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Behavior of EBR FRP Strengthened Beams Exposed to Elevated Temperature
Available online at www.sciencedirect.com
ScienceDirect Procedia Engineering 193 (2017) 297 – 304
International Conference on Analytical Models and New Concepts in Concrete and Masonry Structures AMCM’2017
Behavior of EBR FRP strengthened beams exposed to elevated temperature Rafaá KrzywoĔa* a
Silesian University of Technology, Department of Structural Engineering, Akademicka 5, Gliwice 44-100, Poland
Abstract Externally bonded laminates based on high strength fibers nowadays are often used to strengthen the structures. It is possible, that some of those strengthenings are subjected to direct insolation. The paper presents the dangers appearing with the rise of temperature caused by the infrared solar radiation. Heating conditions were planned on the basis of exposition of concrete samples during the summer months in the southern Poland. The maximum recorded temperatures in the adhesive layer reached 65°C and it was therefore 20°C higher than the glass transition temperature of commercially available adhesives based on epoxy resin. Laboratory tests included a group of fifteen reinforced concrete beams in the real scale, seven of them were strengthened with externally bonded CFRP strip, seven with SRP tape. They were tested in various temperature conditions, from 20°C to 80°C. Beams were heated from the strengthened side with the use of linear infra-red radiators and when the temperature of the adhesive layer reached the predetermined value, beams were loaded to failure. Noticeable impact of temperature appeared from about 50°C. In all cases failure was followed by delamination. Differences in behavior of CFRP and SRP strengthened beams were observed. Delamination of CFRP strip appeared in unexpected way, without any specific symptoms, while SRP failed with grater deflection and lower mid-span strains compared to the beam tested at room temperature. At temperatures above 65°C appears significant bearing capacity degradation. It means that CFRP strengthening, which could be subjected to direct sun exposition, should always be protected by thermal insulation. © 2017 2017The TheAuthors. Authors. Published by Elsevier Ltd.is an open access article under the CC BY-NC-ND license © Published by Elsevier Ltd. This (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the International Conference on Analytical Models and New Peer-review responsibility of the scientific committee of the International Conference on Analytical Models and New Concepts in Concrete and Masonry Structures. Concepts inunder
Concrete and Masonry Structures
Keywords: FRP composites; bond performance; sun exposition, glass transition
* Corresponding author. Tel.: +48-32-2372262; fax: +48-32-2372288. E-mail address: [email protected]
1877-7058 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the International Conference on Analytical Models and New Concepts in Concrete and Masonry Structures
doi:10.1016/j.proeng.2017.06.217
298
Rafał Krzywoń / Procedia Engineering 193 (2017) 297 – 304
1. Introduction Majority of externally bonded fiber reinforced strengthening solutions are based on the polymer systems. Because of the price the most widely used are epoxides. Epoxy polymers are not only adhesives bonding external strengthening with the structure. Usually they are playing a role of matrix of FRP laminate, sometimes also they are applied as a primer for the substrate. One of the drawbacks of epoxides is generally low temperature of glass transition. For commercially available epoxy resins used in structural engineering it could occur around 40÷50°C [1]. Near that temperature epoxy polymer changes its rigid characteristic into a soft and viscous one and therefore the adhesion between the FRP laminate and the strengthened element decreases. For that reason most of EBR FRP solutions are not resistant to elevated temperatures. What is not always obvious, is that the problem concerns not only the fire conditions but may occur after warming by the sun rays. The danger of such situation is associated not only with the loss of bearing capacity, but primarily with the deterioration of strengthening effectiveness resulting from the phenomenon of stress relaxation in the composite after slip in the adhesive layer. Transition from a solid, glasslike state to a rubberlike or viscous one is a continuous process passing gradually in the range of 10÷20°C [2]. The glass transition temperature of an adhesive depends upon the degree of chemical cure understood as the proportion of potential cross-links formed between polymer chains [3] and the physical configuration of the polymer chains within the adhesive [4]. In practice, the glass transition conditions are dependent upon the age of the adhesive, the temperature and humidity to which it has been exposed. Curing can be accelerated at high temperature, which enables stronger chain cross-linking [3]. Thanks to that phenomenon, the heating can be effective post-curing method, enhancing the glass transition temperature. Optimum cure temperature varies around 60°C [5]. Also dry conditions during curing are important for final glass transition temperature [3] because the moisture content affects the Van der Waals bonds between polymer chains. Despite the popularization of external reinforcements, knowledge about the decrease of the properties of the FRP–concrete interface at elevated temperatures still is very limited. The rigid glass into liquid transition in practice means the reduction of mechanical properties of the polymer and consequently reduction of adhesion forces between concrete and FRP laminate. Rapid loss of the adhesive bond strength appears in the temperature range of 60°C and 70°C [6]. Double face shear tests provided by Tadeu demonstrated the 55.3% decrease in bond capacity at a temperature of 60°C and 72.2 at 90°C [7]. Bending tests of strengthened concrete beams performed by Aguiar [8] have shown 50% decrease in strengthening efficiency, while at 80°C total achieved capacity was similar to not strengthened beam. Interestingly, at temperatures of about 50°C bond capacity may slightly grow [9, 10]. It may be the effect of secondary curing during heating of the sample. The decrease of adhesion is also dependent on the type of strengthening overlay. Leone [11] indicated that CFRP laminate behaves a little better than CFRP sheet. Weakening in the adhesive can cause increased deflection of beams. That phenomenon was observed by Ulaga [12] already about 45°C. Another interesting effect is different image of failure. At normal temperatures failure is initiated in the concrete - adhesive interface, leaving small layer of concrete attached to the debonded plate, while at temperatures of 50°C and higher specimen fails without any concrete remained attached to the adhesive [9]. Concluding described researches it can be stated that changes related to glass transition may occur at relatively low temperatures, just above 45°C. As will be shown in the paper such a temperature may occur after several minutes of sun exposition. Nomenclature Tg
