FIBRE REINFORCED POLYMER COMPOSITE

1. INTRODUCTION

A structure is designed for a specific period and its design life varies depending on the nature of that structure. Now a day, deterioration in concrete structures is a major challenge faced worldwide by the infrastructure and bridge industries. The deterioration of structures is mainly due to environmental effects , which includes corrosion of steel, gradual loss of strength with aging, repeated high intensity loading, variation in temperature, freeze-thaw cycles, contact with chemicals or saline water, and exposure to ultra violet radiation. Since the complete replacement or reconstruction of such structures will be cost effective, strengthening or retrofitting is an effective way to strengthen the same. The most popular techniques for strengthening of RC beams is the use of external epoxy-bonded steel plates which increases the flexural strength of the beam. Even though steel bonding technique is simple, cost-effective and efficient, it has the drawback that steel will corroide and hence deterioration of bond occurs at the steel and concrete interphase.

Other common strengthening technique involves construction of steel jackets is quite effective but it increases the overall cross-sectional dimensions, leading to increase in self weight of the structure and is labour intensive. To eliminate these problems, steel plates were replaced by corrosive resistant and light weight FRP composite plates. Also, such material could be designed to meet specific requirements by adjusting placement of fibers. By wrapping FRP sheets, retrofitting of concrete structure provide a more economical and technically superior alternative to the traditional techniques in many structures because it offers high strength, low weight, corrosion resistance, high fatigue resistance, easy and rapid installation and minimal change in structural geometry. FRP systems can also be used in areas with limited access where traditional techniques would be impractical. Successful retrofitting of concrete structures with FRP needs a thorough knowledge on the subject and available user friendly technologies and unique guidelines.

Beams are the critical member which is subjected to bending, torsion and shear in all type of structures. Similarly, columns are also important elements subjected to axial load combined with or without bending. Therefore, extensive research works are being carried out all over the world on retrofitting of these concrete members with externally bonded FRP composites. And several studies were conducted on retrofitting beams with carbon fibre reinforced polymer (CFRP) or glass fibre reinforced polymer (GFRP) composites in order to study the enhancement of strength and ductility, durability, effect of confinement, preparation of design guidelines and experimental investigation of these members.

2. FIBRE REINFORCED POLYMER COMPOSITE

Polymer composites are multi-phase materials and are produced by combining polymer matrix with fillers and reinforcing fibers to produce a bulk material with properties better than those of individual base materials. The matrix can be thermoplastics like polypropylene, polyethylene, polystyrene, PVC etc. or thermosetting like polyester, vinyl ester, and epoxy resins etc. Fillers are often used in these composite in order to bulk the material, to reduce cost, to lower bulk density or to produce aesthetic features. Fibers are used in order to reinforce the polymer and improve its mechanical properties such as stiffness and strength. High strength fibers of glass, aramid and carbon are used as primary means of carrying load, while the polymer matrix protects the fibers and bind them into a cohesive structural unit. These are commonly called fiber-reinforced polymer composite materials (FRPs). Onwards 1960s, advanced composite material is found to have expanded uses in aerospace, marine and automobile engineering due to their engineering properties including specific strength and stiffness, lower density, high fatigue endurance, high damping and low thermal coefficient in fiber direction. Recently, civil engineers and construction engineer begun to realize the potential of these composite material as strengthening material for many problems associated with the deterioration of infrastructures. And also its use in construction field is increased over the last decade. Since there is a continuous drop in the cost of these materials, these being considered as a replacement to the conventional steel in reinforced concrete structures. First application of these materials composite is in the form of rebars and structural shapes. Later, FRP laminates were used for strengthening of concrete bridge girders by bonding them to tension face of girder as well as retrofitting of concrete columns. Now they are available in the form of rods, grids, sheets and winding strands.

2.1 HISTORY OF FRP COMPOSITE

Bakelite was the first fibre-reinforced plastic. Dr. Baekeland first produced a soluble phenol-formaldehyde shellac called “Novolak”, then turned to developing a binder for asbestos which, at that time was moulded with rubber. By controlling the pressure and temperature applied to phenol and formaldehyde, in 1905, he produce the first synthetic plastic Bakelite.

The development of fibre-reinforced plastic for commercial use was being extensively researched in the 1930s.a suitable resin for combining the “fiberglass” with a plastic to produce a composite material was developed in 1936 by du Pont. With the combination of fiberglass and resin, the air content of the material was replaced by plastic. This reduced the insulation properties to values typical of the plastic, but now for the first time the composite showed great strength and behaves as structural and building materials. Carbon fibre production began in the late 1950s and was used in British industry begins in the early 1960s. Aramid fibers were being produced around this time also, and are appearing first under the trade name Nomex by du Pont. Today, each of these fibers is used widely in industry for any applications that require plastics with specific strength or elastic qualities. Now, these three fibers continue to be the important categories of fibre used in FRPs.

