Abstract
Two categories of GFRP specimens, in nominal diameters of 11.2 mm and 15.6 mm respectively, made of E-glass fibers and a vinylester resin were immersed in alkaline solution, saline solution and tap water solution at room temperature. The mechanical and durability properties of tested GFRP bar specimens were determined through direct tensile test and short-beam shear test after exposure to aggressive solutions for different days. The obtained results indicated that for all tested GFRP bar specimens the failure modes of fiber rupture in the tensile test and horizontal cracking in the short-beam shear test would not obviously change with the increasing of exposure period. However, the impact of immersion solutions on the durability degradation of mechanical properties of GFRP bars appeared in the ultimate strength and displacement at failure, and at room temperature the alkaline solution had a greater impact than another two solutions. Under the same exposure conditions, the resistance to the strength degradation of the GFRP bars with a larger diameter was better than that of the smaller GFRP bars. The comparison results indicated that the strength degradation trend of conditioned GFRP bars presented in our study was basically in agreement with the data given in previous studies.
1 Introduction
Reinforcement corrosion has been recognized as a prime factor of performance deterioration of reinforced concrete (RC) structures/infrastructures suffering from chloride attack, such as marine and deicing salt environments [1]. As a result, the damage to the structure with cracking and spalling of cover concrete would commonly appear during its service life and finally the bearing capacity of RC element would be harmed. To overcome these issues, fiber reinforced polymer (FRP) bars have been regarded as a desirable material to resist corrosion and applied in some countries like Canada as internal reinforcing bars of bridge structures [2, 3]. Furthermore, due to the low cost and high strength-to-weight ratio, glass FRP (GFRP) bars have been encouraged to be used in civil engineering applications [4] and they have played a more and more role in some concrete structures exposed to above aggressive conditions [5].
Although GFRP bars embedded in concrete don’t suffer from corrosion like steel reinforcing bars, the uncertainties in their strength degradation and durability are the upcoming problems before their widespread use in civil engineering. Generally, GFRP bars are comprised of two different phases, glass fibers and a resin matrix. As a result, the investigation on the properties of both ingredients and fiber-resin interface has to be made with respect to the influence of environmental conditions [6]. It is significant to find that the glass fibers themselves are exceedingly sensitive to deterioration in the water and moist environments and in the alkaline (hydroxyl ions) environment due to leaching and etching actions [7,8,9], as shown in Eqs. (1) and (2). And the resin matrix, such as vinylester, inclines to degradation by hydrolysis in moist environments as presented in Eq. (3). Therefore, some solution conditions, such as water and alkaline solutions, have a pronounced impact on the GFRP bars, especially with high temperature [10].
Several experimental works [7,8,9, 11,12,13,14,15] have been carried out to explore the long-term mechanical properties of GFRP bars under various aggressive solution conditions. Furthermore, in order to shorten the exposure period, the solution temperature was usually evaluated up to 60~80°C in these studies. Based on these experimental investigations, it could be concluded that the degradation of mechanical and durability properties of GFRP bars would be mainly influenced by the composites (glass fiber and resin matrix), aggressive solution, temperature and exposure time. For the GFRP bars with the same composites (E-glass/vinylester), it seemed that the degradation of mechanical properties (tensile strength or interlaminar shear strength) under alkaline exposure condition was severer than other conditions and that the elevated temperatures accelerated the degradation for the same exposure condition [7, 8, 11,12,13,14]. By comparison with the results given in some literatures [13,14,15], it could be indicated that the impact of resin matrix on the deterioration of GFRP bars would be changed as pH value and temperature increase. Under the room temperature, GFRP bars with E-CR glass fibers [9] showed higher resistance to strength degradation than the ones with E-glass fibers [7, 8] exposed to the same solution conditions with the same exposure period, however, this phenomenon would exist until the environmental temperature reached up to 60°C.
