- Email: [email protected]
Experimental Study on High Temperature Properties of Carbon Fiber Sheets Strengthened Concrete Cylinders Using Geopolymer as Adhesive
Available online at www.sciencedirect.com
ScienceDirect Procedia Engineering 135 (2016) 47 – 55
Experimental study on high temperature properties of carbon fiber sheets strengthened concrete cylinders using geopolymer as adhesive Hai-yan Zhanga,b,*, Xu Haob, Wang Fana a
State Key laboratory of Subtropical Architecture Science, South China University of Technology, Guangzhou 510640, China b Department of Civil Engineering, South China University of Technology, Guangzhou, 510640, China
Abstract Twenty plain concrete cylinders, including unconfined cylinders and confined cylinders with 1, 2 and 3 layers of carbon fiber sheets using geopolymer as adhesive, were tested in axial compression at ambient temperature and after exposure to elevated temperatures, to investigate the strengthening effect of carbon fiber sheet-geopolymer system. The failure modes, load-displacement curves, axial and hoop strains of confined cylinders were compared with that of unconfined cylinders. The failure modes of confined cylinders after exposure high temperature are similar with those of ones at room temperature. When temperature is up to 300oC, the compressive strength of confined cylinders is a little more than that of ones at room temperature, and the ratio of increase on compressive strength which is 106.4% is double as that of at room temperature. However, the ductility of confined cylinders decreases significantly after exposure elevated temperatures. From stress-strain curves, it can be found that there is no obvious degradation on mechanical property after exposure to high temperatures. It shows that carbon fiber sheet-geopolymer system has an excellent resistance to high temperatures. by Elsevier Ltd. This is an openLtd. access article underunder the CCCC BY-NC-ND license © 2016 2016Published The Authors. Published by Elsevier Open access BY-NC-ND license. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of School of Engineering of Sun Yat-sen University. .Peer-review under responsibility of the organizing committee of ICPFFPE 2015 Keywords: Concrete cylinders; Geopolymer; Carbon fiber sheets; High temperature; Compression
1. Introduction Externally bonded fiber reinforced polymer (EB-FRP) is widely used for strengthening concrete structures, due to its advantages of light weight, high strength, corrosion resistance, and ease to application. In EB-FRP system, organic matrices, such as epoxy resin, are used as a primary adhesive. Organic matrix exhibits high strength at ambient temperature, but low resistance to high temperatures. The glass transition temperature (Tg) of epoxy resin is only about 82 oC [1], which results in its great strength degradation at high temperatures. In addition, emission of poisonous gas occurs when epoxy resin is in fire [1]. To deal with the drawbacks of organic matrix, inorganic matrices were developed recently [2,3,4,5]. Among the many types of inorganic matrices, a new class of material called geopolymers, attracted researchers’ attention in the past decades [6-16]. Geopolymer is synthesized by alkaline solutions (such as sodium silicate and potassium silicate [6-16]) activating aluminosilicate source materials (such as metakaolin, fly ash, and slag [6-16]). Compared to organic matrix, inorganic geopolymers have advantages of resistance to high temperature, resistance to UV radiation and minimal toxic smoke under fire exposure.
* Corresponding author. Tel.: +86-20-8711 1030; E-mail address: [email protected]
1877-7058 © 2016 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 organizing committee of ICPFFPE 2015
doi:10.1016/j.proeng.2016.01.078
48
Hai-yan Zhang et al. / Procedia Engineering 135 (2016) 47 – 55
The mechanical properties of geopolymers, such as compressive and bending strength, at ambient temperature and after exposure to high temperatures, were extensively studied in literatures [9]. The bond performance using geopolymer as adhesive is also been explored recently. Menna C. et al. [1] used metakaolin-based geopolymer as adhesive to paste steel cord confining RC beams. Test results showed that geopolymer has good adhesion between concrete substrate and steel cords. Toutanji and Deng [3] compared the strengthening effect of carbon fiber sheets bonded with organic and inorganic matrices on reinforced concrete beams. Results showed the carbon fiber sheet-geopolymer (CFSG) system is effective in increasing strength and stiffness of reinforced concrete beams as that of organic matrix at ambient temperature. Toutanji and Zhao [4] reported that the ductility of reinforced beams strengthened by CFSG system is greatly lower than that of unstrengthened beams, but the load carrying capacity of strengthened beams increases with an increase in layers of carbon fiber sheets. Kurtz S. and Balaguru P. [5] also found that the inorganic and organic systems can provide comparable performance with respect to the increase in strength of RC beams strengthened by carbon fiber sheets. The above researches demonstrated that geopolymers exhibit similar bond properties with carbon fibers sheet and concrete substrate at ambient temperature. However, as for the high temperature properties of CFSG system, there is a lack of experimental data. In this paper, 20 plain concrete cylinders, including unconfined cylinders and confined cylinders with carbon fiber sheets bonded by geopolymers, were tested in compression at ambient temperature and after exposure to elevated temperatures, to investigate the strengthening effect of CFSG system at ambient and high temperatures. The studied parameters include the number of fiber layers and the exposure temperatures. 2. Experimental program The experimental program is composed of a large number of compression tests at ambient temperature and after exposure to elevated temperatures (100, 200, 300 oC), on unconfined plain concrete cylinders and confined cylinders with carbon fiber sheets using geopolymers as adhesive. 2.1. Raw materials The primary aluminosilicate source material used in geopolymer is metakaolin (MK) and fly ash (FA) mixture. The chemical composition of MK and FA is shown in Table 1. Commercially produced metakaolin with an average particle size of 0.017 mm, was supplied by Shanxi Jinkunhengye Ltd., China. Low calcium fly ash, with an average particle size of 0.032 mm, was supplied by Guangzhou Huangpu Power Plant. Potassium silicate solution with SiO 2/K2O molar ratios of 1.0 was used as alkaline-silicate activator. Chopped carbon fibers (CF), with a content of 0.5 percent of MK-FA blend precursor in mass, were added to precursor as reinforcement agent. The length, diameter and density of chopped carbon fibers are 6 mm, 7 μm and 1.76-1.80 g/cm3 respectively. Table 1. Chemical composition of FA and MK (wt, %) SiO2
Al2O3
CaO
Fe2O3
FA
51.35
44.24
0.13
0.98
MK
45.3
41.2
3.77
3.18
TiO2
SO3
MgO
K2O
P2O5
SrO
Na2O
0.9
0
0.48
0.08
1.62
0.75
0.44
0.38
ZrO2
MnO
Loss on ignition
0.45
0
0.36
0.11
0.16
0
0.01
0.72
