A comprehensive review on mechanical properties of pultruded FRP composites subjected to long-term environmental effects

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Abstract

Pultruded fiber-reinforced polymer (FRP) composite is expected to be widely applied for civil infrastructures due to its advantages of lightweight, anti-corrosion, and industrial manufacturing. Its long-term performance has attracted increasing attentions from academia and industry. This paper presents a comprehensive review on experimental studies investigating the mechanical performance of pultruded FRP composites subjected to long-term environmental effects, including water/moisture, alkaline solutions, acidic solutions, low/high temperatures, ultraviolet radiation, freeze-thaw cycles, wet-dry cycles, and in situ environments. Over 1900 experimentally determined mechanical properties of FRP materials were collected, including tensile, compressive, flexural and shear strength and moduli. The reported test data were highly dispersed, and no uniform conclusions could be drawn from these data. Exposure to water and water-based solutions (alkaline and acidic solutions) had detrimental impacts on the mechanical properties of pultruded FRP materials, whereas other environmental effects induced various levels of degradation. The degradation mechanisms for each environmental effect were discussed, and the existing design approaches were presented. Based on the findings from this review, recommendations were proposed for future works. The database presented herein, which is the largest in the available literature, enables a comprehensive understanding of the degradation behavior of pultruded FRP composites. Moreover, this work can serve as a foundation for deriving predictive models for pultruded FRP materials exposed to long-term environmental effects.

Introduction

In recent decades, fiber-reinforced polymer (FRP) composites have gained increasing acceptance and seen wider application in the field of civil engineering [[1], [2], [3]]. FRP composites can be constructed from many different types of fibers, including carbon, glass, aramid and basalt fibers, and various resin matrix materials, including polyester, epoxy and vinyl ester. The selection between fibers and resins is primarily dependent on the balance between the desired performance and cost of the FRP material; in civil engineering applications, FRPs are most commonly composed of carbon and glass fibers. Although differences exist in the mechanical performance of different types of materials, FRP composites are generally known to have high strength- and stiffness-to-weight ratios and excellent corrosion resistance; thus, FRP structures might be used as an alternative to conventional concrete or steel structures, particularly in harsh environments [4]. Nonetheless, existing FRP structures have shown some extent of degradation at the material level after being in service for a certain number of years [5,6]. In addition, an early work by China National Building Materials Bureau [7] have presented the degradations of GFRP composite materials induced by natural weathering, hygrothermal effect and water immersion. Despite of being a preliminary study, this work successfully revealed the degradations of FRP materials and more importantly, anti-degradation measures were also discussed. The durability of FRP composites under long-term environmental effects must be addressed to ensure the safe design of FRP structures and further promote the application of FRP materials in the future [[8], [9], [10], [11]].

