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Bisby, L. A.; Kodur, V. K. R.; Green, M. F.
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Numerical parametric studies on the fire endurance of fibre-reinforced-polymer-confined concrete columns
Bisby, L.A.; Kodur, V.K.R.; Green, M.F.
NRCC-47048
A version of this document is published in / Une version de ce document se trouve dans: Canadian Journal of Civil Engineering, v. 31, no. 6, Dec. 2004, pp. 1090-1100
Doi:10.1139/L04-071
Numerical Parametric Studies on the Fire Endurance
of FRP-Confined Concrete Columns
.L. A. Bisby1, M.F. Green 2, and V.K.R. Kodur 3
1
L.A. Bisby* Ph.D. Candidate
Department of Civil Engineering Queen’s University Kingston, ON K7L 3N6 Phone: 613-533-3086 Fax: 613-533-2128 e-mail: bisby@civil.queensu.ca 2 M.F. Green Professor
Department of Civil Engineering Queen’s University
Kingston, ON K7L 3N6
3
V.K.R. Kodur
Senior Research Officer Fire Risk Management Group
Institute for Research in Construction National Research Council of Canada Ottawa, ON K1A OR6
* Author responsible for correspondence. Luke Bisby is currently an Assistant Professor of Civil Engineering at Queen’s University
ABSTRACT: Confinement of reinforced concrete columns by circumferential FRP wraps is a
promising application of FRP materials for structural strengthening and seismic upgrading of deteriorated or under-strength members. However, if this technique is to be used in buildings, parking garages, and industrial structures, then the ability of FRP materials and FRP-wrapped columns to withstand the effects of fire must be demonstrated and evaluated. This paper presents the results of parametric studies conducted using a previously presented and partially validated numerical fire simulation model to investigate the effects of a number of parameters on the fire behaviour of FRP-wrapped reinforced concrete columns. It is demonstrated that appropriately designed and adequately protected FRP-wrapped reinforced concrete columns are capable of achieving fire endurances equivalent to conventionally reinforced concrete columns.
Furthermore, this study also suggests that a holistic approach to the fire design of FRP-wrapped members is required, rather than an approach based on the specific performance of the FRP materials. Design recommendations for the fire-safe design of FRP-wrapped concrete columns are presented and discussed.
Keywords: Reinforced concrete, rehabilitation, strengthening, fibre reinforced polymer, fire endurance, fire insulation, numerical modelling.
INTRODUCTION
Many structures in Canada and around the world are in need of rehabilitation, upgrade, or renovation. Widespread infrastructure deterioration has forced, among other things, the development of new and innovative structural strengthening solutions, some incorporating fibre reinforced polymers (FRP). One particularly effective application of FRP materials is circumferential wrapping (confinement) of reinforced concrete columns, which has been shown to increase both the strength and ductility of these members (Bisby et al. 2001). However, when FRP are used for column wrapping in interior exposures, such as in buildings, parking garages, and many industrial structures, there is a legitimate concern that FRP materials, which are generally more sensitive to elevated temperatures than either concrete or steel, will perform poorly in fire, and that this could result in unsatisfactory fire endurance for FRP-wrapped structural members. Very little information is currently available in this area, and concerns associated with fire must be addressed before FRP-wrapped columns can be used with confidence in most buildings.
As part of a larger study investigating the fire behaviour of various types of FRP-reinforced concrete members, a numerical model has been developed to study both the heat transfer behaviour and load carrying capacity of FRP-wrapped reinforced concrete columns in a standard fire (Bisby et al. 2003a). In addition, full-scale fire endurance tests have been performed on two circular FRP-wrapped and insulated reinforced concrete columns to investigate their behaviour in fire and to partially validate the numerical model (Bisby et al. 2003b). This paper presents the results of parametric studies performed using the numerical model, and provides rational recommendations for the fire-safe design of FRP-wrapped columns in light of current and past research in this area.
BACKGROUND
FRP materials are sensitive to fire and experience severe deterioration of strength, stiffness, and bond properties at elevated temperatures (Bisby et al. 2001; Blontrock et al. 1999). Fire tests on FRP-plated concrete beams and slabs (Blontrock et al. 2001, 2000) have demonstrated the need for thermal protection of external FRP reinforcement to maintain its structural effectiveness during fire. Furthermore, all FRP materials are combustible when subjected to sufficiently elevated temperatures. Thus, concerns associated with the use of FRP in structures can be divided into two broad categories: environmental and structural. Environmental concerns are associated primarily with the potential for increased flame spread and smoke generation, and may be addressed using protective coatings or by limiting the temperatures experienced by the FRP during fire. Structural concerns, which are the focus of this paper, relate to the loss of structural effectiveness of FRP materials at high temperature.