0.09
0.08
0.05
2.4
Geopolymer pastes were synthesized with fly ash and metakaolin mixture (1:1) and alkali silicate solutions. The liquid/solid (L/S) mass ratio was 0.4 (the liquid consists of solvent in alkali silicate solutions; the solid materials consist of fly ash, matekaolin and solute of alkali silicate solutions). Geopolymer pastes were prepared by hand-mixing MK-FA precursor and short fibers for 2 min, then adding alkaline silicate solution and mixing all ingredients in a mixer about 7 min to ensure they were mixed thoroughly. The materials of concrete cylinders used in this study were Type I Portland cement (Grade 32.5), tap water, crushed limestone with size of 5 to 25 mm and natural sand with size less than 5 mm. The mix proportion of ordinary concrete is listed Table 2. The water-to-binder ratio of concrete was 0.47. Table 2. Mix proportion of concrete: kg/m3 Cement
Fine aggregate
Coarse aggregate
Water
372
593
1260
175
The unidirectional carbon fiber sheets were used to confine concrete cylinders. The main parameters of the carbon fiber
49
Hai-yan Zhang et al. / Procedia Engineering 135 (2016) 47 – 55
sheets used in the tests are presented in Table 3. Table 3. Mechanical properties of carbon fiber sheets Modulus of elasticity (GPa)
Tensile strength (MPa)
Fiber orientation
Thickness (mm)
Width (mm)
Elongation at failure
253
3658
Unidirectional
0.167
300
1.73%
2.2. Test specimens Twenty concrete cylinders of size Ø150h300 mm were cast. These cylinders were naturally cured for more than 3 months. Before wrapping carbon fiber sheets, the lateral surface of cylinders was roughened by a grinder until coarse aggregates were seen. Then voids and deformities on the surface of the concrete were filled using geopolymers. Following that, carbon fiber sheets were attached directly with geopolymers onto the surface of concrete, providing unidirectional lateral confinement in the hoop direction. The overlap length of carbon fiber sheets is 200 mm. Then these confined cylinders were cured at room temperature for 7 days before tests. 2.3. Test apparatus and procedure Before compression tests, high temperature exposure specimens were first heated to the target temperature (100, 200 and 300oC) in an electrical furnace, at a heating rate of 5oC per minute. Once the predetermined target temperature was reached, specimens were kept at the peak temperature for 2 hours to attain thermal stability. Then the heating in furnace was turned off, the door of furnace was opened, and the specimens were allowed to cool naturally. A summary of tested specimens with considered temperatures is shown in Table 4. In the designation of specimens, the alphabet R and H represent room temperature and high temperature respectively, the number of 1, 2 and 3 after alphabet H denotes the exposure temperature of 100oC, 200oC, and 300oC, respectively. Alphabet G represents the confined cylinders with carbon fiber sheets using geopolymers as adhesive, and the number before alphabet G represents the number of fiber layers. Table 4. Summary of test specimens. Group ID
R
H1
H2
H3
1GR
2GR
3GR
2GH1
2GH2
2GH3
Layer(s) of CFRP
0
0
0
0
1
2
3
2
2
2
Temperature (oC)
25
100
200
300
25
25
25
100
200
300
No. of specimens
2
2
2
2
2
2
2
2
2
2
The compression tests were conducted using a universal material testing machine TYE-3000kE, at a loading rate of 0.5 mm/min. Specimens were placed between two loading panels, capped with gypsum to provide parallel surfaces and uniform compression. On the circumference face of the cylinders, four vertical strain gauges and four horizontal strain gauges were stalled at the mid-height, with a center angle interval of 90 oC, to measure the axial and hoop strains during tests. Four linear variable displacement transducers (LVDTs) were placed to the top loading panel, with a center angle interval of 90 oC, to record the axial deformation of specimens. A data acquisition system DH3816 was used to collect data of strain gauges and LVDTs. The test set-up is shown in Fig. 1.
Fig. 1 Test set-up
50
Hai-yan Zhang et al. / Procedia Engineering 135 (2016) 47 – 55
3. Results and Discussion Axial compression tests were conducted on 2 unconfined cylinders and 6 confined cylinders wrapped with 1, 2 and 3 layers of carbon fiber sheets at ambient temperature, and on 6 unconfined cylinders and 6 confined cylinders wrapped with 2 layers of carbon fiber sheets after exposure to high temperatures (100, 200 and 300 oC). The observations during tests were recorded, and the data on axial load, axial displacement, axial strain and hoop strain were measured. 3.1 Comparison of confined cylinders with unconfined cylinders at room temperature A comparison of failure modes, load-displacement curves and strain data from tests at ambient temperature on unconfined cylinders and confined cylinders were made, to investigate the strengthening effect of CFSG system at ambient temperature. 3.1.1 Failure modes Fig. 2 shows the appearance of unconfined cylinders at failure. It can be seen that a typical compressive failure mode occurs on unconfined cylinders. The concrete at the mi-height of the cylinder expanded and fell off. During the compression tests on confined cylinders, a sound of "hiss" was heard, when the imposed load on confined cylinder approached the peak load of the unconfined specimen. This is because that geopolymers and carbon fiber sheets began to burden tensile force, which was induced by concrete expansion at the mid-height. When a peak load was imposed on the confined cylinders, the specimen abruptly lost its carrying capacity. A great drop occurred in the load, accompanied with a loud noise. Fig. 3 presents the failure modes of confined cylinders wrapped with 1, 2 and 3 layers of carbon fiber sheets using geopolymers as adhesive. Slippage failure between geopolymers and carbon fiber sheets occurred in the lap joint, which implies the bond strength between geopolymers and carbon fiber sheets is not enough strong. Uncovering the peripheral carbon fiber sheets of confined cylinders, it can be found that the damage of core concrete aggravated with an increase in the number of layers of the carbon fiber sheets.
Fig. 2. Failure modes of unconfined concrete at room temperature
(a) One layer of CFSG
(b)Two layers of CFSG
(c)Three layers of CFSG
Fig. 3. Failure of confined cylinders wrapped with carbon fiber sheets at room temperature
3.1.2 Load-displacement curves The test results on compressive strength and peak displacement were listed in Table 5. From Table 5, it can be seen that when the concrete cylinders were confined with 1, 2 and 3 layers of carbon fiber sheets, an increment rate of 16.5%, 41.8% and 72.4% in compressive strength and an increment rate of 153.3%, 212.4% and 336.7% in peak displacement occurred.
51
Hai-yan Zhang et al. / Procedia Engineering 135 (2016) 47 – 55
The increase in compressive strength and peak displacement of confined cylinders with the layer of carbon fiber sheets is due to the increase in the restriction effect of external carbon fiber sheets on core concrete. Table 5. Test results of carbon fiber sheets confined concrete.