3. COMPONENTS OF FRP COMPOSITES

An FRP is a specific type of two- component composite material consists of high strength fibers embedded in a polymer matrix. The physical properties and mechanical properties of this composite are mainly controlled by the properties of its materials and the micro-structural configuration. The fibers of the composite are mainly responsible for strength and the stiffness properties, but the polymeric matrix contributes to the load transfer and provides environmental protection. In addition to these fillers are used in the FRP composite to reduce cost and sometimes to improve performance, imparting benefits as shrinkage control, surface smoothness and crack resistance. Additives and modifiers ingredients can expand the usefulness of polymeric matrix, enhance their process ability or extend composite durability.

The reinforcing of low modulus polymeric matrix with high strength and modulus fibers utilizes the viscoelastic displacement of the matrix under stress to transfer the loads to the fiber, which results in a high strength and high modulus composite material. The aim of this combination is to produce a two phase material in which the primary phase, that determines stiffness, is in the form of fibers and is well disperse and bonded and protected by a weak secondary phase, the polymeric matrix. FRP composites are anisotropic. Therefore they are directional, means that the best mechanical properties are in the direction of the fibre placement.

3.1 REINFORCING FIBERS

The properties of FRP composite such as strength and stiffness depends on the fibers provided. Because the fibers used in most structural FRP composite are continuous and are oriented in specific directions. According to Halliwell, the functional requirements of fibers in a composite are:

a) High modulus of elasticity to give stiffness.
b) High ultimate strength
c) Low variation of strength between individual fibers
d) Stability during handling
e) Uniform diameter

The fibers can be used in different ways according to which the performance of the composite changes.

· The highest performance in terms of strength and stiffness in one direction comes from unidirectional composites, when the fibers are parallel and gives their maximum possible performance in this single direction.
· By arranging this fiber in a weave or mat form, the strength can be gained in more directions, although the limit strength is reduced.
· By chopping the fibers into short lengths and arranging them randomly, equal strength is achieved in all directions. This is generally the cheapest technique, used for the least structurally demanding cases.

In civil engineering mainly three types of fibers dominate; these are carbon, glass, and aramid fibers and the composite is often named by reinforcing fiber. They have different properties. All the fibers have generally higher stress capacity than ordinary steel and have linear elastic property until failure. The most important properties that differ between the fiber types are stiffness and tensile strain. The mechanical properties of different types of reinforcing fibers are shown in the table 3.1 below.

Table 3.1. Properties of different reinforcing fibers of FRP composites

 The stress strain graph of all fibers is shown in the figure 3.1 in comparison with the ordinary steel bars and steel tendons.

Figure 3.1. Properties of different fibers and typical reinforcing steel.

3.1.1.GLASS FIBER

"E" glass produced fibers are considered the predominant reinforcement for polymer matrix composites due to their high electrical insulating properties, low susceptibility to moisture and high mechanical properties. Glass fibers used for
reinforcing composites generally range in diameter from 9 to 23 microns. Fibers are drawn at high speeds, approaching 200 miles per hour, through small holes in electrically heated bushings. These bushings form the individual filaments. The filaments are gathered into groups or bundles called "strands." The filaments are attenuated from the bushing, water and air cooled, and then coated with a proprietary chemical binder or sizing to protect the filaments and enhance the composite laminate properties. The sizing also determines the processing characteristics of the glass fiber and the conditions at the fiber-matrix interface in the composite.

Glass is generally a good impact resistant fiber but weighs more than carbon or aramid. Glass fibers have excellent characteristics, equal to or better than steel in certain forms. The lower modulus requires special design treatment where stiffness is critical. Composites made from this material exhibit very good electrical and thermal insulation properties. Glass fibers are also transparent to radio frequency radiation and are used in radar antenna applications.

3.1.2. CARBON FIBER

Carbon fiber is created using poly acryl nitrile (PAN), pitch or rayon fiber . PAN based fibers have good strength and modulus value and also excellent compression strength. Carbon fibers are more expansive than glass fibers. However, carbon fiber offers excellent combination of strength, low weight and high modulus. The tensile strength of carbon fiber is equal to glass while its modulus is about three to four times higher than glass. Carbon fibers are supplied in a number of different forms, from continuous filament to chopped fibers and mats. The highest strength and modulus are obtained by using unidirectional continuous reinforcement. Twist-free tows of continuous filament carbon contain 1,000 to 75,000 individual filaments, which can be woven or knitted into woven roving and hybrid fabrics with glass fibers and aramid fibers.

Carbon fiber composite are more brittle than glass or aramid. They can cause galvanic corrosion when used next to metals. Barrier material like glass and resin is used in order to prevent this corrosion.