glass transition temperature
Rafał Krzywoń / Procedia Engineering 193 (2017) 297 – 304
299
2. Effect of sun exposition on the temperature in the adhesive layer of EBR The objective of this part of the study was to determine the potential of temperature increase in the layer of adhesive between composite overlay and concrete. Two concrete prisms based on typical pavement tiles with dimensions of 350 mm × 350 mm × 60 mm were heated in the sun rays. On their upper surface there were adhered samples of: • CFRP strip 60×1,4mm (further named sCFRP), • laminate of single layer CFRP sheet (lCFRP), • SRP tape 3X2-12 type (Steel Reinforced Polymer) with unfinished surface (tSRP), • SRP tape 3X2-12 type finished with sand plaster (tSRPsand). The thickness of the adhesive layer was about 1 mm. CFRP sheet was laminated with S&P Resin 55, other samples adhered with the use of SikaDur®330. Temperature changes were measured using resistant thermocouples Pt100, placed inside the adhesive layer. Measures were recorded by digital thermometer type CHY 804. Figure 1 shows the schematic set-up and the samples during the measures. Described test were provided in the warmest days of June, July and August 2015. Generally (according to the position of the sun) samples were sunny hours on average from 6.00 AM to 6.00 PM. The temperature growth readings were carried out every 5÷15 minutes, the daily temperature changes readings, every 30 minutes.
Fig. 1. The view of test samples.
During the days of sunny weather, black CFRP specimens were able to heat up over 60°C. The highest temperature observed for the lCFRP model reached 63,3°C. Samples based on carbon fibers heat up a little more than SRP. Noticeable impact on heating speed had wind conditions and presence of clouds. Especially wind was able to cool effectively the surface of the strengthening (similar phenomenon was described by Aguiar [8]). What is also important, concrete samples exposed to sunlight at 10.15 AM were able to achieve glass transition temperature limit equal to 45°C after around an hour of exposition. Another interesting, observed phenomenon was that temperature is more dependent on the intensity of sun exposure (the height of the sun above the horizon, purity of the sky) than the air temperature [13]. The highest temperature was recorded during the day of the air temperature not exceeding the 30°C (Fig. 2a). As a result of the relatively high thermal inertia of concrete, adhesive layer needed more than one hour to exceed the glass transition temperature. Figure 2b shows the rise of samples temperature.The fastest growth can be observed for lCFRP laminate.
Rafał Krzywoń / Procedia Engineering 193 (2017) 297 – 304
70
b) 70
60
measured temperature of adhesive layer °C
300
50 40 30 20 10 0 June 6
June 12
July 22 August 6 August 7 August 8
60
50 45 40
30 lCFRP tSRPsand air
20
sCFRP tSRP
10 10 lCFRP
sCFRP
tSRPsand
tSRP
11
12
air
13
14
15 hour
Fig. 2. (a) Maximum temperatures during selected days; (b) Sample temperature rise.
3. Bending tests of strengthened RC beams subjected to external heating 3.1. Test procedure The aim of this part of the experiment was to determine the effect of increased temperature on the behavior of strengthened RC beam under the flexure. Experimental program took 13 beams, including: • one reference unstrengthened beam, • six beams strengthened using CFRP strip S&P CFK 200/2000 type 60×1,4mm, • six beams strengthened using Hardwire SRP tape type 3X2-20, 150mm wide, All beams had a rectangular cross section (200 x 300 mm) and were made of this same concrete mixture. The mean cube compressive strength of concrete was 44.7 MPa and the tensile strength of concrete estimated from the Brazilian probe had a mean value of 3.2 MPa. Average modulus of elasticity was equal to 30.7 GPa. Beams were reinforced with three Ø12 bottom ribbed bars (yields strength 570.1MPa) and two Ø8 top ribbed bars (yields strength 588.2 MPa). Stirrups were spaced at 100 mm. The same adhesive was used to adhere both types of strengthening. Due to requirement of intermediate viscosity necessary for proper impregnation of SRP tape, Sikadur®330 was applied. To minimize the effect of ageing of adhesive (epoxy polymer), beams were tested 40÷50 days after application of strengthening. Curing took place at room temperature. Laboratory trials were performed in four-point bending test (scheme shown in Figure 3). All beams were loaded monotonically until failure. Force was applied using a hydraulic press. Beams were heated by three linear infra-red radiators. This method was selected as the most similar to sunlight, when heating is caused by radiation rather than convection. The most efficient heating was possible after placing the heater directly under the beam. This method of heating could not be continued during the test, so radiators were removed immediately before the beginning of test. Thanks to a quite large volumetric heat capacity of concrete drop of laminate temperature during the test did not exceed 10°C. To determine deflections, strains and ultimate failure force beams were instrumented with linear displacement transducers in five points along the beam length (support, force application points, a middle of the span), strain gages
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Rafał Krzywoń / Procedia Engineering 193 (2017) 297 – 304
(Fig. 3) and dynamometer under the press. Additionally each strengthened beam was equipped with 5 Pt100 thermocouples to control the heating and test temperature.