Compressive strength (MPa)
Group ID
Standard deviation
Average
Peak displacement (0.01mm)
Ratio of increase (%)
Average
Standard deviation
Ratio of increase (%)
R
47.9
0.2
-
92.9
1.9
-
1GR
55.8
0.85
16.5
235.3
6.5
153.3
2GR
67.9
0.7
41.8
290.2
7.65
212.4
3GR
82.6
0.2
72.4
405.7
18.85
336.7
Fig. 4 presents the load-displacement curves of an unconfined cylinder and three confined cylinders wrapped with 1, 2 and 3 layers of carbon fiber sheets respectively. It can be seen that four curves almost coincide at the early linear elastic stage. However, higher strength and displacement development were seen in confined cylinders with an increase in the layer numbers of carbon fiber sheets. This is because at the early linear elastic stage, the external carbon fiber sheets have not play a role yet. When the imposed load increased gradually, the core concrete began to expand. However, its expansion was restricted by the external carbon fiber sheets, and then tensile stress developed in carbon fiber sheet-geopolymer system, which in turn made the core concrete in compression in radius direction. This leads to the increase in compressive strength and displacement of confined cylinders, compared to the unconfined specimens. 1600
Axial load (kN)
1400 1200 1000 1GR 2GR 3GR R
800 600 400 200 0
0
50
100 150 200 250 300 350 400 450
Axial displacement (0.01mm)
Fig. 4. Axial load-displacement curves
3.1.3 Data on strains Fig. 5 shows the average axial strain and average hoop strain of an unconfined cylinder and three confined cylinders, as a function of axial stress calculated by the imposed load dividing the compressive area. Due to damage occurred in some strain gauges at the late stage, some strain data were lost. It can be seen from Fig. 5 that the stress-strain curves could be approached by using two regions. The first region is from 0 to core concrete expands, and the second region is after concrete expands to confined concrete fails, two regions shown as two straight lines with different slopes. It coincides with the trend of load-displacement curves. Group 1GR and group 2GR match well with group R in the previous linear elastic phase. Group 3GR deviating group R, the reason of strain data delay between them is that the thickness of geopolymers is too big (about 3 mm). In general, because of the first region stiffness mainly provided by the core concrete, so slope of the first straight line is steep. With the increase of axial load, core concrete damaged and stiffness decreased, carbon fiber sheets would play a role in confining cracked concrete, and specimens would continue to carry at a smaller slope. The constraint
52
Hai-yan Zhang et al. / Procedia Engineering 135 (2016) 47 – 55
stems from carbon fiber sheets improves the strength of concrete cylinders, at the same time, also greatly improve the ductility of unconfined cylinders. 70 70
60
60
Stress (MPa)
Stress (MPa)
50 40 R 1GR 2GR 3GR
30 20 10 0
50
30 20 10 0
0
800
1600
2400
3200
4000
Axial strain (micron)
4800
R 1GR 2GR 3GR
40
0
(a) Axial stress-Axial strain curves
300
600
900
1200 1500 1800 2100
Hoop strain (micron)
(b) Axial stress-hoop strain curves
Fig. 5 Stress-strain curves
3.2 Effects of different temperatures on concrete confined with 2 layers of carbon fiber sheets A comparison of failure modes, load-displacement curves and strain data were made on unconfined cylinder with confined cylinders wrapped by 2 layers of carbon fiber sheets, after exposure to different elevated temperatures, to investigate the strengthening effect of CFSG system of geopolymers at high temperature. 3.2.1 Failure modes The failure modes of unconfined and confined cylinders after exposure to high temperature are similar with that at room temperature, as shown in Figs. 6-8. For confined cylinders, slippage failure between geopolymers and carbon fiber sheets occurred in the lap joint. After exposure to 100 oC, the lap joint failure of confined specimens happened. After exposure to 200 oC, the lap joint failure of confined specimen named 2GH2-2 still happened but the failure of confined specimen called 2GH2-1 was the lap joint failure and ruptured of carbon fiber sheets. After exposure to 300 oC, two specimens were failed for carbon fiber sheets were ruptured.
(a) Unconfined specimens
(b) Confined specimens
Fig.6 Failure modes of specimens after exposure to 100oC
(a) Unconfined specimens
(b) Confined specimens
Fig. 7 Failure modes of specimens after exposure to 200 oC
53
Hai-yan Zhang et al. / Procedia Engineering 135 (2016) 47 – 55
(a) Unconfined specimens
(b) Confined specimens
Fig. 8 Failure modes of specimens after exposure to 300 oC
1200
1350
1050
1200
Axial load (kN)
Axial load (kN)
3.2.2 Load-displacement curves Table 6 lists the compressive strength and peak displacement of unconfined and confined cylinders wrapped with two layers of CFSG systems after exposure to 100, 200 and 300 oC. Compared to the compressive strength of confined cylinder with two layers of CFSG system (2RG) at ambient temperature, listed in Table 5), confined cylinders exhibit no significant strength degradation after exposure to elevated temperatures. In comparison to unconfined cylinders, confined cylinders have an increment rate in compressive strength of 56.2%, 90.9% and 106.4% after exposure to 100, 200 and 300 oC respectively, which is higher than the strength increment rate at ambient temperature (41.8%). The higher strengthening effect of CFSG system at high temperatures is mainly due to the difference in thermal expansion between carbon fiber sheets and core concrete, and the good resistance of geopolymers to high temperatures. The expansion coefficient of carbon fiber sheets and concrete is almost 0 oC-1 and (0.008T+6)h10-6 oC -1 [18] respectively, which makes carbon fiber sheets prestress the surface of core concrete. The prestressing force effectively confines the concrete expansion and leads to a better strengthening effect in compressive strentth at elevated temperatures than that at ambient temperature. Geopolymers, with excellent resistance to high temperature, provide effective bond between carbon fiber sheets and core concrete, and thus CFSG system can sustain the confinement on core concrete. Comparing Table 5 and 6, a small decrease in peak displacement is seen on confined cylinders wrapped with two layers of CFSG system after exposure to elevated temperature. And the increment rate in peak displacement of confined cylinder to unconfined cylinder at high temperatures is also lower than that at ambient temperature. The comparison of load-displacement curves between confined cylinder and unconfined cylinder after exposure to elevated temperatures was presented in Fig. 9.
900 750 600
H1 2GH1
450 300 150 0
1050 900 750 600
H2 2GH2
450 300 150
0
40
80
120
160
200
240
0
280
0
Axial displacement (0.01mm)
40
80
120
160
200
240
Axial displacement (0.01mm)
280
1350
1350
1200
1200
Axial load (kN)
Axial load (kN)
(a) Axial load - displacement curves after exposure to 100 oC. (b) Axial load - displacement curves after exposure to 200 oC.
1050 900 750 600
H3 2GH3
450 300
900 2GR 2GH1 2GH2 2GH3
750 600 450 300 150
150 0
1050
0
40
80
120
160
200
240
280
Axial displacement (0.01mm)
(c) Axial load-displacement curves after exposure to 300 oC.
320
0
0
40
80
120 160
200 240
280 320
Axial displacement (0.01mm)
(d) Axial load-displacement curves after exposure to 100, 200 and 300 oC.
x Fig. 9 Axial load-displacement curves after exposure to elevated temperatures.
