fibre and the matrix during the curing stage. Coir fibres are short in length and are found to be sensitive to moisture change. Brown fibres are thick, strong and have high abrasion resistance. White fibres are smoother and finer, but also weaker. Sisal fibres even though relatively strong, are not durable in alkaline environment. Extraction of elephant grass fibres is difficult but they are stable under varying moisture conditions and alkali resistant. These fibres have high tensile strength and young’s modulus and process of extraction is very well developed. Natural Fibre

18. Mechanical Properties of FRC Compressive strength- Improves(0-15 %). 1% increase in fibre content increases 3% of Modulus of Elasticity . Addition of 4% of fibres report 2.5 times more increase in flexural strength . Presence of 3% of fibres develop 2.5 times more splitting tensile strength . Toughness is about 20-40 times that of plain concrete. Addition of fibres increase fatigue strength of about 90%. Impact strength is 5 to 10 times of plain concrete and improves wear and tear. Shear strength increases to 100%. Randomly Distributed fibres develop shear friction and ultimate strength. Use of fibres produce ductility, tensile strength, moment capacity and stiffness .

19. Factors affecting the Properties of FRC Volume of fibers Aspect ratio of fiber Orientation of fiber Relative fiber matrix stiffness

20. Difference between FRC and NRC

21. Advantages of FRC Main role of fibres is to bridge the cracks that develop in concrete and increase the ductility of concrete elements. High modulus of elasticity. Does not rust nor corrode and requires no minimum cover Controls shrinkage cracking Ideal Aspect Ratio makes them excellent. Easily placed, cast, sprayed and less labour intensive than placing rebar . Greater retained toughness in conventional concrete mixes. Possesses enough plasticity to undergo large deformation. Cost savings of 10% to 30% on conventional concrete.

22. Increase in specific gravity of the concrete. This means that the concrete will be heavier than normal concrete in case of some fibres. Proportioning the exact amount of fibres in the batch of concrete. Greater reduction of workability. High aspect ratio also effect workability. Distribution of fibres effect the engineering properties. Disadvantages of FRC

23. Application of FRC Over laying of air fields, road pavements, bridge decks, etc. Canal linings, tunnel linings, refractory linings. Fabrication of precast pipes, boats, beams, wall panels, roof panels, man-hole covers, etc. Blast resistant. Dams and hydraulic structures. It is used for exterior and interior floors, slabs and parking areas, roadways, etc. It includes application in driveways, sidewalks, pool construction, basements, colored concrete, foundations, drainage, etc.