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As against short fibers of steel that are sometimes used to improve the tensile strength and postcracking behavior of the brittle concrete matrix, continuous long fibers can be used as a replacement of the the steel as a reinforcing material for reinforced and pre-stressed concrete construction. From the schematic representation of the cross section of the continuous FRP composite shown in Figure 19 it is clear that they can be seen as made up of extremely fine fibers embedded in a matrix. The volume fraction of the fibers in commonly available FRPs ranges from about 50 to 65 %. The properties of FRP composite products are related to 1) properties of the fiber used 2) the properties of the matrix used and 3) the volume fraction of the fibers (Jeffrey, 2003).
Steps of execution
Application of FRP strips is carried out in the following steps.
Mechanical cleaning of substance (e.g. by sand blasting) in order to reach slightly rough surface.
Removal of dust from the concrete surface.
Cutting of FRP strips to design size.
Cleaning of FRP strips from carbon dust.
Mixing of adhesive (A+ B components).
Application of adhesive both to the substance ad to the FRP (potlife of adhesive should be considered)
Rolling or pushing of FRP in to the adhesive (elimination of air bubbles). Final thickness of adhesive should be 1 to 1.5 mm.
Cover layer to the FRP by the adhesive (if required).
Fire protection (if required).
UV protection (if required).
Application of wraps is similar to that of strips, however above point 4 is irrelevant and in case of point 6 the adhesive to FRP is applied after placing it.
An important application rule is that FRP does not need supporting during hardening of adhesive owning to its low weight.
The behaviour of FRP composites under shear loading is dominated by the matrix properties and, local stress distributions. The specialized composite literature is particularly dedicated in-plan shear of lamina and laminated structures, but FRP reinforcing bars are mainly subjected to transverse shear. Therefore shear properties should be evaluated with respect to shear loading.
Fig 17. Circular FRP bar subjected to transverse shear
Fig 18. Rectangular FRP bar subjected to transverse shear
In case of FRP bars a significant increase in shear resistance can be achieved by winding or braiding fibers transverse to the main reinforcing fibers. Pultruded bars can be strengthened in shear by using continuous strand mat in addition to longitudinal fibers (ACI, 2006). Test methods for the characterization of the shear behaviour of FRP bars, in terms both dowel action and interlaminar shear, have been developed by various committees and are now available in the literature [JSCE-E 540 (1995), ACI (2004), ASTM (2002)]. The properties needed for a particular application should be obtained from the bar manufacture who should also provide information on the test method used to determine the reported shear values.
Effects of loading on direction on mechanical properties
FRP bars are orthotropic and their best properties are in the fiber direction. When FRP reinforcement is utilized in the stirrups the strength in an inclined direction x with an angle Î¸ to the fiber direction (so called of axis strength) is required. Formulas have been developed for both stiffness and strength in off-axis direction (Gay et al, 2003).
Adequate quality of the execution can be reach by considering the following requirements:
a) Certified material properties both for FRP and adhesive.
b) Qualified and trained workers for execution.
c) Appropriate cleaning of surface (dust free surface is needed).
d) Continuous bond should be provided (ckecks by destructive or non-destructive testing).
Existing codes and guidelines
Currently, design guides exist in Japan, Canada, the USA and the UK. In Norway, provisional design recommendations have been developed. Table 4 summarizes the durability-related strength reduction or stress limiting factors assumed for non pre-stressed FRP reinforcement in the various guidelines.
The main point to note here is that these guidelines have a single "environmental effect" factor for each FRP material depending in its fiber type, only. However the main environmental effects are moisture, alkali, temperature and time.
Table 4. Reduction factors used in existing guideline to take account of tensile strength reduction due to environmental actions and sustained stress (fib, 2007)
Design guidelines in Europe
The first research committee on continues fiber reinforcing material was established in 1989 by the Japan society of civil engineers (JSCE). A commission in Canada regarding for concrete also began in 1989.The Canadian society of civil engineers created a technical committee on the use of advanced composite materials in bridge and structures.In 1991 the American concrete institute (ACI) formed committee 440 on fibre reinforced polymer reinforcement, whose effort lead to the publication of a state-of-the-art report in 1996  followed by provisional design recommendations last drafted in January 2000 .