54
Hai-yan Zhang et al. / Procedia Engineering 135 (2016) 47 – 55
Table 6. Test results of confined concrete with 2 layers of carbon fiber sheets after exposure to high temperatures. Compressive strength (MPa) Group ID Average
Peak displacement (0.01mm)
Ratio of increase (%)
Standard deviation
Average
Standard deviation
Ratio of increase (%)
H1
40.9
1.2
-
138.3
9.7
-
2GH1
63.9
0.05
56.2
279.85
8.25
101.7
H2
35.0
1.12
-
88
3
-
2GH2
66.8
4.38
90.9
248.9
23.9
182.8
H3
34.5
4.26
-
102.2
12.5
-
2GH3
71.2
1.60
106.4
255.4
23.9
149.9
x 3.2.3 Data on strains As shown in Table 6, ultimate axial strain and its ratio of increase were 6687.5 microstrains and 156.8% (after exposure to 100 oC), 8083.4 microstrains and 223.3% (after exposure to 200 oC), and 7652 microstrains and 210.2% (after exposure to 300 oC), respectively. From 100 oC to 300 oC, the ultimate axial strains and the ratios of increase of ultimate axial strain present a fast growth, and then slowly decreasing trend. Fig. 12, which was chose data of a specimen from each group to draw curves, shows that the stress-strain curves of confined concrete can be approached by using two regions. The first region is from 0 to a point when core concrete expands, the second region is after core concrete expands to specimen fails. In the first region, curves of confined cylinders coincided with those of unconfined ones. This is due to core concrete was subjected main compressive load in the early stage of loading. In the second region, because core concrete was expanded and peripheral carbon fiber sheets provided the lateral restraint on the expansion of the core concrete, the stress of confined concrete could continue to increase with a smaller slope until specimens were failed. 70
70
60
60
Stress (MPa)
Stress (MPa)
50 40 H1 2GH1
30 20
H2 2GH2
40 30 20 10
10 0
50
0 0
1000 2000 3000 4000 5000 6000 7000
Axial strain (micron)
0
(a) Exposure to 100 oC.
1500
3000
4500
6000
7500
Axial strain (micron)
9000
(b) Exposure to 200 oC. 80
70
70
Stress (MPa)
Stress (MPa)
60 50 40 H3 2GH3-2
30 20 10 0
60 50
2GR 2GH1 2GH2 2GH3
40 30 20 10 0
0
1000 2000 3000 4000 5000 6000 7000 8000
Axial strain (micron)
(c) Exposure to 300 oC.
0
1500
3000
4500
6000
7500
Axial strain (micron)
9000
(d) Comparison of curves after exposure to 100, 200 and 300 oC.
Fig. 10. Axial stress -axial strain curves after exposure to elevated temperatures.
Hai-yan Zhang et al. / Procedia Engineering 135 (2016) 47 – 55
4. Conclusions A large number of compression tests were conducted on unconfined plain concrete cylinder and confined concrete cylinders with carbon fiber sheet-geopolymer system, at room temperature and after exposure to 100, 200 and 300 oC. Based on the test results, the following conclusions can be drawn: (1) The enhancement in compressive strength at ambient temperature of confined cylinder with carbon fiber sheets using geopolymer as adhesive is not great as expected. Slippage occurred between carbon fiber sheets and geopolymers. And the strength of carbon fiber sheets is not full used. (2) The confined cylinders with carbon fiber sheet-geopolymer system exhibit a significant enhancement in compressive strength, but a decrease in ductility, after exposure to elevated temperatures. With the exposure temperature increased from 100 to 300 oC, failure modes of confined cylinders with two layers of carbon fiber sheets changed from slippage failure at the lap joint to the rupture failure of carbon fiber sheets. (3) With the exposed temperatures increasing, the stress-strain curves of confined cylinders are basically in coincidence. This shows that inorganic geopolymers adhesive has a good resistance to high temperatures lower than 300 oC.
Acknowledgements The research presented in this paper is supported by National Natural Science Foundation of China (Grant No. 51108193), State Key laboratory of Subtropical Architecture Science, South China University of Technology (Grant No. 2011ZC30 and 2013ZC21), Technology Fund of Guangzhou Baiyuan District (2011-KZ-48), Technology innovation fund of Guangzhou scientific and technological minor enterprise (2012J4200032).
References [1] Menna C., Asprone D., Ferone C., Colangelo F., Balsamo A., Prota A., 2013. Use of geopolymers for composite external reinforcement of RC members. Compos Part B-Eng 45(1), p. 1667-1676. [2] Xu M., Chen Z.F., Xiao D. H., 2013. Experimental study on RC cylinders confined by CFRP sheets under uniaxial compression after elevated temperature, Engineering Mechanics 30(11), p. 214-220. [3] Toutanji H., Deng Y., 2007. Comparison between organic and inorganic matrices for RC beams strengthened with carbon fiber sheets, Journal of Composites for Construction 11(5), p. 507-513. [4] Toutanji H., Zhao L., Zhang Y., 2006. Flexural behavior of reinforced concrete beams externally strengthened with CFRP sheets boned with an inorganic matrix, Engineering Structures 28, p. 557-566. [5] Kurtz S., Balaguru P., 2001. Comparison of inorganic and organic matrices for strengthening of RC beams with carbon sheets, Journal of Structural Engineering 127(1), p. 35-42. [6] William D.A.R., Arie V.R., 2014. Performance of solid and cellular structured fly ash geopolymers exposed to a simulated fire, Cement and Concrete Composites 48,p. 75-82. [7] Jadambaa T., William R., Melissa L., Arie V.R., 2011. Preparation and thermal properties of fire resistant metakaolin-based geopolymer-type coatings, Journal of Non-Crystalline Solids 357, p. 1399-1404. [8] Monica A.V., Ruby M.G., Soumitra S., Calvin D., Juan C.N., 2015. Thermal properties of novel binary geopolymers based on metakaolin and alternative silica sources, Applied Clay Science, p. 1-7. [9] Zhang H.Y., Kodur V., Qi S.L., Cao L., Wu B., 2014. Development of metakaolin-fly ash based geopolymers for fire resistance applications, Construction and Building Materials 55, p. 38-45. [10] Davidovits J., 1989. Geopolymers and geopolymeric materials. Journal of Thermal Analysis and Calorimetry 35(2). P. 429-441. [11] Zheng W.Z., Chen W.H., Xu W., Wang Y., 2009. Experimental research on alkali-activated slag high temperature resistant inorganic adhesive pasting CFRP sheets on surface of concrete, Journal of Building Structures 30(4), p. 138-144, 157. [12] Zhang H. Y., Kodur V., Qi S. L., Wu B., 2015. Characterizing the bond strength of geopolymers at ambient and elevated temperatures, Cement and Concrete Composites 58, p. 40-49. [13] Zhang H.Y., Kodur V., Wu B., Cao L., Qi S.L., 2015. Effect of carbon fibers on thermal and mechanical properties of metakaolin fly-ash-based geopolymer, ACI Materials 112(3), p. 375-384. [14] Bakharev T., 2005. Durability of geopolymer materials in sodium and magnesium sulfate solutions, Cement and Concrete Composites 35,p. 12331246. [15] Bakharev T., 2005. Resistance of geopolymer materials to acid attack, Cement and Concrete Composites 35, p. 658-670. [16] Duan P., Yan C. J., Zhou W., Luo W.J., Shen C.H., 2015. An investigation of the microstructure and durability of a fluidized bed fly ash-metakaolin geopolymer after heat and acid exposure, Materials and Design 74, p. 125-137. [17] Gu X.L., Li Y.P., Zhang W.P., 2006. Compressive stress-strain relationship of concrete confined by carbon fiber composite sheets, Structural Engineer 22(2), p. 50-56. [18] Xu M., Chen Z.F., Xiao D. H., 2013. Experimental study on RC cylinders confined by CFRP sheets under uniaxial compression after elevated temperature, Engineering Mechanics 30(11), p. 214-220.
