Page 1 of 8 FIRE TESTS ON NSM FRP STRENGTHENED AND INSULATED BEAMS Aniello PALMIERI Title Ph_D University or Affiliation Gent University Address Magnel Laboratory for concrete research, department of struct. Eng., Gent Belgium email address* [email protected]Stijn Matthys Title Prof. University or Affiliation Gent University Address Magnel Laboratory for concrete research, department of struct. Eng., Gent Belgium email address [email protected]Luc Taerwe Title Prof. University or Affiliation Gent University Address Magnel Laboratory for concrete research, department of struct. Eng., Gent Belgium email address [email protected]Abstract Fiber reinforced polymer (FRP) strengthened members experience loss of strength and stiffness properties under fire exposure. This can be attributed to the relatively poor performance of both adhesive and FRP matrix polymers at temperatures in the range of their glass transition temperatures. To evaluate the feasibility of achieving a fire-rated FRP system an investigation was undertaken to examine and document the performance of near-surface mounted (NSM) FRP strengthened concrete beams under fire conditions. The proposed paper discusses experimental results on 6 full-scale reinforced concrete beams exposed to fire for a time equivalent to 1 hour. All the specimens were pre-loaded to their service load. Tests results indicate that insulated NSM FRP strengthened beams can achieve a satisfactory fire endurance of one hour. The proposed paper also discusses the residual strength tests on fire tested beams, tested at room temperature up to failure. Results of this study suggest that if insulation system is able to maintain the adhesive temperature at relatively low temperature, the FRP concrete bond degradation under fire is limited and the FRP strengthened beam is still able to retain part of the original strength. Keywords: NSM, Fire, insulation, strengthening, RC beams. 1. Introduction Fiber reinforced polymers (FRPs) materials are currently produced in different configurations and are widely used for the strengthening and retrofitting of concrete structures. The use of FRP bars and strips as near surface mounted (NSM) is, lately, emerging as a promising strengthening technique. FRP bars or strips are installed by grooving the surface of the member and embedding the FRP reinforcement in the grooves with high strength adhesive (epoxy or mortar). This method is relatively simple and considerably enhances the bond of the FRP reinforcements, thereby using the material more efficiently. One of the main concerns in implementing FRP materials in buildings is their weak
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FIRE TESTS ON NSM FRP STRENGTHENED AND INSULATEDBEAMS
Aniello PALMIERITitle Ph_DUniversity or Affiliation Gent UniversityAddress Magnel Laboratory for concrete research, department of struct. Eng., Gent Belgiumemail address* [email protected]
Stijn MatthysTitle Prof.University or Affiliation Gent UniversityAddress Magnel Laboratory for concrete research, department of struct. Eng., Gent Belgiumemail address [email protected]
Luc TaerweTitle Prof.University or Affiliation Gent UniversityAddress Magnel Laboratory for concrete research, department of struct. Eng., Gent Belgiumemail address [email protected]
AbstractFiber reinforced polymer (FRP) strengthened members experience loss of strength andstiffness properties under fire exposure. This can be attributed to the relatively poorperformance of both adhesive and FRP matrix polymers at temperatures in the range of theirglass transition temperatures.To evaluate the feasibility of achieving a fire-rated FRP system an investigation wasundertaken to examine and document the performance of near-surface mounted (NSM) FRPstrengthened concrete beams under fire conditions. The proposed paper discussesexperimental results on 6 full-scale reinforced concrete beams exposed to fire for a timeequivalent to 1 hour. All the specimens were pre-loaded to their service load. Tests resultsindicate that insulated NSM FRP strengthened beams can achieve a satisfactory fireendurance of one hour. The proposed paper also discusses the residual strength tests on firetested beams, tested at room temperature up to failure. Results of this study suggest that ifinsulation system is able to maintain the adhesive temperature at relatively low temperature,the FRP concrete bond degradation under fire is limited and the FRP strengthened beam isstill able to retain part of the original strength.
