# Microstructure Characteristics of GFRP Reinforcing Bars in Harsh Environment

Authors: Hilal El-Hassan; Tamer El Maaddawy
Journal: Advances in Materials Science and Engineering
 (2019)

---

## Abstract

Fiber reinforced polymer (FRP) composites have been suggested as corrosion-resistant alternatives to traditional steel reinforcement in concrete structures. Within this family of composites, glass fiber reinforced polymers (GFRPs) have been gaining momentum as the primary selection of FRP for construction applications. Despite being advantageous, its wide adoption by the industry has been hindered due to the degradation of its performance in severe environmental conditions. As such, significant studies have been carried out to assess the mechanical properties of GFRP bars subject to different conditioning schemes. However, the inconsistencies and wide variations of results called for more in-depth microstructure evaluation. Accordingly, this paper presents a critical review of existing research on the microstructure of GFRP reinforcing bars exposed to various conditioning regimes. The review analysis revealed that sustained load limits set by codes and standards were satisfactory for nonaggressive environment conditions but should be updated to include different conditioning regimes. It was also found that conditioning in alkaline solutions was more severe than concrete and mortars, where test specimens experienced irreversible chemical degradation, more hydroxyl group formation, and more intense degradation to the microstructure. The progression of hydrolysis was reported correlatively through an increase in hydroxyl groups and a decrease in the glass transition temperature. While moisture uptake was the primary instigator of hydrolysis, restricting it to 1.6% could limit the reduction in tensile strength to 15%. Further, the paper identifies research gaps in the existing knowledge and highlights directions for future research.


---

## Body

## 1. Introduction

Reinforced concrete structures are designed with certain life expectancy under normal service conditions. However, exposure to severe environments may reduce the design service life due to unanticipated durability problems, including corrosion of steel reinforcement. The corrosion of steel reinforcing bars causes cracking and spalling of concrete, creating major reductions in performance and a significant increase in cost for rehabilitation. In fact, Koch [1] reported that 3-4% of each nation’s gross domestic product is dedicated to corrosion-related expenditures. As a form of corrosion management, glass fiber reinforced polymer (GFRP) reinforcing bars have been proposed as a replacement to steel equivalents. This alternative reinforcement is characterized by its light weight, high strength-to-weight ratio, and corrosion resistance [2–4]. However, conflicting results on the degradation of its properties upon exposure to alkaline or acidic solution, moisture/water, and elevated temperatures have been a major setback in its adoption by the construction industry [2–4].For the last few years, extensive research has been carried out to investigate the durability performance of GFRP reinforcement exposed to different environmental conditions. Research findings have identified the major contributors to deterioration of GFRP as moisture uptake, alkaline environment, and temperature. Moisture penetrates the matrix by the flow of solution into the microgaps between polymer chains and capillary transport through fiber/matrix interfacial microcracks and in microcracks formed during manufacturing [5]. As a result, the matrix exhibits plasticisation, leading to a reduction in strength and glass transition temperature. On the other hand, alkaline solutions break the bond between the oxygen and carbon atoms in the molecular chain of the GFRP matrix. This breakage creates microcracks and fractures in the matrix structure, leading to more moisture uptake and a significant reduction in strength. Further, to expedite investigative work conducted on GFRP reinforcing bars, higher temperatures have been employed. Vijay and GangaRao [6] reported that an increase in the conditioning temperature led to an exponential amplification in the actual age. Nevertheless, it should be noted that the elevated temperatures employed in most work may have affected the physical properties of the bars by increasing the coefficient of thermal expansion [7] and, thus, may have altered the behavior of the GFRP bars, rendering the results, in some cases, unrealistic.With the majority of past work characterizing the durability performance of GFRP bars, several researchers have reviewed and summarized these publications to highlight the gaps in existing knowledge and propose future direction of research. In contrast, much fewer and more recent investigations have correlated the degradation in GFRP to changes in the microstructure. Yet, there is no such review and summary of these in-depth investigations. Accordingly, this paper aims to present an overview of recent studies examining the effect of conditioning on the microstructure of GFRP reinforcing bars. Research findings obtained help to shed light on the morphological and microstructure changes in GFRP bars subject to different exposure conditions using various microstructure characterization techniques, including differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). A correlation among these microstructural evaluation techniques is reported. The influence of moisture uptake on the microstructure of conditioned GFRP bars is also evaluated.


## 2. Taxonomy

GFRP reinforcing bars are made of glass fibers impregnated with an organic resin or matrix. Typical diameters range between 8 and 20 mm with smooth surfaces for smaller sizes and ribbed and/or sand-coated surfaces for larger counterparts. GFRP reinforcement falls within the family of FRP composites. The taxonomy on the studies of FRP composites is presented in Figure1. They can be classified based on their fiber type, resin type, and application. Different combinations of fibers and resins have been employed in the production of FRPs, of which composites made with glass fibers and vinylester are the most common. Also, FRPs have been used across many industries, including aerospace, automotive, sports, musical, and construction. In the latter industry, they have been employed in new and existing buildings as internal and external reinforcing bars and for repair, strengthening, and retrofitting purposes. The literature has especially focused on the ability of FRPs to replace steel as the main reinforcement in concrete structures [8–15]. However, before being widely adopted by the construction industry, their durability performance was rigorously investigated. Accordingly, GFRP reinforcing bars have been conditioned in various media, including acidic and alkaline solutions, concrete, tap water, and seawater. Elevated temperatures were also employed during conditioning to accelerate the degradation process. Further, the validity of the environmental reduction factors and creep rupture stress limits adopted by the current international guidelines and standards were assessed by applying a sustained load to GFRP bars exposed to severe environments. Such specimens were examined for performance retention by measuring the tensile properties after conditioning. Correlations between the properties and microstructural changes were evaluated using DSC, TGA, SEM, FTIR, and moisture uptake.Figure 1
Taxonomy on the studies of fiber reinforced polymer composites.


## 3. GFRP Reinforcing Bars

The properties of GFRP reinforcing bars and conditions to which they are exposed are primary factors affecting their performance and microstructure. As such, a comprehensive review of the literature on the physical and mechanical properties and conditioning of GFRP bars is presented in the sections below. It should be noted that the literature reviewed in this paper focuses on examining the microstructure characteristics of GFRP bars exposed to different conditioning regimes.


### 3.1. Physical Properties

Prior to conducting any experimental testing on GFRP reinforcing bars, their physical properties were determined. Various tests, shown in Table1, were typically carried out to measure the diameter, void content, fiber content, moisture uptake, glass transition temperature, and thermal expansion coefficient. Physical properties reported in past studies are summarized in Table 2. The most commonly used glass and matrix materials were E-glass and vinylester, with 90 and 74% of the conducted research having used this fiber and matrix, respectively, as shown in Figure 2. Epoxy, on the other hand, has been only recently employed as the resin of GFRP reinforcing bars, with 14% of published work having explored its combination with E-glass [8, 10, 11, 23, 41, 42]. The diameter has also been varied between 8 and 44.5 mm, with the majority being approximately 12 mm. Yet, Benmokrane, et al. [24] concluded that the bar diameter had limited influence on the physical and mechanical properties of GFRP reinforcing bars. Furthermore, very limited work reported the thermal expansion coefficients, as it was not typically measured by researchers, but provided by the manufacturer [8, 13, 14, 24–27]. In contrast, the moisture uptake has been essential in characterizing the performance and assessing its retention capacity. It ranged between 0.01 and 1.59%, by mass, for unconditioned/control specimens. The fiber content was measured by volume and mass. However, the latter was more commonly reported with the availability of standardized test procedures (ASTM D3171 and ASTM E1868). Measured values ranged between 70 and 83%. The volumetric fiber content, on the other hand, was mainly provided by the manufacturer and could only be determined at the initial stages of production.Table 1
Summary of standardized testing for physical properties.
Property (unit)
Standardized test
Reference
Diameter (mm)
ACI 440.3R & ASTM D7205
[3, 16]
Void content (%)
ASTM D3171 & ASTM D2734
[17, 18]
Fiber content, by mass (%)
ASTM D3171 & ASTM E1868
[17, 19]
Moisture uptake (%)
ASTM D570
[20]
Glass transition temperature (°C)
ASTM E1356
[21]
Thermal expansion coefficient (/°C)
ASTM E831
[22]Table 2
Physical properties of GFRP reinforcing bars.
Reference
Glass material
Matrix material
Diameter (mm)
Long. thermal expansion coef. (x 10−6/°C)
Trans. thermal expansion coef. (x 10−6/°C)
Moisture absorption (%)
Fiber content (%, by volume)
Fiber content (%, by mass)
[7]
E-glass
Vinylester
9.5
—
—
0.1
—
74.5
[8]
E-glass
Vinylester polyester epoxy
12
—
17.7–20.8
0.23–1.15
—
78.8–83.9
[9]
Short Glass E-glass
Vinylester
15.0–20.0
—
—
—
—
83
[10]
E-glass
Epoxy
8.0–9.0
—
—
0.1
—
75.5–78.3
[11]
E-glass
Epoxy
8.0–9.0
—
—
0.10
—
75–78
[12]
E-glass E-glass 366 Advantex 366 Advantex 712
Polyester vinylester
9.3–12.0
—
—
0.01–0.05
—
—
[13]
E-glass
Vinylester
19.0
6.1
23.5
0.48
65.4
74.5
[14]
E-glass
Vinylester
12.7
5.5
29.5
0.38
60.3
77.9
[15]
E-glass
Vinylester
9.5
—
—
—
65
—
[23]
E-glass
Vinylester polyester epoxy
12
—
—
0.23–1.15
—
78.8–83.9
[24]
E-glass
Vinylester
9.5–25.4
—
20.5–22.0
0.02–0.15
80.9–83.0
—
[25]
E-glass
Vinylester
12.7
6.7
27.2
0.62
64.3
81.5
[26]
E-glass
Vinylester
12.7
5.5
29.5
0.38
—
77.9
[27]
E-glass
Vinylester polyester
25.4–44.5
6.9–7.8
21.0–27.1
0.05–1.59
—
79.6–81.5
[28]
E-glass
Vinylester
8.0
—
—
—
—
—
[29]
E-glass
Vinylester
9
—
—
—
70
—
[30]
E-glass
Vinylester
12
—
—
—
—
83
[31]
E-glass
Epoxy
14
—
—
0.01
—
—
[32]
E-glass
Vinylester
12
—
—
—
—
83
[33]
E-glass
Vinylester
9.5
—
—
—
—
—
[34]
E-glass
Vinylester
12.7
—
—
0.37
—
81.5
[35]
E-glass
Vinylester
9.5–16.0
—
—
—
75
—
[36]
E-glass
Vinylester
12.7
—
—
—
—
69.1–73.3
[37]
E-glass
Vinylester
12
—
—
—
—
83
[38]
E-glass
Vinylester
—
—
—
—
—
—
[39]
E-glass
Vinylester
10
—
—
0.4
—
—
[40]
E-glass
Vinylester
—
—
—
—
—
—Figure 2
Percentage distribution of GFRP components. (a) Glass fiber material. (b) Matrix material.
(a)
(b)


