Reinforced concrete (RC) structures can
deteriorate due to poor maintenance, corrosion of steel reinforcement as
well as aging related problems. In addition, the need for strengthening
of existing structures due to natural and man-made disasters (seismic,
hurricanes and blasts) is ever growing. Fiber reinforced polymers (FRP)
have emerged as an alternative for retrofitting and strengthening of
concrete structures due to properties such as light weight, corrosion
resistance, and high strength.
Fire represents a hazard for built
infrastructure, and thus provision of an appropriate fire resistance for
structural systems is a design requirement. The fire safety provisions
for structural members are specified in terms of fire resistance
ratings. The fire resistance rating requirements depend on the type of
structural member, occupancy and other factors.
When an RC member is strengthened with
FRP, the resulting fire resistance will depend on the properties of the
original concrete member as well as the properties of FRP. Currently,
FRP is mainly used in bridges and parking garages where the fire hazard
is not a major consideration in design. However, when used in buildings,
FRP-strengthened structural members have to meet stringent fire
resistance requirements specified in building codes and standards.
FRP UNDER FIRE
Currently, little is known about the
performance of FRP-strengthened concrete structures under fire
conditions. This knowledge gap has emerged as a primary reason that
limits widespread application of FRP in building applications. When used
in buildings, structural members have to satisfy flame spread, smoke
generation, and fire resistance ratings prescribed in building codes.1 The
flame spread and toxic smoke generation, largely depend on the type of
FRP formulation (composition). The third requirement to be satisfied by
FRP-strengthened structural members is the fire resistance rating
specified in building codes. Fire resistance is the actual duration
during which a structural member exhibits resistance with respect to
and stability. Fire resistance depends on many factors, including
structural geometry, material used in construction, and fire
Like other construction materials, FRP
loses its strength and stiffness properties with temperature. However,
the degradation in FRP properties is faster as compared to concrete or
steel since the properties of the FRP matrix start to deteriorate even
at modest temperature. Figure 1 illustrates the degradation in strength
of FRP as compared to concrete, steel, and wood. Also, bond degradation
is another concern with externally bonded FRP. FRP is a combustible
material. Thus, it is susceptible to loss of bond strength and stiffness
above its glass transition temperature (Tg).
The glass transition temperature is the
temperature at which a material changes from relatively stiff material
to viscous, leading to a drop in strength and stiffness. Typically,the
glass transition temperature for commonly used polymers (adhesive)varies
between 60 to 82C.2 In FRP-strengthened members, the main
load carrying mechanism is through transfer of stresses from the
concrete substrate to the FRP reinforcement. This transfer of forces to
FRP reinforcement occurs through development of shear stresses at the
interface of FRP and concrete. However, when the temperature at the
interface reaches Tg, the bond properties of
the adhesive (shear modulus and bond strength) deteriorate considerably
and introduce bond-slip at the interface.3 This bond-slip reduces force transfer from the concrete to the FRP composite, and subsequently leads to debonding of the FRP.
FIRE RESISTANCE EXPERIMENTS
In the last decade, there have been
limited studies to investigate the fire behavior of FRP-strengthened
concrete members. The notable experimental studies on FRP-strengthened
RC beams were conducted by Kodur et al.,1 and Deuring,5 Blontrock et al.,6 Barns and Fidell,7 Kodur et al., and Williams et al.8 Most
of these tests have been conducted under the standard fire exposure
without any consideration to realistic fire, loading, and restraint
scenarios. All these studies recommended use of supplemental insulation
for externally bonded FRP to maintain the bond between the FRP and the
To address shortcomings in previously
conducted research and to develop needed test data for validating
numerical models, four FRP-RC beams were tested under fire conditions at
Michigan State University (MSU). A summary of test parameters and
results are tabulated in Table 1.
