Mechanical Properties and Microstructures of GFRP Rebar after Long-term Exposure to Chemical Environments

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1 Mechanical Properties and Microstructures of GFRP Rebar after Long-term Exposure to Chemical Environments Mechanical Properties and Microstructures of GFRP Rebar after Long-term Exposure to Chemical Environments Hae-Kyun Park a, Su-Jin Lee b, Yoon-Jeong Kim b, Chang-Il Jang b and Jong-Pil Won b, * a Samsung Corporation Engineering & Construction Group, Sungnam, Republic of Korea b Department of Civil & Environmental System Engineering, Konkuk University, Seoul , Republic of Korea Received: 13 October 2006 Accepted: 25 January 2007 SUMMARY This study describes accelerated degradation tests on glass fibre-reinforced polymer (GFRP) rebar to evaluate its long-term durability. The tests used simulated building acid, sulphate and de-icing environments. We measured the resulting reduction in the mechanical properties of the rebar and performed a microstructural degradation analysis of the rebar. The acidic environment consisted of a 0.6% acetic acid solution at a ph of 2.92, whereas the sulphate environment contained 3% sodium sulphate solution, and the de-icing environment was composed of 4% calcium chloride solution. Tensile strength tests were performed to measure the mechanical properties of the rebar after it was immersion in one of the environments for 50 to 100 days. We also measured the change in the internal pore distribution and in the shape of the rebar surface morphology using mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM) technology to observe the microstructure degradation. The tests demonstrated that GFRP rebar exposed to degradation environments exhibited only slight reductions in mechanical properties with time. Areas of surface degradation were difficult to observe in the SEM analysis, and the MIP pore values were very small, indicating a dense structure. Since the durability of GFRP rebar was not adversely affected to an appreciable degree by exposure to the degradation environments, it was deemed suitable for use in concrete reinforcements. 1. INTRODUCTION When a material is exposed to an environment that causes degradation, its durability and dynamic loadcarrying capacity is reduced. We therefore performed accelerated degradation tests to judge the influence of various environmental factors on the durability of fibre-reinforced (FRP) rebar. Accelerated tests, in which the rebar was immersed in environmental solutions, were used to shorten the time required to observe the material degradation that normally occurs over a very long period of time 1,2. Typical environments that cause degradation include salt water for sea structures and calcium chloride, CaCl 2, which is found in de-icers applied during the winter to bridge decks 3. *Corresponding author. Tel ; Fax: jpwon@konkuk.ac.kr (J.-P. Won) Rapra Technology, 2007 Glass fibre-reinforced polymer (GFRP) rebar is used as a concrete reinforcement material mainly when a concern exists regarding the durability of steel, especially in an environment that causes corrosion. GFRP rebar strongly resists salt. This, combined with its superior mechanical properties, means that it can be used as replacement material for reinforcement steel in concrete sea structures. The primary test method used to evaluate the effect of GFRP rebar on saltwater consists of measuring the tensile strength reduction after the material is exposed to a saltwater solution for a given number of days 4 6. The GFRP rebar may also be installed in concrete or attached to a test sample and then exposed to a salt water solution 7. The test can be accelerated by increasing the concentration and temperature of the salt solution. Typically, a sodium chloride (NaCl) solution is used to evaluate the effect of saltwater, whilst magnesium chloride (MgCl 2 ) is used to evaluate the effect of de-icers and sodium sulphate (Na 2 ) is used to simulate an environmental processing structure. Steckel et al. 8 studied the effects of a salt water environment on three types of GFRP rebar. They measured the tensile strength and shear strength of the rebar after exposing it to seawater at 23 C and did not observe any substantial reduction. The shear strength of the material decreased slightly, but not to a degree that would have an effect on the durability of the rebar. The strength reduction was a result of water absorption, not the influence of salt. Water absorbed into the GFRP rebar caused plasticisation of the polymer matrix, which reduces Polymers & Polymer Composites, Vol. 15, No. 5,