1. IntroductionFiber reinforced polymers (FRPs) materials are currently produced in different configurationsand are widely used for the strengthening and retrofitting of concrete structures. The use ofFRP bars and strips as near surface mounted (NSM) is, lately, emerging as a promisingstrengthening technique. FRP bars or strips are installed by grooving the surface of themember and embedding the FRP reinforcement in the grooves with high strength adhesive(epoxy or mortar). This method is relatively simple and considerably enhances the bond of theFRP reinforcements, thereby using the material more efficiently.One of the main concerns in implementing FRP materials in buildings is their weak
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performance under elevated temperature and fire exposure. Temperature changes are expectedto affect the material properties of the concrete, FRP and the adhesive, but also the bondbetween these materials. As the temperature of the polymer matrixes approaches its glasstransition temperature Tg, the matrix transform to a soft, rubbery material with reducedstrength and stiffness [1-3]. Due to the degradation of FRP materials at high temperature,guidelines for FRP design available in codes and standard [4-5] required that the strength ofFRP be ignored unless a fire-protection system is used that can maintain the FRP temperaturebelow its critical temperature (defined as the lowest Tg of its components). Therefore, loadcarrying capacity of FRP strengthened structural member under fire exposure is influenced bythermo-mechanical properties of the adhesive.
To the knowledge of the authors few research projects have investigated the fire performanceof bonded FRP strengthening system subjected to fire exposure [6-9]. The fire performance ofFRP, however, has yet to be fully addressed and is a key area requiring further research. Toevaluate the feasibility of achieving a fire-rated FRP system an investigation was undertakenat Gent University to examine and document the performance of near-surface mounted(NSM) FRP strengthened concrete beams under fire conditions. Eighteen reinforced concretebeams were strengthened in flexure with NSM FRP bars and insulated with differentinsulation systems. The specimens were subsequently exposed to a standard fire of one andtwo hours, while submitted to their service load. In this paper, experimental results of six ofthe eighteen tested beams, exposed to fire for one hour, are presented and discussed. Thiscorresponds to the third furnace test, for the others furnace test reference is made to Palmieriet al. [10]
2. Experimental investigation
2.1 Test Specimen and Parameters
Figure 1 represents the geometry of the beams, the reinforcement arrangement and thenumber and position of the FRP bars or strips. The tested beams have a height of 300 mm, awidth of 200 mm and a length of 3150 mm. The dimensions of the beam specimens werechosen based on the dimensions of the floor furnace at the Warringtonfiregent laboratorywhich has a chamber 6 m long and 3 m wide. The lower reinforcement consist in 2 bars 16mm, the upper reinforcement consisted of 2 bars 10 mm. Closed stirrups 8 mm were usedfor the shear reinforcement.
3150
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25 Dial Gauges and LVDT
1000 500 100050075
3000
Strain Gauges
18
50 181857 57
2 GFRP rods 12 mm
2 CFRP rods 9 mm
50 15 601560
15
Figure 1. Test specimen.
The testing program involved the design and fabrication of ten beams. Four reference beams(an un-strengthened one and three strengthened beams) were tested at room temperature(more details about reference beams in terms of test set-up, mechanical properties of concrete,properties of FRP and reinforcement application can be found in [10]). Six insulated andstrengthened beams were subjected to fire. An overview of the fire test matrix is given in
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Table 1 in terms of FRP reinforcement, FRP diameter Df, FRP tensile strength ff and FRP E-modulus Ef (as reported by manufacturers), type of epoxy resin adhesive, adhesive’s glasstransition temperature (as reported by manufacturers), concrete batch number, concretecompressive strength fc at age of testing and age of fire testing.
The fire protection systems investigated were: a calcium silicate protection board (typePromatect L-500 supplied by Promat) and one insulation system composed by two ceramicbased coatings (type Hot Pipe Coating and Omega Fire supplied by Superior Product EuropeN.V.). The fire system protections were applied to the beams for a total length of 2900 mm.Figure 2 provides the layout of the different fire protection systems.
30+30 mm
20 m
m
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3015
30
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10 +
10
mm
25 + 20 mm
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35 + 20 mm
B1-F3-4
20 + 15 mm
B4-F3-1
10 + 10 mm
10 +
10
mm
10 +
10
mm
50+50 mm
20 m
m
B1-F3-1
3025
30
Figure 2. Layout fire protection system.