3.1.3. ARAMID FIBER

Aramid fiber is an aromatic polyamid that is a man-made organic fiber for composite reinforcement. Aramid fibers offer good mechanical properties at a low density with the added advantage of toughness or damage/impact resistance. They are characterized as having reasonably high tensile strength, a medium modulus, and a very low density as compared to glass and carbon. The tensile strength of aramid fibers are higher than glass fibers and the modulus is about fifty percent higher than glass. These fibers increase the impact resistance of composites and provide products with higher tensile strengths. Aramid fibers are insulators of both electricity and heat. They are resistant to organic solvents, fuels and lubricants. Aramid composites are not as good in compressive strength as glass or carbon composites. Dry aramid fibers are tough and have been used as cables or ropes, and frequently used in ballistic applications.

The table 3.2 below shows the comparison of mechanical properties between common strengthening materials and carbon fiber.

Table 3.2. Mechanical properties of different strengthening material and carbon fiber.

3.2. MATRIX

The matrix is the binder of the FRP and place many important functions as follows:

· To bind the fibers together
· To protect the fiber from abrasion and environmental degradation
· To separate and disperse fibers within the composite
· To transfer force between the individual fibers
· To be chemically and thermally compactable with fiber

According to Hollaway and Head (Elesvier 2001), the requirements for a good FRP matrix are the following:

i. Wet out the fiber and cure satisfactory in the required conditions

ii. Bind together the fiber and protect their surface from abrasion and environmental ageing

iii. Disperse the fiber and separate them in order to avoid any catastrophic propagation of cracks

iv. Transfer stresses to the fibers efficiently v. Be chemically and thermally compatible with fibers

vi. Have appropriate fire resistance and limit smoke propagation

vii. Provide good aesthetic finish

There are mainly two types of polymer which determine the method of manufacturing and the properties of the composite: (a) thermoplastic and (b) thermosetting.

3.2.1. THERMOPLASTIC MATRIX

Thermoplastics are polymers composed of long-chain molecules that are held together by relatively weak Van der Waals forces. But that have extremely strong bonds within individual molecules. These polymers can be amorphous, which implies a random structure with a high concentration of entanglement, or crystalline, with a high degree of molecular order. The molecules in this material can be slide over one another at high temperature, so these materials can be repeatedly softened and hardened by heating and cooling without significant change in their molecular structure. The semi-crystalline polypropylene and nylon are especially popular as matrices.

3.2.2. THERMOSETTING MATRIX

Thermosetting polymers are also long-chain molecules built from monomers, but for these materials the molecular chains are cross-linked through primary chemical bonds. Thus, thermosets cannot be reversibly softened and will deteriorate irreversibly at elevated temperature. These are usually made from liquid or semi-solid precursors. These polymers generally have good thermal stability at service temperatures, good chemical resistance, and display low creep and relaxation properties in comparison with most thermoplastics. Since it is difficult to reversibly soften thermosets, FRP components made from thermosets matrices must be bent or formed during the manufacturing process. This may become a problem in some specific application.

Three specific types of thermosetting resins are commonly used in the manufacture of infrastructure composites: polyester resin, epoxy resin and vinyl esters resin.

i. Polyester resin: They are the mostly used polymers in the manufacture of FRP components for infrastructure applications due to their relatively low cost and ease of processing. Numerous specific types of polyesters are available for use, with varying degrees of thermal and chemical stability, moisture absorption, and shrinkage during curing.

ii. Epoxy resin: Epoxies are often used in wet lay-up applications of FRP plates and sheets because of their ability to cure well at room temperature and owing to their outstanding adhesion (bonding) characteristics. Epoxies have high strength, good dimensional stability, relatively good high-temperature properties, strong resistance to chemicals (except acids), and superior toughness. Epoxies, however, cost significant more than polyesters or vinyl esters.

iii. Vinyl ester resin: Vinyl esters have similar mechanical and in-service properties to those of the epoxy resins and equivalent processing techniques to those of the unsaturated polyesters. Vinyl esters are resistant to strong acids and alkalis, which is one reason that they are commonly used in the manufacture of FRP reinforcing bars for concrete (the environment inside concrete is highly alkaline). They also offer reduced moisture absorption and shrinkage as compared with polyesters. Vinyl esters cost slightly more than polyester.

3.3. FILLER

Fillers are inorganic materials added to the composite in order reduce cost and also to impart performance improvements that might not otherwise be achieved by the reinforcement and resins alone. They can improve mechanical properties including fire and smoke performance by reducing organic content in composite laminates. Also the important properties such as water resistance, weathering, surface smoothness, stiffness, dimensional stability and temperature resistance, can all be improved through the proper use of fillers. There are a number of inorganic materials which can be used as fillers, which includes calcium carbonate, kaolin (hydrous aluminum silicate), alumina trihydrate, calcium sulphate.