2 Hae-Kyun Park, Su-Jin Lee, Yoon-Jeong Kim, Chang-Il Jang and Jong-Pil Won the shear strength and glass transition temperature. The same results were obtained when the rebar was immersed in pure water. Tannous et al. 9 performed resistance tests on eight types of FRP rebar using a saltwater environment. They used a 3% NaCl solution at 25 C to simulate seawater. De-icers were simulated by mixing NaCl and MgCl 2 solutions, also at a ratio of 2:1. The tensile strength of the rebar was tested until the material yielded after 6 months of testing. FRP rebar that used a vinyl ester resin as the polymer matrix material performed better than the FRP rebar that used a polyester resin. The GFRP rebar showed a 16% reduction in durability but exhibited excellent resistance to saltwater. Another durability test on rebar exposed to saltwater was performed by Rahman et al. 10. They varied the temperature from 0 to 70 C whilst exposing different GFRP composites to 4% w/w seawater for 220 to 240 days. The strength and hardness of the material decreased by a maximum of 17%. A durability test on GFRP rebarreinforced concrete samples was performed by Tannous et al. 9. The samples were tested after they had been immersed in either a 7% NaCl + CaCl 2 solution or a 7% NaCl + MgCl 2 solution. Both solutions were mixed at a ratio of 2:1 and maintained at 25 C. After the material was exposed to the saltwater solutions for 2 years, the reduction in tensile strength and allowable bending stress of the reinforced concrete was 9 11% for both solutions. The reduction in the tensile strength of GFRP rebar when it was exposed to a saltwater environment directly was 25 30%. These results indicated that the GFRP rebar reinforcements increased the resistance of concrete to saltwater. of GFRP rebar-reinforced concrete after it was exposed to salt water. The results indicated an 18.4% reduction. However, further analysis revealed that this result was caused by cracks in the concrete and the influence of the alkali environment, not by failure of the GFRP rebar. Accelerated degradation tests of GFRP rebar in acidic and saltwater environments have been performed in many studies. However, most only judged the degree of degradation after measuring the reduction in the mechanical properties of the exposed rebar 12 or observed the degradation of the surface morphology of the GFRP rebar under scanning electron microscopy (SEM) In this study, we investigated the mechanisms causing the mechanical property reductions in GFRP rebar subjected to accelerated degradation tests by examining the surface microstructures and analysing the degradation process through changes in the pore distribution. We measured the amount of pores inside the GFRP rebar and performed a SEM analysis to determine quantitatively the amount of degradation that occurred in GFRP rebar exposed to accelerated degradation environments. Figure 1. Tensile test set-up 2. EXPERIMENTAL DESCRIPTION Materials and Test Method The GFRP rebar used for the accelerated degradation tests (Aslan GFRP rebar; Hughes Brothers Company, Seward, NE, USA) consisted of 70% E-glass fibre and 30% vinyl ester resin and had a tensile strength of 690 MPa, a modulus of elasticity of 40.8 GPa, and a diameter of 12.7 mm. The decrease in mechanical properties was evaluated from a tensile strength test based on the ASTM D 3916 standard after the rebar was exposed to the accelerated degradation environments 16. In contrast to the standard suggested by ACI Committee 440, the inside of the grip in which the tensile test sample was fixed was a half-circle in shape, and made of steel. The GFRP rebar was pressed into the grip. The test set-up is shown in Figure 1. We used a 250 kn capacity universal testing machine (ETH, Zurich, Switzerland) to control the displacement. The LVDT (Sokki Kenkyujo, Tokyo, Japan) used to measure the rate of transformation had a 50-mm gauge length and was fixed to the rebar, as shown in Figure 1. The tensile test was performed at a speed of 5 mm/min. The ASTM D 3916 standard Gangarao et al. 11 performed tests to evaluate the allowable bending stress 404 Polymers & Polymer Composites, Vol. 15, No. 5, 2007