In Sweden a report has been established on 'strengthening of existing concrete structures with carbon fibre fabrics or laminates-Dimensioning, material and execution' which has been expected as a Swedish national rail road and road code in 1999 .
In Germany, different so called expert opinions, which are product related, have been issued. These documents are commission by the manufacturers and assess the suitability and design of the FRP product for use as adhesive-bonded reinforcement to strengthen concrete members.
In the UK a technical report called "Design guidance for strengthening concrete structures using fiber composite materials" has been published by the concrete society in end 2000 .
In the Netherlands, a CUR working commission has reached its final stage in preparing recommendation on the use and design of externally bonded CFRP reinforcement for concrete members, to be published by the end of 2001.
In Switzerland, a commission has been initiated which aims in the development of a SIA code of FRP EBR.
However, the interested in FRP reinforcement in the world is considerable and its use is getting more generally known, mainly with respect to externally bonded FRP reinforcement. IN different countries, design guideline are available or under development.
------------------------------------------------------------------------------------Applications of FRP-----------------------
Techniques for strengthening with composite
The basic composite material strengthening technique, which is most widely applied, involves the manual application of either wet lay-up (so-called hand lay-up) or prefabricated systems by means of cold cured adhesive bonding. This is the so-called classic FRP strengthening technique. Common in this technique is that the external reinforcement is bonded on to the concrete surface with the fibers as parallel as particularly possible to the direction of principle tensile stresses. Typical applications of hand lay-up and prefabricated systems are illustrated in Figure 4.
Fig 4. (a) Hand lay-up of CFRP sheets or fabrics, (b) application of prefabricated strips (fib Bulletin, 2001)
Beside the basic technique, several special techniques have been developed. Two of them are briefly explained in he following.
The strengthening technique through winding of tow or tape was first developed in Japan in the early 90s and a little later in the USA. The technique shown in the Figure 5, involves continues winding of wet fibers under a slight angle around columns or other structures (e.g. chimneys, as has been done in Japan) by means of robot. Key advantage of the technique, apart from good quality control, is the rapid installation.
Fig 5. Aotumated RC colomn wrapping. (a) Schematic. (b) photograph of robot-wrapper.
Near-surface mounted reinforcement
Near surface mounted reinforcement may be thought of as an especial method of supplementing reinforcement to concrete structures. According to this method, the concrete materials in the form of strips or rods are placed in to slits or grooves, respectively, which are cut in to the concrete structure with a depth smaller than the concrete cover. Typically CFRP strips e.g. with a thickness of 2 mm and a width of 20 mm are bonded in to these slits (Figure 7)
The tensile strength of the CFRP can be reached in beams with additional reinforcement consisting of strips in slits, if there is enough lad carrying capacity of the compression zone in the concrete and for shear. The bond behaviour with high strength and ductility allows bridge wide crack peeling-off. Hence, the FRP material can be used more efficiently if it is glued in to slits instead of on the surface.
Fig 7. CFRP strips glued in to slits
A strengthening method has been developed in the past few years (e.g. Lamanna et al.2001) where the strengthening strips are entirely mechanically attached to the concrete surface using multiple small, distributed powder actuated fasteners, sometimes in combination with anchor bolts at the strip ends, without any bonding. This system requires simple and hand tools, lightweight materials and minimally trained labour. Unlike the conventional method of adhesively bonding FRP strips to the concrete surface, this strengthening technique does not requires significant surface preparation and allows for the immediate use of the strengthened structure. RC elements strengthened with conventional method (of adhesively bonding FRP strips) exhibit a tendency to fail in a brittle fashion, with a sudden debonding of the strip. However, suitably design machinery fastener strips enable a more ductile failure, due to the partial shear connection at the strip concrete-interface as a result of strip compression failure at the point of contact with the fasteners, possibly combined with fastener pull-out and/or bending. One of the key requirements for this desirable failure mechanism to be activated is the proper design of strips with fibers in many directions, so that sudden shearing type of failures in the strips may be avoided.
Fig 8. (a) Mechanically fastened FRP. (b) Detaile of end anchorage with a combination of anchors and powder actuated nails.
