Use of Textile Material in earth quake Pron area

Nilesh Tharval

Abstract

The present paper highlights some of the important developments in the field of application of textile materials in earth quake resistance constructions. Cement concrete reinforced with Steel bars is an extremely popular construction material. One major flaw, namely its susceptibility to environmental attack, can severely reduce the strength and life of these structures. External reinforcements using steel plates have been used in earlier attempts to rehabilitate these structures. The most important problem that limited their wider application is corrosion: Recent developments in the field of fiber reinforced composites (FRCs) have resulted in the development of highly efficient construction materials. The (FRCs) are unaffected by electro-mechanical deterioration and can resist corrosive effects of acids, alkalis, salts and similar aggregates under a wide range of temperatures. This novel technique of rehabilitation is very effective and fast for earthquake affected structures and retrofitting of structures against possible earthquakes. This technique has been successfully applied in the earthquake-affected Gujarat.

Key Word: earthquake, Fiber reinforced composites, High Tenacity Polyester fiber, Glass fiber, PVC

Introduction

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. Also from the recent earthquake of Gujarat (2001) it is clear that the 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. The removal and transportation of large amounts of concrete and masonry material causes concentrations of weight, dust, excessive noise, and requires long periods of time to gain strength before the building can be reopened for service.

On the other hand, Fiber Reinforced Composite (FRC) materials, originally developed for the aerospace industry, are being considered for application to the repair of buildings due to their low-weight, ease of handling and rapid implementation. A major development effort is underway to adapt these materials to the repair of buildings and civil structures. Appropriate configurations of fiber and polymer matrix are being developed to resist the complex and multi-directional stress fields present in building structural members. At the same time, the large volumes of material required for building repair and the low cost of the traditional building materials create a mandate for economy in the selection of FRP materials for .building repair. Analytical procedures for reinforced and prestressed concrete and masonry reinforced with FRC materials need to be developed, validated, and implemented, through laboratory testing, computational analysis, full-scale prototyping, and monitoring existing installations. This paper reports recent developments in research, especially experiments that have been carried out to determine the efficacy of the system. The authors understood that a good proportion of the audience is looking for a sound rehabilitation and retrofitting technique for earthquake affected and vulnerable areas.

Use of Textile in Earthquake Pron Area

Structural Damages Due to Earthquake

Earthquake generates ground motion both in horizontal and vertical directions. Due to the inertia of the structure the ground motion generates shear forces and bending moments in the structural framework. In earthquake resistant design it is important ensure ductility in the structure, i.e. the structure should be able to deform without causing failure. The bending moments and shear forces are maximum at the joints. Therefore, the joints need to be ductile to efficiently dissipate the earthquake forces. Most failures in earthquake-affected structures are observed at the joints. Moreover, due to the existing construction practice, a construction joint is placed in the column very close to the beam-column joint (fig. 1a). 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 reinforcements. The high compressive stress in concrete may also cause crushing of concrete. If the concrete lacks confinement the joint may disintegrate and the concrete may spall (fig. 1 b, c). All these create a hinge at the joint and if the number of hinges is more than the maximum allowed maintaining 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. 1d). This may also lead to failure of the joint. Bond failure is also observed, in case, lap splices are too close to the joints1. Indian codes suggest methods that attempt to delay all these failures through a sound reinforcement detailing (IS 13920:1993). However, in many structures these details have not been followed due to perceived difficulties at site. In most of the structures in Gujarat lack of confinement and shear cracks have been found to be most common causes of failure. Rehabilitation and retrofitting strategy must alleviate these deficiencies from the structures.

Composite Materials as Post-Reinforcement

Recent developments in the field of fiber reinforced composites (FRCs) have resulted in the development of highly efficient construction materials. They have been successfully used in a variety of industries such as aerospace, automobile and ship building. The FRCs are unaffected by electro-mechanical deterioration and can resist corrosive effects of acids, alkalis, salts and similar aggregates under a wide range of temperatures. FRCs thus holds a very distinct advantage over steel plates as an external reinforcing device. Moreover, FRCs

2

(a) (b) (c) (d) (e)

Fig. 1 (a) Failure at construction joint (b) Crushing of concrete (c) Spalling of concrete (d) Diagonal shear crack (e) Bending of steel


is available in the form of laminas and different thickness and orientation can be given to different layers to tailor its strength according to specific requirements.