Fig. 3. Test set-up.
3.2. Test results The evaluation of the flexural behavior was made by recording failure load and analysis of beam deformations during the test. Table 1 shows summary of results achieved for all beams. In view of the earlier mentioned temperature drop during the test, temperature measures are given at the start and at the time of failure. The average test time was 30 minutes. To show the effect of temperature on the flexibility of the joint, the comparison also includes the beam’s deflection and deformation of the composite in the middle of the span measured for bending moment equal 50 kNm. Table 1. Selected test results. Beam
Strengthening
Max. temp. [°C]
REF
no
CFRP_1 CFRP_2
Temp. at failure
Failure moment
[°C]
[kNm]
Deflection
Deflection
Midspan strain
at failure
at 50kNm
at 50kNm
[mm]
[mm]
[‰]
Failure type
room
room
49.6
71
-
-
yield of steel
room
room
69.8
20.5
11.1
2.64
concr. debond.
room
room
72.5
21.2
10.8
2.71
concr. debond.
CFRP_T51
CFRP strip
51
44
67.8
20.4
11.4
2.67
concr. debond.
CFRP_T56
60x1.4
56
45
67.3
20.6
11.6
2.71
adhes. debond.
CFRP_T62
62
51
65.8
21.3
11.6
2.67
adhes. debond.
CFRP_T73
73
67
55.4
13.3
11.6
2.39
adhes. debond.
SRP_1
room
room
76.4
29.1
11.1
2.57
concr. debond.
SRP_2
room
room
75.3
28.5
11.4
2.74
concr. debond.
SRP_T37
SRP
37
32
83.4
35.2
10.5
2.47
concr. debond.
SRP_T64
3X2-20 150
64
56
72.6
30.5
11.3
2.04
adhes. debond.
SRP_T69
69
59
63.8
24.7
11.5
2.08
adhes. debond.
SRP_T80
80
68
64.9
23.7
11.6
2.17
adhes. debond.
Figures 4 and 5 represent curves of bending moment vs. deflection of selected SRP and CFRP beams under different thermal exposures.
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Rafał Krzywoń / Procedia Engineering 193 (2017) 297 – 304
80
Bending moment [kNm]
70 60 50 CFRP_1 40
CFRP_T56
30
CFRP_T62
20
CFRP_T73
10 0 5
0
10
15
20
25
Midspan deflection [mm]
Fig. 4. Variation of the bending moment vs. deflection of CFRP strengthened beams.
90
Bending moment [kNm]
80 70 60 SRP_1 50
SRP_T37
40
SRP_T64
30
SRP_T69
20
SRP_T80
10 0 0
5
10
15
20
25
30
35
40
45
Midspan deflection [mm]
Fig. 5. Variation of the bending moment vs. deflection of SRP strengthened beams.
3.3. Analysis of results The analysis of Table 1 shows that first influence of elevated temperature could appear near 50°C. The bearing capacity of CFRP_T51 beam is slightly lower than both CFRP beams tested in room temperature. The size of the phenomenon of decrease of bearing capacity growths with temperature, however, to a temperature of about 65°C reaches only a few percent. Rapid decrease appears over that temperature, above 70°C the beam reaches less than 80% of its nominal capacity. Interestingly, the beam SRP_T37 tested at 37°C achieved a load capacity greater than both SRP beams tested in room temperature. It could be the effect of reported by Blontrock [10] growth of adhesive bonding capacity at temperatures of about 40°C.
Rafał Krzywoń / Procedia Engineering 193 (2017) 297 – 304
303
Temperature only slightly affects the growth of deflection of beams strengthened with CFRP strip. This demonstrates shown in Table 1 comparison of deflections at 50kNm and the evolution of mid-span deflections shown in Figure 4. In case of the CFRP strip mid-span strains a clear difference appears only for specimen tested at 73°C (CFRP_T73). This may be a result of a slip in the adhesive layer and strain distribution into the less strenous zone. Dependence between temperature and development of deflection is better visible for SRP strengthened beams. As can be seen in Figure 5, the difference becomes particularly evident above 45kNm (bending moment corresponding to yield of internal steel reinforcement). This applies especially to the beams tested above 65°C (SRP_T69 and SRP_T80). Measured strains of the SRP tape at mid-span are smaller at higher temperatures. Strain differences are also greater than observed for the CFRP strengthening.