### 3.2. Mechanical Properties

The performance of GFRP reinforcing bars is typically characterized by its ability to retain its mechanical properties. For this reason, as-received and unconditioned specimens were tested for various performance indicators, as summarized in Table3, including tensile and flexural strength, strain, and modulus. Shear strength was also reported in relevant work. Experimental results of a number of studies are shown in Table 4. Over 90% of the studies focused on tensile properties, with values of tensile strength, modulus, and strain in the range of 432–1321 MPa, 31–65 GPa, and 1–3%, respectively. Given the large database of results for the former two properties, it was possible to develop a relation between them. Figure 3 presents a correlation between tensile modulus (Et) and square root of tensile strength (ft) of GFRP bars. A linear trend line, in the form of Equation (1) could provide a prediction of tensile modulus from the strength with reasonable accuracy. On the other hand, flexural and shear properties were recorded on few occasions, mainly in research work related to bending and shear testing. Yet, the available data have shown a linear relationship between flexural (fr) and tensile strength (ft), as shown in Figure 4 and Equation (2). This linear relation provides a reasonably accurate prediction of the former from the latter without the need for experimental testing. It is also worth noting that tensile properties of unconditioned GFRP samples made with epoxy as the matrix material seem to be, on average, 14% higher than those of vinylester counterparts. Additionally, a correlation was made between the physical and mechanical properties of GFRP specimens made with E-glass and vinylester. From Figure 5, it is clear that the tensile strength and modulus are generally proportional to the fiber content, up to a maximum value of 83%, by mass. Similarly, Micelli and Nanni [44] reported that GFRP bars with lower fiber content may experience inferior mechanical performance.(1)Et=1.67ft,(2)fr=1.28ft.Table 3
Summary of standardized testing for mechanical properties.
Mechanical property (unit)
Standardized test
Reference
Tensile strength (MPa)
ACI 440.3R & ASTM D7205
[3, 16]
Tensile strain (%)
ACI 440.3R & ASTM D7205
[3, 16]
Tensile modulus (MPa)
ACI 440.3R & ASTM D7205
[3, 16]
Flexural strength (MPa)
ASTM D4476
[43]
Flexural strain (%)
ASTM D4476
[43]
Flexural modulus (MPa)
ASTM D4476
[43]
Shear strength (MPa)
ACI 440.3R
[3]Table 4
Mechanical properties of unconditioned GFRP reinforcing bars.
Reference
Tensile strength (MPa)
Tensile modulus (GPa)
Tensile strain (%)
Flexural strength (MPa)
Flexural modulus (GPa)
Flexural strain (%)
Shear strength (MPa)
[7]
653
38.5
—
888
—
—
—
[8]
—
—
—
1150–1573
56.9–663.3
2.02–2.54
250–270
[9]
1105–1184
62.6–64.7
1.71–1.89
—
—
—
—
[10]
816–1321
52–53
—
—
—
—
—
[11]
816–1321
52–53
—
—
—
—
—
[12]
612–958.3
31.6–51.8
—
—
—
—
—
[13]
728
47.6
1.53
—
—
—
—
[14]
786
46.3
1.7
1005
46.8
2.15
212
[15]
644.7
53.4
1.2
—
—
—
—
[23]
1015–1220
—
—
—
—
—
—
[24]
1237.4–1315.3
60.0–62.5
2.1–2.3
1406.3–1757.5
—
—
—
[25]
788
47.2
1.7
1095
52.6
2.15
185
[26]
786
46.3
1.7
—
—
—
—
[27]
—
—
—
759–1324
49.3–54.1
—
151–197
[28]
821
44.4
1.84
—
—
—
—
[29]
1200
42.9
2.8
—
—
—
—
[30]
1478
60.4
2.45
—
—
—
—
[31]
714.6
55
1.62
—
—
—
—
[32]
1478
60.4
2.45
—
—
—
—
[33]
700
40.8
—
—
—
—
—
[34]
854
43.0
—
—
—
—
—
[35]
580–658
40–42
1.4–1.6
—
—
—
—
[36]
660.6–692.8
38.5–42.7
—
—
—
—
36.9–42.3
[37]
432–1478
41.9–60.4
1.04–2.45
—
—
—
—Figure 3
Relationship between tensile modulus and strength of GFRP bars.Figure 4
Correlation between flexural and tensile strength of GFRP bars.Figure 5
Correlation between fiber content and mechanical properties. (a) Tensile strength. (b) Tensile modulus.
(a)
(b)


### 3.3. Conditioning

The durability performance of GFRP reinforcing bars exposed to different environmental conditions has been evaluated using accelerated aging tests. Four main exposure parameters were investigated: surrounding medium, conditioning duration, conditioning temperature, and presence of a sustained load. Table5 summarizes the exposure conditions employed by various studies on the microstructure of GFRP bars. It should be noted that only one study reviewed in this work, assessing the microstructure, has investigated the effect of acidic solutions on the performance of GFRP. Seawater exposure has also received little attention, although it is critical to assess the properties and microstructure of GFRP bars for employment in offshore structures and bridge decks in coastal cities and seaports [10, 11, 42]. According to Figure 6(a), alkaline and water solutions have been utilized in 39 and 25% of the studies shown herein. It seems that the ease of preparing such media renders them most favourable among researchers, especially that the former is proposed to simulate concrete alkalinity and behavior. Concrete, on the other hand, has been used by 23% of presented research, even though it better represents real-life scenarios.Table 5
Summary of conditioning regimes in studies investigating the microstructure of GFRP reinforcing bars.
Reference
Surrounding media
Conditioning
Loading
Alkaline
Concrete
Water
Seawater
Acidic
Duration (days)
Temperature (°C)
Yes (%, UTS1)
No
[7]
x
40 to 120
23 to 80
x
[8]
x
41.7 to 208.3
60
x
[9]
x
x
30 to 60
−30 to 60
x
[10]
x
x
150 to 450
20 to 60
x
[11]
x
x
150 to 450
20 to 60
25
[12]
x
X
Up to 1445
–
20–65
[13]
x
60 to 180
23 to 50
x
[14]
x
x
60 to 240
23 to 50
x
[15]
x
100 to 660
40
x
[23]
x
41.67 to 208.33
22 to 60
x
[42]
x
90 to 365
23 to 65
x
[24]
x
90
60
x
[25]
x
0.0208 to 0.1285
−100 to 325
x
[26]
x
x
60 to 365
23 to 70
x
[27]
x
30 to 180
23 to 60
x
[28]
x
30 to 270
23
0–25
[29]
x
x
x
x
28 to 50
60 to 80
x
[30]
x
x
x
180 to 540
23 to 50
x
[31]
x
30 to 720
60
x
[32]
x
0.0417 to 0.125
100 to 300
x
[33]
x
x
x
90 to 730
60
20 & x2
[34]
x
x
60 to 240
23 to 50
20–80
[35]
x
x
30 to 120
20 to 73
19–29
[36]
x
x
x
30 to 132
−25 to 80
x
[37]
x
x
x
180 to 540
23 to 50
x
[38]
x2
1825 to 2920
−24 to 30
x2
[39]
x
30 to 365
23 to 75
x
[40]
x
x
54.2
22 to 90
x
[44]
x
x
21 to 42
−18 to 49
x
[45]
x
x
15 to 60
60
x
[46]
x
x
42 to 365
20 to 60
15
[47]
x
30 to 270
20 to 60
10–15
[48]
x
30 to 180
60
x
[49]
x
41.7
60
x
[50]
x
20 to 120
20 to 70
x
[51]
x
x
41.7
20 to 70
x
[52]
x
x
x
90 to 730
60
20 & x1
1Ultimate tensile strength. 2Natural weathering conditions.Figure 6
Percentage distribution of conditioning parameters. (a) Surrounding media. (b) Maximum conditioning duration (days). (c) Maximum conditioning temperature (°C). (d) Presence of sustained loading.
(a)
(b)
(c)
(d)The duration of conditioning has been widely altered in research studies and ranged between 0.02 and 7200 days. Figure6(b) shows that 28, 18, 10, 23, and 21% of studies have conditioned GFRP bars for a maximum of 0–90, 91 to 180, 181 to 240, 241 to 365, and more than 365 days, respectively. Clearly, the majority of research has focused on examining the effects of early age and prolonged conditioning.The conditioning temperature has been pivotal in accelerating aging of GFRP specimens. Work by Vijay and GangaRao [6] highlighted the relationship between actual age in calendar days at Morgantown, WV, and chamber conditioning days in the form of Equation (3), where T is the temperature in °F. It is worth noting that in their experiments, they [6] placed the specimens in a weathering chamber, i.e., a controlled temperature environment, and in alkaline solutions. Effectively, an increase in chamber conditioning temperature (T) led to an amplification in the actual age. Thus, the conditioning temperature has been altered within and among different studies. In general, they have ranged between 20°C and 90°C, with the exceptions of [25, 32], which investigated the performance of GFRP bars subject to freezing and elevated temperatures. Based on Equation (3), such conditioning temperatures (20°C–90°C) could accelerate the aging by at least four times when specimens are conditioned in alkaline solutions. For instance, alkaline conditioning for 30 days in a chamber at 50°C is equivalent to an actual age of 2660 days (approx. 7 years) at Morgantown, WV. Yet, this equation may not be applicable to specimens encased in concrete or conditioned in regimes other than alkaline solutions. Figure 6(c) highlights the distribution of conditioning temperatures, with 77% of studies utilizing conditioning temperatures above 50°C. Nevertheless, it should be noted that such temperatures may affect the physical properties of the bars by increasing the coefficient of thermal expansion [7] and, thus, may alter the behavior of the GFRP bars, rendering unrealistic results.(3)Age in calendar daysChamber conditioning days=0.098e0.0558T.


### 3.4. Loading

The degradation of GFRP reinforcing bars could be intensified in the presence of a sustained load during conditioning [53–55]. ACI 440.1R [2] limits the tensile stress in GFRP under service conditions to 14% of the ultimate tensile strength (UTS), considering the strength reduction factor for environmental exposure. Higher stress levels caused a shift in the degradation mechanism of GFRP from being affected by the rate of diffusion of alkaline solutions to being controlled by solution transport through resin cracks [56]. Nevertheless, it can be noted from Figure 6(d) that only 27% of investigated microstructure studies applied a sustained load to GFRP bars, with values ranging from 0 to 80% UTS. The objectives of these works, nonetheless, were different. While some researchers applied sustained loads within the limits specified by codes and standards [2, 4, 57–60], others explored the possibility of increasing this allowable limit [11, 12, 28, 33–35, 52]. Some of these latter studies concluded that the limits recommended by codes and standards were conservative for specific reinforcing bars and conditions. In fact, elevated temperatures could accelerate the degradation induced by conditioning. As a result, the limits were found to be inappropriate and required revision through further research data.


### 3.5. Reduction Factors

The effects of service time, relative humidity, and temperature are incorporated into an environmental reduction factor for GFRP bars used as internal reinforcement in concrete structures. ACI 440.1R [2] relates the design (ffu∗) and guaranteed tensile strength (ffu) using the proposed reduction factors (CE). The relationship is shown as Eq. (4). The reduction factors for environmental conditions, shown in Table 6, ranged between 0.3 and 1.0. Lower values were associated with higher relative humidity, service time, and temperature, while a reduction factor of 1 was typically attributed to less severe conditioning [61]. Another load reduction factor was introduced by several researchers and codes to account for the presence of a sustained load [2, 4, 58–60]. Its value ranged between 0.2 and 1.0, with more recent codes suggesting lower reduction factors. This is associated to the wide variations of results reported and somewhat concerning impact of accelerated aging, even though extensive research has been conducted in this area. The combined effect of both factors has also been noted, as shown in the last column of Table 6, to vary between 0.13 and 0.50.(4)ffu=CE·ffu∗.Table 6
Summary of reduction factors for GFRP reinforcing bars.
Reference
Country
Year
Reduction factor
Environmental
Creep/sustained load
Combined
ACI 440.1R [2]
USA
2015
0.7–0.8
0.2
0.14–0.16
FIB [4]
UK
1999
0.72
0.3
0.216
Ali, et al. [23]
Canada
2018
0.75–1.00
—
—
JSCE [57]
Japan
1997
0.77
—
—
CSA S6 [58]
Canada
2006
0.55
0.25
0.1375
NS 3473 [59]
Norway
1998
0.5
0.80–1.00
0.40–0.50
CSA S806 [60]
Canada
2012
0.75
0.3
0.225
Huang and Aboutaha [61]
USA
2010
0.31–1.00
—
—