55
ScienceDirect Procedia Engineering 135 (2016) 47 – 55
Experimental study on high temperature properties of carbon fiber sheets strengthened concrete cylinders using geopolymer as adhesive Hai-yan Zhanga,b,*, Xu Haob, Wang Fana a
State Key laboratory of Subtropical Architecture Science, South China University of Technology, Guangzhou 510640, China b Department of Civil Engineering, South China University of Technology, Guangzhou, 510640, China
Abstract Twenty plain concrete cylinders, including unconfined cylinders and confined cylinders with 1, 2 and 3 layers of carbon fiber sheets using geopolymer as adhesive, were tested in axial compression at ambient temperature and after exposure to elevated temperatures, to investigate the strengthening effect of carbon fiber sheet-geopolymer system. The failure modes, load-displacement curves, axial and hoop strains of confined cylinders were compared with that of unconfined cylinders. The failure modes of confined cylinders after exposure high temperature are similar with those of ones at room temperature. When temperature is up to 300oC, the compressive strength of confined cylinders is a little more than that of ones at room temperature, and the ratio of increase on compressive strength which is 106.4% is double as that of at room temperature. However, the ductility of confined cylinders decreases significantly after exposure elevated temperatures. From stress-strain curves, it can be found that there is no obvious degradation on mechanical property after exposure to high temperatures. It shows that carbon fiber sheet-geopolymer system has an excellent resistance to high temperatures. by Elsevier Ltd. This is an openLtd. access article underunder the CCCC BY-NC-ND license © 2016 2016Published The Authors. Published by Elsevier Open access BY-NC-ND license. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of School of Engineering of Sun Yat-sen University. .Peer-review under responsibility of the organizing committee of ICPFFPE 2015 Keywords: Concrete cylinders; Geopolymer; Carbon fiber sheets; High temperature; Compression
1. Introduction Externally bonded fiber reinforced polymer (EB-FRP) is widely used for strengthening concrete structures, due to its advantages of light weight, high strength, corrosion resistance, and ease to application. In EB-FRP system, organic matrices, such as epoxy resin, are used as a primary adhesive. Organic matrix exhibits high strength at ambient temperature, but low resistance to high temperatures. The glass transition temperature (Tg) of epoxy resin is only about 82 oC [1], which results in its great strength degradation at high temperatures. In addition, emission of poisonous gas occurs when epoxy resin is in fire [1]. To deal with the drawbacks of organic matrix, inorganic matrices were developed recently [2,3,4,5]. Among the many types of inorganic matrices, a new class of material called geopolymers, attracted researchers’ attention in the past decades [6-16]. Geopolymer is synthesized by alkaline solutions (such as sodium silicate and potassium silicate [6-16]) activating aluminosilicate source materials (such as metakaolin, fly ash, and slag [6-16]). Compared to organic matrix, inorganic geopolymers have advantages of resistance to high temperature, resistance to UV radiation and minimal toxic smoke under fire exposure.
* Corresponding author. Tel.: +86-20-8711 1030; E-mail address: [email protected]
1877-7058 © 2016 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 organizing committee of ICPFFPE 2015
doi:10.1016/j.proeng.2016.01.078
48
Hai-yan Zhang et al. / Procedia Engineering 135 (2016) 47 – 55
The mechanical properties of geopolymers, such as compressive and bending strength, at ambient temperature and after exposure to high temperatures, were extensively studied in literatures [9]. The bond performance using geopolymer as adhesive is also been explored recently. Menna C. et al. [1] used metakaolin-based geopolymer as adhesive to paste steel cord confining RC beams. Test results showed that geopolymer has good adhesion between concrete substrate and steel cords. Toutanji and Deng [3] compared the strengthening effect of carbon fiber sheets bonded with organic and inorganic matrices on reinforced concrete beams. Results showed the carbon fiber sheet-geopolymer (CFSG) system is effective in increasing strength and stiffness of reinforced concrete beams as that of organic matrix at ambient temperature. Toutanji and Zhao [4] reported that the ductility of reinforced beams strengthened by CFSG system is greatly lower than that of unstrengthened beams, but the load carrying capacity of strengthened beams increases with an increase in layers of carbon fiber sheets. Kurtz S. and Balaguru P. [5] also found that the inorganic and organic systems can provide comparable performance with respect to the increase in strength of RC beams strengthened by carbon fiber sheets. The above researches demonstrated that geopolymers exhibit similar bond properties with carbon fibers sheet and concrete substrate at ambient temperature. However, as for the high temperature properties of CFSG system, there is a lack of experimental data. In this paper, 20 plain concrete cylinders, including unconfined cylinders and confined cylinders with carbon fiber sheets bonded by geopolymers, were tested in compression at ambient temperature and after exposure to elevated temperatures, to investigate the strengthening effect of CFSG system at ambient and high temperatures. The studied parameters include the number of fiber layers and the exposure temperatures. 2. Experimental program The experimental program is composed of a large number of compression tests at ambient temperature and after exposure to elevated temperatures (100, 200, 300 oC), on unconfined plain concrete cylinders and confined cylinders with carbon fiber sheets using geopolymers as adhesive. 2.1. Raw materials The primary aluminosilicate source material used in geopolymer is metakaolin (MK) and fly ash (FA) mixture. The chemical composition of MK and FA is shown in Table 1. Commercially produced metakaolin with an average particle size of 0.017 mm, was supplied by Shanxi Jinkunhengye Ltd., China. Low calcium fly ash, with an average particle size of 0.032 mm, was supplied by Guangzhou Huangpu Power Plant. Potassium silicate solution with SiO 2/K2O molar ratios of 1.0 was used as alkaline-silicate activator. Chopped carbon fibers (CF), with a content of 0.5 percent of MK-FA blend precursor in mass, were added to precursor as reinforcement agent. The length, diameter and density of chopped carbon fibers are 6 mm, 7 μm and 1.76-1.80 g/cm3 respectively. Table 1. Chemical composition of FA and MK (wt, %) SiO2
Al2O3
CaO
Fe2O3
FA
51.35
44.24
0.13
0.98
MK
45.3
41.2
3.77
3.18
TiO2
SO3
MgO
K2O
P2O5
SrO
Na2O
0.9
0
0.48
0.08
1.62
0.75
0.44
0.38
ZrO2
MnO
Loss on ignition
0.45
0
0.36
0.11
0.16
0
0.01
0.72
0.09
0.08
0.05
2.4