3.4. ADDITIVES AND MODIFIERS

Additives are used in composites to modify materials properties and tailor the
laminate’s performance. Although, these materials are generally used in relatively low quantity by weight compared to resins, reinforcements and fillers, they perform critical functions. Additives and modifier ingredients expand the usefulness of polymers, enhance their processibility or extend product durability. Catalyst, promoters, inhibitors etc. are the ingredients added as additives and modifiers.

4. FABRICATION OF FRP COMPOSITES

 There is a wide variety of techniques by which FRP composites can be fabricated, even though there are differences between the techniques available for thermosetting and thermoplastics, due to their intrinsic properties. The table 4.1 shows the commonly used process for fabrication of FRP composites applied in civil engineering, their principle and typical application.

Table.4.1. Fabrication process of FRP composites

 

 

5. TYPES OF FIBER REINFORCED POLYMER

5.1. GLASS FIBER REINFORCED POLYMER

Glass fibers are basically made by mixing silica sand , limestone, folic acid and other minor ingredients. The mix is heated until it melts about 12600C. The molten glass is then allowed to flow through fine holes in a platinum plate. The glass strands are cooled, gathered and wound. The fibers are drawn to increase the directional strength. The fibers are then woven into various forms for the use in composites.

 Based on aluminium lime borosilicate composition glass produced fibers are considered the predominant reinforcement for the polymer matrix composites due to their high electrical insulating properties, low susceptibility to moisture and high mechanical properties. Glass fibres have excellent characteristics equal to or better than steel in certain forms. Glass is generally a good impact resistant fibre but weighs more than carbon or aramid.

Figure 5.1. Glass fiber reinforced polymer sheet

5.2. CARBON FIBER REINFORCED POLYMER

Carbon fibers have a high modulus of elasticity, 200-800GPa. The ultimate elongation is 0.3-2.5 % where the lower elongation corresponds to the higher stiffness and vice versa. Carbon fibers donot absorb water and are resistant to many chemical solutions. They with stand fatigue excellently, do not stress corrode and do not show any creep or relaxation, having less relaxation compared to low relaxation high tensile prestressingsteel strands.

Carbon fiber is elastically conductive and there fore might give galvanic corrosion in the direct contact with steel.

Figure 5.2. Carbon fiber reinforced sheets

5.3. ARAMID FIBER REINFORCED POLYMER

Aramid is the short form for aromatic polyamide. A well known trademark of aramid fibres is Kevlar but there exists other brands too,e.g Twaron, Technora and SVM.The modulli of the fibres are 70-200 GPa with ultimate elongation of 1.5-5% depending on the quality.Aramid has a high fracture energy and is therefore used for helmets and bullet-proof garments.Aramid fibres are sensitive to elevated temperatures, moisture and ultraviolet radiation and therefore not widely used in civil engineering applications.Further aramid fibres do have problems with relaxation and stress corrosion.

6. FRP SHEETS AS EXTERNAL REINFORCEMENT IN RC BEAMS

The technique based on the externally bonded fiber reinforced polymer (FRP) materials is one of the most widely used for retrofitting existing damaged structures, since they have high mechanical properties in lightweight, high strength, ease to install, and do not change significantly the original geometry of the strengthened elements. The studies have shown that the beams strengthened with FRP in flexural strengthening would avoid the debonding failure mode when carefully designed anchorage is applied, which gives good flexural performance in terms of strength and ductility.

6.1. FRP EBR SYSTEM

 Different systems of externally bonded FRP reinforcement exist, related to constituent materials, the form and the technique of the FRP strengthening. In general, these can be subdivided into “we lay-up” (or “cured in- situ”) systems and “prefab” (or “pre-cured”) systems.

6.1.1. WET LAY-UP SYSTEMS

· Dry unidirectional fibre sheet and semi-unidirectional fabric (woven or knitted), where fibers run predominantly in one direction partially or fully covering the structural element. Installation on the concrete surface requires saturating resin usually after a primer has been applied. Two different processes can be used to apply the fabric:

Ø The fabric can be applied directly into the resin which has been applied uniformly onto the concrete surface,

Ø The fabric can be impregnated with the resin in a saturator machine and then applied wet to the sealed substrate.

· Dry multidirectional fabric (woven or knitted), where fibers run in at least two directions. Installation requires saturating resin. The fabric is applied using one of the two processes described above.

· Resin pre-impregnated uncured unidirectional sheet or fabric, where fibers run predominantly in one direction. Installation may be done with or without additional resin.

· Resin pre-impregnated uncured multidirectional sheet or fabric, where fibers run predominantly in two directions. Installation may be done with or without additional resin.

· Dry fibre tows (untwisted bundles of continuous fibers) that are wound or otherwise mechanically placed onto the concrete surface. Resin is applied to the fibre during winding.

· Pre-impregnated fibre tows that are wound or otherwise mechanically placed onto the concrete surface. Product installation may be executed with or without additional resin.