It can be concluded from the above research works that the impact of environmental conditions on the mechanical properties of GFRP bars is obvious and unavoidable, and that the typical environments used to assess degradation rate of GFRP bars are aggressive solutions, such as water (moisture), alkaline solution and saline solution, and temperature. D’Antino et al. [10] collected 356 tensile results of GFRP bars from 20 different research groups and completed the comparative analysis of the effect of different exposure conditions of hot dry and humid air, alkaline solutions, salt solutions and water. The comparative analysis of residual strength of GFRP bars subjected to certain exposure condition was conducted based on the main influencing factors of exposure time and temperature. Besides, some other factors, such as the bar fiber volume fraction, the matrix and resin type, and the failure mode, were also considered and discussed in that paper. The authors proposed that the comparative analysis presented in many instances contradictory results and incompatible trends and that the first issue was the high temperatures used to accelerate aging tests; and they believed that the temperatures higher than 40°C in water solutions are not practical and the highest temperature of reinforcing bars placed in concrete never exceeds 45°C [10]. Therefore, the investigation on the long-term performance of GFRP bars is clamant and significant when they are adopted in civil engineering environment with natural temperatures. Lu et al. [16] compared the degradation of tensile properties of three categories of FRP bars, including BFRP, CFRP and GFRP, immersed in water, seawater solution and alkaline solution under room temperature (around 20°C) and found that GFRP bars showed more decline of tensile strength in water and alkaline solutions than BFRP and CFRP bars. Besides, with the development of new GFRP bars, the producers claim that their current GFRP bars have better mechanical properties and show higher resistance to aggressive environments especially the water and alkaline solutions [10, 14]. The study of mechanical and durability properties of newly developed GFRP bars after exposure to natural aggressive environments is still required.
The aim of this paper is to investigate the degradation of mechanical properties (tensile strength and interlaminar shear strength) of two commercial GFRP bars after immersion in alkaline, saline and water solutions under room temperature. During the immersion period, the tensile test and short-beam shear test of conditioned GFRP specimens were conducted several times to evaluate the degradation process of their mechanical performances. The test outcomes of tensile and shear strength retentions were discussed and compared with the results recorded in existing studies.
2 Experimental program
2.1 GFRP bars
Two types of GFRP bars were adopted in this study and both of them were manufactured by a company named Fenghui located at Nanjing, China, as presented in Figure 1. The GFRP bars were made of continuous longitudinal E-glass fibers, which were impregnated in a vinylester resin using the pultrusion process. Both of them have the fiber volume fraction of approximately 64% by weight. The bar's surface was grooved during pultrusion with the purpose of improving the bond performance between the bar and the concrete. The nominal diameters of GFRP1 and GFRP2 bars are 11.2 mm and 15.6 mm, respectively. As presented in Figure 1(b), the groove gap, spacing and depth of GFRP1 bar are 2.5 mm, 7 mm and 1 mm, while those of GFRP2 bar are 2.5 mm, 3 mm and 1 mm. The inner and external diameters of GFRP1 bar are 10 mm and 12 mm respectively, and those of GFRP2 bar are 15 mm and 16 mm respectively, as shown in Figure 1(c) and 1(d). Therefore, the mechanical properties of GFRP bars were calculated based on their nominal diameters and cross-sectional areas in the study. According to the international standard ISO 10406-1 [17], the immersion testing for GFRP bars was completed and the measurements of nominal diameter and cross-sectional area were summarized in Table 1.
GFRP bars adopted in this study
Physical properties of GFRP bars.
| Property | Bar type | |
|---|---|---|
| GFRP1 | GFRP2 | |
| Resin type | Vinylester | Vinylester |
| Surface treatment | Grooved | Grooved |
| Fiber volume by weight (%) | 64 | 64 |
| Glass transition temperature, Tg (°) | 73.2 | 71.6 |
| Nominal diameter, d (mm) | 11.2 | 15.6 |
| Nominal cross-sectional area, A (mm2) | 98.1 | 190.5 |
2.2 Natural aggressive environmental conditions
It has been proved in existing studies that the direct immersion of GFRP bars in conditioned solutions is believed to be more aggressive than any practical civil engineering application [9]. However, it is an effective way to obtain the valuable results within a relatively short period of testing time and some solution conditions have been included and recommended in some test guidelines [18, 19]. The aggressive solution conditions considered in this study included:
Alkaline solution (AS), which aimed at simulating the pore solution of normal concrete. According to the recommendations of the ACI 440 committee [19], this alkaline solution was prepared with deionized water by adding 118.5 g/L of Ca(OH)2, 0.9 g/L of NaOH and 4.2 g/L of KOH. Finally, the pH value of this solution was measured to be 12.9.
Saline solution (SS), which was designed to simulate the ocean water. Based on the ASTM D665 standard [20], the saline solution mainly had the following compositions: 24.54 g/L of NaCl, 5.2 g/L of MgCl2, 4.09 g/L of Na2SO4, 1.16 g/L of CaCl2 and 0.69 g/L of KCl. The measured pH value of this solution was 7.1.
Tap water solution (TWS), which was designed to simulate the high humidity environment.