Geopolymer pastes were synthesized with fly ash and metakaolin mixture (1:1) and alkali silicate solutions. The liquid/solid (L/S) mass ratio was 0.4 (the liquid consists of solvent in alkali silicate solutions; the solid materials consist of fly ash, matekaolin and solute of alkali silicate solutions). Geopolymer pastes were prepared by hand-mixing MK-FA precursor and short fibers for 2 min, then adding alkaline silicate solution and mixing all ingredients in a mixer about 7 min to ensure they were mixed thoroughly. The materials of concrete cylinders used in this study were Type I Portland cement (Grade 32.5), tap water, crushed limestone with size of 5 to 25 mm and natural sand with size less than 5 mm. The mix proportion of ordinary concrete is listed Table 2. The water-to-binder ratio of concrete was 0.47. Table 2. Mix proportion of concrete: kg/m3 Cement
Fine aggregate
Coarse aggregate
Water
372
593
1260
175
The unidirectional carbon fiber sheets were used to confine concrete cylinders. The main parameters of the carbon fiber
49
Hai-yan Zhang et al. / Procedia Engineering 135 (2016) 47 – 55
sheets used in the tests are presented in Table 3. Table 3. Mechanical properties of carbon fiber sheets Modulus of elasticity (GPa)
Tensile strength (MPa)
Fiber orientation
Thickness (mm)
Width (mm)
Elongation at failure
253
3658
Unidirectional
0.167
300
1.73%
2.2. Test specimens Twenty concrete cylinders of size Ø150h300 mm were cast. These cylinders were naturally cured for more than 3 months. Before wrapping carbon fiber sheets, the lateral surface of cylinders was roughened by a grinder until coarse aggregates were seen. Then voids and deformities on the surface of the concrete were filled using geopolymers. Following that, carbon fiber sheets were attached directly with geopolymers onto the surface of concrete, providing unidirectional lateral confinement in the hoop direction. The overlap length of carbon fiber sheets is 200 mm. Then these confined cylinders were cured at room temperature for 7 days before tests. 2.3. Test apparatus and procedure Before compression tests, high temperature exposure specimens were first heated to the target temperature (100, 200 and 300oC) in an electrical furnace, at a heating rate of 5oC per minute. Once the predetermined target temperature was reached, specimens were kept at the peak temperature for 2 hours to attain thermal stability. Then the heating in furnace was turned off, the door of furnace was opened, and the specimens were allowed to cool naturally. A summary of tested specimens with considered temperatures is shown in Table 4. In the designation of specimens, the alphabet R and H represent room temperature and high temperature respectively, the number of 1, 2 and 3 after alphabet H denotes the exposure temperature of 100oC, 200oC, and 300oC, respectively. Alphabet G represents the confined cylinders with carbon fiber sheets using geopolymers as adhesive, and the number before alphabet G represents the number of fiber layers. Table 4. Summary of test specimens. Group ID
R
H1
H2
H3
1GR
2GR
3GR
2GH1
2GH2
2GH3
Layer(s) of CFRP
0
0
0
0
1
2
3
2
2
2
Temperature (oC)
25
100
200
300
25
25
25
100
200
300
No. of specimens
2
2
2
2
2
2
2
2
2
2
The compression tests were conducted using a universal material testing machine TYE-3000kE, at a loading rate of 0.5 mm/min. Specimens were placed between two loading panels, capped with gypsum to provide parallel surfaces and uniform compression. On the circumference face of the cylinders, four vertical strain gauges and four horizontal strain gauges were stalled at the mid-height, with a center angle interval of 90 oC, to measure the axial and hoop strains during tests. Four linear variable displacement transducers (LVDTs) were placed to the top loading panel, with a center angle interval of 90 oC, to record the axial deformation of specimens. A data acquisition system DH3816 was used to collect data of strain gauges and LVDTs. The test set-up is shown in Fig. 1.
Fig. 1 Test set-up
50
Hai-yan Zhang et al. / Procedia Engineering 135 (2016) 47 – 55
3. Results and Discussion Axial compression tests were conducted on 2 unconfined cylinders and 6 confined cylinders wrapped with 1, 2 and 3 layers of carbon fiber sheets at ambient temperature, and on 6 unconfined cylinders and 6 confined cylinders wrapped with 2 layers of carbon fiber sheets after exposure to high temperatures (100, 200 and 300 oC). The observations during tests were recorded, and the data on axial load, axial displacement, axial strain and hoop strain were measured. 3.1 Comparison of confined cylinders with unconfined cylinders at room temperature A comparison of failure modes, load-displacement curves and strain data from tests at ambient temperature on unconfined cylinders and confined cylinders were made, to investigate the strengthening effect of CFSG system at ambient temperature. 3.1.1 Failure modes Fig. 2 shows the appearance of unconfined cylinders at failure. It can be seen that a typical compressive failure mode occurs on unconfined cylinders. The concrete at the mi-height of the cylinder expanded and fell off. During the compression tests on confined cylinders, a sound of "hiss" was heard, when the imposed load on confined cylinder approached the peak load of the unconfined specimen. This is because that geopolymers and carbon fiber sheets began to burden tensile force, which was induced by concrete expansion at the mid-height. When a peak load was imposed on the confined cylinders, the specimen abruptly lost its carrying capacity. A great drop occurred in the load, accompanied with a loud noise. Fig. 3 presents the failure modes of confined cylinders wrapped with 1, 2 and 3 layers of carbon fiber sheets using geopolymers as adhesive. Slippage failure between geopolymers and carbon fiber sheets occurred in the lap joint, which implies the bond strength between geopolymers and carbon fiber sheets is not enough strong. Uncovering the peripheral carbon fiber sheets of confined cylinders, it can be found that the damage of core concrete aggravated with an increase in the number of layers of the carbon fiber sheets.
Fig. 2. Failure modes of unconfined concrete at room temperature
(a) One layer of CFSG
(b)Two layers of CFSG
(c)Three layers of CFSG
Fig. 3. Failure of confined cylinders wrapped with carbon fiber sheets at room temperature
3.1.2 Load-displacement curves The test results on compressive strength and peak displacement were listed in Table 5. From Table 5, it can be seen that when the concrete cylinders were confined with 1, 2 and 3 layers of carbon fiber sheets, an increment rate of 16.5%, 41.8% and 72.4% in compressive strength and an increment rate of 153.3%, 212.4% and 336.7% in peak displacement occurred.
51
Hai-yan Zhang et al. / Procedia Engineering 135 (2016) 47 – 55
The increase in compressive strength and peak displacement of confined cylinders with the layer of carbon fiber sheets is due to the increase in the restriction effect of external carbon fiber sheets on core concrete. Table 5. Test results of carbon fiber sheets confined concrete.
