Reinforced concrete has been used as an essential material in main load-carrying system of various structures in several countries. Reinforced concrete is recognized to be durable and capable of withstanding a variety of environmental conditions. In the beginning, life of RCC was expected to be about 100 years. However in practice, RCC needs lot of maintenance to even complete 50 years of service. Nevertheless, failures of structures still do occur as a result of premature steel reinforcement corrosion. The corrosion of steel in reinforced concrete deteriorates the strength of such a structure. The effects of corrosion is even more pronounced in flexural reinforced concrete member as nearly all of the tension force is exerted on steel reinforcement. It has been estimated that only transportation agencies across USA invest more than $5 billion on concrete bridge repairs and renovation annually [Tikalsky, 2000].

Fibres have been implemented in concrete structures to enhance tensile characteristics by inhibiting crack growth and improving mechanical behaviour. Fibres in concrete substantially improve toughness or energy absorption capacity, tensile strength, flexural strength, fatigue resistance, and ductility. With reduction of surface and internal cracks, entry of water and harmful liquids is restricted. It reduces corrosion of tensile reinforcement and increases service period of building with lesser maintenance.

Fibres are short threadlike, thin tension materials. They are manufactured from different types of materials and in different shapes and sizes. They are commercially available in different types, steel fibers of different shapes (straight, twisted, deformed with hooked or paddled ends), glass fibers, natural organic or mineral fibers (wood, coconut, asbestos), and finally, synthetic fibers such as nylon, polyester, kevlar and polypropylene fibers (plain, twisted, fibrillated, buttoned ends). The most widely used fibers are typically of steel, glass, and polypropylene. Fibers may take many shapes. Their cross sections include circular, rectangular, half round, and irregular or varying cross sections. They may be straight or bent.

Many research studies carried out over the past three decades have also focused on the shear behaviour of Fibre Reinforced Concrete. Most of the published work, however, has focused exclusively on steel fiber-reinforced concrete (SFRC). The results showed that steel fibers can be used to boost the shear capacity of concrete and to improve the shear crack distribution; therefore they are capable of replacing some of the vertical stirrups in RC structural members. This helps to reduce the problems associated with congestion of shear reinforcement such as interference with concrete compaction that results in honeycombing and poor quality of concrete, particularly at critical sections such as beam-column junctions [Salah Altoubat, 2009].

The shear reinforcement in beams typically consists of steel bars bent in the form of stirrups or hoops, the addition of deformed steel fibers to the concrete shows to enhance shear resistance and ductility in reinforced concrete beams. The use of short fibers in concrete offers noticeable advantages, such as limited cracking and increased toughness. It can also increase shear strength, allowing reduction of stirrup reinforcement, and improve ductility and safety [Shah S.P.]. The influence of fibers on shear strength has been studied by several authors to quantify the increase of resistance and to associate a model for calculation [Batson, 1972].

Fibres cannot entirely replace the conventional shear reinforcement when the structural elements are subjected to very high shear stress. However, the use of fibres reduces the severity of the failure mode, which can change from a brittle shear into a ductile flexural failure [Sharma, 1986]. These fibers are typically hooked or crimped. When used in reinforced concrete beams without transverse reinforcement, fibers increase shear strength by providing post-cracking diagonal tension resistance [Swamy, 1993].

The fibers also enhance cracking distribution, similar to the effect of stirrups. This leads to reduced crack widths, and thus an increase in shear resistance through aggregate interlock. The addition of steel fibers to a reinforced concrete beam is known to increase its shear strength and, if sufficient fibers are added, a brittle shear failure can be suppressed in favour of more ductile behaviour. The increase in shear strength attributable to steel fibers varied from 13 to 170% [Narayan, 1987].