Structural fire endurance modelling requires a detailed understanding of material behaviour at high temperatures. However, information on the deterioration of mechanical properties of FRP is extremely scarce, and a great deal of further research is required to fill all the gaps in knowledge. For purposes of the current study, a series of semi-empirical analytical expressions have been derived, based on information available in the literature, to describe the thermomechanics of carbon, glass, and aramid FRP at elevated temperatures. Figure 1 shows the deterioration in strength of FRP materials with increasing temperature, in comparison with reinforcing steel and concrete, based on semi-empirical relationships developed by Bisby (2003). Steel and concrete have been treated herein as recommended by Lie (1992).
NUMERICAL MODEL
A numerical model has been developed (Bisby et al. 2003a) to predict the behaviour in fire of FRP-wrapped columns. The model, which has been partially validated against experimental data from full-scale fire endurance tests on both unwrapped and wrapped circular reinforced concrete columns, uses a coupled heat-transfer/load-capacity analysis to determine the distribution of temperatures throughout a column during fire, and subsequently to estimate its load carrying capacity.
The heat transfer analysis discretizes the column cross-section into a number of ring elements (Fig. 2), and uses an explicit finite difference procedure similar to that used previously by researchers at the National Research Council of Canada (NRC) for modelling conventionally reinforced concrete members (Kodur and Lie 1997; Lie 1992; Lie and Celikkol 1991). The heat transfer model accounts for the variation in thermal properties of all the materials involved, and explicitly accounts for the evaporation of moisture from the concrete. The analysis assumes that the column is infinitely long, and that the contribution of the internal reinforcing steel to the heat transfer within the column is negligible.
The load capacity analysis consists of a strain-equilibrium approach in which the column cross-section is further discretized into a series of annular elements (Fig. 2). The analysis accounts for the deterioration in mechanical properties of all materials involved at high temperature. The numerical model is unique in that it accounts for the beneficial confining effect of an FRP wrap at an elevated temperature using a modified version of the Spolestra and Monti (1999) iterative confinement scheme. Failure of the column can be assumed to occur by crushing of the concrete under uniaxial compression or by buckling under the combined effects of axial compression and buckling. The model can also handle various types and sizes of
columns, and can account for the presence of internal steel reinforcement, FRP wraps, and supplemental insulation (applied to the exterior of the FRP wrap). A more detailed description of the model is given by Bisby et al. (2003a).
TEST PROGRAM AND MODEL VALIDATION
The numerical model described above has been compared against data from fire endurance tests on reinforced concrete columns available in the literature (Franssen and Dotreppe 2003; Lie and Celikkol 1991) and against data obtained from fire endurance tests on FRP-wrapped and insulated reinforced concrete columns conducted by the authors (Bisby et al. 2003b). Tests conducted by the authors consisted of full-scale fire endurance tests on two 400 mm diameter, 3810 mm long, spirally-reinforced concrete columns, wrapped with a single layer of a carbon/epoxy FRP strengthening system (the Tyfo SCH® system supplied by Fyfe Co., San Diego), and insulated with a unique two-component fire protection system (the Tyfo® EI/VG insulation system, also supplied by Fyfe Co.). The EI/VG insulation system is a unique two-component fire protection system consisting of a modified spray-applied cementitious plaster (VG) with a surface hardening intumescent epoxy coating (EI). For the two columns tested to date, different overall thicknesses of the insulation system were used: Column 1 was protected with an overall insulation thickness of approximately 57 mm, and Column 2 with approximately 32 mm. Details of the tested columns are presented in Fig. 3.
The columns were tested in the column furnace at NRC under exposure to the CAN/ULC S101 standard fire (CAN/ULC 1989). During testing, both columns were subjected to a sustained load equivalent to their full unfactored strengthened service load. The unfactored service load, 2515 kN, was determined by back-calculating from the ultimate design load for the
FRP-wrapped columns, which was calculated using the recommendations of the ISIS Canada Design Manual No. 4 (ISIS 2001) assuming a live-to-dead load ratio of 1-to-1.
Figure 4 shows predicted and observed temperatures during fire exposure at several key locations for an FRP-wrapped and insulated reinforced concrete column tested by Bisby et al. (2003b). A good agreement is observed between the model predictions and test observations, although the model does not precisely capture some of the subtleties observed in the experimental thermal profiles. For the purpose of fire endurance prediction, however, the model adequately predicts temperatures within the member. Figure 5 shows a comparison of predicted load capacity and observed failure loads for an FRP-wrapped reinforced concrete column tested by Bisby et al. (2003b), two unwrapped reinforced concrete columns tested by Lie and Celikkol (1991), and two unwrapped reinforced concrete columns tested by Franssen and Dotreppe (2003). The model appears to be capable of conservatively predicting the fire endurance of these members.
Based on comparisons with available test data, the numerical model was deemed to be conservative and satisfactorily predict the overall trends observed in the fire behaviour of FRP-wrapped and insulated circular reinforced concrete columns, such that it could be used to conduct qualitative parametric studies to investigate the behaviour of these members in fire.
