10-06-2017, 07:09 AM
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INTRODUCTION
In the last 200 years, rapid advances in construction materials technology have enabled civil engineers to achieve impressive gains in the safety, economy, and functionality of structures built to serve the common needs of society. Through such gains, the health and standard of living of individuals are improved. The use of FIBRE REINFORCED POLYMERS (FRP) as reinforcement for structures is rapidly gaining appeal. Fiber reinforced polymer (FRP) systems are high-strength, lightweight reinforcement in the form of paper-thin fabric sheets, thin laminates, or bars that are bonded to concrete members with epoxy adhesive to increase their load carrying capacity. The earliest Fibre Reinforced Polymer (FRP) materials used glass bres embedded in polymeric resins that were made available by the burgeoning petrochemical industry following World War II. The combination of high-strength, high-stiffness structural bres with low-cost, light weight, environmentally resistant polymers resulted in composite materials with mechanical properties and durability better than either of the constituents alone.
Fibre materials with higher strength, higher stiffness, and lower density, such as boron, carbon, and aramid, were commercialized to meet the higher performance challenges of space exploration and air travel in the 1960s and 1970s. At rst, composites made with these higher performing bres were too expensive to make much impact beyond niche applications in the aerospace and defence industries. Work had already begun in the 1970s, however, to lower the cost of high performance FRPs and promote substantial marketing opportunities in sporting goods. By the late 1980s and early 1990s, as the defence market waned, increased importance was placed by bre and FRP manufacturers on cost reduction for the continued growth of the FRP industry. As the cost of FRP materials continues to decrease and the need for aggressive infrastructure renewal becomes increasingly evident in the developed world, pressure has mounted for the use of these new materials to meet higher public expectations in terms of infrastructure functionality.
Aided by the growth in research and demonstration projects funded by industries and governments around the world during the late 1980s and throughout the 1990s, FRP materials are now nding wider acceptance in the characteristically conservative infrastructure construction industry. The existing applications of FRP reinforcement in buildings, bridges and tunnel linings have demonstrated that the FRP bonding technique is remarkably efficient. Due to the fact that structural strengthening is achieved by the interfacial stress transfer from FRP sheets to concrete matrix through the adhesive layer, in the recent years, many efforts have been made to study the interfacial bonding/debonding behaviors. The use of FRP composite materials represent an alternative to steel as it can avoid corrosion of steel. Their light weight, high strength-to-weight ratio, ease of handling and application, lack of requirement for heavy lifting and handling equipment, noncorrosive properties, speed and ease of installation, lower cost, and aesthetic appeal are some factors that are advantageous in repair, retrofitting and rehabilitation of civil engineering structures.
FRP COMPOSITE MATERIALS
General Introduction
In their broadest form, composites are materials consist of two or more constituents. The constituents are combined in such a way that they keep their individual physical phases and are not soluble in each other or not to form a new chemical compound. One constituent is called reinforcing phase and the one in which the reinforcing phase is embedded is called matrix. Historical or natural examples of composites are abundant namely brick made of clay reinforced with straw, mud wall with bamboo shoots, concrete, concrete reinforced with steel rebar, granite consisting of quartz, mica and feldspar, wood (cellulose fibres in lignin matrix), etc.
Composites consist of two or more phases that are usually processed separately and then bonded, resulting in properties that are different from those of either of the component materials. Polymer matrix composites generally combine high-strength, high-stiffness fibres (graphite, kevlar, etc.) with low-density matrix materials (epoxy, polyvinyl, etc.) to produce strong & stiff materials that are lightweight. Laminates are generally built up from multiple layers of lamina, the fibres within each lamina are generally parallel, but laminates usually contain lamina with their fibres oriented in various directions. Each lamina is an anisotropic layer with properties varying as a function of fibre angle.
A composite material is a material system composed of a mixture or combination of two or more micro or macro constituents that differ in form and chemical composition and which are essentially insoluble in each other. Composites involve two or more component materials that are generally combined in an attempt to improve material properties such as stiffness, strength, toughness, etc., the resulting properties are largely dependent on the distribution, relative amounts and geometries of the constituents.
Structure of Composites
Structure of a composite material determines its properties to a significant extent. The structure factors affecting properties of composites are as follows:
Bonding strength on the interface between the dispersed phase and matrix.
Shape of the dispersed phase inclusions (particles, flakes, fibres, laminates).
Orientation of the dispersed phase inclusions (random or preferred).
Interfacial bonding
Good bonding (adhesion) between matrix phase and dispersed phase provides transfer of load, applied to the material to the dispersed phase via the interface. Adhesion is necessary for achieving high level of mechanical properties of the composite. There are three forms of interface between the two phases:
1. Direct bonding with no intermediate layer. In this case adhesion ( wetting ) is provided by either covalent bonding or Van der Waals force.
2. Intermediate layer (inter-phase) is in form of solid solution of the matrix and dispersed phases constituents.
3. Intermediate layer is in form of a third bonding phase (adhesive).
Shape and orientation of dispersed phase
Importance of these structure parameters is confirmed by the fact, that one of the systems of classification of composites is based on them.
Particulate composites
Fibrous composites
Laminate composites