Experimental Study on the Mechanical Properties of Basalt Fibres and Pultruded BFRP Plates at Elevated Temperatures

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1 Experimental Study on the Mechanical Properties of Basalt Fibres and Pultruded BFRP Plates at Elevated Temperatures Experimental Study on the Mechanical Properties of Basalt Fibres and Pultruded BFRP Plates at Elevated Temperatures Zhongyu Lu, Guijun Xian*, and Hui Li Key Lab of Structures Dynamic Behaviour and Control (Harbin Institute of Technology), Ministry of Education, Heilongjiang, Harbin, , China School of Civil Engineering, Harbin Institute of Technology, Heilongjiang, Harbin, , China Received: 12 June 2013, Accepted: 24 July 2014 SUMMARY Basalt fibre reinforced polymer (BFRP) composites have been widely used for structural strengthening and reinforcement in civil engineering recently. Since fire risk is unavoidable for most civil structures, the degradation of BFRP at elevated temperatures needs to be known for the safe design of its strengthened or reinforced structures under fire risk. In this study, the mechanical properties of basalt fibre roving and pultruded basalt fibre reinforced epoxy plates were investigated at elevated temperatures. As the temperatures increasing from room temperature to 200 C, the tensile strength and modulus of the fibre roving were reduced by 8.3% and 9.7%, respectively. Meanwhile, the Weibull shape parameter (m) of the fibre roving decreased by 20.5%. As regards the BFRP plates, however, the elevated temperatures show more adverse influence. The tensile strength and modulus of the BFRP plates is reduced by 37.5% and 31% as temperature rising to 200 C. Compared to the tensile properties, the short beam shear strength (SBS) was reduced by around 90%, more susceptible to the elevated temperatures. The glass transition temperature (~93 C) plays a key role on the inflexion of the variation of the mechanical properties, especially for the tensile strength. In a wide temperature range, the relationship between the tensile strength and the SBS showed a good linearity, indicating the reduction of the tensile strength comes from the deterioration of the interlaminar shear strength at elevated temperatures. Keywords: Elevated temperature, Basalt fibre, Pultruded plate, Tensile properties, Fibre roving 1. Introduction In recent years, FRPs have already been widely used in civil engineering, such as replacing steel bars to reinforce concrete structures, or strengthening and upgrading existed concrete structures. Fibre reinforced polymer (FRP) composites possess many advantages over traditional structural materials in terms of high specific strength and stiffness, flexibility of design, resistance to corrosion, etc. However, the poor fire resistance and higher flammability of FRPs are always a key concern since the FRP reinforced or strengthened structures are less suitable to face a fire risk 1. Although various protective coatings Smithers Information Ltd., 2015 were applied to isolate the FRPs from direct contact the fire 2, FRPs were still subjected to the elevated temperatures 3. To determine the design parameters of FRP strengthening or reinforcing structures with a fire risk, it is a prerequisite to understand the mechanical degradation of FRP materials at elevated temperatures. The polymer matrix of FRP composites, generally, shows a low temperature resistance. When heated to moderate temperatures ( C), the polymer matrix will rapidly lose its strength and stiffness 4, while FRP materials start to soften 3,5. This speeds up the degradation of the composites 6. As a result, FRPs tend to lose their reinforcement effect and the concrete beams or columns lose their structural capacity. Thus, the safety of the FRP reinforced or strengthened structures is significantly deteriorated. Therefore, FRPs cannot reach their full potential until the behaviour under elevated temperature is fully understood 7. Although the resin matrix starts to soften at T g, the mechanical properties of unidirectional FRPs at the temperatures above T g still retained to a high extent. As reported recently, for a GFRP composite, almost half of the tensile strength was lost near its T g. However, 40% of the room temperature strength of the GFRP was still retained at 200 C 8. At 300 C the ultimate strength was approximately 50% of the room-temperature strength 9. Wu et al. 4,10 studied the tensile properties of CFRP and hybrid FRP composites Polymers & Polymer Composites, Vol. 23, No. 5,

