Fibre Reinforced Polymer Composites to Strengthen Structures
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The project aim is to identify the use of FRP (Fibre Reinforced Polymers) composites in strengthening of structures. The general process and methodology considered in achieving this objective is by externally bonding Fibre reinforced plastics to the metal structure/plate and thus testing the specimen under 3 point bend as well as 4 point bend tests. The preliminary report will focus on the project introduction, literature review related to the project topic, project plan and further to be carried out.
Analysis of Tasks
As mentioned the projectâ€™s main focus is strengthening of structures. So the question is why is there a need for strengthening of structures? The reason behind the strengthening of structures is that the structure will be able to support greater magnitude of loads than the values for which the structures are originally designed. Strengthening may become necessary in course of time in order to overcome the damage caused due to environmental factors such as corrosion as well as fatigue cracking. The method of bonding reinforcement represents an attractive solution to the problem as it can be achieved with relatively small impact on the structure. The next stage of the preliminary thesis will focus on the literature review. The literature review section is divided in to two sections firstly it will focus on the topics related to the Composite Materials, which will involve the following topics.
Definition of Composite Material
Classification of Composite Materials
Types of Matrix
Types of Reinforcement
Advantages and Disadvantages of Composite Materials
Manufacturing of Composite Materials
The next section of the literature review will focus on the topics related to strengthening of structures. For this section following topics will be considered.
Different Methods for Structure Strengthening
Different Methods for Structure Strengthening using FRP Composites.
The preliminary thesis will also look at the comparison between the structural strengthening using FRP Composites and Steel plates. This will help in giving a suitable reason as to why Composites are preferred over Steel plates for structural strengthening.
Composites are considered as one of the most promising material for reducing the weight as well as increasing the strength of the material. The theory of hybrid material has been successfully exploited when studying sandwich panel technology. Now the concept of integrated multi materials is extended to a wider variety of structures, components and applications. Fibre reinforced plastics (FRPâ€™s) have been successfully used for the post strengthening of structures over a number of years. The design and manufacturing of composites has led to its application in variety of industries such as automotive, aerospace, sporting goods, construction and in the marine as well as oil and gas industries.
Moreover the method of bonded reinforcements also comes into account when there have been cases where errors in design or construction of a structure have questioned safety aspect. This can be achieved by externally bonding reinforcements at a cost of very small impact to the structures.
Project Time-Line (Gantt chart)
Composite materials also known as composites are defined as a combination of two or more materials to give a unique combination of properties . This definition is very general and thus includes metals, alloys, plastic co-polymers, minerals and wood. A material is only classified as a composite if the material satisfies the following conditions :
The material must be manufactured
It should consist of two or more physically and/or chemically distinct, suitably arranged or distributed phases with an interface separating them.
The characteristics of the composites are not depicted by any of the components in the isolation.
Fibre reinforced composite materials differ slightly from the general definition because in this FRP the constituent materials are different at the molecular level and at mechanically separable. However the final material properties of the composite are better in comparison to that of the constituents. The figure 1 below gives a basic idea of how the structure of composite material looks like. The composites can be classified as fibre reinforced, particle reinforced, dispersion strengthened and laminates composites.
Figure Structure of Composite Material
The composite material consists of two main elements the matrix and reinforcements (fibre). The classification of Composite materials is as shown in figure 2.
Classification of Composite Material
Types of Matrix
There are three main types of matrix considered when studying composites materials namely Polymer, Metal and Ceramic.
Polymer Matrix composites are the most common types of matrix composites. They are also known as FRP (Fibre reinforced polymers). The resin used in here is polymer based combined with a variety of fibres such as Glass, Carbon and Aramid as reinforcements.
Metal Matrix Composites are mainly used in the automotive industry. The materials use a metal for example Aluminium as the matrix which is then reinforced with fibres such as silicon carbide.
