Since the presentation of fibre reinforced polymer reinforcement in the concrete structures, the utilizing of non-ferrous reinforcement has been increased rapidly. Although many researchers have been done in the world, the development of codes of practice containing advanced materials is still seen as a restriction for its more large use. Nowadays, the market for externally bonded FRP has a dynamic situation. Several types of manufactures are working in this market part, while researcher and institutes are trying to obtain design guidelines for FRP. The ranges of FRP applications in civil structures are very wide. In this regard different difficult projects have been done, such as tripling the bearing capacity of a floor slab in Belgium, strengthening of silos in Sweden, seismic strengthening in Greece and Italy, etc. In this review paper, FRP advantages and disadvantages, its manufacture, different form of FRP, causes of deterioration of FRP, FRP durability and also prestressed FRP will be probed.
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Keywords: Polymers Fibres Composites, All-composite structures, New structural forms, Sustainable structures, FRP durability, FRP manufacture
The history of composites is relative to few thousand years ago. For example natural fibrous composites have used by ancient Egyptians for building small houses (straw-reinforced-clay bricks), in fabricating natural compression designed cross-ply papyrus papers as well as in improving the linen wrapping method for their mummies to increase the flexibility of the dried brittle dead bodies .
Figure 1. Ancient Egyptians utilization of natural composites: a) Fibrous clay blocks, b) Compression moulded cross-ply papyrus sheets, c) Mummies linen wrapping systems (Leonard Hollaway et al, 2004).
Recent developments have concentrated on the mixing of continued fibre-base textiles with mortars instead of resins as in the case of the FRP, leading to the extending of so-called textile-reinforced mortars (TRM). Both FRP and TRM material may be called, continuous fibre composites or advanced composites or simply composites. In compare to steel, typical strain-stress diagrams for unidirectional composites with short term monotonic loading are showed in Figure 2. The applications of FRP as structural reinforcement have been become more in construction all over the world. A lot of researches on the use of FRP began in Europe around 25 years ago. The most part of the European activities concentrated on externally bonded FRP reinforcement (FRP EBR). In Switzerland alone, the amount of FRP material used for strengthening in structures reached 30 to 50 km a year up to 2000 (Thomas Telford, 2001).
Figure 2. Uniaxial tension stress-strain diagram for steel and different unidirectional FRPs (FIB Bulletin 35, 2006).
In Switzerland alone, the amount of FRP material used for strengthening in structures reached 30 to 50 km a year up to 2000 (Thomas Telford, 2001).
Table 1. To comparison typical properties of the prefabricated FRP strips with steel (FIB Bulletin 35, 2006)
Ultimate tensile strain É›fu(%)
with low modulus CFRP strips
with high modulus CFRP strips
Moreover, new types of products and methods have come in the market to develop the feasibility of FRP external bonded reinforce strengthening method such as L shaped carbon FRP strips for shear strengthening, methods for improving anchorage capacity by mechanical devices, methods to use prestressed FRP external bonded reinforce, modular systems including prefab wood/carbon FRP beams to improve the stiffness and efficiency of FRP external bonded reinforce, cut-in FRP strips which are positioned inside (near the surface) the concrete or wood and have good anchorage capacity, etc (Thomas Telford, 2001).
FRPs have lots of advantages in outdoor, indoor for structure applications. First they are very rigid and strong, offering and outstanding strength in compare to weight. Secondly they present a high creep resistance in the long term, high resistance to temperature changes (no softening or brittleness), humidity and atmospheric pollution and high impact strength and also a good resistance to UV radiation. Moreover these materials present a low flammability (depending on choice of resin), a good thermal resistance and a good dimensional stability. Users use a wide range of possibilities in terms of colour and from good design changeable. FRPs can be used in construction as lightweight materials. They are easy to carry and also can be as a prefabricated elements, which are easily and rapidly join with no need for special handling equipment.
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FRPs suffer from disadvantages which can not to be ignored by engineers. Contrary to steel which behaves in an elastoplastic manner, FRPs are generally linear elastic to failure without any yielding or plastic deformations, reaching to decrease deformability at ultimate. In addition, the cost of FRP materials on a weight basis is several times more than steel. Furthermore some fibre materials such as aramid and carbon have inconsistent thermal expansion coefficients with concrete. Finally the reaction of FRPs to high temperatures (e.g. in case of fire) may lead to premature reduction and peeling. (Some epoxy resin start softening at about 45-70 ÌŠ C). This is not the case with textile-reinforced mortars which contain inorganic cement-based binders instead of resins. So FRPs should not be mistaken of as a blind replacement of steel in construction (FIB Bulletin 35, 2006).