The four rectangular RC beams were of 3.96 m in span length and designed as per ACI 31810 specifications
to represent typical beams in buildings. The RC beams were fabricated
with concrete having a design compressive strength of 42 MPa. The
measured compressive strength of concrete at 28 days was 52 MPa, while
on the day of the test (after 2 years), it was 55 MPa.The beams had
three 19 mm dia. rebars as flexural reinforcement and two 13 mm rebars
as compressive reinforcement. The RC beams were strengthened with FRP
sheets (2 mm thick and 203 mm width) to enhance the flexural strength
capacity by 50%. The resin used was two-component epoxy material with a
glass transition temperature (Tg) of 82C. For
Beams B1 and B2, FRP was applied on the entire unsupported length of the
beam terminating at a distanced (352.4 mm) from the supports to study
the influence of anchorage zone on fire response of FRP-strengthened RC
beams. While for Beams B3 and B4, fire exposed beam length (central 2.44
m) was retrofitted with FRP sheets to evaluate the effect of debonding
on fire resistance FRP-RC beams. Unlike in previous studies,6, 8, 11 no
shear strengthening was provided in these beams to study the failure
patterns under flexural strengthening effect only. The beams were
insulated with a vermiculite-based insulation (VG insulation) and epoxy
coating. The insulation layout comprised of 25 mm of insulation at the
bottom surface of the beam extending 100 mm on the two sides (refer to
The beams were instrumented at various
locations: Type K thermocouples to measure temperature distribution
throughout the cross section, high temperature foil strain gauges to
measure strain in the reinforcing steel and FRP and electro-mechanical
displacement gauges to measure overall beam deflection. The
thermocouples were distributed throughout the beam cross-section at
three different sections along the span (cross-sections A, B, C in Fig. 2
(a)) and also at several points along the unexposed face of the beam,
as required by ASTM E119.12
Beams B1 and B2 tested under a design
fire that comprised of a growth phase followed by a cooling phase, while
Beams B3 and B4 were tested under the ASTM E119 standard fire. The
beams were subjected to two point loads (P in Fig. 2(a)), each of 70 kN,
which represents 50% of the strengthened beam's nominal capacity
according to ACI 440.2 Data generated from these fire tests
was valuable in developing failure patterns, as well as in developing
numerical models for tracing the fire response of FRP-strengthened RC
RESULTS FROM FIRE TESTS
Data from fire tests can be used to
illustrate the fire behavior of FRP-strengthened RC beams. The
time-temperature progression at various locations of tested
FRP-strengthened RC beams is shown in Fig. 3. It can be noticed that
temperature at various beam cross sections, including in rebar and
concrete, increases throughout the test duration for beams B3 and
B4,which were exposed to the standard fire. However, in beams B1 and B2,
which were exposed to a design fire, the measured temperatures increase
to a maximum value and then start to decrease. This decrease in
temperature can be attributed to the decay (cooling) phase in
time-temperature curve of the design fire. For both types of insulation
(Type A and B), cracks appeared in early stages of the fire exposure
that allowed transfer of heat flux (refer to Fig. 4).This led to a
temperature increase in the FRP and resulted in localized burning of
epoxy, since Tg for epoxies is low (Tg=82C).
However, the slow temperature increase at
FRP/concrete interface is due to formation of a protective char layer
as a result of the pyrolysis process of the matrix.13 In Beam
B3, a sudden increase in temperature at the FRP/concrete and
FRP/insulation interfaces is due to delamination of the FRP from the
beam soffit around 38 minutes (refer to Figs. 3 and 5).
The insulation played a key role in
limiting the temperature rise in the rebars. This is mainly attributed
to the low thermal conductivity of the insulation. The average rebar
temperature in all the tested beams remained below 400C for the complete
test duration (refer to Fig. 6). Since rebars do not lose any
significant strength up to 400C,14 the steel reinforcement
maintained full strength capacity for the test duration. This led to
achieving high fire resistance in these beams.
Fig. 7 shows the variation of mid-span
deflection as function of time for the four tested beams. The mid-span
deflections increased slowly up to 20 minutes and then all beams
experienced a sudden increase in deflections. This can be attributed to
loss of bond at the FRP/concrete interface with temperature,which occurs
at Tg (83C). After debonding of the FRP, the behavior of Beams B1, B2 and B4 was different than that of Beam B3.
