Thermal and mechanical characterization of fibre reinforced polymers, concrete, steel, and insulation materials for use in numerical fire endurance modelling

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Thermal and mechanical characterization of fibre reinforced polymers,

concrete, steel, and insulation materials for use in numerical fire

endurance modelling

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T h e r m a l a n d m e c h a n i c a l c h a r a c t e r i z a t i o n o f

f i b r e r e i n f o r c e d p o l y m e r s , c o n c r e t e , s t e e l ,

a n d i n s u l a t i o n m a t e r i a l s f o r u s e i n n u m e r i c a l

f i r e e n d u r a n c e m o d e l l i n g

N R C C - 4 9 6 8 4

C h o w d h u r y , E . U . ; G r e e n , M . F . ; B i s b y , L . A . ;

B é n i c h o u , N . ; K o d u r , V . K . R .

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Thermal and Mechanical Characterization of Fibre Reinforced

Polymers, Concrete, Steel, and Insulation Materials for use in

Numerical Fire Endurance Modelling

E. U. Chowdhury1, M. F. Green1, L.A. Bisby1, N. Bénichou2 and V. K. R. Kodur3 1

Queen’s University, Canada 2

National Research Council, Canada 3

Michigan State University, U.S.A

Abstract

The greatest impediments to using fibre reinforced polymer (FRP) composites in buildings and parking garages are their susceptibility to degradation when exposed to elevated temperatures and the limited knowledge on the thermal and mechanical properties of these composites at elevated temperatures. Small-scale material tests are performed to study the thermal and mechanical properties of FRP composites, as well as concrete and steel, at elevated temperatures. Among the thermal properties that are investigated in this study are mass loss with temperature, thermal conductivity, specific heat, and glass transition temperature of different types of currently available FRP composites for infrastructure. The stress-strain behaviour of FRP composites, steel and concrete, and FRP-to-FRP and FRP-to-concrete bond strength, are investigated at elevated temperatures. This paper presents preliminary results from the thermal analyses conducted on carbon/epoxy FRP composites, and three types of insulation materials (intumescent, gypsum, and cementitious). Results from these material tests will be used in refining and calibrating numerical fire endurance models developed by the authors.

Keywords: FRP, insulation, elevated temperatures, thermal and mechanical properties.

Mark F. Green

Civil Engineering, Queen’s University 58 University Avenue

Kingston, ON K7L 3N6 Canada

Email: greenm@ce.queensu.ca Tel: 613-533-2147

1.0 Introduction

Fibre reinforced polymers (FRPs) are increasingly being applied in many areas of construction, particularly for rehabilitation of concrete, steel, and timber structures. FRP repair has emerged as an alternative to rehabilitating deteriorated reinforced concrete structural members with steel plates because FRPs offer numerous advantages over steel, including high strength to weight ratios, resistance to electrochemical corrosion, and ease and speed of application. Carbon and glass fibre FRP composites are predominantly used in the rehabilitation of concrete structures. For external strengthening of reinforced concrete members, the FRP sheets or plates are bonded to the exterior of the members using adhesives such as epoxy resins.

FRPs have been widely used in the rehabilitation of bridges; however, concerns associated with fire remain an obstacle to applications of FRPs in buildings and parking garages, largely due to their susceptibility to degradation of mechanical and bond properties at elevated temperatures. At high temperature, all polymer resins will soften and eventually ignite, causing the resin matrix to weaken and raising potential concerns regarding the structural integrity of FRP strengthened concrete structures during fire. In addition, little is known about the mechanical and thermal properties of materials used in FRP strengthened concrete members, especially currently available FRP systems and fire insulation materials. Research is being conducted at Queen’s University in conjunction with the National Research Council (NRC) of Canada and industry partners to investigate the effects of fire on the mechanical and thermal properties of FRPs, concrete, reinforcing steel, and various insulation materials used in fireproofing FRP strengthened concrete structures. Small-scale material tests are being conducted to characterize the mechanical and thermal properties of these materials. This paper presents the results of an initial test program to study the material properties of FRPs and insulation materials at elevated temperatures.