PARAMETRIC STUDIES
While the behaviour of FRP-wrapped concrete columns in fire is extremely complex, the numerical model and experimental data demonstrate that it is possible to approximately predict the thermal and structural behaviour of both unwrapped and FRP-wrapped reinforced concrete columns during fire. However, information on the thermal properties of insulation systems and FRP wraps at high temperature is scarce, and the properties used in the analyses have been
assumed based on information available in the literature and from material suppliers. A more complete understanding of the thermal and mechanical properties of the materials involved (particularly with respect to the thermal properties of the insulation and the thermomechanical properties of the FRP wraps) is required before the numerical model can be used with confidence for specific fire-safe design of FRP-wrapped members and insulation schemes.
In the following discussion, the numerical model has been used to conduct parametric studies to investigate the effects of a number of parameters on the fire endurance and performance of FRP-wrapped and insulated reinforced concrete columns. The goal herein is to provide guidance as to which factors are likely to be important in the future design and testing of insulation schemes for FRP-wrapped members.
Failure Criteria
Three distinct failure criteria have been considered for FRP-wrapped reinforced concrete columns in fire:
• Criterion 1: The average temperature of the FRP wrap shall not exceed the glass-transition temperature of the polymer matrix. This criterion is assumed to provide some degree of protection against loss of the FRP confinement effect, and has been suggested as a design criterion to allow the use of FRP in buildings.
• Criterion 2: The temperature at the outside face of the FRP wrap shall not exceed the ignition temperature of the polymer matrix. This is thought to guard against evolution of toxic gas and smoke, and to reduce the potential for increased flame spread.
• Criterion 3: The load-bearing capacity of the column shall not fall below the full unfactored service load of the strengthened member. This is equivalent to the traditional ULC S101 failure criterion currently used for reinforced concrete columns in buildings in Canada.
Only criterion 3 is applicable to current North-American fire design guidelines, although flame spread and smoke generation characteristics are also important from an environmental fire protection standpoint. In conducting the parametric studies presented herein, the following assumptions have been made, unless otherwise stated:
• Supplemental Insulation, applied to the exterior of the FRP-wrap, consists of a modified spray-applied cementitious plaster (Tyfo® EI/VG). The thermal properties assumed for the insulation are presented by Bisby (2003). The room temperature thermal conductivity of EI/VG insulation is about 0.12 W/m°C. The insulation is assumed not to crack or debond.
• The FRP matrix consists of a specific epoxy resin, with a glass transition temperature (GTT) of 93°C (typical for wet lay-up FRP applications) and an ignition temperature of 450°C (Bisby 2003).
• The degradation of mechanical properties of FRPs at high temperature is assumed to be described by the semi-empirical analytical relationships shown in Fig. 1, which are based on a multi-variable least squares regression analysis of test data available in the literature. The thermal properties assumed for the FRP depend primarily on the matrix material, and are also presented by Bisby (2003).
• For FRP-wrapped columns, service loads have been determined by back-calculating from ISIS Design Manual No. 4 (ISIS 2001) with a live-to-dead load ratio of 1-to-1. The wrap is assumed to consist of a single layer of a typical carbon/epoxy FRP system available in industry. The assumed FRP properties are given in Table 1.
• The concrete columns are assumed to have dimensions and internal reinforcement details equivalent to those tested previously by Bisby et al. (2003b) (refer to Fig. 3).
Table 2 presents a summary of the various column, wrap, and insulation configurations that were analyzed during the parametric studies.
Comparison of Member Types
Before discussing results of parametric studies, it is instructive to examine the predictions of the model for various column types. Figure 6 shows predicted structural fire endurance curves (plots of load capacity versus time of fire exposure) for concrete columns with unwrapped, wrapped, and wrapped-and-insulated configurations.
The strength of the unwrapped column is predicted to decrease steadily during exposure to fire, as should be expected. The reduction in strength is such that the unwrapped column is predicted to fail under its 1996 kN service load after approximately 270 minutes of fire exposure (with service loads calculated in accordance with CSA A23.3-94 (CSA 1994) assuming a factored live-to-dead load ratio of 1-to-1). Adding a single layer of FRP wrap is predicted to increase the initial strength of the column by only about 5%. This can be attributed to the fact that the model assumes failure by buckling, and wrapping with FRP cannot be expected to increase the elastic modulus of concrete substantially. The service load on the FRP-wrapped column is 2515 kN, again calculated assuming a factored live-to-dead load ratio of 1-to-1. The increase in strength due to wrapping is predicted to be lost within minutes of fire exposure, and the strength of the column is subsequently reduced to slightly higher than that of the unwrapped column for the remainder of the fire exposure. Hence, the FRP-wrapped column displays a predicted structural fire endurance of only 195 minutes. This is 75 minutes less than the unwrapped column, and such a decrease in fire endurance would almost certainly be deemed unsatisfactory in most building applications. By insulating the column with a 25 mm thickness
of gypsum plaster insulation, the predicted fire endurance of the FRP-wrapped column is increased dramatically, to more than 5 hours.
