Saturday, December 18, 2010

STRENGTHENING OF CONCRETE STRUCTURES USING FIBRE REINFORCED POLYMER

1. INTRODUCTION

FRP composite materials hold great promise for the future of construction industry. Strengthening of reinforced concrete and prestressed concrete structures may be required as a result of increase in service loads, change in usage pattern, structural degradation and defects in design or construction. Repair with externally bonded FRP reinforcement is a highly practical strengthening system, because of ease and speed of installation, efficiency of the structural repair and corrosion resistance of the materials. The application of FRP poses minimal modification to the geometry, aesthetics and utility of the structure. Several studies on the behavior of reinforced concrete beams strengthened with FRP composite sheets provided valuable information regarding the strength, deformation, ductility and long-term performance of the FRP strengthening systems. The advantages of using FRP include light weight, ease of installation, minimal labour costs and site constraints, high strength-to-weight and durability. FRP plating is a versatile technique which can be applied equally well for existing RC beams and new ones. Plating of FRP laminates results in increase of composite moment of inertia of the section, thus making it behave with more stiffness after plating.

2. FIBRE REINFORCED POLYMER

Fibre reinforced polymer is a composite material generally consisting of a fibre in a polymeric resin matrix.FRP composites are as the name suggests, a composition of two or more materials which, when properly combined, form a different material with properties not available from the ingredients alone. Depending on the ingredients chosen and the method of combining them, properties of FRP can be controlled. Reinforced Concrete is a good example of a composite. The steel rebars provide excellent tensile strength and the concrete provides compressive strength and transfers the load between the steel bars. The polymer industry matured in the late 1970’s when world polymer production surpassed that of steel. Fibre reinforced polymers have been a significant aspect of this industry since the beginning.

2.1 Ingredients Of FRP

The major constituents of FRP are the fibre and the resin. The mechanical properties of FRP are controlled by the type of fibre and the durability characteristics are affected by the type of resin. The three important categories of fibre used in FRP are

2.1.1 Glass Fibre

FRP use textile glass fibres, textile fibres are different from other forms of glass fibres used for insulating applications. Textile glass fibres begin as varying combinations of SiO2, Al2O3, B2O3, CaO,MgO in powder form. These mixtures are then heated through a direct melt process to temperatures around 1300 degree Celsius, after which dies are used to extrude filaments of glass fibre in diameter ranging from 9 to 17 micrometer. These filaments are then wound into larger threads and spun onto bobbins for transportation and further processing. Glass fibre is by far the most popular means to reinforce plastic and thus enjoys a wealth of production processes, some of which are applicable to aramid and carbon fibres as well owing to their shared fibrous qualities.

2.1.2 Carbon Fibre

Carbon fibres are created when polyacrylonitrile fibres (PAN), Pitch resins, or Rayon are carbonized at high temperatures. Through further processes of graphitizing or stretching the fibres strength or elasticity can be enhanced respectively. Carbon fibres are manufactured in diameters analogous to glass fibres with diameters ranging from 9 to 17 micrometer. These fibres wound into larger threads for transportation and further production processes. Further production processes include weaving or braiding into carbon fabrics, cloths and mats analogous to those described for glass that can then be used in actual reinforcement processes.

2.1.3 Aramid Fibre

Aramid fibres are most commonly known Kevlar, Nomex and Technora. Aramids are generally prepared by the reaction between an amine group and a carboxylic acid halide group(aramid).commonly this occurs when an aromatic polyamide is spun from a liquid concentration of sulphuric acid into a crystallized fibre. Fibres are then spun into larger threads in order to weave into large ropes or woven fabrics. Aramid fibres are manufactured with varying grades to based on varying qualities for strength and rigidity, so that the material can be somewhat tailored to specific design needs concerns, such as cutting the tough material during manufacture.