Ceramic Matrix Composites are used in very high temperature environments. This type of materials uses Ceramic as the matrix and the matrix is then reinforced with short fibres such as Silicon Carbide and Boron Nitride. A matrix plays an important role in a Composite structure. There are several functions of Matrix most of which are very important to the satisfactory performance of the structure. The following points outline the important functions of the matrix.
The matrix binds the fibre together and thus transfers the load to the fibres. The matrix provides the rigidity and shape to the structure.
The matrix isolates the fibres such that each individual fibre can perform separately, due to this crack propagation process slows down.
The surface finish quality of the structure is provided by the matrix.
The matrix acts as a protection to the reinforced fibres from chemical attack and mechanical wear and tear.
The type of material selected as matrix affects the ductility and failure mode of the structure together with the fibreâ€™s compatibility.
Types of Reinforcements
The other constituent in the composite material is called the Reinforcement. This gives the composite the necessary strength and stiffness. The structure of the reinforcement is thin rod like. The most commonly used reinforcements are Glass, Carbon, Aramid, and Boron fibres. The diameters of these fibres range from 5Âµm to 20 Âµm. 
Due to the thin diameter of the fibre, the fibres are flexible and can be formed easily into any shapes. Fibres can come into many forms such as continuous fibre, discontinuous fibre, short fibres, long fibres, organic fibres and inorganic fibres. Fibre brings out the high performance of the material; this is due to three important characteristics of the fibres. The orientation of the fibres also has an impact on the performance of the composite. The fibres can be unidirectional, cross ply or random in its arrangement.
Small diameter in comparison to the grain size. As a result of this higher fraction of theoretical strength can be attained.
High aspect ratio (length/diameter); this allows a larger amount of load to be transferred.
High degree of flexibility.
The main functions of fibres are as listed below ;
To carry the load. Hence fibres are made from materials with high tensile strength and high elastic modulus.
Provides strength, stiffness, thermal stability and other structural properties in the composites.
Provide electrical conductivity or insulation, depending on the type of fibre used.
Advantages of Composites
Composites are designed to perform in applications which require lighter weight and higher performance. The advantages of using composites are listed below.
High resistance to Corrosion; due to this the application in marine, infrastructure and chemical is very good.
High specific stiffness and high specific strength; this gives a weight reduction so is used for the application in aerospace, automotive and manufacturing of sporting goods.
The impact resistance is high compared to metal.
Higher fatigue strength. Unidirectional carbon/epoxy composites have good fatigue strength of almost 90% of its static strength.
Composite materials offer increased amount of design of flexibility. For example the coefficient of thermal expansion of composite can be made zero if suitable constituents and lay up sequence is selected. As the coefficient of thermal expansion is relatively low compared to metals, the composite structure thus provides a good dimensional stability. Also due to the design flexibility, composite materials can be formed into any shapes.
Process cycle times and costs are also reduced because of the use of composites in production of net shape and near net shape parts.
Composite materials dampen the vibrations an order of magnitude better than metals.
Glass reinforced and aramid reinforced composite meet the FAA and JAR requirements for low smoke and toxicity and thus are used in interior panels of aircrafts, stow bins and galley walls.
Disadvantages of Composites
There will always be limitations to the benefits of the composites. The disadvantages are as listed below.
Weaker in transverse direction and low in toughness.
Material cost is high compared to that of steel and aluminium.
The lack of high volume production limits the wide spread use of composite materials. However this is changing as modern production methods such as Pultrusion, Resin transfer Moulding and other methods have been automated to increase the rate of production.
The knowledge through books and database is limited when comes to designing parts with composite.
Composites absorb moisture, which affects the properties and dimensional stability of the composite.
Difficult to join together with other material due to its anisotropic properties and high sensitivity to damage. For example when drilling holes for mechanical fastening.
Brittle like behaviour.
Difficult to repair as most composites use thermo set matrices that can not be re shaped. On the contrary thermoplastics can be repaired how ever they are rare .
Solvent resistance, chemical resistance and environmental stress cracking of composite depend on the properties of polymers.
