Rehabilitation and Repairs of Distressed structures

V. Sadhan kumar

1. Chaitanya Bharathi Institute of Technology,

Gandipet-500075, Andhra Pradesh, India.

noel_4u@yahoo.com

Abstract

Cement concrete reinforced with steel bars is an extremely popular construction material. One of the

major flaws, namely its susceptibility to environment attacks, can severely reduce the strength and life of

the structures. External reinforcement 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

development in the field of fiber reinforcement composites (FRCs) have resulted in the development of

highly efficient construction materials. The (FRCs) are unaffected by electro-mechanical deterioration

and can resists corrosion effects of acids, alkalis, salts and similar aggregates under a wide range of

temperature. This novel technique of rehabilitation very effective and fast for earthquake affected

structures and retrofitting of the structure against possible earthquake. This technique has been

successfully applied in the earthquake-affected GUJRAT. In the present paper important developments in

this field from its origin to the recent times have been presented.

Since most of the damage is done to the beams, columns and slabs. In this paper I have also mentioned

about the repairs for structural failure in concrete structures.

1.0 Introduction

Although hundreds of thousands of successful reinforced concrete and masonry buildings are annually

constructed worldwide, there are large numbers of concrete and masonry structure that deteriorate, or

becomes unsafe due to changes in loading, changes in use, or changes in configuration. Also from the

recent earthquake of Gujarat it is clear that old structure designed for gravity loads are not able withstand

seismic forces and caused wide spread damages. Repair of this structure with like materials is often

difficult, expensive, hazardous and disruptive to the operation of the building. The removal and

transportation of large amount concrete and masonry material causes concentration of weight, dust,

excessive noise, and requires long period of time to gain strength before the building can be reopened for

service. We understood that a good of the audience is looking for a sound rehabilitation and retrofitting

techniques for affected and vulnerable areas. Therefore, we conclude a brief review of damages that have

occurred in recent earthquake at Gujrat.

On the other hand, Fiber Reinforced Composite (FRC) materials, originally developed for the aerospace

industry are being considered for the application to the repair of buildings due to their low weight, ease of

handing and rapid implementation. A major development effort is underway to adapt this material to the

repair of building and civil structure. So appropriate configurations of fibers and polymers matrix are

being developed to resist the complex and multi-directional stress fields present in building structure

members.

Since rehabilitation is done to the whole structure, so I took in interest in repair also which is done local

damages and to the particular elements.

2.0 Rehabilitation

Rehabilitation: rehabilitation means to restore to make suitable, to put back in good condition, to re-

establish in a firm sound basis, to bring back to full use, to reinstate, to renew and to revive.

Rehabilitation includes whole bridge structure, including its primary structure member.

2.1 Structure damage due to earth quake

Earthquake generates ground motion both in horizontal and vertical direction. Due to the inertia of the

structure ground motion generates shear force and bending moments in the structure framework. I.e. in

the earthquake resistant design it is important ensure ductility in the structure, i.e. the structure able to

deform without causing failure. The bending moments and shear are maximum at the joints, so the joints

should be ductile to dissipate the earthquake forces. Most of the earthquake affected structures are

observed at the joints. If the concrete lacks confinement the joints may disintegrate and the concrete may

spall (fig. 1(a, b)). If the shear reinforcement in the in the beam is insufficient there may be diagonal

cracks near the joints (fig. 2(c, d)).

Figure. 1 (a) spalling of concrete

Figure. 1 (b) spalling of concrete at supports

Figure. 2 (c) diagonal cracks near the joints

Figure. 2 (d) failures at construction joints

3.0 Advantages of fiber reinforced composite materials

1. FRCs are unaffected by electro-mechanical deterioration and can resist corrosive effects

acids, alkalis, salts and similar aggregate under a wide range of temperature.

2. FRCs thus holds a very good distinct advantage over steel plates as an external reinforcement

device.

3. Moreover FRCs is available in laminates and different thickness and orientation can be given

to different layers to tailor its strength according to strength requirement.

4. Composite materials are easy handle and light in weight.

5. Corrosion of the of the reinforcement can be avoided completely.

3.1 Application of fiber reinforcement in structure

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

1. Plates- at a face to improve the tension capacity: FRC for strengthening of structure can be glued

to an old and deterioted concrete surface to improve its strength. This method is more convenient

and durable than epoxy bounded steel plates. It is observed that prestressed laminates are

effective in closing the crack in the damaged structure, therefore increases the serviceability of

the strengthened structure. Prestress also reduces the stress in the reinforcing steel. This is more

advantageous when the steel is weakened due to corrosion. Another significant advantage of

prestressing is that it reduces the tendency of delamination at the crack front (fig 4(a)).

Figure 4 (a) fiber plates

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

concrete structure is often largely responsible in early corrosion and deterioration of concrete

structure. The steel reinforcement is susceptible to corrosion and corrosion leads to spalling in

concrete. As FRCs rebars are nonmagnetic and non corrosion behaviour. Another major problem

in FRCs rebars is their lower bond strength. Bond strength is being improved by mechanical

anchorage and coating the surface of the bar with sand (fig 5 (a)).

Figure 5 (a) Fiber reinforced bars in construction

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

problems are very severe in transportation structure, especially those which are exposed to marine

conditions. This encourages the use of FRCs in bridges. FRCs cables, post-tension tendons and

plating can be used to improve the durability of the bridge. Moreover FRCs cables are much

lighter than the conventional steel cables leading to the lesser in self weight hence longer spans

can be designed using FRCs (fig 6 (a)).