Fig. 6. Debonding failure in the concrete (a) and in the concrete-adhesive interface (b).
Depending on the temperature, two failure models were observed. At temperatures up to 50°C failure occurred as a delamination of strengthening in the concrete near the interface with adhesive, leaving a small layer of concrete attached (Figure 6a – beam SRP_T37). Above that temperature amount of concrete particles left on detached strengthening successively decreased. Beams tested over 70°C failed at the interface between adhesive and concrete (Figure 6b – beam SRP_T80). These observations are consistent with Klamer findings [9] and confirm the weakening of the bond properties of the epoxy adhesive at elevated temperature. 4. Conclusions Carried out studies show that the adhesion between concrete and externally bonded reinforcement is sensitive on the temperature increase. Significant degradation in bearing capacity can be observed over 65°C. Temperature influence is visible for CFRP and SRP reinforcement, but the course of changes is different in both cases. Impact on beam strengthened with CFRP strip is visible above the 60°C and results in unexpected and rapid failure under load much lower than the nominal. Besides a decrease in load capacity there is no visible effect of change in strengthening efficiency. The values of deflections and strains in laminate are very similar. The differences of those values are noticeable in elements strengthened with SRP tapes. In addition to the decrease in load capacity (however, smaller than in the case of CFRP), at higher temperatures the deflection is greater and mid-
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Rafał Krzywoń / Procedia Engineering 193 (2017) 297 – 304
span laminate strains are lower. Especially the latter phenomenon may indicate a progressive slippage in the adhesive layer. These effect can unfortunately lead to relaxation of SRP strengthening and weakening of its effectiveness. The reasons of described differences in behavior of CFRP and SRP laminate is probably a different construction of both laminates. Epoxy adhesive for CFRP strips serves only their fixing to the concrete while for SRP tape, it also acts as a matrix joining the steel wires thus affecting the mechanical properties of the entire laminate. Deterioration of epoxy adhesive bond properties can appear even under solar exposure of strengthened concrete element. Therefore, the external application of FRP laminates adhered with epoxies in warm locations should be carried out with protective insulation systems. References [1] J. Michels, R. Widmann, C. Czaderski, R. Allahvirdizadeh, M.Motavalli, Glass transition evaluation of commercially available epoxy resins used for civil engineering applications, Composites Part B 77, 2015, pp. 484-493. [2] G. Hülder, C. Dallner, G.W. Ehrenstein, Curing of epoxy-adhesives for the supplementary reinforcement of buildings with bonded CFRPstraps (in German), Bauingenieur 81, 2006, pp. 449-454. [3] D. Othman, T.J. Stratford, L.A. Bisby, A Comparison of On-Site and Elevated Temperature Cure of an FRP Strengthening Adhesive, Proceedings of the FRPRCS11, UM, Guimarães, 2013. [4] G.M. Odegard, A. Bandyopadhyay, Physical aging of epoxy polymers and their composites, J. Polymer Sci. Part B: Polymer Physics, 49, 2011, pp. 1695-1716. [5] R.J.C. Carbas, E.A.S. Marques, A.M. Lopes, L.F.M. da Silva, Effect of cure temperature on the glass transition temperature of an epoxy adhesive, Proceedings of the 15th International Conference on Experimental Mechanics ICEM2015, University of Porto 2012.. [6] J.C.P.H. Gamage, M.B. Wong, R. Al-Mahaidi, Performance of CFRP strengthened concrete members under elevated temperatures, Proceedings of the International Symposium on Bond Behaviour of FRP in Structures (BBFS 2005), pp. 113-118. [7] A. Tadeu, F. Branco, Shear tests of steel plates epoxy bonded to concrete under temperature, Journal of Materials in Civil Engineering 2000, 12(1), pp. 74-80. [8] J. B. Aguiar, A. Camoes, N. F. Vaz, Effect of temperature on RC elements strengthened with CFRP. Materials and Structures (2008) 41, pp. 1133–1142. [9] E.L. Klamer, D.A. Hordijk, C.S. Kleinman, Debonding of CFRP laminates externally bonded to concrete specimens at low and high temperatures, Proceedings of Third International Conference on FRP Composites in Civil Engineering (CICE 2006), Miami, Florida, USA, pp. 35-38. [10] H. Blontrock, L. Taerwe, H. Vanwalleghem, Bond testing of externally glued FRP laminates at elevated temperature, Proceeding of the international conference: Bond in concrete- from research to standard, Budapest, Hungary, 2002, pp. 648–54. [11] M. Leone, S. Matthys, M.A. Aiello, Effect of elevated service temperature on bond between FRP EBR systems and concrete, Composites: Part B 40 (2009), p. 85–93. [12] T. Ulaga, U. Meier, Long-term Behaviour of CFRP-laminate-strengthened Concrete Beams at Elevated Temperatures, Proceedings of the FRPRCS-5 Conference, Cambridge, 2001, pp. 147-156. [13] R. KrzywoĔ, Temperature in the adhesive layer of externally bonded composite reinforcement heated by the sun, ACEE Archit. Civ. Eng. Environ. 2016, Vol. 9 No. 1, pp. 79-84.