## 3.1. Physical Properties

Prior to conducting any experimental testing on GFRP reinforcing bars, their physical properties were determined. Various tests, shown in Table1, were typically carried out to measure the diameter, void content, fiber content, moisture uptake, glass transition temperature, and thermal expansion coefficient. Physical properties reported in past studies are summarized in Table 2. The most commonly used glass and matrix materials were E-glass and vinylester, with 90 and 74% of the conducted research having used this fiber and matrix, respectively, as shown in Figure 2. Epoxy, on the other hand, has been only recently employed as the resin of GFRP reinforcing bars, with 14% of published work having explored its combination with E-glass [8, 10, 11, 23, 41, 42]. The diameter has also been varied between 8 and 44.5 mm, with the majority being approximately 12 mm. Yet, Benmokrane, et al. [24] concluded that the bar diameter had limited influence on the physical and mechanical properties of GFRP reinforcing bars. Furthermore, very limited work reported the thermal expansion coefficients, as it was not typically measured by researchers, but provided by the manufacturer [8, 13, 14, 24–27]. In contrast, the moisture uptake has been essential in characterizing the performance and assessing its retention capacity. It ranged between 0.01 and 1.59%, by mass, for unconditioned/control specimens. The fiber content was measured by volume and mass. However, the latter was more commonly reported with the availability of standardized test procedures (ASTM D3171 and ASTM E1868). Measured values ranged between 70 and 83%. The volumetric fiber content, on the other hand, was mainly provided by the manufacturer and could only be determined at the initial stages of production.Table 1
Summary of standardized testing for physical properties.
Property (unit)
Standardized test
Reference
Diameter (mm)
ACI 440.3R & ASTM D7205
[3, 16]
Void content (%)
ASTM D3171 & ASTM D2734
[17, 18]
Fiber content, by mass (%)
ASTM D3171 & ASTM E1868
[17, 19]
Moisture uptake (%)
ASTM D570
[20]
Glass transition temperature (°C)
ASTM E1356
[21]
Thermal expansion coefficient (/°C)
ASTM E831
[22]Table 2
Physical properties of GFRP reinforcing bars.
Reference
Glass material
Matrix material
Diameter (mm)
Long. thermal expansion coef. (x 10−6/°C)
Trans. thermal expansion coef. (x 10−6/°C)
Moisture absorption (%)
Fiber content (%, by volume)
Fiber content (%, by mass)
[7]
E-glass
Vinylester
9.5
—
—
0.1
—
74.5
[8]
E-glass
Vinylester polyester epoxy
12
—
17.7–20.8
0.23–1.15
—
78.8–83.9
[9]
Short Glass E-glass
Vinylester
15.0–20.0
—
—
—
—
83
[10]
E-glass
Epoxy
8.0–9.0
—
—
0.1
—
75.5–78.3
[11]
E-glass
Epoxy
8.0–9.0
—
—
0.10
—
75–78
[12]
E-glass E-glass 366 Advantex 366 Advantex 712
Polyester vinylester
9.3–12.0
—
—
0.01–0.05
—
—
[13]
E-glass
Vinylester
19.0
6.1
23.5
0.48
65.4
74.5
[14]
E-glass
Vinylester
12.7
5.5
29.5
0.38
60.3
77.9
[15]
E-glass
Vinylester
9.5
—
—
—
65
—
[23]
E-glass
Vinylester polyester epoxy
12
—
—
0.23–1.15
—
78.8–83.9
[24]
E-glass
Vinylester
9.5–25.4
—
20.5–22.0
0.02–0.15
80.9–83.0
—
[25]
E-glass
Vinylester
12.7
6.7
27.2
0.62
64.3
81.5
[26]
E-glass
Vinylester
12.7
5.5
29.5
0.38
—
77.9
[27]
E-glass
Vinylester polyester
25.4–44.5
6.9–7.8
21.0–27.1
0.05–1.59
—
79.6–81.5
[28]
E-glass
Vinylester
8.0
—
—
—
—
—
[29]
E-glass
Vinylester
9
—
—
—
70
—
[30]
E-glass
Vinylester
12
—
—
—
—
83
[31]
E-glass
Epoxy
14
—
—
0.01
—
—
[32]
E-glass
Vinylester
12
—
—
—
—
83
[33]
E-glass
Vinylester
9.5
—
—
—
—
—
[34]
E-glass
Vinylester
12.7
—
—
0.37
—
81.5
[35]
E-glass
Vinylester
9.5–16.0
—
—
—
75
—
[36]
E-glass
Vinylester
12.7
—
—
—
—
69.1–73.3
[37]
E-glass
Vinylester
12
—
—
—
—
83
[38]
E-glass
Vinylester
—
—
—
—
—
—
[39]
E-glass
Vinylester
10
—
—
0.4
—
—
[40]
E-glass
Vinylester
—
—
—
—
—
—Figure 2
Percentage distribution of GFRP components. (a) Glass fiber material. (b) Matrix material.
(a)
(b)


## 3.2. Mechanical Properties

The performance of GFRP reinforcing bars is typically characterized by its ability to retain its mechanical properties. For this reason, as-received and unconditioned specimens were tested for various performance indicators, as summarized in Table3, including tensile and flexural strength, strain, and modulus. Shear strength was also reported in relevant work. Experimental results of a number of studies are shown in Table 4. Over 90% of the studies focused on tensile properties, with values of tensile strength, modulus, and strain in the range of 432–1321 MPa, 31–65 GPa, and 1–3%, respectively. Given the large database of results for the former two properties, it was possible to develop a relation between them. Figure 3 presents a correlation between tensile modulus (Et) and square root of tensile strength (ft) of GFRP bars. A linear trend line, in the form of Equation (1) could provide a prediction of tensile modulus from the strength with reasonable accuracy. On the other hand, flexural and shear properties were recorded on few occasions, mainly in research work related to bending and shear testing. Yet, the available data have shown a linear relationship between flexural (fr) and tensile strength (ft), as shown in Figure 4 and Equation (2). This linear relation provides a reasonably accurate prediction of the former from the latter without the need for experimental testing. It is also worth noting that tensile properties of unconditioned GFRP samples made with epoxy as the matrix material seem to be, on average, 14% higher than those of vinylester counterparts. Additionally, a correlation was made between the physical and mechanical properties of GFRP specimens made with E-glass and vinylester. From Figure 5, it is clear that the tensile strength and modulus are generally proportional to the fiber content, up to a maximum value of 83%, by mass. Similarly, Micelli and Nanni [44] reported that GFRP bars with lower fiber content may experience inferior mechanical performance.(1)Et=1.67ft,(2)fr=1.28ft.Table 3
Summary of standardized testing for mechanical properties.
Mechanical property (unit)
Standardized test
Reference
Tensile strength (MPa)
ACI 440.3R & ASTM D7205
[3, 16]
Tensile strain (%)
ACI 440.3R & ASTM D7205
[3, 16]
Tensile modulus (MPa)
ACI 440.3R & ASTM D7205
[3, 16]
Flexural strength (MPa)
ASTM D4476
[43]
Flexural strain (%)
ASTM D4476
[43]
Flexural modulus (MPa)
ASTM D4476
[43]
Shear strength (MPa)
ACI 440.3R
[3]Table 4
Mechanical properties of unconditioned GFRP reinforcing bars.
Reference
Tensile strength (MPa)
Tensile modulus (GPa)
Tensile strain (%)
Flexural strength (MPa)
Flexural modulus (GPa)
Flexural strain (%)
Shear strength (MPa)
[7]
653
38.5
—
888
—
—
—
[8]
—
—
—
1150–1573
56.9–663.3
2.02–2.54
250–270
[9]
1105–1184
62.6–64.7
1.71–1.89
—
—
—
—
[10]
816–1321
52–53
—
—
—
—
—
[11]
816–1321
52–53
—
—
—
—
—
[12]
612–958.3
31.6–51.8
—
—
—
—
—
[13]
728
47.6
1.53
—
—
—
—
[14]
786
46.3
1.7
1005
46.8
2.15
212
[15]
644.7
53.4
1.2
—
—
—
—
[23]
1015–1220
—
—
—
—
—
—
[24]
1237.4–1315.3
60.0–62.5
2.1–2.3
1406.3–1757.5
—
—
—
[25]
788
47.2
1.7
1095
52.6
2.15
185
[26]
786
46.3
1.7
—
—
—
—
[27]
—
—
—
759–1324
49.3–54.1
—
151–197
[28]
821
44.4
1.84
—
—
—
—
[29]
1200
42.9
2.8
—
—
—
—
[30]
1478
60.4
2.45
—
—
—
—
[31]
714.6
55
1.62
—
—
—
—
[32]
1478
60.4
2.45
—
—
—
—
[33]
700
40.8
—
—
—
—
—
[34]
854
43.0
—
—
—
—
—
[35]
580–658
40–42
1.4–1.6
—
—
—
—
[36]
660.6–692.8
38.5–42.7
—
—
—
—
36.9–42.3
[37]
432–1478
41.9–60.4
1.04–2.45
—
—
—
—Figure 3
Relationship between tensile modulus and strength of GFRP bars.Figure 4
Correlation between flexural and tensile strength of GFRP bars.Figure 5
Correlation between fiber content and mechanical properties. (a) Tensile strength. (b) Tensile modulus.
(a)
(b)


## 3.3. Conditioning

The durability performance of GFRP reinforcing bars exposed to different environmental conditions has been evaluated using accelerated aging tests. Four main exposure parameters were investigated: surrounding medium, conditioning duration, conditioning temperature, and presence of a sustained load. Table5 summarizes the exposure conditions employed by various studies on the microstructure of GFRP bars. It should be noted that only one study reviewed in this work, assessing the microstructure, has investigated the effect of acidic solutions on the performance of GFRP. Seawater exposure has also received little attention, although it is critical to assess the properties and microstructure of GFRP bars for employment in offshore structures and bridge decks in coastal cities and seaports [10, 11, 42]. According to Figure 6(a), alkaline and water solutions have been utilized in 39 and 25% of the studies shown herein. It seems that the ease of preparing such media renders them most favourable among researchers, especially that the former is proposed to simulate concrete alkalinity and behavior. Concrete, on the other hand, has been used by 23% of presented research, even though it better represents real-life scenarios.Table 5
Summary of conditioning regimes in studies investigating the microstructure of GFRP reinforcing bars.
Reference
Surrounding media
Conditioning
Loading
Alkaline
Concrete
Water
Seawater
Acidic
Duration (days)
Temperature (°C)
Yes (%, UTS1)
No
[7]
x
40 to 120
23 to 80
x
[8]
x
41.7 to 208.3
60
x
[9]
x
x
30 to 60
−30 to 60
x
[10]
x
x
150 to 450
20 to 60
x
[11]
x
x
150 to 450
20 to 60
25
[12]
x
X
Up to 1445
–
20–65
[13]
x
60 to 180
23 to 50
x
[14]
x
x
60 to 240
23 to 50
x
[15]
x
100 to 660
40
x
[23]
x
41.67 to 208.33
22 to 60
x
[42]
x
90 to 365
23 to 65
x
[24]
x
90
60
x
[25]
x
0.0208 to 0.1285
−100 to 325
x
[26]
x
x
60 to 365
23 to 70
x
[27]
x
30 to 180
23 to 60
x
[28]
x
30 to 270
23
0–25
[29]
x
x
x
x
28 to 50
60 to 80
x
[30]
x
x
x
180 to 540
23 to 50
x
[31]
x
30 to 720
60
x
[32]
x
0.0417 to 0.125
100 to 300
x
[33]
x
x
x
90 to 730
60
20 & x2
[34]
x
x
60 to 240
23 to 50
20–80
[35]
x
x
30 to 120
20 to 73
19–29
[36]
x
x
x
30 to 132
−25 to 80
x
[37]
x
x
x
180 to 540
23 to 50
x
[38]
x2
1825 to 2920
−24 to 30
x2
[39]
x
30 to 365
23 to 75
x
[40]
x
x
54.2
22 to 90
x
[44]
x
x
21 to 42
−18 to 49
x
[45]
x
x
15 to 60
60
x
[46]
x
x
42 to 365
20 to 60
15
[47]
x
30 to 270
20 to 60
10–15
[48]
x
30 to 180
60
x
[49]
x
41.7
60
x
[50]
x
20 to 120
20 to 70
x
[51]
x
x
41.7
20 to 70
x
[52]
x
x
x
90 to 730
60
20 & x1
1Ultimate tensile strength. 2Natural weathering conditions.Figure 6
Percentage distribution of conditioning parameters. (a) Surrounding media. (b) Maximum conditioning duration (days). (c) Maximum conditioning temperature (°C). (d) Presence of sustained loading.
(a)
(b)
(c)
(d)The duration of conditioning has been widely altered in research studies and ranged between 0.02 and 7200 days. Figure6(b) shows that 28, 18, 10, 23, and 21% of studies have conditioned GFRP bars for a maximum of 0–90, 91 to 180, 181 to 240, 241 to 365, and more than 365 days, respectively. Clearly, the majority of research has focused on examining the effects of early age and prolonged conditioning.The conditioning temperature has been pivotal in accelerating aging of GFRP specimens. Work by Vijay and GangaRao [6] highlighted the relationship between actual age in calendar days at Morgantown, WV, and chamber conditioning days in the form of Equation (3), where T is the temperature in °F. It is worth noting that in their experiments, they [6] placed the specimens in a weathering chamber, i.e., a controlled temperature environment, and in alkaline solutions. Effectively, an increase in chamber conditioning temperature (T) led to an amplification in the actual age. Thus, the conditioning temperature has been altered within and among different studies. In general, they have ranged between 20°C and 90°C, with the exceptions of [25, 32], which investigated the performance of GFRP bars subject to freezing and elevated temperatures. Based on Equation (3), such conditioning temperatures (20°C–90°C) could accelerate the aging by at least four times when specimens are conditioned in alkaline solutions. For instance, alkaline conditioning for 30 days in a chamber at 50°C is equivalent to an actual age of 2660 days (approx. 7 years) at Morgantown, WV. Yet, this equation may not be applicable to specimens encased in concrete or conditioned in regimes other than alkaline solutions. Figure 6(c) highlights the distribution of conditioning temperatures, with 77% of studies utilizing conditioning temperatures above 50°C. Nevertheless, it should be noted that such temperatures may affect the physical properties of the bars by increasing the coefficient of thermal expansion [7] and, thus, may alter the behavior of the GFRP bars, rendering unrealistic results.(3)Age in calendar daysChamber conditioning days=0.098e0.0558T.