Figure 6 (a) Cables of FRCs used in suspension bridges

4. Wraps-around concrete member to confine concrete and improve the compressive strength: The

tensile strength of concrete is much less in comparison compared to its compressive strength.

Often these structures fail due to tensile stress that develops in the perpendicular direction to that

of compressive load. If such a concrete element is confined using wrapping failure due to tensile

cracks can be prevented (fig 7 (a)).

Figure 7 (a) Wrapping of column by FRCs

2.2 Use of composite material as post-reinforcement

Recent development in the fields of fiber reinforced composites (FRCs) has resulted in the development

of highly efficient constructional materials. They have been successfully used in a variety of industries

such as aerospace, automobiles and ship building.

The difficulties encountered using steel plates as reinforcement lead us to the use of fiber reinforced

composite material. They are used because of their high specific strength (strength /weight)

2.3 Materials for strengthening of structures

A comparison of mechanical behavior of material that is available for strengthening of structures. It can

be seen that non-metallic fibers have strengths that are 10 times more than that of the steel. It addition,

density of these materials is approximately one-third that of the 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. By considering steel, polyesters, glass, Aramid and carbon. Carbon is considered as

beneficial as compared with other composites (fig 3 (d,e)).

Figure. 3 (d) carbon fiber reinforced composite

Figure. 3 (e) carbon fiber available in rolls

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

1. no corrosion and therefore, no corrosion protection are necessary: when compared with carbon,

steel is susceptibility to environment attacks

2. no problem of transportation as it available in rolls

3. higher ultimate strength

4. higher Young’s modulus

5. very good fatigue properties

6. low weight

7. endless tapes available, therefore, no joints

2.5 Disadvantages

1. Erratic plastic behavior and less ductility

2. susceptible to local unevenness

3. high cost.

4.0 Repairs

Repair: repair means, to put into good shape or working order again, to recondition, to renovate, to restore

and to correct. Repair concerns rather the local damages of structural members or bridge elements than

the overall structure.

4.1 Repairs for structural failures in concrete structures

4.1.1 Techniques for strengthening of beams:

1. Adding new members and enlarging sections: If the beams span is too long, in the mid span

added steel beams are bolted to the existing beams to increasing its strength. In other method we

just drill hole on either side of the existing beam and a new rigid steel channels are attached and

bolted (fig 8 (a)).

Figure 8 (a) Adding new member for strengthening

2. Shortening the span: shortening of span is nothing but constructing columns in-between long

beams, so that by avoiding more sagging in the beam. So there by strengthening the beam from

more sagging (fig 9 (a)).

Figure 9 (a) Shortening of span

3. Adding bolted steel tension reinforcement: In the same way as adding new member, the earlier

beam is roughened new stirrups are being drilled in adhesive anchor and new concrete is

plastered.

4. Adding bolted FRS plates and wraps: As the beam is susceptible to spalling to avoid these, the

surface of the beam is pasted with adhesive and FRS plates are being wrapped around the beam.

5. For distress due to shear can be improved by following ways: The new plates cover the sides of

the existing beams and are through bolted at least two places. Other way is by adding new

stirrups. Use epoxy-bonded FRP composites to wrap the ends of the beam in thin laminates or

FRP plates.

4.12 Techniques for strengthening of column:

1. Section enlargement: Section can be enlarged by adding new stirrups and plastering the column

with new concrete to the earlier column. By this way we can enlarge section thereby increasing

the strength.

2. Adding columns: Adding column in nothing but constructing a new column besides the older

column.

3. Reinforcing with structural steel: In this method the earlier column, plates are fixed around the

surface and they are drilled with holes, the holes are filled with epoxy or grout. They are bolted.

4. Reinforcing with FRP wraps: As similar to that beam the column is also wrapped with FRP

(fig 10 (a)).

5. Jacketing around: Jacketing is the method in which, the previous column is roughened and new

ties and longitudinal reinforcement is assembled and concreting is done by micro concrete. An it

is made into a new column of increased dimension. (fig (d,e)).

4.13 Techniques foe strengthening of slabs:

1. By shortening of span by introducing steel beams at the mid spans: As similar to adding new

member in beams, if the slab is too long. In the mid spans steel beams are introduced in the lower

part of the slab, the steel beam is fixed onto the slab by epoxy or resin.

2. By adding steel plates to improve the flexural resistance of the existing slab: in this method steel

plates are being applied to the existing beams to improve flexural resistance

3. By applying various fiber composites like CFRP: in this method Carbon Fiber Reinforced

Polymer is being used as now a days it is a popular composite for repairing.

Conclusion

So the methods used for the Rehabilitation, is mostly done by Fiber Reinforced composite as it is

universally accepted and it is corrosion free, they are unaffected by electro-mechanical deterioration and

they are easily available in the market. Rehabilitation is done to reconstruct the structure without

demolishing.

Reference

1. Deerendra Babu, M.R., “Construction technology and valuation”, Falcon Publisher.

2. IS Code 456-2000, plain and reinforced concrete.

3. Repair and rehabilitation, civil engineering and construction review, volume 21, pages 38-41.

4. Punmia, B.C.,”Building construction”, Laxmi Publications.

5. Mallick, P. K., “Fiber-Reinforced Composites: Materials, Manufacturing, and Design”, CRC

Press.