ScienceDirect Procedia Engineering 193 (2017) 297 – 304
International Conference on Analytical Models and New Concepts in Concrete and Masonry Structures AMCM’2017
Behavior of EBR FRP strengthened beams exposed to elevated temperature Rafaá KrzywoĔa* a
Silesian University of Technology, Department of Structural Engineering, Akademicka 5, Gliwice 44-100, Poland
Abstract Externally bonded laminates based on high strength fibers nowadays are often used to strengthen the structures. It is possible, that some of those strengthenings are subjected to direct insolation. The paper presents the dangers appearing with the rise of temperature caused by the infrared solar radiation. Heating conditions were planned on the basis of exposition of concrete samples during the summer months in the southern Poland. The maximum recorded temperatures in the adhesive layer reached 65°C and it was therefore 20°C higher than the glass transition temperature of commercially available adhesives based on epoxy resin. Laboratory tests included a group of fifteen reinforced concrete beams in the real scale, seven of them were strengthened with externally bonded CFRP strip, seven with SRP tape. They were tested in various temperature conditions, from 20°C to 80°C. Beams were heated from the strengthened side with the use of linear infra-red radiators and when the temperature of the adhesive layer reached the predetermined value, beams were loaded to failure. Noticeable impact of temperature appeared from about 50°C. In all cases failure was followed by delamination. Differences in behavior of CFRP and SRP strengthened beams were observed. Delamination of CFRP strip appeared in unexpected way, without any specific symptoms, while SRP failed with grater deflection and lower mid-span strains compared to the beam tested at room temperature. At temperatures above 65°C appears significant bearing capacity degradation. It means that CFRP strengthening, which could be subjected to direct sun exposition, should always be protected by thermal insulation. © 2017 2017The TheAuthors. Authors. Published by Elsevier Ltd.is an open access article under the CC BY-NC-ND license © Published by Elsevier Ltd. This (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the International Conference on Analytical Models and New Peer-review responsibility of the scientific committee of the International Conference on Analytical Models and New Concepts in Concrete and Masonry Structures. Concepts inunder
Concrete and Masonry Structures
Keywords: FRP composites; bond performance; sun exposition, glass transition
* Corresponding author. Tel.: +48-32-2372262; fax: +48-32-2372288. E-mail address: [email protected]
1877-7058 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the International Conference on Analytical Models and New Concepts in Concrete and Masonry Structures
doi:10.1016/j.proeng.2017.06.217
298
Rafał Krzywoń / Procedia Engineering 193 (2017) 297 – 304
1. Introduction Majority of externally bonded fiber reinforced strengthening solutions are based on the polymer systems. Because of the price the most widely used are epoxides. Epoxy polymers are not only adhesives bonding external strengthening with the structure. Usually they are playing a role of matrix of FRP laminate, sometimes also they are applied as a primer for the substrate. One of the drawbacks of epoxides is generally low temperature of glass transition. For commercially available epoxy resins used in structural engineering it could occur around 40÷50°C [1]. Near that temperature epoxy polymer changes its rigid characteristic into a soft and viscous one and therefore the adhesion between the FRP laminate and the strengthened element decreases. For that reason most of EBR FRP solutions are not resistant to elevated temperatures. What is not always obvious, is that the problem concerns not only the fire conditions but may occur after warming by the sun rays. The danger of such situation is associated not only with the loss of bearing capacity, but primarily with the deterioration of strengthening effectiveness resulting from the phenomenon of stress relaxation in the composite after slip in the adhesive layer. Transition from a solid, glasslike state to a rubberlike or viscous one is a continuous process passing gradually in the range of 10÷20°C [2]. The glass transition temperature of an adhesive depends upon the degree of chemical cure understood as the proportion of potential cross-links formed between polymer chains [3] and the physical configuration of the polymer chains within the adhesive [4]. In practice, the glass transition conditions are dependent upon the age of the adhesive, the temperature and humidity to which it has been exposed. Curing can be accelerated at high temperature, which enables stronger chain cross-linking [3]. Thanks to that phenomenon, the heating can be effective post-curing method, enhancing the glass transition temperature. Optimum cure temperature varies around 60°C [5]. Also dry conditions during curing are important for final glass transition temperature [3] because the moisture content affects the Van der Waals bonds between polymer chains. Despite the popularization of external reinforcements, knowledge about the decrease of the properties of the FRP–concrete interface at elevated temperatures still is very limited. The rigid glass into liquid transition in practice means the reduction of mechanical properties of the polymer and consequently reduction of adhesion forces between concrete and FRP laminate. Rapid loss of the adhesive bond strength appears in the temperature range of 60°C and 70°C [6]. Double face shear tests provided by Tadeu demonstrated the 55.3% decrease in bond capacity at a temperature of 60°C and 72.2 at 90°C [7]. Bending tests of strengthened concrete beams performed by Aguiar [8] have shown 50% decrease in strengthening efficiency, while at 80°C total achieved capacity was similar to not strengthened beam. Interestingly, at temperatures of about 50°C bond capacity may slightly grow [9, 10]. It may be the effect of secondary curing during heating of the sample. The decrease of adhesion is also dependent on the type of strengthening overlay. Leone [11] indicated that CFRP laminate behaves a little better than CFRP sheet. Weakening in the adhesive can cause increased deflection of beams. That phenomenon was observed by Ulaga [12] already about 45°C. Another interesting effect is different image of failure. At normal temperatures failure is initiated in the concrete - adhesive interface, leaving small layer of concrete attached to the debonded plate, while at temperatures of 50°C and higher specimen fails without any concrete remained attached to the adhesive [9]. Concluding described researches it can be stated that changes related to glass transition may occur at relatively low temperatures, just above 45°C. As will be shown in the paper such a temperature may occur after several minutes of sun exposition. Nomenclature Tg
