Advanced Sesmic Retrofitting Technique Fiber Composite

PAPER PRESENTATION

ON

“ADVANCED SESMIC RETROFITTING TECHNIQUE – FIBER COMPOSITE”

By

NAGA RAJU.P KOTESWARARAO.R

III/IV ME III/IV ME

Email :

DEPARTMENT OF MECHANICAL ENGINEERING,

GUDLAVALLERUENGINEERINGCOLLEGE,

GUDLAVALLERU-521356, KRISHNA Dist.

TO

VIGNANENGINEERINGCOLLEGE

‘ VIGNAN MAHOSTAV-06’

vadlamudi,Guntur

ABSTRACT:

This paper discusses a novel technique of rehabilitation of earthquake-affected structures and retrofitting of structures against possible earthquakes using fiber composites. This technique has been successfully applied in earthquake-affected Gujarat; it introduces high-strength non-metallic fibers along with polymeric resins in repair. As non-metallic fibers are hitherto unused in structural repairs in India, a brief account on these materials has been included. Design methods, field application techniques and their suitability have also been discussed.

The Gujarat earthquake on January 26, 2001 has caused widespread damage of structures and a substantial portion of them require extensive structural rehabilitation. The structural rehabilitation community is in search of techniques that are reliable, fast, cost effective and easy to implement. In addition, the earthquake has exposed the vulnerability of the existing structures, especially in highly seismic regions. A large number of unaffected structures in the region require retrofitting to avoid future loss of property. A vast majority of these structures is reinforced concrete (RC) buildings. Existing practices of repair go little beyond cosmetic treatment of the structure. Such methods neither strengthen the structure nor extend its life. This paper discusses a novel rehabilitation and retrofitting technique that has been successfully implemented in rehabilitation and seismic qualification of RC buildings in the Gujarat region. The method has been in use in other seismically active regions of the world.

INTRODUCTION:

An earthquake generates ground motion in both the horizontal and the vertical directions. Due to the inertia of the structure, the ground motion generates shear force and bending moments in the structural framework. Most failures in earthquake-affected structures are observed at the joints. Moreover, due to existing construction practice, a construction point is placed in the column very close to the beam-column joint, Fig. 1(a). This leads to shear or bending failure at or very close to the joint. The onset of high bending moments may cause yielding or buckling of the steel reinforcement. The high compressive stress in concrete may also cause crushing of the concrete. If the concrete lacks confinement, the joint may disintegrate and the concrete may spall, Fig. 1(b) and (c). All these create a hinge at the joint and if the number of hinges is more than the maximum allowed to maintain the stability of the structure, the entire structure may collapse. If the shear reinforcement in the beam is insufficient, there may be diagonal cracks near the joints, Fig. 1(d). This may also lead to failure. Bond failure is also observed in cases where lap splices are too close to the joints.

The conventional strengthening methods for reinforced concrete structures attempt to compensate the lost strength by adding more material around the vulnerable sections. These methods (column retrofitting by concrete and steel jacketing, L & T beam retrofitting, foundation by rebar) include section enlargement, polymer modified concrete filling and polymer grouting.

The methods that involve concrete in strengthening are time consuming, dusty and laborious. They require a long time to implement, and therefore, a longer period of evacuation. They also increase the dead load on the structure. In some cases, especially in bridges, external post-tensioning bonded steel plates and steel jacketing have been used. These techniques often apply steel reinforcement that remains exposed to environmental attack. Therefore, they are vulnerable to corrosion that limits their lives. Moreover, the quality of the strengthening depends heavily upon the skill of the personnel. It is difficult to strengthen complex areas such as beam-column connections using these methods.

Recent developments in Fiber Reinforced Composites (FRC) can solve many of these problems. These materials are extremely strong, with high ultimate strain. They are chemically inert and corrosion resistant. Moreover, they are very light and that facilitates easy implementation at site with less supporting structures. These methods are cleaner and the materials used cure very quickly. This leads to shorter down time of the affected structure. As these materials are relatively new to concrete users, a brief description has been given below.

1. FIBRE REINFORCED COMPOSITES:

FRCs have two components-matrix and fiber, Fig. 1. In the present context, thermosetting resins like epoxy or polyethylene are used as matrix, while aramid, carbon and glass fibres reinforce the matrix and lend strength to the composite. The resin coheres and gives shape to the object, while fibres reinforce it. The result of such combination is a light, flexible and strong composite material.

Unlike conventional materials, composites are not homogeneous. Their properties are dependent on position and angle under consideration. Generally, composites are elastic up to failure and exhibit no yield point or region of plasticity. The properties are dependent on fibre and matrix, their relative quantity and orientation of fibre.

If all the fibres are aligned in one direction then the composite becomes very stiff and strong in that direction but it will have low strength and low modulus in the transverse direction.