2 Zhongyu Lu, Guijun Xian, and Hui Li from room temperature to 200 C. When the temperature exceeded its glass transition temperature (T g ), the tensile strength of CFRP sheets remains about 60% of its original strength at room temperatures, and the strength of the dry fibre sheets showed no obvious change. Tensile properties were fibre dominated, and elevated temperature had little influence on the fibre properties. However, the effect of the fibre uniformity and interface of the composites under elevated temperatures wasn t reported in the above papers. Bai et al. 11,12 studied the stiffness change of the FRP composites under elevated temperatures. A new model was proposed to calculate the temperature dependent E-modulus of FRP composites. Chowdhury 8 studied the GFRP mechanical degradation between room temperature and 200 C, and proposed an analytical model to characterize the mechanical properties at elevated temperature. Wang et al. 9 studied the mechanical properties of pultruded CFRP plates under elevated temperatures. An equation that related the tensile strength of the plate to the temperatures was proposed. An understanding of the variation of the mechanical properties as a function of temperatures is essential to set the design parameters in civil engineering structures. Recently, basalt fibre and its reinforced polymer composites have been widely investigated as a new structural material in civil engineering as an alternative of glass- or carbon- fibre reinforced composites. It is known that the basalt fibres have better tensile strength than the E-glass fibres, greater failure strain than the carbon fibres as well as good resistance to chemical attack, impact load and fire with less poisonous fumes 13. Basalt fibres are from basalt rocks through melting process. The basalt rocks can be so finely divided into small particles that it is possible to process them into the form of fibres. In addition, the basalt fibres do not contain any other additives in a single producing process, which makes additional advantage in cost 14. From these advantages, the basalt fibre and its reinforced FRP materials are increasingly applied. The fundamental properties of the basalt fibre as a reinforcing steel replacement in the concrete structures have been already studied 15,16. To understand the mechanical properties of a pultruded BFRP plate at elevated temperatures for their safe and economic application in engineering structures, tensile and short beam shear properties of the basalt fibre roving and BFRP plates were studied as a function of elevated temperatures ranging from room temperature to 200 C. The possible mechanisms of the degradation were analyzed. It is worth noting that the present study was performed on both the fibre and the BFRP composites at elevated temperatures, and the degradation of the bonding strength between fibre and resin matrix was paid more attention to. The study is different from the previous study, where the fibre and the bonding properties at elevated temperatures were neglected. 2. Experimental 2.1 Raw Materials The unidirectional basalt fibre reinforced epoxy composite plates were manufactured with a pultrusion process at the Laboratory for FRP Composites and Structures (LFCS), Harbin Institute of Technology, China. The thicknesses and widths of the plates were 1.4 mm and 15 mm, respectively. Glass transition temperature (T g ) of the plate tested with DSC (Differential Scanning Calorimetry) was 93.3 C, and the fibre volume fraction (V f ) was approximately 70.7% tested with the burning-off method. The tensile strength, modulus and elongation at room temperature were 2.7 GPa, 85.4 GPa and 3.7%, respectively. The diameter of the used basalt fibres were 15.5 µm, and the density of the fibre was 2.60 g/cm 3. The resin is a high temperature curable epoxy system. 2.2 Tensile Properties Testing of Basalt Fibre Roving In the present study, the tensile properties of the basalt fibre roving used for the below BFRP plates were tested. The gauge length was set as 260 ± 2 mm, and the specimen was loaded in tension at a rate of 5 mm/min. The testing temperature was controlled with an electrically heated kiln (Figure 1), which was purpose-built at LFCS, and the control precision was within ± 2 C. The temperature of the roving was measured with thermocouples, which was in touch with the fibre roving during heating. The fibre roving was kept at the predetermined elevated temperatures for 10 minutes, and then was tested in tension with a hydraulic machine (WDW-100D model, Jinan SJ Company, China). Note, the nominal modulus was used and the Weibull distribution model were used to calculate the tensile strength, which is indicated in literature Mechanical Testing of BFRP Plates Tensile tests of BFRP plates were performed according to ASTM D 3039/D 3039M (Standard Test Method for Tensile Properties of Polymer Matrix Composite Materials). The length of the composite panels was cut into 400 mm ± 3 mm in length. An extensometer having a 50 mm gauge length was used to measure strains. When a level of 0.3% strain was reached the extensometer was removed from test equipment so as to avoid the damage from the specimens rupture. Tests were conducted under a displacement controlled mode. The tensile rate was set as 5 mm/min. All testing specimens were pretreated in an oven at 60 C for 24 hours before the 278 Polymers & Polymer Composites, Vol. 23, No. 5, 2015