The increased shear strength and ductility of fiberreinforced beams stems from the post- cracking tensile strength of fiber-reinforced concrete. The addition of 2% to 5% steel fiber lead to improvements in compressive strength of 3.7 to 25% compared to mixtures without fibers, and it significantly increases in both tensile strength and shear strength [Sadegh Kazemi, 2012]. Compressive strength is also affected by increase in temperature. The compressive strength of Conventional High strength concrete, degrades with temperature. However, the addition of polypropylene fibers slows down this degradation [Wasim Khaliq, 2012].

Bilal S. Hamad (2011) showed that the addition of steel fibers in the concrete mix lead to increase in the ultimate load of the specimen, increase in the corresponding steel stress, and increase in the ductility of the load-deflection history. Niwa J (2012) experimented on eight one-sixth scaled specimens that include four T-joint and four kneejoint specimens to compare in terms of crack patterns, load-displacement relationships, ductility, energy dissipation capacity, and stiffness degradation and found that specimens with 1.5% of steel fibers reduced steel rebars as compared to control specimen.

Yoon- Keun Kwak (2002) had taken two types of concrete beams namely FNB (Fiber Reinforced Normal Strength Concrete Beam of Compressive Strength 31 MPa) and FHB (Fiber Reinforced High Strength Concrete Beam of compressive strength 65 MPa) with three steel fibervolume fractions (0, 0.5, and 0.75%) and suggested that, as the fiber content increased, the failure mode changed from shear to flexure. Bencardino F (2008) evaluated the reliability of the models available in literature, a critical comparative study was carried out between the experimental data and the various proposed theoretical stress-strain relationships to define the stress-strain behaviour in compression. Cunha V (2010) gives the experimental results of both straight and hooked-end steel fibers pullout tests on a self-compacting concrete medium.

Dinh (2010) showed that the use of hooked steel fibers in a volume fraction greater than or equal to 0.75% led to multiple diagonal cracking and a substantial increase in shear strength compared to reinforced concrete (RC) beams without stirrup reinforcement. As the addition of fibre reinforcement increases, the mode of failure changes from shear to flexure. Ou, Y (2012) performed a compression test on cylinders to characterize the compressive stress-strain behavior of steel fiberreinforced concrete (SFRC) with a high reinforcing index and suggested that adding steel fibers to SFRC increased both its toughness and its strain at the peak stress. However, this improvement reached a limit at a fiber volume fraction of approximately 2%. Calogero Cucchiara (2004) explained the inclusion of fibres which modifies the brittle shear mechanism into a ductile flexural mechanism, thus allowing a larger dissipation of energy, as can be seen by observing the crack pattern and the load–deflection curves.Chen H. (2011) prepared an analytical model of a freely supported reinforced concrete slab. It was first developed to simulate the shrinkage and thermal stress distributions in concrete owing to the restraint provided by GFRP rebars in comparison with the stresses induced by steel rebars.

Xiaobin Lu (2006)studied the behavior of high strength concrete and steel fiber reinforced high strength concrete under uniaxial and triaxial compression and had shown that, with an increase of the lateral confining pressure σ₃ , the axial strains at peak stress for both HSC and SFHSC increase remarkably. Allena, S (2012) had shown that the total shrinkage of fiber reinforced ultra high strength concrete specimens that occurred during 30 days was 3,006 μ, with early age shrinkage (first 24 hours) contributing 58.5% of the total shrinkage. Fatih Altun (2007) concluded that both the ultimate loads and the flexural toughnesses of reinforced-concrete beams produced with concrete classes of C20 and C30 with shear forces at a dosage of 30 kg/m₃ increase appreciably as compared to those RC beams without steel fibers. S. Goel (2012) investigated the flexural fatigue performance of Self Consolidating Concrete (SCC) and Self Consolidating Fiber Reinforced Concrete (SCFRC) containing round corrugated steel fibers, using two parameter Webull Distribution. Sujivorakul C. (2011) investigated on the glass fiber reinforced specimens for water absorption, bending strength, bending strain, and toughness at 7, 28, 56, and 180 days by replacing cement with fly ash, rice husk ash and palm oil fuel ash. The electrical resistivity and mechanical properties of carbon fiber-reinforced cement mortar (CFRCM) were investigated by Vipulanandan C (2008). Empirical relations were developed to relate the specific electrical resistivity to unit weight, Young's modulus, and pulse velocity.