Effect of strengthening method for opening on CFRP sheet
When strengthening reinforced concrete buildings with carbon fiber-reinforced plastic (CFRP) sheet, the portions to be strengthened by CFRP sheet normally have applied finishing materials. If a dry finishing process is adopted, then bolt holes should be drilled through the sheets. Receptacles and boxes embedded in the building body also require opening in the sheet. Since the strengthening effect of CFRP sheet is produced by its continuous adhesion on to the member, such opening which causes discontinuity of the sheet, may reduce the strengthening effect. Because of the directional properties of CFRP sheet, uncertainly remains about the method of strengthening around openings.
A limited value of the failure strain may also be considered as a simplified design alternative. In this case, the ULS verification restricts excessive deformation in the composite materials, rather than verifying the related failure mode itself. If the ULS verification involves bond failure, it is expected that this will develop through shearing in the concrete. In this case the material safety factor is taken as that for concrete failure. In the following table type A is relative to application of prefab systems under normal quality control conditions. Type B is relative to application of wet lay-up system under normal quality control conditions.
Application type A
Application type B
Table 2. Composite material safety factors Î³f (fib, 2001)
There are basically four types of failure in a RC beam strengthened with FRP:
Flexural failure generally occurs when there is no debonding at the ends of FRP sheets. In a flexural failure either the FRP sheet is rupture or the concrete is crushed in compression. Also this failure is very similar to the flexural failure of RC beams, it is a very brittle failure. Actually flexural strengthening of RC beams using FRP sheets leads to a strength gain (up to 76%) but causes a reduction in ductility.
In a RC beam strengthened in flexural using FRP sheets, the shear failure mode can be more critical. FRP sheets placed at the tension zone of RC beam have little contribution to shear resistance. Thus the shear capacity of an RC beam dictates the failure mode. In such cases the shear capacity of RC beam must be increased, so that flexural failure precedes shear failure. Although in a RC beam strengthened with FRP sheets flexural failure is brittle, it is still more ductile than shear failure.
Debonding of plate-end failure
Before ultimate capacity of the strengthened beam is reached, premature failure may occur due to end debonding (Figure 13). Separation of the concrete cover at one of the two ends is the most commonly seen failure mode. In this mode of failure, first a crack forms, it propagates up to the tension reinforcement and then progresses horizontally along the steel. This presses leads to the separation of the concrete cover.
Fig 14. Plate-end debonding failure
Intermediate crack-induced interfacial debonding failure
Debonding may occur at a flexural crack near mid-span that propagates towards one end. This is intermediate crack-induced interfacial debonding failure (Figure 15). It is also a very premature and brittle mode of failure.
Fig.15 Intermediate crack-induced interfacial debonding failure (Teng et al, 2002)
Design of concrete members strengthened with externally bonded FRP reinforcement
In contrast to ACI-440, the Eurocode 2 still does not address especially the use of FRP reinforcement and the equations derived for steel bars, which may will be inappropriate in FRP RC design, where used in the design example. It becomes apparent that the values adopted for the EC 2 coefficients taking in to account bond properties of the steel reinforcing bond are not appropriate for the FRP reinforcement. The ACI-440 equation for effective moment of inertia Icr,e includes modification parameter Î²d which takes in to account bond properties of the FRP bars. However the implied relation between the bond properties of the FRP reinforcement and their young's modulus of elasticity Ef is not so obvious. From viewpoint of Triantafillou (2001) the differences in results between the two codes of practice emerged mainly due to different considerations of bond properties of the FRP bars and any further code of practice should address in a more consistent way.
Guidelines for strengthening concrete structures with FRP laminates
Bonding FRP to the tension face increases the flexural strength of beams and slabs. Failure of the element may then occur as a result of either the concrete reaching its ultimate compressive strain or FRP reaching its ultimate tensile strain. Laboratory tests have shown that the latter rarely occurs in practice. The element generally fails prematurely as a result of plate separation. This is undesirable since the failure load is difficult to predict. In design, anchoring the RP and the design strain in the FRP below its ultimate value normally voids this condition. The design procedure is a function of failure mode. Calculating the design ultimate moment and comparing with the balanced moment of resistance of section can predict the failure mode. Balanced failure in a strengthened beam is said to occur when the concrete and the FRP reach their ultimate design strains simultaneously. When the design moment is less than the balance moment, the FRP will reach its design tensile strains before the concrete crushes whereas when the design moment exceed the balance moment, the concrete will crush before the FRP reaches its design tensile strain.