6.1.2. PREFABRICATED ELEMENTS

· Pre-manufactured cured straight strips, which are installed through the use of adhesives. They are typically in the form of thin ribbon strips or grids that may be delivered in a rolled coil. Normally strips are Pultruded. In case they are laminated, also the term laminate instead of strip may be used.

· Pre-manufactured cured shaped shells, jackets or angles, which are installed through the use of adhesives. They are typically factory-made curved or shaped elements or split shells that can be fitted around columns or other elements.

6.2. TECHNIQUES FOR FRP STRENGTHENING

6.2.1 BASIC TECHNIQUE

The basic FRP strengthening technique involves manual application of either wet lay-up or fabricated systems by means of cold cured adhesive bonding. Common in this technique is that the external reinforcement is bonded onto the concrete surface with the fibers as parallel as practically possible to the direction of principal tensile stresses.

Figure 6.1. Hand lay-up of CFRP sheets or fabrics

6.2.2. SPECIAL TECHNIQUE

Besides the basic techniques, several special techniques have been developed. Some of them are a follows.

i. Automated wrapping: This technique was first developed in Japan. This involves continuous winding of wet fibers under a slight angle around columns or other structures.

ii. Prestressed FRP: In some cases it may be advantageous to bond the external FRP reinforcement onto the concrete surface in a prestressed state. Both laboratory and analytical research shows that prestressing represents a significant contribution to the advancement of the FRP strengthening technique, and methods have been developed to prestress the FRP composites under real life condition.

iii. Fusion-bonded-pin-loaded straps: this technique involves replacing solid and relatively thick strips known as pin- loaded strap.

iv. In-situ fast curing using heating devices: Instead of cold curing of the bond interface (curing of the two-component adhesive under environmental temperature), heating devices can be used. In this way it is possible to reduce curing time, to allow bonding in regions where temperatures are too low to allow cold curing, to apply the technique in winter time, to work with prepreg FRP types, etc.

v. CFRP inside slits: CFRP in concrete slits is considered as a special method of supplementing reinforcement to concrete structures. The slits are cut into the concrete structure with a depth smaller than the concrete cover. CFRP strips e.g. with a thickness of 2 mm and a width of 20mm are bonded into these slits. vi. Prefabricated shapes: Prefab type of FRP EBR systems are mostly applied in the form of straight strips. However, these can be produced in other forms, depending upon their application. By shaping them, prefab systems can be employed in applications where normally the more flexible wet lay-up systems can be used. vii. FRP impregnation by vacuum: it is quite common in the plastic industry. Vacuum impregnation is, to some extent, comparable with wet lay-up. The concrete element to be strengthened according to this method is pre-treated in the same manner as for the other method.

7. THE EXPERIMENTAL PROGRAM

7.1. GEOMETRY OF BEAMS AND TEST SET-UP

The experimental program is made of flexural tests carried out on 7 RC beams having width and length as 150mm, 1700mm and depth of 250mm or 300mm. the concrete cover thickness was 25 or 35 mm for the beam tested. Two bars of diameter 8mm as internal steel reinforcement in compression, two 10 or 14 mm diameter bars as longitudinal reinforcement in tension. For the beams tested, 6 mm diameter steel stirrups were placed along the entire length of the beam with a spacing of 100 mm. For the 7 RC beams tested, one beam WR1 was kept without retrofitting and considered as a reference beam. Rest 6 beams were reinforced with either single layer or two layers of CFRP sheets. Here WR2 was reinforced with one layer of CFRP sheets and WR3 with two layers of CFRP sheets, without pre-crack. The other four beams, WR4, WR5, WR6 and WR7, were pre-cracked with the maximum crack widths of 0.51, 0.59, 0.56 and 0.53 mm, respectively, and subsequently strengthened with two layers of CFRP sheets. Finally, two strips of the U-shaped CFRP sheets were bonded onto both sides of the beam near the supports as external anchorage to reduce the edge stresses and prevent the delamination of other CFRP sheets.

All the beams are simply supported over a clear span of 1500mm and tested under four- point flexural load by using a servo controlled hydraulic actuator having a maximum capacity of 200KN. The strain on concrete, steel, FRP sheets of all beams is measure using instruments. In addition, three linear voltage displacement transducers (LVDTs) were also used to measure deflections at the mid-span and loading points.

7.2. MATERIAL USED

The yield strength (fy), ultimate strength (fu), elastic modulus (E) and the ultimate elongation of the steel reinforcement and that of GFRP sheets are shown in the table 7.1 below. Here the GFRP sheets of thickness 0.111mm are used.