Beams B1-F3-1 and B2-F3-1 were protected with Promatect L -500 fixed in a U shaped form.The thickness of the plate at the bottom was 100 mm (composed of four plates Promatect L-500 with a thickness of 50 mm and a length of 1450 mm, screwed together) and 60 mm(composed of four plates Promatect L-500 with a thickness of 30 mm and a length of 1450mm, screwed together) respectively for beam B1-F3-1 and B2-F3-1. The thickness of theplates at the side faces of the element was 20 mm for both beams. Silicate glue (typePromacol k84 supplied by Promat) was applied along the entire length of the beams surfacewhere the fire board protection was applied, in between the board joints and for covering thescrews. The lower bottom plate was fixed with screws (with a distance of 150 mm) to theside-plates that were mechanically fixed to the side of the concrete beam with a spacing of200 mm. Beam B1-F3-2 and B1-F3-3 and B1-F3-4 were protected with Hot Pipe Coating(HPC), which was spray-applied to a thickness of 25 mm (beam B1-F3-2), 35 mm (beam B1-F3-3) and 20 mm (beam B1-F3-4) at bottom and of 10 mm to the side. The total thicknesseswere applied in different layers in a range of 0.2-1 mm each one. Moreover a layer of OmegaFire was spray applied to a thickness of 20 mm (beam B1-F3-2 and beam B1-F3-3) and 15mm (beam B1-F3-4) to the bottom and 10 mm to the side for all three beams. For beam B2-F3-1, in which the NSM FRP bar was embedded with an expansive mortar, a thin layer of
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HPC and Omega fire (10 + 10 mm at the bottom and the side) was provided for the insulation.According to the manufactures, the density and the thermal conductivity of the insulationsystems were, respectively, 500 kg/m3 and 0.09 W/mK for Promat L-500 and 527 kg/m3 and0.07 W/mK for the HPC/Omega fire.
2.3 Fire test procedure
The beams were tested simultaneously in a horizontal furnace of 6 m long by 3 m wide. Thebeams were exposed to fire from below, and the top surface was exposed to ambienttemperature. Fire testing standards require that structural elements need to resist only serviceloads during fire. Thus the ultimate flexural capacity and service load of the strengthenedbeams was determined according to Eurocode 2 [11] and fib Bullettin 14 [4]. The load wasapplied in four point bending and kept constant during fire test. Beams B1-F3-1, B1-F3-2,B1-F3-3 and B1-F3-4 were subjected to a service load of 72 kN and beams B2-F3-1 and B4-F3-1 were subjected to a service load of 81 kN. The service load, for all the strengthenedbeams, is restricted by the allowable compressive stress in the service limit state (SLS). Thebeams were instrumented with thermocouples (type K) at various depths in the concrete, onthe unexposed surface of the beams, on the lower steel reinforcement and in the epoxyadhesive. In addition, a displacement transducer (LVDT) was instrumented at the unexposedsurface of each beam to measure the deflection at midspan in the pre-load phase and duringfire testing. Details of the fire testing are presented elsewhere [10]. After all the beams wereloaded to their service load the fire test was started; a maximum of 1 hour fire exposure waschosen, during which the beams can be unloaded if one of the beams reach a criticalspan/deflection ratio of about l/30. During the fire tests the furnace temperature wascontrolled to follow the standard time-temperature curve according to ISO 834 [12]. Thisstandard prescribes the heating by the combustion gases as function of the time and is givenby equation 1:
Tgs =T0 +345log10(8t+1) (1)
Where Tgs = the temperature of the combustion gases [°C], T0 = the initial temperature [°C]and t = the time [min].
3. Fire endurance test results and discussionsVisual observations during the fire test were done through a number of small view portsaround the furnace walls. Upon completion of the test, the fire exposed side of the beams wasexamined; figure 3 shows, as reference, pictures of beam B1-F3-1 and B1-F3-3 after fireexposure.
a) b)
Figure 3. Beams a) B1-F3-1 and b) B1-F3-3 after fire exposure
Except for some discoloration and small cracks in the lower bottom protection board ofbeams insulated with Promat L-500, the fire insulation board system was completely intact
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and no signs of deterioration were observed (see figure 3a). After fire exposure theHPC/Omega Fire protection coating was consumed in portions of the exposed face withconsiderable cracks, but a layer of HPC was intact all along the length of the beams (seefigure 3b). After fire exposure the insulation system was carefully removed and no signs ofdamage were observed into the adhesive (resin or mortar) for all the tested beams.To obtain a fire endurance rating, according to the EN 1363-1 [13], the structural membersmust sustain the applied service load without structural failure, the temperature in thereinforcing steel should not increase more than 570 °C and the average unexposedtemperatures should not increase the initial average temperature by more than 140 °C. Theexperimental data demonstrate that all the beams obtained the fire endurance ratings of onehour by satisfying both thermal and load bearing criteria described above. A summary of themaximum temperature recorded at the unexposed concrete surface, at the bottom steelreinforcement and at the adhesive (epoxy or mortar) after 1 hour of fire exposure and the timewhen the epoxy reach the Tg are reported in table 2.