3 Experimental Study on the Mechanical Properties of Basalt Fibres and Pultruded BFRP Plates at Elevated Temperatures tensile test. The testing temperatures were set as 40 C, 80 C, 120 C, 160 C and 200 C. Before testing at predetermined elevated temperatures, the specimens were kept in the kiln for10 minutes to make sure the temperature in the plate to be uniform 19. The interlaminar shear properties of BFRP plates were conducted following ASTM D 2344/D 2344M (Standard Test Method for Short-Beam Strength of Polymer Matrix Composite Materials and Their Laminates).The dimensions of the specimens were 1.4 mm 3 mm 9 mm (thickness width length). A crosshead speed of 1 mm/min was applied for all the tests. The pretreatment procedure of the specimens was the same as that used for tensile testing. Table 1. Tensile properties of basalt fibre roving at various temperatures. Note that the testing data were analyzed with Weibull distribution model, S 0 is the scale parameter, m is the shape parameter No. Temperatures ( C) S 0 (MPa) m Tensile Strength (MPa) Tensile Modulus (GPa) Figure 1. Tensile test setup for fibre roving and BFRP plates with a kiln for temperature control All mechanical tests were conducted with the same tensile testing machine as mentioned for fibre roving testing. 2.4 SEM Analysis Tensile fractured surfaces of the BFRPs were platinum coated and observed by scanning electron microscopy (SEMQUANTA 200F, FEI Company, America). 3. Results and discussion 3.1 Tensile Properties of Basalt Fibre Roving at Elevated Temperatures Table 1 summarizes the tensile properties of basalt fibre roving at various testing temperatures, which were analyzed on the raw testing values according to the Weibull distribution model 17. Figure 2 shows the strength degradation of the fibre roving with increase of the testing temperatures. As shown in Table 1 and Figure 2, as the temperature increased from room temperature to 200 C, the tensile strength of the fibre roving decreased from 2515 MPa to 2306 MPa, reduced by 8.3%. It is worth noting that the effects of temperatures lower than 120 C is negligible on the tensile strength. The tensile modulus of the fibre roving (See in Table 1) is slightly lower than that tested with the single fibre tensile test. This can be explained by the fibre slack or debonding of anchoring during testing [21]. The effects of the testing temperatures on the tensile modulus of the fibre roving are shown in Figure 3 (also in Table 1). The modulus decreases slightly with increase of the testing temperatures. The reduction ratio is about 9.7% as the temperature increases from room temperature to 200 C. It is worth noting that the shape parameter (m) of the tensile strength of the fibre roving appears to be very susceptible to the testing temperatures. As the temperature rose to 200 C, the value of m is reduced by 20.5%. As known, the value of m is related Polymers & Polymer Composites, Vol. 23, No. 5,

4 Zhongyu Lu, Guijun Xian, and Hui Li Figure 2. Variation of tensile strength of fibre roving as a function of testing temperatures Figure 3. Variation of tensile modulus of fibre roving as a function of testing temperatures testing temperatures when it is lower than the glass transition temperature (T g ) of the BFRP plate, i.e., 93.3 C. As the testing temperatures exceeded the value of T g, the modulus of the BFRP plates dropped sharply, due to the softening of the matrix. When the temperatures increased from 80 C to 160 C, the tensile modulus started to reduce abruptly to about 69% of its room temperature values. Further increase of the test temperatures did not vary the tensile modulus. The modulus of the BFRP plates is mainly dependent on the fibres since the modulus of resin is much lower than the fibre roving. As shown in Figure 4, however, rather than small change of the modulus of the fibre roving (as shown in Figure 3), the modulus of the BFRP decrease remarkably as the testing temperature exceeds T g. This can be attributed to the softening of the resin matrix, which can not bind the fibres together and make them to bear the loading simultaneously. The fact that the modulus of BFRP is reduced at elevated temperatures, indicates that the resin system still plays a key role on the degradation of the tensile modulus of the BFRP plate. Besides, it is worth noting that high variation of the modulus is also found for the case of the high temperatures. This is also attributed to the assumption above, that the fibres cannot bear the loading simultaneously due to the resin matrix softening. to the variation of the tensile strength of the fibre 22. Therefore, it can be concluded that increasing the testing temperatures leads to more fluctuation in the tensile strength of the basalt fibre roving. 3.2 Tensile Modulus of BFRP at Elevated Temperatures The temperature dependence of the tensile modulus of BFRP plate is shown in Figure 4. As shown, the modulus was slightly influenced by the 3.3 Tensile and Short Beam Shear Strength of BFRP Plates The temperature dependence of the tensile strength of BFRP plates is presented in Figure 5. As shown, when the testing temperatures increased from the room temperature to 40 C, the tensile strength is enhanced slightly. The post-curing effect is expected to be responsible for this. With further increase of the testing temperatures, however, the tensile strength decreased almost linearly. As the temperature 280 Polymers & Polymer Composites, Vol. 23, No. 5, 2015

Experimental Study on the Mechanical Properties of Basalt Fibres and Pultruded BFRP Plates at Elevated Temperatures reach 200 C, the tensile strength reduced by 37.