The requirement of ductility is of great importance for earthquake design and for structures exposed to all forms of dynamic loading. In addition, an inherently ductile concrete is essential to produce slender members of reinforced and pre stressed concrete which utilise greater quantities of high strength steel reinforcement. The demand for such members for high rise building structures and bridges is constantly becoming greater. The practical ease with which steel fibres can be incorporated in concrete is likely to provide ductility more economically than very closely spaced stirrups which may cause additional problems due to congestion of reinforcement.

Moataz Badawi (2005) explained the main causes for corrosion in reinforced concrete structures as carbonation and chloride penetration. Reinforcing steel bars in concrete structures are de-passivated when the chloride concentration reaches threshold levels on the rebar surface or when the pH of the concrete cover drops below critical levels due to carbonation. Corrosion of iron is an oxidation process that produces hydrated ferric oxide (red rust) as one of the final products. Bonacci J. F. (2000) explained that this reaction reduces the effectiveness of reinforced concrete in two ways. One is loss of bar strength and other is loss of bond strength. Loss of bar strength results from the reduced cross sectional area of the bar caused by the transformation of iron to rust. Since the density of rust is less than iron, internal pressures build up, and radial cracks grow outward from the bar. In the absence of confinement, a layer of corrosion products forms at the bar surface and acts as a lubricant, which reduces the frictional force along the bond length. Secondly, corrosion reduces the size of the bar ribs. As a result, the mechanical interlock force that provides much of the bond strength for a deformed bar is reduced, and consequently leads to the premature failure of the concrete member.

The total life span of conventional reinforcement of a RCC structure can be divided into two periods of time. They are initiation and propagation. The time required by the external aggressive agents to penetrate into the concrete and cause the depassivation of the reinforcing steel in RCC is known as initiation period. The period for the steel reinforcement corrodes, and the safety of the structure reduces and is known as propagation period. A schematic representation of both initiation and propagation of service life of a structure, according to Tuutti´s model, is shown in Figure 1.

Figure 1. Tuutti's Service Life Model [K. Tuutti (1982)]

Bischoff (2003) concluded that, steel fibres transfer the tension force across the crack and led to reduce the crack spacing and increased tension stiffening. The effect of fibres on crack width and crack spacing are schematically depicted in Figure 2. Figure 2 (a) shows that when tensile force acts on the plane concretes, it fails after first cracking but in FRC the total tension transfers to fibres when first crack develops in concrete. The fibre bears the total load itself until it slips away from the concrete surface. Due to this property, multiple cracks will develop which is shown in Figure 2(b), which increases the ductility property of FRC. Figure 2 (c) and (d) show the internal cracks of tie elements of RC and FRC.

Figure 2. a) and b): Crack pattern in RC and SFRC elements subjected to tension. c) Sketch of main internal crack for tie elements for RC; d) Sketch of main internal crack for tie elements for SFRC [Bischoff (2003)]

Dimitri V. Val and Leonid Chernin (2009) described the influence of corrosion of reinforcing steel on deflections and cracking. The factors that control the serviceability of RC beams is examined by numerical simulations and the behaviour of corroding RC beams is predicted using a nonlinear finite element model. The model was verified against available experimental data and the result showed that, serviceability failure caused by corrosion would initially occur due to cracking and spalling of the concrete cover, while the probability of excessive deflection at that time would be negligible. Comparing with strength (flexural) failure, the probability of serviceability failure due to excessive deflection would reach its target value at approximately the same level of corrosion as the probability of strength failure. Abdullah (1996) explained that ultimate flexural strength of slabs decreased progressively with the degree of corrosion of the embedded steel. The reduction in the ultimate flexural strength of slabs with 5% reinforcement corrosion was 25%, while it was 60% in the slabs with 25% reinforcement corrosion. Xiao-Hui Wang and Xi-La Liu (2010) proposed compatibility conditions of the RC beams with partial length of complete loss of bond and partial length corrosion using the concept of the “equivalent plastic region length” of the unbonded pre stressed beam. Those compatibility conditions of deformations of the beam and the principles of equilibrium of forces were used to predict the flexural capacity of RC beams with partial length of complete loss of bond and partial length corrosion and good result was found in between the analytically predicted results and corresponding experimental results.