The difficulties encountered in using steel plates as reinforcement lead us to the use of fiber reinforced composite materials as post-reinforcements. Due to their high specific strength (strength/weight ratio) the composite reinforcements are very light and easy to handle. The composite materials are available as unidirectional fibers of a huge length. Therefore, joints in the reinforcement can be avoided very easily. Moreover, the corrosion of the reinforcements can be avoided completely. Research work is gaining momentum on the application of composite materials as post-reinforcement.The potential use of fiber reinforced composites in civil structures is manifold. The scenario of the application of composites is shown in Fig.2

FRCs can be used in the concrete structures in the following forms:

Plates -at a face to improve the tension capacity.

Bars -as reinforcement in beams and slabs replacing the steel bars.

Cables -as tendons and post tension members in suspension and bridge girders.

Wraps -around concrete members to confine concrete and improve the compressive strength

3

Bonded Unbonded

PlatingPlating Wrapping Wrapping

Stand Alone Hybrid FRP-Concrete

FIBER REINFORCED COMPOSITES APLLICATIONS

Nonpretressed

RepairNew

Prestressed

New Repair

Fig. 2 Application of fiber reinforced composites in structures


Materials for Strengthening of Structures

Carbon fibre, Kevlar®, Glass these fiber are the widely used in composite used for the earthquake resistance constructions. Fig. 3 & Table 1 present a comparison of mechanical behavior of materials that are available for strengthening of structures. It can be seen that the nonmetallic fibers have strengths that are 10 times more than that of steel. The ultimate strain of these fibers is also very high. In addition, density of these materials is approximately one-third that of steel. Due to its corrosion resistance FRCs can be applied on the surface of the structure without worrying about its deterioration due to environmental attack. They in turn protect the concrete core from environmental attack. FRPC sheets, being malleable, can be wrapped around the joints very easily. We shall discuss the investigations on application FRP on concrete.

Fig. 3 material for strengthening of concrete

Table 1: Properties of commercially representative reinforcement fibers

MaterialDensity

g/m3Initial

ModulusTenacity (mN/tex)

Decomposition melt (0C)

Aramid standard 1.44 55 2065 550Carbon HT 1.78 134 1910 3700Carbon HM 1.83 256 1230 3700E-Glass 2.58 28 780 825Steel cord 7.85 20 330 1600

%

2500

2000

1500

1000

500

5 10 15

MPa

Carbon

Aramid

Polyester

Glass

Steel

4


FRC Plates as Reinforcement to Concrete Beams

FRC for strengthening of structures can be glued to an old and deteriorated concrete surface to improve its strength. This method is more convenient and durable than epoxy bonded steel plates. Meier (1987) has examined the suitability of carbon fiber reinforced epoxy laminates for rehabilitation of concrete bridges.

The main advantages of carbon fiber composite laminates have been found to be

no corrosion and therefore, no corrosion protection is necessary no problem of transportation as it is available in rolls higher ultimate strength higher Young's modulus very good fatigue properties low weight endless tapes available, therefore no joints

The main disadvantages are

Erratic plastic behavior and less ductility -susceptible to local unevenness high cost

The first repair work of a concrete bridge using CFC laminates has been carried out at Ibach Bridge, Lucerne, Switzerland (Meier and Deuring, 1991, Meier 1988). The 228m long bridge was designed as a continuous beam of span 39m several prestressing tendons of the bridge were accidentally severed preventing the bridge to operate at its full capacity. The bridge was repaired with a 2mm thick 150mm wide CFRP laminate. It was found that the repair work became particularly easy due to the use of composite materials. Owing to its light weight 175 kg steel could be replaced by only 6.2 kg of CFC. As a result the work could be carried out from a traveling hydraulic lift and the cost of scaffolding could be avoided. The composite is held in position by means of a vacuum bag, thereby avoiding pressers required in case of steel plates. Although CFC was 40 times more expensive than steel plates, it was estimated that the process saved 20% in cost.