2.0 Research Needs

As part of this research study, the authors are involved in developing numerical heat transfer and structural response models for flexural and axial concrete members strengthened with FRPs [1,2,3]. These numerical methods of analysis could considerably reduce the costs incurred in standard fire testing of full-scale specimens, provided that accurate material behaviour at high temperature is properly accounted for in the analysis. Information on the thermal and mechanical behaviour of currently available externally bonded FRP strengthening systems at high temperature is scarce. As a result, externally-bonded FRP strengthened concrete members must currently be able to meet fire endurance requirements without any strength contribution from the FRP [4]. Tests have shown that application of insulating fire protection can increase the fire endurance of FRP strengthened concrete members [1,2], but limited information is available regarding the thermal properties of these specific insulating materials.

3.0 Background

Unlike concrete and steel, little is known about the mechanical and thermal properties of FRP composites at high temperature. The thermal and mechanical properties of FRPs depend on the type of fibre and resin matrix, fibre volume ratio, and modulus of elasticity of the fibres and matrix materials [5]. Thus, numerous different formulations of FRP materials are available, making it difficult to generalize their thermal and mechanical behaviour in fire.

3.1 Thermal Properties of FRP Strengthening Systems at High Temperature

Carbon fibres tend to oxidize at temperatures above 300 to 400°C and melt at temperatures around 4000°C [6]. Although glass fibres are not susceptible to oxidation, they begin to soften around temperatures of 650 to 970°C and melt above 1225°C [6,7]. Except for CFRP composites in the direction of the fibres, which are highly conductive, FRP products usually have low thermal conductivity in both the longitudinal and transverse directions. The values reported in Table 1 have all been measured at room temperature and may change considerably at high temperature. The extent of the dependence of thermal conductivity on temperature is currently unknown. Table 1: Thermal conductivities of common construction materials [8,9]

Thermal Conductivity (W/m-°C) Material Longitudinal Transverse Glass/Epoxy FRP 3.46 0.35 Carbon/Epoxy FRP 48.44 to 129.75 0.865 to 0.04 Steel 15.6 to 46.7 Aluminium 138 to 216 Concrete - Epoxy 0.346

3.2 Mechanical and Bond Properties of FRP Strengthening Systems at High Temperature The fire behaviour of FRP composites depends predominantly on the behaviour of the polymer resin matrix/adhesive. Polymer resins soften at their glass transition temperature, Tg, thus limiting

the transfer of stress between the fibres [10]. The Tg of resin systems typically used in

construction applications are in the range of 65 to 120°C [11]. Blontrock et al. [12] state that the strength and stiffness of FRP composites start degrading rapidly at temperatures close to the glass transition temperature of their constituent polymer resin. Furthermore, both epoxy and polyester based composites will quickly ignite when they are exposed to fire, typically at temperatures in the range of 300 to 400°C [13], thus mechanical properties of the composites will significantly and irreversibly deteriorate due to combustion of the polymer resin at these temperatures [14]. For thermally thick FRPs (greater than about 5 or 10 mm), once the polymer matrix near the surface of the FRP has burned, it forms a char layer of low thermal conductivity; however, the thin layer of char provides minimal protection from fire [1,2,11]. Such a protective char layer is not expected to form on thin composites typically used for repair of concrete.

Rehm and Franke [15] and Sen [16] have investigated the tensile strength of different types of glass fibres at varying temperatures. From these studies, it was concluded (independent of the glass fibre types) that glass fibres lose about 50% of their original tensile strength above 550°C. Carbon fibres have demonstrated much more resistance to high temperatures [7,17]; they experience little to no change to their tensile strength up to temperatures of more than 1000°C. Gates [18] has shown that changes in the transverse and shear moduli of carbon FRPs were more pronounced at elevated temperatures than those observed for the longitudinal modulus, which showed no significant changes up to 200°C. It should be noted that the glass transition temperature quoted in the study was 220°C, which would be unusually high for an infrastructure composite. Gates [18] also observed a reduction in strength of 40 to 50% at temperatures around 125°C and 80 to 90% at 200°C, which was well below Tg.