Table 7.1.Mechanical properties of materials

7.3. FAILURE MODE AND LOADS

The failure mode of the reference beam WR1 is a typical bending failure pattern. For the beams strengthened with one or two layers of CFRP sheets, appearing of the cracks was delayed, also the width of those cracks and the inter-space between cracks were reduced. There were two main failure modes for the beams strengthened, i.e. snapping and debonding of CFRP sheets, and shear cracks propagated toward the loading point accompanied by debonding of the CFRP sheets from the concrete. The failure mode of the beams such as WR2, WR3 and WR6 was characterized by the snapping of one layer or two layers of CFRP sheets were bonded on the tensile side of beams. However, the beams of WR4, WR5 and WR7 were failed by the debonding of CFRP sheets from mid span or loading point. The table shows below summarizes the outcomes of the test conducted:

Table 7.2. Test results of beam tested

7.4. LOAD-DEFLECTION BEHAVIOUR

Figure below shows that the initial stiffness of beams strengthened increases significantly in comparison to the reference beam due to the contribution from the CFRP reinforcement. The beam WR3 gives a smaller deflection at the given load than that of beam WR4 from zero loads to the load of 52KN, since the cracks developed during the preloading degraded the beams stiffness. The beams with the higher longitudinal reinforcement ratio (WR5) or a deeper cross section depth (WR6) may lead to a higher stiffness than that of the same reinforcing beam (WR4) before the yielding of steel bars. However, the beam WR5 gives a higher stiffness but a lower ultimate strength than that of beam WR6, this is due to the highest longitudinal reinforcement ratio contributed to highest strength and the deepest cross-section depth lead to highest stiffness. The stiffness of beams WR4 and WR7 which have the same reinforcement but different concrete cover thickness is almost the same. The result indicated that the concrete cover thickness may not be an influential factor to the bending stiffness.

Figure 7.1. Load versus deflection curves for the beams tested

7.5. CRACKING BEHAVIOUR

During the test the maximum crack widths where measured at every loading increment by the PTS-C10 crack width measuring device. The relationship between the load and the corresponding crack width is shown in the table and the figure below.

The strengthened beam gives a smaller crack width than that of reference beam. This was due to the constraining effect of the attached CFRP sheets on the strengthened beam. But during the initial loading up to approximately 40 KN, the beams WR2 and WR3 shows a larger crack width than that of reference beam, this might be caused by possible slipping and engaging between FRP and concrete. The beam WR5, with the most longitudinal reinforcement ratio and WR6 with the deepest cross-section depth gives a smaller maximum crack width than that of beam WR3, WR4 and WR7. The result shows that longitudinal reinforcement is more effective than the cross-section depth and concrete cover thickness in controlling the crack development.

Figure 7.2.Load versus cracked width measured before the failure

Table 7.3. Data of cracks at failure for flexural beams tested

7.6. RESULTS OF THE EXPERIMENT

i. The ultimate load carrying capacity of all the strengthened beams is higher when compared to the control beams.

ii. The initial cracks in strengthened beams are formed at higher load compared to control beam.

iii. The flexural strengthening capacity of the beams strengthened with externally bonded of CFRP increases significantly. The increase on overall flexural capacity of CFRP strengthened beams varies between 41%nand 125% over the reference beam. Hence applying FRP in the flexure zone is quite effective method to enhance the load carrying capacity.

iv. The flexural strength could be diminished by increasing the concrete cover thickness, but the deflection at mid span could be increased greatly.

v. Both the ductility and strength for FRP strengthened beams could be enhanced significantly by increasing the longitudinal reinforcement ratio, but the preloading before the failure test might results in little decrease in flexural strength, stiffness and ductility.

vi. Strengthening of continuous beam by providing U-wrap of FRP sheets is a new and effective way of enhancing the load carrying capacity.

vii. Flexural failure at the intermediate support can be prevented by the application of FRP sheets

8. APPLICATION OF FRP COMPOSITE IN CIVIL ENGINEERING

Over the last decade there has been significant growth in the use of FRP composites as construction material in structural engineering. There are three broad divisions in which applications of FRP in civil engineering can be classified: repair and rehabilitation applications, application in construction engineering and architecture, application in new constructions.

8.1. REPAIR AND REHABILITATION

Majority of repair and rehabilitation works consists of repair of old deteriorating structures, damage due to seismic activities and other natural hazards. Structural strengthening is also required because of degradation problems which may arise from environmental exposure, inadequate design, poor quality construction and a need to meet current design requirement. Generally, FRP composites can be utilized for structural rehabilitation in the following situations:

• Deficiencies at the design stage, including: design errors, inadequate factors of safety, use of inferior class materials and poor construction quality.

• Change of use, in service, namely, increased safety requirements (upgrading of structural design standards), modernization that causes redistribution of stresses and increase of the applied load.

• Ageing of materials that compromise the load capacity of the structure: for example concrete degradation in hostile marine or industrial environments.

• Accidents, as fire or seismic events.

8.1.1. REPAIR AND STRENGTHENING

Repair with FRP composites has been used successfully on concrete, timber, metal and masonry structures. The predominant role of concrete as a structural construction material simulated the application of FRP composite in repairing of concrete structures such as bridges and large structural elements.