Some polymers have low resistance to the solvents and environmental stress cracking.
Manufacturing of Composite Materials
Before looking into manufacturing of composite materials; it is important to look at manufacturing of fibre performs briefly. Fibre performs is how fibre are manufactured before being bonded to the matrix to form a composite material. Fibre performs are often manufactured in sheets or filaments in case of spraying applications. The fibre manufacturing process is carried out by adopting the technique used in textile industry. The techniques used are weaving, knitting, braiding and stitching .
The manufacturing process of composite material in general can be divided into two main processes.
The process of prepreg moulding can be further categorized as:
Autoclave/Vacuum bag Moulding
Bladder Moulding: In this process the sheets of prepreg material are laid up and placed in female style mould along with a balloon like bladder. The mould is then closed and placed in the heated press. Eventually, the bladder is pressurized which then forces the layers of material against mould walls. The part is then cured and removed from the hot mould. The process is ideally suited for complex hollow shapes. Also the process has great cost to performance balance. Typical example of equipment using bladder moulding technique is the manufacturing of tennis racquet. Average cure cycle range is 15-60 minutes.
Compression Moulding: A process where a â€œperformâ€? or â€œchargeâ€? of single moulding compound (SMC) or bulk moulding compound (BMC) or sometimes prepreg fabric is placed in the mould cavity. Once the mould is closed, the material is compacted and cured inside by heat and pressure. The process also offers excellent detailing for geometric shapes. The average cure cycle range is 2-20 minutes. The tooling is process is often more expensive.
Auto Clave/Vacuum Bag Moulding: The figure below gives an idea of vacuum bagging for prepreg lay-up process.
Figure Vacuum bagging for prepreg lay up process
Once all the prepregs are laid out in the desired sequence, vacuum bagging preparations are set up as per the figure 3 for curing of the part. First step is application of release film on the top of all the prepreg. The release film is a perforated film that allows captured air, excess resin and volatiles to escape. Secondly is the application of bleeder on top of release film; it is a porous fabric that absorbs moisture and excess resin coming from stack of prepregs/laminates. Thirdly is the application of non porous and non-perforated film on top of bleeder. After this a breather layer is applied; this is a porous fabric which creates even pressure around the part and at the same time allowing air and volatiles to escape. Then final layer is of vacuum bag. The vacuum bag is an expendable polyamide film. The film is sealed on all sides of laminate using a seal tape. A nozzle is inserted into the vacuum bag and is then connected to hose vacuum pump for creating vacuum.
Mandrel Wrapping: In these process sheets of prepreg material is wrapped around steel or aluminium mandrel. The prepreg material is compacted by nylon or polypropylene cello tape. Parts are cured by hanging in the oven. Once the curing process is completed, the cello and mandrel are removed which results in a hollow carbon tube.
Advantages of prepreg lay up process are that it is simple process when manufacturing complex parts. Also strong and stiff parts can be fabricated using this process. Also allows production of high fibre volume fraction. Prepregs usually have more than 60% fibre volume fraction.
Limitations of the process being that labour is intensive as a result of which the process is not suitable for high volume production applications. Also parts manufactured by this process are very expensive.
Wet moulding process can be divided into following processes.
Spray Up process
Resin transfer Moulding (RTM) & Resin transfer moulding under vacuum (VARTM)
Wet Lay-up: in this process a fabric is placed in open mould which is then hand saturated with wet resin. The curing occurs normally at room temperature. However it can be cured at higher temperatures based on the heat resistance of the mould. The advantage of this process is that the material cost and tooling cost is low. Because of this the process is considered as low tech process.
Spray-up process: The processing steps are similar to that of the wet lay-up process except for the method of creating the laminates. The basic steps are as follows .
The mould is waxed and polished for easy de-moulding
Gel coat is applied to the mould surface and allowed to harden before building any other layer.
The barrier coat is applied to avoid fibre print through the gel coat surface.
Oven curing of barrier coat.