Kumahara et al. [19] conducted an investigation of the tensile strength of various available glass and carbon FRP composite reinforcing bars for concrete at elevated temperatures. The findings in this study included a tensile strength reduction of up to 25% at about 100°C and 50% at 250°C for carbon/epoxy reinforcing bars, and a tensile strength reduction of up to 20% at about 100°C and 40% at about 250°C for glass/vinylester reinforcing bars. Below temperatures of 250°C, there were only negligible changes to the elastic moduli of the FRP rebars. Alsayed et al. [20] demonstrated that strength was lost more rapidly than stiffness with increasing temperature in CFRP. In this study, CFRP bars retained about 35% of their original strength and 40% of their original stiffness at 350°C.

The bond between FRP laminates and concrete is an important issue because it is typically the limiting parameter when strengthening concrete structures with FRP [21]. Studies [22,23,24] have shown that, under room temperature conditions and without specific preventative measures, premature failure due to debonding of FRP laminates is the most common type of failure in FRP strengthened concrete beams and slabs. External application of FRP laminates requires them to develop and transfer high shear forces through the interface between the adhesive or polymer resin and the concrete substrate. The bond properties between concrete and FRPs deteriorate with increasing temperature, which will eventually lead to the delamination of the FRP laminate and the ensuing loss of interaction between the FRP and the concrete [25].

Gamage et al. [25] performed an experimental investigation on the shear bond between CFRP and concrete at elevated temperature. The bond was unaffected at temperatures ranging between 22 and 36°C, however, the shear bond strength deteriorated rapidly when the temperature increased between 60 and 70°C. At temperatures greater than 60°C, the CFRP laminate peeled off from the concrete surface in contrast to concrete rupture at temperatures below 50°C. Although, Gamage et al. [25] did not report the glass transition temperature of the resin matrix, the manufacturer of the FRP system used in this study reports the glass transition temperature to be between 70 and 75°C. Katz et al. [26] and Sumida et al. [27] investigated the bond strength of FRP bars in concrete at elevated temperature by conducting pullout tests. Up to temperatures of 100°C, there was no significant loss of bond, but beyond 100°C, the bond strength decreased drastically. Katz et al. [26] tested glass FRP bars with different surface textures having glass transition temperatures ranging from 60 to 124°C. Katz et al. [26] reported that the bond strength decreased to about 10% of the original strength at a temperature between 200 and 250°C.

Residual mechanical and bond properties of externally-bonded carbon and glass FRP systems for concrete after exposure to elevated temperatures were investigated by Bisby and Foster [28,29]. In these studies, severe degradation of the residual tensile strength of the FRPs was observed at temperatures less than the thermal decomposition temperature (TDT) of the polymer resin, which occurred around 367°C, but more than Tg, which was about 75°C. The residual bond properties of

these FRPs to concrete were severely degraded after exposure to temperatures at about only 150°C. The loss of bond strength was attributed to moisture evaporation causing damage to the adhesive layer.

3.3 Thermal Properties of Fire Insulation Materials

Whatever fire insulation material is used in protecting the FRP strengthening system, the protection is generally not sufficient to prevent the FRP temperature from rising above Tg in a

combust within a few minutes of fire [31]. The most common types of fire insulation materials used in civil and structural applications are gypsum board, spray-applied cementitious or gypsum based mortars and intumescent coatings. Bénichou and Sultan [32] have reported the thermal properties of gypsum and various other insulation materials. Material tests were performed to determine thermal conductivity, thermal expansion/contraction, specific heat, and mass loss with temperature of fire insulation materials. Results from this study are not presented here.

4.0 Experimental Procedure

4.1 Mechanical and Bond Tests

The experimental procedure for the current study is based on earlier research performed at Queen’s on the residual properties of FRPs after exposure to elevated temperature [28,29]. The experimental program will include (1) tension tests on FRP coupons to investigate the stress-strain behaviour of FRP, (2) tension tests on single-lap FRP-to-FRP bonds, and (3) pull-apart bond tests on FRP-to-concrete bonds. During these tests, the specimens will be loaded while exposed to elevated temperature. The tests will be conducted in an INSTRON Universal Testing Machine, which has a custom designed thermal chamber with an internal dimension of 250 mm (width) by 250 mm (depth) by 300 mm (height), and a maximum load capacity of 600 kN. Material tests can be conducted at temperatures ranging from ambient to 625°C. An EPSILON Model 3448 self-supporting high temperature axial strain gage extensometer will be used to measure the strains experienced by the specimens during the tension or compression tests. This high-temperature contact extensometer can operate equally well in tension and compression at temperatures up to 1200°C. An advanced photogrammetry system will also be used to record deformations and strains.