The basic FRP strengthening technique, which is most widely applied, involves the manual application of either wet lay-up or prefabricated systems by means of cold cured adhesive bonding. Common in this techniques is that the external reinforcement is bonded onto the concrete surface with the fibers as parallel as practically possible to the direction of principal tensile stresses. Besides the basic techniques, several special techniques have been developed, namely the automated wet lay-up wrapping of columns or chimneys, use of pre-stresses FRP to close open cracks in bridge decks. Near-surface mounted (NSM) technique may also be thought as a special method of reinforcement of concrete structures. IN this method, grooves are first cut into the concrete cover and the FRP reinforcement, usually a laminate strip, is bonded therein with appropriate groove filler, typically epoxy paste or cement grout.

8.1.2. SEISMIC RETROFIT

The problem of structural deficiency of existing constructions is especially acute in seismic regions, as, even there, seismic design of structures is relatively recent. The enhancement of confinement in structurally deficient concrete columns in seismically active regions of the world has proven to be one of the most significant applications of FRP materials in infrastructure applications.

Seismic retrofit of reinforced concrete structures, namely bridges, using conventional steel techniques has found to time consuming, it cause significant traffic disruption, rely on field welding and is susceptible to corrosion. Additionally, many of the method increases the stiffness and strength capacity of the columns putting adjacent structural elements at risk from higher transmitted seismic forces. The use of FRP composites in this application, not only provides a means of confinement, without the associated increase in stiffness, but also enables the rapid fabrication of cost effective and durable jackets with little traffic interference.

Figure 8.1. Seismic retrofitting of column with FRP sheets

8.2. CONCRETE STRUCTURES REINFORCED WITH FIBER COMPOSITE

Concrete reinforced with fiber reinforced polymer (FRP) materials has been
under investigation since the 1960’s. The predominant role of concrete as a construction material and the problems associated with corrosion of steel reinforcement stimulated the development of fiber composites for internal and external reinforcement of concrete and pre-stressing cables and tendons. Unstressed FRP reinforcement has been developed in a number of forms including ribbed FRP rod similar in appearance to deformed steel reinforcing bar, under formed E-glass and carbon fiber bar bound with polyester, vinyl ester or epoxy resin, E-glass mesh made from flat FRP bars and prefabricated reinforcing cages using flat bars and box sections. Stressed FRP reinforcement is also available, usually consisting of bundles of rods or strands of fiber-reinforced polymer running parallel to the axis of the tendon. These are used in a similar fashion to conventional steel tendons.

The durability of FRP composite is considered by engineers to offer a possible solution to the problem of corrosion of steel reinforcement, a primary factor in reduced durability of concrete structures. Other advantages of FRP rebar include enhanced erection and handling speeds and suitability to applications which are sensitive to materials which impede radio wave propagation and disturb electromagnetic fields.

8.2.1 FRP BARS, RODS AND GRIDS

The use of FRP reinforcing bars and grids for concrete is a growing segment of application of FRP composites in structural engineering. For an effective reinforcing action, it is necessary to develop bond strength between FRP and concrete. This is achieved in FRP rods by having various types of deformation systems, including exterior wound fibers, sand coatings and separately formed deformations. FRP reinforcing bars and grids for concrete with both glass and carbon fibers are produced by a number of companies in USA, Asia and Europe. Applications have become routine for certain specialized environments, namely in bridge decks and in underground tunnels.

8.2.2 FRP CABLES FOR PRESTRESSING AND POSTSTRESSING APPLICATION

Composite cable applications in the infrastructure are used in the construction of suspension and stay cables for bridges, pre-stressed tendons for various concrete structures and external reinforcements for structural beams. All these applications require materials that incorporate high tensile strength and, in addition, require characteristics such as corrosion resistance and light weight.

Corrosion of steel pre-stressing tendons can lead to the concrete degradation and the deterioration of structural integrity. In cable-stay applications, both corrosion and fatigue make the replacement of conventional cables a significant life cost. FRP composites have good corrosion, durability and fatigue characteristics and therefore the utilization of these materials does make good engineering sense. The initial cost of this cables is higher than other types of cables but this will nullify against their reduced transportation cost, handling cost, maintenance cost etc.

FRP cables are unidirectional reinforced structural elements made from glass, aramid or carbon fibers embedded in polymer matrix. Different shapes such as bars, cables, rectangular strips and braided reinforcements are available. Carbon fiber and aramid cables are used for pre-tension and post- tension concrete, however glass fiber cables are not recommended for pre- tension due to the low resistance to alkaline environments.