For the GFRP bar samples prepared for the tensile test, the length of the samples was 1000 mm with the anchorage zone at the two ends to avoid shear failure (see Figure 3). The anchor with a length of 300 mm was manufactured by a galvanized steel pipe. The inner diameters of the galvanized steel pipes for GFRP1 bars and GFRP2 bars were 13 mm and 17 mm, respectively, and the thickness of the pipes for both types of GFRP bars was 5 mm. During the period of anchor installation, aluminum caps were settled at the edge of the anchors in order to close both ends of the steel pipe and ensure the bar placed in the center of the steel pipe. For the purpose of establishing an enough adhesive force between the GFRP bar and the anchor [15, 16, 21], the galvanized steel pipes were grouted with an epoxy resin of WSR618 (E-51) with the mass ratio (the epoxy resin to the curing agent) of 4:1. After a week, the conditioned GFRP bars were immersed in three prepared solution containers, as shown in Figure 2. All containers were put in the Civil Engineering Laboratory at Jiangsu University. During the exposure period, the temperature and relative humidity (RH) were not controlled and the natural indoor temperature and RH, which were measured at noon, were in the range of 15–26°C and 70–80%, respectively. As a result, the degradation of mechanical properties of conditioned GFRP bars can be approximately assumed to be determined at room temperature with around 20°C.
GFRP bars immersed in different solutions
In this study, the tensile and short-beam shear tests were conducted to evaluate the degradation of mechanical properties of unconditioned and conditioned GFRP bars. Except for the unconditioned reference specimens, the tensile tests for conditioned specimens were conducted after exposure periods of 45, 90, 135 and 180 days; while the short-beam shear tests were completed after periods of 12, 36 and 72 days. Before testing, the bar samples were taken out from the solution containers and cleaned.
2.3 Mechanical testing plan
In the tensile test, total 78 GFRP specimens (6 unconditioned reference specimens and 72 conditioned specimens) were prepared. Before the tensile testing, both side parts of the bar samples should be prepared with the designed adhesive anchors, see Figure 3(a). The thickness of the steel pipes was 5 mm and their inner diameters were 13 mm and 17 mm for GFRP1 and GFRP2 bars. Each tensile sample was 1000 mm long and the length of the test part between two anchored ends was 400mm. The prepared tensile specimens of GFRP bars were shown in Figure 3(b).
Tensile specimens of GFRP bars
Based on the guide of ACI 440.3R-04 [19] and China national standard of GB/T 13096-2008 [22], the tensile tests of all unconditioned and conditioned GFRP bars were carried out using a WAW-1000D electro-hydraulic universal testing machine (UTM) with a maximum capacity of 1000 kN (see Figure 4(a)). The tensile process was controlled by displacement with 2 mm/min [21], and the loading duration for each specimen was in the range of 3 to 5 minutes. The strain of GFRP bars was measured by the clip-on extensometer and all testing data were collected automatically by the system.
Experimental setups of mechanical tests
The short-beam shear test was performed to evaluate the degradation of the interlaminar shear strength of all tested GFRP specimens. According to the standards of GB/T 13096-2008 [22] and ASTM D4475 [23], the short-beam specimens of unconditioned and conditioned GFRP bars were prepared with span to diameter ratio of 5. Total 60 specimens (6 unconditioned reference specimens and 54 conditioned specimens) were tested using a UTM5305 machine, as presented in Figure 4(b). The loading rate was 1.3 mm/min and the loading duration was within 200 s for each specimen. The loads and movements of the loading arbor were recorded automatically.
3 Results and discussions
3.1 Tensile properties
3.1.1 Failure mode and tensile results
In the tensile test, the tested GFRP bars broke suddenly at the ultimate loads and all of them failed in a similar mode with fiber rupture regardless of the immersion solution and exposure time. The failure occurred with a sudden longitudinal delamination of the fibers at center due to the surface debonding between the fibers and matrix. Therefore, the broken fibers were divergent and fan-shaped as shown in Figure 5. This failure mode of all tested GFRP bars was brittle fracture and similar results had been reported in previous studies [4, 12, 14].
