Fiber Reinforced Polymer Composites : A Novel Way for Strengthening Structures

To meet up the requirements of advanced infrastructure, new innovative materials/ technologies in construction industry has started to making its way. Any technology or material has its limitations. To meet the new requirements, new technologies have to be invented and put to use. With structures becoming old and the increasing bar corrosion, old buildings have started to demand additional retrofits to increase their durability and life.

Use of FRP for confinement has proved to be effective retrofitting and strengthening application. The confinement in seismically active regions has proved to be one of the early applications of FRP materials in infrastructure applications. Confinement may be beneficial in non-seismic zones too, where, for instance, survivability of explosive attacks is required or the axial load capacity of a column needs to be increased due to higher vertical loads, e.g. if new storey's have to be added to an existing building or if an existing bridge deck has to be widened. In any case, confinement with FRP may be provided by wrapping RC columns with prefabricated jackets or in situ cured sheets, in which the principal fiber direction is circumferential. Beams, Plates and columns may be strengthened in flexure through the use of FRP composites bonded to their tension zone using epoxy as a common adhesive for this purpose. The direction of fibers is parallel to that of high tensile stresses. Both prefabricated FRP strips, as well as sheets (wet-layup) are applied. Hence, FRP composites are finding ways to prove effective and economical at the same time.

Dr. Gopal Rai,
R & M International, and
Yogesh Indolia,
21 SHM Consultant, Mumbai

Introduction

In today's growing economy, Infrastructure development is also raising its pace. Many reinforced concrete and masonry buildings are constructed annually around the globe. With this, there are large number of them which deteriorate or become unsafe to use because of changes in use, changes in loading, change in design configuration, inferior building material used or natural calamities. Thus repairing and retrofitting these structures for safe usage of these structures has a great Market.

There are several situations in which a civil structure would require strengthening or rehabilitation due to lack of strength, stiffness, ductility and durability. Some common situations where a structure needs strengthening during its lifespan are:

• Seismic retrofit according to current code requirements.
• Upgraded loading requirements; damage by accidents and environmental conditions.
• Initial design flaws
• Change of usage.

Depending on the desired properties, usage and level of damage in structural members, these can be repaired and/or strengthened by several widely used methods some of widely used repair techniques are presented below.

Concrete jacketing can be applied to locally damaged or heavily damaged structures. When concrete is slightly damaged, the loose concrete is removed the surfaces are roughened and the dust is cleaned. Now depending on the amount of concrete removed, some additional ties or reinforcement can be added and jacketing is carried out i.e. new concrete is filled. Non shrinkage concrete or concrete with low shrinkage properties should be used. Special attention is paid to achieve a good bond between old and new concrete.

Jacketing should also be applied in cases of heavily damaged columns or in cases of insufficient column strength. This is actually a strengthening procedure but can be used for repair purposes. The additional concrete and reinforcement added contribute to increase in strength.

Concrete jacketing has a lot of limitations. The jacket should be of minimum 100mm thickness. The sizes of members are increased and the free available usable space becomes less. Also adding a huge dead mass and increasing the stiffness which reduces the efficiency of the structure. Its durability has also been often found to be limited. Furthermore the whole process is slow and takes lot of time for completion.

Jackets may also be made of steel. It is a popular technique to use steel plates bonded with epoxy to external surfaces of columns, beams and slabs. This technique is simple and effective as far as both cost and mechanical performance is concerned but suffers major disadvantages. Corrosion of steel plates hurdles its use in structures in/near river, lake and sea. Furthermore difficulty in manipulating heavy steel plates in tight construction sites, need for scaffolding, and limitations in available plate lengths which results in need of joints. Sometimes steel's high young's modulus causes it to take large portion of axial load resulting in premature buckling.

The conventional jackets, sheets, plates may be replaced with FRP fabrics, sheet and laminates in view of above limitations.

A Fiber Reinforced Polymer (FRP) composite is defined as a polymer (plastic) matrix, either thermo set or thermoplastic, that is reinforced (combined) with a fibre or other reinforcing material with a sufficient aspect ratio (length to thickness) to provide a discernable reinforcing function in one or more directions. FRP composites are different from traditional construction materials such as steel or aluminium. FRP composites are anisotropic (properties apparent in the direction of the applied load) whereas steel or aluminium is isotropic (uniform properties in all directions, independent of applied load). Therefore, FRP composite properties are directional, meaning that the best mechanical properties are in the direction of the fiber placement.

Composites are composed of:

• Epoxy - The primary functions of the resin are to transfer stress between the reinforcing fibers, act as a glue to hold the fibers together, and protect the fibers from mechanical and environmental damage. The most common resins used in the production of FRP grating are polyesters (including orthop- hthalic-"ortho" and isophthalic-"iso"), vinyl esters and phenolics.
• Reinforcements - The primary function of fibers or reinforcements is to carry load along the length of the fiber to provide strength and stiffness in one direction. Reinforcements can be oriented to provide tailored properties in the direction of the loads imparted on the end product. The largest volume reinforcement is glass fiber.
• Fillers - Fillers are used to improve performance and reduce the cost of a composite by lowering compound cost of the significantly more expensive resin and imparting benefits as shrinkage control, surface smoothness, and crack resistance.
• Additives - Additives and modifier ingredients expand the usefulness of polymers, enhance their processability or extend product durability.

The following are major pros and cons of using Composites: