Durability of glass fibre-reinforced polymer (GFRP) bars embedded in concrete under various environments. I: Experiments and analysis

Journal Pre-proofs Durability of glass fibre-reinforced polymer (GFRP) bars embedded in concrete under various environments. I: Experiments and analysis Daoguang Jia, Qingyong Guo, Jize Mao, Jianfu Lv, Zailin Yang PII: DOI: Reference:

S0263-8223(19)30648-8 https://doi.org/10.1016/j.compstruct.2019.111687 COST 111687

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Composite Structures

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21 February 2019 1 November 2019 6 November 2019

Please cite this article as: Jia, D., Guo, Q., Mao, J., Lv, J., Yang, Z., Durability of glass fibre-reinforced polymer (GFRP) bars embedded in concrete under various environments. I: Experiments and analysis, Composite Structures (2019), doi: https://doi.org/10.1016/j.compstruct.2019.111687

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Durability of glass fibre-reinforced polymer (GFRP) bars embedded in concrete under various environments. Ⅰ: Experiments and analysis Daoguang Jia, Qingyong Guo, Jize Mao*, Jianfu Lv, Zailin Yang College of Aerospace and Civil Engineering, Harbin Engineering University, Harbin, 150001, China Abstract This paper presents the tensile strength of glass fibre-reinforced polymer (GFRP) reinforcing bars embedded in concrete that is exposed to tap water, saline solutions and ambient humidity under accelerated conditions. These conditions were used to simulate the effect of river water, seawater or atmospheric moisture on the GFRP bars. In addition, the electrical impedance method was employed to evaluate the water content at different depths of a concrete cover under a variety of ambient humidity conditions. The effects of seawater, water-to-cement ratio (W/C) of the concrete cover, environmental humidity and cover depth were studied. The results revealed that the GFRP bars embedded in concrete exhibited a substantial strength loss when exposed to tap water or high ambient humidity. The degradation of concrete-wrapped GFRP bars immersed in saline solution could be different. This discrepancy is due to the variation of the degradation mechanism after exposure to chloride ions and is related to the cover depth. The depth of the concrete cover also had an evident influence on the durability results of GFRP bars subjected to various humid environments. Furthermore, reducing the W/C of the concrete cover had negative effects on the GFRP reinforcing bars. Keywords: GFRP bar; durability; tensile strength; saline solutions; relative humidity; resistivity. 1. Introduction The corrosion of steel in concrete is a major cause of deterioration of conventional steel-reinforced concrete structures and has led to substantial economic losses due to more frequent maintenance and retrofitting. Glass fibre-reinforced polymer (GFRP) bars are widely used in civil engineering structures due to their primary resistance to corrosion, high strength-to-weight ratio, ease of handling, and low cost [1]. However, due to the widespread application of concrete in different areas, GFRP-reinforced concrete structures may be subjected to harsh environments, such as high-temperature and high-humidity climates and marine conditions. In addition to severe climate attack, GFRP bars used in concrete structures are also attacked by free hydroxyl ions (OH-) that diffuse through concrete pore solutions. Therefore, evaluating and predicting the long-term performance of GFRP reinforcing bars under harsh environments is required to increase their use in civil engineering structures. To obtain durability results of GFRP bars used in concrete structures, accelerated testing methods have generally been used in many studies by immersing bare GFRP bars in simulated alkaline solutions. It is well known that simulated concrete environments show a significant influence on the long-term performance of GFRP bars [2-4]. However, Robert and Benmokrane [5] reported that the durability of

*

Corresponding author E-mail address: [email protected] (J. Mao). 1

GFRP bars exposed to simulated alkaline solutions is more affected than when exposed to moist mortar, which could in turn lead to unrealistic durability predictions. Moreover, simulated alkaline solutions can only provide saturated concrete conditions, whereas partly saturated concrete conditions may be required to simulate partly moist environments, such as high-moisture conditions and areas with wet and dry (WD) cycles. Thus, it is reasonable to study the long-term performance of GFRP bars by embedding bars in concrete and then exposing them to solutions or humid conditions to more accurately simulate actual situations in the field. Many studies in the literature discuss degradation of concrete-wrapped GFRP bars that are intended for use in a moist environment. Chen [2] tested GFRP bars embedded in concrete with a water-to-cement ratio (W/C) of 0.55. The loss in tensile strength of these specimens was 10% and 39% after exposure to WD cycles and alkaline solution (pH of 12.7), respectively, for 90 days at 40℃. Almusallam and Al-Salloum [6] conducted the tests on GFRP bars embedded in high alkaline cement paste, and Na2O increased from 0.2 to 1%. After immersion in tap water for 4, 8, and 16 months at 40℃, GFRP bars demonstrated 4.9%, 13.1%, and 16.3% strength reductions, respectively. In a concrete environment with a 0.45 W/C, GFRP reinforcing bars were studied by immersion in tap water at 60℃ [7]. The tensile strength reduction was approximately 55% for a maximum of 210 days. Robert and Benmokrane [5] demonstrated the loss of tensile strength of GFRP bars embedded in a cement mortar that had a W/C of 0.40. The loss was equal to 16, 10, and 9% at 50, 40, and 23℃, respectively, after 240 days of ageing. The long-term performance of concrete specimens reinforced with GFRP bars exposed to an ocean environment has also been investigated. Robert and Benmokrane [8] described the results of exposure of GFRP bars to saline solutions at 23, 40, 50, 70℃ for a maximum of 12 months. EI-Hassan et. [9] reported the results of GFRP reinforcing specimens at 20, 40, and 60℃ for a maximum of 15 months. It was concluded that the saline environment had a clear influence on the performance of the GFRP reinforcing bars. Moreover, only a comparative study of concrete-wrapped GFRP bars subjected to salt water and tap water was based on a high alkaline environment [6]. The reductions in the tensile strength of GFRP bars immersed in salt water were slightly higher than those of GFRP bars immersed in tap water for each ageing period, from 4 to 16 months. Although a number of durability studies on GFRP reinforcing bars have been conducted by many researchers, there is still a lack of information to assess the durability in various environments. Additional aspects should include (1) a comparative study between concrete-wrapped GFRP bars subjected to salt solution and those subjected to tap water, (2) the effect of W/C of concrete cover, and (3) a reliable test methodology for GFRP specimens under ambient relative humidity (RH) conditions. It is difficult to represent the real environment using certain conditions, as in previous reports [10, 11]. The moisture equilibrium between external RH and internal water content depends, in particular, on the specimen thickness, RH gradient, and time [12]. Therefore, a new test mothed should be proposed and applied to detect the fluctuation of internal water content as a function of testing time, to study the effect of the concrete cover and develop a new prediction model that combines all of the factors above. The present work improves the understanding of the degradation of GFRP reinforcing bars in terms of salt level or humidity in the environment, the thickness of the concrete cover, and W/C of the concrete. The electrical impedance method was employed for the tests of GFRP reinforcing bars under humid conditions. Additionally, a refined prediction method that considered these terms was proposed. Taking into account the experimental results of ageing tests and the proposed model, the long-term performance of GFRP reinforcing bars in the field was predicted. 2