2.1.4 Resin

There are two basic types of polymers: thermoplastic and thermoset. In general, FRP composites utilize a thermoset plastic. A plastic in which the polymer molecules are not crosslinked is a thermoplastic. Since the molecules are not connected by crosslinks, it allows the molecules to spread farther apart when the plastic is heated. This is the basic characteristic of a thermoplastic; the plastic will soften, melt, or flow when heat is applied. Melting the plastic and allowing it to cool within a mold will form the finished product. Typical thermoplastics are: polyethylene (PE) used in making garbage bags; polyvinyl chloride (PVC) used for house siding. A plastic in which the polymer molecules are crosslinked (chemically bonded) with another set of molecules to form a "net like" or "ladder-like" structure is a thermoset. Once crosslinking has occurred, a thermoset plastic does not soften, melt, or flow when heated. However, if the crosslinking occurs within a mold, the shape of the mold will be formed. Typical thermoset plastics are: unsaturated polyester (UP) used for bowling balls and boats; epoxy– used for adhesives and coatings; and polyurethanes (PURs) used in foams and coatings. In addition to these basic characteristics, polymers provide the FRP composite designer with a myriad of characteristics that can be selected, depending on the application. Combined with reinforcement of the polymer matrix, a vast range of characteristics are available for FRP composites. The matrix must also meet certain requirements in order to first be suitable for the FRP process and ensure a successful reinforcement of itself. The matrix must be able to properly saturate and bond with the fibres within a suitable curing period. The matrix should preferably bond chemically with the fibre reinforcement for maximum adhesion. The matrix must also completely envelope the fibres to protect them from cuts and notches that would reduce their strength and to transfer forces to the fibres. The fibres must also be kept separate from each other so that if failure occurs it is localized as much as possible and if failure occurs the matrix must also debond from the fibre for similar reasons. Finally the matrix should be of a plastic that remains chemically and physically stable during and after reinforcement and moulding processes. To be suitable for reinforcement material fibre additives must increase the tensile strength and modulus of elasticity of the matrix. Moreover the strength and rigidity of fibres itself must exceed the strength and rigidity of the matrix alone and there must be optimum bonding between fibres and matrix. Frp products are extremely durable versus many traditional products. The thermosetting resin properties provide chemical, moisture, and temperature resistance, while the fiber reinforcement increases strength and provides good performance over a wide temperature range.

3. MANUFACTURING OF FRP

There are two distinct categories of moulding processes using FRP; this includes composite moulding and wet moulding. Composite moulding uses Prefab system, meaning the plastics are fibre reinforced before being put through further through the moulding process. Sheets of Prefab FRP are heated or compressed in different ways to create geometric shapes. Wet moulding combines fibre reinforcement and the matrix during the moulding process. The commonly used types of producing FRP are

3.1. Pultrusion

Fibre bundles and slit fabrics are pulled through a wet bath of resin and formed into the rough part shape. Saturated material is extruded from a heated closed die curing while being continuously pulled through die. This process is mainly used for roadside reflector poles and ladder rails.

3.2 Filament Winding

Machines pull fibre bundles through a wet bath of resin and wound over a rotating steel mandrel in specific orientations. Parts are cured either in room temperature or elevated temperatures. Mandrel is extracted, leaving a final geometric shape but can be left in some cases.

4. APPLICATIONS OF FRP

Fibre reinforced polymers are best suited for any design program that demands weight savings, precision engineering, finite tolerances and the simplification of parts in both production and operation. A moulded polymer artifact is cheaper, faster and easier to manufacture than cast aluminium or steel artifact and maintains similar and sometimes better tolerances and material strengths. The Mitsubhishi Lancer Evolution IV also used FRP for its spoiler material. Utility structures: Utility structures like poles, crossarms, sports materials, car accessories are nowadays made of FRP.

4.1 Structural Strengthening

FRP can be applied to strengthen the beams, columns and slabs in buildings. It is possible to increase strength of these structural members even after these have been severely damaged due to loading conditions. For strengthening beams, two techniques are adopted. First one is to paste FRP plates to the bottom of a beam. This increases the strength of beam, deflection capacity of beam and stiffness. Alternately, FRP strips can be pasted in 'U' shape around the sides and bottom of a beam, resulting in higher shear resistance. Columns in building can be wrapped with FRP for achieving higher strength. This is called wrapping of columns. The technique works by restraining the lateral expansion of the column. Slabs may be strengthened by pasting FRP strips at their bottom. This will result in better performance, since the tensile resistance of slabs is supplemented by the tensile strength of FRP. In the case of beams and slabs, the effectiveness of FRP strengthening depends on the performance of the resin chosen for bonding.

5. CASE STUDY 1

5.1 Test Plan

Experimental investigation was carried out on fifteen beam specimens having three steel ratios, wrap thicknesses and wrap materials. The specimens were tested under fourpoint bending. Sufficient data was obtained for GFRP laminated as well as control beams. The details of the fifteen specimens prepared for experimental work are shown in Table 1.