## 3.4. Loading

The degradation of GFRP reinforcing bars could be intensified in the presence of a sustained load during conditioning [53–55]. ACI 440.1R [2] limits the tensile stress in GFRP under service conditions to 14% of the ultimate tensile strength (UTS), considering the strength reduction factor for environmental exposure. Higher stress levels caused a shift in the degradation mechanism of GFRP from being affected by the rate of diffusion of alkaline solutions to being controlled by solution transport through resin cracks [56]. Nevertheless, it can be noted from Figure 6(d) that only 27% of investigated microstructure studies applied a sustained load to GFRP bars, with values ranging from 0 to 80% UTS. The objectives of these works, nonetheless, were different. While some researchers applied sustained loads within the limits specified by codes and standards [2, 4, 57–60], others explored the possibility of increasing this allowable limit [11, 12, 28, 33–35, 52]. Some of these latter studies concluded that the limits recommended by codes and standards were conservative for specific reinforcing bars and conditions. In fact, elevated temperatures could accelerate the degradation induced by conditioning. As a result, the limits were found to be inappropriate and required revision through further research data.


## 3.5. Reduction Factors

The effects of service time, relative humidity, and temperature are incorporated into an environmental reduction factor for GFRP bars used as internal reinforcement in concrete structures. ACI 440.1R [2] relates the design (ffu∗) and guaranteed tensile strength (ffu) using the proposed reduction factors (CE). The relationship is shown as Eq. (4). The reduction factors for environmental conditions, shown in Table 6, ranged between 0.3 and 1.0. Lower values were associated with higher relative humidity, service time, and temperature, while a reduction factor of 1 was typically attributed to less severe conditioning [61]. Another load reduction factor was introduced by several researchers and codes to account for the presence of a sustained load [2, 4, 58–60]. Its value ranged between 0.2 and 1.0, with more recent codes suggesting lower reduction factors. This is associated to the wide variations of results reported and somewhat concerning impact of accelerated aging, even though extensive research has been conducted in this area. The combined effect of both factors has also been noted, as shown in the last column of Table 6, to vary between 0.13 and 0.50.(4)ffu=CE·ffu∗.Table 6
Summary of reduction factors for GFRP reinforcing bars.
Reference
Country
Year
Reduction factor
Environmental
Creep/sustained load
Combined
ACI 440.1R [2]
USA
2015
0.7–0.8
0.2
0.14–0.16
FIB [4]
UK
1999
0.72
0.3
0.216
Ali, et al. [23]
Canada
2018
0.75–1.00
—
—
JSCE [57]
Japan
1997
0.77
—
—
CSA S6 [58]
Canada
2006
0.55
0.25
0.1375
NS 3473 [59]
Norway
1998
0.5
0.80–1.00
0.40–0.50
CSA S806 [60]
Canada
2012
0.75
0.3
0.225
Huang and Aboutaha [61]
USA
2010
0.31–1.00
—
—


## 4. Microstructure Evaluation Methods

Microstructural characterization was employed to evaluate changes to GFRP reinforcing bars after conditioning and provide explanations to the degradation in mechanical properties. Of the many procedures available, TGA, SEM, FTIR, and DSC have been the most commonly used. Table7 summarizes the types of microstructure tests conducted in each of the studies presented herein. While moisture uptake is not a typical microstructure evaluation method, it has been proven pivotal in understanding the causes of degradation in the properties of GFRP specimens [10]. Clearly, SEM has been the most utilized of all tests. In fact, it has been employed in 92% of the studies reviewed, while DSC, FTIR, and moisture uptake have been used in only 40–43%, as shown in Figure 7. This is mainly due to SEM being able to highlight and identify morphological changes due to conditioning, which are typically characterized by matrix degradation, interfacial deterioration, and/or fiber etching. TGA, on the other hand, has been used in 5% of the studies reviewed herein. Its primary use was to determine the thermal degradation of the polymer matrix [12, 25]. Nevertheless, it is worth noting that the majority of researchers have resorted to more than one microstructure test to confirm experimental results and findings. It should also be noted that the results reported hereafter for conditioned samples are associated to the most severe conditioning scheme utilized in each study.Table 7
Summary of microstructure tests conducted on GFRP reinforcing bars.
Reference
Microstructure tests
Moisture uptake
TGA
SEM
FTIR
DSC
[7]
x
[8]
x
x
x
x
[9]
x
x
x
[10]
x
x
x
[11]
x
x
x
x
[12]
x
x
x
[13]
x
[14]
x
x
x
[23]
x
x
x
[42]
x
x
[24]
x
x
[25]
x
x
x
[26]
x
x
x
[27]
x
x
x
[28]
x
[29]
x
[30]
x
[31]
x
x
x
x
[32]
x
[33]
x
[34]
x
x
x
x
[35]
x
x
x
[36]
x
[37]
x
[38]
x
x
x
[39]
x
x
x
[40]
x
x
[44]
x
[45]
x
[46]
x
x
[47]
x
[48]
x
x
x
[49]
x
[50]
x
[51]
xFigure 7
Percentage distribution of microstructure tests across investigated past research.


### 4.1. Moisture Uptake

Moisture penetrates into a composite material by the flow of solution into the microgaps between polymer chains, capillary transport through fiber/matrix interfacial microcracks, and in microcracks formed during manufacturing [5]. The diffusion process mainly depends on the type of matrix and surrounding environment. It can be measured as per the procedure of ASTM D570 [20]. Typical moisture absorption/uptake tests involve weighing samples before and after immersion. However, the hydrolysis reaction that may have occurred during conditioning could cause mass dissolution and, thus, affect the results. To account for this mass loss, samples are oven-dried for 24 hours at 100°C. This oven-dried mass would then be used to find the moisture uptake:(5)moisture uptake%,by mass=conditioned mass−oven dried massinitial mass×100.Figure8 is a typical moisture uptake curve that presents the increase in mass over conditioning time. Clearly, the diffusion rate of moisture is different for temperatures below and above 60°C. At temperatures of 60°C and below, the majority of the uptake occurs within the first 30 days of immersion. At higher temperatures, the uptake increases at a steady rate up to 50 days. Robert, et al. [7] explained that such behavior is owed to the amplification of thermomechanical degradation mechanisms at a temperature of 80°C. Further, the moisture uptakes for conditioned and unconditioned GFRP reinforcing bars tested in the past literature are summarized in Table 8. These values correspond to the most severe conditioning regime employed in each study. Results for conditioned samples ranged between 0.06 and 5.1, with vinylester-GFRP presenting the widest variations. Past researchers have discussed several factors that affected the uptake, namely, presence of a sustained load, void content, pH of conditioning medium, conditioning duration, and conditioning temperature [8–11, 24, 30, 32, 35, 38, 47, 56, 64, 65]. Each factor will be discussed in the sections below.Figure 8
Typical moisture uptake curve of GFRP bars (adapted from [7]).Table 8
Results of moisture uptake of GFRP reinforcing bars.
Reference
Unconditioned
Conditioned
Within high durability limits
Uptake (%)
Medium
Temperature (°C)
Time (days)
Sustained load
Uptake (%)
ACI 440.6 [62]
CSA S807 [63]
[7]
0.10
Water
80
58
1.80
Not Acceptable
Not Acceptable
[8]
0.23–1.15
Alkaline
50
21
0.25–1.36
Not Acceptable
Not Acceptable
[9]
0.00
Concrete
50
Saturation
0.44
Acceptable
Acceptable
[10]
0.10
Concrete
60
450
1.23–1.50
Not Acceptable
Not Acceptable
[11]
0.10
Concrete
60
288 & 372
x
1.95–2.54
Not Acceptable
Not Acceptable
[12]
0.05
Concrete
60
48.2
x
0.50
Acceptable
Acceptable
[23]
0.23–1.15
Alkaline
60
208.3
0.25–1.36
Not Acceptable
Not Acceptable
[42]
0.00
Seawater
65
365
5.10
Not Acceptable
Not Acceptable
[24]
0.02–0.15
Water
23
Saturation
0.20
Acceptable
Acceptable
[27]
0.05–1.59
Alkaline
50
21
0.06–1.63
Not Acceptable
Not Acceptable
[32]
0.01
Alkaline
60
720
0.74
Acceptable
Not Acceptable
[35]
0.37
Concrete
50
30
x
0.57
Acceptable
Acceptable
[40]
0.40
Alkaline
50
94
1.20
Not Acceptable
Not Acceptable
[44]
0.00
Alkaline
22
16.67
0.55
Acceptable
Acceptable
[46]
0.00
Alkaline
60
360.4
x
1.10
Not Acceptable
Not Acceptable
[48]
0.01
Alkaline
60
176
0.38
Acceptable
AcceptableThe effect of sustained load can be noted by comparing the results of [10, 11]. The only difference between these two researches is the addition of a sustained load of 25% UTS in [11]. Results, presented in Table 8, show that even with an average 26% shorter conditioning durations, the uptake was, on average, 68% higher. It is thus obvious that the addition of a sustained load can aggravate the moisture uptake.Recent work by El-Hassan, et al. [10] compared two types of GFRP bars under similar exposure conditions. The void contents were measured as 0.10 and 0.23%. The authors reported higher moisture uptake in the latter GFRP bar with higher void content. However, with limited information on the void content or fiber content in the existing literature, it is difficult to create a general conclusion. Further work would be needed to investigate the effect of void content on performance retention and microstructure changes.To simulate severe environments, researchers have utilized alkaline solutions with different pH. Values of pH ranged between 12 and 13.6. A recent study by D’Antino, et al. [66] reported that higher pH resulted in less tensile strength retention. Yet, immersion in simulated concrete pore (alkaline) solution is not a proper reproduction of real-life concrete. The main difference is the degree of contact of alkaline solution with the GFRP reinforcing bars. In the former case, the surface of the bar is entirely coated by the solution, resulting in maximum contact area, while in the latter case, alkaline solutions remain in the concrete pores, causing limited contact with the imbedded GFRP bar. In the case of moist concrete, representing underwater concrete, the degree of contact is intermediary between the dry concrete and wet alkaline solution.Conditioning temperature and duration have been widely investigated. Figure8 shows the effect of each of these conditions on the moisture uptake. It is clear that higher temperatures and longer exposure durations resulted in more uptake. Yet, it is worth noting that the former was the more influential factor of the two, thus, causing a more exacerbated degradation phenomenon. Robert, et al. [7] concluded that a thermomechanical degradation mechanism would occur at an elevated temperature of 80°C, owing to higher solution diffusion in a more porous resin matrix. Nevertheless, such conditioning temperature does not correspond to service life conditions of GFRP bars, justifying the maximum aging temperature recommended by ACI 440.3 [3] and CSA S806 [60] being 60°C.ACI 440.6 [62] and CSA S807 [63] propose durability limits in the form of maximum allowable moisture/water uptake. These limits are 1 and 0.75%, by mass, respectively, regardless of the exposure conditions. Table 8 shows the comparison of conditioned GFRP reinforcing bars with each limit. It is clear that more than half of the bars tested were not classified of high durability, based on moisture uptake. In fact, Figure 9 shows that 56 and 62% of bars, tested under the worst conditions in each study, failed to fall within the high durability limits of ACI 440.6 [62] and CSA S807 [63], respectively. In general, samples conditioned in alkaline solution at temperatures beyond 50°C were deemed unacceptable [8, 23, 27, 39, 46]. GFRP bars imbedded in concrete resulted in uptakes within the acceptable range, except for El-Hassan, et al. [11], where a sustained load of 25% UTS was applied, which is higher than the maximum allowable load recommended by ACI 440.1R [2]. A general comparison between alkaline and concrete conditioned GFRPs shows higher uptake in the former case and lower degree of satisfactoriness by the codes. Therefore, it should be noted that the moisture uptake of GFRP bars fell within the acceptable limits if conditioned in concrete and for shorter durations, lower temperatures, and/or without a sustained load. Nevertheless, in real-life scenarios, where GFRP bars are imbedded in concrete, it is possible to limit the moisture uptake by creating an impermeable concrete. This can be achieved by adding supplementary cementitious materials, as fly ash, ground granulated blast furnace slag, silica fume, or others.Figure 9
Percentage of GFRPs within acceptable durability limits. (a) ACI 440.6. (b) CSA S807.
(a)
(b)


