Comparative durability of GFRP composite reinforcing bars in concrete and in simulated concrete environments

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Abstract

Many studies suggest that the durability of glass-fiber-reinforced-polymer (GFRP) bars in a simulated concrete pore solution is very different than the same bars in an actual concrete environment. This study conducted a comparative evaluation of the durability of GFRP bars in concrete and in simulated concrete environments by investigating their interlaminar shear strength. It focused on evaluating the physical, mechanical, and microstructural properties of GFRP bars subjected to high moisture, saline, and alkaline environments. Bare GFRP bars and cement-embedded GFRP bars were immersed in solutions at different temperatures (23 °C, 60 °C, and 80 °C) and for different exposure times (28, 56, and 112 days). The results show that the percentage water uptake and the apparent diffusivity of the GFRP bars were strongly dependent on the type and temperature of the immersion solution. The interlaminar shear strength of the GFRP bars directly immersed in a solution degraded more significantly than those embedded in concrete and immersed. Moreover, the alkaline solution was more aggressive to the GFRP bars than tap water or saline solution, affecting bar fiber, matrix interface, and chemical structure. As a result of this study, master curves and time-shift factors were developed to correlate the retention of interlaminar shear strength from the accelerated aging test to the service life of GFRP bars in an actual concrete environment.

Introduction

Glass-fiber-reinforced-polymer (GFRP) bars have emerged as a promising and cost-effective replacement for steel to increase the useful life of reinforced-concrete structures exposed to severely aggressive environments. Extensive research and development has been conducted in the US, Canada, Europe, and Japan [1] on this reinforcing material, which has a number of characteristics, such as high tensile strength; light weight; noncorroding, nonmagnetic, and nonconductive properties. This work has led to the many successful field applications of GFRP-reinforced concrete structures, including highway bridges and barriers, pavements and parking garages, storage facilities for chemical and wastewater-treatment plants, magnetic-resonance-imaging facilities, detector loops in railway lines, and temporary structures, such as soft-eyes in underground excavations and tunneling works. Composite reinforcing material is now also being increasingly investigated and used in Australia [[2], [3], [4], [5], [6]] as the main reinforcement in concrete structures near the coastline and in aggressive soils. As internal reinforcement, GFRP bars are continuously subjected to alkaline environments provided by the surrounding concrete and other environmental conditions that might affect the physical, mechanical, and long-term durability properties of the bars. Many researchers have suggested that the high alkalinity of concrete pore solutions constitutes an aggressive environment for GFRP bars, which could damage the glass fibers and/or degrade the fiber–resin interface [[7], [8], [9], [10], [11]]. Similarly, infrastructure systems are exposed to external agents during their service life, including high moisture, alkalinity, and saline environments that can degrade the material properties [12] and the bond of GFRP bars in concrete [13,14]. Given the limited understanding about GFRP-bar durability, Ceroni et al. [15] and Karbhari and Zhang [9] indicated that designers apply very conservative safety factors to account for the unquantified detrimental effects which eliminates the high-strength performance of this reinforcing material. Thus, Micelli and Nanni [16] and Gooranorimi and Nanni [17] highlighted the importance of understanding the long-term and durability performance of GFRP bars under different aggressive environmental conditions, since this performance is critical to the widespread acceptance of GFRP reinforcement for civil-infrastructure applications.

