Accelerated ageing test and durability prediction model for mechanical properties of GFRP connectors in precast concrete sandwich panels

Construction and Building Materials 237 (2020) 117632

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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat

Accelerated ageing test and durability prediction model for mechanical properties of GFRP connectors in precast concrete sandwich panels Weichen Xue a,⇑, Ya Li a, Kai Fu a, Xiang Hu a, Yan Li b a b

Department of Structural Engineering, Tongji University, Shanghai 200092, China School of Aerospace Engineering and Applied Mechanics, Tongji University, Shanghai 200092, China

h i g h l i g h t s
Durability of GFRP connectors in alkaline solution at elevated temperatures was tested.
SEM micrographs confirmed degradation in fibre/matrix interfacial bonding after conditioning.
Ageing conditions degraded tensile and compressive properties of GFRP connectors.
A prediction model for tensile and compressive strength of GFRP connectors was proposed based on Arrhenius equation.

a r t i c l e

i n f o

Article history: Received 12 July 2018 Received in revised form 30 September 2019 Accepted 16 November 2019

Keywords: Precast concrete sandwich panel GFRP connector Durability Tensile property Compressive property Arrhenius equation Prediction model

a b s t r a c t Glass fiber-reinforced polymer (GFRP) connectors are the key components in precast concrete sandwich panels, because they not only provide continuity between two concrete wythes but also promote thermal efficiency of the panels. This paper presents an experimental study on durability of GFRP connectors in alkaline solution (simulated concrete environment) with variable parameters of temperature (40, 60, and 80 °C) and conditioning time (3.65, 18, 36.5, 92 and 183 days). A total of 160 specimens were immersed in the alkaline solution with a composition specified in ACI 440.3R-12 at elevated temperatures. Scanning electron microscope (SEM) was used to observe the degradation mechanism of the specimens, and the phenomenon of debonding between fibers and resin could be observed more obvious with the increase of erosion time and temperature. The test results also showed that, after 183 days of immersion at 60 °C, tensile strength and modulus of elasticity of the specimens decreased about 38.85% and 21.45% respectively, while compressive strength and modulus of elasticity of the specimens decreased about 40.18% and 27.62%. Finally, a prediction model of tensile and compressive strength after conditioning over time at varied temperatures was proposed based on Arrhenius equation. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction Precast concrete sandwich panel (PCSP) is a significant development for energy efficiency, environmental protection and life-cycle economy in precast concrete structures. It is produced in factory and assembled in the construction site, which could obviously reduce on-site construction period. Since its appearance in the 1960s, PCSPs have been widely used in residual, commercial and industrial buildings in north America and Europe [1,2]. As one of

⇑ Corresponding author at: Department of Structural Engineering, Tongji University, Siping Road 1239, Shanghai 200092, China. E-mail addresses: [email protected] (W. Xue), [email protected] (Y. Li), [email protected] (K. Fu), [email protected] (X. Hu), [email protected] (Y. Li). https://doi.org/10.1016/j.conbuildmat.2019.117632 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

the most appropriate panels for exterior wall systems, PCSPs have been booming in China since 2010. A typical PCSP consists of interior wythe, exterior wythe, insulation layer and connectors, as shown in Fig. 1. It is well known that the connectors, installed to penetrate the insulation layer and join the two concrete wythes, are the key components of PCSP because they are used to bear the inner force in terms of tension, compression and shear between the interior and exterior wythes. As a result, the safety of PCSP during the expected design life mainly depends on the mechanical properties and durability of connectors. Besides, the thermal efficiency of PCSPs is significantly influenced by thermal conductivity of connectors. According to literature review, it could be seen that solid concrete zones between the two wythes were used as connectors to provide continuity at the beginning [2]. PCSPs with solid concrete region usually had high strength and construction efficiency but sacrificed the

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Exterior Concrete wythe Interior Concrete wythe Connector Insulation layer

Fig. 1. A typical precast concrete sandwich panel.