Compressive strength (MPa)
Group ID
Standard deviation
Average
Peak displacement (0.01mm)
Ratio of increase (%)
Average
Standard deviation
Ratio of increase (%)
R
47.9
0.2
-
92.9
1.9
-
1GR
55.8
0.85
16.5
235.3
6.5
153.3
2GR
67.9
0.7
41.8
290.2
7.65
212.4
3GR
82.6
0.2
72.4
405.7
18.85
336.7
Fig. 4 presents the load-displacement curves of an unconfined cylinder and three confined cylinders wrapped with 1, 2 and 3 layers of carbon fiber sheets respectively. It can be seen that four curves almost coincide at the early linear elastic stage. However, higher strength and displacement development were seen in confined cylinders with an increase in the layer numbers of carbon fiber sheets. This is because at the early linear elastic stage, the external carbon fiber sheets have not play a role yet. When the imposed load increased gradually, the core concrete began to expand. However, its expansion was restricted by the external carbon fiber sheets, and then tensile stress developed in carbon fiber sheet-geopolymer system, which in turn made the core concrete in compression in radius direction. This leads to the increase in compressive strength and displacement of confined cylinders, compared to the unconfined specimens. 1600
Axial load (kN)
1400 1200 1000 1GR 2GR 3GR R
800 600 400 200 0
0
50
100 150 200 250 300 350 400 450
Axial displacement (0.01mm)
Fig. 4. Axial load-displacement curves
3.1.3 Data on strains Fig. 5 shows the average axial strain and average hoop strain of an unconfined cylinder and three confined cylinders, as a function of axial stress calculated by the imposed load dividing the compressive area. Due to damage occurred in some strain gauges at the late stage, some strain data were lost. It can be seen from Fig. 5 that the stress-strain curves could be approached by using two regions. The first region is from 0 to core concrete expands, and the second region is after concrete expands to confined concrete fails, two regions shown as two straight lines with different slopes. It coincides with the trend of load-displacement curves. Group 1GR and group 2GR match well with group R in the previous linear elastic phase. Group 3GR deviating group R, the reason of strain data delay between them is that the thickness of geopolymers is too big (about 3 mm). In general, because of the first region stiffness mainly provided by the core concrete, so slope of the first straight line is steep. With the increase of axial load, core concrete damaged and stiffness decreased, carbon fiber sheets would play a role in confining cracked concrete, and specimens would continue to carry at a smaller slope. The constraint
52
Hai-yan Zhang et al. / Procedia Engineering 135 (2016) 47 – 55
stems from carbon fiber sheets improves the strength of concrete cylinders, at the same time, also greatly improve the ductility of unconfined cylinders. 70 70
60
60
Stress (MPa)
Stress (MPa)
50 40 R 1GR 2GR 3GR
30 20 10 0
50
30 20 10 0
0
800
1600
2400
3200
4000
Axial strain (micron)
4800
R 1GR 2GR 3GR
40
0
(a) Axial stress-Axial strain curves
300
600
900
1200 1500 1800 2100
Hoop strain (micron)
(b) Axial stress-hoop strain curves
Fig. 5 Stress-strain curves
3.2 Effects of different temperatures on concrete confined with 2 layers of carbon fiber sheets A comparison of failure modes, load-displacement curves and strain data were made on unconfined cylinder with confined cylinders wrapped by 2 layers of carbon fiber sheets, after exposure to different elevated temperatures, to investigate the strengthening effect of CFSG system of geopolymers at high temperature. 3.2.1 Failure modes The failure modes of unconfined and confined cylinders after exposure to high temperature are similar with that at room temperature, as shown in Figs. 6-8. For confined cylinders, slippage failure between geopolymers and carbon fiber sheets occurred in the lap joint. After exposure to 100 oC, the lap joint failure of confined specimens happened. After exposure to 200 oC, the lap joint failure of confined specimen named 2GH2-2 still happened but the failure of confined specimen called 2GH2-1 was the lap joint failure and ruptured of carbon fiber sheets. After exposure to 300 oC, two specimens were failed for carbon fiber sheets were ruptured.
(a) Unconfined specimens
(b) Confined specimens
Fig.6 Failure modes of specimens after exposure to 100oC
(a) Unconfined specimens
(b) Confined specimens
Fig. 7 Failure modes of specimens after exposure to 200 oC
53
Hai-yan Zhang et al. / Procedia Engineering 135 (2016) 47 – 55
(a) Unconfined specimens
(b) Confined specimens
Fig. 8 Failure modes of specimens after exposure to 300 oC
1200
1350
1050
1200
Axial load (kN)
Axial load (kN)
3.2.2 Load-displacement curves Table 6 lists the compressive strength and peak displacement of unconfined and confined cylinders wrapped with two layers of CFSG systems after exposure to 100, 200 and 300 oC. Compared to the compressive strength of confined cylinder with two layers of CFSG system (2RG) at ambient temperature, listed in Table 5), confined cylinders exhibit no significant strength degradation after exposure to elevated temperatures. In comparison to unconfined cylinders, confined cylinders have an increment rate in compressive strength of 56.2%, 90.9% and 106.4% after exposure to 100, 200 and 300 oC respectively, which is higher than the strength increment rate at ambient temperature (41.8%). The higher strengthening effect of CFSG system at high temperatures is mainly due to the difference in thermal expansion between carbon fiber sheets and core concrete, and the good resistance of geopolymers to high temperatures. The expansion coefficient of carbon fiber sheets and concrete is almost 0 oC-1 and (0.008T+6)h10-6 oC -1 [18] respectively, which makes carbon fiber sheets prestress the surface of core concrete. The prestressing force effectively confines the concrete expansion and leads to a better strengthening effect in compressive strentth at elevated temperatures than that at ambient temperature. Geopolymers, with excellent resistance to high temperature, provide effective bond between carbon fiber sheets and core concrete, and thus CFSG system can sustain the confinement on core concrete. Comparing Table 5 and 6, a small decrease in peak displacement is seen on confined cylinders wrapped with two layers of CFSG system after exposure to elevated temperature. And the increment rate in peak displacement of confined cylinder to unconfined cylinder at high temperatures is also lower than that at ambient temperature. The comparison of load-displacement curves between confined cylinder and unconfined cylinder after exposure to elevated temperatures was presented in Fig. 9.
900 750 600
H1 2GH1
450 300 150 0
1050 900 750 600
H2 2GH2
450 300 150
0
40
80
120
160
200
240
0
280
0
Axial displacement (0.01mm)
40
80
120
160
200
240
Axial displacement (0.01mm)
280
1350
1350
1200
1200
Axial load (kN)
Axial load (kN)
(a) Axial load - displacement curves after exposure to 100 oC. (b) Axial load - displacement curves after exposure to 200 oC.
1050 900 750 600
H3 2GH3
450 300
900 2GR 2GH1 2GH2 2GH3
750 600 450 300 150
150 0
1050
0
40
80
120
160
200
240
280
Axial displacement (0.01mm)
(c) Axial load-displacement curves after exposure to 300 oC.
320
0
0
40
80
120 160
200 240
280 320
Axial displacement (0.01mm)
(d) Axial load-displacement curves after exposure to 100, 200 and 300 oC.
x Fig. 9 Axial load-displacement curves after exposure to elevated temperatures.