In recent years, significant efforts have gone into studying the effects of highly aggressive environments on GFRP-bar durability. Most studies have focused on high moisture, saline, and alkaline environments as these types of environments are considered environmental problems in reinforced-concrete elements [15]. In most cases, the durability of this reinforcing material is determined based on the changes in bar mechanical properties as the result of accelerated testing and evaluation programs using bare GFRP bars [18]. Kim et al. [19] exposed GFRP bars directly to different solutions at room and elevated temperatures to accelerate degradation. They found that the alkaline solution reduced the tensile strength of E-glass/vinyl-ester FRP bars by almost 60% after 132 days. Based on the model presented by Davalos et al. [20], GFRP bars made of E-glass and vinyl-ester resin can retain only 38% of their tensile strength after 50 years of exposure in saturated loaded concrete at 10 °C. In most of these durability studies, the GFRP bars experienced significant loss of mechanical properties and it was impossible to make full use their superior properties. Almusallam et al. [21], however, highlighted that the accelerated laboratory experiments were harsher than actual field conditions. Robert et al. (2009) found that GFRP bars embedded in moist concrete and exposed to tap water at elevated temperature could retain up to 90% of their tensile strength after 240 days of exposure. Moreover, in-field durability testing indicated that there was no degradation of the GFRP bars in the concrete structures. Mufti et al. [23] found no degradation of the GFRP bars in five concrete bridge structures across Canada after exposure to natural environmental conditions for 5–8 years. Gooranorimi and Nanni [17] further validated the long-term durability of GFRP bars extracted from the concrete deck of the Sierra de la Cruz Creek Bridge in Texas (US) after 15 years of service. More recently, Benmokrane et al. [24] reported no significant changes in the physico-chemical properties and microstructure of GFRP bars extracted from a concrete bridge barrier on the Val-Alain Bridge in Canada after 11 years of service exposure to wet–dry cycles, freeze–thaw cycles, and deicing salts. Mufti et al. [23] pointed out that the durability results for GFRP bars in actual concrete structures are very different from those produced by accelerated laboratory testing in which the bars are immersed in alkaline solution because the concrete itself shields the GFRP bars from direct exposure to various environmental conditions. Thus, a more realistic, simple study should be conducted to better understand the durability of GFRP bars in concrete environments. Moreover, predicting the service life and long-term performance of GFRP bars subjected to different environmental factors is of immense importance to the further use of these non-corrodible reinforcing materials.

The above studies clearly indicate that the accelerated testing of bare GFRP bars subjected to simulated concrete pore solutions yields very different durability results than those of bars in actual concrete environments over a long period of time. In fact, D'Antino et al. [25] highlighted that the environmental aging of GFRP bars by different research groups often produced contradictory results. Most of these studies were conducted by characterization of the tensile-test properties of GFRP bars, flexural investigation of concrete beams reinforced with GFRP bars subjected to accelerated aging testing, or extraction of GFRP bars from actual concrete structures for in-field service evaluation. These investigations required significant resources and time, thereby limiting the test parameters and data obtained to evaluate GFRP-bar durability. Similarly, most durability investigations focusing on tensile-strength tests measured very little change because this property is mostly defined by fiber mechanical properties [26,27]. Ceroni et al. [15] and Kim et al. [19] highlighted that accelerated aging involves exposure to moisture and elevated temperatures, which affects resin properties more than fiber properties. Field conditions limit the length of bar extraction from existing concrete structures. Consequently, ILSS characterization using the short-beam shear test method might be the only feasible option. Adams [28] indicated that the short-beam shear test is one of the most important mechanical tests for composites and an excellent choice for comparative testing. This is due to the test-method's simplicity, but it measures the interior integrity of bars, especially fiber–matrix adhesion. Thus, evaluating the interface property of GFRP bars using the short-beam shear test can give a straightforward, reliable indication of the resistance of the fiber–matrix interface after exposure to aggressive environments.

Karbhari and Zhang [9] suggested that glass fibers and vinyl-ester resin systems are preferred for civil infrastructure due to cost considerations and ease of processing. Tannous and Saadatmanesh [8] reported that vinyl-ester-based GFRP bars offer higher fiber protection and provide high resistance to chemical attack. Moreover, Benmokrane et al. [1,11] found that the vinyl-ester-based GFRP bars tested could retain almost all their original tensile strength and stiffness properties, even after long-term exposure to alkaline solutions. Based on the short-term test results for concrete beams immersed in tap water in temperature-controlled tanks, Davalos et al. [20] found that the dominant degradation mechanism for the GFRP bars was deterioration of the fiber–matrix interface. Wang et al. [29] also indicated that the fiber–resin interface is more generally easily destroyed by the aggressive solution and is commonly believed to be the weakest component in composite materials. Micelli and Nanni [16] suggested that the short-beam shear test could yield good representative results in evaluating the fiber–resin interface of GFRP bars, and these results could provide indications of potential effects on longitudinal properties. This is due to the interlaminar shear strength (ILSS) of GFRP bars, which is primarily related to resin properties and governed by the fiber–matrix interface [1]. Furthermore, Benmokrane et al. [11] suggested that the fiber–resin interface was one of the important issues in manufacturing GFRP bars. As a result, this bar property was added to the recently approved CSA S807 [30] as a new test requirement for quality-assurance testing. Therefore, variations in interlaminar shear strength could be a good indicator of deterioration in GFRP bars [15] and provide a measurement of resin damage caused by fluid penetration, which occurs during bar aging [19]. Despite its simplicity, few investigations of the durability of GFRP bars using ILSS have been conducted [1,11,15,19]; none have explored ILSS for evaluating the durability of GFRP bars under accelerated aging conditions and exposed to simulated concrete environments.