Reinforced concrete elements, such as beams and slabs may be strengthened in flexure through the application of composites to their tension zones with the direction of fiber parallel to that of high tensile stresses. The analysis for limit states for such elements may follow well-established procedures for reinforced concrete structures provided that (a) the contribution of external reinforcement is taken in to account properly and (b) special consideration is given to the issue of bond between the concrete and the external reinforcement, through the use of an appropriate bond model. Failure modes may be divided in to two types: (a) those where full composite action of concrete and external reinforcement is maintained until concrete reaches crushing in compression or the composite material fails in tension. Such failure modes may also be characterized as classical and (b) those where composite action is lost due to debonding of the composite material.
Typical load-deflection curves for RC elements strengthened in flexural are given in Figure 9.
Fig 9. Load-deflection curves for beams strengthened with FRP in flexure.
Methods of flexural strengthening
Flexural strengthening of RC beams is generally done by bonding an FRP sheet to the beam as shown in Figure 12. It is very important that the RC beam should be prepared prior to the application of FRP sheet. Unevenness of the beam surface must be correct. Bonding of FRP sheets to the bottom surface of the beam is the most common strengthening technique of RC beams for flexure. There are basically two schemes for the adhesive of FRP the sheets: a) wet lay-up b) adhesive bonding of prefabricated FRP plate. The former method is the most commonly used due to its greater flexibility for field application. Epoxy resin is applied to the concrete surface and FRP sheets are impregnated in place using rollers. In the later method prefabricated FRP plates are cut according to the application and bonded to the RC beam by using epoxy. The wet lay-up method is very sensitive to unevenness of the beam surface, which leads to debonding. On the other hand, the prefabricated FRP plate method, due to material uniformity and quality control is not sensitive to unevenness of beam surface. To prevent debonding FRP U-shaped strips can be bonded to the ends of the sheets (Figure 13). However, in the most of the beam cases wrapping is not possible. It is noted that wrapping can delay debonding only up to a certain limit (Smith and Teng 2001).
Fig 12. Flexural strengthening of RC beam by FRP sheet
Fig 13. Strengthened RC beam with FRP U-strip
Shear strength of columns can be easily improved by wrapping with a continuous sheet of FRP to form a complete ring around the member. Shear strengthening of beam is more problematic since they are normally cast monolithically with slabs. This increases the difficulty of anchoring the FRP at the beam/slab junction and exacerbates the risk of debonding failure. Nevertheless bonding FRP on either the side faces or the side faces and soffit, will provide some shear strengthening for such members. In both cases it is recommended that the FRP be paced such that the principal fiber orientation, Î², is either 45 ÌŠ or 90 ÌŠto the longitudinal axis of the members.
To calculate the shear resistance of the FRP, the design strain in the FRP must be evaluated. Its value depends on the failure mode of the strengthened member. Basically failure can arise due to one of the three possible mechanisms, namely:
a) loss of aggregate interlock
b) FRP rupture
c) Delamination of the FRP from concrete surface
Shear strengthening of RC members (e.g. column, beam, shear walls) using composites may be provided by bonding the external reinforcement with the principle fiber direction as parallel as practically possible to that of maximum principal tensile stresses, so that the effectiveness of the external reinforcement is maximized.
Fig 10. Dependence of the composite material elastic modulus, Ef, on the fiber orientation
For the most common case of structural members subjected to transverse loads, that is perpendicular to the member axis. The maximum principal stress trajectories in the shear-critical zones form an angle with the member axis which may be taken roughly equal to 45 ÌŠ. However it is normally more practical to attach the external reinforcement with the principal fiber direction perpendicular to the member axis.
Fig 11. Schematic illustration of RC element strengthened in shear with externally bonded composites: (a) sheets bonded to the web of beam (b) Wrapped strips applied to beam (c) Four sided wrapping of columns
Detailing with respect to strengthening lay-out
Flexural strengthening is provided by axially oriented fabrics of pultruded strips or cured in situ fabrics bonded to the top or bottom faces of the member or to the sizes. In the anchorage zones no additional transverse reinforcement is required if adequate anchorage is provided by bond stresses and debonding is resisted by concrete tensile stresses.
