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ABSTRACT Compared to glass fiber reinforced polymer (GFRP) rebars, basalt fiber reinforced polymer (BFRP) rebars possess many advantages as internal ...
CICE 2010 - The 5th International Conference on FRP Composites in Civil Engineering September 27-29, 2010, Beijing, China

Influence of Elevated Temperature on the Mechanical and Thermal Performance of BFRP Rebar Jingyu Wu, Hui Li & Guijun Xian ([email protected]) School of Civil Engineering, Harbin Institute of Technology, Harbin, China

ABSTRACT Compared to glass fiber reinforced polymer (GFRP) rebars, basalt fiber reinforced polymer (BFRP) rebars possess many advantages as internal reinforcement of concrete structures, in terms of the enhanced corrosion resistance. In the present study, mechanical and thermal properties of BFRP rebar at elevated temperatures and after elevated temperature treatment were conducted. The study is believe to offer the basic mechanical and thermal property data of BFRP rebars during and post- fire, which is helpful for the the safe design of BFRP in rehabilitation when considering the fire hazarder. The tensile properties of BFRP rebars were performed in the temperature ranging from room temperature to 350ć. Three stages of deterioration of the tensile strength and modulus are observed based on the degradation rate. It was found that the strength and stiffness of BFRPs still remain high values (e.g., more than half of the room temperature values) even at the temperature much higher than Tg (glass transition temperature) of the system. The deterioration is attributed to the decreased force transferring capacity of the resin. In addition, BFRP rebars were treated at elevated temperatures for various periods in an oven. The oxidation of the resin and the deterioration of the interface between fiber and resin bring in remarkable degradation of the mechanical and thermal properties. KEY WORDS

1 INTRODUCTION Basalt fiber reinforced polymer (BFRP) rebar has many advantages such as high strength, excellent anticorrosion performances, etc. As reported BFRP rebar possesses (Sim et al. 2005, Liu et al. 2006) a good alkali resistance, and thus it may be suitable to replace steel bar, reinforcing concrete structures. Since BFRP composites were applied in civil engineering only for several years, the study of BFRP on its basic physic-chemical properties as well as long term durability performance are very limited. This lack of comprehensive understanding of BFRP composites will hinder its wide and safe application. In view of this, the FRP composite and structure group at the school of civil engineering, Harbin institute of technology has been conducting a series of researches to illuminate the advantage and disadvantage of BFRP composites used in external or internal strengthening system for concrete structures. As known, due to the low temperature deflection point of polymer matrix, FRP composites, generally, exhibit a sharp decrease in the stiffness and strength at the temperatures, which excesses the glass transition temperatures of the polymer matrix, e.g., in the range of 60 ~ 200ć. Besides, it is also very important to evaluate the safety of the FRP related elements / structures which have been undergone a fire. In the present paper, performances of BFRP rebar at

elevated temperatures or after elevated temperature treatment have been investigated.

2 EXPERIMENT 2.1 Specimen Preparation The studied BFRP rebar is spirally wound with glass fiber rovings and coated with sand to improve the bonding performance between the rebar and concrete. The tensile properties were tested according to ACI 440.3R-04. The tensile strength is 899MPa, and the elastic modulus is 50.8GPa, which is acquired based on 25 testing specimens. The glass transition temperature (Tg) of the BFRP rebar is 132ć, which is tested with differential scanning calorimetry (Linseis, PT10) at heating rate of 10ć/min. The gauge length of the specimen is 320 mm, and the anchor length is 140 mm. Griping anchor is selected as the anchorage system, which consists of binding material (mixture of epoxy resin and plugging compound) and a circular steel tube confined the binding material. This kind of anchorage system can make sure the rupture occurred in the middle of the rebar. 2.2 Elevated temperature testing The tensile properties of BFRP rebar at various elevated temperatures (ranging from 100ćto 350ć) is tested. The testing apparatus include following parts: WDW10E computer controlled electronic universal tensile

L. Ye et al. (eds.), Advances in FRP Composites in Civil Engineering © Tsinghua University Press, Beijing and Springer-Verlag Berlin Heidelberg 2011

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machine (UTM), electrically heating kiln, thermocouple and temperature controlling device. The maximum capacity of the UTM is 100 kN and its relative error reading is less than 1%. The electrically heating kiln (Figure 1), which is purpose-built, contains 10 heating rods, which are arranged in a curved line. The kiln can heat rebars up to 500ć. The thermocouple has two probes, which can record the temperature of air in the kiln (T1) and the temperature of rebar (T2), which will be fed into computer directly.