Tensile failure mode of GFRP2 bars
During the tensile test, the applied load was exerted by a WAW-1000D electro-hydraulic universal testing machine (UTM), and the strain of GFRP bar was measured by a clip-on extensometer, which was connected with a computer. The corresponding strain could be recorded automatically by its data acquisition system. Using the original nominal cross-sectional areas given in Table 1, the load-related nominal stress of the tested GFRP bar was calculated. Figure 6 draws the typical tensile stress-strain relationships of GFRP1 bars with different exposure periods. It can be illustrated from Figure 6 that the stress-stain curves were almost linear up to failure at peak load without the influences of environmental solution and exposure time. The similar phenomenon was observed for the GFRP2 bars. It can be concluded that the tensile performance of all tested GFRP bars showed a nearly linear elastic behavior without considering the effect of the conditioned solutions.
Tensile stress-strain curves of GFRP1 bars immersed in different solutions
Table 2 summarizes the tensile results of the tested GFRP bars, including the mean values and the coefficients of variation (COV) of tensile strength, elastic modulus and ultimate strain. Here, the elastic modulus of GFRP bar was determined as the slope of stress-strain curve (shown in Figure 6) in the range of 20% to 60% of the ultimate tensile capacity. From Table 2, it can be found that the tensile strength of the GFRP bars conditioned in certain aggressive solution would decrease with exposure period. At the same time, the bar's ultimate strain at failure usually decreased accordingly. These phenomena indicated that the solution conditions in this study have harmful impact on the tensile strength and ultimate strain of GFRP bars conditioned at room temperature.
Results of tensile test of all tested GFRP bars.
| Bar type | Environmental solution | Period (Days) | Tensile strength | Elastic modulus | Ultimate strain | |||
|---|---|---|---|---|---|---|---|---|
| Mean (MPa) | COV (%) | Mean (GPa) | COV (%) | Mean (%) | COV (%) | |||
| GFRP1 | Unconditioned Alkaline solution (AS) | 0 | 971.8 | 1.4 | 40.2 | 2.5 | 2.417 | 3.3 |
| 45 | 824.1 | 2.4 | 39.6 | 2.8 | 2.080 | 2.8 | ||
| 90 | 785.2 | 2.8 | 41.8 | 3.5 | 1.880 | 1.9 | ||
| 135 | 745.8 | 4.4 | 36.6 | 7.8 | 2.038 | 3.4 | ||
| 180 | 680.4 | 5.2 | 38.5 | 5.3 | 1.766 | 2.8 | ||
| Saline solution (SS) | 45 | 895.7 | 2.9 | 37.7 | 5.7 | 2.373 | 2.6 | |
| 90 | 883.3 | 3.9 | 44.4 | 4.6 | 1.990 | 2.5 | ||
| 135 | 874.5 | 4.2 | 44.5 | 4.5 | 1.967 | 3.7 | ||
| 180 | 774.6 | 5.4 | 41.1 | 2.6 | 1.883 | 2.9 | ||
| Tap water solution (TWS) | 45 | 931.9 | 3.0 | 44.2 | 3.2 | 2.110 | 2.8 | |
| 90 | 867.5 | 4.3 | 42.2 | 2.5 | 2.058 | 3.1 | ||
| 135 | 870.2 | 4.4 | 41.8 | 1.9 | 2.082 | 2.9 | ||
| 180 | 872.8 | 4.9 | 41.3 | 3.2 | 2.116 | 5.1 | ||
| GFRP2 | Unconditioned Alkaline solution (AS) | 0 | 1058.2 | 2.3 | 45.7 | 2.2 | 2.316 | 3.8 |
| 45 | 890.8 | 5.4 | 42.7 | 4.1 | 2.086 | 4.2 | ||
| 90 | 876.1 | 3.6 | 44.2 | 2.3 | 1.981 | 3.6 | ||
| 135 | 861.7 | 4.9 | 46.9 | 2.9 | 1.836 | 3.7 | ||
| 180 | 811.2 | 4.2 | 42.8 | 5.2 | 1.897 | 4.6 | ||
| Saline solution (SS) | 45 | 1000.7 | 6.2 | 44.2 | 4.6 | 2.264 | 5.5 | |
| 90 | 998.0 | 5.4 | 45.8 | 6.3 | 2.179 | 4.1 | ||
| 135 | 964.9 | 5.2 | 46.1 | 5.9 | 2.093 | 3.6 | ||
| 180 | 935.8 | 4.9 | 43.9 | 4.2 | 2.132 | 4.7 | ||
| Tap water solution (TWS) | 45 | 1021.8 | 5.3 | 44.2 | 3.3 | 2.311 | 5.1 | |
| 90 | 983.1 | 4.2 | 44.5 | 5.2 | 2.208 | 6.2 | ||
| 135 | 952.7 | 5.3 | 43.6 | 6.3 | 2.185 | 4.7 | ||
| 180 | 931.9 | 4.4 | 45.8 | 4.6 | 2.036 | 3.4 | ||
Note: The mean and coefficient of variation (COV) were determined based on three specimens tested at same time.