The most significant observation that can be gleaned from Fig. 6 is that, for the wrapped and insulated column, the predicted fire endurance of the overall concrete member is significantly improved as compared with the unwrapped column, even at a higher applied load level. Thus, although the structural benefits of the FRP wrap may be difficult to maintain in fire, the overall fire endurance of the member is superior to that of the unwrapped member, and an increase in the fire endurance rating of the wrapped and insulated member certainly appears warranted.
Unprotected FRP-Wrapped Columns
The above discussion demonstrates that unprotected FRP wraps will be rendered ineffective within minutes of exposure to fire. Thus, during fire, unprotected wraps should be considered completely ineffective at providing confining reinforcement to concrete columns. In addition, there are significant flame spread and smoke generation concerns, not discussed in detail here, associated with the use of fire-exposed FRP in buildings.
FRP-wrapped and Insulated Columns
The numerical models and fire tests conducted to date suggest that, for insulated FRP-wrapped concrete columns, fire endurance can be improved by applying a layer of fire insulation to the exterior of the FRP wrap. However, a variety of fire insulation materials are currently available in industry, and it is unclear which of these are preferable for fire protection of FRP wraps. It seems reasonable that any insulation material which can be shown to provide good thermal protection for an FRP wrap, and which is known to remain intact during fire exposure, can be effectively used to increase the fire endurance of FRP-wrapped reinforced concrete
columns. Thus, in an attempt to determine which parameters are most important in the fire behaviour of FRP-wrapped and insulated reinforced concrete columns, and in the selection of insulation materials and schemes for these members, parametric studies have been conducted investigating various factors using the numerical fire simulation model described earlier. Parameters investigated to date include: the fibre type, insulation thickness, thermal conductivity, density and specific heat, FRP matrix GTT and ignition temperature, and the concrete aggregate type.
Effect of Fibre type
Figure 7 shows fire endurance curves (plots of axial load capacity versus time of exposure to the standard fire) for three identical reinforced concrete columns wrapped with carbon FRP (CFRP), glass FRP (GFRP), or aramid FRP (AFRP), and protected with 25 mm of gypsum plaster insulation. To generate the plots, it was assumed that the GFRP and AFRP wraps were applied in such a manner as to provide the same confinement ratio (fl/f’c) in pure
compression as a single layer of a typical CFRP sheet, which was accomplished by adjusting the sheet thickness. Table 1 provides the assumed room-temperature mechanical properties of the different FRP materials. Hence, the assumed thicknesses of GFRP and AFRP were 2.0 mm and 0.57 mm respectively, to give a lateral confinement pressure of 5.7 MPa at ultimate, as was calculated for a single ply of CFRP.
The thermal properties of the FRP wraps were assumed to be identical regardless of fibre type, since the transverse thermal conductivity of FRPs depends primarily on the thermal characteristics of the polymer matrix (assumed to be epoxy for all 3 systems). As such, there is no significant difference in the times to reach failure Criteria 1 and 2 for columns wrapped with different fibre types. In terms of Criterion 3, however, which relates to the load carrying
capacity of the column during fire, minor differences are evident in the predicted behaviour. Referring to Fig. 7, the predicted load capacity of the GFRP-wrapped column decreases comparatively rapidly under exposure to fire, whereas the load capacity of the AFRP-wrapped column actually increases briefly before decreasing to match that of the CFRP-wrapped column. This behaviour can be attributed to the different longitudinal coefficients of thermal expansion (CTEs) assumed for the respective FRP materials (refer to Table 1). GFRP has a positive longitudinal CTE, and hence the wrap becomes less effective (relaxes) due to heating. Conversely, AFRP has a mildly negative longitudinal CTE (it contracts on heating), so aramid wraps briefly become more effective under heating. CFRP displays negligible longitudinal thermal expansion, so the carbon-wrapped column displays behaviour that is intermediate between glass and aramid. The effect or fibre type is of only minor consequence in any case, since all three wrapped and insulated columns display outstanding structural fire endurance.
Effect of Insulation thickness
Figure 8 shows the relationship between insulation thickness and fire endurance based on the 3 failure criteria outlined previously. In Figure 8a, the Criterion 1 and 2 fire endurances of the members are plotted as a function of insulation thickness. As expected, the fire endurance increases for the increasing insulation thickness. In addition, the model suggests that maintaining the wrap temperature below its GTT for longer than 120 minutes will be difficult for insulation thicknesses that would be used in practice. The criterion 2 fire endurance displays a similar trend, although in this case fire endurances in excess of 4 hours could be achieved with as little as 15 mm of insulation.
The design service load for the FRP-wrapped column used in the development of Fig. 8 is 2515 kN, so the criterion 3 fire endurance is reached in about 195 minutes without insulation.
Figure 8b, which plots fire endurance curves for various insulation thicknesses, demonstrates that increasing the insulation thickness even slightly drastically improves the load carrying capacity of the FRP-wrapped column during fire. For instance, providing only 5 mm of gypsum plaster is predicted to increase the fire endurance to more than 300 minutes. The reader will note that the increased fire endurance is due not to the prolonged effectiveness of the FRP wrap, but to the temperatures within the concrete being maintained at sufficiently low values to prevent strength loss of the overall member.
