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Retrofitting And Strengthening Reinforced Engineering Essay

Introduction: - In recent years the construction industry has seen an increased demand to reinstate, rejuvenate, strengthen and upgrade existing concrete structures (Garg and Rajesh Kumar, 2004). Although hundreds of thousands of successful reinforced concrete and masonry buildings are annually constructed worldwide, there are large numbers of concrete and masonry structures that deteriorate, or become unsafe due to changes in loading, changes in use, or changes in configuration. Old structures designed for gravity loads are not able to withstand seismic forces and caused wide spread damages. Repair of these structures with like materials is often difficult, expensive, hazardous and disruptive to the operations of the building. Therefore in recent years there is vast use of fibre reinforced polymer which has proved to be less expensive and more durable.

Fibre Reinforced Polymer (FRP) is a composite made of high strength fibres and a matrix for binding these fibres to fabricate structural shapes. Development of FRP materials in various forms and configurations offers an alternative design approach for construction of new and rehabilitation of the existing civil Structures. The first use of FRP products was in reinforced concrete structures in the mid-1950. Since their early application, many FRP materials with different types of fibres have been developed. FRP product can take the form of the bars, cables, two and three dimensional grids, sheet materials and laminates. FRP product may achieve the same or better reinforcement objectives as commonly used metallic products such as steel reinforcing bars, prestressing tendons, and bonded plates. There are various types of fibres available in the market for wrapping technique. The following 3 are the most commonly used,

Glass Fibre

Electrical Glass (E-Glass)

Structural Glass (S-Glass)

Chemical Glass (C-Glass)

Aramid Fibre

Carbon Fibre

Fig. Fibre Composite Materials (Ash Ashbee, 1989)

Arguments:-

Glass Fibre:-

Nanni, A., (Ed. 1993) Glass fibres are silica based glass compounds that contain several metal oxides which can be tailored to create different types of glass. The main oxide is silica in the form of silica sand; the other oxides such as calcium, sodium and aluminium are incorporated to reduce the melting temperature and impede crystallization. Glass fibre is the most common fibre used in FRPC because its low cost, high strength, high chemical resistance, and good insulating properties. Despite being widely used in marine applications, glass fibre is subjected to strength loss under moisture and load.

The most important grades of glass are:

E-glass: has low alkali content of the order of 2%. It is used for general purpose structural applications and is the major one used in the construction industry; it also has good heat and electrical resistance.

S-glass: is a stronger and stiffer fibre with a greater corrosion

resistance than the E-glass fibre. It has good heat resistance. The S-2-glass has the same glass composition as S-glass but differs in its coating. The S-2-glass has good resistance to acids such as hydrochloric, nitric and sulphuric acids.

Aramid Fibre ( Kevlar)

(AZom.com, 2002) Aramid fibre is an aromatic polyamide, better known by trade name such as Kevlar (DuPont). Aramid has the lowest specific gravity and the highest tensile strength to weight ratio among other reinforced fibres used. Aramid fibre is 43% lighter then glass and around 20% lighter than carbon fibres. It also offers good resistance to abrasion and impact, as well as chemical and thermal degradation.

Some applications for aramid fibres are listed below. It is usually used as fibre reinforcement for polymer matrix composites.

Ballistic protective applications such as bullet proof vests.

Protective apparel such as gloves, motorcycle protective clothing and hunting gaitors, chaps and pants.

Sails for sailboats, yachts etc.

Belts and hosing for industrial and automotive applications.

Aircraft body parts.

Boat hulls.

Fibre optic and electromechanical cables.

Friction linings such as clutch plates and brake pads.

Gaskets for high temperature and pressure applications.

Adhesives and sealants

Carbon Fibres:-

FRPCs offer many advantages over other materials used in construction and rehabilitation (Mukherjee, A., and Joshi, M., 2001) as follows:

FRPC are non- metallic. Therefore, they are resistant to corrosion.

They have high strength to weight ratio. Therefore, for the same strength FRPC is considerably lighter. This eliminates requirements of heavy construction equipment and supporting structures.

FRPC are available in rolls of very long length. Therefore, they need very few joints, avoiding laps and slices. Its transportation is also very easy.

They have a short curing time; therefore, the application takes a shorter time, this reduces the project duration and down time of the structure to a great extent.

Application of FRPC does not require bulky and dusty materials in a large quantity, therefore, the site remains tidier.

FRPC have high ultimate strain; therefore they offer ductility to the structure and they are suitable for earthquake resistant application.