2. Experimental procedures 2.1 Materials In this study, GFRP bars that were helically wrapped and underwent a sand-coated surface treatment were used, as shown in Fig. 1 (a). All of the bars were made of E-glass fibres and vinyl ester resin due to their widespread application in civil infrastructure and potential cost savings. The volume fraction of glass was 70% and they had a nominal diameter of 9.5 mm. All bars were cut into 620 mm lengths as specified by ACI 440.3R-04 B2. The embedded concrete was cast only in the middle third of the bars to avoid any degradation at the ends, which were used as grips during the tensile tests (ACI 440.3R-04 B2). Three different concrete mixtures were prepared with the W/C of 0.31, 0.4, and 0.6. Table 1 describes the mix design. Type I Portland cement was used for the concrete mixture. The main chemical composition and physical properties of cement were listed in Table 2 Dredged river gravel and fines were used. The nominal maximum size of the coarse aggregate was 10 mm, and the fine aggregate size was approximately 0-4.5 mm. The mean concrete compressive strength of each mixture were determined according to ASTM C39/C 39M [13]. Five specimens were tested for each mixture. As listed in table 1, the compressive strengths (28-day) were 75 ± 8, 64 ± 3, and 44 ± 6 MPa for the mixtures with W/C of 0.31, 0.4, and 0.6, respectively. Cylindrical specimens were used in this study. The GFRP bars were placed at the centre of the cylinders so that the concrete cover was equal around the bars and the same relative conditions existed for the bar in every direction under various ageing environments, as shown in Fig. 1 (c). A total of 138 GFRP specimens were cast using the specially designed mould, as shown in Fig. 1 (b). The upper part of the mould comprised polyvinyl chloride (PVC) pipes with a length of 200 mm. Two external diameters of 96 and 154 mm were used to form GFRP specimens with two different concrete covers, namely, 47 and 62 mm, respectively. The lower part of the mould was plywood with a hole in the centre. 2.2 Environmental groups The GFRP specimens were subdivided into 5 groups based on their environmental exposure after casting and curing. The seawater used here consisted of sodium chloride (30 g/L) and sodium sulfate (5 g/L) to simulate ocean water. Four tanks (see Fig. 1 (d)), which were designated T1, T2, T3, and T4, were fabricated for the tap and sea water groups and fitted with electrical heaters and thermostats to maintain the water temperature at approximately 40 or 60℃. The elevated temperatures were applied to accelerate attack by the simulated environments on the GFRP reinforcing bars because the degradation rate of GFRP bars in moist concrete mainly depends on diffusion rate and chemical reaction rate, both of which can be accelerated by elevated temperatures [2]. Three different concrete mixtures were all immersed in T2 (tap water – continuous exposure at 60℃) to investigate the influence of concrete cover on degradation of the GFRP bars. Other exposure conditions were used only on GFRP specimens with a W/C of 0.4. Three chambers (see Fig. 1 (e)), designated C1, C2, and C3, were employed to provide different relative humidity conditions, namely, 98%, 85%, and 75%, respectively, at a constant 3

temperature of 60°C. It is worth mentioning here that the relative humidity provided by C3 changed from 75% to 65% to better understand the variations of water content in concrete influenced by the outside relative humidity. The GFRP specimens with a concrete cover of 62 mm were placed in all three chambers, whereas those with a concrete cover of 47 mm were placed in all containers except for the third one. For clarity, the details for all the specimens in all containers are given in Table 3. 2.3 Pull-out test design The protruding portions of the reinforcing bars were coated with epoxy before immersion to protect them from moisture or conditions in the solutions. After ageing for certain periods, which were 1, 2, 3, and 4 months, the specimens were prepared for pull-out tests. The protruded portions of specimens were sheathed with thick-wall hollow steel pipes, as shown in Fig. 1 (f). The pipes were 150 mm in length and had a 19.05 mm inside diameter and were further processed by internally threading along the length to improve the roughness for anchorage. The gap between a pipe and the GFRP bar were filled with epoxy. To achieve sufficient bond strength, the curving time of epoxy for each specimen was no less than 24 hours. The portions of the specimens embedded with a concrete cover of 62 mm were cut into small pieces to avoid the potential influence of a heavy concrete cover on tensile tests of the GFRP reinforcing bars. After cutting, the cross-section shape of the surrounding concrete was rectangular with approximate dimensions of 40 × 90 mm. The pull-out tests of GFRP reinforcing bars were carried out using the 1000 kN capacity universal testing machine, as shown in Fig. 1 (g). The loading rate for the tests was 1.3 mm/min. 2.4 Resistivity test design The degradation mechanism of the GFRP bars must correspond to the water content of the wrapped concrete [10, 14, 15]. Notably, the movement and distribution of free water in cement-based materials after storage in humid conditions varies with the depth of concrete [16] and storage history [17]. To assess the available water content surrounding the bars, it was therefore necessary to understand the water content at different depths of the concrete cover, where the degradation rate of the GFRP bars differs. Additionally, weighing each depth of concrete is difficult during monthly testing periods. Thus, the electrical impedance method was used to identify the free water content at each depth under in situ humidity conditions. Many studies have focused on the resistivity of cement-based materials [18-21]. The resistivity of cement paste is mainly related to two aspects [22]. One is the ionic conductivity of the pore solution, which depends on the concentration and type of ions in solution; the other is the conductivity through the free water in the cement, based on its microstructure. For concrete, the presence of aggregates does not influence the water content inside the cement [12]. Based on the above understanding, the electrical response of concrete can be utilized as an indicator of water content when the type and concentration of ions in solution is stable. Measuring the resistivity on a very local scale inside concrete using small embedded electrodes has been shown to be a viable method for measuring and monitoring the effects of moisture changes [23]. The specimen used for the resistivity test in this study had three pairs of 8 mm diameter copper electrodes inserted into the concrete cylinder. Each pair of electrodes was placed at different depths of concrete cover, namely, 18, 47, and 62 mm, as shown in Fig. 2. The electrode was 10 cm in and 2 cm 4