Effect of Insulation Thermal Conductivity
To investigate the effect of the insulation’s thermal conductivity, it was assumed that the thermal conductivity remained constant with the increasing temperature. While this assumption is false for most insulation materials, it is useful for the purposes of illustration.
Figure 9 shows the effect of varying the thermal conductivity of a 25 mm thick layer of insulation on the fire endurance of an FRP-wrapped reinforced concrete column with respect to the three previously defined failure criteria. The thermal conductivity of the insulation is observed to play a crucial role in its effectiveness at prolonging fire endurance. A reduction in the insulation’s thermal conductivity from 0.5 W/m·K to 0.1 W/m·K results in an increase of the Criterion 1 fire endurance from 9 minutes to 41 minutes, and from 85 minutes to over 5 hours for Criterion 2. Within the range of thermal conductivities of currently available structural fire insulation materials (0.1 to 0.5 W/m·K), even slight reductions in thermal conductivity result in large increases in fire endurance.
Figure 9b shows fire endurance curves for columns with various insulation thermal conductivities. The benefits of low thermal conductivity insulation are immediately evident, particularly in the 0.1 to 0.5 W/m·K range. For example, a decrease in the insulation thermal
conductivity from 0.5 to 0.1 W/m·K results in a 35% higher load capacity at 4 hours of fire exposure.
Effect of Insulation Specific Heat
The effect of the insulation’s specific heat was investigated by assuming that the specific heat of the insulation remained constant with temperature. An essentially linear trend was observed between specific heat and fire endurance for Criteria 1 and 2, and materials with higher specific heats were found to be preferable. However, the effect of the insulation’s specific heat on predicted fire endurance was not nearly as dramatic as the effect of thermal conductivity. For instance, for a 25 mm thick insulation, an increase in the specific heat from 500 to 2500 J/kg·K results in Criterion 1 and 2 fire endurance increases of 16 minutes and 22 minutes respectively. The effect of increased specific heat was observed to be even less significant for smaller insulation thicknesses. Furthermore, low thermal conductivity and high specific heat are contradictory thermal properties.
The effect of the insulation’s specific heat on the load carrying capacity of the column during fire exposure was also predicted to be minimal, and only extremely mild improvements in structural behaviour were predicted for substantial increases in specific heat. For example, with all other parameters kept constant, an increase in the specific heat of the insulation from 500 to 2500 J/kg·K resulted in a load capacity increase of less than one percent after 4 hours of fire exposure. The insulation specific heat was thus deemed to be a factor of secondary importance.
Effect of Insulation Density
As was the case for the specific heat of the insulation, a linear trend was predicted between insulation density and fire endurance for Criteria 1 and 2. It was evident that higher density materials are preferable for insulation, although low thermal conductivity and high
density are also contradictory thermal properties for most insulating materials. For a 25 mm thick layer of insulation, an increase in density from 250 to 2500 kg/m3 results in Criterion 1 and 2 fire endurance increases of 50 minutes and 83 minutes respectively. The effect of increased density is less pronounced for smaller insulation thicknesses, with Criterion 1 and 2 fire endurance increases of only 10 minutes and 17 minutes, respectively, for the aforementioned increase in insulation density with a 10 mm thick layer of insulation.
The effect of the insulation’s density on the load carrying capacity of the column is minimal. Thus, while high density and high specific heat are preferred properties for insulation materials, thermal conductivity is the dominant parameter, and it should therefore be the primary consideration in the selection of insulation materials for FRP-strengthened concrete members.
Effect of Matrix GTT
A potential method to increase the fire endurance of FRP-wrapped columns is with the use of low thermal conductivity insulating materials, as discussed above. Another technique is to use FRP wrap materials with superior retention of mechanical properties at high temperature. While the GTT of currently used epoxy matrices is generally in the range of 65-150°C (ACI 2002), various polymer materials are available that have GTTs in excess of 300°C and are resistant to ignition and flaming. Indeed, a new generation of highly thermally resistant composite materials called Geocomposites are currently under investigation by several researchers (Balaguru et al. 1997; Kurtz and Balaguru 2001; Lyon et al. 1996). These materials are composed of carbon fibres, identical to those used in FRP composites, in conjunction with an inorganic geopolymer matrix (an alumino-silicate powder that reacts with water to form a solid material). Research into the use of Geocomposites has indicated that they demonstrate similar room-temperature physical and mechanical properties to FRPs, but that they display a superior
resistance to high temperatures. Kurtz and Balaguru (2001) reported retention of 63% of the room temperature tensile strength for a carbon/geopolymer composite after exposure to 800°C for one hour. Geocomposites are non-combustible up to at least 800°C.