3.1.2 Tensile strength retention
On the basis of the failure mode and stress-strain curves shown in Figure 5 and Figure 6, the nominal tensile strength of all tested GFRP bars can be determined with the peak stress. In order to show the impact of exposure period on the degradation of tensile strength of GFRP bars, the tensile strength retention η (%) was defined as the ratio of the nominal tensile strength of the conditioned specimen, fu, to that of unconditioned reference specimen, fu0, as given in Eq. (4).
Figure 7 plots the tensile strength retentions of all tested GFRP bars. The results given in Table 2 and Figure 7 showed that the obvious reductions of tensile strength were obtained within the exposure period for all conditioned bars tested here. Under the same condition, the strength losses of GFRP1 bars were slightly greater than those of GFRP2 bars. By comparison with the retentions got from the three aggressive solutions, it can be noticed that for both categories of GFRP bars the average reduction of tensile strength in AS was higher than those in SS and TWS. As shown in Figure 7, after 180 days of exposure, the average strength retentions of two categories of GFRP bars were 73.3%, 84.1% and 88.9% in AS, SS and TWS, respectively. As indicated by Eqs. (1)~(3), the strength losses of GFRP bars immersed in these solutions were expected to increase with increasing the exposure period. In particular, based on the facts that glass fibers are deteriorated by the moisture and alkaline environments and that the deterioration of vinylester matrices appears due to hydrolysis, plasticization and swelling, it is reasonable to get the result that at the room temperature the largest loss in the tensile strength of GFRP bars was recorded in the AS environment, which was in good agreement with several previous studies [7, 8, 11, 12].
Relationships between tensile strength retention and exposure period of GFRP bars tested in this study and previous researches
For the purpose of verifying the effectiveness of test data proposed here, several experimental results conducted on GFRP bars exposed to similar conditions in other literatures [8, 13, 14] were collected and also compared with our results in Figure 7. It should be noticed that all collected data were gained around 20°C in order to approximately meet the temperature condition adopted in this experiment. The GFRP bars studied in these works were all made of E-glass and vinylester and had the similar nominal diameters of 12 mm [14] and 12.7 mm [8, 13]. Besides, the tensile strengths of E-glass/vinylester composites pultruded in 1.6 mm thick strips of 1524 mm width reported by Chu et al.[11] were also chosen to compare the results after exposure to water. The comparison shown in Figure 7(a) indicated that there was a great discreteness in the tensile strength retention of GFRP bars after exposure to AS. Our investigations in AS fell in the middle of the results obtained on the G1 rods by Kim et al. [8] and reported by Won et al. [13], and they agreed well with the findings tested on the G2 rods with modified vinylester [8]. From the comparisons indicated in Figure 7(b) and (c), it can be concluded that the difference in strength degradation of GFRP bars after exposure to SS and TWS was not as great as that got from AS. The corresponding degradation trends of tensile strength proposed in this study were in good agreement with the results obtained by Kim et al. [8] and Chu et al. [11].
With the increasing of exposure period, all test results given in Figure 7 showed that the reduction in the tensile strength was fastest within the initial stage of 45 to 60 days of exposure in the three tested solutions. After that, the strength degradation became slow in the further exposure period. This phenomenon was consistent with the exponential degradation models which believe that the degradation rate of tensile strength of GFRP bars is high in the initial stage, and then decreases with increasing exposure time [15, 16, 24]. Their degradation mechanisms related to GFRP bars are assumed to be debonding at the interface between fibers and matrix and they had been successfully applied to fit the short-term experimental data of tensile strength retention of GFRP bars immersed in environmental solutions [16, 25].
3.1.3 Tensile elastic modulus
Based on the calculations of elastic modulus given in Table 2, Figure 8 displays the elastic modulus retentions of conditioned GFRP bars after 45, 90, 135 and 180 days of exposure. From Figure 8, It can be illustrated that the elastic modulus of conditioned GFRP bars mainly fluctuated around the initial modulus of unconditioned specimens. For conditioned GFRP1 bars, the elastic modulus retentions varied in the range of 90% to 110%; while the modulus retentions of conditioned GFRP2 bars were in a smaller scope between 95% and 105%. Silva et al. [26] pointed out that the elastic modulus of GFRP composites was mainly influenced by the environmental temperature. The test results reported by Najafabadi et al. [27] indicated that the elastic modulus of GFRP bars started to descend when the environmental temperature reached the glass transition temperature (Tg). As the Tg values of tested GFRP bars were slightly greater than 70°C (see Table 1), the impact of room temperature on the modulus of elasticity of GFRP bars in this study can be neglected and the error caused by manual measurement might be a possible reason that induced the slight change of bars’ elastic modulus [28].