All the beams were loaded to their full service load and this load was kept constant during theone hour of fire exposure. Time-deflection curves have been recorded for all the tested beamsin order to record the increase of deflection of the beams under fire exposure. Figure 4 showsthe time-deflection curves of all the beams. From the diagrams it is clear that all the insulatedbeams were able to support the service load throughout the 1 hour fire tests without any signsof impending failure; no significant changes in the slope of the time–deflection curves wereobserved.
0
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ion
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B1-F3-1B1-F3-2B1-F3-3B1-F3-4
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Def
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Figure 4. Time-deflection curves.
Finally, code provisions limiting the temperature of the adhesive during fire event do notcurrently exist. It is however important to recognize that experimental results demonstratesthat even if the recorded temperature of the adhesive, for all the tested beams, exceeded its
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glass transition temperature in a time range between 33 and 49 minutes, no impending failureof the strengthened and insulated beams was observed. Then all the insulation materials wereable to keep the temperature of the adhesive at relatively low temperature in order that theinsulating beams were able to withstand the full service load for one hour of fire exposure. Itis noted that the temperatures recorded in the compression zone of the concrete and tensilesteel reinforcement were sufficiently low during fire exposure as to not significantly affecttheir mechanical properties. To know if the FRP strengthening is still active in some degreeafter one hour of fire exposure, residual strength testing has been conducted as discussed inthe next section.
4. Residual strength testsThe fire tested beams were stored for approximately one month at ambient temperature andthen tested up to failure to determine their residual strength. The test set-up was the sameadopted for the fire test and to test the reference beams at ambient temperature (T= 25ºC)[10]. Hence the fire damaged beams were all tested up to failure in 4 point bending, and wereinstrumented with LVDTs and dial gauges in order to measure electronically and manuallythe beams deflection at midspan, under the point loads and at both supports. Table 3summarize the experimental results in terms of ultimate load capacity (Qu), increase offlexural strength with respect of the unstrengthened beam tested at room temperature(Qu/QuB0), percentage of residual strength with respect of the strengthened beam tested atroom temperature (Qu/Qu,25) , failure mode and temperature of adhesive after one hour of fireexposure in function of the adhesive glass transition temperature (Tg= 62 ºC for beams B1-F3-1, B1-F3-2, B1-F3-3 and B1-F3-4 and Tg= 65 ºC for beam B2-F3-1).
B4-F3-1 67.3 1.17 0.91 Concrete Crush -*Nt: No tested
Experimental results shown that, for all the beams, the insulation systems were able to keepthe adhesive at relatively low temperature (in a temperature range 101 ºC and 131 ºC) so thatthey retained a significant part of the original strength at ambient condition. Indeed even afterone hour of fire exposure all the tested beams were able to increase their flexural strength upto 56 % in comparison to that of the unstrengthened beam B0 showing a residual strength in arange between 86% and 92% in comparison to that of the FRP strengthened beams tested atambient temperatures. Thereby residual strength tests effectively demonstrated that, if wellinsulated, the adhesive start to lose effectiveness above the Tg but is still able to retain part ofits strength and stiffness until reaching a certain critical temperature (more research is neededto define the critical temperature of the adhesive).The experimental load-midspan deflection curves of beams B1-F3-1 and B1-F3-3, shown infigure 5 as reference, were in close agreement with that of the FRP strengthened beam B1until they fail under the applied loads.
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0
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0 10 20 30 40 50 60 70 80 90 100
Loa
d [
kN
]
displacement [mm]
B1_20
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83 kN concretecrush started
Debonding FRP bar
0
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Lo
ad
[k
N]
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B1-F3-3
B1_20
B1-F3-3
84 kN concretecrush started 89 kN debonding FRP bar
Figure 5. Load-deflection curves beams B1-F3-1 and B3-F3-3.