potential thermal savings. Considerable bowing was also observed owing to high stiffness, followed by cracking in concrete wythes. Solid concrete regions were eliminated from many panels and were replaced by stainless steel ties and fiber-reinforced polymer (particularly glass fiber-reinforced polymer, GFRP) connectors in order to reduce thermal bridging and bowing in the 1980’s [2–4]. Compared to stainless steel connectors, GFRP connectors have high strength-weight ratio and excellent thermal efficiency. It has about 1/150 of the thermal conductivity of steel and 1/40 of that of concrete [4]. Conventionally, GFRP connectors are made of high strength, alkali-free glass fibers impregnated in vinyl ester resin or epoxy resin using the pultrusion process [5]. Three types of GFRP connectors are widely used in global construction projects, namely, plate-shaped, rod-shaped and truss-shaped GFRP connectors (Fig. 2). GFRP connectors have been used for more than 30 years in North America [3]. For now, more than 10 million GFRP connectors have been applied in PCSPs with an area larger than 2 million square meters in China. Over the last three decades, a number of studies have been carried out to investigate the pull-out strength and shear capacity of GFRP connectors as well as mechanical behavior of PCSPs with GFRP connectors [6–11]. Although the GFRP connectors are considered to be desirable in PCSPs because of good mechanical behavior, concern still remains about their durability. GFRP connectors are anchored in concrete; this condition was found to be aggressive for GFRP, due to the high pH level of the pore water solutions and presence of alkaline ions. The internal concrete environment has high alkalinity and moisture with a pH between 10.5 and 13.5 [12,13]. Hence the durability of GFRP connectors under alkaline conditions (simulated concrete environment) has a great influence on their mechanical properties, thus need to be investigated. Up to now, considerable experimental studies have been devoted to investigate durability of GFRP materials, mainly focusing on GFRP rods and GFRP laminates. An important issue

for the ageing tests was simulating the alkaline environment of concrete. Katsuki and Uomoto [14] used 1.0 mol/L NaOH solution with pH of 13 to simulate concrete environment at 1995. Micelli et al. and Aboelseoud et al. [15,16] made a simulated concrete pore solution by dissolving 0.16% Ca(OH)2, 1% NaOH and 1.4% KOH by weight into distilled water (pH = 12.6 ~ 13). At 2006, Chen et al. [17] mixed NaOH, KOH, and Ca(OH)2 with different proportions and obtained two types of solutions with pH values of 13.6 and 12.7, respectively, to simulate the pore solutions of normal concrete and high-performance concrete. The composition of simulated pore-water solution varies widely in the above-mentioned researches, in consequence it is hard to compare results obtained from different tests. Although no consensus was reached on using a particular solution to simulate the alkaline environment of concrete, a simulated pore-water solution suggested by ACI 440.3R12 [18], in which 118.5 g of Ca(OH)2, 0.9 g of NaOH, and 4.2 g of KOH were mixed in per liter of deionized water, was adopted by most researchers. Recently lots of tests were conducted according to ACI 440.3R-12. For instance, Benmokrane et al. [19,20] exposed GFRP bars to this alkaline solution at 22 and 60 . After 140 days exposure at 22 , a reduction of up to 17% in tensile strength was recorded for GFRP bars with stress levels equal to 30% of the ultimate tensile strength. The reduction ratios for the flexural and short-beam shear strength were 7% and 5% after 5000 h (208.3 days) exposure at 60 , respectively. Wu et al. [21] immersed GFRP laminates in this solution for 66 days at 55 at 2014. The tensile strength decreased by 29.3%, while no reduction in tensile modulus of elasticity was recorded. Generally, most of the tests focused on tensile strength while relatively few investigations of the compressive strength and elastic modulus have been conducted. In addition, existing experiments continued for a short time or were conducted at low temperatures. Note that there is no experimental study on durability of GFRP connectors by now. Following the accelerated tests, theoretical analyses have been carried out on the short-term data to predict durability of GFRP materials in alkali environment. Most prediction models were established based on Arrhenius equation or Fick’ Law, among which Arrhenius equation was often used to develop prediction models by regression analysis of test results at varied temperatures. The application of Arrhenius equation to civil engineering materials was first described by Litherland et al. at 1981 [22]. Following the procedure used by Litherland et al., prediction models were presented by Bank et al. [23] and Dejke et al. [24], in which the property retention is a linear function of the logarithm of exposure time. It is the most widely used degradation model for FRP materials. The other model based on Arrhenius equation was developed by Chen et al. [17], in which the property retention is an exponential function of the negative of exposure time. In general, regression constants derived from a specific test may not be applicable to GFRP composites immersed in simulated porewater solution with different composition. To promote the universality of prediction models, it is a good selection to conduct ageing tests following standard test method recommended by ACI 440.3R12. Moreover, regression models in the above-mentioned models

Fig. 2. GFRP connectors in PCSPs (a) plate-shaped GFRP connector (b) rod-shaped GFRP connector (c) truss-shaped shaped GFRP connector.