Elastic modulus retention of conditioned GFRP bars after different exposure periods
As a general observation, it can be concluded that no obvious deterioration of elastic modulus had been found for tested GFRP bars after exposure to alkaline, saline and tap water solutions under room temperature. Similar observations were also found for GFRP bars after exposure to various environmental conditions [3, 14, 15, 24].
3.1.4 Degradation model of tensile strength
According to the results collected in Table 2, the degradation models of tensile strength for the conditioned GFRP bars in various environmental solutions were proposed based on Eq. (4) with curve fitting, as given in Eqs. (5)~(7).
where, t is the exposure period (d), which is less than 180 days in this study.
With the aim of verifying the validity of Eqs. (5)~(7), the ratios of predicted tensile strength to measured tensile strength obtained from this study and existing works [8, 11] were compared and plotted in Figure 9. It can be found that the obtained mean and coefficient of variation (COV) of these ratios are 1.07 and 8.1%, respectively. It can be proved that the proposed degradation models of tensile strength for the conditioned GFRP bars are reasonable and practical.
Relationship between predicted tensile strength and measured tensile strength of conditioned GFRP bars
3.2 Shear properties
3.2.1 Failure mode and load-displacement curves
In the short-beam shear test, the GFRP specimens showed bending deformation under the loading arbor during the initial loading phase. With an increasing applied load, some crackle caused by the debonding of interface between fibers and matrix could be heard. Then the specimen failed suddenly in a shear mode in which a principal horizontal crack developed along the mid-plane at the end of the bar, as shown in Figure 10. For GFRP2 bars with larger diameter than GFRP1 bars, two or more horizontal cracks appeared when the specimens failed (see Figure 10(b)). This shear failure in the short-beam test was also a mode of brittle fracture and would not be affected by the exposure environment and period conducted in this study. Similar phenomenon can be found from the previous research work in which the ratio of span to diameter of GFRP bars was 4 [7].
Typical failure mode of conditioned GFRP bars in the short-beam shear test
Figure 11 displays the typical load-displacement curves for GFRP2 bars with different exposure periods. It can be found from Figure 11 that for the unconditioned bar the horizontal crack started when the vertical displacement reached the value of around 2 mm where the load, that is breaking load, was maximum. For the conditioned ones, both of the breaking load and the corresponding deflection decreased with increasing the exposure periods, which was in accordance with the phenomenon that the horizontal cracks in these bars occurred earlier than that in the unconditioned one. Figure 11 also shows that the stiffness of tested bars began to degrade gradually after the horizontal cracks occurred and the degradation rate appeared in the conditioned bars was faster than that measured from the unconditioned one. The similar appearance was observed from the tested GFRP1 bars.
Load-displacement curves of GFRP2 bars in the short-beam shear tests
3.2.2 Shear strength
According to the standard of ASTM D4475-02 [23], the interlaminar shear strength, fv, of GFRP bars determined in the short-beam shear test can be expressed as:
where, P is the breaking load (N); and d is the nominal diameter of GFRP bar (mm).
The measured interlaminar shear strength of the conditioned GFRP bars are collected in Table 3. For the unconditioned reference GFRP1 specimens (d=11.2 mm), the obtained mean value and COV of interlaminar shear strength were 41.6 MPa and 4.7%, respectively. While a higher shear strength of 48.8 MPa with a COV of 4.0% was gained for the unconditioned reference GFRP2 specimens (d=15.6 mm), which indicated that the initial interlaminar shear strength of GFRP bars can be improved with increasing bar diameter. Using the above two initial interlaminar shear strengths, the retention of interlaminar shear strength, ηv, of conditioned GFRP bars was calculated and also listed in Table 3. After 12-day exposure to three aggressive solutions at room temperature, the shear strength of GFRP1 specimens presented a slight increase with 3%~15%, especially in TWS. While no increase in shear strength was observed for GFRP2 bars with a larger diameter. After that, a fast decline of shear strength retention was found for two kinds of GFRP bars within the exposure time of 36 days. Comparatively, during the exposure period of 36–72 days, the degradation of shear strength became slow and stable. By comparison with the effects of the three environmental solutions on the reduction in interlaminar shear strength, it can be observed that AS caused a greater decline of shear strength retention of tested GFRP bars, especially for the GFRP1 specimens with a smaller diameter. This finding is similar to the outcome of tensile strength retention determined on the same GFRP bars exposed to the same solution environments. Besides, the diameter of GFRP bars also has obvious influence on the shear strength retention. Except the retention measured at 12 days of exposure, the shear strength retention of GFRP2 specimens was higher than that of GFRP1 bars after the same exposure time to aggressive solutions, which indicates that the GFRP bars with larger diameter have better resistance to the degradation of interlaminar shear strength under these solution environments.