### 4.2. TG Analysis

TG analysis was employed in the past literature to assess the decomposition of the polymer matrix under heat exposure. ASTM E1868 [19] was used to examine the change in mass as a function of temperature (20°C–800°C) and evaluate the thermal stability of the matrix. A typical mass variation curve as a function of temperature measured by TGA is shown in Figure 10. It is clear that a major mass loss of 18% occurred in the range of 300°C–450°C. This is attributed to the thermal degradation of the polymer matrix, as the molecular chains broke, leading to microcrack formation at the fiber/matrix interface and matrix itself. Such mass loss also indicates a critical loss of mechanical properties due to an irreversible degradation mechanism.Figure 10
Mass variation of GFRP bar specimen when heated between 20°C and 800°C (adapted from [25]).


### 4.3. FTIR Analysis

A number of past studies explored the degradation mechanism of GFRP reinforcing bars using FTIR analysis. Samples were placed in an FTIR spectrometer and analysed from 400 to 4000 cm−1 in transmittance or absorbance mode. Figure 11 shows typical FTIR spectra for conditioned and unconditioned samples. Five zones could be identified: 900–1200, 1400–1600, 1600–1800, 2800–3100, and 3200–3600 cm−1. These spectral zones are, respectively, attributed to O–H bending, C–O stretching, C=O stretching, C–H stretching, and O–H stretching. The past literature studies have identified and reported these peaks, as shown in Table 9. It is clear that most conducted research has focused on the O–H and C–H stretching bonds. This is owed to the ability to evaluate the hydrolysis reaction by measuring the peak in the OH band, which increases due to hydrolysis and relating it to the constant peak of CH. As such, the relative quantity of hydroxyl groups is characterized by the ratio of maximum peaks of OH-to-CH [13, 14, 27, 34]. Table 10 presents the OH-to-CH ratio of conditioned and control specimens of numerous studies. It should be noted that the reported ratio for conditioned samples is a result of the most severe conditioning scheme utilized in each study. This is a typical approach adopted by several researchers [8, 9, 13, 14, 24, 26, 31, 34]. The ratio for control GFRP bars ranged between 0.21 and 2.60, while that of conditioned counterparts was between 0.25 and 14.30. With such wide variations in the degradation, i.e., OH-to-CH ratio, the percentage increase was calculated and reported in Table 10. It ranged between 0 and 450%. Thus, to provide a more conclusive interpretation of the results, a more isolated investigation is performed. GFRP bars made with epoxy and vinylester are separated and each correlated to its associated moisture uptake. Results of Figure 12(a) show that the higher the moisture uptake, the higher the OH/CH increase due to conditioning of epoxy-GFRP bars. These test samples were all conditioned at 60°C, but for different durations. In fact, the left marker (0.74% moisture uptake increase) was conditioned in alkaline solution for up to 720 days, but did not show any increase in OH/CH ratio, indicating no degradation due to hydrolysis reaction [31]. This shows that the degradation only initiated after 1% moisture uptake. Further increase in uptake up to 2% did not cause significant impact on the change in OH/CH ratio, after which it dramatically increased up to 115%. Thus, it can be concluded that epoxy-GFRP bars could experience limited degradation if the moisture uptake is maintained below 2%, by mass.Figure 11
Glass transition temperature of conditioned and reference GFRP bars (adapted from [24]).Table 9
Summary of FTIR wavelengths detected in conditioned GFRP reinforcing bars.
Reference
Wavelength (cm−1)
O–H stretch
C=O stretch
C–H stretch
C–O stretch
O–H bend
[8]
3200–3650
—
2800–3000
—
—
[9]
3300–3600
1600–1800
2800–3100
1400–1600
900–1100
[10]
3200–3600
—
2800–3000
—
—
[11]
3200–3600
—
2800–3000
—
—
[13]
3300–3600
—
2800–3100
—
—
[14]
3300–3600
—
2800–3000
—
—
[23]
3200–3650
—
2800–3000
—
—
[24]
3300–3600
1600–1800
2800–3100
1400–1600
900–1200
[26]
3200–3500
—
2800–3000
—
—
[27]
3300–3600
—
2850–3100
—
—
[31]
3400
—
3026
—
—
[34]
3200–3400
—
2800–3100
—
—
[35]
3430
—
2900
—
—
[38]
3430
—
2900
—
—
[39]
3540
—
2900
—
—
[40]
3334
1557
2900
1420
1019Table 10
Results of OH/CH ratios in control and conditioned GFRP reinforcing bars.
Reference
Resin type
Conditioning
OH/CH
Medium
Temperature (°C)
Time (days)
Sustained load
Control
Conditioned
% Increase
[8]
Polyester
Alkaline
60
208.3
2.6 & 1.6
14.3 & 3.5
450 & 118.8
[9]
Vinylester
Concrete
50
60
No change
—
[10]
Epoxy
Concrete
60
450
0.59 & 1.07
1.15 & 1.24
94.9 & 15.9
[11]
Epoxy
Concrete
60
288 & 372
x
0.59 & 1.07
1.27 & 1.27
115.3 & 18.7
[13]
Vinylester
Water
50
180
0.45
0.51
13.3
[14]
Vinylester
Concrete
50
240
0.21
0.25
19.0
[23]
Polyester
Alkaline
60
208.3
2.6 & 1.6
14.3 & 3.5
450 & 118.8
[24]
Vinylester
Water
23
90
No change
—
[26]
Vinylester
Alkaline
70
365
No change
—
[27]
Vinylester polyester
Alkaline
60
180
0.49 & 0.48
0.54 & 0.87
22.7 & 81.3
[31]
Epoxy
Alkaline
60
720
No change
—
[34]
Vinylester
Concrete
50
30
x
0.51
0.53
3.9
[35]
Vinylester
Alkaline
73
120
x
1.05
1.45 & 1.30
38.1 & 23.8
[38]
Vinylester
Concrete
30
2920
x
No change
—
[39]
Vinylester
Alkaline
50
365
1.05
1.18
12.4
[40]
Vinylester
Alkaline
22
16.67
No change
—Figure 12
Correlation between OH/CH increase (%) and moisture uptake due to conditioning (%). (a) GFRP with epoxy matrix. (b) GFRP with vinylester matrix.
(a)
(b)Figure12(b) presents a relation between OH/CH percent increase and moisture uptake in conditioned GFRP bars made with vinylester matrix. The OH/CH increase remained below 20%, indicating a lower degree of hydrolysis reaction, when the moisture uptake was below 1.5%. Beyond 1.5%, degradation was much more significant due to conditioning, with OH/CH increasing by up to 95% compared to the control samples. In comparison and based on the studies revised in this paper, GFRP bars made with vinylester resin have been found to be more generally susceptible to hydrolysis than their epoxy counterparts.The effect of the initial tensile strength of unconditioned GFRP bars on their resistance to degradation was investigated in [10, 11]. In these studies, two different types of GFRP bars with dissimilar tensile strengths were compared. FTIR analysis results of [10], presented in Table 10, show that the GFRP bar with lower void content had an increase in OH/CH ratio of 15.9% compared to 94.9% for the sample with the higher void content. Obviously, as the void content increased, the hydrolysis reaction progressed, leading to higher OH/CH ratios. The degradation was further intensified upon the application of a sustained load, as shown in the results of [11].