The long-term performance of FRP composites may not be accessible through real-time experimental tests, since this type of test often requires long durations, which are not achievable in laboratory conditions. Therefore, accelerated aging tests are typically adopted to evaluate the degradation of FRP materials within a reasonable amount of time. Numerous experimental and analytical studies have been conducted to investigate the physical, mechanical and chemical properties of FRP composites exposed to various environmental effects, including water, high humidity, alkaline and acidic solutions, low/high temperature, ultraviolet (UV) radiation, freeze-thaw cycles and wet-dry cycles, and the combinations of these effects, such as the combination of water/humidity and high temperature, which is also referred to as the hygrothermal effect. Among these effects, water and water-based solutions have drawn greater attention in the field, as they are able to induce detrimental degradation of FRPs. The earliest study on the effect of water dates back to the 1970s, when Ishai [12] carried out a test to evaluate the tensile behavior of glass FRPs (GFRPs) immersed in water. It was observed a reduction in tensile strength from the water exposure; this material degradation was attributed to the water ingress and its resulting damage at the fiber-matrix interface. Later, Selzer and Friedrich [13] immersed carbon FRPs (CFRPs) in distilled water. It was found that the absorbed moisture reduced the matrix-dominated and fiber-matrix interface-dominated properties of the CFRPs, whereas the corresponding influence of the fiber-dominated properties was not detectable. Nishizaki and Meiarashi [14] performed similar tests with GFRPs in heated water and found that fiber-matrix debonding occurred when the GFRPs were immersed in higher temperature water, which resulted in a reduction in flexural strength. A similar test was carried out by Ellyin and Maser [15]; in which the resin of the GFRPs were seen to be gradually detached from the fibers as the immersion temperature increased. Moreover, Soykok et al. [16] concluded that extending the immersion time and elevating the immersion temperature could increase the water uptake in the material, which would lead to greater degradation in mechanical properties. In addition to freshwater, seawater exposure, wherein the pH value is often approximately 8, is also of interest since marine constructions using FRP materials have seen developmental progress in many countries. Wood and Bradley [17] immersed GFRP and CFRP materials in seawater and investigated their interfacial strength. Damage was observed to initiate at the boundaries of the resin-rich zone. Chu et al. [18] compared the effects of distilled water and seawater and concluded that the material degradation was more severe in alkaline solutions than in distilled water. Recently, Tual et al. [19] carried out a test on CFRP materials used in turbine blades operating in ocean environments, wherein seawater-induced corrosion is inevitable. This study identified that the seawater exposure significantly degraded the strength of the CFRP material, whereas the corresponding degradation in the moduli of the material was not prominent. Moreover, Afshar et al. [20] found that in marine environments, the degradation in the transverse flexural modulus was more substantial than that in the longitudinal direction; however, both the longitudinal flexural strength and the transverse flexural strength were significantly decreased by the exposure to the marine environment. The damage was attributed to UV-induced microcracks on the material surface and the degradation of the fiber-matrix interface. Recently, Li et al. [21] immersed CFRPs in the artificial seawater and found that a higher temperature could lead to a greater water absorption, while the degradation behavior of the tested CFRPs was not that sensitive to the alkalinity. Jesthi and Nayak [22] tested the mechanical properties of the hybrid carbon-glass fiber reinforced composites under seawater immersion. It was found that the hybrid composites showed a lower seawater diffusivity as compared to plain GFRPs and also, improved mechanical properties can be achieved through the hybrid composites.

To quantify the effect of water or water-based solutions on FRP composites, some efforts have been made to develop predictive models for assessing the water diffusion in FRP composites. Shen and Springer [23], as a benchmark for the field, proposed a set of equations to predict the moisture distribution in both absorption and desorption processes of FRP materials exposed to water or high humidity. Later, Shen and Springer [24] used their equations to predict the residual tensile strength of materials, through which only the trend could be captured. Bonniau and Bunsell [25] also developed a model to predict water diffusion in FRP composites in which both water immersion and high humidity were considered. Zheng and Morgan [26] investigated the water absorption of CFRP materials exposed to hygrothermal conditions. In contrast to previous studies, a reverse thermal effect was observed: due to water molecule condensation within elastic cavities, the epoxy resin absorbed more water at low temperatures than at high temperatures. In addition, Akay et al. [27] revealed that oven-cured aramid FRP (AFRP) laminates with a greater number of voids exhibited increased moisture diffusivity, which resulted in decreased residual mechanical properties. Gellert and Turley [28] studied the seawater uptake in GFRP materials and found that the flexural and interlaminar shear strength values were reduced. Ben Daly et al. [29] compared the water diffusion of pultruded GFRPs immersed in distilled water and seawater. It was found that at high temperatures, 25–85 °C, the moisture diffusivity was higher in distilled water than in seawater, whereas at a low temperature, 5 °C, the opposite trend was observed, which was attributed to the icy state of water at low temperatures. Additionally, Dhakal et al. [30] tested polymer composites reinforced with organic hemp fibers, wherein they reported a similar finding to Akay et al. [27]; in which the water uptake increased as the fiber volume fraction Vf increased. This phenomenon occurred because the number of voids in the material increased as the fiber volume fraction increased. In an early study by Loos and Springer [31]; CFRPs were tested after immersion in two uncommon solutions: jet fuel and aviation oil. The moisture diffusion of CFRP materials immersed in such solutions was not as sensitive to temperature as the corresponding diffusion when immersed in water.