Figure 2 Thermocouple probes and high-temperature extensometer fixed to the rebar

2.3 Elevated temperature treated specimens The BFRP rebar are kept in an oven at temperatures of 150, 200 and 250ć for 1, 2, 4, 8 hours, respectively. In each condition, 5 specimens were tested. In this test, the UTM and extensometer are the same as used in the above section. Figure 1 The elevated temperature tensile testing setup

3 RESULT AND DISCUSSION

To do the elevated temperature testing, the following procedure was followed. Firstly, two thermocouple probes and extensometer was fastened on the rebar, as shown in Figure 2. Then, fix the rebar to the UTM and install the electrically heating kiln. Be sure that the extensometer doesn’t contact with the kiln. Thirdly, raise the temperature to the target values. When the rebar reaches the target temperature (T2), hold this temperature for 10 minutes (f5ć). Then, test the rebar to failure with a speed of 5mm/min. The UMT will record the stress vs. strain curve. The softening of the superficial resin at elevated temperature makes it difficult to get the stiffness of the rebar. In view of this, a high-temperature extensometer is selected to measure the stiffness of rebar. Using high-temperature extensometer, the stress-strain curves at the temperatures ranging from 100ć to 250ć can be obtained. However, at the temperature of 300ć and 350ć, a serious slippage between extensometer and the rebar happens. The modulus was not obtained in the latter cases.

When BFRP rebars is heated to the temperature above 200ć (Figure 3), a smoke is rising from the kiln. At 250ć and 350ć, the rebar started to burn with flame. The resin at the location of fracture of the rebar above temperature of 300 ć is found to be decomposed completely. The fibers of FRP start to break at much lower stress at high temperatures compared with room temperature. This is obtained from the sound of fiber rupture at elevated temperature which comes out at lower stress levels (before the failure of rebar) than at room temperature. This may be due to the fact that the resin can’t transfer tension effectively at such high temperatures. Figure 4 shows the variation of the tensile strength as a function of temperatures. The initial tensile strength is 899 MPa, which is acquired based on testing date of 25 specimens at room temperature. From the curve, it is founded that below the temperature of 250ć, the strength doesn’t suffer from remarkable deterioration. However, when the temperature exceeds 250ć, there is a sharp drop of the strength. The residual strength is less

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Figure 5 Variation of the elastic modulus as a function of temperature

Figure 3 Smoke comes out from kiln at 200ć

Figure 6 Stress-strain curve of BFRP rebar at 250ć

Figure 4 Variation of the tensile strength vs. temperature

Figure 7 shows the variation of strength as a function of heating durations. It is found that the strength increases with the duration of 8 hour. This may come from the post-curing effect of the resin at high temperatures, which increases the force transferring capacity of the

than 1/3 at 350ć. In addition, it should be noted that the standard deviation of the testing data rises with the temperature. Figure 5 Shows the variation of the stiffness as a function of the temperature ranging from 100 ć to 250ć. It is found that there are two temperature ranges that the elastic modulus suffers a great loss. At the temperature of 250ć, the residual stiffness of rebar is less than 1/2 of the stiffness at room temperature. Compared with the relationship between strength and temperature, it is noted that the stiffness is much sensitive to the temperature change. The stress-strain curves at 150ć and 200ć show a linear dependence. While at 250ć, the degree of linear dependence reduced. Figure 6 shows the stress-strain relationships at the temperature of 250ć.

Figure 7 Tensile strength of BFRP rebar after exposed to high temperature for various durations

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resin to fibers. While at the temperature of 250ć, the strength decreased due to the oxidation of the resin and possible degradation of the interfaces between fiber and matrix. At temperature of 250ć, a terrible smoke is rising from the oven, which indicates the serious decomposition process of the resin.

4 CONCLUSIONS From room temperature to 350ć, BFRP rebar exhibits a readily reduction in the tensile modules, and reaches about 40% of the residual modulus at 250ć. The tensile strength of the BFRP shows a better temperature resistance. When the temperature increases up to 250ć, the tensile strength is slightly altered, but dramatic reduce to 1/3 of

its room temperature value at 350ć due to the ignition of the resin at such high temperature. Treated at elevated temperature for up to 8 hours, BFRP shows a slight decrease of the strength, less than 10% for the temperature ranging from 150 to 250ć.

REFERENCES Liu, O. & Shaw, M. T. 2006. Investigation of basalt fiber composite aging behavior for applications in transportation. Polymer Composites 27(5): 475-483. Sim, J. & Park, C. 2005. Characteristics of basalt fiber as a strengthening material for concrete structures. Composites Part B-Engineering 36(6-7): 504-512.