FRC’S as Wrapping on Concrete Elements

The tensile strength of concrete is much less in comparison to its compressive strength As a result; even the compression members often fail due to the tensile stress that develops in the perpendicular direction of the compressive load. If such a concrete element is confined using a wrapping (Fig 4) the failure due to tensile crack can be prevented. The compressive strength of the wrapped concrete element is several times higher than the unwrapped concrete element. Although this is known for a long time effective application of confinement could not be achieved due to a lack of suitable wrapping material. If the wrapping is torn the capacity of the element reduces dramatically. Therefore, the durability of the wrapping material is of utmost importance. In addition, the wrapping material remains exposed to environmental attack. Therefore, steel is unsuitable for this purpose.

5


FRCs due to their non-corrosive nature offers an attractive alternative. Moreover, the light weight FRC fibers can be very easily wrapped around an old concrete column.

However, typical stress-strain curve of cylindrical specimens wrapped with FRPC of varying number of layers is presented in figure 5. 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 structures. 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. Glass fibers that have considerably lower stiffness than the carbon fibers and higher ultimate strain is desirable. The unfavorable creep behavior of glass fiber poses little adversity in earthquake resistant applications as earthquake forces are seldom encountered. Moreover, glass fiber is much less expensive than carbon fiber. Therefore, glass fiber has been used in rehabilitation and retrofitting of structures in Gujarat.

6

Tension crack

Wrapping

Concrete section

Fig.4. FRC wrapping around concrete elements


Fig. 5: Axial strain circular columns with different. Levels of confinement

Glass Fiber Jackets to Protect Bridge from Earthquake

Thousands of bridges columns in California are to be wrapped with jackets containing high performance glass fiber in order to protect the structure from sever earthquakes. The jacket are made form industries glass fibers & isotophthalic polyester resin in contrast to the conventional sheet steel wraps and are designed to reduce the risk of serious seismic damage. Previous technology consisted of concrete or steel jackets. Although effective, these techniques are costly, time consuming, and require their own maintenance as well. Following major earthquake damage to bridges and overpasses in 1989, the California Department of Transportation identified 1,039 bridges that were in need of seismic retrofitting to prevent spalling and catastrophic failure during earthquakes4. After the 1994 Northridge earthquake, another 1,325 bridges were added to the list. The need for a more cost effective and user friendly system was imminent.

TechnologyThe Snap Tite Composite Column Reinforcement strengthens a concrete column by confining it in an external composite jacket, which prevents the concrete from expanding during seismic activity or prolonged freeze-thaw cycles3. The pre-manufactured fiberglass jacket is comprised of glass fibers and corrosion resistant isopolyester resins. The resin

7

Str

ess

in

MPa

0.020.00 0.03 0.04

50

0.01 0.05

100

200

0

Bare

t=3.2 mm

t=1.6 mm

t=0.8 mm

t=0.4 mm

“t” is thickness of wrap

Axial Strain

150


completely encapsulates the reinforcing fiber network, which, for most applications, is conventional E-glass woven roving and bi-directional fabric. Each Snap Tite component is a single-seamed, cylindrical jacket that "snaps on" the column. The column is cleaned and prepared with a high performance urethane adhesive before the first jacket is applied. More jackets are applied until the desired thickness for the job is achieved. Adhesive is applied between layers, and the vertical and horizontal jacket seams are symmetrically alternated. A typical column will require 3 to 4 layers of jackets, with a nominal jacket thickness of around 1/8 inch thick. Each nested jacket is bound with belt clamps until the adhesive cures.

Benefits of snap tileSnap Tite is recognized as one of the most cost effective and user friendly solutions for rehabilitating or upgrading existing steel reinforced concrete columns or structures. Snap Tite replaces steel, the conventional material used for column reinforcement, reducing installation and long-term maintenance costs. For example, Snap Tite, because of its light weight, can typically be installed in three hours vs. three days for steel, and can be lifted in place by workers using only a few pieces of light, mobile equipment. Snap Tite won't rust and never needs to be painted, even when installed in corrosive environments.