In addition to the above studies independently addressing the effects of water/moisture, some studies have also considered the effects of external loads on immersed FRP materials. Marom and Broutman [32] applied external stresses on GFRP and CFRP materials immersed in water and observed that the moisture absorption rate and the maximum moisture content both increased due to the existence of external stresses. Abdel-Magid et al. [33] also confirmed that applied stresses could have a positive short-term impact on the material, wherein the strength of the material could increase due to fiber straightening. However, this same impact did not hold from a long-term perspective. Humeau et al. [34] directly evaluated the coupled effects of water diffusion and external loading on CFRP materials and concluded that the coupling of these effects could have a greater impact on water uptake and damage development in the materials.

The influence of high temperatures has already been reported in several tests studying the effects of water. In addition to those studies, some studies have been specifically conducted to define the effects of high temperatures. Tsotsis [35] exposed CFRP materials to high temperatures in air and evaluated their corresponding residual mechanical properties. This study recommended that the compressive properties of FRP materials should be used as a measure of degradation since the compressive properties exhibited a more pronounced decrease than tensile properties after high-temperature exposure. Later, Tsotsis [36] and Tsotsis and Lee [37] suggested that weight loss could lead to large errors in estimating the mechanical properties of FRP materials; thus, mechanical properties cannot be used as an indicator of the degradation of FRP materials. Akay et al. [38] evaluated the interlaminar shear strength and impact performance of CFRP materials at elevated temperatures and revealed that high temperatures could cause a progressive loss of resin matrix, resulting in a gradual degradation of the material. Muliana et al. [39] also applied high temperatures to FRP materials and emphasized that the high temperatures, together with external loads, could impact the microstructure of CFRP composites. In addition, Atas and Dogan [40] concluded that thermal aging could significantly affect the impact resistance of GFRP composites.

In contrast to high temperatures in normal conditions, some researchers have focused on the performance of FRP materials exposed to fire. Bai et al. [41,42] experimentally tested the mechanical properties of pultruded GFRP materials exposed to high temperatures up to 700 °C, and they proposed predictive models to characterize the degradation of the materials. Later, Correia et al. [43] and Bai et al. [44] experimentally and analytically investigated the effectiveness of a fire protection system for pultruded GFRP beams. When exposed to fire, the compressive properties of GFRP materials were more susceptible to degradation than the corresponding tensile properties: the tested beams all failed at the compressive flanges rather than at the tensile flanges. Then, Correia et al. [45] reviewed previous works on the fire performance of pultruded GFRP materials and concluded that the fire resistance of this type of material could significantly vary, as the loading scheme (compressive or tensile loading), cross-sectional geometries, and the number of exposed surfaces could all have an impact on the overall fire resistance of the GFRP members. In addition, Morgado et al. [46,47] continued previous works on fire protection systems and recommended that GFRP members should be integrated into floor systems, as the number of exposed surfaces could greatly affect the fire resistance of the material. In all these studies addressing the effects of fire exposure, the faster degradation of the compressive properties of the FRP materials was highlighted. This finding is in agreement with the study by Tsotsis [35]; who found that the compressive behavior of FRP materials was a better measure of material degradation than the corresponding tensile behavior.

Low/subzero temperatures had a different influence on FRP materials than high temperatures. Dutta and Hui [48] reported that at subzero temperatures, the stiffness of GFRP materials increased, particularly the flexural properties, which are matrix-dominated properties. Wu et al. [49] evaluated GFRP materials in a bridge deck system subjected to freeze-thaw cycling and low temperatures, in which it was found that these conditions had an insubstantial effect on the flexural properties of the materials. Sousa et al. [50] reviewed some of the previous works on the effects of freeze-thaw cycling on FRP materials. It was concluded that under the effects of thermal cycling, the flexural behavior, particularly the flexural modulus, of FRPs was more affected than the corresponding tensile and interlaminar shear behaviors. Additionally, matrix cracking and fiber-matrix debonding were identified as the main reasons for the rupture of the materials. Recently, Grammatikos et al. [51] applied freeze-thaw cycles to pultruded GFRP materials exposed to air and soaked in water. The soaked samples exhibited greater degradation in the matrix and the fiber-matrix interface, whereas the properties of the soaked samples were able to be recovered after drying; this phenomenon was due to the reversible plasticization process. Based on the above studies, it can be seen that low temperatures generally have a negligible effect on the mechanical properties of FRP composites. However, note that when moisture exists, the resulting combined effects of low temperatures and water could cause serious material degradation, as the residual stress due to these combined effects could lead to microcracks in the matrix and the fiber-matrix interface [52].