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were derived from test results of GFRP rods and laminates, hence they may not be applicable to GFRP connectors. The ICC Evaluation Service (ICC-ES) promulgated Acceptance Criteria AC320 [25] at 2006, which is the only standard providing specifications for durability of GFRP connectors. In AC320-06, limits on minimum tensile strength retention values after conditioning are stipulated. The minimum tensile strength retentions of 85% and 90% are accepted after 1000 h (41.67d) and 3000 h (125d) exposure to alkaline solution (pH = 12) at 23 . However, no minimum retentions of compressive and shear strength are specified. Besides, the composition and pH value of alkaline solution suggested in the criteria is inconsistent with the recommended solution in ACI 440.3R-12. In addition, the ageing tests are not accelerated by elevated temperature and the exposure time is too short compared with the design life of structures (at least 50 years), thus it is hard to predict long-term performance of GFRP connectors during the design life. From the above literature review, one can conclude as follows:
Previous studies on durability of GFRP composites mainly focused on GFRP rods and GFRP laminates. GFRP connectors are different with rods and laminates in volume fraction of each ingredient and manufacturing process, therefore concern remains about their durability in alkaline solution. Besides, most tests focused on tensile strength rather than compressive strength and elastic modulus, and usually continued for a short time or was conducted at low temperatures.
Prediction models based on Arrhenius equation have been used to predict time-dependent properties of GFRP composites at varied temperatures. However, the application of the regression equations was limited due to simulated pore-water solution inconsistent with ACI 440.3–12 in aging tests. Analogously, regression constants obtained from the test results of GFRP rods and laminates may not be applicable to GFRP connectors.
Limits on minimum tensile strength retentions after 1000 h and 3000 h of exposure at 23 are stipulated in AC320-06. However, the ageing tests are not accelerated by elevated temperature and the exposure time is too short compared with the design life of PCSPs and connectors (at least 50 years), consequently it is hard to predict long-term performance of GFRP connectors during the design life. Since 2007, our research group has conducted a series of investigations on mechanical properties of a kind of plate-typed GFRP connector which has been widely used in China [26]. Based on results of these researches, we carried out accelerated ageing tests on this kind of connector in accordance with ACI 440.3R-12, mainly focusing on degradation of mechanical properties with increase of temperature and conditioning time. Considering the length limitation, parts of the investigations are presented in this paper, including the degradation of tensile strength, tensile modulus of elasticity, compressive strength and compressive modulus of elasticity. In addition, scanning electron microscope (SEM) was used to investigate the degradation mechanism of the specimens. Finally, a prediction model of durability of GFRP connectors at varied temperatures was proposed based on Arrhenius equation.