glass transition temperature
Rafał Krzywoń / Procedia Engineering 193 (2017) 297 – 304
299
2. Effect of sun exposition on the temperature in the adhesive layer of EBR The objective of this part of the study was to determine the potential of temperature increase in the layer of adhesive between composite overlay and concrete. Two concrete prisms based on typical pavement tiles with dimensions of 350 mm × 350 mm × 60 mm were heated in the sun rays. On their upper surface there were adhered samples of: • CFRP strip 60×1,4mm (further named sCFRP), • laminate of single layer CFRP sheet (lCFRP), • SRP tape 3X2-12 type (Steel Reinforced Polymer) with unfinished surface (tSRP), • SRP tape 3X2-12 type finished with sand plaster (tSRPsand). The thickness of the adhesive layer was about 1 mm. CFRP sheet was laminated with S&P Resin 55, other samples adhered with the use of SikaDur®330. Temperature changes were measured using resistant thermocouples Pt100, placed inside the adhesive layer. Measures were recorded by digital thermometer type CHY 804. Figure 1 shows the schematic set-up and the samples during the measures. Described test were provided in the warmest days of June, July and August 2015. Generally (according to the position of the sun) samples were sunny hours on average from 6.00 AM to 6.00 PM. The temperature growth readings were carried out every 5÷15 minutes, the daily temperature changes readings, every 30 minutes.
Fig. 1. The view of test samples.
During the days of sunny weather, black CFRP specimens were able to heat up over 60°C. The highest temperature observed for the lCFRP model reached 63,3°C. Samples based on carbon fibers heat up a little more than SRP. Noticeable impact on heating speed had wind conditions and presence of clouds. Especially wind was able to cool effectively the surface of the strengthening (similar phenomenon was described by Aguiar [8]). What is also important, concrete samples exposed to sunlight at 10.15 AM were able to achieve glass transition temperature limit equal to 45°C after around an hour of exposition. Another interesting, observed phenomenon was that temperature is more dependent on the intensity of sun exposure (the height of the sun above the horizon, purity of the sky) than the air temperature [13]. The highest temperature was recorded during the day of the air temperature not exceeding the 30°C (Fig. 2a). As a result of the relatively high thermal inertia of concrete, adhesive layer needed more than one hour to exceed the glass transition temperature. Figure 2b shows the rise of samples temperature.The fastest growth can be observed for lCFRP laminate.
Rafał Krzywoń / Procedia Engineering 193 (2017) 297 – 304
70
b) 70
60
measured temperature of adhesive layer °C
300
50 40 30 20 10 0 June 6
June 12
July 22 August 6 August 7 August 8
60
50 45 40
30 lCFRP tSRPsand air
20
sCFRP tSRP
10 10 lCFRP
sCFRP
tSRPsand
tSRP
11
12
air
13
14
15 hour
Fig. 2. (a) Maximum temperatures during selected days; (b) Sample temperature rise.
3. Bending tests of strengthened RC beams subjected to external heating 3.1. Test procedure The aim of this part of the experiment was to determine the effect of increased temperature on the behavior of strengthened RC beam under the flexure. Experimental program took 13 beams, including: • one reference unstrengthened beam, • six beams strengthened using CFRP strip S&P CFK 200/2000 type 60×1,4mm, • six beams strengthened using Hardwire SRP tape type 3X2-20, 150mm wide, All beams had a rectangular cross section (200 x 300 mm) and were made of this same concrete mixture. The mean cube compressive strength of concrete was 44.7 MPa and the tensile strength of concrete estimated from the Brazilian probe had a mean value of 3.2 MPa. Average modulus of elasticity was equal to 30.7 GPa. Beams were reinforced with three Ø12 bottom ribbed bars (yields strength 570.1MPa) and two Ø8 top ribbed bars (yields strength 588.2 MPa). Stirrups were spaced at 100 mm. The same adhesive was used to adhere both types of strengthening. Due to requirement of intermediate viscosity necessary for proper impregnation of SRP tape, Sikadur®330 was applied. To minimize the effect of ageing of adhesive (epoxy polymer), beams were tested 40÷50 days after application of strengthening. Curing took place at room temperature. Laboratory trials were performed in four-point bending test (scheme shown in Figure 3). All beams were loaded monotonically until failure. Force was applied using a hydraulic press. Beams were heated by three linear infra-red radiators. This method was selected as the most similar to sunlight, when heating is caused by radiation rather than convection. The most efficient heating was possible after placing the heater directly under the beam. This method of heating could not be continued during the test, so radiators were removed immediately before the beginning of test. Thanks to a quite large volumetric heat capacity of concrete drop of laminate temperature during the test did not exceed 10°C. To determine deflections, strains and ultimate failure force beams were instrumented with linear displacement transducers in five points along the beam length (support, force application points, a middle of the span), strain gages
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(Fig. 3) and dynamometer under the press. Additionally each strengthened beam was equipped with 5 Pt100 thermocouples to control the heating and test temperature.