Due to their malleability, fibre reinforced plastics are easy to fabricate. Recent developments in this field have indicated that they can be used as highly efficient construction materials in various civil engineering activities. Fibre Reinforced Polymer Composites (FRPC) have already been successfully used in industries like aerospace, automobile and shipbuilding. Recently, civil engineers and construction industry have begun to realize that these materials have potential to provide remedies for many problems associated with the deterioration and strengthening of infrastructure. Effective use of these materials could significantly increase the life of structures, minimizing the maintenance requirements.

FRPC MATERIAL:

GLASS FIBRE:

E-glass fibre sheets that have a minimum tensile strength of 1700 MPa and an average elastic modulus of 75000 MPa with a density 900 g/m2 are used. Sheets of width 250 mm and 500mm and a length of 50 m were found to be convenient to use and they also resulted in very little wastage.

RESINS:

Resin impregnation is necessary to obtain good mechanical properties for glass fibre. For standard fibre wrapping, resin is impregnated at the construction site under ordinary temperature and pressure. One of the important properties regarding the workability of resin is optimum viscosity that simultaneously enables good impregnation into the fibres and keeps the fibres in place. A viscosity of around 1000 cps was found to be suitable.

2.FRPCs IN STRUCTURAL APPLICATIONS:

Fig.3 shows different applications of FRPCs in structures. It can be seen that composite materials are used in a variety of forms—both in new construction and repairs. However, in this paper the discussion is restricted to non-prestressed applications of FRPCs in repair and retrofitting of structures. This form is most interesting in the context of earthquake resistant constructions of Gujarat. In non-prestressed applications FRPCs can be used in the following forms.

Plates: - These pre-cured FRPC members are used mainly to increase the bending and shear capacity of concrete sections, Fig. 3(a). These sections are produced by pultrusion in factories with high reliability of performance. However, the shape of the FRPC element must be known at the time of its production. It is unsuitable when the FRPC element needs to be bent at site.

Bars: - These are also produced in factory by pultrusion Fig. 3(b) . These bars can be used as near-surface reinforcement with little risk of corrosion, as tension reinforcement in beams and slabs to replace the steel bars.

Sheets: - These are uncured fiber tapes with unidirectional fibers or bi-directional woven roving, Fig. 3(c). The main advantage of this form is that it can be laid in any form at site. Therefore, they are most suitable in wrapping around deteriorated concrete members. The main application of sheets is in wrapping around concrete sections to increase confinement and shear strength. However, their strength is not as reliable as that of the plates and the bars. The FRP sheets have been used most widely in Gujarat.

There are a few other less popular forms of FRPC such as grids, cells and honeycombs. These are beyond the scope of this paper.

3. REHABILITATION AND RETROFITTING WITH FRPC:

The two main advantages of FRPC in earthquake resistant applications are its high strength and high ultimate strain. Due to its high strain at failure, FRPC wrapped columns exhibit a high level of confinement and shear strength. Due to its corrosion resistance, FRPC can be applied on the surface of the structure without worrying about its deterioration due to environmental attack. As FRPC sheets are malleable, they can be wrapped around the joints very easily. An exhaustive test programme has been undertaken at the Indian Institute of Technology (IIT), Bombay to evaluate the efficacy of FRPC in structural strengthening, with collaboration from the PennsylvaniaStateUniversity and Cold Regions Research and Engineering Laboratory, USA. A detailed account of the research is beyond the scope of the present paper. However, the strengthening achieved using FRPC wrap is highlighted here. Fig. 4 presents a typical axial stress versus strain curve of cylindrical specimens wrapped with FRPC using a varying number of layers. It may be noted that with one layer of FRPC wrap, the ultimate strength of the specimens increased by a factor of 2.5. The ultimate strength went on to increase up to 8 times when 8 layers of the wrap were used. The ultimate strain increased by 6 times with one layer of wrap. This feature is particularly attractive for earthquake resistant structure. Due to higher ultimate strain the ductility of the structure also increases.

It may be noted that the ultimate strain of the specimens is insensitive to the number of layers of wrap. Therefore, for earthquake resistance a thin wrap that offers high ultimate strain but low stiffness is desirable. The unfavorable creep behavior of glass fibre does not pose problems in earthquake-resistant applications as earthquake forces are seldom encountered. Moreover, glass fibre is much less expensive than carbon fibre. Therefore, glass fibre has been used in rehabilitation and retrofitting of structures in Gujarat.

The resin must be able to hold all the fibres together. It is also important that the resin maintains a bond between the concrete and the FRP.

4. PREPARATION OF SUBSTRATE:

The procedure of fibre wrapping is shown in, Fig. 5. Before application of wrap, the substrate has to be prepared. In the case of damaged members, the first step is to rebuild the damaged member. The step in rebuilding consists of:

• removing all loose materials and exposing the concrete surface
• treating all internal cracks and voids with suitable grouts
• replacing the spelled concrete with epoxy mortar or epoxy concrete
• Preparing a smooth concrete surface that is suitable for wrapping.