Tension tests and Mode II (shear bond) FRP-to-FRP bond overlap tests will be conducted at elevated temperatures on FRP coupon specimens having a width of 25 mm. The length of the FRP coupon specimens must be greater than the height of the thermal chamber, which is 300 mm. This will ensure that the coupon gripping system will be outside the thermal chamber, thus preventing anchorage failure. Glass FRP tabs will be attached on each end of the FRP coupons, as shown in Figure 1, to prevent failure of the coupons within the mechanical wedge action grips. For Mode II FRP-to-FRP bond tests, FRP coupon specimens will have an overlap of 51 mm as shown in Figure 1(b). Both tension and Mode II tests will be conducted at various temperatures between ambient temperature and 400°C. Once the FRP specimens achieve the desired temperature inside the thermal chamber of the INSTRON testing machine, tension load will be applied to the specimens until failure occurs. Five specimens will be tested at each temperature to permit statistical analysis of the experimental data. The details of specimens for the Mode II FRP-to-concrete pull-apart bond test are yet to be determined since the specimens tested by Bisby and Foster for residual mode II FRP-to-concrete bond [29] would be too large for the current apparatus.

Compressive strength tests will be conducted at a range of temperatures on concrete cylinders having 102 mm diameter and 203 mm height. Steel reinforcing bars will also be tested at various temperatures in tension. Similar to the FRP test regime, five sample specimens will be tested to failure at each desired temperature.

4.2 Thermal Analysis

Thermal investigations for the FRP composite will include thermogravimetric analysis (TGA), which will be conducted independently on the fibres, resin and cured FRP laminate, and differential scanning calorimetry (DSC) and dynamic mechanical thermal analysis (DMTA),

which will be conducted only on the resin to determine its glass transition temperature. The thermal conductivity of the FRP laminate will be established using a transient-state measurement technique (Hot Disk System). Such a transient measuring system is more representative of temperature changes during a fire scenario than are more conventional steady-state temperature tests [33]. The temperature will range from ambient temperature to about 400°C, at which point the polymer resin will be in danger of igniting. In a similar manner, TGA and thermal conductivity tests will be conducted on concrete and various insulation systems to obtain the thermal properties with increasing temperatures.

> 300 mm

63.5 mm 63.5 mm

FRP specimen _{GFRP tabs} FRP specimen _{GFRP tabs}

> 300 mm

63.5 mm 63.5 mm

(a) (b)

51.0 mm overlap

Figure 1: FRP coupon schematic for (a) tensile tests and (b) Mode II FRP-to-FRP bond-overlap test (adapted from previous works [28,29])

5.0 Preliminary Results

Initial results from thermal analyses of carbon/epoxy FRP and insulation materials from two different manufacturers are presented in this paper. Results from the thermogravimetric analyses are shown in Figure 2. The epoxy sample specimens retained 95% of their room temperature mass until 300°C as shown in Figure 2(a). Between 320 and 510°C, there was a rapid drop in the mass of the epoxy sample specimens, which indicated severe degradation of their material properties. The mass loss of the epoxies occurred between temperatures where Griffis et al. [34] have previously observed an increase in the specific heat of carbon FRP.

In Figure 2(b), both carbon and glass fibres retained 90% of their room temperature mass until 750°C. On the other hand, there was a variation in the mass loss between the two different types of carbon FRP sample specimens. As shown in Figure 2(c), carbon FRP 2 retained 35% of its original mass until 800°C. There was a rapid drop of mass between the same temperatures as observed for the epoxy sample specimens. This drop in mass of the FRP sample specimens indicates the decomposition of the epoxy. However, carbon FRP 1 was able to retain 10% more of its original mass in comparison to carbon FRP 2 at the same temperature. This is likely due to a difference in the sizing on the specific carbon fibres tested. Also shown in Figure 2(c) is the variation in mass of a carbon/epoxy FRP (carbon FRP 3) tested by Dimitrienko [35]. The difference between the three data sets can be explained by the different carbon/epoxy combination of the three FRP specimens.