Mixing of resin with fillers and the mixture is then pumped to a holding tank.
Spraying of resin, catalyst and chopped fibres on the mould surface. This is done with the aid of hand held spray gun. The spraying is carried out in typical pattern to create uniform thickness of the laminate.
A roller is then used in order to make the fibre and resin material compact as well as create a smooth and even surface.
The laminate is then cured in the oven. The part is then de-moulded and sent for finishing work.
The process of spray lay-up is very economical, maximizes the use of low cost tooling as well as low cost material systems however it is not suitable for making parts that have high structure requirements, fibre volume fraction can be difficult to control as well as the thickness. The surface finish on both the sides is not same. Also dimensional tolerance is poor.
Filament Winding: This is a process in which resin-impregnated fibres are pulled and wound over a rotating mandrel at desired angle. The fibre pulled is from a wet bath of resin. Curing occurs at room temperature or higher temperature. The performance is limited and it is difficult to obtain uniform fibre distribution and resin content through out the thickness of the laminate. The process is very suitable for tubular parts such as pressure vessels.
Figure Filament Winding Process
The advantages of this process being its ability to utilize low cost raw material and low cost tooling systems. It can be automated for the production of high volume composite parts. The figure 4 gives an indication of the filament winding process.
Pultrusion: As the name suggests, it is a process which will have pulling and extrusion (cutting). In this process resin impregnated fibres are pulled through to make a part. Saturated material is pulled through a heated closed die and cured while continuously moving through the die. The figure 5 gives an indication of the process.
Figure Pultrusion process
The advantage of pultrusion process is that it is a continuous process and can be completely automated to get the finished product. The process is suitable for making high volume composite parts. Utilizes low cost fibre and resin systems thus provide a low production cost of products.
The limitations being that the thin wall parts, tapered and complex parts cannot be produced.
Resin Transfer Moulding
A resin and catalyst are placed in two separate tanks A and B. A release agent such as gel coat is then applied to mould for good surface finish.
The preform is placed inside the mould and the mould is then clamped. The mould is then heated to a specified temperature.
Mixed Resin is then injected at selected temperature and pressure. Vacuum is also created to remove air bubbles as well as assist in resin flow.
The injection continues until the mould is completely filled. The vacuum is turned off. The pressure inside the mould is increased to ensure that remaining porosity is collapsed.
After curing for certain period of time depending on resin, the composite part is removed from mould.
In this process, fabrics are placed in to a mould and then wet resin is injected. Resin is pressurized and is then forced into the cavity which is under vacuum. In the VARTM process resin is completely pulled in to cavity under vacuum. This moulding process allows precise tolerance and detailed shaping, however this may result in at times failure to fully saturate fabric leading to weak shape in the final product. The figure and steps following the figure describe the process.
Figure Resin Transfer Moulding
Strengthening of Structures
The next stage of literature review will now focus on the strengthening of structures. As explained earlier the need for strengthening of structures; it is important to look at first the different methods adopted in order to strengthen the structure. The structure strengthening methods include the following.
Structure Strengthening using FRP Composites.
This is method in which length of a beam is shortening. This is achieved by installing additional supports underneath the existing members. Materials used in this process are mainly structural steel members and cast in place reinforced concrete members. The members are connected mechanically using bolts and adhesive anchors. Span shortening reduces the deflection in the beam. This can be proved by means of simple calculation.
Let us consider a beam of length â€œL (m)â€? under a load P kN undergoing a 3 point bend test.
Where Î´ is the deflection in the beam. Let us consider a pair of equations in order to calculate deflection in the beam.
If ratio of the deflection is taken in to account, and doubling the span length meaning l2 = 2l1. Then,
Then Î´2 = 8 x Î´1; this means that central deflection will become 8 times.
So to conclude the method shorter the span, less deflection will occur at the centre.
Pre stressed Concrete
Pre stressed concrete is a method used for overcoming concreteâ€™s natural weakness in tension. Pre-stressing tendons generally made of high tensile steel rods are used to provide clamping load that generates compressive stresses which balances out the tensile stress that would be experienced by the concrete beams due to bending .