out of the concrete cylinder. The W/C of the cylinder was 0.4, whereas the diameter of the cylinder was 154 mm. Every two resistivity specimens were used for each ageing humidity condition, as listed in Table 3. The bulk electrical resistance of the specimens was measured every 5 days during humidity ageing testing. The equipment used for the uniaxial bulk resistance testing was a Miller-400D digital resistance metre operating at a voltage of 12 V with an accuracy of ± 0.01 Ω [24]. The measured resistance (𝑅𝑚𝑒𝑎𝑠𝑢𝑟𝑒𝑑) for each depth was adjusted for the resistance of electrodes (𝑅𝑒) according to Eq. (1) to obtain the resistance of the specimens (𝑅𝑏): 𝑅𝑏 = 𝑅𝑚𝑒𝑎𝑠𝑢𝑟𝑒𝑑 ― 𝑅𝑒

(1)

The bulk resistivity can then be calculated as follows: 𝜌 = 𝑅𝑏 ⋅ 𝐴/𝐿

(2)

where 𝜌 is the electrical resistivity (𝑘𝛺/𝑐𝑚), 𝑅𝑏 is the bulk electrical resistance (𝑘𝛺), 𝐴is the cross-sectional area between a pair of electrodes (𝑐𝑚2), and 𝐿 is the distance between a pair of electrodes (𝑐𝑚). 3. Results and discussion 3.1 Failure mode The tensile strength of GFRP bars is much higher than that of concrete, and the concrete cover adhered the failure zone of GFRP bars were broken before the rupture of the fibres, which are essential requirements to carry out tensile tests of GFRP bars with a concrete cover. Tensile testing of all the GFRP specimens displayed an linear behaviour up to failure, and no yielding occurred. This is due to the brittle nature of GFRP bars. The tensile testing behaviour of the bare GFRP bars was the same as that reported in other studies [8, 25]. The typical failure mode for GFRP specimens is shown in Fig. 3. As the load increased, the tensile strength migrated to the GFRP bar only. Initially, both the concrete cover and the GFRP bar developed low tensile stresses; however, under increasing load, the stresses raised, and the concrete cover cracked. A cross-section cracking showed in the middle of the concrete cover, then the cover below this cracking was splitting into pieces and dropped on the floor. The damaged part of the concrete was shown in Fig. 3 through the dash lines. The load ceased to be carried by the corresponding fraction of concrete cover and was carried out further by the GFRP bar alone. Following the splitting of the concrete cover, the GFRP specimens eventually failed from fibre rupture. The failure was accompanied by delamination of fibres and resin. The failure of the bar occurred at the place where the concrete was previously wrapped, as indicated in Fig. 3. 3.2 The effect of saturated immersion Tap and salt water immersions are two common ways to form saturated alkali solutions in concrete and are understood to seriously degrade the performance of GFRP reinforcing bars [3]. The tensile strength of these samples was obtained after immersion in three environments, namely, tap water at 40 or 60℃ and saltwater at 60℃. Fig. 4 displays the retention of the tensile strength of aged GFRP bars according to the duration of its immersion. The retention of the tensile strength was defined as the ratio of the tensile strength of GFRP bars after a certain aging period to the tensile strength of unaged GFRP bars. Three specimens of GFRP bars without aging were tests, the average tensile strength of these bars was 997 ± 49 MPa. The 5

recorded results show that the longer the period of immersion was, the larger the loss of strength resistance for both the tap and salt water ageing conditions. Furthermore, it is clear that the immersion temperature affects the retention of tensile strength. For the duration of 120-day immersion, the loss of tensile strength for the water immersion was 39.0% and 19.8% at 60℃ and 40 °C, respectively. This phenomenon was due to the increase in the diffusion rate of the alkali solution and chemical reaction of degradation that were caused by the elevated temperature during immersion. The tensile strength retention after salt water immersion was smaller than that of tap water for each ageing period at 60℃. However, after 120 days of exposure, the tensile strength retention after tap and salt water immersion was very close, namely, 61.6% and 60.7%, respectively. The comparative results indicate that immersion in saline solution has only a slightly greater impact on the durability of GFRP bars than immersion in tap water in this study. In three other studies, [6, 8], the influence of tap and salt water immersion on the durability of GFRP bars were compared. To make a clear comparison, the difference in the results between GFRP bars subjected to salt water and exposed to tap water are listed in Fig. 5. The configuration of specimens embedded in cement-based materials is also shown in this figure. The results are discussed in the following section based on two main principles of the degradation of GFRP bars: moisture gain and activity of alkali ions. The data points are all within a range of approximately 10%. The values above zero indicate that the influence of salt water is larger than that of tap water, whereas the values below zero indicate the opposite trend. The results of GFRP specimens tested in [8] indicated that salt water could show less influence on the durability of GFRP bars than that of tap water. This outcome may be due to the decrease in the pore solution activity resulting from the penetration of chloride-bearing salts [26]. The lower the activity of the pore solution surrounding the glass fibre composite material is, the lower the moisture gain of the composites [26-28]. Thus, the degradation of GFRP reinforcing bars exposed to salt environment could be lower than that of the bars exposed to the freshwater environment. However, the decrease in pore solution activity leads to an increase in the effective solution diffusivity into composites [26, 29, 30]. Additionally, the penetration of chloride-bearing salts alters the chemical composition of the pore solution [31]. For instance, sodium chloride increases the hydroxyl concentration and thus the pH of pore solutions. Therefore, the pore solution with chloride ions could penetrate deeply into composites with a high hydroxyl concentration. The penetration of pore solutions with and without the influence of chloride ions are demonstrated in Fig. 6. When the influence of the moisture gain on GFRP bars enables a high penetrating depth and hydroxyl concentration, the degradation of the bars immersed in tap water is larger than that in salt water. Otherwise, the degradation of the bars immersed in salt water is larger than that in tap water, as shown in Fig. 5. Moreover, it is reasonable to ignore the influence of penetrate depth on the durability of GFRP reinforcing bars immersed in solution compared to the influence of the total moisture gain because the distribution area of the solution that penetrates immersed GFRP bars is the key influences on the tensile durability. Determining the influence of moisture gain and hydroxyl concentration is therefore important for immersed ageing tests. Based on the testing results summarized in Fig. 5, it can be concluded that the original hydroxyl concentration of the pore solution is a larger influence than the chloride penetration. When the original hydroxyl concentration reaches a certain level, a salt solution immersion could damage the GFRP bars more than a tap water immersion. For the test specimens listed in Fig. 5, the covers used were 18 mm, 47 mm, and a minimum of 20 mm and a maximum of 70 mm (bars were placed on the bottom side of the beam) in [8], this study, and [6], respectively. In particular, in [6], part of the concrete was replaced by cement with 5 times the alkaline content. This replacement 6