Table 1 Volume fraction of each ingredient in the GFRP connector. Glass fiber

Vinyl ester resin

Hardeners and mold release agent

75%

22%

3%

The TM-glass fiber, which was provided by Chongqing Polycomp International Corp., is a kind of free-boron glass fiber. Compared to conventional E-glass fiber, TM-glass fiber has higher tensile strength, tensile modulus, alkaline resistance and acid resistance, which was shown in Table 1. The volume fraction of each ingredient is showed in Table 1. Tensile properties and compressive properties before and after corrosion were tested to evaluate durability of the connectors. Mechanical properties of specimens before erosion are summarized in Table 2. Furthermore, an investigation through scanning electron microscopy (SEM) was conducted to evaluate the effects of alkali on the fibers and matrix. Tensile specimens were dumb-bell-shaped types, as specified in ISO 5272:2012 [27], which is a widely accepted standard test method for the tensile strength of composite materials, while compressive specimens were made according to ISO 604:2002 [28]. 2.2. Environmental conditions and testing methods Accelerated aging tests were conducted in accordance with ACI 440.3R-12 [18]. The conditioning of GFRP specimens included the combined exposure to a harsh alkaline environment and elevated temperature. Three elevated temperatures were selected, among which 60 °C was the accelerated aging temperature specified by ACI 440.3R-12, while 40 °C and 80 °C were set to research the impact of different temperatures on durability of GFRP connectors. All the three temperatures were well below the glass transition temperature of selected GFRP connectors. Exposure periods of 3.65, 18, 36.5, 92 and 183 days were considered, among which 183 days (6 months) was the longest exposure period specified by ACI 440.3R-12. In all cases 5 specimens were tested at each time period and each temperature to allow for statistical valid samples. A total of 160 specimens were tested in this paper. Test parameters of specimens are summarized in Table 3. The alkaline solution in this study consisted of 118.5 g of Ca (OH)2, 4.2 g of KOH and 0.9 g of NaOH in 1 L of deionized water, as recommended by ACI 440.3R-12. This solution provided a high pH value of 12.6 to 13, which is a representative pH value of mature concrete pore solution. During the immersion, the pH value of the alkaline solution was monitored regularly and adjusted to keep the constant constituents and pH value. Tensile tests were conducted to measure tensile strength and tensile modulus of elasticity of the specimens according to ISO 5272:2012 [27]. An electronic universal testing machine was used to conduct tensile strength tests, as shown in Fig. 3(a). Compressive tests were conducted to measure compressive strength and compressive modulus of elasticity of the specimens in accordance with ISO 604:2002 [28] on a hydraulic testing machine, see Fig. 3(b). Furthermore, observation and image analysis were performed by scanning electron microscopy (SEM), as shown in Fig. 4, to detect changes in microstructure between the specimens before and after conditioning.

2. Experimental program 3. Results and discussion 2.1. Materials and test specimens 3.1. Observation and image analysis GFRP connector specimens examined in this study were commercial plate-shaped GFRP connectors manufactured by Nanjing Spare Composites Co, Ltd, as shown in Fig. 2(a). The connectors were made of unidirectional continuous TM-glass fibers impregnated in a vinyl ester resin matrix using the pultrusion process.

3.1.1. Surface condition Fig. 5 illustrates the surface condition of the GFRP specimens before and after being exposed to alkaline solution. The changes in surface condition were as follows:

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Table 2 Mechanical properties of the GFRP connector. Tensile strength (MPa)

Tensile modulus of elasticity (GPa)

Compressive strength (MPa)

Compressive modulus of elasticity (GPa)

875.01

58.93

429.10

85.82

(1) The specimens had smooth surface before exposure, while small white blisters and pits were observed in consequence of hydrolysis of resin after exposure. (2) With the temperature increased, corrosion pits on the surface became more obvious. No visible pits were found after 18 days of exposure at 40 and 60 , while bulges began to appear on the surface at 80 . (3) The surfaces conditions of the specimens changed over time. After 36.5 days of exposure to alkaline solution, the specimens tarnished to some degree. The color turned from translucent white to opaque milky white and slightly yellowed. As for GFRP specimens immersed in alkaline solution for 183 days at 80 , considerable bulges were observed and hairline cracks appeared subsequently.

Fig. 4. Scanning electron microscopy.

surrounding resin. Many gaps were observed between fibers and at fiber-matrix interfaces which affected the bond between glass fibers and vinyl ester resin. Degradation of the GFRP connector gradually increased with time and temperature. For instance, there was no visible debonding between the fibers and the vinyl ester resin after 18 days exposure in alkaline solutions at 40 °C. However, specimens at 80 showed more voids in the matrix after 18 days. Significant damage to the matrix and fiber-matrix interfaces was observed after 183 days of immersion. GFRP connectors in alkaline solution were totally damaged at 80 for 183 days so that it was hard to made samples for SEM observation from the conditioned specimens.