The current study provides a comparative evaluation of the durability of GFRP bars in concrete and in simulated concrete environments by investigating their interlaminar shear strength. It focuses on the evaluation of the physical, mechanical, and microstructural properties of GFRP bars in high moisture, saline, and alkaline environments. These types of environments were selected as they are considered environmental problems in reinforced-concrete elements [15]. The results from this study provide a better understanding of the durability and long-term performance of GFRP bars for safe design and application as internal reinforcement in concrete structures. It also provides a prediction of the service life of GFRP bars in an actual concrete environment based on the temperature time-shift factor determined from accelerated aging tests.

Section snippets

Materials

Sand-coated Grade III GFRP bars [30] with a nominal diameter (db) of 9.53 mm were used in the study. These GFRP bars were considered as they have been successfully applied in many infrastructure projects and are classified as highly durable reinforcement by the CSA S807 [30]. The bars were manufactured using ECR-glass fibers in a modified vinyl-ester resin in a pultrusion process. Small bar size was considered in this study due to faster moisture absorption compared to larger bar sizes [1]. The

Moisture uptake

Moisture uptake is an important factor that can significantly affect the mechanical and durability properties of GFRP bars [11]. Micelli and Nanni [16] pointed to strong evidence that the rate of degradation of GFRP bars exposed to fluid environments is related to the rate of fluid sorption. GFRP bars embedded in reinforced-concrete elements can absorb moisture and water, which penetrate the resin and affect the fiber–resin interface. Moreover, the water volume expansion at low temperatures

SEM and FTIR observations

Scanning-electron-microscopy (SEM) observations were performed to assess the microstructure of the GFRP bars before and after conditioning after 112 days using the JEOL JSM-840A SEM (JEOL, Akishima, Tokyo, Japan). All of the specimens observed under SEM were cut, polished, and coated with a thin layer of gold–palladium using a vapor-deposit process. SEM observations were performed on both the bar cross section and at the fiber–matrix interface. Similarly, Fourier transform infrared spectroscopy

Prediction of the long-term behavior and service life of GFRP-Bar ILSS

Aiello et al. [26] stated that reliably predicting the long-term behavior of civil infrastructure subject to the action of environmental factors is a complex problem. It requires accelerated aging through hygrothermal exposure for the long-term assessment of materials durability and relies on the superposition of temperature and moisture to enhance and speed up environmental degradation. Moreover, Davalos et al. [20] highlighted that only a few studies focused on the development of life-cycle

Conclusions

This study comparatively evaluated the durability of GFRP bars in concrete and in a simulated concrete environment by investigating their interlaminar shear strength. It focused on evaluating the physical, mechanical, and microstructural properties of GFRP bars in high-moisture, saltwater, and alkali environments. The following conclusions can be drawn from this study:

  • The percentage water uptake and the apparent diffusivity of the GFRP bars were strongly dependent on the type of solution and

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

The first author acknowledges the scholarship granted by the Australian Government Endeavour Executive Leadership Award to undertake his professional development at the Centre for Integration of Composites into Infrastructure (CICI), the Natural Science and Engineering Research Council (NSERC) of Canada, West Virginia University, Morgantown, USA, and University of Sherbrooke, Sherbrooke, QC, Canada. The authors are grateful to Inconmat V-ROD Australia and Pultrall Inc. (Thetford Mines, QC,

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