Many studies have revealed the significance of investigating the in situ environmental effects on FRP composites. Indeed, in situ environments include combined effects that could facilitate the degradation process of FRP composites, and more importantly, FRP structures are always exposed to combined effects in practical applications. Roylance and Roylance [53] performed the earliest tests that employed in situ environmental conditions, in which the environments in Panama, Puerto Rico, the United States and Australia were all considered. Although their findings were suggested as preliminary, they highlighted the need to understand the environmental degradation of composites. Al-Bastaki and Al-Madani [54] studied the effect of natural weathering and compared this effect to that from seawater immersion. A reduced flexural strength was observed in both conditions, whereas a higher deterioration rate was identified in seawater than in natural weathering. To simulate an in situ environment under laboratory conditions, Mouzakis et al. [55] developed an environmental chamber that could generate various aging effects, including temperature, moisture and UV radiation. Post-curing induced by high temperatures and UV radiation was observed to increase the stiffness of the resin matrix. Although it was not a direct experimental test, Keller [56] examined an existing FRP bridge that was in service in Switzerland for eight years and reported different extents of degradation at the material level, which again demonstrated the need to assess the mechanical performance of FRP composites exposed to in situ environmental effects.

To date, many studies have been carried out to address the degradation of FRP composites subjected to long-term environmental effects. Nonetheless, the conducted experimental tests were highly dependent on the choices of the researchers, and the reported results were extremely dispersed [57]. Thus, it is difficult to draw a uniform conclusion. The observed dispersion in test methods and the corresponding results is attribute to a number of reasons. Bank et al. [58]; in their review of studies from 1988 to 1995, highlighted that the degradation of composite materials may occur in multiple locations, including in the fibers, matrix and fiber-matrix interface; thus, experimental tests must be tailored regarding the specific features of each phase of the material. In addition, Micelli and Nanni [59] also pointed out that each FRP composite has unique constituents and manufacturing techniques; therefore, the conclusions drawn for one material are not necessarily applicable to others. This may partially explain the variations in test methods and test results. As a result of the dispersive nature of the obtained test data, there is still a lack of experimental databases on the durability of FRP composites, particularly for those tests conducted over 18 months [60].

Moreover, FRP composites can be manufactured via many techniques, such as wet layup, autoclave processing, vacuum-assisted resin transfer molding (VARTM), and pultrusion. The FRP products created through different manufacturing techniques often differ in terms of their mechanical properties, since these techniques could create significantly different fiber-matrix architectures and fiber volume ratios. For example, pultruded FRP composite materials, manufactured via pultrusion process (see Fig. 1a), have unidirectional fiber-matrix architecture and can be made virtually in any shapes (see Fig. 1b). Pultruded FRP composites have a relatively higher fiber volume ratio, typically ranging from 60% to 75%, whereas the wet-layuped laminates typically have several layers of fibers in multiple directions and a lower fiber volume ratio. The effect of the fiber volume ratio has been investigated by many researchers. Akay et al. [27] found that increasing the fiber volume ratio Vf could lead to increased water uptake in FRP materials since a greater number of voids exist at higher fiber volume ratios. In terms of the fiber architecture, Allred [61,62] observed that AFRP materials with different fiber orientations exhibited different degradation behaviors when exposed to moisture, high temperatures, and freeze-thaw and wet-dry cycles. Aronhime et al. [63] also identified that AFRP materials with different fiber orientations, 0° and 90°, had different water diffusivities. The diffusivity increased as a shorter diffusion path was provided; hence, the diffusion coefficient in the longitudinal direction of the composite material was higher than that in the transverse direction. In addition, this study again demonstrated that diffusivity increased as the fiber volume ratio Vf increased. Karbhari and Zhang [60] tested the moisture diffusivity of GFRP materials in both distilled water and an alkaline solution with a pH value of 10. The material thickness and fiber architecture were found to significantly affect the degradation behavior of the material. Despite having various FRP products available in the market, pultruded FRP composites are currently the most commonly used in large-scale civil engineering structures, such as cooling towers, bridges, modular buildings and ocean platforms. In this regard, this work investigates only pultruded FRP composites, whereas all other types of FRP materials, such as wet-layuped laminates, are not discussed. In addition, focusing on pultruded FRP materials neglects the complex degradation behavior induced by nonuniform fiber architectures.