They have high fatigue resistance. So they do not degrade, which easily alleviates the requirement of frequent maintenance.

They have low thermal conductivity.

They are bad conductors of electricity and are non-magnetic.

Due to their lightweight prefabricated components, they can be easily transported. They encourage prefabricated construction; reduce site erection, labour cost and capital investment requirement.

(Nanni, A., (Ed), 1993):- Carbon Fibre Reinforced Polymer (CFRP) is a more costly material than its counterparts also used in the construction industry: glass Fibre reinforced polymer (GFRP) and aramid Fibre reinforced polymer (AFRP), though CFRP is generally regarded as having superior properties. Carbon Fibre reinforced plastic has over the past two decades become an increasingly notable material used in structural engineering applications. Its use in industry can be either for retrofitting: to strengthen an existing structure, or as an alternative reinforcing to steel from the outset of a project.

Since it is the most widely used Fibre in strengthening, the detailed explanation is given further. Studied in an academic context as to its potential benefits in construction, it has also proved itself cost-effective in a number of field applications strengthening concrete, masonry, steel and timber structures. Retrofitting has become the increasingly dominant use of the material in civil engineering, and applications include increasing the load capacity of old structures (such as bridges) that were designed to tolerate far lower service loads than they are to today, seismic retrofitting and repair of damaged structures. Retrofitting is popular in many instances as the cost of replacing the deficient structure can greatly exceed its strengthening using Carbon Fibre Reinforced Polymer (CFRP).

Critical Appraisal:-

Benefits of FRPs over traditional construction materials:

Low density

Good Durability Characteristics

Adaptability in Relation to finishes

Aesthetics

Ability to meet special requirements relating to resistance and conductivity

Corrosion resistance

(Nanni, A., (Ed), 1993) Author emphasis more on use of Carbon Fibre Reinforced Polymer (CFRP) as it is more durable, easy to apply, high in tensile strength. Although CFRP is more expensive than other fibre reinforced polymer it proved to be more effective material especially in retrofitting. Due to the incredible stiffness of CFRP, it can be used underneath bridge spans to help prevent excessive deflections, or wrapped around beams to limit shear stresses. When used as a replacement to steel, CFRP sheets are used to reinforce concrete structures. More commonly they are used as prestressing materials due to their high stiffness and strength. The advantages or CFRP over steel as a prestressing material, namely its light weight and corrosion resistance, enable the material to be used for niche applications such as in offshore environments.

Glass Fibre: Good Compressive strength and stiffness, good tensile strength and excellent electrical properties at relatively low cost. It’s economical

Aramid Fibre: High Strength, good impact, high abrasion resistance and medium stiffness.

Carbon Fibre: Very high strength in both tensile and compression, high stiffness and high resistance to corrosion, creep and fatigue, but poor impact strength. It’s expensive.

Property

Aramid

Carbon

Glass

High Tensile Strength

B

A

B

High Tensile Modulus

B

A

C

High Compressive Strength

C

A

B

High Compressive Modulus

B

A

C

High Flexural Strength

C

A

B

High Flexural Modulus

B

A

C

High Impact Strength

A

C

B

High Interlaminar Shear Strength

B

A

A

High In-plane Shear Strength

B

A

A

Low Density

A

B

C

High Fatigue Resistance

B

A

C

High Fire Resistance

A

C

A

High Thermal Insulation

A

C

B

High Electrical Insulation

B

C

A

Low Thermal Expansion

A

A

A

Low Cost

C

C

A

(Key: C=Poor, B=Good, A=Excellent)

Due to the incredible stiffness of CFRP, it can be used underneath bridge spans to help prevent excessive deflections, or wrapped around beams to limit shear stresses. When used as a replacement to steel, CFRP sheets are used to reinforce concrete structures. More commonly they are used as prestressing materials due to their high stiffness and strength. The advantages or CFRP over steel as a prestressing material, namely its light weight and corrosion resistance, enable the material to be used for niche applications such as in offshore environments.

Much research continues to be done on using CFRP both for retrofitting and as an alternative to steel as a reinforcing or prestressing material. Cost remains an issue and long term durability questions still remain. Some are concerned about the brittle nature of the material, in contrast to the ductility of steel.

One of the main advantages of applying CFRP wraps is that it significantly decreases corrosion activity when applied over the entire specimen. The application of the wraps before corrosion propagation will prevent corrosion from taking place, while the application of the wraps after corrosion occurrence will drop the rate of corrosion sharply. This effect is probably a virtue of the epoxy saturant used to apply the CFRP sheets and not of the fibres themselves.


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