3 Mechanical Properties and Microstructures of GFRP Rebar after Long-term Exposure to Chemical Environments does not recommend a length for the test sample, so we adopted a length of 800 mm based on the standard proposed by ACI Committee The dimensions and shape of each sample are shown in Figure 2. Figure 3. Tensile strength of the GFRP rebar after immersion in the chemical solutions Accelerated Ageing To evaluate the long-term durability of GFRP rebar, we must analyse its properties using an accelerated test method to shorten the environmental degradation period 18. We considered an acidic environment and saltwater environments created by CaCl 2 and Na 2. The samples were kept in a room maintained at a constant relative humidity of 50±2% and temperature of 23±2 C to minimise the possibility of a defect caused by moisture absorption or temperature changes. The damage caused by transportation and cutting was also minimised to avoid influencing the mechanical test results. We immersed a sample for 100 days in a 0.6% acetic acid solution (ph 2.92) at 20 C to evaluate the effects of an acidic environment. A polypropylene sheet was used to seal the container, to minimise any changes due to evaporation. Samples were immersed for 50 days in either a 4% CaCl 2 solution or a 10% Na 2 solution to emulate the effects of the salt water environment found in environmental processing facilities and de-icers used for bridges in the winter, respectively. Microstructure Analysis We measured the amount of pores in the samples using mercury intrusion porosimetry (MIP) to obtain a quantitative measure of the degradation. We then estimated the relationship between the amount of pores and the surface degradation by observing the surface of the rebar under SEM. 3. RESULTS AND DISCUSSION Tensile Test The tensile strength results of GFRP rebar exposed to various accelerated degradation environments are shown in Figure 3. Figure 4 gives the residual tensile strength of GFRP rebar as a percentage of the tensile strength of the control specimen. The results show that the residual tensile strength of GFRP rebar exposed to an acidic environment was 96% after 50 days and 95% after 100 days. This was the lowest value obtained for the different chemical environments considered in Figure 2. Dimensions of the GFRP rebar specimens used for the tensile test this study. The longer the exposure time was, the smaller the residual tensile strength became, although the degree of reduction between 50 and 100 days was small. The rebar exposed to the de-icing and sulphate environments had a residual tensile strength of 98 and 99%, respectively. Therefore the chemical environments considered in this study had little adverse effect on the durability of GFRP rebar. Microstructure Analysis Figure 5 shows the change in the pore distribution of the rebar immersed in the chemical solutions. For the rebar exposed to an acidic environment, a longer exposure time increased the amount of pressurised mercury. According to Purnell et al. 19, the degradation of the glass fibre matrix plays a role in increasing the amount of pressurised mercury, resulting in delamination and the formation of microcracks. Ions penetrate through the delaminated areas and the microcracks, impacting the glass fibres directly, thereby accelerating the process. However, since the acidic environment used in this study did not have a great effect on the durability of the glass fibres, we cannot use this finding to establish that degradation of the GFRP rebar had Polymers & Polymer Composites, Vol. 15, No. 5,

4 Hae-Kyun Park, Su-Jin Lee, Yoon-Jeong Kim, Chang-Il Jang and Jong-Pil Won Figure 4. Residual tensile strength of the GFRP rebar after immersion in the chemical solutions Figure 5. Change in pore distribution of the GFRP rebar immersed in the chemical solutions; (a) Control, (b) 50 days, (c) 100 days in the chemical solutions dispersed through the pores until the material became saturated. The amount of pressurised mercury in the GFRP rebar exposed to an acidic environment was greater than that in the rebar exposed to either a de-icing or sulphate environment, which explains the results of the residual tensile strength tests. The rebar samples exposed to a de-icing or sulphate environment had a dense structure with almost no pressurised mercury; therefore, their tensile strength did not decrease very much compared to the control sample. Table 1 shows the amount of mercury that was intruded into the samples subjected to the accelerated test environments. Figure 6 shows SEM images illustrating the change in surface morphology of the rebar specimens immersed in the acidic solution. No change occurred in the surface morphology when the rebar was immersed for 50 days. The interfaces between the fibres and matrix were clear and no indications of any degradation of the fibres themselves were seen. After the sample was immersed for 100 days, separation of the polymer matrix occurred because of plasticisation. The interfaces between the fibres and matrix were not as clear and visual evidence existed of fibre degradation due to cracks in the matrix, which explains why the amount of pressurised mercury increased with the period of immersion. However, the degree of fibre degradation was small and it did not appear to have any effect on the mechanical properties of the rebar. Table 1. Volume