durability of carbon fiber reinforced polymer (cfrp)

APFIS2017 - 6th Asia-Pacific Conference on FRP in Structures Singapore, 19-21st July 2017

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DURABILITY OF CARBON FIBER REINFORCED POLYMER (CFRP) WITH COMBINED USE OF SEAWATER SEA SAND CONCRETE (SWSSC) F.Guo1, S.Al-Saadi1, R.K. Singh Raman1,2 and X.L.Zhao3 1

Department of Mechanical & Aerospace Engineering, Monash University, Victoria, Australia Email: [email protected], [email protected] 2 Department of Chemical Engineering, Monash University, Victoria, Australia Email: [email protected] 3 Department of Civil Engineering, Monash University, Victoria, Australia Email: [email protected]

Keywords: CFRP, SWSSC, durability, SEM, moisture uptake Abstract This paper presents the results of an experimental investigation into carbon fiber reinforced polymer (CFRP) exposed to simulated environments of different types of seawater sea sand concrete (SWSSC) for 3 months under an accelerating condition. Scanning electron microscopy (SEM) revealed interface debonding and matrix degradation in the case of all simulated concrete solutions at 60 °C. However, continuous cracks were found along the interface exclusively in the case of seawater sea sand normal concrete (SWSSNC) and normal concrete (NC) solutions. Such cracks are quite likely to facilitate the penetration of solution into the bulk. The CFRP specimens suffered less weight gain in SWSSC solutions (i.e. SWSSNC, SWSSHPC) than in conventional concrete solutions (i.e. NC, HPC). 1. Introduction The increasing awareness of environmental and sustainability issues has generated considerable interest in the research of seawater sea sand concrete (SWSSC) in recent years [1-3]. However, corrosion of common reinforcing materials, steels, in SWSSC is a critical concern that needs to be addressed for practical engineering applications. To mitigate the severe corrosion that steels are likely to suffer in high chloride environment of seawater in SWSSC, fiber reinforced polymer (FRP) is proposed to be an alternative to steels as the reinforcement for SWSSC. Compared with other fibers (e.g. glass and basalt), carbon fibers has superior mechanical properties, and they do not chemically react with alkali and moisture [4]. Epoxy, the most common matrix for CFRP, possesses good mechanical properties, corrosion resistance and adhesion to the fibers [4]. A number of previous studies [5-8] on durability performance of CFRP in both alkaline and marine environments have found CFRP, in general, to be reasonably resistant to both moisture and alkali attack. For example, epoxy based CFRP was found [8] to exhibit good retention of mechanical properties after exposure to simulated concrete solution at 60 °C for 42 days with only 0.54% weight increase and no visiable damage on fibers and fiber/matrix interface. When immersed in the simulated seawater solution [5], the water uptake of epoxy based CFRP was found to reach a saturation stage after 365 days with an average weight gain of 0.625%, as well as a superior retention of mechanical properties than GFRP. However, to the best knowledge of authors, there is little reported on the durability of CFRP in SWSSC environment. This paper presents the preliminary results of the degradation mechanism of F.Guo, S.Al-Saadi, R.K. Singh Raman and X.L.Zhao


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CFRP in the simulated SWSSC environments, comparing with the performance of CFRP in the simulated conventional concrete environments. 2. Experimental Program 2.1. Materials and Specimens The specimen coupons (dimensions, 30 x 40 x 3.5 mm) were cut from filament wound CFRP tubes (outer diameter 50 mm, thickness 3.5 mm) by using a diamond saw. According to the supplier, the resin system is a blend of bisphenol A and bisphenol B with an amine hardener, and the fiber winding pattern is 20% 15°+ 40% 45°+ 40% 75°. Prior to the immersion test, specimens were cleaned with distilled water, sealed with epoxy at cutting edges, and oven dried at 60 °C for 24 hours. 2.2. Immersion tests Specimens were immersed in solutions at 60 °C to accelerate the degradation process. There are five test solutions (Table 1), including two simulated fresh water and river sand concrete pore solutions, i.e., simulated normal concrete (NC) solution and simulated high performance concrete (HPC) solution, to each of which 35 g/l NaCl was added to simulate two different SWSSC solutions, i.e., simulated seawater sea sand normal concrete (SWSSNC) solution and simulated seawater sea sand high performance concrete (SWSSHPC) solution respectively, and the fifth environment was just a plain distilled water (DW), as a reference. High performance concrete generally contains cementitious materials, such as fly ash, slag or silica fume. As a result of fly ash/ slag/ silica fume content, the pore solutions of high performance concretes (i.e. HPC, SWSSHPC) have lower pH than those of normal concrete pore solutions (i.e. NC, SWSSNC). Chemical compositions and pH of the five test solutions are described in Table 1. Table 1. Chemical composition of testing solutions Solution No. S1 S2 S3 S4 S5