5 Experimental Study on the Mechanical Properties of Basalt Fibres and Pultruded BFRP Plates at Elevated Temperatures reach 200 C, the tensile strength reduced by 37.5% compared to the strength at room temperature. Similarly, the softening of the resin cannot effectively share the tensile force to each filament, and thus, the fibres cannot fracture in the same time, leading to lowered tensile strength. Besides, as expected, degradation of the mechanical properties of fibres (as shown in Figure 2) is also contributed to the decrease of the tensile strength. Figure 6 shows the photographs of the BFRP plate samples after tension. When the testing temperatures are lower than its glass transition temperature, the failure mode looks brittle, and fibres were still bound together by the resin matrix. However, as the temperature is close to or higher than the glass transition temperature, the fibres seem to fracture separately, due to the lack of the binding effect of the softening resin. Figure 4. Effect of test temperatures on the tensile modulus of the BFRP plate Figure 5. Effect of test temperatures on the tensile strength of the BFRP plate Figure 7 shows SEM photographs of the tension rapture surfaces of BFRP plates. A good bonding characteristic between fibre and matrix at room temperature is indicated by much resin attached on the fibre surfaces. With increasing the testing temperatures, resin starts to soften, and less resin was found on the surfaces of the fibres. Figure 8 presents the short beam strength (SBS) of the BFRP plate as a function of the testing temperatures. As shown, when the temperature increases by T g, the SBS decreases remarkably. At room temperature, the SBS is 70.5 MPa. Increasing the temperatures leads to an almost linear decrease of SBS. At 160 C, SBS is reduced to 7.2 MPa, only 10% of its original value at room temperature. Figure 6. Photograph of the BFRP samples after tensile test at elevated temperatures Figure 9 shows the relationship between the tensile strength of BFRP plates and the SBS values at various testing temperatures. As shown, in the temperature range of 40 C to 160 C, a good linear curve is observed. As known, the SBS of a Polymers & Polymer Composites, Vol. 23, No. 5,

$Zhongyu Lu, Guijun Xian, and Hui Li Figure 7. SEM photographs of tensile fractured BFRP sample tested at room temperature (a), 120 C (b) and 200 C (c) Figure 8.$ characteristics of the fibre and resin matrix.

characteristics of the fibre and resin matrix.

Conclusions In the present paper, the effects of elevated temperatures on basalt fibre roving and pultruded unidirectional BFRP plates were investigated.

Correlation between SBS and tensile strength The tensile results on the fibre roving indicated that temperatures ranging from room temperature to 200 C show

As regards the BFRP plates, however, elevated temperatures show more adverse effects on the mechanical properties.

6 Zhongyu Lu, Guijun Xian, and Hui Li Figure 7. SEM photographs of tensile fractured BFRP sample tested at room temperature (a), 120 C (b) and 200 C (c) Figure 8. Variation of the short beam shear strength as a function of testing temperatures fibre reinforced polymer composites can be used to characterize the bonding characteristics of the fibre and resin matrix. The results given in Figure 9 clearly indicate that the degradation of the bonding strength readily brings in the reduction of the tensile strength. 4. Conclusions In the present paper, the effects of elevated temperatures on basalt fibre roving and pultruded unidirectional BFRP plates were investigated. The following conclusions were drawn, based on the experimental results. Figure 9. Correlation between SBS and tensile strength The tensile results on the fibre roving indicated that temperatures ranging from room temperature to 200 C show adverse effects on the tensile properties. The tensile strength and modulus are reduced by 8.3% and 9.7%, respectively. As regards the BFRP plates, however, elevated temperatures show more adverse effects on the mechanical properties. Up to 200 C, the tensile strength and modulus are reduced by 37.5% and 31%, respectively, while the short beam shear strength was reduced by around 90%. The inflexion points of the variation of the mechanical properties as a function of temperatures was closely related to the glass transition temperatures (~93 C) of the resin matrix. The correlation between the tensile strength and SBS shows a good 282 Polymers & Polymer Composites, Vol. 23, No. 5, 2015