The process of pre stressing can be achieved in three ways: pre tensioned concrete, bonded or un-bonded post-tensioned concrete. However the method is mainly used for civil engineering and construction projects. The figure below gives an indication of the method.
Figure Pre Stressing Method
In simple terms, it is when the second moment of area is changed. The method involves placing an additional bonded reinforcement concrete to an existing structure member in the form of an overlay or a jacket. The main advantage of this technique is that it increases the load bearing capacity or stiffness. The technique reduces bending and shear forces on overstressed beams.
Structure Strengthening using FRP Composites
Fibre reinforced polymers are applied to strengthen structure. There method can be primarily divided in to two categories. One is the strengthening of structure at manufacturing phase and other is after manufacturing. The technique used to strengthen structure at manufacturing phase is known as Near Surface Mounted Reinforcement (NSMR) and the strengthening of structure can be achieved once the manufacturing is completed is by external plate bonding method.
Near Surface Mounted Reinforcement
Near surface mounted reinforcement is one of the latest and most promising techniques to be considered when strengthening concrete structures. Using FRP instead of steel in this technique has many advantages such as its better resistance to corrosion, quick and easy installation due to light weight. When compared with external bonding method, the NSMR has many advantages such as amount of site installation work is reduced for example removal of plaster, etc. second advantage being that NSMR is less prone to de-bonding from concrete substrate. NSMR bars can be easily anchored into adjacent members to prevent failures due to de-bonding. This feature is very important when considering flexural strengthening. NSMR bars can be easily pre-stressed. As the bars are covered by cover of concrete, so they are less exposed to accidental impact or mechanical damage such as fire or wear and tear.
As the NSMR technique is new, the knowledge on this technique is limited than that of externally bonded reinforcement.
In recent studies CFRP (carbon fibre reinforced polymer) NSM reinforcements have been widely used to strengthen structures. The following general steps must be performed during the strengthening.
Sawing up slots in concrete cover, depth of the structure is dependent on the product used.
After sawing of slots, careful cleaning is required.
If using an epoxy system, the slots where the bars are to be installed must be dry before bonding. If cement system is used then the surface must be wet.
Adhesive is applied in the slot when using epoxy system or cement mortar is used when using cement system.
The figure below gives an indication of the technique.
Figure Near Surface mounted Reinforcement technique
In the above figure it can be seen that a carbon fibre rope is placed in the centre of the brick wall and an epoxy paste is applied. The figure on the left is for vertical reinforcement and the figure on the right is for horizontal reinforcement. The second part of the images is the finished version of structure once the method is completed. The application of this technique is mainly on flat surfaces and is suitable for strengthening in bending. The process is also helpful in increasing the shear capacity of the beams.
The next method considered for structure strengthening using FRP composites is external plate bonding. However it is first important to compare the advantage of using composites over steel plates. These are explained in the next segment of the report.
External Plate Bonding
The general principle of external plate bonding method is that in this method transfer of stresses takes place from the structural element to the additional plates that are adhered or bolted. Whilst strengthening structure many problems are faced such as.
Load Increases: Structure with externally bonded plates has capacity to accept higher live loads for example in factories where heavy machines are installed. External plate bonding also reduces deformation and is also helpful where vibrations are an issue.
Damage to Structural Parts: it can be utilized in cases where the building has been damaged due to fire or vehicle collision.
Improvement in suitability: This can be achieved by limiting deflection and reducing crack widths and stresses.
Modification of Structural System: Plate bonding provides a cost effective solution where structure has been weakened by removal of walls and columns or opening cuts.
Errors in planning or construction: Plate bonding provides solution to earlier design errors and calculation errors.
In general terms external plate bonding can reduce deflection thus limit cracking as well as increase the load bearing capacity and also increase the flexural strength and finally improve resistance to shear in certain cases.