At the start of the test,
FRP-strengthened RC beams were loaded to 50% of their strengthened
nominal capacity at room temperature. This loading corresponds to about
80% load ratio for an unstrengthened RC beam. Therefore, beam B3, which
behaved as a reinforced concrete beam after the FRP was lost
(debonding), experienced an increase in deflection due to the higher
load ratio, as shown in Fig. 7. The factor that contributed towards the
lower deflections in beams B1 and B2 after the FRP debonded is the
"cable action" (similar to tensile membrane action is slabs) provided by
unbonded continuous carbon fibers at the beam soffit held by a cool
anchorage zone towards the end supports (refer to Fig. 8).13 In beam B4, development of the axial restraint force mainly limited the deflection increase for the entire duration of the test.
In general, this axial restraint force
acts below the neutral axis of the beam section; this leads to the
development of an arch action,which counteracts the moment due to
applied loading. A closer examination of Fig. 7 indicates that, similar
to cable action mechanism, the fire induced axial restraint force is
beneficial in enhancing the fire resistance of FRP-RC beams. It helps to
slow down progression of deflections in the beam under a higher load
ratio, and this in turn leads to high fire resistance in the beams.
Therefore, an effective insulation scheme in FRP-strengthened RC beams
is critical for achieving good fire resistance.
As part of ongoing research at MSU, a
numerical model has been developed for tracing the fire response of
FRP-strengthened RC beams exposed to fire.15 The model is
based on a macroscopic finite element (FE) approach and uses
moment-curvature relationships to trace the response of an
FRP-strengthened RC beam. The model accounts for fire induced debonding
of FRP, axial restraint, high temperature material properties and
various failure criteria. The model has been validated against fire
tests on FRP-RC beams.
This fire resistance model takes into
account the elevated temperature properties of constitutive materials to
generate moment-curvature (M-k) relationships for different beam
segments at various time steps. These time dependant M-k relationships
are utilized to trace the response of FRP-strengthened RC beams up to
collapse under fire conditions. A flow chart illustrating various steps
in fire resistance analysis of FRP-RC beams is shown in Fig. 9.
The model has been validated against
tests performed on rectangular FRP-strengthened RC beams conducted by
Blontrock et al.6 as well as T-beams by Williams et al.8 The
thermal and structural response predictions from the model compared
well with the measured test parameters throughout the range of fire
exposure. Thus, the model can be applied to evaluate fire response of
FRP-strengthened RC beams.
The above developed macroscopic finite
element based numerical model can be applied to develop optimum
insulation schemes for FRP-strengthened RC beams. To illustrate the
usefulness of the model, fire resistance analysis was carried out on an
FRP-strengthened RC beam (380 ~ 610 mm). The beam was provided with
insulation of varying thicknesses and configuration schemes. On the
sides of the beam, 20 mm thick insulation was applied with a depth of
105 mm. The insulation thickness at the beam soffit was varied to be 15,
25, 40, and 50 mm, respectively. The analysis was carried out by
exposing the beams to the standard ASTM E119 fire from three sides. The
applied load ratio on the beam was kept constant at 52% for all the
cases. Typical results that are generated from fire resistance analysis
are shown in Fig. 10.
The failure of the fire exposed beams was
computed based on strength, rebar critical temperature and deflection
limit states. No failure occurred under deflection or rate of deflection
limit states. This can be attributed to lower deflections (resulting
from reduced ductility) in the beams when strengthened with FRP. Closer
examination of Fig. 10 indicates that increasing insulation beyond an
optimum thickness does not contribute much to fire resistance. This can
be attributed to the fact that the strength (moment capacity) of the
beam, under fire conditions, is controlled by the tension forces in the
FRP (up to a certain fire exposure time) and steel reinforcement.
Increased insulation thickness helps to reduce temperature in rebars,
and this in turn helps to achieve a higher moment capacity at a given
fire exposure time. However, beyond the optimum insulation thickness, at
which rebar temperatures reach about 400C, any further reduction in
rebar temperatures does not result in higher tension force or capacity
of the beam. This is because the strength loss in rebars occur only
above 400C13 and any measure to decrease temperature below
400C, through increased insulation thickness, does not contribute to
increased tension force. This is illustrated in Fig. 9, where rebar
temperature and corresponding yield strength ratio (fy,r /fy,20),
obtained from parametric studies was plotted as a function of
insulation thickness for 3 hours of fire exposure. It can be seen that
increasing insulation thickness from 0 to15 mm has maximum benefit, and
beyond this thickness, the beneficial effect gradually decreases. Beyond
the optimum insulation thickness of 40 mm, there is no advantage to
increasing the insulation thickness.