Interlaminar shear strength of GFRP bars got from this study and other researches.
| Environmental solution | Period (Days) | Bar type | Shear strengtha | Retention ηv (%) | ||||
|---|---|---|---|---|---|---|---|---|
| Mean (MPa) | COV (%) | This study | Chen et al. [7] | Kim et al. [8] | Fergani et al. [9] | |||
| Alkaline solution (AS) | 12 | GFRP1 (d=11.2mm) | 42.9 | 3.6 | 103.0 | |||
| GFRP2 (d=15.6mm) | 45.6 | 4.6 | 93.3 | |||||
| 36 | GFRP1 (d=11.2mm) | 27.7 | 9.3 | 66.7 | ||||
| GFRP2 (d=15.6mm) | 39.3 | 4.0 | 80.4 | |||||
| 45 | GFRP1 (d=9.53mm) | 92c | ||||||
| GFRP2 (d=9.53mm) | 79c | |||||||
| 60 | G1 (d=12.7mm) | 83.2b | ||||||
| G2 (d=12.7mm) | 76.1b | |||||||
| 72 | GFRP1 (d=11.2mm) | 27.4 | 15.8 | 65.9 | ||||
| GFRP2 (d=15.6mm) | 38.5 | 12.1 | 78.8 | |||||
| 270 | GFRP (d=8mm) | 95/93/74e | ||||||
| Saline solution (SS) | 12 | GFRP1 (d=11.2mm) | 45.0 | 0.7 | 108.2 | |||
| GFRP2 (d=15.6mm) | 47.0 | 1.1 | 96.3 | |||||
| 36 | GFRP1 (d=11.2mm) | 34.0 | 3.0 | 81.7 | ||||
| GFRP2 (d=15.6mm) | 45.7 | 6.3 | 93.6 | |||||
| 45 | GFRP2 (d=9.53mm) | 90c | ||||||
| 72 | GFRP1 (d=11.2mm) | 34.1 | 5.4 | 81.8 | ||||
| GFRP2 (d=15.6mm) | 44.1 | 1.9 | 90.4 | |||||
| 90 | G1 (d=12.7mm) | 87.5d | ||||||
| G2 (d=12.7mm) | 79.9d | |||||||
| Tap water solution (TWS) | 12 | GFRP1 (d=11.2mm) | 47.9 | 3.8 | 115.1 | |||
| GFRP2 (d=15.6mm) | 46.7 | 8.6 | 95.5 | |||||
| 36 | GFRP1 (d=11.2mm) | 37.0 | 0.8 | 89.1 | ||||
| GFRP2 (d=15.6mm) | 40.6 | 9.0 | 83.2 | |||||
| 45 | GFRP1 (d=9.53mm) | 96c | ||||||
| GFRP2 (d=9.53mm) | 90c | |||||||
| 72 | GFRP1 (d=11.2mm) | 34.8 | 3.5 | 83.7 | ||||
| GFRP2 (d=15.6mm) | 41.2 | 7.9 | 84.3 | |||||
| 90 | G1 (d=12.7mm) | 89.4d | ||||||
| G2 (d=12.7mm) | 76.6d | |||||||
| 270 | GFRP (d=8mm) | 90/86/75e | ||||||
Note:
- a
The mean and coefficient of variation (COV) were determined based on three specimens tested at same time.
- b
The data tested at 40°C.
- c
The data tested at 60°C.
- d
The data tested at 80°C and
- e
the data tested at 20°C, 40°C, 60°C, respectively.