As the steel bar corrodes, the increased volume of the corrosion products results in a "bursting" pressure, which causes longitudinal cracks in the specimens. Dario Coronelli (2011) explained that with increase in corrosion level, crack width increases causing the breakdown of adhesion and friction at the steel-concrete interface. At low corrosion levels exclusive of longitudinal cracking, the corrosion products have a beneficial effect of improving the bond characteristics at the steel-concrete interface. At higher corrosion levels, the steel bars display localized pitting and loss of some of the ribs over the bar length, thereby weakening the rib-concrete mechanical interlocking force transfer mechanism and found loss of bond to be very critical. The experimental results indicated that after 2% of diameter loss, there was loss in bond strength and also a reduction in flexural capacity.

John Newman and Ban Seng Choo (2003) explained that Steel fibres provide virtually no increase in the compressive or uniaxial tensile strength of concrete. The main benefits in uniaxial tension result from the control of crack widths due to shrinkage or thermal effects in slabs and tunnel linings and this is not an easily quantifiable parameter, but relates to post-cracking fibre pull-out or fracture forces. Post cracking uniaxial tensile strengths of 0.5–1.5 MPa is possible at commonly used fibre volumes, at crack openings up to about 2 mm. For the lower fibre volumes, the post cracking flexural strength calculated from elastic theory is generally less than the matrix cracking strength but nevertheless exceeds the postcracking tensile strength as a result of the increased area of the tensile part of the stress block.

The main objectives of the modern engineer in attempting to modify the properties of concrete by the inclusion of fibres are as follows:

• To improve the plastic cracking characteristics of the material in the fresh state or up to about 6 hours after casting.
• To improve the tensile or flexural strength.
• To improve the impact strength and toughness.
• To control cracking and the mode of failure by means of post-cracking ductility.
• To improve durability.

One of the principal functions of the fibres in FRC is to control both the extent of cracking, and the crack widths. This may be accomplished with a high enough volume percentage of fibres in unreinforced FRC, or by the appropriate combination of fibres and conventional reinforcement.

Moataz Badawi and Khaled Soudki (2005) investigated the efficiency of using CFRP laminates to confine the corrosion-induced cracking and to reduce the rate of corrosion activity in reinforced concrete (RC) specimens. They found that CFRP U-wrap confinement reduced the corrosion mass loss in the post repaired beams by 35% and 33% for the shear-span and uniform corrosion beams, respectively, compared to the unwrapped corroded beams. The reason is due to penetration of the fibers that, build up around the reinforcing bar surface through the surrounding concrete pores. Thus it forms a physical barrier to the ingress of water and oxygen and slows their movement toward the bar. Also they explained that, the uniform corrosion beams exhibited high measured mass loss values than those of the shear-span corrosion beams which may be the lower resistance of salted concrete and longitudinal corrosion cracks forming along the whole length of the beam, which lead to more accessibility for water and oxygen.

A.P. Singh and Dhirendra Singhal (2011) investigated to study the effects of different fibre parameters on the permeability of steel fibre reinforced concrete. They concluded that, the permeability of concrete decreases significantly with the inclusion of steel fibres in concrete and continues to decrease with increasing weight fractions of fibres by comparing the values of coefficient of permeability of fiber reinforced concrete and plane cement concrete. The decrease in permeability with the addition of steel fibres is mainly attributed to the reduction in shrinkage cracks as well as corrosion.