3.1.2. Microstructural analysis SEM observations were performed to investigate microstructural changes in the GFRP connectors before and after immersion. Fig. 6 shows the SEM micrographs of the reference and conditioned specimens at the 1000 magnification. SEM analysis of the reference specimens indicated that internal structure of GFRP connectors was compact before corrosion. Fibers and resin exhibited a strong bond. For conditioned specimens, there was a distinct increase in the number and size of voids, which developed progressively with exposure time. It can be clearly seen that while there was almost no deterioration in the glass fibers, the matrix around the glass fibers in specimens were significantly deteriorated. Exposure of specimens to alkaline solution resulted in separation of fiber surfaces from the

3.2. Tensile strength and modulus of elasticity During the tensile test, GFRP connector specimens showed an approximately linear behaviour up to failure and failed by the rupture of fibers. The tensile test results of specimens at all three

Table 3 Experimental parameters of specimens. Number of specimens Temperature (°C)

Before exposure

After 3.65 days after exposure

After 18 days after exposure

After 36.5 days after exposure

After 92 days after exposure

After 183 days after exposure

40 60 80

10

10 10 10

10 10 10

10 10 10

10 10 10

10 10 10

Fig. 3. Setup for (a) tensile testing and (b) compressive testing.

W. Xue et al. / Construction and Building Materials 237 (2020) 117632

5

Fig. 5. Surface condition of the specimens before and after exposure (a) before exposure (b) after 3.65 days of exposure (c) after 18 days of exposure (d) after 36.5 days of exposure (e) after 92 days of exposure (f) after 183 days of exposure.

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(a)

(b)

(c)

(d)

(e)

(f) Fig. 6. Micrographs of the specimen sections before and after exposure (a) before exposure (b) after 3.65 days of exposure (c) after 18 days of exposure (d) after 36.5 days of exposure (e) after 92 days of exposure (f) after 183 days of exposure.

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Residual tensile strength (MPa)

(1) The tensile strength of GFRP connector specimens decreased with an increase in ageing days at all three temperatures. For example, GFRP specimens immersed in alkaline solutions at 60 showed a decrease in tensile strength of about 8.69%, 19.42%, 26.61%, 32.91% and 38.85% after 3.65, 18, 36.5, 92 and 183 days, respectively. The degradation rate (slope of curves) decreased quickly as the exposure time increased. A likely explanation for this phenomenon is decrease of diffusion velocity resulting from accumulation of by-products of chemical reactions. (2) The degradation of the tensile strength of the specimens exposed to higher temperature was severer. After 183 days of exposure to alkaline solution at 40 , specimens experienced only 28.90% reduction in the tensile strength. A reduction which was nearly twice of former was observed at 60 . The tensile strength of GFRP specimens decreased by 53.11% after 92 days immersion in alkaline solution at 80 . The specimens were totally damaged after 183 days of exposure at 80 so that the tensile strength was undetectable. These test results indicate that higher temperatures can accelerate the degradation due to fast hydrolysis reaction of resin in GFRP connector.

40ºC 60ºC 80ºC

1000 800 600 400 200 0 0

50

100

150

200

Exposure time (days)

(3) The tensile modulus of elasticity also decreased over time. Average residual modulus of 95.52%, 86.37%, 85.49%, 79.98% and 78.55% were recorded after 3.65, 18, 36.5, 92 and 183 days of exposure to alkaline solution at 60 , respectively. Like the phenomenon observed in degradation of tensile strength, the degradation rate of tensile modulus of elasticity decreased quickly over time. (4) The tensile modulus of elasticity of specimens was affected by conditioning in an alkaline solution less than the tensile strength. The residual tensile modulus of specimens after 183 days of immersion at 40 °C and 60 °C were 84.93% and 78.46%, respectively. This observation indicates that the localized degradation does not lead to a remarkable change in the stress–strain relationship, which is also concluded by Wu et al. [21]. 3.3. Compressive strength and modulus of elasticity Fig. 8 shows the residual compressive strength and modulus of elasticity of the tested FRP connector specimens after 3.65, 18, 36.5, 92 and 183 days of immersion in the alkaline solution at 40 °C, 60 °C and 80 . Test results indicate the degradation characteristics of compressive properties as follows: (1) The compressive strength of GFRP connector specimens decreased and the rate slowed down over time. For instance,

Residual tensile modulus of elasticity (MPa)

temperatures are summarized in Fig. 7. Note that each data in the figure represents an average of five test results. Based on test results, the following characteristics of degradation can be drawn:

40ºC 60ºC 80ºC

70 60 50 40 30 20 10 0 0

50

100

150

200

Exposure time (days)

(a)

(b)

40ºC 60ºC 80ºC

500 400 300 200 100 0 0

50

100

150

Exposure time (days)

(a)

200

Residual compressive modulus of elasticity (MPa)

Residual compressive strength (MPa)

Fig. 7. Residual tensile properties after exposure (a) residual tensile strength (b) residual tensile modulus of elasticity.

