19-05-2014, 11:27 AM
STATIC BEHAVIOUR OF PLAIN AND STEEL FIBRE REINFORCED CONCRETE
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ABSTRACT
Plain concrete possesses a very low tensile strength, limited ductility and poor resistance to cracking. Owing to its poor tensile strength, crack-propagation with the application of load eventually leads to brittle failure in plain concrete. So to improve the physical properties fibre reinforcements of steel is used. A critical review of the existing literature indicates that extensive studies have been carried out to investigate the behaviour of plain and fibrous concrete. Most of the static properties of concrete, such as compressive strength and flexural strength, studies have mainly focused on the type, size, shape, and aspect ratio of fibres. Critical investigation for M-40 grade of concrete having mix proportion 1:1.50:1.90 with water cement ratio 0.45 to study the compressive strength and flexural strength of steel fibre reinforced concrete (SFRC) containing fibres of 0%, 0.5, 1% and 1.5% volume fraction of steel fibres. Two aspect ratios of flat corrugated - type steel fibre and round corrugated fibres were proposed to be investigated with mixed aspect ratio by weight of the longer and shorter fibres in the proportions of 100%-0%, 50%-50% and 0%-100% at each of the fibre volume fractions of 0.5%, 1.0% and 1.5%. Round steel fibres of length 20mm and diameter 0.5mm and flat steel fibres of length 40mm and width 2mm, 0.6mm thick are used. A result data obtained has been analyzed and compared with a control specimen (0% fibre). A relationship between volume fraction vs. compressive strength and volume fraction vs. flexural strength, are represented graphically.
What is a Fibre Reinforced Concrete?
Fibre reinforced concrete (FRC) is concrete containing fibrous material which increases its structural integrity. It contains short discrete fibres that are uniformly distributed and randomly oriented. Fibres include steel fibres, glass fibres, synthetic fibres and natural fibres each of which lend varying properties to the concrete. In addition, the character of fibre reinforced concrete changes with varying concretes, fibre materials, geometries, distribution, orientation, and densities.
Historical background
The concept of using fibres as reinforcement is not new. Fibres have been used as reinforcement since ancient times. Historically, horse-hair was used in mortar and straw in mud bricks. In the 1900s, asbestos fibres were used in concrete. In the 1950s, the concept of composite materials came into being and fibre reinforced concrete was one of the topics of interest. Once the health risks associated with asbestos were discovered, there was a need to find a replacement for the substance in concrete and other building materials. By the 1960s, steel, glass (GFRC), and synthetic fibres such as polypropylene fibres were used in concrete. Research into new fibre reinforced concretes continues today.
Effect of fibres in concrete
Fibres are usually used in concrete to control cracking due to plastic shrinkage and to drying shrinkage. They also reduce the permeability of concrete and thus reduce bleeding of water. Some types of fibres produce greater impact, abrasion, and shatter resistance in concrete. Generally fibres do not increase the flexural strength of concrete, and so cannot replace moment resisting or steel reinforcement. Indeed, some fibres actually reduce the strength of concrete.
The amount of fibres added to a concrete mix is expressed as a percentage of the total volume of the composite (concrete and fibres), termed "volume fraction" (Vf). Vf typically ranges from 0.1 to 3%. The aspect ratio (l/d) is calculated by dividing fibres length (l) by its diameter (d). Fibres with a non-circular cross section use an equivalent diameter for the calculation of aspect ratio. If the fibre’s modulus of elasticity is higher than the matrix (concrete or mortar binder), they help to carry the load by increasing the tensile strength of the material. Increasing the aspect ratio of the fibres usually segments the flexural strength and toughness of the matrix. However, fibres that are too long tend to "ball" in the mix and create workability problems.
Some recent research indicated that using fibres in concrete has limited effect on the impact resistance of the materials. This finding is very important since traditionally, people think that ductility increases when concrete is reinforced with fibres. The results also indicated that the use of micro fibres offers better impact resistance to that of longer fibres.
Fibre Material Properties
There are several properties that a good reinforcing fibre must have to be effective: tensile strength, ductility, high elastic modulus, elasticity, and Poisson’s ratio. Several are key to the mechanical behaviour of fibre-reinforced concrete.
To see significant improvements in tensile capacity of concrete, the fibre must be much stronger than the concrete matrix in tension, since the load bearing area is much 74 less than the matrix. For ductility improvements, the fibre must be able to withstand strains much greater than the matrix. Fibres subject to creep have a reduced effectiveness.
The most important, though, is the elastic modulus. The proportion of the load carried by the fibre depends directly upon the comparative elastic modulus of the fibre and matrix. If the elastic modulus of the fibres is less than that of the concrete matrix, the fibres will contribute relatively little to the concrete behaviour until after cracking. In addition, the composite strain after cracking will be higher. This is a primary problem afflicting polymer fibres: a relatively low elastic modulus. (Johnston, p. 25-26)
Tensile Strength
One would think that adding fibres to concrete would increase the tensile strength of the concrete since the tensile strength of concrete is so low. However, the modulus of elasticity of the polymeric fibres is less than that of the concrete matrix, so the fibres do not take much load until cracking. Once cracking occurs, sometimes the tensile strength of the fibres bridging the crack is higher than that of the concrete, causing the ultimate tensile strength to be reached after cracking, when the fibres alone provide the strength. However, this does not actually increase the cracking strength of the mix. Ductility is obviously greatly increased.
Balaburu and Khajuria (1996) also tested the splitting tensile strength of lightweight concrete with polymer fibres. The strengths were not appreciably different at 28 days; they were slightly higher at 7 days. However, the difference was not statistically significant. A major difference was that after failure, the fibre cylinders maintained their coherence, while the plain concrete cylinders fractured into two pieces.
Combined
Chen and Carson (1971) designed an investigation to measure the influence of the length of randomly oriented wire fibres (38.2, 25.4, 12.7-mm), the percentage by volume of reinforcement and the age and curing conditions on the tensile strength, compressive strength and ductility of the material. Standard cylinders were tested in direct compression and indirect tension to obtain the complete stress-strain curves for the materials. The compressive and tensile strength increased 60% for the 12.7-mm long fibre at 0.75% volume fraction in the mortar. The best tension and compression strengths were obtained for concrete with 2.0 percent volume fraction of 25.4 and 12.7-mm long fibres, respectively. The age of the materials had no significant influence on the tensile and compressive strength.
The mechanical properties of concrete and mortar reinforced with randomly distributed smooth steel fibres were investigated by Shah and Rangan (1971). Different volumes, lengths, orientations and types of fibres were used. The effect of fibres was compared with that of conventional reinforcement in flexure, tension and compression. It was observed that significant reinforcing effect of fibres is derived after the cracks are initiated in the matrix, similar to conventional tensile and stirrup reinforcement. The post-cracking resistance of fibres was considerably influenced by their length, orientation and stiffness.
CONCLUDING REMARKS
The testing programme as planned has been explained in this chapter to achieve the objectives of the present investigation. The basic properties of the various constituents of concrete such as cement, fine aggregates, coarse aggregates, water and steel fibres are presented. Concrete mix details along with method of casting and curing have been reported. The procedure adopted for testing hardened concrete in compression, flexure and split tensile has been described.
The test data and results on hardened plain and fibre reinforced concrete with different fibre volume fractions and different aspect ratios as obtained in this investigation are analyzed and discussed in detail in the next chapter.