5.2 Materials And Methods

M20 grade concrete was used for casting the specimens. The designed mix proportion was 1:1.54:3.01 with water cement ratio of 0.5. Fine aggregates passing through 4.75 mm IS sieve and coarse aggregates passing through 20 mm IS sieves were used for concreting. The compressive strength of cubes tested after 28 days was 23.54 MPa. Glass fibre types such as Chopped Strand Mat and Woven Rovings were used for this investigation. A tilting type drawn mixer was used for mixing fresh concrete. The cement, sand and coarse aggregate were placed inside the wet drawn and then dry mixed. Concrete was placed in three layers up to the top of rectangular beam and compacted. Curing was carried for a period of 28 days. The soffit of the rectangular beam was well cleaned with a wire brush and roughened with a surface-grinding machine. Two part epoxy adhesive consisting of epoxy resin and silica filler was used to paste the FRP laminates. The glue was spread over the pasting surface with the help of a spread. A thickness of more than 2 mm was maintained throughout the length of the pasting area. The laminate was pasted by gently pressing it by hand from one end and solely moving toward the other end. A nominal weight to keep in position was placed over the laminates. The oozing out compound was removed. The final thickness of the glue will be around 2 mm thick. The beam is left undisturbed for one week and then subjected to testing.

Table 1: Specimen Specifications

Sl.No

Beam Designation

%steel reinforcement

Type of GFRP

Thickness of GFRP

Composite Ratio(Area of FRP/Area of steel)

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

SR1

SR1CSM3

SR1CSM5

SR1WR3

SR1WR5

SR2

SR2CSM3

SR2CSM5

SR2WR3

SR2WR5

SR3

SR3CSM3

SR3CSM5

SR3WR3

SR3WR5

.419

.419

.419

.419

.419

.603

.603

.603

.603

.603

.905

.905

.905

.905

.905

-

CSM

CSM

WR

WR

-

CSM

CSM

WR

WR

-

CSM

CSM

WR

WR

-

3

5

3

5

-

3

5

3

5

-

3

5

3

5

-

2.387

3.979

2.387

3.979

-

1.562

2.653

1.562

2.653

-

1.231

2.051

1.231

2.051

CSM-Chopped Strand Mat; WR-Woven Rovings

5.3 Test Procedure

The beams were tested under two point loading after curing for 28 days at room temperature. All the beams were tested in a loading frame of 750 KN capacities and 100 mm bearing was given on both ends, resulting in a effective test span of 2800 mm as shown in Fig.1

Fig.1 Details of test beam

The deflections were measured at midspan and load points using mechanical dial gauges of 0.01 mm accuracy. The crack widths were measured using a crack defection microscope with a least count of 0.02 mm. The deflections and crack width were measured at different load levels until failure.

Fig.2 Test Setup of Case study 1

5.4 Results

The effect of steel ratio on any property was evaluated by comparing the performance levels of beams with steel ratio 0.603% and 0.905% against the beam with steel ratio of 0.419%. The effect of thickness of GFRP plate on performance parameters was measured by comparing the performance of plated beam with that of unplated beam having the same steel ratio. For ascertaining the effect of type of fibre on the performance of beams, comparison was made between CSM and WR plated beams of similar thickness plating.

Strengthening of reinforced concrete beams using GFRP plates resulted in higher load carrying capacity for beams. The WRGFRP plating showed higher improvement in first crack load, while the CSMGFRP showed limited enhancement in first crack load. The increase in first crack loads was up to 88.89% for 3 mm thick WRGFRP plates and 100.00% for 5 mm WRGFRP plated beams.

The deflections at which first cracks appeared at the tension zone of the beams were higher for GFRP plated beams. The maximum reductions in first crack load were up to 50.59% for 3 mm thick plating and up to 58.59% for 5 mm thick plating. Yield loads increased substantially due to the bonding of GFRP plates. The increase level of achieved by WRGFRP plates was higher than those achieved by CSMGFRP plates. The increase in yield load was up to 40.00% for 3 mm thick CSMGFRP and 128.57% for 5 mm CSMGFRP, 103.33% for 3 mm WRGFRP and 200.00% for 5mm WRGFRP plating. Yield deflection values were marginally lower for GFRP plated beams compared to the unplated beams. The reduction in yield deflection ranged from 7.99-28.03% for 3 mm GFRP plated beams and from 5.19% to 28.54% for 5 mm GFRP plated beams. The increase in ultimate load ranged from 28.57-40.00% for 3 mm CSMGFRP and 28.57-128.57% for 5 mm CSMGFRP plating. WRGFRP plating resulted in substantially higher

ultimate load levels compared to CSMGFRP plating. Increase of ultimate strength ranged from 42.86-103.33% for 3 mm WRGFRP plating and from 60.00-200.00% for 5 mm WRGFRP plating.