TGA results of a gypsum-based insulation material, a cementitious-based insulation material, and one intumescent material are presented in Figure 2(d). There was no significant mass loss from the gypsum-based or cementitious-based insulation specimens until about 100°C. Beyond 100°C,

the mass started gradually decreasing with the evaporation of moisture. At 800°C, these two insulation specimens had lost about 14% of their original mass. The intumescent paint started losing mass after its activation temperature, which was approximately 235°C. This corresponded to the onset of intumescence (i.e. foaming and charring). Rapid expansion and charring was observed between temperatures 325 and 525°C. The thickness of the intumescent paint, during expansion, was at least 50 times its original thickness. The intumescent paint maintained its shape well until 600°C, after which point it contracted into a thin layer of residual char.

Temperature (°C) 0 200 400 600 800 No rm ali zed Mass 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Sp ecific H eat (kJ/kg ·K) 1 2 3 4 5 6 Epoxy 1 Epoxy 2 Specific Heat for CFRP [34] Temperature (°C) 0 200 400 600 800 Nor m alized Mass 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Carbon fibres 1 Glass fibres 1 Carbon fibres 2 Temperature (°C) 0 200 400 600 800 Nor m alized Mass 0.0 0.2 0.4 0.6 0.8 1.0 1.2 CFRP 1 CFRP 2 CFRP 3 [35] (a) (b) (c) (d) Temperature (°C) 0 200 400 600 800 Norm alized Ma ss 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Gypsum-based insulation Cementitious-based insulation Intumescent material

Figure 2: Thermogravimetric analysis of (a) resin systems (b) bare carbon and glass fibres, (c) carbon FRP systems, and (d) insulation systems

Results from the DSC analysis for the epoxy of carbon FRP 2 are shown in Figure 3. During the DSC, the epoxy specimen went through a heating and cooling cycle, where the maximum temperature was 180°C and the minimum temperature was -50°C. The glass transition temperature of the epoxy was characterized by a gradual decrease in the heat flow as a function of temperature, which was observed between 70 and 90°C. This suggests that heat was being absorbed by the specimen, and thus the heat capacity was increasing. The midpoint of the transition region, which was 82°C, was taken to be the glass transition temperature of the epoxy. The glass transition temperature obtained from the DSC analysis in this study was higher than the manufacturer stated value of 71°C [30]. The glass transition temperature for the epoxy of carbon FRP 1 was reported to be 93°C [13]. Based on the TGA data and the specific heat data from

Griffis et al. [34], shown in Figure 2, the thermal decomposition temperature (TDT) of the carbon/epoxy FRP would be approximately 320°C, which is much higher than its glass transition temperature. In a fire, FRPs will lose strength starting at some temperature slightly below Tg. At

some critical temperature, likely between Tg and the TDT, the FRP will no longer have the

capacity to provide sufficient strength to be considered to be performing structurally. To identify such critical temperatures with respect to the mechanical and bond properties of FRPs, further small-scale tests are being performed by the authors.

Figure 3: Differential scanning calorimetry of Epoxy 2 6.0 Concluding remarks

To properly understand the behaviour of FRP strengthened concrete structures in fire, better information regarding the properties of FRP at high temperatures is needed. At temperatures above the glass transition temperature of the resin (60 to 120°C), the mechanical properties and bond strength of the FRP reduce. At temperatures between 300 and 500°C, resins typically decompose or combust. At some point between these two extremes, a critical temperature will be attained, above which the composite will have inadequate structural strength. A test program to determine such critical temperatures for FRP composites as well as thermal properties of insulating materials for fire protection of FRP strengthened structures was detailed. These material properties will be incorporated into numerical models for predicting fire endurance of FRP strengthened concrete structures.

7.0 Acknowledgements

The authors are members of the ISIS Canada Research Network and would like to thank Fyfe Corp. and BASF for their support of this project.

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