Recent Advances in Repair Concrete

Recent Advances in Repair and Rehabilitation of RCC
Structures with Nonmetallic Fibers

Abhijit Mukherjee
l
, Mangesh Joshi
2


ABSTRACT

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. In the present paper important developments in
this field from its origin to the recent times have been presented.

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 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 re-
opened 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,
------------------------------------------------------------
1 Proffesor, Department of Civil Engineering, Indian Institute of Technology,
Bombay, Mumbai 400076,India
2 Research Student, Department of Civil Engineering, Indian Institute of
Technology, Bombay, Mumbai 400076, India

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 of the audience
is looking for a sound rehabilitation and retrofitting technique for
earthquake affected and vulnerable areas. Therefore, we include a brief
review of damages that have occurred in recent earthquake.

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, ie. 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. 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
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 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 of
the joint. Bond failure is also observed, in case, lap splices are too close
to the joints. 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. A rehabilitation and retrofitting strategy must alleviate these
deficiencies from the structures.




Figure 1 (a) Failure at construction
joint


Figure 1(b) Crushing of concrete









Figure 1 (c) Spalling of concrete






Figure 1 (d) Diagonal shear crack



CORROSI0N PROBLEMS IN INDIA

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. In humid conditions, atmospheric moisture percolates through the
concrete cover and reaches the steel reinforcements. The process of rusting
of steel bars is then initiated. The steel bars expand due to the rusting and
force the concrete cover out resulting in spalling of concrete cover. This
exposes the reinforcements to direct environmental attack and the rusting
process is accelerated. Along with unpleasant appearance it weakens the
concrete structure to a high degree. The spalling reduces the effective
thickness of the concrete. In addition, rusting reduces the cross sectional
area of steel bars, thereby reducing the strength of the reinforcements.
Moreover, the bond between the steel-and the concrete is reduced which
increases the chances of slippage. The rusting related failure of reinforced
concrete is more frequent in a saline atmosphere because salinity leads to a
faster corrosion of the steel reinforcements. In a tropical country like
India, where approximately 80% of the annual rainfall takes place in the two
monsoon months, rusting related problems are very common, especially in
residential and industrial structures. India also has a very long coastline
where marine weather prevails. Typically, a building requires major
restoration work within fifteen years of its construction.


EARLY METHODS OF REPAIR

From the above discussion we can conclude that the three main weaknesses of
RCC structures that requires attention are:
• Loss of reinforcement due to corrosion
• Lack of confinement in concrete especially at the joints.
• Deterioration of concrete due to attack of multiple environmental
agencies.

The present practice of repairs in India is focused towards delaying the
deterioration. However, there have been some attempts to strengthen the
dilapidated structures. A review on the methods of strengthening highway
bridges is available in Dudek et al. (1985). In the last two decades the
attempts on rehabilitation of damaged RC structures have been mainly
concentrated in two methods external post tensioning, and the addition of
epoxy bonded steel plates to the tension flange. High strength steel strands
are used in external post tensioning to increase the strength of damaged
concrete structures. Early investigation on this method has been reported by
Berridge and Donovan ( 1968). The work was experimental in nature. Similar
method has later been applied by Dunker et al. (1985) and Mancarti (1984).
Saadatmanesh et al. (1989) have performed an experimental study on pre-
stressed composite beams They also presented an analytical method for design
of such beams. However, the main obstacle faced in this method is difficulty
in providing anchorage in post-tensioning strands. The lateral stability of
the girder may become critical due to post-tensioning. Moreover, the strands
are to be protected very carefully against corrosion.

An alternative to the post tensioning method is the use of epoxy bonded steel
plates(Fig 2). This alleviates the main difficulties of using the post-
tensioning method -anchorage and lateral stability. This method has been
applied to increase the carrying capacity of existing structures and to
repair damaged structures as well. Several field applications of the epoxy
bonded steel plate have been reported recently. In South Africa, the
reinforcing steel of a few beams was accidentally omitted. These beams were
strengthened with epoxy-bonded steel plates at the tension face (Dussek,
1980). Several cracked slabs and girders of the elevated highway bridges in
Japan have been repaired using this method (Maeda et al., 1980). A number of
damaged reinforced concrete bridges in Poland and erstwhile USSR have been
repaired by bonding steel plates The main advantage of using this method in
repairing bridges is that it does not need closing down of the traffic during
the repair.


Experimental studies on post-reinforcing concrete beams by steel plates have
been conducted by Jones et al. (1980) and Swamy et al ( 1987). Two series of
beams of different dimensions were tested using two different glues. The
yield strength of the steel plates were also varied. Several aspects such as
glue thickness, pre-cracking prior to bending, plate lapping etc were
studied. It was observed that the steel plate reinforced beam increases the
allowable load on the structure and delays the usual cracks. The
reinforcement improves overall mechanical characteristics of the Beam.
McDonald and Calder ( 1982) studied the behavior of concrete beams of
rectangular cross section reinforced with steel plates. They observed an
improvement in performance in the ultimateload, stiffness and crack control.
But the exposure tests revealed that considerable corrosion takes place in
steel plates with natural exposure causing a loss of strength at the
interface. Ladner et al (1981, 1989, 1990) have also observed a substantial
increase in short term strength through steel plate post- reinforcing, but
they also have reported a few disadvantages. Handling of heavy steel plates
for long span beams can be very difficult Problems were faced in forming
clean butt joints in steel plates at small intervals In addition, they also
reported the possibility of high corrosion at the steel epoxy interface.