Fig. 3. Test set-up.
3.2. Test results The evaluation of the flexural behavior was made by recording failure load and analysis of beam deformations during the test. Table 1 shows summary of results achieved for all beams. In view of the earlier mentioned temperature drop during the test, temperature measures are given at the start and at the time of failure. The average test time was 30 minutes. To show the effect of temperature on the flexibility of the joint, the comparison also includes the beam’s deflection and deformation of the composite in the middle of the span measured for bending moment equal 50 kNm. Table 1. Selected test results. Beam
Strengthening
Max. temp. [°C]
REF
no
CFRP_1 CFRP_2
Temp. at failure
Failure moment
[°C]
[kNm]
Deflection
Deflection
Midspan strain
at failure
at 50kNm
at 50kNm
[mm]
[mm]
[‰]
Failure type
room
room
49.6
71
-
-
yield of steel
room
room
69.8
20.5
11.1
2.64
concr. debond.
room
room
72.5
21.2
10.8
2.71
concr. debond.
CFRP_T51
CFRP strip
51
44
67.8
20.4
11.4
2.67
concr. debond.
CFRP_T56
60x1.4
56
45
67.3
20.6
11.6
2.71
adhes. debond.
CFRP_T62
62
51
65.8
21.3
11.6
2.67
adhes. debond.
CFRP_T73
73
67
55.4
13.3
11.6
2.39
adhes. debond.
SRP_1
room
room
76.4
29.1
11.1
2.57
concr. debond.
SRP_2
room
room
75.3
28.5
11.4
2.74
concr. debond.
SRP_T37
SRP
37
32
83.4
35.2
10.5
2.47
concr. debond.
SRP_T64
3X2-20 150
64
56
72.6
30.5
11.3
2.04
adhes. debond.
SRP_T69
69
59
63.8
24.7
11.5
2.08
adhes. debond.
SRP_T80
80
68
64.9
23.7
11.6
2.17
adhes. debond.
Figures 4 and 5 represent curves of bending moment vs. deflection of selected SRP and CFRP beams under different thermal exposures.
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80
Bending moment [kNm]
70 60 50 CFRP_1 40
CFRP_T56
30
CFRP_T62
20
CFRP_T73
10 0 5
0
10
15
20
25
Midspan deflection [mm]
Fig. 4. Variation of the bending moment vs. deflection of CFRP strengthened beams.
90
Bending moment [kNm]
80 70 60 SRP_1 50
SRP_T37
40
SRP_T64
30
SRP_T69
20
SRP_T80
10 0 0
5
10
15
20
25
30
35
40
45
Midspan deflection [mm]
Fig. 5. Variation of the bending moment vs. deflection of SRP strengthened beams.
3.3. Analysis of results The analysis of Table 1 shows that first influence of elevated temperature could appear near 50°C. The bearing capacity of CFRP_T51 beam is slightly lower than both CFRP beams tested in room temperature. The size of the phenomenon of decrease of bearing capacity growths with temperature, however, to a temperature of about 65°C reaches only a few percent. Rapid decrease appears over that temperature, above 70°C the beam reaches less than 80% of its nominal capacity. Interestingly, the beam SRP_T37 tested at 37°C achieved a load capacity greater than both SRP beams tested in room temperature. It could be the effect of reported by Blontrock [10] growth of adhesive bonding capacity at temperatures of about 40°C.
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Temperature only slightly affects the growth of deflection of beams strengthened with CFRP strip. This demonstrates shown in Table 1 comparison of deflections at 50kNm and the evolution of mid-span deflections shown in Figure 4. In case of the CFRP strip mid-span strains a clear difference appears only for specimen tested at 73°C (CFRP_T73). This may be a result of a slip in the adhesive layer and strain distribution into the less strenous zone. Dependence between temperature and development of deflection is better visible for SRP strengthened beams. As can be seen in Figure 5, the difference becomes particularly evident above 45kNm (bending moment corresponding to yield of internal steel reinforcement). This applies especially to the beams tested above 65°C (SRP_T69 and SRP_T80). Measured strains of the SRP tape at mid-span are smaller at higher temperatures. Strain differences are also greater than observed for the CFRP strengthening.