The above case study illustrates the
usefulness of the model in evolving optimum insulation schemes for
FRP-strengthened RC beams for a specified fire resistance. Numerical
models, like the one discussed in this article, will help practitioners
to arrive at optimum fire insulation scheme for a given fire resistance
application. Typical application of the research presented in this paper
for arriving at fire resistance strategies in buildings is illustrated
in Fig. 11.
The information presented in this paper
is based on the research funded by the National Science Foundation
(Grant No. CMMI 0855820) and Michigan State University through Strategic
Partnership Grant (Award No. SPG 71-4434). The authors would also like
to thank Fyfe Company LLC, USA, for supplying TyfoWR Advanced Fire
Protection system. Any opinions, findings, and conclusions or
recommendations expressed in this paper are those of the authors and do
not necessarily reflect the views of the sponsors.
Venkatesh Kodur and Aqeel Ahmed are with the Michigan State University.
- Kodur, V., Bisby, A. and Green, M. FRP Retrofitted Concrete Under Fire Conditions. Concrete International, 2006. 28 (12), p. 37-44.
- ACI 440, Guide for the Design And Construction of Externally Bonded FRP Systems For Strengthening Concrete Structures, American Concrete Institute Farmington Hills, MI, 2008.
- Ahmed, A. and Kodur, V. Factors
Governing Fire Resistance of FRP- Strengthened Reinforced Concrete
Beams in Composite & Polycon, American Composites Manufacturers
Association (ACMA). Las Vegas, NV, 2010.
- Kodur, V. & Harmathy, T. "Properties of building materials": SFPE Handbook of Fire Protection Engineering, 4th edition, National Fire Protection Association, Quincy, MA, 2008.
- Deuring, M., Brandversuche An Nachtraglich Verstarkten Tragern Aus Beton, in Research Report EMPA No. 148,795. Swiss Federal Laboratories for Materials Testing and Research: Dubendorf, Switzerland, 1994.
- Blontrock, H., L. Taerwe, and P. Vandevelde. Fire Tests On Concrete Beams Strengthened With Fibre Composite Laminates in Third PhD Symposium. Vienna, Austria, 2000.
- Barnes, R. and Fidell, J. Performance in Fire of Small-Scale CFRP Strengthened Concrete Beams. Journal of Composites for Construction, 10: p. 503-508, 2006.
- Williams, B., Kodur, V., Green, M. and Bisby, L. Fire Endurance of Fiber-Reinforced Polymer Strengthened Concrete T-Beams. ACI Structural Journal, 105(1): p. 60-67, 2008.
Kodur, V. & Ahmed, A., "Behavior of
FRP-strengthened reinforced concrete beams exposed to design fire
scenarios", Proceedings of 2011 NSF Engineering Research and Innovation Conference, National Science Foundation, Washington, 2010.
- ACI 318, Building Code Requirements for Reinforced Concrete and Commentary. American Concrete Institute, Farmington Hills, MI, 2008.
Deuring, M., Verst rken von Stahlbeton
mit gespannten Faserverbundwekstoffen. (Post-Strengthening of Concrete
Structures with Pretensioned Advanced Composites), EMPA Research Rep.
No. 224, Dubendorf, Switzerland, 1993.
ASTM E119, Standard Test Methods for
Fire Tests of Building Construction and Materials, West Conshohocken,
- Ahmed, A., Kodur, V. Performance
of FRP-strengthened Reinforced Concrete Beams under Design Fire
Exposure in Proceedings of Sixth International Conference on Structures
in Fire. East Lansing, MI, 2010.
- Eurocode 2, EN 1992-1-2: Design of concrete structures. Part 1-2: General rules - Structural fire design. European Committee for Standardization: Brussels, Belgium. 2004.
- Kodur, V. and Ahmed, A. A Numerical Model For Tracing The Response Of FRP-Strengthened Reinforced Concrete Beams Exposed To Fire. ASCE Journal of Composites, 2010.