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 hold a very
distinct advantage over steel plates as an external reinforcing device.
Moreover, FRCs are 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 (Nanni et
al., 1995). The potential use of fiber reinforced composites in civil
structures is manifold. The scope of the present paper is limited to the
repair of - existing concrete structures only.

The scenario of the application of composites is shown in Fig.3

Fig. 3 Application of fiber reinforced composites in structures

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.


Materials For Strengthening Of Structures

Figure 4 presents a comparison of mechanical behavior of materials that are
available for strengthening of structures. It can be seen that the non-
metallic 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.





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.
Early research works focused on this area and a large volume of literature
exists.
Meier (1987) has examined the suitability of carbon fiber reinforced epoxy
laminates for rehabilitation of concrete bridges. The first doctoral
dissertation on the field has been presented by Kaiser (1989) of the same
institute. The topics discussed in his work are as follows:

1. Method of fixing the laminate with RC beams.
2. Effect of temperature and frost cycles on concrete laminate bond.
3. Effect of bending and shear cracks in concrete.
4. Analytical model of beam-laminate combination.
5. Safety factor to be used for such constructions.
6. Fatigue characteristics

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

Meier and Kaiser ( 1991) have reported the performance of CFC laminates in
post-strengthening of cracked concrete beams. The load deflection graph of a
post strengthened beam has been compared with that of an unstrengthened beam
in Figure 5. It was observed that a 0.3 mm thick CFC laminate has doubled the
ultimate load of a 15O x 200 beam of 2m span. They also have presented an
account of the failure modes in such beams. It is observed that the tensile
failure of the laminate occurred suddenly with a sharp explosive snap.
However, it was announced in advance by cracking sound. They stressed on the
importance of an even bonding surface and guarding against shear cracks. They
also have indicated the high potential of such repair work in wood and other
metal structures and in prestressed girders as well.



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. The potential or use or
CFCs in suspension cables or bridges is also discussed by Meier (1991).

Saadatmanesh and Ehsani(1990) have reported experimental results on epoxy
bonded GFRP plates. They have used beams strengthened with different epoxies
to test the importance of forces on the mechanical behavior of the
strengthened beam. It was observed that the epoxy should have sufficient
stiffness and strength to transfer the shear force between the composite
plate and concrete. It should also be tough enough to prevent brittle bond
failure as a result of cracking of concrete. They recommended use of rubber
toughened epoxies for this purpose.

Prestressing of the reinforcing laminate is advantageous for several
applications. It has been observed that the prestressed laminates are
effective in closing the crack in damaged structures and therefore, increase
the serviceability of the strengthened structure. Prestressing also reduces
the stress in the reinforcing steel. This is 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. 6).

Deuring (1993) has conducted experiments on beams of 2m and 6m span under
static and dynamic loading. The prestressing favorably influenced the number
and the width of cracks. Therefore, the prestressed beams had a very good
fatigue behavior. He observed that the FRP sheet has no plastic reserve
strength. Therefore, maximum flexural strength of the beam is obtained when
the failure of the laminate occurs at the instant of plastic yielding of the
steel.






Triantafillou et al (1992) have developed non-linear relationships for
calculating the ultimate load of concrete beams with pre-stressed FRC post-
strengthening. The expressions have been validated with experimental work.
Some of the other notable research works include Shahawy et al (1996a,b).

Research on repairing masonry structures using CFRP has been reported by
Schwegler (1994). Both woven fabric and unidirectional tapes connecting the
to and the bottom slabs have been employed in strengthening. Theoretical work
was done to develop design charts to carry out the post-strengthening of load
bearing masonry walls. The durability tests carried out at IIT Bombay and by
Bakis et al (2002) on the technique have been extremely encouraging.

FRC BARS AS REINFORCEMENTS IN SLABS AND BEAMS

The steel reinforcement in concrete structures is often largely responsible
in early corrosion and deterioration of concrete structures The steel
reinforcements are susceptible to corrosion and corrosion leads to spalling
in concrete Moreover, modern scanning equipment that use magnetic
interferometers require a nonmagnetic environment This has led to the
development of FRC rebars that are nonmagnetic and non-corrosive in nature.
However, FRC bars have much less ductility and unpredictable plastic
behavior. Another major problem in FRC rebars is their lower bond strength.
The bond strength is improved by mechanical anchorages and coating the
surface of the bar by sand. However, the behavior of FRC bar reinforced
concrete element largely depends on the bond behavior between the concrete
and composite bars. (Nanni, 1992). As a result a number of researchers have
investigated this aspect (Daniali 1990, Ehshani et al., 1995).