54
Hai-yan Zhang et al. / Procedia Engineering 135 (2016) 47 – 55
Table 6. Test results of confined concrete with 2 layers of carbon fiber sheets after exposure to high temperatures. Compressive strength (MPa) Group ID Average
Peak displacement (0.01mm)
Ratio of increase (%)
Standard deviation
Average
Standard deviation
Ratio of increase (%)
H1
40.9
1.2
-
138.3
9.7
-
2GH1
63.9
0.05
56.2
279.85
8.25
101.7
H2
35.0
1.12
-
88
3
-
2GH2
66.8
4.38
90.9
248.9
23.9
182.8
H3
34.5
4.26
-
102.2
12.5
-
2GH3
71.2
1.60
106.4
255.4
23.9
149.9
x 3.2.3 Data on strains As shown in Table 6, ultimate axial strain and its ratio of increase were 6687.5 microstrains and 156.8% (after exposure to 100 oC), 8083.4 microstrains and 223.3% (after exposure to 200 oC), and 7652 microstrains and 210.2% (after exposure to 300 oC), respectively. From 100 oC to 300 oC, the ultimate axial strains and the ratios of increase of ultimate axial strain present a fast growth, and then slowly decreasing trend. Fig. 12, which was chose data of a specimen from each group to draw curves, shows that the stress-strain curves of confined concrete can be approached by using two regions. The first region is from 0 to a point when core concrete expands, the second region is after core concrete expands to specimen fails. In the first region, curves of confined cylinders coincided with those of unconfined ones. This is due to core concrete was subjected main compressive load in the early stage of loading. In the second region, because core concrete was expanded and peripheral carbon fiber sheets provided the lateral restraint on the expansion of the core concrete, the stress of confined concrete could continue to increase with a smaller slope until specimens were failed. 70
70
60
60
Stress (MPa)
Stress (MPa)
50 40 H1 2GH1
30 20
H2 2GH2
40 30 20 10
10 0
50
0 0
1000 2000 3000 4000 5000 6000 7000
Axial strain (micron)
0
(a) Exposure to 100 oC.
1500
3000
4500
6000
7500
Axial strain (micron)
9000
(b) Exposure to 200 oC. 80
70
70
Stress (MPa)
Stress (MPa)
60 50 40 H3 2GH3-2
30 20 10 0
60 50
2GR 2GH1 2GH2 2GH3
40 30 20 10 0
0
1000 2000 3000 4000 5000 6000 7000 8000
Axial strain (micron)
(c) Exposure to 300 oC.
0
1500
3000
4500
6000
7500
Axial strain (micron)
9000
(d) Comparison of curves after exposure to 100, 200 and 300 oC.
Fig. 10. Axial stress -axial strain curves after exposure to elevated temperatures.
Hai-yan Zhang et al. / Procedia Engineering 135 (2016) 47 – 55
4. Conclusions A large number of compression tests were conducted on unconfined plain concrete cylinder and confined concrete cylinders with carbon fiber sheet-geopolymer system, at room temperature and after exposure to 100, 200 and 300 oC. Based on the test results, the following conclusions can be drawn: (1) The enhancement in compressive strength at ambient temperature of confined cylinder with carbon fiber sheets using geopolymer as adhesive is not great as expected. Slippage occurred between carbon fiber sheets and geopolymers. And the strength of carbon fiber sheets is not full used. (2) The confined cylinders with carbon fiber sheet-geopolymer system exhibit a significant enhancement in compressive strength, but a decrease in ductility, after exposure to elevated temperatures. With the exposure temperature increased from 100 to 300 oC, failure modes of confined cylinders with two layers of carbon fiber sheets changed from slippage failure at the lap joint to the rupture failure of carbon fiber sheets. (3) With the exposed temperatures increasing, the stress-strain curves of confined cylinders are basically in coincidence. This shows that inorganic geopolymers adhesive has a good resistance to high temperatures lower than 300 oC.
Acknowledgements The research presented in this paper is supported by National Natural Science Foundation of China (Grant No. 51108193), State Key laboratory of Subtropical Architecture Science, South China University of Technology (Grant No. 2011ZC30 and 2013ZC21), Technology Fund of Guangzhou Baiyuan District (2011-KZ-48), Technology innovation fund of Guangzhou scientific and technological minor enterprise (2012J4200032).
References [1] Menna C., Asprone D., Ferone C., Colangelo F., Balsamo A., Prota A., 2013. Use of geopolymers for composite external reinforcement of RC members. Compos Part B-Eng 45(1), p. 1667-1676. [2] Xu M., Chen Z.F., Xiao D. H., 2013. Experimental study on RC cylinders confined by CFRP sheets under uniaxial compression after elevated temperature, Engineering Mechanics 30(11), p. 214-220. [3] Toutanji H., Deng Y., 2007. Comparison between organic and inorganic matrices for RC beams strengthened with carbon fiber sheets, Journal of Composites for Construction 11(5), p. 507-513. [4] Toutanji H., Zhao L., Zhang Y., 2006. Flexural behavior of reinforced concrete beams externally strengthened with CFRP sheets boned with an inorganic matrix, Engineering Structures 28, p. 557-566. [5] Kurtz S., Balaguru P., 2001. Comparison of inorganic and organic matrices for strengthening of RC beams with carbon sheets, Journal of Structural Engineering 127(1), p. 35-42. [6] William D.A.R., Arie V.R., 2014. Performance of solid and cellular structured fly ash geopolymers exposed to a simulated fire, Cement and Concrete Composites 48,p. 75-82. [7] Jadambaa T., William R., Melissa L., Arie V.R., 2011. Preparation and thermal properties of fire resistant metakaolin-based geopolymer-type coatings, Journal of Non-Crystalline Solids 357, p. 1399-1404. [8] Monica A.V., Ruby M.G., Soumitra S., Calvin D., Juan C.N., 2015. Thermal properties of novel binary geopolymers based on metakaolin and alternative silica sources, Applied Clay Science, p. 1-7. [9] Zhang H.Y., Kodur V., Qi S.L., Cao L., Wu B., 2014. Development of metakaolin-fly ash based geopolymers for fire resistance applications, Construction and Building Materials 55, p. 38-45. [10] Davidovits J., 1989. Geopolymers and geopolymeric materials. Journal of Thermal Analysis and Calorimetry 35(2). P. 429-441. [11] Zheng W.Z., Chen W.H., Xu W., Wang Y., 2009. Experimental research on alkali-activated slag high temperature resistant inorganic adhesive pasting CFRP sheets on surface of concrete, Journal of Building Structures 30(4), p. 138-144, 157. [12] Zhang H. Y., Kodur V., Qi S. L., Wu B., 2015. Characterizing the bond strength of geopolymers at ambient and elevated temperatures, Cement and Concrete Composites 58, p. 40-49. [13] Zhang H.Y., Kodur V., Wu B., Cao L., Qi S.L., 2015. Effect of carbon fibers on thermal and mechanical properties of metakaolin fly-ash-based geopolymer, ACI Materials 112(3), p. 375-384. [14] Bakharev T., 2005. Durability of geopolymer materials in sodium and magnesium sulfate solutions, Cement and Concrete Composites 35,p. 12331246. [15] Bakharev T., 2005. Resistance of geopolymer materials to acid attack, Cement and Concrete Composites 35, p. 658-670. [16] Duan P., Yan C. J., Zhou W., Luo W.J., Shen C.H., 2015. An investigation of the microstructure and durability of a fluidized bed fly ash-metakaolin geopolymer after heat and acid exposure, Materials and Design 74, p. 125-137. [17] Gu X.L., Li Y.P., Zhang W.P., 2006. Compressive stress-strain relationship of concrete confined by carbon fiber composite sheets, Structural Engineer 22(2), p. 50-56. [18] Xu M., Chen Z.F., Xiao D. H., 2013. Experimental study on RC cylinders confined by CFRP sheets under uniaxial compression after elevated temperature, Engineering Mechanics 30(11), p. 214-220.
55