means that the equivalent depth of the cover used in this study was the largest among all the listed studies. The ageing results from these studies show that the larger the section of the cement-based material cover was, the stronger the influence of saltwater compared to that of tap water. The larger the volume of the whole cover was, the greater the amount of hydroxyl ions during the long-term temperature-elevated accelerated test, and the higher the original hydroxyl concentration that was maintained during the immersion tests. It should also be mentioned that a concrete cover of 150 mm is recommended for the ageing test of concrete-wrapped GFRP bars immersed in water by ACI [32]. The purpose of this recommendation is to provide a sufficient supply of hydroxyl ions. Overall, the use of GFRP reinforcing bars is more dangerous in a salt-water environment than in a freshwater environment, especially where huge concrete structures are concerned. And the comprehensive mechanism of this area needs to be further studied. The influence of penetrating depth must consider the durability of GFRP reinforcing bars under wet and dry (WD) ageing cycles. The evaporation of water leads to the enrichment of chloride ions in the outer zones of the concrete cover [33] and even in the GFRP bars. This enrichment induces a high effective solution diffusivity and moisture absorption at greater depths of the GFRP bars during the wetting cycle. In addition, the moisture that is embedded deeply in the bars could stay a relatively long time during the drying stage. If the drying stage is not long enough to evaporate the deep moisture in the bars, then the moisture could penetrate further during the next wetting stage.

Where increasing

degradation could occur and lead to a substantial decrease in tensile strength during the WD cycles, as shown in Fig. 5. However, a more accurate analysis of the behaviour of GFRP bars during WD cycling needs to be carried out because reported experimental data are sparse. The main discussion in this section concerned testing the immersion ageing of GFRP bars. 3.3 The effect of humidity Conductivity measurements were recorded across electrode pairs within the concrete cover under continuous humidity environments at 60℃. The failure tensile strength of GFRP bars was also recorded during humidity ageing periods. Based on the data collected, the relationship between the conductivity and the degradation of tensile strength is presented to highlight the durability of GFRP bars that can be obtained from electrical monitoring. 3.3.1 Electrical resistivity The variation of electrical conductivity under various outside relative humidity (RH) conditions was monitored. The electrical resistivity response from depths of 17, 47, and 62 mm for each RH environment is displayed in Fig. 7-9. As unsaturated RHs are subjected to saturated specimens, the fluctuation of moisture in the concrete cover is evident for all depths of the concrete cover and is characterized by a general increase in electrical resistivity, where the prominence of this increase diminishes with ageing time. A resistivity increase signifies that the moisture transported out of the zone of influence of the electrical field between that particular electrode pair. For the unsaturated specimens that were placed in a 75% RH chamber, when the outside humidity suddenly changed on the 45th day of the ageing period, the resistivity changed immediately and then maintained a relatively constant value for a long period. This result was similar to the resistivity profiles of saturated specimens. Therefore, the change of outside humidity from 65% to 95% could be detected by the 7

resistivity of the concrete, regardless of the moisture absorbed in it. Additionally, it is sensible to present the moisture influenced by various RHs using resistivity measurements. For example, the electrode pair at 47 mm displayed large differences in resistivity values on the 120th day and were 52.77, 22.4, and 9.78 kΩ/cm for RH values of 65%, 85%, and 95%, respectively. In studying the response as a function of time, it is informative to present the variation in resistivity through the concrete cover at salient points in time. Fig. 10 presents the bar charts of resistivity during the ageing periods of 40 and 120 days presented in Fig. 7-9. The general increase in electrical resistivity, which was described above, occurs in 15 days because the proportion of the resistivity increase for 0-15 days to the resistivity increase for 0-40 days is approximately 75% for all testing humidity conditions. This outcome indicates that the moisture transport can be considered effectively finished after 15 days. Similar results regarding moisture transport were reported from the tests of water content under humidity environments in terms of cover depths and time [16]. However, the resistivity continued to increase after the 15th day and even until the 120th day. This increase was mainly attributed to the on-going hydration reaction, indicating a continual refinement in pore structure in the cover zone during the post-curing period [23, 34]. It can be seen that the increase in resistivity in the first 40 days is related to the decrease in outside RH. This proportion increases from approximately 45% to approximately 75%, compared to the results for 95% and 85% RH during the testing period of 120 days. Thus, using the resistivity value obtained from the final days at 75% and 65% RH is a reasonable representation of the influence of the outside RH. Because the process of RH exposure involves the transfer of water vapor from within the concrete cover to the exposed surface, a moisture gradient is established through the cover zone. The resistivity profiles decrease with distance from the exposed surface and reflect the increasing degree of pore saturation from the exposed surface. Resistivity values measured at a depth of 17 mm can be up to 1.5 times higher than those measured at 62 mm at the same RH. Furthermore, the final resistivity values for 65%, 75%, 85%, and 95% RH along with the depths of concrete cover are displayed in Fig. 11. A comparison of slopes between 17 and 47 mm and between 47 and 62 mm are taken as a way to estimate the depth of the surface zone most influenced by water vapor desorption. This region is termed the RH-sensitive zone here. The electrode pair at 17 mm displays a greater response than other electrode pairs under RH 65% and 75%, showing a sharper slope of resistivity values from 47 mm to 17 mm, compared to those between 47 mm and 62 mm, as shown in Fig. 11. While the electrode pairs at 17 and 47 mm both exhibit greater response compared to the electrode pair at 17 mm for RH 85% and 95%, evidenced by the sharp slope of resistivity values from 62 mm to 47 mm. Therefore, the RH-sensitive zone is at a depth of approximately 25 (approximately 17) mm when the RH is below 80%, where it is approximately 50 (between 47 and 62) mm when the RH is above 80%. Moreover, the results for 75% and 85% RH on the 40th day are quite close, especially the values for 47 and 62 mm, as shown in Fig. 11. This is due to the distribution of pores in concrete. The process of moisture moving out of concrete under humid conditions is related to the pore size [35]. Large pores could lose moisture at high RHs, whereas small pores could lose moisture only at low RHs, but there is an RH value where no further moisture could be lost, and this specific value needs to be validated by additional work [12, 17]. Due to the moisture gradient along the concrete cover, the outside RH could be assumed to be more than 85% for a concrete layer at a depth of 47 or 62 mm in the concrete cover subjected to 85% RH. In addition, the gaps between the results of 75%, 85% and 95% RH (at 17 mm) are relatively clear. Therefore, the specific value mentioned above should be considered between 85% 8

