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Engineers Use Fiber Reinforced Polymer to Strengthen Concrete Structures

A large number of structures were devastated during the earthquakes last year. Reconstruction of those structures is going on. The expensive process of maintaining, repairing and rebuilding infrastructures/structures has led owners and contractors to seek more efficient solutions to the problem by the use of Fiber Reinforced Polymer (FRPs). Light weight, resistant to corrosion, ease in installation and durability are the major reasons behind its popularity among the engineering professionals.

Illustration: Ratna Sagar Shrestha/THT

It consists of high strength fibers such as glass, steel wires embedded in a polymer matrix where the main fiber acts as a reinforcing element and polymer matrix such as epoxy resin acts as a binder in between them. They are rapidly becoming the material of choice over steel in reinforced concrete structures. FRP material can be prefabricated in the manufacturing agency in which it can be transformed into various forms and sizes which can be used in terms of strengthening applications bars, rods and plates, whereas on-site it is prepared by mixing  dry fabric from glass or carbon with resins such as epoxy which is bonded to prepare concrete substrate.  Multiple buildings and bridges are being constructed in large numbers  using FRP in recent days around the world because of its versatility and better working environment.  Field application covers a diverse range of structures over corrosive and cold climatic conditions. From the blade of large wind mills to doors of houses and other home renovating elements, FRP is spreading its charm. FRP doors are extremely strong and hard to break in. After curing, FRP behaves as an integral part of a structural system. Generally used fiber for FRPs is carbon, glass and aramid. Properties of final FRP product depend on the quality of fiber, shape, orientation, adhesion to the polymer matrix and the process of manufacturing.

The most common FRP system is carbon fiber based (CFRP). The matrix not only coats the fibers and protects them from various accidental mechanical abrasions, but also transfer stresses between the fibers. Thermosetting and thermoplastics are two types of polymer matrices widely used for FRP composites. Thermosetting polymers are more used than thermoplastics. Thermosetting polymers are processed in a liquid state to obtain good output of fibers. Some of them are polyesters, vinyl esters and epoxies. Prefabricated FRP elements are typically stiff due to which CFRP bar is limited to straight and slightly curved surfaces in terms of orientation. On the other side, FRP fiber is available on the rolls that can easily fit into any geometry and be wrapped in almost any type of profile.

In the context of bridge design, FRP is used where longer unsupported spans are desirable with reduced overall weight combined with increased strength with greater seismic resistance. On the other hand, FRP reinforced structure can reduce the cost of columns and foundations and can tackle the increasing demands of heavier possible traffic loads. It has been successfully applied for seismic upgrading of structures in a variety of genres. From beam-column joints, it’s applicable during shear failure in beam/columns, buckling of longitudinal steel bars in columns, energy dissipation capabilities of reinforced concrete structures and its overall performance regarding various temperatures and seismic attributes.

Reinforcing bars is another product that’s gaining a lot of interest in the field of designing and construction of safer and sound reliable structure. Surface of the FRP bars can be of various choices such as spiral, straight and deformed. The bond of these bars with concrete is equal to, or better than, the bond of steel. Property of resistant to corrosion has made it applicable in both interior and exterior surfaces in almost all favorable environmental conditions. Surface preparation is a must for the proper bond between FRP and pre-existing concrete. Before application of FRP system, the existing deterioration or corrosion in the concrete must be removed.  Failure in performing this will ultimately damage the FRP system because of delamination of concrete structure. In steel-reinforced concrete, reduction of crack width is a must in order to inhibit the corrosion of steel reinforcement.  In terms of FRP reinforced concrete this is not required due to the corrosion resistant property of FRP materials. If we evaluate this on a large scale, it has long-term advantageous behavior.

Adhesive is used with FRP including a primer, which is used to penetrate the concrete and improve bonding of the system. Resin such as epoxy is use in filling of the void in the concrete structure to give a finishing surface over which FRP is coated.  From handling and application point of view, FRP bars can be transferred to places with less amount of effort than conventional steel bars. Construction and application of FRP bar is similar to that of conventional steel reinforcement format. Because of low weight of FRP, casting of formwork of FRP is much less time-consuming than that of steel reinforcement. As we do not have our own repair and strengthening guidelines in Nepal, little information regarding applications of FRP is listed in Indian Standard codes. Numerous structures and infrastructures can be strengthened in a better way if devastated structures are technically supervised with their regular and proper implementation.


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