### 4.4. DSC Analysis

The thermal behavior and characteristics of conditioned and control GFRP samples is analysed using DSC. The main properties obtained from DSC are glass transition temperature (Tg), curing process, melting temperature, crystallinity, relaxation, and thermal stability [25]. Of these properties, glass transition temperature is the most reported in the past literatures. It is determined by performing two scans, each from 20°C to 250°C, in accordance with ASTM E1356 [21]. The first scan compares the Tg of the conditioned and control samples. A reduction in Tg is indicative of matrix plasticization. The second scan identifies the degradation mechanism, whereby a similar Tg for conditioned and control samples indicates a reversible plasticizing effect, while a lower Tg for the former signifies an irreversible chemical degradation. Figure 13 illustrates typical DSC curves for alkaline-conditioned and unconditioned GFRP reinforcing bars. The step shown in the range of 90°C–110°C is used to find the Tg. While it appeared to be an endothermic or melting peak, there was no melting involved in this work [31]. In fact, the authors associated this peak to “the thermal stress relaxation phenomenon of polymer chains of the GFRP rebar during long-term ageing”.Figure 13
Differential scanning calorimetry curve for conditioned and control GFRP bars (adapted from [31]).Furthermore, a summary ofTg temperatures and cure ratio of various studies is presented in Table 11. These temperatures are results of the GFRP samples subject to the most severe conditioning scheme utilized in each study. It is clear that most studies experienced some decrease in the Tg of the first scan, ranging from 0 to 46%. This wide variation in results is owed to the different curing regimes, fiber content, matrix type, and presence of sustained loading, which all contribute to lowering the Tg and the deterioration of the GFRP bars. However, it is possible to isolate some values by selecting GFRP bars made with epoxy matrix and correlating them to other parameters. Figure 14 shows that higher changes in OH/CH ratios led to more intense reductions in the Tg. This shows a clear correlation between results of FTIR and DSC, signifying the progression of a hydrolysis reaction. In other terms, the factors that affect the OH/CH ratios have a similar effect on the Tg.Table 11
Results ofTg temperatures and cure ratio in control and conditioned GFRP reinforcing bars.
Reference
T
g
Cure ratio (%)
1st scan (°C)
2nd scan (°C)
Control
Conditioned
Decrease (%)
Control
Conditioned
Decrease (%)
[8]
93, 113, 126
98, 100, 112
5.4, 11.5, 11.1
—
—
—
98–100
[9]
145
140
3.4
—
—
—
—
[10]
101 & 125
95 & 90
5.9 & 28
106 & 125
105 & 126
0.0
95.3 & 100
[11]
101 & 125
90 & 90
10.9 & 28
106 & 125
105 & 125
—
95.3 & 100
[13]
110
103
6.4
129
128
0.8
85.3
[14]
105
104
1.0
134
129
3.7
78.4
[42]
116
62
46.6
120
83
30.8
96.7
[24]
105–125
—
—
105–125
—
—
100.0
[25]
112
67
40.2
113
68
39.8
99.1
[26]
116
117
—
117
118
—
99.1
[27]
123, 124, 90
122, 123, 84
0, 0, 6.6
124, 123, 90
123, 122, 85
0, 0, 5.5
100
[31]
105
96
8.6
—
—
—
—
[34]
112
95
15.2
131
130
0.8
85.5
[35]
114
105
7.9
120
120
0.0
95.0
[38]
124
120
3.2
125
120
4.0
99.2
[48]
115–125
112–124
2.4–2.6
—
—
—
—Figure 14
Percent increase ofTg of the first scan as a function of percent increase of OH/CH after conditioning of epoxy-GFRP bars.In the second scan, studies that employed concrete as the conditioning medium did not note any change in theTg between conditioned and control samples [10, 11, 14, 34, 38]. On the other hand, specimens conditioned in alkaline solutions and water showed some signs of irreversible chemical degradation, with the former conditioning scheme being more detrimental [25, 27, 42]. This provides evidence to the severity of conditioning in alkaline solutions, rendering results somewhat unrealistic and unreliable.Additionally, the cure ratio is shown in Table11. It is the ratio of the Tg of the first to second scan for the control samples. Other than [13, 14, 34], which are conducted by the same authors and probably using the same GFRP bars, the reported cure ratios were above 95%. Lower values signify that the samples were not fully cured and that postcuring occurred during the second scan.


### 4.5. SEM Analysis

SEM was employed to evaluate the microstructure and morphological changes to the GFRP reinforcing bars after conditioning. The fiber-matrix interface is considered one of the most vulnerable areas in GFRP reinforcing bars. It is merely a few microns thick and plays a critical role in transfer of the load between fiber and matrix [67]. In the presence of moisture and alkalis, the bond between fiber and matrix is weakened, causing damage to the interface itself and the GFRP as a whole.Micrographs typically highlight different damages or deteriorations that may be induced by conditioning, including matrix degradation, fiber/material interface degradation, microcrack formation, and fiber etching and leaching. Table12 shows a summary of deteriorations that have been identified in different studies using SEM. Clearly, matrix degradation, fiber/matrix interfacial degradation, and microcrack formation are interlinked, as they are reported simultaneously in multiple studies; they are denoted collectively hereafter as damage A. Fiber etching and leaching, on the other hand, is designated as damage B. Of the literature reviewed in this work, 58, 15, and 27% reported damage A, damage B, and no damage, respectively, as shown in Figure 15. This demonstrates that the matrix and the fiber/matrix interface are indeed the most vulnerable components of GFRP reinforcing bars, with much lesser deterioration of the fiber.Table 12
Summary of deteriorations identified by SEM in conditioned GFRP reinforcing bars.
Reference
Deterioration/Damages
Matrix degradation
Fiber/matrix interface damage
Microcracks
Fiber etching and leaching
[8]
x
x
x
[9]
No deterioration reported
[10]
x
x
x
[11]
x
x
x
[12]
x
x
x
[13]
No deterioration reported
[14]
No deterioration reported
[15]
x
x
x
x
[23]
x
x
x
[42]
x
x
[24]
No deterioration reported
[25]
x
x
x
[26]
No deterioration reported
[27]
x
x
x
[28]
x
x
x
x
[29]
x
x
x
[30]
x
x
x
[31]
No deterioration reported
[32]
x
x
x
[33]
x
x
x
[34]
No deterioration reported
[35]
x
x
x
[36]
x
x
x
x
[37]
x
x
x
[38]
No deterioration reported
[39]
x
x
x
[44]
x
x
x
[45]
x
x
x
[46]
x
x
x
x
[47]
x
x
x
[49]
x
x
x
[50]
x
x
x
x
[51]
No deterioration reportedFigure 15
Distribution of damage/deterioration in conditioned GFRP reinforcing bars.Figure16(a) shows a micrograph from past work that identified damage A in a conditioned GFRP reinforcing bar [10]. Samples in this study were placed in seawater-contaminated concrete for 15 months at 60°C. Clearly, the fiber/matrix interface weakened to the extent that fiber delamination occurred. This is primarily owed to the progression of hydrolysis reaction.Figure 16
Micrograph of conditioned GFRP reinforcing bar. (a) Damage A (adapted from [10]). (b) Damage B (adapted from [28]).
(a)
(b)Fiber etching and leaching were only noticed in research work that examined the durability performance of GFRP reinforcing bars made with E-glass/vinylester and conditioned in alkaline environment; they are represented as damage B. Figure16(b) presents a micrograph of a GFRP bar that suffered damage B [28] with circumferential damage to the fiber. In this work, GFRP samples were conditioned in an alkaline solution under a 25% UTS sustained load. The severity of the testing conditions resulted in all samples failing within 20 days of conditioning. No tensile strength could be retained at that point. The degree of deterioration was too harsh that it caused matrix degradation, fiber/matrix interfacial debonding, microcrack formation, and fiber damage simultaneously.In some of the reviewed studies, no damage or deterioration was noticed. Though some degradation may have been reported in the microstructure study, the mechanical properties of GFRP reinforcing bars were not significantly affected. It is thus critical for researchers to correlate the tensile strength, moisture uptake, OH/CH ratio, and SEM micrographs in a distinct study to provide a better understanding of the effect of the microstructure on the mechanical properties. Yet, while the scope of this work is to focus on microstructure characteristics of GFRP bars exposed to different conditioning regimes, a correlation based on the reviewed studies has been developed. For this matter, results from [10, 11, 13, 14, 27, 35] are employed. It is worth noting the GFRP samples were conditioned differently in these studies. The SEM micrographs of Figures 17(a)–17(f) show little to no cracks at the fiber/matrix interface, indicating limited degradation. The GFRPs of [10, 11, 27] are associated with moisture uptakes of 1.20, 1.95, and 1.63%, respectively. The respective increases in OH/CH ratios of [10, 11, 13, 14, 27, 35] are 15.9, 18.5, 13.3, 19.0, 22.7, and 3.9%. The strength retention of these samples was 85, 73, 93, 84, 100, and 84%, respectively. Yet, samples in [11, 35] were conditioned under sustained loads. Accordingly, it is possible to accept an increase in moisture uptake and OH/CH ratio up to a threshold of 1.6 and 18%, respectively, while anticipating up to 15% reduction in tensile strength.Figure 17
Micrograph of conditioned GFRP reinforcing bar with no damage. (a) Adapted from [10]. (b) Adapted from [11]. (c) Adapted from [13]. (d) Adapted from [14]. (e) Adapted from [27]. (f) Adapted from [35].
(a)
(b)
(c)
(d)
(e)
(f)


## 4.1. Moisture Uptake

Moisture penetrates into a composite material by the flow of solution into the microgaps between polymer chains, capillary transport through fiber/matrix interfacial microcracks, and in microcracks formed during manufacturing [5]. The diffusion process mainly depends on the type of matrix and surrounding environment. It can be measured as per the procedure of ASTM D570 [20]. Typical moisture absorption/uptake tests involve weighing samples before and after immersion. However, the hydrolysis reaction that may have occurred during conditioning could cause mass dissolution and, thus, affect the results. To account for this mass loss, samples are oven-dried for 24 hours at 100°C. This oven-dried mass would then be used to find the moisture uptake:(5)moisture uptake%,by mass=conditioned mass−oven dried massinitial mass×100.Figure8 is a typical moisture uptake curve that presents the increase in mass over conditioning time. Clearly, the diffusion rate of moisture is different for temperatures below and above 60°C. At temperatures of 60°C and below, the majority of the uptake occurs within the first 30 days of immersion. At higher temperatures, the uptake increases at a steady rate up to 50 days. Robert, et al. [7] explained that such behavior is owed to the amplification of thermomechanical degradation mechanisms at a temperature of 80°C. Further, the moisture uptakes for conditioned and unconditioned GFRP reinforcing bars tested in the past literature are summarized in Table 8. These values correspond to the most severe conditioning regime employed in each study. Results for conditioned samples ranged between 0.06 and 5.1, with vinylester-GFRP presenting the widest variations. Past researchers have discussed several factors that affected the uptake, namely, presence of a sustained load, void content, pH of conditioning medium, conditioning duration, and conditioning temperature [8–11, 24, 30, 32, 35, 38, 47, 56, 64, 65]. Each factor will be discussed in the sections below.Figure 8
Typical moisture uptake curve of GFRP bars (adapted from [7]).Table 8
Results of moisture uptake of GFRP reinforcing bars.
Reference
Unconditioned
Conditioned
Within high durability limits
Uptake (%)
Medium
Temperature (°C)
Time (days)
Sustained load
Uptake (%)
ACI 440.6 [62]
CSA S807 [63]
[7]
0.10
Water
80
58
1.80
Not Acceptable
Not Acceptable
[8]
0.23–1.15
Alkaline
50
21
0.25–1.36
Not Acceptable
Not Acceptable
[9]
0.00
Concrete
50
Saturation
0.44
Acceptable
Acceptable
[10]
0.10
Concrete
60
450
1.23–1.50
Not Acceptable
Not Acceptable
[11]
0.10
Concrete
60
288 & 372
x
1.95–2.54
Not Acceptable
Not Acceptable
[12]
0.05
Concrete
60
48.2
x
0.50
Acceptable
Acceptable
[23]
0.23–1.15
Alkaline
60
208.3
0.25–1.36
Not Acceptable
Not Acceptable
[42]
0.00
Seawater
65
365
5.10
Not Acceptable
Not Acceptable
[24]
0.02–0.15
Water
23
Saturation
0.20
Acceptable
Acceptable
[27]
0.05–1.59
Alkaline
50
21
0.06–1.63
Not Acceptable
Not Acceptable
[32]
0.01
Alkaline
60
720
0.74
Acceptable
Not Acceptable
[35]
0.37
Concrete
50
30
x
0.57
Acceptable
Acceptable
[40]
0.40
Alkaline
50
94
1.20
Not Acceptable
Not Acceptable
[44]
0.00
Alkaline
22
16.67
0.55
Acceptable
Acceptable
[46]
0.00
Alkaline
60
360.4
x
1.10
Not Acceptable
Not Acceptable
[48]
0.01
Alkaline
60
176
0.38
Acceptable
AcceptableThe effect of sustained load can be noted by comparing the results of [10, 11]. The only difference between these two researches is the addition of a sustained load of 25% UTS in [11]. Results, presented in Table 8, show that even with an average 26% shorter conditioning durations, the uptake was, on average, 68% higher. It is thus obvious that the addition of a sustained load can aggravate the moisture uptake.Recent work by El-Hassan, et al. [10] compared two types of GFRP bars under similar exposure conditions. The void contents were measured as 0.10 and 0.23%. The authors reported higher moisture uptake in the latter GFRP bar with higher void content. However, with limited information on the void content or fiber content in the existing literature, it is difficult to create a general conclusion. Further work would be needed to investigate the effect of void content on performance retention and microstructure changes.To simulate severe environments, researchers have utilized alkaline solutions with different pH. Values of pH ranged between 12 and 13.6. A recent study by D’Antino, et al. [66] reported that higher pH resulted in less tensile strength retention. Yet, immersion in simulated concrete pore (alkaline) solution is not a proper reproduction of real-life concrete. The main difference is the degree of contact of alkaline solution with the GFRP reinforcing bars. In the former case, the surface of the bar is entirely coated by the solution, resulting in maximum contact area, while in the latter case, alkaline solutions remain in the concrete pores, causing limited contact with the imbedded GFRP bar. In the case of moist concrete, representing underwater concrete, the degree of contact is intermediary between the dry concrete and wet alkaline solution.Conditioning temperature and duration have been widely investigated. Figure8 shows the effect of each of these conditions on the moisture uptake. It is clear that higher temperatures and longer exposure durations resulted in more uptake. Yet, it is worth noting that the former was the more influential factor of the two, thus, causing a more exacerbated degradation phenomenon. Robert, et al. [7] concluded that a thermomechanical degradation mechanism would occur at an elevated temperature of 80°C, owing to higher solution diffusion in a more porous resin matrix. Nevertheless, such conditioning temperature does not correspond to service life conditions of GFRP bars, justifying the maximum aging temperature recommended by ACI 440.3 [3] and CSA S806 [60] being 60°C.ACI 440.6 [62] and CSA S807 [63] propose durability limits in the form of maximum allowable moisture/water uptake. These limits are 1 and 0.75%, by mass, respectively, regardless of the exposure conditions. Table 8 shows the comparison of conditioned GFRP reinforcing bars with each limit. It is clear that more than half of the bars tested were not classified of high durability, based on moisture uptake. In fact, Figure 9 shows that 56 and 62% of bars, tested under the worst conditions in each study, failed to fall within the high durability limits of ACI 440.6 [62] and CSA S807 [63], respectively. In general, samples conditioned in alkaline solution at temperatures beyond 50°C were deemed unacceptable [8, 23, 27, 39, 46]. GFRP bars imbedded in concrete resulted in uptakes within the acceptable range, except for El-Hassan, et al. [11], where a sustained load of 25% UTS was applied, which is higher than the maximum allowable load recommended by ACI 440.1R [2]. A general comparison between alkaline and concrete conditioned GFRPs shows higher uptake in the former case and lower degree of satisfactoriness by the codes. Therefore, it should be noted that the moisture uptake of GFRP bars fell within the acceptable limits if conditioned in concrete and for shorter durations, lower temperatures, and/or without a sustained load. Nevertheless, in real-life scenarios, where GFRP bars are imbedded in concrete, it is possible to limit the moisture uptake by creating an impermeable concrete. This can be achieved by adding supplementary cementitious materials, as fly ash, ground granulated blast furnace slag, silica fume, or others.Figure 9
Percentage of GFRPs within acceptable durability limits. (a) ACI 440.6. (b) CSA S807.
(a)
(b)