40ºC 60ºC 80ºC

100 80 60 40 20 0 0

50

100

150

200

Exposure time (days)

(b)

Fig. 8. Residual compressive properties after exposure (a) residual compressive strength (b) residual compressive modulus of elasticity.

Test-40ºC Calculation-40ºC Test-60ºC Calculation-60ºC Test-80ºC Calculation-80ºC

100

80

60

40

20

0 0

50

100

150

200

Exposure time (days)

Degradation of compressive strength (%)

W. Xue et al. / Construction and Building Materials 237 (2020) 117632

Degradation of tensile strength (%)

8

Test-40ºC Calculation-40ºC Test-60ºC Calculation-60ºC Test-80ºC Calculation-80ºC

100

80

60

40

20

0 0

50

100

150

200

Exposure time (days)

(a)

(b)

Fig. 9. Experimental and calculated degradation rate of tensile strength and compressive strength (a) degradation of tensile strength (b) degradation of compressive strength.

the specimens showed a decrease in compressive strength at 60 °C by 15.75%, 22.65%, 28.92%, 34.17% and 40.18% after 3.65, 18, 36.5, 92 and 183 days of exposure, respectively. As the erosion time increases, hydrolysis reaction in resin results in the degradation. (2) Reductions of 30.32% and 40.18% in compressive strength of specimens were observed after 183 days of exposure to alkaline solution at 40 °C and 60 °C. Meanwhile, the higher temperature accelerated the degradation of compressive properties due to acceleration of hydrolysis reaction in resin. (3) The compressive modulus of elasticity also decreased over time. Average reduction of 3.12%, 17.06%, 17.39%, 22.86%, 25.31% and 27.62% were recorded after 3.65, 18, 36.5, 92 and 183 days of exposure to alkaline solution at 60 °C, respectively. The reduction values were obviously lower than those in compressive strength. (4) The compressive modulus of GFRP specimens decreased by 27.62% after 183 days of immersion at 60 , while it only decreased by 18.19% at 40 . That means degradation of compressive modulus also increases as temperature increases.

4. Prediction model for mechanical properties of GFRP connectors

Ea

8 40  C > < 4:96lnðtÞ Dt ¼ 7:15lnðtÞ 60  C > : 11:25lnðtÞ 80  C

ð2Þ

8 40  C > < 6:15lnðtÞ Dc ¼ 8:01lnðtÞ 60  C > : 11:60lnðtÞ 80  C

ð3Þ

where Dt = degradation rate of tensile strength (%); Dc = degradation rate of compressive strength (%); t = exposure time (days). Based on Arrhenius equation, the temperature could be introduced to the prediction model, thus equations (2) and (3) can be summarized as follows:

(

Arrhenius equation states that a chemical process is accelerated as an exponential function of temperature. The primary assumptions of this model are that the single dominant degradation mechanism of FRP material will not change with time and temperature during the exposure, but the rate of degradation will be accelerated with the increase of temperature. And the degradation can be accelerated by immersing FRP materials to elevated temperature and measuring the change in tensile and compressive strength values as a function of corrosion time. The rate of tensile and compressive strength losses at each temperature can be estimated by a regression equation. In Arrhenius equation, the degradation rate of GFRP connectors over temperature is expressed as follows [29]:

k ¼ A  eð RT Þ

According to the procedure proposed by Bank et al. [23], the logarithm of exposure time versus property retention. Based on this relationship, which has been accepted by lots of investigators, the degradation of tensile strength and compressive strength of GFRP connector in different temperature over time were fitted, respectively, and the degradation model can be expressed as:

ð1Þ

where k = degradation rate (1/time); A = constant related to material and degradation process; Ea = activation energy; R = universal gas constant; and T = Kelvin temperature.