7 Experimental Study on the Mechanical Properties of Basalt Fibres and Pultruded BFRP Plates at Elevated Temperatures linearity in a wide temperature range, indicating that the deterioration of the adhesion between the fibres and resin is responsible for the reduction of the tensile strength. Acknowledgements This work is financially supported by NSFC with Grant No , the National Key Basic Research Program of China (973 Program) with Grant No. 2012CB026200, and supported by Guangdong Provincial Key Laboratory of Durability for Marine Civil Engineering (Shenzen University, China) with grant No. GDDCE References 1. Ji, G., G. Li, and W. Alaywan, A new fire resistant FRP for externally bonded concrete repair. Construction and Building Materials, : p Kandola, B.K., W. Bhatti, and E. Kandare, A comparative study on the efficacy of varied surface coatings in fireproofing glass/epoxy composites. Polymer Degradation and Stability, (11): p Kandare, E., et al., Fibre-reinforced epoxy composites exposed to high temperature environments. Part II: modeling mechanical property degradation. Journal of Composite Materials, (14): p Shenghu Cao, Z.W., Xin Wang, Tensile Properties of CFRP and Hybrid FRP Composites at Elevated Temperatures. Journal of Composite Materials, (4): p Azwa, Z., et al., A review on the degradability of polymeric composites based on natural fibres. Materials & Design, : p Jiang, X., H. Kolstein, and F.S. Bijlaard, Moisture diffusion in glass fibre-reinforced polymer composite bridge under hot/wet environment. Composites Part B: Engineering, (1): p Harries, K.A., M.L. Porter, and J.P. Busel, FRP Materials and Concrete: Research Needs. Concrete international, (10). 8. E. U. Chowdhury, R.E., L. A. Bisby, M. F. Green, N. Benichou, Mechanical Characterization of Fibre Reinforced Polymers Materials at High Temperature. Fire Technology, (4): p Ke Wang, B.Y., Scott T. Smith, Mechanical properties of pultruded carbon fibre-reinforced polymer (CFRP) plates at elevated temperatures. Engineering Structures, (7): p Shenghu Cao, X.W.a.Z.W., Evaluation and prediction of temperature-dependent tensile strength of unidirectional carbon fibre-reinforced polymer composites. Journal of Reinforced Plastics and Composites, (9): p Yu Bai, T.K., Till Vallée, Modeling of stiffness of FRP composites under elevated and high temperatures. Composites Science and Technology, 2008: p Bai, Y., T. Vallée, and T. Keller, Modeling of thermal responses for FRP composites under elevated and high temperatures. Composites Science and Technology, (1): p Wei, B., H. Cao, and S. Song, Environmental resistance and mechanical performance of basalt and glass fibres. Materials Science and Engineering: A, (18): p Kencanawati, N.N. and M. Shigeishi, Acoustic Emission Hit Generation Behavior of Basalt Fibre High Strength Mortar under Compression. Applied Mechanics and Materials, : p Borhan, T.M. and C.G. Bailey, Structural behaviour of basalt fibre reinforced glass concrete slabs. Materials and Structures, (1-2): p Sarasini, F., et al., Effect of basalt fibre hybridization on the impact behavior under low impact velocity of glass/basalt woven fabric/epoxy resin composites. Composites Part A: Applied Science and Manufacturing, : p Mohamed R Mili, T.B., Paul Merle Estimation of Woefully Parameters From Loose-Bundle Tests. Composites Science and Technology, : p ASTM, D., 3039/D Standard test method for tensile properties of polymer matrix composite materials, Wu, J., H. Li, and G. Xian, Influence of Elevated Temperature on the Mechanical and Thermal Performance of BFRP Rebar, in Advances in FRP Composites in Civil Engineering. 2011, Springer. p ASTM, D., 2344/D2344M-00. Standard Test Method for Short- Beam Strength of Polymer Matrix Composite Materials and Their Laminates, J. Andersons, R.J., M. Hojo,S. Ochiai, Glass fibre strength distribution determined by common experimental methods. Composites Science and Technology, : p Weibull.W, A Statistical Theory of the Strength of Materials. Proc Royal Swedish Inst Eng Res, : p Polymers & Polymer Composites, Vol. 23, No. 5,

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Mechanical Characterization of Basalt Fibre Reinforced Polymer Bars for Reinforced Concrete Structures

Mechanical Characterization of Basalt Fibre Reinforced Polymer Bars for Reinforced Concrete Structures P.Thiyagarajan 1, V.Pavalan 2 and R. Sivagamasundari 3 1 Research scholar, Department of Structural