Fig. 6. Debonding failure in the concrete (a) and in the concrete-adhesive interface (b).
Depending on the temperature, two failure models were observed. At temperatures up to 50°C failure occurred as a delamination of strengthening in the concrete near the interface with adhesive, leaving a small layer of concrete attached (Figure 6a – beam SRP_T37). Above that temperature amount of concrete particles left on detached strengthening successively decreased. Beams tested over 70°C failed at the interface between adhesive and concrete (Figure 6b – beam SRP_T80). These observations are consistent with Klamer findings [9] and confirm the weakening of the bond properties of the epoxy adhesive at elevated temperature. 4. Conclusions Carried out studies show that the adhesion between concrete and externally bonded reinforcement is sensitive on the temperature increase. Significant degradation in bearing capacity can be observed over 65°C. Temperature influence is visible for CFRP and SRP reinforcement, but the course of changes is different in both cases. Impact on beam strengthened with CFRP strip is visible above the 60°C and results in unexpected and rapid failure under load much lower than the nominal. Besides a decrease in load capacity there is no visible effect of change in strengthening efficiency. The values of deflections and strains in laminate are very similar. The differences of those values are noticeable in elements strengthened with SRP tapes. In addition to the decrease in load capacity (however, smaller than in the case of CFRP), at higher temperatures the deflection is greater and mid-
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span laminate strains are lower. Especially the latter phenomenon may indicate a progressive slippage in the adhesive layer. These effect can unfortunately lead to relaxation of SRP strengthening and weakening of its effectiveness. The reasons of described differences in behavior of CFRP and SRP laminate is probably a different construction of both laminates. Epoxy adhesive for CFRP strips serves only their fixing to the concrete while for SRP tape, it also acts as a matrix joining the steel wires thus affecting the mechanical properties of the entire laminate. Deterioration of epoxy adhesive bond properties can appear even under solar exposure of strengthened concrete element. Therefore, the external application of FRP laminates adhered with epoxies in warm locations should be carried out with protective insulation systems. References [1] J. Michels, R. Widmann, C. Czaderski, R. Allahvirdizadeh, M.Motavalli, Glass transition evaluation of commercially available epoxy resins used for civil engineering applications, Composites Part B 77, 2015, pp. 484-493. [2] G. Hülder, C. Dallner, G.W. Ehrenstein, Curing of epoxy-adhesives for the supplementary reinforcement of buildings with bonded CFRPstraps (in German), Bauingenieur 81, 2006, pp. 449-454. [3] D. Othman, T.J. Stratford, L.A. Bisby, A Comparison of On-Site and Elevated Temperature Cure of an FRP Strengthening Adhesive, Proceedings of the FRPRCS11, UM, Guimarães, 2013. [4] G.M. Odegard, A. Bandyopadhyay, Physical aging of epoxy polymers and their composites, J. Polymer Sci. Part B: Polymer Physics, 49, 2011, pp. 1695-1716. [5] R.J.C. Carbas, E.A.S. Marques, A.M. Lopes, L.F.M. da Silva, Effect of cure temperature on the glass transition temperature of an epoxy adhesive, Proceedings of the 15th International Conference on Experimental Mechanics ICEM2015, University of Porto 2012.. [6] J.C.P.H. Gamage, M.B. Wong, R. Al-Mahaidi, Performance of CFRP strengthened concrete members under elevated temperatures, Proceedings of the International Symposium on Bond Behaviour of FRP in Structures (BBFS 2005), pp. 113-118. [7] A. Tadeu, F. Branco, Shear tests of steel plates epoxy bonded to concrete under temperature, Journal of Materials in Civil Engineering 2000, 12(1), pp. 74-80. [8] J. B. Aguiar, A. Camoes, N. F. Vaz, Effect of temperature on RC elements strengthened with CFRP. Materials and Structures (2008) 41, pp. 1133–1142. [9] E.L. Klamer, D.A. Hordijk, C.S. Kleinman, Debonding of CFRP laminates externally bonded to concrete specimens at low and high temperatures, Proceedings of Third International Conference on FRP Composites in Civil Engineering (CICE 2006), Miami, Florida, USA, pp. 35-38. [10] H. Blontrock, L. Taerwe, H. Vanwalleghem, Bond testing of externally glued FRP laminates at elevated temperature, Proceeding of the international conference: Bond in concrete- from research to standard, Budapest, Hungary, 2002, pp. 648–54. [11] M. Leone, S. Matthys, M.A. Aiello, Effect of elevated service temperature on bond between FRP EBR systems and concrete, Composites: Part B 40 (2009), p. 85–93. [12] T. Ulaga, U. Meier, Long-term Behaviour of CFRP-laminate-strengthened Concrete Beams at Elevated Temperatures, Proceedings of the FRPRCS-5 Conference, Cambridge, 2001, pp. 147-156. [13] R. KrzywoĔ, Temperature in the adhesive layer of externally bonded composite reinforcement heated by the sun, ACEE Archit. Civ. Eng. Environ. 2016, Vol. 9 No. 1, pp. 79-84.