Some recommendations which should apply (Deutsches Institut fur Bautechnik, 1998):
Maximum spacing sf,max between strips should follow these limitations:
Sf,max â‰¤ 0.2 l (l = span length)
â‰¤ 5h (h = total depth)
â‰¤ 0.4 lc (lc = length of cantilever)
Minimum distance to the edge of the beam should equal to the concrete cover of internal reinforcement.
Shear strengthening can be provided by (a) factor made L-shaped CFRP strips (b) continues sheets. The externally bonded shear reinforcement generally covers four or three sides of the elements, but in some cases only two sides. Appropriate encourage is strongly recommended. It is important to note that in principle there are two different cases: (a) proper anchorage of the shear strengthening system. (b) Side or U-shaped shear strengthening system. Anchorage failure, debonding failure and FRP fracture are accounted for in design through the effective FRP strains.
Proper anchorage means a fully wrapped or a system that is properly anchored in the compression zone. Where partially possible, it is recommended to use for anchoring the whole height of compression zone to guarantee an anchoring as good as possible. FRP strips at the only sides of the beam are not recommended as in this case there is a lack of anchorage in both the compression and tension zone.
Shear strengthening of RC beam with FRPs
Methods of shear strengthening
Strengthening of RC beams in shear is done basically in three different schemes: a) by bonding of FRP sheets to the sides of the RC beam only, b) by bonding FRP U-strips to both the sides and the tension face of the RC beam (U jacketing), c) by wrapping FRP around the whole cross section of the RC beam. While strengthening RC beams in shear, fiber orientation must be carefully chosen to control the shear cracks. FRPs are strong only in the fiber direction. A suitable strengthening method must be selected according to the: 1) accessibility i.e. whether wrapping is possible or not 2) strengthening requirement (reversed cyclic loading or monotonic) 3) How much increase in shear capacity is needed.
Among three different schemes, side bonding only is easiest to apply and needs the least amount of FRP, but it is the most vulnerable to debonding and the least effective. In U jacketing beam ends must be rounded. This scheme is less vulnerable to debonding compared to side bonding. Although it is acting as mechanical anchor for flexural strengthening with FRP, it may needs mechanical anchors at the free ends of the U. Among the three different schemes wrapping is the most effective and less vulnerable to debonding. However, in most of the cases it is not feasible or very difficult due to the inaccessibility of at least one side of the beam.
Shear failure modes
There are basically three types of shear failure mode occur in a RC beam strengthened with FRP:
a) Shear failure with FRP rupture
This failure generally occurs with a diagonal shear crack. First a vertical flexural crack occurs and propagates diagonally towards the loading point. As the width of the crack increase, the strain in the FRP increase and the FRP ruptures when it reaches its ultimate strain. Rupture of the FRP leads to brittle failure of the RC beam.
b) Shear failure without FRP rupture
This is very similar to shear failure whit rupture, except that FRP does not rupture and can carry loads after he concrete fails (Chajes et al. 1995).
c) Shear failure due to FRP debonding
This is the most commonly seen failure mode for side bonding and U jacketing (Figure 16). On the side of the beam debonding of FRP occurs first, and then beam fail in a brittle manner.
Fig 16. Shear failure due to FRP debonding
In the FRP installation is to remains effective a survey of the structure focusing on the condition of the existing substrate and probable cause of deterioration is an essential requirement. Prior to installation, the surface of concrete must be cleaned so that it is free of laitance and other material that are likely to affect the bond strength between the FRP and the concrete. Also any sharp edges that are likely to damage the FRP should be removed. The surface of fiber composite material may also requires light abrasion prior to the application of the adhesive. It is generally necessary to control the environment in work area during the preparation of the surface, the application of the adhesive and the subsequent curing period. Also the work site should be protected from moisture and humidity. The bonding adhesive should be applied evenly and the FRP material wrapped tightly over, avoiding any wrinkles and rolled to expel and air.
Effect of corner radius on the performance of externally bonded FRP reinforcement
Externally bonded FRP reinforcement is wrapped around concrete members in order to provide confinement and/or shear strengthening. The need for bending the fiber over the member corners affects the performance of the FRP laminate and the efficiency of its confining/strengthening action.
Based on literature review (Xinbo Yang, Antonnio Nanni, Genda Chen, 2000) , it was found that only a portion of the CFRP laminate capacity was developed when failure occurred at the corner. Increasing the number of plies from one to two slightly improved the efficiency of the laminate.