Many high-temperature composites currently exist or are under development, and it is instructive to examine the effect of improving the thermomechanical properties of FRP on the overall fire performance of FRP-wrapped columns. Figure 10 shows the predicted fire endurance (based solely on the Criterion 1 fire endurance) as a function of matrix GTT, for various insulation thicknesses. An increase in the matrix GTT is seen, not surprisingly, to increase the Criterion 1 fire endurance, and the effect of increasing the matrix GTT is more pronounced for greater insulation thicknesses. Hence, a combination of increased matrix GTT in conjunction with supplementary fire insulation has the potential to drastically increase the Criterion 1 fire endurance of FRP-wrapped columns. The development of affordable and effective high-temperature composites is an area in which a great deal of further research is required.
Effect of Matrix Ignition Temperature
Figure 11 shows the predicted fire endurance of an FRP-wrapped column based on Criterion 2 failure for various matrix ignition temperatures. This plot is similar to Figure 10, in that an increase in the matrix GTT will increase the Criterion 2 fire endurance. Again, the effect of increasing the matrix ignition temperature on the fire endurance is more pronounced at greater insulation thicknesses. The development of ignition-resistant composites is another area in which further research is required.
In the fire safety design of conventionally reinforced concrete structures, the aggregate type is often an important consideration in assigning fire endurance ratings. The dependence on the aggregate type is mainly caused by the higher heat capacity of carbonate aggregate concrete, which increases to a value about 10 times that of siliceous aggregate concrete at temperatures near 700°C. This can be attributed to an endothermic reaction caused by the dislocation of dolomite that is present in carbonate aggregate. However, results of parametric studies on columns with carbonate or siliceous aggregates show that the predicted Criterion 1 and 2 fire endurances would be within 5 minutes of each other with an assumed insulation thickness of 25mm. Furthermore, siliceous and carbonate aggregate concrete columns were predicted to perform similarly during fire exposure from a structural fire endurance standpoint (Criterion 3). Hence, the effect of the aggregate type was assumed to be minimal for FRP-wrapped and insulated columns, likely because the concrete temperatures remain sufficiently low that differences due to the aggregate type have only a minor effect.
Unwrapped but Insulated Reinforced Concrete Columns
It is interesting to consider that the primary factor contributing to the predicted enhancement of fire endurance for FRP-wrapped and insulated concrete columns is the presence of supplemental insulation, rather than maintenance of the confining effect of the FRP wrap. Thus, an unwrapped but insulated column could be expected to achieve a similar level of fire endurance as a wrapped and insulated column under the same applied load. This is because both columns are assumed herein to fail by buckling at an elevated temperature, which depends largely on the modulus of the concrete and reinforcing steel in the column. Wrapping with FRP can be expected to increase the modulus of the concrete only very slightly, and thus the FRP wrap does not significantly increase the buckling strength of the column. Consequently, the loss
of effectiveness of the wrap has only minor consequences for the column, which is assumed to be loaded only to service load levels during fire.
However, FRP-wrapping is not performed to increase members’ fire endurance, and in most cases columns cannot practically be provided with insulation sufficient to prevent a loss of effectiveness of the wrap. Columns are wrapped for increased axial load capacity (or to improve seismic performance), and fire insulation is required in these cases to ensure that the column can carry the increased (wrapped) service load for an adequate period of fire exposure. Thus, it is important when assessing the fire endurance of FRP-wrapped concrete columns to use a holistic approach, and to keep the goals of structural fire engineering clearly in mind.
CONSEQUENCES FOR DESIGN
The above discussions indicate that it is unlikely that currently available FRP-wrapping materials will perform structurally for any significant period of time during exposure to fire unless they are provided with a substantial thickness of supplemental fire insulation. Depending on the failure criterion used, the critical factors in the fire design of FRP-wrapped reinforced concrete columns appear to be: the presence and thickness of supplementary insulation, the insulation’s thermal conductivity, and, depending on the thermal fire endurance criteria selected, the glass transition and ignition temperatures of the FRP polymer matrix.
Given the wide variety of column sizes, lengths, end-conditions, reinforcing details, concrete strengths, aggregate types, fibre and matrix types, and possible insulation materials, it is neither practical nor possible at this juncture to provide detailed guidelines for the fire design of FRP-wrapped reinforced concrete columns. It is more appropriate to examine these members on a case-by-case basis. However, test results presented by Bisby et al. (2003b), and the numerical parametric studies presented in the current paper, indicate that satisfactory fire endurances can be
obtained for FRP-wrapped concrete members provided that supplemental insulation is applied to the exterior of the FRP wrap, and that the insulation remains intact during fire.
Based on research conducted to date, the following guidelines for FRP-wrapped concrete members are suggested for fire-safety, with failure defined as structural collapse:
1. Under no circumstances should the strengthened (increased) service load on the upgraded column exceed the ultimate design strength of the unstrengthened column. Thus, maximum allowable strength upgrades for FRP-wrapped columns can be suggested for various dead-to-live load ratios using the following expression:
[1]
( )
φRn existing ≥(
SDL+SLL)
newwhere
( )
φRn existing is the factored strength of the existing member (before strengthening), Sis the strengthened (wrapped) service dead load, and is the strengthened service live load.