## 4.2. TG Analysis

TG analysis was employed in the past literature to assess the decomposition of the polymer matrix under heat exposure. ASTM E1868 [19] was used to examine the change in mass as a function of temperature (20°C–800°C) and evaluate the thermal stability of the matrix. A typical mass variation curve as a function of temperature measured by TGA is shown in Figure 10. It is clear that a major mass loss of 18% occurred in the range of 300°C–450°C. This is attributed to the thermal degradation of the polymer matrix, as the molecular chains broke, leading to microcrack formation at the fiber/matrix interface and matrix itself. Such mass loss also indicates a critical loss of mechanical properties due to an irreversible degradation mechanism.Figure 10
Mass variation of GFRP bar specimen when heated between 20°C and 800°C (adapted from [25]).


## 4.3. FTIR Analysis

A number of past studies explored the degradation mechanism of GFRP reinforcing bars using FTIR analysis. Samples were placed in an FTIR spectrometer and analysed from 400 to 4000 cm−1 in transmittance or absorbance mode. Figure 11 shows typical FTIR spectra for conditioned and unconditioned samples. Five zones could be identified: 900–1200, 1400–1600, 1600–1800, 2800–3100, and 3200–3600 cm−1. These spectral zones are, respectively, attributed to O–H bending, C–O stretching, C=O stretching, C–H stretching, and O–H stretching. The past literature studies have identified and reported these peaks, as shown in Table 9. It is clear that most conducted research has focused on the O–H and C–H stretching bonds. This is owed to the ability to evaluate the hydrolysis reaction by measuring the peak in the OH band, which increases due to hydrolysis and relating it to the constant peak of CH. As such, the relative quantity of hydroxyl groups is characterized by the ratio of maximum peaks of OH-to-CH [13, 14, 27, 34]. Table 10 presents the OH-to-CH ratio of conditioned and control specimens of numerous studies. It should be noted that the reported ratio for conditioned samples is a result of the most severe conditioning scheme utilized in each study. This is a typical approach adopted by several researchers [8, 9, 13, 14, 24, 26, 31, 34]. The ratio for control GFRP bars ranged between 0.21 and 2.60, while that of conditioned counterparts was between 0.25 and 14.30. With such wide variations in the degradation, i.e., OH-to-CH ratio, the percentage increase was calculated and reported in Table 10. It ranged between 0 and 450%. Thus, to provide a more conclusive interpretation of the results, a more isolated investigation is performed. GFRP bars made with epoxy and vinylester are separated and each correlated to its associated moisture uptake. Results of Figure 12(a) show that the higher the moisture uptake, the higher the OH/CH increase due to conditioning of epoxy-GFRP bars. These test samples were all conditioned at 60°C, but for different durations. In fact, the left marker (0.74% moisture uptake increase) was conditioned in alkaline solution for up to 720 days, but did not show any increase in OH/CH ratio, indicating no degradation due to hydrolysis reaction [31]. This shows that the degradation only initiated after 1% moisture uptake. Further increase in uptake up to 2% did not cause significant impact on the change in OH/CH ratio, after which it dramatically increased up to 115%. Thus, it can be concluded that epoxy-GFRP bars could experience limited degradation if the moisture uptake is maintained below 2%, by mass.Figure 11
Glass transition temperature of conditioned and reference GFRP bars (adapted from [24]).Table 9
Summary of FTIR wavelengths detected in conditioned GFRP reinforcing bars.
Reference
Wavelength (cm−1)
O–H stretch
C=O stretch
C–H stretch
C–O stretch
O–H bend
[8]
3200–3650
—
2800–3000
—
—
[9]
3300–3600
1600–1800
2800–3100
1400–1600
900–1100
[10]
3200–3600
—
2800–3000
—
—
[11]
3200–3600
—
2800–3000
—
—
[13]
3300–3600
—
2800–3100
—
—
[14]
3300–3600
—
2800–3000
—
—
[23]
3200–3650
—
2800–3000
—
—
[24]
3300–3600
1600–1800
2800–3100
1400–1600
900–1200
[26]
3200–3500
—
2800–3000
—
—
[27]
3300–3600
—
2850–3100
—
—
[31]
3400
—
3026
—
—
[34]
3200–3400
—
2800–3100
—
—
[35]
3430
—
2900
—
—
[38]
3430
—
2900
—
—
[39]
3540
—
2900
—
—
[40]
3334
1557
2900
1420
1019Table 10
Results of OH/CH ratios in control and conditioned GFRP reinforcing bars.
Reference
Resin type
Conditioning
OH/CH
Medium
Temperature (°C)
Time (days)
Sustained load
Control
Conditioned
% Increase
[8]
Polyester
Alkaline
60
208.3
2.6 & 1.6
14.3 & 3.5
450 & 118.8
[9]
Vinylester
Concrete
50
60
No change
—
[10]
Epoxy
Concrete
60
450
0.59 & 1.07
1.15 & 1.24
94.9 & 15.9
[11]
Epoxy
Concrete
60
288 & 372
x
0.59 & 1.07
1.27 & 1.27
115.3 & 18.7
[13]
Vinylester
Water
50
180
0.45
0.51
13.3
[14]
Vinylester
Concrete
50
240
0.21
0.25
19.0
[23]
Polyester
Alkaline
60
208.3
2.6 & 1.6
14.3 & 3.5
450 & 118.8
[24]
Vinylester
Water
23
90
No change
—
[26]
Vinylester
Alkaline
70
365
No change
—
[27]
Vinylester polyester
Alkaline
60
180
0.49 & 0.48
0.54 & 0.87
22.7 & 81.3
[31]
Epoxy
Alkaline
60
720
No change
—
[34]
Vinylester
Concrete
50
30
x
0.51
0.53
3.9
[35]
Vinylester
Alkaline
73
120
x
1.05
1.45 & 1.30
38.1 & 23.8
[38]
Vinylester
Concrete
30
2920
x
No change
—
[39]
Vinylester
Alkaline
50
365
1.05
1.18
12.4
[40]
Vinylester
Alkaline
22
16.67
No change
—Figure 12
Correlation between OH/CH increase (%) and moisture uptake due to conditioning (%). (a) GFRP with epoxy matrix. (b) GFRP with vinylester matrix.
(a)
(b)Figure12(b) presents a relation between OH/CH percent increase and moisture uptake in conditioned GFRP bars made with vinylester matrix. The OH/CH increase remained below 20%, indicating a lower degree of hydrolysis reaction, when the moisture uptake was below 1.5%. Beyond 1.5%, degradation was much more significant due to conditioning, with OH/CH increasing by up to 95% compared to the control samples. In comparison and based on the studies revised in this paper, GFRP bars made with vinylester resin have been found to be more generally susceptible to hydrolysis than their epoxy counterparts.The effect of the initial tensile strength of unconditioned GFRP bars on their resistance to degradation was investigated in [10, 11]. In these studies, two different types of GFRP bars with dissimilar tensile strengths were compared. FTIR analysis results of [10], presented in Table 10, show that the GFRP bar with lower void content had an increase in OH/CH ratio of 15.9% compared to 94.9% for the sample with the higher void content. Obviously, as the void content increased, the hydrolysis reaction progressed, leading to higher OH/CH ratios. The degradation was further intensified upon the application of a sustained load, as shown in the results of [11].