Simulated environment SWSSNC NC SWSSHPC HPC DW

NaOH 2.4 2.4 0.6 0.6 -

Quantities (g/l) KOH Ca(OH)2 19.6 2.0 19.6 2.0 1.4 0.037 1.4 0.037 -

NaCl 35 35 -

pH 13.4 13.4 12.7 12.7 7.5

2.3. Weight gain measurements and scanning electron microscopy(SEM) The weight gain of specimens were determined according to ASTM D5229M-14 [9], and the morphological changes of specimens after 3-month immersion in the different solutions (Table 1) at 60 °C were examined by SEM JOEL 7001. 3. Results and Discussion 3.1. Weight gain The weight gain of CFRP specimens immersed in five different test solutions at 60 ˚C was calculated by using Eq.1, where W0 is the oven-dried specimen weight before immersion and Wt is the specimen weight at time t. Wgain (%)= (Wt –W0)/W0 × 100

F.Guo, S.Al-Saadi, R.K. Singh Raman and X.L.Zhao

(1)


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The averaged weight gain of three replications in each solution was plotted with normalised immersion time (i.e. √immersion days), as shown in Figure 1. It can be seen from the Figure 1 that after 3-month immersion, CFRP specimens in NC exhibited the highest weight gain (approx. 0.7%), followed by those in SWSSNC, DW, HPC and SWSSHPC. The observed trend of less weight gain in seawater sea sand concrete solutions (i.e. SWSSHPC, SWSSNC) than in conventional concrete solutions (i.e. HPC, NC) is similar to the trend reported previously for GFRP in seawater and distilled water [10]. Al-Salloum et al. [10] attributed this trend to the existence of osmotic effect. The composite matrix itself acts as a semi-permeable membrane for the osmosis process to operate, the ion concentration within the composites is lower than that in the exposure solution, driving water out of composites to balance the concentration difference [11-12]. In this case, the less moisture uptake of specimens in SWSSC environments might result in less degradation of the composite than those in conventional concrete solutions. Additionally, it is also noticeable that specimens in normal concrete solutions (i.e. SWSSNC, NC) demonstrated higher weight gain than those in high performance concrete solutions (i.e. SWSSHPC, HPC). The reason could be that the high concentration of OH- in normal concrete solutions has caused irreversible degradation to the specimens, allowing more moisture to penetrate; whereas the lower alkalinity level of high performance concrete solutions resulted in less degradation.

Figure 1. 3-month CFRP weight gain results in different solutions at 60 °C 3.2. Scanning Electron Microscopy (SEM) Figure 2 presents the SEM images of specimens exposed to SWSSNC and NC at a magnification of 500. Continuous cracks were found along fiber/matrix interface at the margin area of the specimens exposed to these two solutions. However, these were not observed in the case of the specimens in DW, HPC, SWSSHPC solutions. These cracks provide easy paths and more space for solution to penetrate, resulting in a higher weight gain. Apart from the cracks, minor fiber/matrix interface debondings and matrix degradation were observed in the case of specimens immersed in each of the simulated concrete solutions but not in the case of those exposed to distilled water. These debondings could presumably develop into continuous cracks under a combination of physical attack (i.e. swelling and plasticization) and chemical attack by hydroxides. Although no significant fiber degradation was found, the considerable degradation along the fiber/matrix interface could impair the mechanical properties (e.g. tensile strength), as actually found in a separate study (not included here). F.Guo, S.Al-Saadi, R.K. Singh Raman and X.L.Zhao


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Figure 2. SEM images for specimesn in SWSSNC (left) and NC (right) at 60°C for 3 months 4. Conclusion After pre-exposure to simulated seawater sea sand concrete solutions (SWSSC) and conventional concrete solutions at 60 °C for 3 months, carbon fiber reinforced polymer (CFRP) specimens were found to undergo the degradations as described below: 

Interface debonding and matrix degradation were found in each of the simulated concrete solutions. However, continuous cracks were found along fiber/matrix interface exclusively in the case of normal concrete solutions (i.e. SWSSNC, NC), and they could provide easy paths and more spaces for solution to penetrate.

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The highest weight gain (approx. 0.7%) was found in the case of normal concrete (NC) solution, followed by seawater sea sand normal concrete (SWSSNC), distilled water (DW), high performance concrete (HPC) and seawater sea sand high performance concrete (SWSSHPC) solutions. The observed less weight gain in SWSSC solutions (i.e. SWSSNC, SWSSHPC) than those in conventional concrete solutions (i.e. NC, HPC) can be attributed to osmotic effect.

Acknowledgments This project is funded by Australian Research Centre (ARC) under the SWSSC durability project. References [1] J.G. Teng, T. Yu, J.G. Dai and G.M. Chen, "FRP composites in new construction: current status and opportunities", in Proceedings of 7th National Conference on FRP Composites in Infrastructure (Supplementary Issue of Industrial Construction), keynote presentation, 2011. [2] Z.K. Wang, X.L. Zhao, G.J Xian, G. Wu, R. Singh and S. Al-Saadi, "Tensile properties of basaltfibre reinforced polymer (bfrp) bars within seawater and sea sand concrete environment", in 7th International Conference on Fibre-Reinforced Polymer (FRP) Composites in Civil Engineering (CICE 2016), Hong Kong, China, 2016. [3] Y.L. Li, X.L. Zhao, R. Singh and S. Al-Saadi, "Tests on seawater and sea sand concrete-filled CFRP, BFRP and stainless steel tubular stub columns", Thin-Walled Structures, vol. 108, pp. 163-184, 2016. [4] D. Chung, Carbon fiber composites, 1st ed. Boston: Butterworth-Heinemann, 1994.



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