Fig.3 Yield Load

Fig.4 Ultimate Load

6. CASE STUDY 2

6.1 Test Plan

Beams bonded with four different types of Glass Fibre Reinforced Polymer (GFRP) having 3.50 mm thickness were used. Totally five rectangular beams of 3 m length were cast. One beam was used as reference beam and the remaining beams were provided with GFRP laminates on their soffit. The variable considered for the study is type of GFRP laminate. The study parameters of this investigation included first crack load, yield load, ultimate load, first crack deflection, yield deflection, ultimate deflection, crack width. The main objectives of the study were:

To study the impact of externally bonded Chopped Strand Mat (CSM), Woven Roving (WR), CSMWRGFRP and Uni-directional (UD) GFRP laminates on strength and deformation of the test beams and to examine the composite action of the GFRP laminates at all load levels; and to understand the associated cracking and failure.

The significance of the research is that in a developing country like India, the cost of FRP system is also a major concern. Since the cost of GFRP is the lowest and since it is the most commonly available material GFRP was considered suitable for the study. Hence, this research study investigated the characteristics of RC rectangular beams strengthened with externally mounted GFRP laminates.

6.2 Materials And Methods

Cement concrete having characteristic compressive strength of 33.50 MPa was used for casting the beams. The longitudinal steel reinforcement was provided using Fe 415 grade steel rods and shear stirrups were provided using Fe 250 grade steel rods of 8 mm diameter. The tensile steel reinforcements were provided at 0.40% of the gross cross sectional area of the beam.

6.3 Test Procedure

A total of five reinforced concrete beams were cast. One without plating and four with CSMGFRP, WRGFRP, Uni-directional GFRP and combination of CSMWRGFRP plating of 3.5 mm thickness. The details of the specimen are presented in Table-2.

Table 2: Specimen specifications

Fig.6 Test Setup of Case Study 2

The beams were tested under four point bending by applying two equal loads dividing the span into three equal parts. Deflectometers were fixed at the mid span and below the loading points to measure the deflection. Two deflectometers were fixed on top of the beam near a support at a spacing of 100 mm in order to measure the curvature. The load was applied through a hydraulic jack placed on top of a spreader beam. The failure of reference beams without any GFRP plating was preceded by high levels of deformation after yield point.

Fig.7 Load deflection behaviour

Table 3: Loads, deflections and crack width at salient stages

In all the cases, the beams with GFRP plating reached higher load levels. The stiffness of the GFRP plated beams was higher than that of the unplated beams, resulting in higher load carrying capacity at lower deformation levels.

6.4 Results

For WRGFRP plated beams, the first crack loads showed increase of 71.43% over the corresponding reference specimens. The increase in yield load was higher for WRGFRP plated beams when compared to the CSMGFRP plated beams. The application of WR fibre reinforced laminate resulted in higher ultimate strength values compared to CSM reinforced laminates. The ultimate load for GFRP plated RC beams increased by a maximum of 42.84% for SRWRGFRP plated beam, by 71.40% for SRUDGFRP plated beam and by 85.70% for SRCSMWRGFRP plated beam, when compared to the reference beam. The type of GFRP influenced the performance of the GFRP plated beams. SRUDGFRP resulted in better performance when compared to SRCSMGFRP.

7. CONCLUSIONS

FRP can be applied for strengthening a variety of structural members like beams, columns, slabs and masonry walls. Beams and slabs may be strengthened in flexure by bonding FRP strips at the soffit portion along the axis of bending. Shear strengthening of beams may be achieved by bonding vertical or inclined strips of FRP at the side faces of beams. Strengthening of beams in both flexure and shear may be achieved by wrapping around the cross section of beams in U-shape.

Fibre Reinforced Polymer (FRP) composite materials have been successfully used in new construction and for rehabilitation of existing structures. FRP composite materials hold great promise for the future of construction industry.

8. REFERENCES

1. Pannirselvam N, Nagaradjane V., Chandramouli K., (2009) Strength Behaviour Of Fibre Reinforced Polymer Strengthened Beam, Asian Research Publishing Network Journal Of Engineering And Applied Sciences, 9, 4, pp 35-39

2. Pannirselvam N., Raghunath P.N., Suguna K., (2008) Strength Modeling Of Reinforced Concrete Beam With Externally Bonded Fibre Reinforcement Polymer Reinforcement, American Journal Of Engineering And Applied Sciences, 1, 3, pp 192-199

3. Goli Nossoni, Ronald Harichandran S., (2010) Improved Repair Of Concrete Structures Using Polymer Concrete Patch And FRP Overlay, Journal Of Materials In Civil Engineering, 4, 22, pp 314-322

4. ShaoY., Wu Z.S., Bian J., (2005) Wet Bonding between FRP Laminates And Cast In Place Concrete, International Symposium On Bond Behaviour of FRP In Structures.

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