## 4.4. DSC Analysis

The thermal behavior and characteristics of conditioned and control GFRP samples is analysed using DSC. The main properties obtained from DSC are glass transition temperature (Tg), curing process, melting temperature, crystallinity, relaxation, and thermal stability [25]. Of these properties, glass transition temperature is the most reported in the past literatures. It is determined by performing two scans, each from 20°C to 250°C, in accordance with ASTM E1356 [21]. The first scan compares the Tg of the conditioned and control samples. A reduction in Tg is indicative of matrix plasticization. The second scan identifies the degradation mechanism, whereby a similar Tg for conditioned and control samples indicates a reversible plasticizing effect, while a lower Tg for the former signifies an irreversible chemical degradation. Figure 13 illustrates typical DSC curves for alkaline-conditioned and unconditioned GFRP reinforcing bars. The step shown in the range of 90°C–110°C is used to find the Tg. While it appeared to be an endothermic or melting peak, there was no melting involved in this work [31]. In fact, the authors associated this peak to “the thermal stress relaxation phenomenon of polymer chains of the GFRP rebar during long-term ageing”.Figure 13
Differential scanning calorimetry curve for conditioned and control GFRP bars (adapted from [31]).Furthermore, a summary ofTg temperatures and cure ratio of various studies is presented in Table 11. These temperatures are results of the GFRP samples subject to the most severe conditioning scheme utilized in each study. It is clear that most studies experienced some decrease in the Tg of the first scan, ranging from 0 to 46%. This wide variation in results is owed to the different curing regimes, fiber content, matrix type, and presence of sustained loading, which all contribute to lowering the Tg and the deterioration of the GFRP bars. However, it is possible to isolate some values by selecting GFRP bars made with epoxy matrix and correlating them to other parameters. Figure 14 shows that higher changes in OH/CH ratios led to more intense reductions in the Tg. This shows a clear correlation between results of FTIR and DSC, signifying the progression of a hydrolysis reaction. In other terms, the factors that affect the OH/CH ratios have a similar effect on the Tg.Table 11
Results ofTg temperatures and cure ratio in control and conditioned GFRP reinforcing bars.
Reference
T
g
Cure ratio (%)
1st scan (°C)
2nd scan (°C)
Control
Conditioned
Decrease (%)
Control
Conditioned
Decrease (%)
[8]
93, 113, 126
98, 100, 112
5.4, 11.5, 11.1
—
—
—
98–100
[9]
145
140
3.4
—
—
—
—
[10]
101 & 125
95 & 90
5.9 & 28
106 & 125
105 & 126
0.0
95.3 & 100
[11]
101 & 125
90 & 90
10.9 & 28
106 & 125
105 & 125
—
95.3 & 100
[13]
110
103
6.4
129
128
0.8
85.3
[14]
105
104
1.0
134
129
3.7
78.4
[42]
116
62
46.6
120
83
30.8
96.7
[24]
105–125
—
—
105–125
—
—
100.0
[25]
112
67
40.2
113
68
39.8
99.1
[26]
116
117
—
117
118
—
99.1
[27]
123, 124, 90
122, 123, 84
0, 0, 6.6
124, 123, 90
123, 122, 85
0, 0, 5.5
100
[31]
105
96
8.6
—
—
—
—
[34]
112
95
15.2
131
130
0.8
85.5
[35]
114
105
7.9
120
120
0.0
95.0
[38]
124
120
3.2
125
120
4.0
99.2
[48]
115–125
112–124
2.4–2.6
—
—
—
—Figure 14
Percent increase ofTg of the first scan as a function of percent increase of OH/CH after conditioning of epoxy-GFRP bars.In the second scan, studies that employed concrete as the conditioning medium did not note any change in theTg between conditioned and control samples [10, 11, 14, 34, 38]. On the other hand, specimens conditioned in alkaline solutions and water showed some signs of irreversible chemical degradation, with the former conditioning scheme being more detrimental [25, 27, 42]. This provides evidence to the severity of conditioning in alkaline solutions, rendering results somewhat unrealistic and unreliable.Additionally, the cure ratio is shown in Table11. It is the ratio of the Tg of the first to second scan for the control samples. Other than [13, 14, 34], which are conducted by the same authors and probably using the same GFRP bars, the reported cure ratios were above 95%. Lower values signify that the samples were not fully cured and that postcuring occurred during the second scan.


## 4.5. SEM Analysis

SEM was employed to evaluate the microstructure and morphological changes to the GFRP reinforcing bars after conditioning. The fiber-matrix interface is considered one of the most vulnerable areas in GFRP reinforcing bars. It is merely a few microns thick and plays a critical role in transfer of the load between fiber and matrix [67]. In the presence of moisture and alkalis, the bond between fiber and matrix is weakened, causing damage to the interface itself and the GFRP as a whole.Micrographs typically highlight different damages or deteriorations that may be induced by conditioning, including matrix degradation, fiber/material interface degradation, microcrack formation, and fiber etching and leaching. Table12 shows a summary of deteriorations that have been identified in different studies using SEM. Clearly, matrix degradation, fiber/matrix interfacial degradation, and microcrack formation are interlinked, as they are reported simultaneously in multiple studies; they are denoted collectively hereafter as damage A. Fiber etching and leaching, on the other hand, is designated as damage B. Of the literature reviewed in this work, 58, 15, and 27% reported damage A, damage B, and no damage, respectively, as shown in Figure 15. This demonstrates that the matrix and the fiber/matrix interface are indeed the most vulnerable components of GFRP reinforcing bars, with much lesser deterioration of the fiber.Table 12
Summary of deteriorations identified by SEM in conditioned GFRP reinforcing bars.
Reference
Deterioration/Damages
Matrix degradation
Fiber/matrix interface damage
Microcracks
Fiber etching and leaching
[8]
x
x
x
[9]
No deterioration reported
[10]
x
x
x
[11]
x
x
x
[12]
x
x
x
[13]
No deterioration reported
[14]
No deterioration reported
[15]
x
x
x
x
[23]
x
x
x
[42]
x
x
[24]
No deterioration reported
[25]
x
x
x
[26]
No deterioration reported
[27]
x
x
x
[28]
x
x
x
x
[29]
x
x
x
[30]
x
x
x
[31]
No deterioration reported
[32]
x
x
x
[33]
x
x
x
[34]
No deterioration reported
[35]
x
x
x
[36]
x
x
x
x
[37]
x
x
x
[38]
No deterioration reported
[39]
x
x
x
[44]
x
x
x
[45]
x
x
x
[46]
x
x
x
x
[47]
x
x
x
[49]
x
x
x
[50]
x
x
x
x
[51]
No deterioration reportedFigure 15
Distribution of damage/deterioration in conditioned GFRP reinforcing bars.Figure16(a) shows a micrograph from past work that identified damage A in a conditioned GFRP reinforcing bar [10]. Samples in this study were placed in seawater-contaminated concrete for 15 months at 60°C. Clearly, the fiber/matrix interface weakened to the extent that fiber delamination occurred. This is primarily owed to the progression of hydrolysis reaction.Figure 16
Micrograph of conditioned GFRP reinforcing bar. (a) Damage A (adapted from [10]). (b) Damage B (adapted from [28]).
(a)
(b)Fiber etching and leaching were only noticed in research work that examined the durability performance of GFRP reinforcing bars made with E-glass/vinylester and conditioned in alkaline environment; they are represented as damage B. Figure16(b) presents a micrograph of a GFRP bar that suffered damage B [28] with circumferential damage to the fiber. In this work, GFRP samples were conditioned in an alkaline solution under a 25% UTS sustained load. The severity of the testing conditions resulted in all samples failing within 20 days of conditioning. No tensile strength could be retained at that point. The degree of deterioration was too harsh that it caused matrix degradation, fiber/matrix interfacial debonding, microcrack formation, and fiber damage simultaneously.In some of the reviewed studies, no damage or deterioration was noticed. Though some degradation may have been reported in the microstructure study, the mechanical properties of GFRP reinforcing bars were not significantly affected. It is thus critical for researchers to correlate the tensile strength, moisture uptake, OH/CH ratio, and SEM micrographs in a distinct study to provide a better understanding of the effect of the microstructure on the mechanical properties. Yet, while the scope of this work is to focus on microstructure characteristics of GFRP bars exposed to different conditioning regimes, a correlation based on the reviewed studies has been developed. For this matter, results from [10, 11, 13, 14, 27, 35] are employed. It is worth noting the GFRP samples were conditioned differently in these studies. The SEM micrographs of Figures 17(a)–17(f) show little to no cracks at the fiber/matrix interface, indicating limited degradation. The GFRPs of [10, 11, 27] are associated with moisture uptakes of 1.20, 1.95, and 1.63%, respectively. The respective increases in OH/CH ratios of [10, 11, 13, 14, 27, 35] are 15.9, 18.5, 13.3, 19.0, 22.7, and 3.9%. The strength retention of these samples was 85, 73, 93, 84, 100, and 84%, respectively. Yet, samples in [11, 35] were conditioned under sustained loads. Accordingly, it is possible to accept an increase in moisture uptake and OH/CH ratio up to a threshold of 1.6 and 18%, respectively, while anticipating up to 15% reduction in tensile strength.Figure 17
Micrograph of conditioned GFRP reinforcing bar with no damage. (a) Adapted from [10]. (b) Adapted from [11]. (c) Adapted from [13]. (d) Adapted from [14]. (e) Adapted from [27]. (f) Adapted from [35].
(a)
(b)
(c)
(d)
(e)
(f)


## 5. Conclusions and Remarks

In this paper, studies examining the microstructure of GFRP reinforcing bars exposed to different environmental conditions and sustained loading were reviewed. Experimental results and findings were collected from past literature, analysed, and compared to provide conclusive remarks on the effect of conditioning on the microstructure of GFRP reinforcing bars. Based on the conducted analysis, the following conclusions can be drawn:(i)
Mechanical properties from the past literature were used to provide correlation equations for unconditioned GFRP bars. Tensile strength and modulus of as-received GFRP bars were correlated in a linear equation. These relationships could provide reasonably accurate prediction of the tensile modulus from the tensile strength.(ii)
Only 14% of reviewed studies investigated the performance and microstructure of GFRP bars made with E-glass and epoxy. On average, the tensile properties of such unconditioned bars seemed to be 14% superior to those of vinylester counterparts. Discrepancies in the testing procedures did not allow for a direct microstructure comparison between the two types. Further testing is needed prior to the adoption of these GFRP reinforcing bars.(iii)
In the majority of reviewed studies, GFRP specimens were conditioned in alkaline and water solutions due to the ease of the experimental setup. Concrete was only employed in 23% of the studies. Conditioning durations varied greatly, with 0–90 and more than 365 days being most commonly used.(iv)
The application of a sustained load to GFRP reinforcing bars during conditioning was adopted in 28% of the studies reviewed. Research studies that exceeded the limits stated by codes and standards concluded that the set limits were only valid for specific nonaggressive environment conditions. In fact, elevated temperatures promoted the degradation mechanism. More research is needed to update these limits to include different conditioning regimes.(v)
Higher temperature, prolonged conditioning, presence of a sustained load, and conditioning in alkaline solution caused an increase in moisture uptake. Most uptake values extracted from the literature exceeded the limits set by ACI 440.6 [62] and CSA S807 [63]. Only specimens that were conditioned in concrete at low temperatures, for short durations, and/or without a sustained load were within the acceptable moisture uptake durability limits. This raises a concern to the aggressiveness of the conditioning utilized in the literature.(vi)
A correlation was developed between the progression of hydrolysis (OH/CH increase) and the decrease inTg (%). Within an increase of 20% OH/CH, the Tg decreased by only 10%. However, greater surges in OH/CH caused much more dramatic reductions in the glass transition temperature. Nevertheless, limiting the moisture uptake could control the increase in hydroxyl groups.(vii)
Conditioning in alkaline solution was found to be too severe and did not accurately represent real-life scenarios. Specimens conditioned in alkaline solutions experienced irreversible chemical degradation, more hydroxyl group formation, and more intense degradation to the microstructure.(viii)
Four types of damages were reported in conditioned GFRP bars: matrix degradation, fiber degradation, fiber/matrix interface degradation, and microcrack formation. Most of concrete-encased GFRP specimens did not experience excessive degradation, as opposed to alkaline conditioning that severely damaged the GFRP.(ix)
The increase in moisture uptake and OH/CH ratio and formation of microcracks does not necessarily have a severe effect on the mechanical performance. In fact, it is possible to allow an increase in moisture uptake and OH/CH ratio up to 1.6 and 18%, respectively, while anticipating up to 15% reductions in tensile strength.The conclusions presented above were based on the past literature reviewed in this work. Further studies are needed to fill the research gaps before GFRP reinforcing bars could be widely adopted by the construction industry. The following are suggestions for future research direction:(i)
A standard testing procedure for GFRP reinforcing bars capable of providing consistent and reliable results that represents real-life scenarios(ii)
Comparative performance and microstructure evaluation of GFRP reinforcing bars in concrete and subjected to different environment conditions and levels of sustained load(iii)
Microstructure characterization of GFRP bars exposed to acidic solution(iv)
The effect of void content on performance retention and microstructure changes of GFRP reinforcing bars(v)
Performance and microstructure comparison between GFRP bars made with different resin, i.e., epoxy and vinylester, subject to the same conditioning schemes(vi)
Performance and microstructure comparison between concrete-encased GFRP specimens exposed to elevated temperatures with an applied sustained load and actual in-service specimens(vii)
GFRP reinforcing bars in concretes made with different supplementary cementitious materials to limit the moisture uptake



---

*Source: 8053843-2019-04-01.xml*