The other market challenge to Snap Tite is the epoxy resin composite column wrap. Although this composite does meet performance requirements, it is much more expensive to manufacture. The current manufacturer of this resin also uses extensive equipment for installation, Snap Tite does not. Full-scale tests at two major universities have verified that columns reinforced with Snap Tite withstand three-to-eight times the deflection of columns without reinforcements. Preliminary tests indicate that Snap Tite can improve earthquake capability three times beyond that of a steel jacket. Snap tite jackets work by increasing the resistance to the phenomenon of spalling which occurs when an earthquake causes the concrete columns to shatter. This expose the steel reinforcement bars which then bend outward. The jacket also allow for greater deflection when a bridge column bends, so reducing the risk shear failure. John stone composite reinforcements then stitches the glass fiber strand into non crimp fabric to form a continuous – strand mat. Finally CMIInc embeds the mat in the isophthalic polyester resin to form what are essentially sheet of reinforced plastic. The material can be molded continuously and custom made to fit snugly around the bridge columns.

Retrofitting Walls for Seismic Loads using Kevlar Fibers

Existing medical facility buildings, particularly those in the eastern United States, commonly have inadequate strength and to resist strong earthquake ground shaking. The retrofit of existing buildings to mitigate seismic loads can be most difficult and expensive.High strength fibers and elastometric polymer bonded to walls can significantly strengthen walls against wind, seismic and blast loads4.

Two solutions exist to protect people on the inside of a building. First, the exterior walls can be strengthened to withstand the blast, wind, seismic forces. Another solution, for non load bearing walls, is to allow the wall to fail and to catch the debris. The application of high strength fibers bonded to walls is an excellent solution to strengthening a wall to

8


prevent failure. Fig. 6 shows a test wall with Kevlarmatting. Kevlarfibers are encased in a 0.03 inch thick layer of resin and placed parallel to the direction of the wall span between supports. Additional vertical Kevlarreinforcement is placed around openings to make up for the area of mats interrupted by the openings. The high strength fiber can be placed on one or both sides of the wall; however, load bearing walls must have the fiber attached to both sides. E-glass or carbon fibers are optional high strength fibers that can be used in place of Kevlar. High strength fiber can be applied to the inside (i.e., non blast loaded side), or to both sides of masonry walls. In the case of load bearing walls, high strength fibers must be attached to both sides of the wall to prevent wall failure during rebound. Concrete masonry units (CMU) walls strengthened with high strength fibers must typically be grouted so that the wall capacity is not controlled by the relatively low shear strength of ungrouted masonry. Ungrouted walls can be grouted by removing face shells at various locations and pumping grout into the voids of the CMU blocks. The shear capacity of grouted walls can limit the strength of retrofitted walls against very high blast loads. The compression strength of the CMU that forms a resisting couple with the high strength fibers in tension may also limit the flexural strength, since the strength of any high strength fibers on the compression side of the wall can be limited by buckling within the surrounding resin mat.

The Mid-America Earthquake Center (MAEC) is conducting an extensive investigation of FRP applications to the seismic strengthening of unreinforced brick masonry walls. Fig. 7 shows the apparatus for testing unreinforced masonry wall component specimen. MAEC has confirmed that because of the high tensile strength of FRP materials, a fairly thin sheet can provide substantial strength and confinement to the masonry. However, MAEC cautions though lateral strength of the unreinforced masonry wall can be enhanced several fold using FRP sheets; inelastic deformation capacity may be limited as a result of debonding of the sheets which occurs suddenly with little ductility. FRP application to both sides of masonry walls may be necessary in seismic retrofits because of reversed out of plane bending demands.

9

Fig. 6 Attachment of KevlarMat with Epoxy Figure 7. Testing Unreinforced Masonry Pier Specimens


A new Fiber-reinforced concrete may enhance earthquake readiness

Now research on Fibre-reinforced concrete (FRC), which can remarkably enhance the seismic properties of buildings, has been initiated by the Japanese government & is making significant progress. In Japan Dr. Fukuyama has developed the ductile concrete to reinforce& enhance the seismic properties of buildings. Generally, concrete is strong against compressive force, but weak against tensile force. Because conventional concrete elements tend to suffer brittle shear failure, in the kobe earthquake it is seen that concrete cracked heavily or spalled, which eventually collapsed the building itself. The beam columns connections are critical region for the seismic safety of buildings. The plastic hinging zones must undergo inelastic deformations when overloaded so, new concrete material which would exhibit ductile behavior like iron. The newly developed FRC offers high ductility in flexural & tensile behavior2.