D¼

2256:65 Dt ¼ 6543:0  lnðtÞ  eð T Þ 1746:4 D ¼ 1588:8  lnðtÞ  eð T Þ

ð4Þ

c

where D = degradation rate (%) and T = Kelvin temperature (K). This model can predict degradation rate of tensile strength and compressive strength considering the influence of conditioning time and elevated temperature. There is no accelerated ageing test on GFRP connectors except tests presented in this paper, therefore the calculated values are compared with the above-mentioned experimental results. Fig. 9 shows the comparison of experimental and calculated degradation rate of tensile strength and compressive strength over time. As listed in Table 4 and Table 5, the average value of the ratio of the calculated values to the experimental result is 1.05 and 0.95 for tensile strength and compressive strength, respectively. Furthermore, the standard deviation is less than 0.16. Overall, the calculated strength retentions have a good agreement with experimental results, which shows that equations (4) is successfully applied to predict time-dependent mechanical properties of GFRP connector at varied temperatures with a reasonable accuracy.

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W. Xue et al. / Construction and Building Materials 237 (2020) 117632 Table 4 Comparison of calculated tensile strength degradations and experimental results. Temperature (°C)

Exposure time (days)

Experimental degradation rate Dtest (%)

Calculated degradation rate Dcal (%)

Dcal/Dtest

40

3.65d 18d 36.5d 92d 183d

5.86 12.47 15.20 23.19 26.39

6.37 14.23 17.70 22.25 25.64

1.09 1.14 1.16 0.96 0.97

60

3.65d 18d 36.5d 92d 183d

8.69 19.42 26.61 32.91 38.85

9.81 21.89 27.25 34.25 39.46

1.13 1.13 1.02 1.04 1.02

80

3.65d 18d 36.5d 92d

17.59 28.261 40.41 44.99

14.37 32.09 39.94 50.20

0.82 1.14 0.99 1.12

Average

1.05

Standard deviation

0.10

Table 5 Comparison of calculated compressive strength degradations and experimental results. Temperature (°C)

Exposure time (days)

Experimental degradation rate Dtest (%)

Calculated degradation rate Dcal (%)

Dcal/Dtest

40

3.65d 18d 36.5d 92d 183d

10.18 18.15 24.26 27.27 30.32

7.78 17.38 21.63 27.19 31.32

0.76 0.96 0.89 1.00 1.03

60

3.65d 18d 36.5d 92d 183d

15.75 22.65 28.92 34.17 40.18

10.88 24.29 30.23 38.00 43.78

0.69 1.07 1.05 1.11 1.09

80

3.65d 18d 36.5d 92d

24.31 28.65 41.71 52.37

14.64 32.68 40.68 51.13

0.60 1.14 0.98 0.98

Average

0.95

Standard deviation

0.16

Table 6 Comparison of experimental or calculated tensile strength retention and limited minimum value. Codes or guidelines

Temperature (°C)

Exposure time (days)

Limited minimum tensile strength retention RS (%)

Experimental or calculated tensile strength retention RE (%)

RE/ RS

ACI 440.2R-02 AC320-06

60 23 23

183 41.67 125

50 90 85

61.15 87.84 84.26

1.22 0.98 0.99

5. Discussion on requirements of durability in ACI 440 and AC320 The durability of GFRP connectors can be tested following the stipulations in ACI 440.3R-12 or AC320-06. This paper conducted accelerated ageing tests in accordance with ACI 440.3R-12 [18]. After 6 months (183 days) of exposure, which is the longest exposure period specified by ACI 440.3R-12, the tensile strength of the specimens decreased by 38.85%. Obviously, the experimental result satisfies the environmental-reduction factor of 0.5 (Table 6), which is stipulated by ACI 440 to determine the design ultimate tensile strength of FRP composites [30]. As for AC320-06 [25], minimum tensile strength retentions after 1000 h and 3000 h exposure to alkaline solution (pH = 12) at 23 were stipulated. On account of a lack in test results at 23 , the tensile retentions

of GFRP connectors were calculated based on the prediction model proposed in this paper. The calculated retentions are a little bit less than the stipulated minimum values in AC320-06, as shown in Table 6. The calculated retentions can be considered to basically satisfy the limited minimum values stipulated by AC320-06 owing to the slightly lower pH value of alkaline solution suggested in it. In summary, the GFRP connectors tested in this paper could satisfy the requirements of durability stipulated by ACI 440 and AC320-06. It should be noted that the direct correspondence between requirements of durability of in these two standards need further study owing to different composition of simulated porewater solution. Considering the short ageing period stipulated by AC320-06, test method and corresponding requirements recommended by ACI 440 is preferred.