and 95% RH for the concrete layer. 3.3.2 Tensile-strength retention under humidity environments The retention of the tensile strength of aged GFRP bars according to the duration of exposure to various humidity conditions is presented in Fig. 12. The initial conclusions are that the tensile strength decreases with increasing RH outside of concrete covers and with time. For specimens aged with 95% RH, the tensile strength decreases markedly until 120 days. For specimens aged with 65% to 85% RH, the tensile strength decreases are much less than for specimens aged with 95% RH and are comparatively close to each other. For instance, the decrease in retention at the 120th day for 95% RH is 42%, whereas that for RHs from 65% to 85% is between 14% and 23%. In addition, the depth of the concrete cover has a negative impact on the durability of the GFRP reinforcing bars because the deeper areas in the cover contain more moisture during exposure to the humid environments. The extra loss in the tensile strength retention for bars placed at 62 mm and other than 42 mm are 5.5% and 5.0% when subjected to RH 85% and 95%, respectively. Moreover, in the cases of 75% and 85% RH, both at 47 mm, the time histories of tensile strength retention during the whole 120 days are similar. This means that the degradation rate of GFRP bars for RH between 75% and 85% can be considered stable when using the bars with a thick concrete cover, such as 47 mm. 3.3.3 Relationship between the wrapped concrete resistivity and the tensile strength degradation of GFRP bars The degradation of GFRP bars is a complicated process associated with many accelerated factors, such as the surrounding solution media, pH, moisture content, and temperature [5]. As the environmental RH decreases, the moisture in the pores of concrete can be removed, leading to a change in the content of saturated pore solution surrounding the bars. Thus, the principal factor for the degradation of GFRP bars is the moisture content of the surrounding concrete when using the bars in humid conditions at a certain temperature. According to the results of the concrete resistivity and the discussion in the sections above, the resistivity results are closely related to the moisture content of the concrete. It is then reasonable to evaluate and predict the degradation of GFRP bars in humid environments by using the technique of electrical resistivity. It is noted that the results of tensile strength retention as a function of ageing time fit the Arrhenius law when these parameters are plotted on a log scale [2, 3, 5, 9, 27, 36, 37]. The retention continues to decrease during ageing. For instance, the decrease of the first month could be larger than that of the third month, even though the bars experience the same ageing conditions. In this study, the resistivity values were prepared to evaluate the degradation of GFRP bars, with a one-to-one correspondence, so that differences in different periods could be avoided. Thus, the “per month degradation (𝐷𝑚)” was defined as the degradation of the current month compared to the former month, instead of the status before ageing, as shown the formula below: 𝑅𝑐 𝐷𝑚 = 1 ― × 100% 𝑅𝑓

(

)

(3)

where 𝑅𝑐 is the retention of the current month, 𝑅𝑓 is the retention of the former month, and when the current month is the first month, the value of this factor is one. The average resistivity during a month was the resistivity corresponding to the per month 9

degradation of the same month. The values of the average resistivity and per month degradation are listed in Table 4 and drawn in Fig. 13. A logarithm regression curve was generated, as shown in Fig. 13. The regression curve shows a high 𝑅 2 value of 0.88, which reveals a good correlation between the resistivity and per month degradation. It should be noted that if the values of degradation and resistivity used here could be adopted from a testing period of less than one month, the fitting accuracy and deviation would be further enhanced. As shown in Fig. 13, the relationship between the resistivity of the concrete cover (𝜌) and tensile strength degradationper month degradation (𝐷 𝑚) of GFRP bars can be expressed as follows: 𝐷 𝑚 = ―3.5033ln (𝜌 ― 4.03) +13.767𝑌 = ―3.5033𝑙𝑛 (𝑥 ― 4.03) +13.767

(4)