of mercury intruded into samples immersed in different chemicals environments Solution Total volume intruded (cm 3 /g) 50 days 100 days Acid Na CaCl occurred. The GFRP rebar exposed to the de-icing and sulphate environments had similar pore distributions. By studying the pore distribution, we noted that more pressurised mercury was present when pores with small diameters were present. The moisture that penetrated through the surface when the GFRP rebar was immersed Figure 7 shows SEM images of the surface morphology of the GFRP rebar exposed to the de-icing and sulphate environments for 50 days. Compared to the surface morphology of the control GFRP rebar shown in Figure 6, both environments caused an increase in the number of small white spots in the matrix part because of the plasticisation of the polymer matrix, but no substantial degradation occurred in the fibres themselves. Occasionally, a 406 Polymers & Polymer Composites, Vol. 15, No. 5, 2007

5 Mechanical Properties and Microstructures of GFRP Rebar after Long-term Exposure to Chemical Environments Figure 6. Surface morphology of the GFRP rebar immersed in acid solution; (a) control, (b) 50 days (c) 100 days (a) (b) (c) Figure 7. Surface morphology of the GFRP rebar immersed in (a) Na 2 and (b) CaCl 2 solutions (a) (b) small amount of degradation of a single fibre was visible, but the amount was so small (and limited to the surface) that little effects were seen on the mechanical properties of the material. The plasticisation of the polymer matrix proceeded slowly compared to that of the acidic environment, so debonding between the fibres and matrix did not occur and the interfaces between fibres and matrix remained clear. This explains the MIP analysis results, which indicated very little pressurised mercury. In the tests, GFRP rebar with a dense microstructure did not show a major decrease in mechanical properties. 4. CONCLUSIONS The following conclusions were drawn from this study. The tensile strength of the GFRP rebar exposed to an acidic environment decreased slightly as the exposure time increased. However, the degree to which the strength decreased was too small to have an effect on its mechanical properties. Using MIP, we determined that the amount of pores in the rebar increased from to cm 3 /g after immersion in the acidic solution for 50 days. This increased to cm 3 /g after immersion for 100 days. Thus, the longer the exposure time, the more pressurised mercury accumulated. As the GFRP rebar was exposed to the acidic solution, water was absorbed into the resin, which caused debonding and microcracks due to plasticisation of the polymer matrix, increasing the amount of pores. However, the accumulated pressurised mercury of GFRP rebar immersed in the Na 2 and CaCl 2 solutions for 50 days was and cm 3 /g, respectively. This indicates very little degradation since the values are much smaller than is observed in the rebar placed in the acidic environment. The SEM analysis showed that the degradation of the GFRP rebar accelerated as the exposure time to the acidic environment increased. However, the surface degradation of the reinforcement fibres was so small that it did not affect the durability of the material. Almost no degradation occurred in the GFRP rebar exposed to the de-icing and sulphate environments; only a small amount of delamination between the fibres and matrix was observed. The results indicated that if the amount of degradation in the actual fibres was small, the mechanical properties of the GFRP rebar were little affected. REFERENCES 1. Chateauminois A., Chabert B., Soulier J.P. and Vincent L., Effects of Hygrothermal Aging on the Durability of Glass/Epoxy Composites, Physico-Chemical Analysis and Damage Mapping in Static Fatigue, Proceedings of the 9th International Conference on Composite Materials (ICCM/9), (1993), Karbhari V.M., Zhao L., Murphy K. and Kabalnova L., Environmental Durability of Glass Fiber reinforced Composites Short Term Effects, Proc. 1st Conference on Durability of FRP Composites for Construction (CDCC 98), Sherbrooke (Canada), (1998), Bank L.C. and Puterman M., Microscopic Study of Surface Degradation of Glass Fiber- Reinforced Polymer Rods Embedded in Concrete Castings Subjected to Environmental Conditioning, High Temperature and Environmental Effects on Polymeric Composites: 2nd Volume, ASTM STP 1302, T. S. Gates and A.-H. Zureick, Eds., American Society for Testing and Materials, West Conshohocken, PA., (1997), Dejke V., Durability of FRP Reinforcement in Concrete, Department of Building Materials, Chalmers University of Technology, Sweden, (2001). Polymers & Polymer Composites, Vol. 15, No. 5,

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