As shown in Fig.8, the concrete plate which is believed to be brittle, shows high flexural capacity when load is added. The result of this test is shown in fig. 9

10

Fig. 9 Third point flexural test result.

Fig. 8 High-flexural deformability of PVA-ECC (over 3% strain)


The concrete also has high tensile ductility. When uni-axial load is added (fig.10 a,), a fine crack develops perpendicular to the load axis (Fig.10 step b)

The reinforcing fibers in the material bridge this crack. If the bridging ability of the reinforcing fiber- like tensile strength, elastic modulus, & bonding strength with matrix- are high enough, this first crack doesn’t cause catastrophic rupture of material but the second fine crack close to the frist crack (c). in this manner, multiple cracks develop close together on the concrete, perpendicular to the load axis (d,e) and pseudo-strain hardening behavior.Secrete to the high ductile concrete material is short cut PVA fibers. The concrete realized unprecedented ductility by way of optimization of matrix, fiber, and fiber/matrix interface through a micromechanical approach. Controlling the interaction between PVA fibers and calcium ions in the cement is important to obtaining the optimal frictional bond, which leads to excellent strain hardening behavior.

According to ACM-MRI in advance fiber-reinforced, cementitious composites & their applications in building, ECC structural members are expected to exhibits ductile behavior due to its unique high tensile shear, high strain capacity, and anti spalling ability of the material. Besides its high ductile, which leads to high tensile strain capacity and high fracture resistance, the PVA-ECC has shown other major properties that makes it more suitable for creating earthquake proof buildings than conventional FRC. Traditionally PVA fiber has been used for FRCs due to its affinity to the cement matrix and its good alkaline

11

Fig. 10 Typical uni- axial tensile behavior of PVA-ECC


resistance. It is hardly damage by an alkaline matrix. It is a very good method, but it is too costly. Using the PVA-ECC for building walls would cost about one tenth of the steel frame system and yet have a similar effect against earth quakes. The ECC wall itself absorbs energy from the earth quake. this is not only reduces the tremble of the, building but also, because walls wont collapse, the hinging zones of columns and beams will not have to bear such large loads which will eventually save buildings. PVA offers cost effectiveness compared to other aramide and carbon fibers. And it has good workability as well4.

Conclusion

The actual choice of fiber type, composite type, and number of layers of composite tiles for an earthquake resistance construction particular depends on the collaborative expertise of textile technologist, engineers and structural engineers. Engineers design the overall structure and the supporting systems, and work out the required levels of tensile properties & ductile properties to be achieved in the enveloping composites. Textile fibers and coating systems to meet these levels and to perform satisfactorily need to be incorporated for use over many years. The choice also, of course, depends on general economics. The collaborative efforts of the two types of experts have enabled revolutionary possibilities to be developed in the earthquake resistance construction, shape, and appearance of major building structures.

Reference1. Mukherjee A., “Recent Advances in Repair and Rehabilitation of RCC Structures

with Nonmetallic Fibers”, Indian Concrete Journal, 2002, Vol.76 (8), pp 496-5022. “Glass fiber jackets to protect bridge from earth quake”, Technical Textile

International, May 1998.3. M. Reza Esfahani, “Axial Compressive Strength of Reinforced concrete columns

Wrapped with FRP”, 1st Conference on Application of FRP Composites in Construction & Rehabilitation of Structure, May 2004, Tehran, Iran

4. Tagawa Kikuko, “Positive reinforcement”, Industrial Fabric Product Review, May 2001, pp 20-27.

5. Henrik Stang, Victor C. Li. “Classification of Fiber Reinforced Cementitious Materials for Structural Applications”, 6thRILEM Symposium on fiber-Reinforced Concretes (FRC)-BEFIB 2004,Italy, pp197-218

6. B. Kasal et al, “Seismic Performance of Laminated Timber Frames with Fibre-Reinforced Joints”, Earthquake Engg. Struct. Dyn., 2004, vol. 33, pp633-646

7. Victor C.Li, “Large volume, High Performance Applications of Fibers in Civil Engineering”, Journal of applied Polymer Science, Vol. 83, pp 660-686.

8. “Glass Reinforced Plastic Bridge Opens”, Technical Textile International, July /Aug 1997.

12


13

Documents

Use of Textile Material in earth quake Pron area