DL
LL S
2. Supplemental fire protection insulation is required in most cases. Depending on the degree of strengthening, an analysis should be conducted to determine the approximate thickness of fire insulation required to achieve the desired load carrying capacity during fire. Tests should be conducted to ensure that the fire insulation will remain in place.
The first design recommendation above is similar to the strengthening limit approach currently suggested in ACI 440.2R-02, Clause 8.2 (ACI 2002), and provides a measure of protection against vandalism and poor workmanship in addition to fire. Figure 12 shows the resulting allowable strength increases for members with various live-to-dead load ratios using the above recommendation. The limits indicate allowable strength increases that range from 25% to 50%.
Although it appears from Figure 12 that ACI 440.2R-02 allows greater strength increases than Eq. 1 above, it is important to remain cognizant of the fact that the load factors used by ACI are larger than those used in Canada, which accounts for much of the discrepancy. In addition, the ACI 440.2 allows greater strength increases for predominantly live-loaded members, since it is assumed that FRP wraps are often applied for increased live-load capacity, and live load is assumed to be reduced during a fire.
The second design recommendation above is essentially a statement of the ULC S101 fire endurance requirement. This concept has also been previously suggested by ACI 440.2R-02 (Clause 8.2.1) and is given by the following expression:
[2]
( )
Rnθ new ≥(
SDL+SLL)
newwhere is the nominal load capacity of the strengthened member at high temperature. Unfortunately, it is extremely difficult to estimate the nominal strength of an FRP-wrapped member at high temperature, and the numerical model described herein is the only existing tool that can be used to provide guidance in this regard.
( )
Rnθ newIf it can be reliably shown that the column can carry the increased service load for the required fire duration without the wrap and without supplemental insulation, then fire insulation may not be required on the basis of structural endurance. However, in such cases a fire-resistant coating would likely be required on the outside surface of the FRP to control the flame spread and smoke generation.
CONCLUSIONS
Based on the limited experimental data and parametric studies presented in this paper, the following conclusions can be drawn:
1. While it appears that it will be difficult in practice to prevent loss of effectiveness of FRP wraps during fire, the overall structural fire endurance of FRP-wrapped members can be enhanced by applying supplemental insulation to the exterior of the FRP wrap. The fire endurance of FRP-wrapped columns protected in this way can meet or exceed the fire endurance of the unwrapped column if properly designed.
2. Parametric studies, conducted with respect to 3 distinct failure criteria, indicate that primary factors to consider in the selection and design of insulation schemes for FRP-wrapped reinforced concrete columns are the insulation thickness and thermal conductivity. In addition, fire performance of FRP-wrapped concrete members can be improved by improving the high temperature mechanical properties of the FRP materials themselves, particularly by increasing the matrix GTT.
3. Simple conservative design recommendations have been suggested, similar to those currently used by Committee 440 of the American Concrete Institute, based on the assumption that FRP-wraps are rendered structurally ineffective during fire.
ACKNOWLEDGEMENTS
The authors are members of the Intelligent Sensing for Innovative Structures Network (ISIS Canada) and wish to acknowledge the support of the Networks of Centres of Excellence Program of the Government of Canada and the Natural Sciences and Engineering Research Council of Canada. The authors would also like to acknowledge the financial and technical contributions of the National Research Council of Canada, Queen’s University, and Fyfe Company.
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LIST OF SYMBOLS
Cpi insulation specific heat
f’c unconfined concrete compressive strength
fl lateral confining pressure at ultimate
kins insulation thermal conductivity
( )
Rnθ new nominal strength of the strengthened member at high temperature DLS service dead load on the strengthened (wrapped) member
LL
S service live load on the strengthened (wrapped) member Tign FRP polymer matrix ignition temperature
ti insulation thickness
( )
φRn existing factored strength of the existing un-strengthened memberρi insulation density