MANUFACTURE OF FIBRE REINFORCED POLYMERS
The applied tension on the fibres within the manufacture removes twists and straightens them. When these products are really stressed during testing or in real application, a more uniform distribution of stresses among all the fifers will get, which prevents static fatigue at low load levels. It should be noted that the tension used at time of manufacture is small, so as not to damage the performance of the composite as a structural martial.
Since the manufacturing of 2D and 3D FRP products is probed, the grids are produced by impregnating bundles of fibres with resins and then laying them in X and Y directions under pressure and tension before shaping them in to grids. The three-dimensional textiles are produced either by alternately laying bundles of fibres in X, Y, and Z directions in several layers with impregnating them with resins. FRP sheets are another form of FRP materials suitable for use usually in works for reinforcement and strengthening of structures. They can be categorized as one way textile, two way textile and one way pre- impregnated textiles.
DIFFERENT FORMS OF FIBRE MATERIALS
The lowest practical form of fibre materials is a bundle of strands, which includes hundreds of monofilament thread assembled to form a thicker strand called a "roving". These are wound on to spools as continuous strands and may be directly applied as constructional reinforcement in processes such as filament winding pultrusion. Composites having only roving aligned in one direction will have highly unidirectional mechanical properties. Care must be taken to stop splitting when drilling the parallel to the continuous roving direction (Raymond W. Meyer, 1987).
Discontinuous roving chopped strand glass maybe cut down in to very small lengths (1/2 in. to 2 in.) and used to make parts using hand-spray method (Figure 4). Spray-up is one of the cheapest and quickest methods for producing a part, but it also causes the lowest stiffness and strength. This form of reinforcement is frequently used where fibre volume is low and low mechanical properties are allowable.
Woven roving is made by intertwining fibreglass in to a fabric. This yields a coarse reinforcement product used in hand lay-up and panel moulding step. A lot of weave patterns are available, such as the plain weave pattern shown in Figure 5 for both fibreglass and carbon fabric. The weave can be made with more strands in more than one direction in order to create highly orthotropic properties.
Figure 3. Spools of continuous fiberglass rovin (Niket M. Telang, 2006)
Figure 4. Chopped strand glass (Niket M. Telang, 2006)
Mats may be produced in form of chopped-strand or continuous mats. A chopped strand mat is made orbitrary depositing chopped strands on to a plate, then assembling them to each other using a small amount of binder. A continuous strand mat is made similarly, but without chopping. Figure 6 shows typical fabric rolls of mat.
By knitting or sewing the reinforcement strands together and using lightweight threads, sheets or fabric can be used without weaving to produce straight, non-crimped layers of fibers (Fig. 7). This form of sheet reinforcement has become common for making deck, because it lets large quantities of fiber reinforcement on single spool. Furthermore, unlike woven fabric, the non-crimped fiber strands keep their straightness. So they have higher stiffness and strength retention. Non-crimped fabrics are produced in multiple layers, so basically, they themselves are sub-laminates. Nevertheless, non-crimp fabric costs more to manufacture than other forms.
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Figure 5. Woven roving fabric (Niket M. Telang, 2006)
Figure 6. Chopped strand mat fabric (Niket M. Telang, 2006)
Figure 7. Non-crimp fabric construction (Niket M. Telang, 2006)
DURABILITY OF FRP REINFORCEMENT
FRP does not corrode in a chloride conditions in compare to steel. It must be noted, how it behaves in other conditions. If FRP properties decline in other conditions, care must be taken when it is used as reinforcement for concrete construction. Because majority of concrete structures with FRP as reinforcement were constructed within 20 years, it cannot be realized whether this material has enough durability. So one of the most important subjects that remains to be noted in using FRP, as concrete reinforcement is durability. Without the knowledge about durability of FRP in different types of situations engineers may easily make mistakes in using them.
IMPORTANT CAUSES OF DETERIORATION OF FRP
FRP is a composite material, composed of millions of resins and fibres. The diameter of fibres is approximately in the range of 6 to 15 microns (Aramid fibres and glass fibres). When tensile load is used for FRP, fibres transfer load and resin carries stress to neighbouring fibres. The resin can also protect fibres from entry of hurtful ions from their environment.
Properties of both fibres and resin and also the transition zone between fibres and resin is controlled by durability of FRP. This makes the deterioration mechanism of FRP more complex in compared to steel. As majority of the mechanical properties are controlled by fibres, if the fibres do not degenerate, FRP can make a stand against load in most cases. But when resin attacked and degenerated the fibres fall off from the surface and the FRP strength decrease. Considering the properties and application of FRP, important items on degeneration to be noted are listed below. Items first to third are for reinforcements inserted in concrete and items fourth to sixth are for surface reinforcements (mainly sheets).
