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Behavior of FRP-Strengthened Reinforced Concrete Members Under Fire Conditions
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Behavior of FRP-Strengthened Reinforced Concrete Members Under Fire Conditions: Making fiber reinforced polymers (FRP) a viable alternative

By Venkatesh Kodur, Ph.D., P.E. and Aqeel Ahmed  | Fire Protection Engineering

INTRODUCTION

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 strength,

 

 
integrity, and stability. Fire resistance depends on many factors, including structural geometry, material used in construction, and fire characteristics.
 

 

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 concrete substrate.

 

 

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 Fig. 2).

 

 

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 beams.

 

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.

 

NUMERICAL MODEL

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.

 

PRACTICAL APPLICATIONS

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.

 

 

ACKNOWLEDGEMENTS

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.

 

References:

  1. Kodur, V., Bisby, A. and Green, M. FRP Retrofitted Concrete Under Fire Conditions. Concrete International, 2006. 28 (12), p. 37-44.
  2. ACI 440, Guide for the Design And Construction of Externally Bonded FRP Systems For Strengthening Concrete Structures, American Concrete Institute Farmington Hills, MI, 2008.
  3. 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.
  4. Kodur, V. & Harmathy, T. "Properties of building materials": SFPE Handbook of Fire Protection Engineering, 4th edition, National Fire Protection Association, Quincy, MA, 2008.
  5. 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.
  6. Blontrock, H., L. Taerwe, and P. Vandevelde. Fire Tests On Concrete Beams Strengthened With Fibre Composite Laminates in Third PhD Symposium. Vienna, Austria, 2000.
  7. 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.
  8. 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.
  9. 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.
  10. ACI 318, Building Code Requirements for Reinforced Concrete and Commentary. American Concrete Institute, Farmington Hills, MI, 2008.
  11. 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.
  12. ASTM E119, Standard Test Methods for Fire Tests of Building Construction and Materials, West Conshohocken, PA, 2001.
  13. 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.
  14. 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.
  15. 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.

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