In this work, to systemically investigate the mechanical performance of pultruded FRP composites subjected to long-term environmental effects and provide a reliable database for the field, a review was conducted on experimental studies from 1996 to 2018. In these studies, the data from over 1100 aging tests, each conducted under specific aging conditions, were collected. In addition, over 1900 experimentally determined residual properties of the pultruded FRP materials were obtained, including tensile, compressive, flexural and shear strength and moduli. In the following sections, the degradation mechanisms of environmental effects are first discussed. Then, experimental tests corresponding to each environmental effect, including water and high moisture, alkaline and acidic solutions, temperatures, UV radiation, freeze-thaw and wet-dry cycles, and in situ environments, are presented. In the end, recommendations for future work are provided, and conclusions are drawn. It is worth noting that this database is the largest known in the available literature, aiming to provide a comprehensive understanding for the field and a solid foundation for future work. Additionally, this work directly supports the ongoing development of the Chinese standard for pultruded FRP composite structures.

Section snippets

Degradation mechanisms

In this work, a total of eight environmental effects are addressed, including water or high moisture, alkaline solutions, acidic solutions, temperatures, UV radiation, freeze-thaw cycles, wet-dry cycles, and in situ environmental conditions. Each effect may induce different extents of degradation in FRP composites via physical and/or chemical reactions. Water or high moisture is of the most interest since it exists in most constructions and could significantly damage FRP materials at the

Environmental effects

This work addresses a total of eight environmental effects that are of the highest interest for civil infrastructures: water/high moisture, alkaline solutions, acidic solutions, low/high temperature, UV radiation, freeze-thaw cycles, wet-dry cycles and in situ environmental conditions. In the practical design of FRP structures, four mechanical material properties are typically needed: tensile, compressive, flexural and shear properties. In the following sections, the collected data from the

Existing design approach

In the past decade, design guides for pultruded FRP composites have been established in many countries and/or regions worldwide, such as the ASCE [115] from the United States, the EUR27666 [116] from Europe, and the CECS [117] from China. The American and European design guides have discussed the degradation of FRP materials due to exposure to moisture and high temperatures. The strength and modulus reduction factors from the American and European guides are shown in Table 1. These guides

Recommendations for future work

In recent decades, a number of studies have been conducted by many researchers to investigate the mechanical performance of pultruded FRP composites subjected to long-term environmental effects. The reported experimental studies have helped acquire a good understanding of the degradation behavior of FRP materials and, more importantly, provided valuable guidance for future work. Based on the tests reviewed in this work, the following recommendations are proposed for future work.

  • 1)

    A longer

Conclusions

A comprehensive review was conducted on experimental studies investigating the mechanical performance of pultruded FRP composites subjected to long-term environmental effects, including water/moisture, alkaline solutions, acidic solutions, low/high temperature, UV radiation, freeze-thaw cycles, wet-dry cycles, and in situ environments. In addition, the degradation mechanisms of these eight environmental effects were discussed. Then, the experimental tests on each environmental effect and the

Data availability

All data, models, and code generated or used during the study appear in the submitted article.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by grants from the China National Key Research and Development Project (No.2017YFC0703000).

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