Based on the regression formula and the resistivity obtained from the previous section, the degradation of GFRP bars using different thicknesses of a concrete cover under various levels of humidity could be predicted. 3.4 The effect of W/C of concrete cover Fig. 14 shows the retention of the ultimate strength of aged GFRP bars as a function of exposure time for bars embedded in concrete with various W/C. The recorded results show that the longer the time of immersion was, the larger the loss of resistance. Furthermore, it is evident that the W/C of the covers affects the loss of durability. For an immersion of 120 days, the resistance losses are equal to 33.7%, 35.7%, and 38.4% for a W/C of 0.6, 0.4, and 0.31, respectively. It can be seen that the concretes with a low W/C have a large influence on the durability of GFRP bars. This trend is unexpected because the concretes with a low W/C have a low porosity [38], which were considered to provide better protection of the rebars. On the one hand, alkalis (𝐾2𝑂 and 𝑁𝑎2𝑂) can undergo expansive reactions with certain aggregates. For concrete with low W/C, more alkaline ions are generated than for a high W/C, which could induce more damage to GFRP bars embedded in concrete. On the other hand, during the immersion of GFRP specimens, the Na+ and K+ ions leach from cement-based materials [38] and induce a new equilibrium of ions and pH in pore solutions. In previous research, the pH was found to be approximately 10.5 after immersion, while the pH inside the similar specimens was 13.1 just after the standard curing [39]. In addition, the low porosity resulting from the small W/C could weaken the transportability of ions out of the concrete covers. Specifically, an order of magnitude decrease in the permeability was shown when the W/C decreased by 0.5 [40]. Thus, the new equilibriums for low W/C concrete could generate high levels of pH. The pH in concrete with a W/C of 0.31 was much higher than that in concrete with a W/C of 0.6. Because more damage occurred on rebars embedded in concrete with a W/C of 0.31, it can be concluded that ion transport prior to porosity is the major factor that influences the durability of embedded GFPR bars immersed in water. 4. Conclusions In this study, GFRP bars were embedded in concrete and exposed to tap and salt water at an elevated temperature to accelerate the effect of tap water and simulated seawater environments. In addition, the GFRP specimens with different depths of concrete cover were tested by subjecting them to four degrees of relative humidity. Furthermore, GFRP bars were embedded in concrete with W/Cs. The preand post-exposure tensile strengths of the bars were deemed indicative of specimen durability in terms 10

of immersion conditions, humidity degree, and W/C of the concrete cover, and the degradation mechanisms for different conditions were analysed. In addition, the electrical resistivity method was introduced to detect the effects of outside RH on the surrounding concrete of rebars and to characterize the effect of cover depths and humidity levels. Based on the results of this study, the main conclusions may be drawn: 1.

The tensile strength of the tested GFRP specimens decreases with the ageing time for both tap and salt-water immersion. In this work, the decrease for salt-water immersion is larger than that for tap water immersion. However, the mechanisms of degradation of GFRP specimens immersed in tap and salt water are different. Normal pore solution resulting from the tap-water has a greater influence on the GFRP bar itself in terms of moisture gain and concentration of alkaline ions than that of salt-water immersed concrete. In contrast, the pore solution resulting from the salt-water immersed concrete has more influence on the depth of penetration of moisture in the bars than that of tap immersed concrete.

2.

The difference in tensile strength of GFRP specimens immersed in tap water and salt water is determined by the depth of the concrete cover for testing specimens. Specimens with a thick cover exhibit a larger decrease in the tensile strength immersed in salt water compared to that in tap water.. In the testing of this study, the change in tensile strength of GFRP specimens in salt water is slightly larger than that of specimens immersed in tap water when the concrete cover is 47 mm.

3.

The remaining tensile strength of GFRP bars is influenced by the degree of outside RH. Overall, the higher RHs show more influence on the durability of GFPR bars than the lower RHs. The decrease for 95% RH is up to three times larger than that for 65% RH after 120 days of ageing. However, the results of tensile strength retention for 75% and 85% RH are quite close, especially for the bars positioned at 47 and 62 mm. This is because the transport of moisture in concrete in humid environments is related to the distribution of concrete pores. A certain degree of RH must be reached until further changes in moisture proceed.

4.

The concrete cover depth has a negative effect on GFRP bars exposed to humid conditions due to the moisture gradient generated along the cover zone. More moisture remains in deeply embedded in the concrete cover. In addition, the impacted depth of the concrete cover, which is easily influenced by outside RH, decreases with decreasing RH. This RH-sensitive zone is approximately 50 mm for RH above 80%, whereas it is approximately 25 mm for RH below 80%.

5.

The electrical method can be used to indicate the degradation of embedded GFRP bars under humid conditions. A logarithmic relationship between the wrapped concrete resistivity and the tensile strength degradation of GFRP bars is established, which can be used to predict the degradation of GFRP bars at different RHs and concrete cover depths.

6.

The W/C of the concrete cover has a negative influence on the durability of GFRP bars. The degradation in concrete with a W/C of 0.31 is slightly higher than that in concrete with a W/C of 0.6.

Acknowledgements The supports of the National Natural Science Foundation of China (51750110494), the Heilongjiang Natural Science Foundation (E201415), and the Fundamental Research Funds for the Central Universities of China (HEUCFP201716) are highly appreciated. 11

References [1] Uomoto T, Mutsuyoshi H, Katsuki F, Misra S. Use of fiber reinforced polymer composites as reinforcing material for concrete. Journal of Materials in Civil Engineering. 2002;14:191-209. [2] Chen Y, Davalos JF, Ray I, Kim HY. Accelerated aging tests for evaluations of durability performance of FRP reinforcing bars for concrete structures. Composite Structures. 2007;78:101-11. [3] Chen Y, Davalos JF, Ray I. Durability prediction for GFRP reinforcing bars using short-term data of accelerated aging tests. Journal of Composites for Construction. 2006;10:279-86. [4] Sawpan MA. Effects of Alkaline Conditioning and Temperature on the Properties of Glass Fiber Polymer Composite Rebar. Polymer Composites. 2016;37:3181-90. [5] Robert M, Cousin P, Benmokrane B. Durability of GFRP Reinforcing Bars Embedded in Moist Concrete. Journal of Composites for Construction. 2009;13:66-73. [6] Almusallam TH, Al-Salloum YA. Durability of GFRP rebars in concrete beams under sustained loads at severe environments. Journal of Composite Materials. 2006;40:623-37. [7] Davalos JF, Chen Y, Ray I. Long-term durability prediction models for GFRP bars in concrete environment. Journal of Composite Materials. 2012;46:1899-914. [8] Robert M, Benmokrane B. Combined effects of saline solution and moist concrete on long-term durability of GFRP reinforcing bars. Construction and Building Materials. 2013;38:274-84. [9] El-Hassan H, El-Maaddawy T, Al-Sallamin A, Al-Saidy A. Durability of glass fiber-reinforced polymer bars conditioned in moist seawater-contaminated concrete under sustained load. Construction and Building Materials. 2018;175:1-13. [10] Huang JW, Aboutaha R. Environmental Reduction Factors for GFRP Bars Used as Concrete Reinforcement: New Scientific Approach. Journal of Composites for Construction. 2010;14:479-86. [11] Dong ZQ, Wu G, Zhao XL, Wang ZK. A refined prediction method for the long-term performance of BFRP bars serviced in field environments. Construction and Building Materials. 2017;155:1072-80. [12] Baroghel-Bouny V. Water vapour sorption experiments on hardened cementitious materials - Part I: Essential tool for analysis of hygral behaviour and its relation to pore structure. Cement and Concrete Research. 2007;37:414-37. [13] C39M AC. Standard test method for compressive strength of cylindrical concrete specimens.: ASTM International; 2016. [14] Dong ZQ, Wu G, Xu B, Wang X, Taerwe L. Bond durability of BFRP bars embedded in concrete under seawater conditions and the long-term bond strength prediction. Materials & Design. 2016;92:552-62. [15] Yan F, Lin ZB. Bond durability assessment and long-term degradation prediction for GFRP bars to fiber-reinforced concrete under saline solutions. Composite Structures. 2017;161:393-406. [16] Rucker-Gramm P, Beddoe RE. Effect of moisture content of concrete on water uptake. Cement and Concrete Research. 2010;40:102-8. [17] Jennings HM, Kumar A, Sant G. Quantitative discrimination of the nano-pore-structure of cement paste during drying: New insights from water sorption isotherms. Cement and Concrete Research. 2015;76:27-36. [18] McCarter WJ, Brousseau R. A.C. response of hardened cement paste. Cement and Concrete Research. 1990;20:891-900. [19] McCarter WJ, Chrisp TM, Starrs G. Early hydration of alkali-activated slag: Developments in 12