Table 3 also collects some experimental results related to the retention of interlaminar shear strength of GFRP bars recorded in other researches [7,8,9]. It should be noticed that there are some differences in the physical property of GFRP bars and aggressive environment adopted in these comparative researches. In Chen et al.'s study [7], the fiber volumes of two types of GFRP bars (E-glass fibers and vinylester resin) were not stated and the measurements obtained from the alkaline solution (Solution 2) with a pH value of about 13.6 and tap water solution were selected. The datum collected from Kim et al. [8] were tested on G1 (E-glass/vinylester) and G2 (E-glass/modified vinylester) rods with fiber volumes of 73.3% and 69.1%, respectively. Some test results reported by Fergani et al. [9] were also compared here, which were conducted on the unstressed GFRP bars (E-CR glass/ninylester) with fiber volume of 88% by weight. By comparison with the retentions given in Table 3, it can be concluded that the degradation trend of interlaminar shear strength of GFRP bars tested in this study and the literatures [7, 8] was quite similar and AS environment had greater influence on the strength decline, although the material composition and exposure temperature existed some differences. Furthermore, it can be confirmed that these solution environments can cause definite impact on the shear strength of GFRP bars made of E-glass fibers and vinylester resin. While the GFRP bars using E-CR glass fibers had much higher resistance to strength degradation in the alkaline and moisture environments with the exposure temperature less than 40°C [9]. Besides, the test datum proposed by Fergani et al. [9] indicated that the glass transition temperature (Tg) of GFRP samples would reduce to around 90% of original Tg value of 164°C after conditioned in AS and TWS at 60°C for 270 days. As a result, a significant reduction of strength retention had been observed due to the change of glass transition temperature at elevated solution temperature of 60°C. However, in this study, the solution temperature was kept at around room temperature (nearly 20°C). It had been testified from Fergani et al. [9] the Tg of GFRP bars would be hardly influenced by the solution immersion at 20°C. Therefore, the effect of the change in Tg on the strength degradation of GFRP specimens studied here can be neglected and the main reason for this degradation might be attributed to the chemical attack caused by the aggressive solutions to fibers, matrix and their interface [7,8,9,10].
4 Conclusions
The experimental investigation on the mechanical and durability properties of GFRP bars was carried out after exposure to the alkaline, saline and tap water solutions at room temperature and the following findings can be made from this work:
During the tensile testing, all unconditioned and conditioned GFRP bars presented a similarly linear elastic behavior and failed in a same mode with fiber rupture without taking the immersion solution and exposure time into account. After exposure of 180 days, the impact of solution environments on the degradation of tensile properties of GFRP bars lied in the tensile strength and ultimate strain at failure, and no obvious deterioration of elastic modulus had been found.
A comparison of the tensile results got from our work and several previous studies showed a similar degradation trend in the tensile strength retention of GFRP bars exposed to the approximate aggressive solutions at room temperature and indicated that the alkaline solution (AS) had the greater impact on the strength loss than another two solutions. The reason for this outcome might be ascribed to the coupled degradation actions of E-glass fibers and vinylester matrices in alkaline environment because of hydrolysis, plasticization and swelling.
In the short-beam shear test, the tested GFRP specimens showed a similar shear failure mode in which one or two principal horizontal cracks developed along the mid-plane at two end sections regardless of the exposure environment and period. The typical load-displacement curves indicated that both of the breaking load and the corresponding deflection decreased with an increasing exposure period, especially in alkaline solution.
The obtained shear strength retentions of all conditioned GFRP specimens indicated that the change of interlaminar shear strength could be divided into three stages within the total exposure period of 72 days. A fast decline was found for two types of GFRP bars between the exposure periods of 12 and 36 days and the degradation of shear strength became slow and stable during the exposure period of 36–72 days. Under the same exposure conditions, the GFRP bars with a larger diameter of 15.6 mm had a better resistance to the degradation of shear strength than the 11.2mm-diameter GFRP bars, and the similar consequence was also found in the degradation of tensile strength.
With the rapid development of manufacturing technique for GFRP bars, there exists various differences in long-term mechanical performance between the various types of GFRP bars manufactured by different corporations. The test results have shown a certain discreteness in the strength decline (tensile strength and interlaminar shear strength) between our work and these comparative studies, which was mainly ascribed to the differences in the material composition (the matrix and the glass fiber) and exposure environment conditions, especially the solution temperature. It is believed that more investigations on the mechanical and durability properties of GFRP bars are required to enrich the relevant achievements.
Acknowledgement
This research was financially supported by the National Natural Science Foundation of China (Grant Nos. 51878319 and 51578267).
Data availability statement: All data presented in this study are available upon request.
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© 2021 Chunhua Lu et al., published by De Gruyter
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