Khaled Soudki etal (2007) explained how CFRP wrapping will reduce the rate of corrosion activity in reinforced concrete. In their study, eight beams were cracked and repaired with CFRP sheets, while the other three beams were kept uncracked as a control. In terms of environmental exposure, three beams were kept at room temperature and eight beams were subjected up to 300 wetting and drying cycles with deicing chemicals (3% NaCl). They performed non-destructive tests to determine the corrosion rate, as well as destructive tests to determine chloride diffusion and reinforcing bar mass loss. At last they concluded that, CFRP sheets and the resin system appeared to decrease chloride ionic diffusion and may reduce the corrosion rate of reinforcing steel in the beams.

In terms of material behaviour, it has been found that under pure shear, Fiber Reinforced Concrete (FRC) fails in a ductile manner, while plain concrete fails in a brittle manner. The shear strength of FRC containing 1% by volume of steel fibres was found to be about 20% higher than that of the plain concrete. These observations help to explain the enhancement of the shear behaviour of structural elements containing fibres.

It is essential that every concrete structure should continue to perform its intended functions, which is to maintain its required strength and serviceability, during the specified or traditionally expected service life. It follows that concrete must be able to withstand the process of deterioration to which it can be expected to be exposed. Durability does not mean an indefinite life, nor does it mean withstanding any action on concrete.

Harald Justin discussed that deterioration caused by reinforcement corrosion is normally divided into two main time periods i) initiation period and ii) propagation period.

The initiation period is defined as the time until the reinforcement becomes depassivated by the presence of chloride ions. During the propagation period, the reinforcement corroding, may lead to deterioration of concrete by expansive corrosion product creating cracks along the reinforcement and subsequently spalling of the concrete cover. Finally the loss of cross section of the reinforcement may lead to reduction of the load bearing capacity.

As per the Mejlbro-Poulsen model, time for initiation of corrosion in years is given by

[1]

where α_t = constant describing time dependence of diffusion coefficient.

Equation 1 predicts service life based on chloride initiated corrosion of rebars with α _t. α_t in the equation is calculated based on the water cement ratio. However fibres in concrete shall greatly reduce cracks in concrete. This in turn should increase the time for initiation of corrosion in years. Considering equation 1, if 5% increase in α_t

It could be seen from the literature that, the addition of fiber in reinforced cement concrete increases the flexural strength, splitting tensile strength as well as shear strength. The use of fibre reinforcement can reduce the amount of shear stirrups required and also the presence of fibres proves to be more effective in beams in which the failure in the absence of adequate shear reinforcement is governed by a beam effect. Adding steel fibers had little effect on the modulus of elasticity and compressive strength of SFRC. Long steel fibers and fibers with a lower aspect ratio resulted in a larger increase of the toughness of SFRC. Besides that, addition of fiber decreases the coefficient of permeability as well as arrest the cracks developed in beam which avoids the corrosion of reinforcement. Also for Self Compacting Fiber Reinforced Concrete (SCFRC), it gives better flexural fatigue performance as compared to Self Compacting Concrete (SCC). It increases the durability of structure and decreases the life cycle cost. So it is recommended that, the addition of fiber in RCC should be mandatory for all important structures for higher durability and lower service life cost. It is seen that the crack spacing and the crack width of the reinforced concrete pavements with the addition of glass fiber reinforced polymer are larger than those of the addition of steel fiber. Test results carried by Sujivorakul, C. showed that, for both glass fiber reinforced concrete panels with and without cement replacement by fly ash, rice husk ash and palm oil fuel ash, the limit of proportionality (LOP) increased with an increase in the age of curing, whereas the modulus of rupture (MOR) increased initially and then started to drop gradually.

It is clear from various literatures discussed in the paper that lot of studies are reported on steel fibres and its effect on the strength and durability of concrete. However use of steel fibres in concrete may be restricted due to reduction of workability and poor dispersion of steel fibres in concrete. These limitations of steel fibres may be overcome by the use of comparatively light glass fibres in concrete. However detailed study on the effect of glass fibre in concrete on workability, strength and more importantly durability, diffusion coefficient and life cycle cost of concrete is needed.