monitoring techniques. Cement and Concrete Composites. 1999;21:277-83. [20] McCarter WJ, Chrisp M. Monitoring water and ionic penetration into cover-zone concrete. Aci Materials Journal. 2000;97:668-74. [21] McCarter WJ, Starrs G, Chrisp TM. Electrical conductivity, diffusion, and permeability of Portland cement-based mortars. Cement and Concrete Research. 2000;30:1395-400. [22] Sanish KB, Neithalath N, Santhanam M. Monitoring the evolution of material structure in cement pastes and concretes using electrical property measurements. Construction and Building Materials. 2013;49:288-97. [23] Chrisp TM, McCarter WJ, Starrs G, Basheer PAM, Blewett J. Depth-related variation in conductivity to study cover-zone concrete during wetting and drying. Cement & Concrete Composites. 2002;24:415-26. [24] Bu YW, Weiss J. The influence of alkali content on the electrical resistivity and transport properties of cementitious materials. Cement & Concrete Composites. 2014;51:49-58. [25] Micelli F, Nanni A. Durability of FRP rods for concrete structures. Construction and Building Materials. 2004;18:491-503. [26] Lekatou F, Ghidaoui, Lyon and Newman. Effect of water and its activity on transport properties of glass/epoxy particulate composites. Composites Part a-Applied Science and Manufacturing. 1997;28:14. [27] Silva MAG, da Fonseca BS, Biscaia H. On estimates of durability of FRP based on accelerated tests. Composite Structures. 2014;116:377-87. [28] Guo F, Al-Saadi S, Raman RKS, Zhao XL. Durability of fiber reinforced polymer (FRP) in simulated seawater sea sand concrete (SWSSC) environment. Corrosion Science. 2018;141:1-13. [29] Romhild S, Hedenqvist MS, Bergman G. The Effect of Water Activity on the Sorption and Diffusion of Water in Thermosets Based on Polyester, Vinyl Ester, and Novolac Resins. Polymer Engineering and Science. 2012;52:718-24. [30] Taylor HFW. Cement Chemistry. 1990. [31] Bertolini L, Elsener B, Pedeferri P, Redaelli E, Polder R. Corrosion of Steel in Concrete: Prevention, Diagnosis, Repair (second edition). 2013. [32] ACI Committee. Guide test methods for fiber-reinforced polymers (FRPs) for reinforcing or strengthening concrete structures. ACI 440.3 R. 2004. [33] Tritthart J. Chloride binding in cement - Ⅱ. The influence of the hydroxide concentration in the pore solution of hardened cement paste on chloride binding. Cement and Concrete Research. 1989;19:586-94. [34] Joshaghani A, Balapour M, Ramezanianpour AA. Effect of controlled environmental conditions on mechanical, microstructural and durability properties of cement mortar. Construction and Building Materials. 2018;164:134-49. [35] Saeidpour M, Wadso L. Moisture equilibrium of cement based materials containing slag or silica fume and exposed to repeated sorption cycles. Cement and Concrete Research. 2015;69:88-95. [36] Liang H, Li S, Lu Y, Yang T. Reliability Study on FRP Composites Exposed to Wet-Dry Cycles. Applied Sciences. 2018;8:892. [37] Li S, Hu J, Ren H. The Combined Effects of Environmental Conditioning and Sustained Load on Mechanical Properties of Wet Lay-Up Fiber Reinforced Polymer. Polymers. 2017;9:244. [38] Goto S, ROY DM. Diffusion of ions through hardened cement pastes. Cement & Concrete Composites. 1981;11:751-7. 13

[39] Benmokrane B, Wang P, Ton-That TM, Rahman H, Robert JF. Durability of glass fiber-reinforced polymer reinforcing bars in concrete environment. Journal of Composites for Construction. 2002;6:143-53. [40] Goto S, ROY DM. The effect of W/C ratio and curing temperature on the permeability of hardened cement paste. Cement & Concrete Composites. 1981;11:575-9.

14

Conflict of interest No potential conflict of interest was reported by the authors.

15

(f) Steel pipes.

(a) Bare GFRP bar.

(c) GFRP specimens. (b) The designed mold. (g) 1000 kN capacity universal testing machine.

(d) Temperature controlled tank.

(e) Temperature and humidity control chamber.

Fig. 1. Flow diagram of experimental process of GFRP specimens.

16

Fig. 2. Resistivity specimen design: (a) schematic and (b) specimen after concrete casting.

17

Fragment part of the cover

Failure occurrence of the bar Cut Fig. 3. Typical failure mode of GFRP specimens.