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6. Conclusions According to the test method specified in ACI 440.3R-12, which is the most common test method for durability of GFRP composites in civil engineering, a total of 160 specimens were immersed in alkaline solution at 40 °C, 60 °C or 80 °C for 3.65, 18, 36.5, 92 or 183 days to investigate degradation of tensile and compressive properties of GFRP connectors with increase of temperature and conditioning time. Based on the results of this study, the followings could be concluded: (1) SEM micrographs of conditioned specimens showed the debonding at the fiber–matrix interface which became severe with the aging time and temperature increased. (2) While tensile strength and compressive strength of GFRP connectors degraded significantly over time due to deterioration at the interface between fibers and resin, elastic modulus was relatively less affected. The tensile strength and elastic modulus of GFRP connector specimens decreased by 38.85% and 21.45% after 183 days of exposure to alkaline solution at 60 , respectively. (3) Degradation of mechanical performance could be accelerated by increasing of temperature. The compressive strength of GFRP bars decreased by 30.32% and 40.18%, and elastic compressive modulus decreased by 18.19% and 27.62%, respectively, after being exposed to alkaline solution for 183d at 40 and 60 . (4) Taking account of influence of temperature and exposure time, a prediction model based on Arrhenius equation was proposed to predict the mechanical performance of GFRP connectors. The model predictions provided good agreement with experimental results. (5) The GFRP connectors tested in this paper could satisfy the limited minimum tensile strength retentions stipulated by ACI 440 and AC320-06. However, the direct correspondence between requirements of durability of GFRP composites stipulated by these two standards need further study because of different composition of simulated pore-water solution. In general, test method and corresponding requirements recommended by ACI 440 is preferred due to the short ageing period stipulated by AC320-06. It should be noted that the presented results have been incorporated in Chinese National Industrial Standard ‘‘Fibre-Reinforced Polymer Connectors for Precast Concrete Sandwich Insulation Walls”. This standard would promote the application of GFRP connectors in China. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors gratefully acknowledge the financial support provided by the National Key R&D Program of China (No. 2016YFC0701400), Natural Science Foundation of China (No. 51678433) and Fundamental Research Funds for the Central Universities, China (No. 0200219151). References [1] PCI Committee on Precast Sandwich Wall Panels, State of the art of precast/ prestressed concrete sandwich wall panels, PCI J. 52 (2 (spring)) (2011) 131–176.

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Kai Fu is a structural engineer in China. He received his Ph.D. from Department of Structural Engineering at Tongji University. His research interests include durability of fiber-reinforced polymer composites.

Weichen Xue is a Professor in Department of Structural Engineering at Tongji University, Shanghai, China. He received his B.Eng. from Harbin Engineering University, Harbin, China in 1990, M.Eng. from Harbin Institute of Technology, Harbin, China in 1992, and PhD from Southeast University, Nanjing, China in 1995. His research interests include precast, prestressed concrete structures, and fiberreinforced polymer composites.

Xiang Hu is a doctor in Department of Structural Engineering at Tongji University. He received his M.Eng and Ph.D. from Tongji University in 2008 and 2019, respectively. His research interests include analysis and design of precast concrete frame structures.

Ya Li is a Ph.D. Candidate in Department of Structural Engineering at Tongji University. Her research interests include analysis and design of precast concrete sandwich panels.

Yan Li is a Professor in School of Aerospace Engineering and Applied Mechanics at Tongji University, Shanghai, China. She received her PhD from the University of Sydney, Sydney, Australia in 2001. Her research interests include durability of FRP composite and development and application of green fiber-reinforced polymer composite.