Bond between concrete and FRP sheet
Strengthening of concrete structures with FRP has emerged as one of the effective techniques. However, the effectiveness of such strengthening system depends largely on the performance of the interface between concrete and FRP in epoxy joint. For strengthening design, the bond between concrete and FRP is the major factor. The bond strength of such joins depends on concrete strength, type and thickness of FRP and bonded length. FRP sheets can be bonded to external surface of structures in different configuration, since they can follow contour of the structure easily. Further the application of FRP sheet is quietly simple and speedy that does not required heavy machinery for lifting and placement. Due to these advantages FRP sheets has emerged as one of the best suited materials for strengthening concrete structure. The performance of the strengthened system largely depends on the behavior of bond between concrete and FRP sheet in epoxy joint. The failure of the system sometimes occurs from the bond line failure. Since FRP is a brittle material once it hardens; the failure due to debonding of sheet from concrete is very sudden, which occurs without any warning. There have been a number of studies about boding stress between concrete and FRP sheet around the world such as Bronsens & Van Germert (1997), Hariguchi & Saeki (1997) and etc.
According to experiments which have done by Bimal Babu Adhikary the debonded CFRP retained some chunks of concrete on their bonded surface. This suggests that the actual failure is somewhat shearing-off of concrete just below the CFRP sheets. Thus the concrete surface tensile strength is one of the most important factors in CFRP strengthened structures. The surface tensile strength of concrete can be related to compressive strength of concrete. Anyway, the bonding stress between concrete and FRP sheets is governed by some parameters such as Modulus of elasticity of FRP, thickness of FRP, number of layers of FRP, concrete compressive strength, etc. There are a few formulas to calculate bonding stress between concrete and FRP and also effective strain of FRP which have suggested by researchers. It needs separate review paper.
Alkali resistance of fibers, FRP rods and epoxy resins
Deterioration of concrete structures is a worldwide problem nowadays. Corrosion of reinforcing bars is a one of the major factors for concrete deterioration. Recently there have been a lot of researches in the use of rods made of fiber reinforced polymers (FRP) as an alternative reinforcement in concrete structure. Much kind of FRP rods have been recently developed to use them as reinforcements in concrete. These rods are generally made of carbon, aramid, glass and vinylon fibers. Since theses rods are corrosion proof and lightweight they are increasingly being used for new concrete structures utilizing their advantages.
There are few experimental results about durability of FRP rods in regard to their alkali resistance. Based on experimental results which have done by Atsushi Sumida in 2000, the relationship between tensile strength and immersion period has different changes for different types of FRP rods. The tensile strength remains somewhat constant in the case of CFRP, AFRP rods, but there is a remarkable decrease in the strength for GFRP rods. VFRP rod shows a little change in its tensile strength. The Young's modulus remains almost the same even up to 2 years of immersion for all kinds of FRP rods. It was found that for CFRP, AFRP and GFRP the ultimate strains after 2 years of immersion were almost the same as the original ones. However GFRP rods, shows decrease in ultimate strain and at the end of 2 years the drop was approximately 75%. These results show that the durability of GFRP rod is very poor, whereas other FRP rods are satisfactory in their durability performance.
The epoxy resin has almost constant strength whereas vinylester resin shows a decreasing trend in strength with time.
Stress-rupture of FRP
Stress-rupture is the process which leads to failure of the material if subjected to a sustained high load. The stress-rupture behaviour is strongly influenced by the sustained stress level, the type of FRP and the environmental conditions.
For a specific failure FRP, the time to failure depends primarily on two factors. The first factor is the ratio of the applied load to the short term tensile strength. If this ratio increases the time to failure will decrease. The environment is the second factor. For example permanent contact of a GFRP element with an alkaline solution-chemically similar to the concrete's pore water, will definitely damaged the fibers resulting in a decrease of the failure time.
Stress-rupture is usually preceded by creep and can be thought of as a creep-to-failure which leads to the alternative name of creep-rupture. The behaviour of most fibrous is characterized by an initial elastic response, followed by a fairly rapid primary creep phase, a long relatively slow secondary creep phase and a rapid tertiary creep towards failure as shown in the fig.2.
Fig.2. Creep behaviour of FRP (Thomas Telford, 2001)