Static Fatigue Fracture
One of the remarkable problems for the application of FRP as reinforcement for concrete is static fatigue fracture of FRP. When sustained tensile load is used for FRP, because of rough stress distribution to individual fibre some of the fibres may fracture. Redistribution of load, transferred by the remaining fibres, increase the stress of some of the fibres and this makes individual fracture of fibres one at a time.
Fatigue behaviour of FRP under cyclic is another noticeable problem. When fatigue load is used, FRP may fracture the same as steel.
When FRP is inserted in concrete and use as reinforcement, FRP must be durable enough against alkali, which caused by hydrated cement, also the solution in the pores of concrete is not the same as in pure NaOH solution. Alkali resistance of FRP can be measured in accelerated test in NaOH solution. Based on literature review (Katsuki, 1996) in the case of CFRP and AFRP rods there is no special change even when they are engrossed in NaOH solution for 1000 hours. But in the case of GFRP rods, the strength decrease suddenly when temperature increases and also concentration of NaOH increases.
When FRP is inserted in concrete, no notification is needed on acidic resistance. But when FRP is used as external reinforcement, such as sheet reinforcement and external cables, acids may affect FRP as in the case of acidic river, acidic rain, etc. A lot of tests (Nishimural et al, 1999) have done on the tensile strength of fibers engrossed in HCl solution. The results present a decrease in strength of all fibers at 80 ÌŠ C. In the case of carbon fiber and glass fiber they do not degenerate much up to 40 ÌŠ C. But in the case of Aramid fibers, strength reduction can be seen also in the case of 40 ÌŠ C.
Ultra-Violet Ray Resistance
When FRP is used as external reinforcement, the influence of ultra-violet ray must not be ignored. FRP is composed of fiber and resin, but resin can easily degenerate by ultra-violet ray when they are not covered in sunlight. In regard to AFRP, the Aramid fiber may be also degenerate in the same manner.To check the influence of ultra-violet rays speeded up testes using ultra-violet ray chamber, have done for both FRP rods and fibers by researchers (Uomoto et al, 1998). In the case of fibers no especial strength decrease was detected for carbon fibers and glass fibers. In the case of Aramid fibers strength reduction was detected. For FRP rods to realize the degeneration of resin under ultra-violet rays, it is assumed that strength reduction of CFRP rods was made by the loss of degenerated resin from the surface.
To test the durability of FRP in cold climates, freeze-thaw experiments are done. The decrease of strength is seen only in the case of GFRP rods, but the decrease was only 8% after 300 cycles (Uomoto, 1998). This research presented that the influence of freeze-thaw is limited only on the surface of FRP.
High Temperature Resistance and Fire Resistance
High temperature may influences on properties of both fibers and FRP. Kobayashi et al (1988) did tensile strength tests for FRP rods at different temperatures in range of -10 ÌŠC to 60 ÌŠC. According to these tests both strength and elastic modulus decreases 20% to 30% as temperature increases from -10 ÌŠ C to 60 ÌŠC. These results show that mechanical properties of both FRP and fiber itself are easily influenced by high temperature, and care must to be taken during using FRP as reinforcement for concrete structures in high temperature environment.FRP is able to burn easy in compared to steel. Both strength and elasticity of FRP decline more after the fire. For GFRP and CFRP in the range of up to 350 ÌŠC, the declination is around 25%, but in the case of AFRP there is large reduction up to 35% in the range above 250 ÌŠC. The results indicate care has to be taken during applying FRP reinforcements to structures that require fire resistance.
HUMIDITY AND MOISTURE ISSUES
When using an FRP system, especially in regard to fabrics that can wrap the total surface of the element, the water can gather at the bond line. So in regard to flexural strengthening of beams or of slabs it is advised to leave a gap to provide vapour transfer space. For shear strengthening, a gap each 300 mm should be left exposed.
FRP prestressing reinforcement is produced in several countries by suppliers or local manufacturer.
Figure 8. CFRP prestresse pole (left) and versus conventional pole (right) (Thomas Telford, 2001)
In some cases it is useful to bond the external reinforcement FRP on to the concrete surface in a prestressed state.
Figure 9. Strengthening with prestressed FRP strips: (a) prestressing; (b) bonding; (c) end anchorage and FRP release upon hardening of the adhesive (FIB Bulletin 35, 2006).
Both analytical and laboratory research (e.g. Triatafillou et al. 1992, Diuring, 1993) have indicated that prestressing results an important contribution to the advancement of the FRP strengthening method. The concept for applying a prestressed FRP strip can be seen schematically in Figure 9.