Brown and Bartholomew (1993) observed that the FRC reinforced bars behaved
the same way as the steel reinforced bars. However, the FRC bars have much
less elastic modulus. Therefore, deflection was the limiting criterion in
case of FRC reinforced beams. FRPC bars are also used as reinforcement in
slabs in the form of composite grids by Danthia et al. ( 1995) and Ahmad et
al ( 1994). Dutta and Daily ( 1995) outlined typical tests that have to be
carried out on grid frame and grid reinforced concrete. Banthia et al. (
1995) compared the behavior of concrete slabs reinforced with FRPC grids to
that of a slab reinforced with steel grid. The ultimate loads supported by
slabs reinforced with FRPC were equal to or higher than that supported by the
companion slab reinforced with steel. It was concluded that, no significant
changes are needed to the various code equations when extending them to slabs
reinforced with FRPC reinforcement. Ahmad et al. (1994) presented results of
punching shear behavior of concrete slabs reinforced with 3D continuous
carbon fiber fabric under central concentrated load. Test results revealed
that the carbon fiber reinforced concrete slabs exhibited a significant non-
linear behavior and reduction in stiffi1ess in the post cracking stage.

FRCs AS CABLES AND TENDONS

Corrosion problems are very severe in transportation structures, especially
those exposed to marine environment. This encourages use of FRCs in bridges.
The FRC cab1es, post-tension tendons and plating can be used to improve the
durability of bridges. Moreover, FRC cables are much lighter than the
conventional steel cables leading to lesser self weight. Therefore, muchl
onger spans can be designed by using FRC cables.

Kim and Meier (1991) and Meier (1995) have reviewed the applicability of FRCs
in cable stayed bridges. McKay and Erki ( 1993) have experimented on the use
of aramid fiber tendons on the post-tensioning of concrete beams Meier (1987)
has observed that use of FRC cables would allow tripling of limiting span of
cable stayed bridges in comparison to steel wires.

FRCS 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 7) the failure due to tensile cracks 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. FRCs
due to their non-corrosive nature offer an attractive alternative. Moreover,
the light weight FRC fibers can be very easily wrapped around an old concrete
column.





An exhaustive test program has been undertaken at the Indian Institute of
Technology, Bombay to evaluate the efficacy of FRPC in structural
strengthening (Mukherjee, et al., 2001) with collaboration from the
Pennsylvania State University and Cold Regions Research and Engineering
Laboratory, USA. A detailed account of the research is beyond the scope of
the present paper.
However, typical stress-strain curve of cylindrical specimens wrapped with
FRPC of varying number of layers is presented in figure 8. 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.

Faza and Ganga Rao (1994) suggested wrapping around damaged concrete elements
to improve the strength of these members. Li et al. (1992) developed semi-
empirical expressions to predict the mechanical behavior of wrapped
compression members. Wrapping can be applied to strengthen concrete beams in
compression and shear. Michael et al. ( 1995) have reported a study on
composite fabric wraps in concrete beams. Mirmiran and Shahaway ( 1996) have
tested the strength or concrete filled hollow FRC sections for compressive
strength.



APPLICATION IN INDIA

Application of FRCs in India is gaining momentum of late. One of the major
applications has been in earthquake damaged structures in Gujarat (Mukherjee
and Joshi, 2001). The edge FRCs enjoyed over the conventional repairing
techniques is speed of execution. A severely damaged fertilizer plant has
been rehabilitated using the technique. The total area of repair was around
5000 sqmt. In about forty-five days the repair was completed, and the plant
was able to come back to full production within three months. All the
components used in the repair were procured in India from Indian
manufacturers. There is a huge potential of application of the technique in
India.

CLOSING REMARKS

From the above discussion it can be observed that the fiber reinforced
composites are a very attractive proposition for repair and upgradation of
damaged concrete structures. However, the success of the method depends on
several factors.
• Understanding mechanics - The mechanics of FRC is fundamentally
different from other construction materials. Therefore, one needs to
understand the material before its use. A large body of research work
exists. However, an efficient dissemination of knowledge is of utmost
importance.
• Availability of design methods - The design methods are already
developed. However, they need wider publicity and possible inclusion in
Indian Standards.

• Availability of materials- GFRP is available locally in adequate
quantities. CFRP needs to be imported. New production facilities will
be extremely important as the use of the technique gains popularity.
• High cost of material- Although per Kg cost of FRCs is substantially
higher than that of steel or cement the cost of repair using these
materials is far cheaper, faster, durable and cleaner.
• Availability of technology- This repair technique needs knowledge about
several new construction materials. Successful application of these
materials would require fundamental understanding of the behavior of
these materials Although a large research base is already available
awareness in India about these materials is scant. There is a need to
include these materials in basic curriculum.
• Durability under Indian conditions- These materials are chemically
inert and show little degradation over time even in harsh conditions.
Our tests show little effect of tropical Indian environment on these
materials.