18

Tensile strength retention (%)

100

40°C for tap water immersion 60°C for salt water immersion 40°C for tap water immersion 60°C for salt water immersion

80

60

40

20

0

0

30

60

90

120

Exposure time (Days) Fig. 4. Tensile strength retention of GFRP bars exposed to tap and salt water at 40℃ and 60℃.

19

Figuration of specimens:

[8]

[this study]

[6]

Immersed

70mm

47mm

Na2O increased by 5 times

45mm

18mm

20mm

The difference between the retentions of GFRP bars (%)

6 4 2 0 0

60

120

180

240

300

360

420

480

-2

[6]-40-20/70mm [6]-40-20/70mm-Wet/Dry [8]-70-18mm [8]-50-18mm [This study]-60-47mm

-4 -6 -8

Reference -10

Aging Cover temperature condition

Exposure time (Days) Fig. 5. The difference between the tensile-strength retention of GFRP bars subjected to salt water and tap water.

20

Penetration area

Matrix

Fibers (a)

(b)

Hydrone Chloride ion

Fig. 6. Schematic diagram of degradation: (a) subjected to tap water and (b) subjected to saltwater.

21

depth of 17 mm depth of 47 mm depth of 62 mm

Resistivity (kcm )

10

5 Subjected to 95%RH

0

0

10 20 30 40 50 60 70 80 90 100 110 120 Exposure time (Days)

Fig. 7. The values of resistivity subjected to 95% RH along with exposure days.

22

30

Resistivity (kcm )

Subjected to 85%RH

20

10 depth of 17 mm depth of 47 mm depth of 62 mm

0

0

10 20 30 40 50 60 70 80 90 100 110 120 Exposure time (Days)

Fig. 8. The values of resistivity subjected to 85% RH as a function of exposure days.

23

80

depth of 17 mm depth of 47 mm depth of 62 mm

Resistivity (kcm )

60

Subjected to 75%RH

40

20

0

Subjected to 65%RH

0

10 20 30 40 50 60 70 80 90 100 110 120 Exposure time (Days)

Fig. 9. The values of resistivity subjected from 75% RH to 65% RH as a function of exposure days.

24

th

0-15 day

th

th

15-40 day

th

0-40 day

95%RH, 62mm 95%RH, 47mm 95%RH, 17mm

40-120 day Period of 0-120th day

85%RH, 62mm 85%RH, 47mm 85%RH, 17mm 95%RH, 62mm 95%RH, 47mm 95%RH, 17mm

82%/18% 79%/21% 76%/24%

85%RH, 62mm 85%RH, 47mm 85%RH, 17mm

Period of 0-40th day 68%/32%

72%/28% 60%/40%

73%/27%

75%RH, 62mm 75%RH, 47mm 75%RH, 17mm

73%/27%

65%RH, 62mm 65%RH, 47mm 65%RH, 17mm

66%/34%

68%/32% 69%/31%

0

10

20

(88%/12%)

30

40

Resistivity ( k cm ) Fig. 10. The proportion of resistivity for exposure periods of 40 and 120 days for different depths of concrete cover under various humidity conditions.

25

80

65%RH 75%RH 85%RH 95%RH

Resistivity (kcm )

60 RH-sensitive zone

40

20

0

0

20

40 Depth (mm)

60

Fig. 11. The values of resistivity along with depth of concrete over for various RHs on the 40th testing day.

26

95%RH , 47mm 85%RH , 62mm

85%RH , 47mm 75%RH , 47mm 75%RH changed to 65%RH , 62mm

95%RH , 62mm

30

120

Tensile strength retention (%)

100 90 80 70 60 50

0

60 90 Exposure time (Days)

Fig. 12. Tensile strength retention of GFRP reinforcing bars exposed to various humidity conditions at 60℃.

27

Fig. 13. The relationship between the wrapped concrete resistivity and the tensile strength degradation of bars.

28

Tensile strength retention (%)

100

0.6 0.4 0.31

90

80

70

60

0

30

60

90

120

Exposure time (Days) Fig. 14. The tensile strength retention of GFRP bars embedded in different W/Cs along the exposure time.

29

Table 1 Concrete mix design. W/C

Cement

Water

Coarse aggregate

Fine aggregate

Compressive strength

(kg/m )

(kg/m )

(kg/m )

(kg/m )

(MPa)

0.31

550

171

974

704

0.4

537

215

1054

593

0.6

367

220

986

657

3

3

3

3

75 ± 8 64 ± 3 44 ± 6

Table 2 Main chemical composition (%) and physical properties of cement. SiO2

Al2O3

Fe2O3

Na2O

MgO

K2O

CaO

MnO

Specific gravity (kg/m3)

Specific surface (m2/kg)

20.9

2.8

4.6

0.5

1.6

0.3

69.0

0.2

3150

350

Table 3

Container designation

Exposure condition

Temperature

W/C

Cover depth

Number of specimens

(mm) Tank 1 (T1)

Tap water

40℃

0.4

47

12

Tank 2 (T2)

(immersion)

60℃

0.31; 0.4; 0.6

47

36

Tank 3 (T3)

Seawater

40℃

0.4

47

12

Tank 4 (T4)

(immersion)

60℃

0.4

47

12

Humidity chamber 1

RH 98%

60℃

0.4

47

12

62

14

(H1)

(including

2

for

2

for

2

for

conductivity test) Humidity chamber 2

RH85%

60℃

0.4

(H2)

47

12

62

14

(including

conductivity test) Humidity chamber 3

RH75% and changed

(H3)

to RH65%

Humidity chamber 4

RH75%

60℃

0.4

62

14

(including

conductivity test) 60℃ Total

0.4

47

12 150

Details of the specimens in various exposure conditions.

30

Table 4 The values of tensile-strength degradation and corresponding resistivity. Cover depth

RH Degradation

47mm

(%) Resistivity (kΩ/cm) Degradation

62mm

(%) Resistivity (kΩ/cm)

95%

85%

75%

65%

4.28

5.67

7.83

9.21

12.94

18.95

22.45

23.21

14.19

17.5

10.89

8.08

6.17

6.00

5.31

3.51

3.49

6.34

4.58

5.61

7.01

8.63

9.47

13.46

14.71

15.98

9.32

18.9

35.54

43.43

18.8

12.31

11.93

7.01

7.94

8.61

2.26

6.11

7.42

2.20

2.56

1.63

31