Table 1 – Assumed properties of FRP materials used in parametric studies
FRP Type Tensile strength (MPa) Elastic modulus (MPa) Thickness (mm) CTE* (×10-6˚C-1) Carbon/epoxy 1510 90.2 0.76 -0.1 Glass/epoxy 575 26.1 2.0 6.3
*
Coefficient of Thermal Expansion
Table 2 – Summary of columns analyzed in parametric studies
Parameter Studied Range
Failure†
Criterion Figure Wrap and insulation
configuration 1. Unwrapped 2. Wrapped 3. Wrapped/Insulated 3 6 Type of FRP wrap* 1. Carbon FRP 2. Glass FRP 3. Aramid FRP 1, 2, 3 7 Insulation thickness (mm) ti = 0-60 1, 2, 3 8 Insulation thermal conductivity (W/m·K) kins = 0.1-5.0 1, 2, 3 9 Insulation specific heat
(J/kg·K) Cpi = 500-2500 1, 2, 3 -- Insulation density (kg/m3) ρi = 250-2500 1, 2, 3 -- Matrix GTT (°C), Insulation thickness (mm) GTT = 50-500, ti = 0-60 1 10 Matrix ignition temp. (°C),
Insulation thickness (mm)
Tign = 200-800,
ti = 0-30 2 11
Aggregate type 1. Carbonate
2. Siliceous 3 --
*
Refer to Table 1
†
1 – Polymer matrix GTT exceeded
2 – Polymer matrix ignition temperature exceeded 3 – Load capacity insufficient to carry service load
LIST OF FIGURE CAPTIONS
Figure 1 – Deterioration in strength of various materials with temperature
Figure 2 – Discretizations of the column cross-section for heat-transfer and load capacity analyses
Figure 3 – Dimensions and reinforcement details of the columns tested by Bisby et al. (2003) Figure 4 – Predicted and observed temperatures in an FRP-wrapped and insulated reinforced concrete column during exposure to a standard fire (after Bisby et al. 2003)
Figure 5 – Predicted fire endurance curves and observed failure loads two FRP-wrapped reinforced concrete columns and two unwrapped reinforced concrete columns
Figure 6 – Predicted fire endurance curves for unwrapped, wrapped, and wrapped and insulated reinforced concrete columns
Figure 7 – Effect of fibre type on fire endurance
Figure 8 – Effect of insulation thickness on fire endurance: (a) Criterion 1 and 2 fire endurance
(b) Criterion 3 fire endurance
Figure 9 – Effect of insulation thermal conductivity on fire endurance: (a) Criterion 1 and 2 fire endurance
(b) Criterion 3 fire endurance
Figure 10 – Effect of matrix GTT on fire endurance
Figure 11 – Effect of matrix ignition temperature on fire endurance Figure 12 – Allowable strength increases for FRP-wrapped columns
Temperature (oC) 0 200 400 600 800 % of I n it ia l St rengt h 0 20 40 60 80 100 Concrete Reinforcing Steel Carbon/Epoxy FRP Glass or Aramid/Epoxy FRP
Column Centreline Fire Insulation Wrap Concrete Line of Symmetry Rc Rw
8 - 19.5 mm diameter bars longitudinal 11.3 mm diameter spiral w/ 50 mm pitch c/c 40 mm cover to spiral *all dimensions in mm A A ELEVATION 50 m m p it c h c/c ConcreteRebar SECTION A-A
Time (min) 0 60 120 180 240 300 T e mperat ure ( o C) 0 200 400 600 800 1000 1200 Test Data Insulation Surface FRP Surface Concrete Surface Model Predictions
Exposure Time (min) 0 60 120 180 240 300 Load C a pac it y (kN ) 0 1000 2000 3000 4000 5000 L&C Pred. L&C Test BISBY Pred. BISBY Test F&D Pred. F&D Test
* L&C - Lie and Celikkol (1991) BISBY - Bisby et al. (2003) F&D - Franssen and Dotreppe (2003)
Exposure Time (min) 0 60 120 180 240 300 Lo ad Ca pa city (kN) 0 1000 2000 3000 4000 5000 Unwrapped FRP-Wrapped
FRP-Wrapped and Insulated
Wrapped Service Load (ISIS 2001) Unwrapped Service Load (CSA 1994)
Exposure Time (min) 0 60 120 180 240 300 Load C apac it y ( k N) 0 1000 2000 3000 4000 5000 Carbon FRP Glass FRP Aramid FRP Unwrapped/Uninsulated Wrapped Service Load (ISIS 2001)
Insulation Thickness (mm) 0 10 20 30 40 50 60 F ire E ndura n c e (m in) 0 50 100 150 200 250 300 Criterion 1 Criterion 2 (a)
Exposure Time (min)
0 60 120 180 240 300 Load Capac it y ( k N) 0 1000 2000 3000 4000 5000 0 mm 5 mm 10 mm 15 mm 20 mm 25 mm Wrapped Service Load (ISIS 2001) (b)
Insulation Thermal Conductivity (W/m.K) 0.0 0.5 1.0 1.5 2.0 2.5 3.0 F ire En du ra n c e (mi n) 0 50 100 150 200 Criterion 1 Criterion 2 (a)
Exposure Time (min)
0 60 120 180 240 300 Loa d Cap a cit y (kN) 0 1000 2000 3000 4000 5000 k = 0.1 W/m.K k = 0.5 W/m.K k = 1.0 W/m.K k = 2.0 W/m.K k = 5.0 W/m.K Wrapped Service Load (ISIS 2001) (b)
Matrix Glass Transition Temperature (o C) 100 200 300 400 500 C riter ion 1 Fire En du ra nce (min ) 0 60 120 180 240 300 0 10 20 30 40 50 60 Insulation Thickness (mm):
Matrix Ignition Temperature (o C) 200 300 400 500 600 700 800 Criterion 2 F ire End uran c e (min) 0 60 120 180 240 300 0 5 10 15 20 25 30 Insulation Thickness (mm):
Live-to-Dead Load Ratio 0 1 2 3 4 5 Al lo w a b le Stre n g th In cre a se (% ) 0 20 40 60 80 100 ACI 440.2R-02 Cl. 8.2 Equation 1