One must remember that the FRPC layer is very thin. Therefore, it is extremely important to prepare a smooth convex surface of concrete before the wrapping is begun. The FRPC becomes ineffective if it is not in contact with the surface of concrete. Care must be taken to avoid wrinkles, voids and sheet deformation. Moreover, sharp edges and corners are potential zones of fibre breakage due to stress concentration. Therefore, all projections are removed and all corners are rounded off. A corner radius of 25mm is found sufficient to avoid stress concentration.

5. FIBRE SHEET WRAPPING:

After preparation of the surface a low viscosity primer is applied on the concrete surface to improve bond between the fibre sheet and the concrete, Fig. 7(a). Fibre sheets are cut to required sizes. An allowance for the length of lap joint must be given while cutting the sheets. The lap length is determined based on test results in the laboratory and the precision that can be maintained in construction. The cut fibre sheets are rolled on a circular spindle to make them easy for wrapping.

It is very important to choose the right epoxy resin for wrapping applications. The resin must be viscous enough to hold the fibres in place. On the other hand, the resin must wet the fibre thoroughly and there should not be any dry pockets. The viscosity of the resin, therefore, is a trade off between these two contradicting requirements. The resin is usually a two-part mix. The mixing of the parts must be thorough. The resin should not entrap air during mixing. Therefore, the speed of the stirrer and the duration of stirring are extremely important parameters. The mixed epoxy resin is applied on to the concrete surface that is to be wrapped.

There are two methods of laying dry lay up and wet lay up. In the dry lay up, the dry fibre sheet is applied on the concrete surface freshly coated with epoxy resin. In the wet lay up, the fibre sheet is wetted with epoxy resin before wrapping. Although wet lay up ensures a better wetting, it is not always convenient to use wet lay up, especially in the hot climate of Gujarat. Therefore, dry lay up has been used in the present work. The sheet should not be slack at the time of wrapping and care must be taken to maintain the intended fibre direction. The sheet is rolled by serrated Teflon rollers, so that the resin oozes out through the sheet and wets the sheet properly. Rolling must always be in the direction avoid anydefect in bond. Spreading some extra resin on the lap area is a good idea. The wrapping must be completed within the pot life period of the resin that is usually 20 to 30 minutes. Therefore, it is advisable to mix small quantities of resin at a time. A thin coat of resin is applied after the wrapping is over. After the resin is completely cured (usually 24 hours), the wrap is inspected to rule out any defect. A micaceous polyamide topcoat is applied on the wrapped surface to protect the resin from deterioration from exposure to ultraviolet rays. The wrapped column is shown in Fig.

6. STRENGTHENING OF BEAMS:

Due to the forces of earthquake, the beams may weaken in shear; bending or they may have crushing of the concrete due to a lack of confinement. Beams require separate treatments for strengthening the above aspects. While the treatment required improving confinement is largely the same as that for columns, the flexural and shear strengthening require separate discussion.

6.1 FLEXURAL STRENGTHENING:

Flexural strengthening of beams and slabs is necessary when the tension steel has yielded or it has deteriorated due to corrosion. Flexural members that are found to have inadequate reinforcement can also be strengthened by this method. In order to improve the flexural capacity of beams and slabs, continuous fiber sheets or plates are bonded to its tension and compression faces. This is the simplest method of improving flexural capacity of a structural member. However, the stiffness of the FRPC is of great importance in this case. The allowable transverse deflection of the flexural members is very small. As a result, we need a stiff FRPC layer for effective improvement of the flexural capacity. The bond between concrete and FRPC is also of immense importance here. Therefore, the adhesive must be chosen with great care.

The method of application of the FRPC in flexural strengthening, however, is the same as that in the case of wrapping. The only difficulty one faces in flexural strengthening is that often the application is overhead. To resist the displacement of FRPC due to gravitational forces, a thyrotrophic adhesive is often used. However, in Gujarat, the same glue that is used in wrapping has been used in flexural strengthening. The application of FRPC also impedes moisture ingress and further corrosion of steel.

6.2 SHEAR STRENGTHENING:

The shear capacities the beams can be improved by placing FRPC on the webs the beams. The same wrapping techniques as that given for columns is employed to strengthen the beam. Wherever possible, the beam is wrapped on all four sides. Along with improving the shear capacity, it improves the confinement the concrete. That, in turn, delays the failure of concrete. For T-beams, where full wrap is not possible due to obstruction from slab, U-wraps are provided. The method of application of shear wraps is identical to that of column wraps.

6.3 STRENGTHENING OF BEAM-COLUMN JOINTS:

In earthquake-affected structures, most of the failures are found at the beam-column junctions, and are combinations of the three primary types of failures discussed earlier. Therefore, a combination of all the above strengthening methods is to be used. Using FRPC sheets, a simple and fast method is developed and employed to strengthened beam-column connections. The step-by-step procedure is explained in fig. 9.