5. ConclusionsBased on the results of fire tests discussed herein, the following conclusions can be made.Despite the higher service load applied to the fire tested beams, the investigative program hasclearly demonstrated the feasibility of providing a fire endurance of 1-hour fire enduranceratings under service loads if adequate protection from the fire is provided. For none of thestrengthened beams FRP NSM detached visibly.
The insulation systems evaluated herein appears to have effectively protected the NSM FRPstrengthened beams from heat penetration; adhesive’s temperature was maintained below itsglass transition temperature for a time ranging between 33 to 49 minutes depending on theinsulation performance and bond properties. All the insulation systems maintained theadhesive temperature to relatively low temperatures and no impending failure was observedfor the 1 hour of fire exposure.
After fire exposure, the insulated beams were able to retain the unstrengthened flexuralcapacity by maintaining the temperature of the concrete and the steel respectively below140°C and 570°C. Moreover residual strength tests on fire tested beams have demonstratedthat, if the insulation is able to maintain the adhesive temperature below 150°C the FRP isable to retain bond strength to the concrete and the beam is still able to retain a considerablypart (in this test program up to 92%) of the flexural capacity of the FRP strengthened beam atambient condition.
6. AcknowledgmentsThe authors wish to acknowledge the support of the FWO and companies Hughes Brothers,Fortius, Schoeck, Promat, and Superior Product Europe N.V. for providing testing materials.Fire tests have been conducted in cooperation with Warrington Fire Gent.
7. References[1] Blontrock H., Taerwe L., Matthys S., Properties of Fibre Reinforced Plastics at Elevated
Temperatures with Regard to Fire Resistance of Reinforced Concrete Memberspresented at the 4th Int. Symposium on FRP Reinforcement for Concrete Structures(FRPRCS-4), Baltimore, USA, October 31- November 5 ,1999.
[2] Kodur, V., Bisby, L., Green, M., Preliminary guidance for the design of FRP-strengthened concrete members exposed to fire, Journal of Fire Protection Engineering,Vol 17, 2007, pp 5-16.
[3] Y.C. Wang, P.M.H. Wong, V. Kodur. An experimental study of the mechanical
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properties of fibre reinforced polymer (FRP) and steel reinforcing bars at elevatedtemperatures, Composite Structures, Vol. 80, Issue 1, September 2007, pp 131-140
[4] CEB-FIP Task Group 9.3, Externally bonded FRP reinforcement for RC structures.Comite’ Euro-International du Beton. Bulletin 14 2001.
[5] American Concrete Institute (ACI). Guide for the design and construction of externallybonded FRP systems for strengthening concrete structures. Farmington Hills, MI: ACI440.2R-08, ACI Committee 440; 2008. p. 1–80.
[6] Burke P.J. Low and High Temperature Performance of Near Surface Mounted FRPStrengthened Concrete Slabs, Queen’s University, Canada, thesis, 2007
[7] Blontrock H., Taerwe L., Vandevelde, P., Fire Testing of Concrete Slabs Strengthenedwith Fibre Composite Laminates, presented at the 5th Annual Symposium on Fibre-Reinforced-Plastic Reinforcement for Concrete Structures (FRPRCS-5), London, UK,July 16-18,2001.
[8] Rein G., Abecassis Empis C., Carvel R., The Dalmarnock Fire Tests: Experiments andModelling, published by School of Engineering and Electronics, University ofEdinburgh, (2007).
[9] Williams B., Kodur V., Green M., Bisby L., Fire endurance of Fiber-ReinforcedPolymer Strengthened Concrete T-Beams, ACI Structural Journal, 105(1),60-67 (2008).
[10] Palmieri A, Matthys S., Taerwe L., Experimental investigation on fire endurance ofinsulated concrete beams strengthened with near surface mounted FRP barreinforcement, Compo: part B- Eng,In Press,2011.
[11] CEN, Eurocode 2: EN 1992-1-1: Design of concrete structures – Part 1-1: General rulesand rules for buildings, ed. CEN, 2004
[12] ISO 834 Fire Resistance Tests – Elements of Buildings Construction – Part I – GeneralRequirements, ISO 1999
[13] UNI EN-1363-1. Fire Resistance Tests – General Requirements – Part I, CEN 1999