8.3 NEW FRP CIVIL STRUCTURES

A small number of new load bearing civil engineering structures have been made predominantly from FRP materials over the last three decades. These include compound curved roofs pedestrian and vehicle bridges decks energy absorbing roadside guardrails(Bank and Gentry 2000), building systems, modular rooftop cooling towers (Barbero and GangaRao 1991), access platforms for industrial, chemical and offshore (Hale 1997), electricity transmission towers, power poles, power pole cross-arms and light poles and marine structures such as seawalls and fenders (Weaver 1999). FRP pultruded structures profiles have been used in a significant number of structures to data, including pedestrian bridges, vehicular bridges, building bridges, building frames, cooling towers, walkways and platforms, etc. (Susana Cabral-Fonseca 2008).

9. ADVANTAGES AND DISADVANTAGES OF FRP COMPOSITE

9.1. ADVANTAGES

 i. Low weight: The FRP is much less dense and therefore lighter than the equivalent volume of steel. This property of FRP makes its installation and handling significantly easier. Also the use of FRP composite does not increase the weight of the structure or dimensions of the member. And because of its light weight, the transport of FRP material has minimal environmental impact.

ii. Mechanical strength: FRP can provide a maximum material stiffness to density ratio of 3.5 to 5 times that of aluminum or steel. FRP is so strong and stiff for its weight, it can perform better than other materials.

iii. Formability: The material can take up irregularities in the shape of the concrete surface. It can be moulded to almost any desired shape. We can create or copy most shapes with ease.

iv. Chemical resistance: FRP is minimally reactive, making it deal as a protective covering for surfaces chemicals are present.

v. Corrosion resistance: Unlike metals, FRP does not rust away and it can be used to make long- lasting structures.

vi. Low maintenance: Once FRP is installed, it requires minimal maintenance. The materials fibers and resins are durable if correctly specified, and require little maintenance. If they are damaged in service, it is relatively simple to repair them, by adding an additional layer.

vii. Long life: it has high resistance to fatigue and has shown excellent durability over the last 50 years.

viii. Easy to apply: The application of FRP plate or sheet material is like applying wallpaper; once it has been rolled on carefully to remove entrapped air and excess adhesive it may be left unsupported. Fibre composite materials are available in very long lengths while steel plate is generally limited to 6 m. These various factors in combination lead to a significantly simpler and quicker strengthening process than when using steel plate.

9.2. DISADVANTAGES

i. Low plastic behaviour

ii. High initial cost of the material

iii. Debonding failure may occur

iv. Susceptible to local unevenness.

v. Lack of accepted design standards.

10. CONCLUSION

The strength properties of FRP composite are the major reason for which it is used in the design of structures by civil engineers. The strength of a material depends on its ability to sustain the loads without excessive deformation or failure. When the specimens reinforced externally with FRP sheets is tested in axial tensions, the applied force per unit cross-sectional area(stress) is proportional to the ratio of change in specimen’s length to its original length(strain). When the load applied is removed, the FRP returns to its original shape or length. In other words, FRP responds linear – elastically to the applied axial stress. The response of FRP to axial compression is reliant on the relative proportion in volume of fibres, the properties of fibre and resin, and the interface bond strength. The compression failure in FRP occurs when the fibres exhibit extreme lateral or side way deflection called fibre buckling. FRP’s responds to transverse tensile stress is very much dependent on the properties of fibre and matrix, the interaction between the fibre and matrix, and the strength of fibre matrix interface. But the tensile strength is very poor in this direction.

Shear stress is included in the plane of an area when external loads tend to cause two segments of a body to slide over the other. The shear strength of the FRP composite is difficult to quantify. The failure will generally occur within the matrix material and is parallel to the fibres. FRP composite have high strength properties and also have the most relevant features which includes excellent durability and corrosion resistant. Also, their high strength to weight ratio is of significant benefit; a member composed of FRP can support larger live loads since its dead weight does not contribute significantly to the loads that it must bear. Other features includes ease of installation, versatility, anti-seismic behaviour, electromagnetic neutrality, excellent fatigue behaviour and fire resistance. FRPs also have a few drawbacks such as high cost, brittle behaviour, susceptibility to deformation under long term loads, UV degradation, photo degradation, temperature and moisture effects, lack of design codes, and most importantly lack of awareness.

REFERENCE

1. Jiangfeng Dong et al (2011), CFRP sheets for Flexural Strengthening of RC Beams, IEEE for Civil Engineering, Page-1000-1003.

2. C. D. Modhera & Kaushal Parikh (2012), Application of GFRP on preloaded retrofitted beam for enhancement in flexural strength, International Journal of
Civil and Structural Engineering, Volume 2, No 4, Pages-1070-1080.

3. E. Rakesh Reddy(2014), Strengthening Of RC Beam Using FRP Sheet,
International Journal of Modern Engineering Research (IJMER), Volume 4, Issue.7, Pages-30-62.

4. J.G. Dai et al (2005), Flexural strengthening of RC beams using externally bonded FRP sheets through flexible adhesive bonding, Proceedings of the
International Symposium on Bond Behaviour of FRP in Structures (BBFS 2005), Page 205-214.