As the method of plate bonding can be achieved by both using Composites as well as steel plates so it is important to understand the advantage of composite over steel plates. This analysis will be carried on three grounds namely technical, practical/application and Economic.
The table on the next page summarizes the technical differences between the composite and steel plates
High tensile strength of Carbon fibre (5650N/mm2)
Tensile strength of steel is (235 N/mm2)
Highly prone to corrosion
Strength to weight ratio is higher
Strength to weight ratio is lower
Table Technical differences between Composite and Steel Plates
When performing practical application on site; installation of steel plates requires an extensive amount of work such as drilling holes in plates, wrapping plates and bolting the plates. Where as the composite plates would require very limited work such as bonding to the metal plate using an epoxy.
Table 2 summarizes the economic differences
Typically 10-50% of steel
smaller area of plates required so less adhesive
Area of plates is high so higher amount of adhesive
Preparation Off site
Table Economic Differences between Composite and steel plates
The application of the plates externally can be varying such as if the surface is horizontal than FRP is applied horizontally or in the U shape. If the structure is a circular column then FRP is applied by wrapping around it. Wrapping sheets have fabrics in same direction or bi-directional. One thing to be considered when using FRP is that it needs to be protected from fire. An FRP plate applied to the bottom of structure (tension face) increases the strength of beam and reduces deflection. Where as application of FRP strips attached in U shape around the sides and bottom of beam increases shear resistance. Wrapping of sheets around column results in higher strength and restrains lateral expansion of columns.
The figures below give an example of application of FRP composite to structure.
Figure Strengthening using FRP Composites
Figure A and B look at plate bonding and figure C gives an indication of wrapping columns.
The project is currently in its research phase. The topics covered in the literature review section of the report are vital as it is important to gain knowledge about the subject and understand its applications in the real engineering world. The future work in the project will involve an additional literature review for structural strengthening and the next stage will be to set up a meeting with the supervisor to discuss the experiment as to what needs to be performed. Once this is completed the project plan will be reviewed to meet the final submission date of the thesis as mentioned earlier in the deliverables section. To conclude the test to be carried out for this purpose will be a 3 point bend test and 4 point bend test and thus the results will be concluded.
Dag Linghoff (2009), Thin Walled Structures, Carbon-fibre Composites for strengthening steel structures, volume 47 pages 1048-1058.
U.Meier (1995), Construction and Building Materials, Strengthening of structures using carbon fibre epoxy composites, volume 9 issue 6 pages 341-351.
A.R.Rahai and M.M.Alinia (2008), Construction and Building Materials, Performance Evaluation and Strengthening of concrete structures with composite bracing members, volume 22 issue 10 pages 2100-2110
J.G.Broughton (1997), International Journal of Adhesion and Adhesives, Carbon fibre reinforced plastic(CFRP) strengthening of aluminium extrusions volume 17 issue 3 pages 269-278
Osman Hag Elsafi (2001), Composite Structures, Application of FRP laminates for strengthening of reinforced concrete T-beam bridge structure, volume 52 issue 3-4 pages 453-466
R El-Hacha (2001), Progress in Structural Engineering and Materials, Pre-stressed fibre reinforced polymer laminates for strengthening structures volume 3 issue 2 pages 111-121 [Internet] available at http://onlinelibrary.wiley.com/doi/10.1002/pse.76/abstract accessed on 22nd November 2010.
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Bjorn Taljsten, (2003) Construction and Building Materials, Strengthening concrete beams for shear with CFRP sheets volume 17pages 15-26.
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Mr H.Nordin, Prof B. Taljsten and Tech Lic.A.Carolin, Department of Civil Engineering, Lulea University of Technology, CFRP Near Surface Mounted Reinforcement for pre-stressing concretes available at http://www.quakewrap.com/frp%20papers/CFRP-Near-Surface-Mounted-Reinforcement-(NSMR)-For-Pre-Stressing-Concrete-Beams.pdf accessed on 19 November 2010.
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