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Laboratory Assessment and Durability Performance of Vinyl-Ester, Polyester, and Epoxy
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Glass-FRP Bars for Concrete Structures
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Brahim Benmokrane,
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Ahmed H. Ali,
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Hamdy M. Mohamed,
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Adel ElSafty,
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and Allan Manalo
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Corresponding author. Professor of Civil Engineering and Tier-1 Canada Research Chair in
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Advanced Composite Materials for Civil Structures and NSERC Chair in FRP Reinforcement for
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Concrete Structures, Department of Civil Engineering, University of Sherbrooke, Quebec,
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Canada, J1K 2R1, Tel.: 1-819-821-7758.
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Brahim.Benmokrane@usherbrooke.ca
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PhD candidate
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Department of Civil Engineering
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University of Sherbrooke, Quebec, Canada
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Ahmed.Ali@usherbrooke.ca
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Postdoctoral fellow
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Department of Civil Engineering
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University of Sherbrooke, Quebec, Canada
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Hamdy.Mohamed@usherbrooke.ca
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Professor
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Civil Engineering, College of Computing, Engineering, and Construction, UNF, Jacksonville,
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FL, USA
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Adel.el-safty@unf.edu
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Senior Lecturer, Centre for Future Materials, Faculty of Health, Engineering and Sciences,
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University of Southern Queensland, Toowoomba, Queensland 4350, Australia.
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manalo@usq.edu.au
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Abstract
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In the last decade, noncorrosive glass fiber-reinforced-polymer (GFRP) bars have become more
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widely accepted as cost-effective alternatives to steel bars in many applications for concrete
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structures (bridges, parking garages, and water tanks). Also, these reinforcing bars are valuable
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for temporary concrete structures such as soft-eyes in tunneling works. The cost of the GFRP
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bars can be optimized considering the type of resin according the application. Yet limited
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research seems to have investigated the durability of GFRP bars manufactured with different
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types of resin. In this study, the physical and mechanical properties of GFRP bars made with
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vinyl-ester, isophthalic polyester, or epoxy resins were evaluated first. The long-term
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performance of these bars under alkaline exposure simulating a concrete environment was then
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assessed in accordance with ASTM D7705. The alkaline exposure consisted in immersing the
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bars in an alkaline solution for 1000, 3000 and 5,000 h at elevated temperature (60
o
C) to
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accelerate the effects. Subsequently, the bar properties were assessed and compared with the
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values obtained on unconditioned reference specimens. The test results reveal that the vinyl-ester
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and epoxy GFRP bars had the best physical and mechanical properties and lowest degradation
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rate after conditioning in alkaline solution, while the polyester GFRP bars evidenced the lowest
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physical and mechanical properties and exhibited significant degradation of physical and
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mechanical properties after conditioning.
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Keywords: Glass fiber; vinyl ester, polyester, epoxy; fiber-reinforced polymer (FRP); glass FRP
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(GFRP) rebars; physical and mechanical properties; durability performance; alkaline; accelerated
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aging; microstructural, concrete structures.
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Introduction
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Fiber-reinforced-polymer (FRP) bars have been well accepted as internal and external
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reinforcement for concrete structures (ACI 440.1R [ACI 2015]; Benmokrane et al. 2016a; Ali et
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al. 2016a; Mohamed et al. 2016). This reinforcing material offers better resistance to
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environmental agents as well as high stiffness-to-weight and strength-to-weight ratios when
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compared with conventional construction materials such as steel. Extensive research and
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development efforts have demonstrated that FRP bars are effective reinforcement in concrete
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members subject to bending (Maranan et al. 2015), shear (Ali et al. 2013 and 2016b),
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compression (Maranan et al. 2016), and impact (Goldston et al. 2016). Material specifications
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and design guidelines (ACI 440.6M [ACI 2008]; CAN/CSA S807 [CSA 2010]) have also been
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developed to encourage the construction industry to use FRP bars. This has resulted in many
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demonstration projects and field applications, such as bridges (Benmokrane et al. 2004), parking
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garages (Benmokrane et al. 2012), water-treatment plants (Mohamed and Benmokrane 2014),
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bridge barriers (El-Salakawy et al. 2005), concrete pavement (Benmokrane et al. 2008), and
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jetties (Manalo et al. 2014).
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Different types of fibers are used in manufacturing FRP bars such as carbon, glass,
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aramid, and basalt. Many studies have been carried out on the performance and use of FRP bars
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made with these different fibers, providing good insight into their physical and mechanical
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properties as well as their durability characteristics (Kocaoz et al. 2005; Banibayat and Patnaik
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2014; Ali et al. 2015; Benmokrane et al. 2016a, b; Li et al. 2015; Abbasi and Hogg 2005;
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Alsayed et al. 2012; Al-Salloum et al. 2013; Hassan et al. 2016; Tanks et al. 2016). Glass is the
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most commonly used fiber type in manufacturing FRP bars due to their relatively low
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comparative cost (ACI 2015). Similarly, Castro et al. (1998) highlighted the importance of the
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resin system used in manufacturing FRP bars to achieve the desired mechanical properties and
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durability characteristics. The resin system is important as it acts as a matrix bonding the fibers
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together and spreading the load applied to the composite between each of the individual fibers.
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The resin system also protects the fibers from abrasion and impact damage as well as from
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severe environmental conditions—such as water, salts, and alkalis—which affect the durability
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of FRP products (SP System 1998). A deterioration of this interface reduces the transfer of the
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loads between fibers and thus weakens the composite materials (Almusallam et al. 2013). The
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interface between the fiber and matrix is a nonhomogeneous region about 1 µm thick. This layer
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is weakly bonded and most vulnerable to deterioration. The three dominant deterioration
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mechanisms include matrix osmotic cracking, interfacial debonding, and delamination (Chen et
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al., 2007). Moisture diffusion into FRP composites could be influenced by the material’s
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anisotropic and heterogeneous character. Along with diffusion into the matrix, wicking through
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the fiber/matrix interface in the fiber direction could be a predominant mechanism of moisture
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ingress (Apicella et al., 1982). Nonvisible dissociation between fibers and matrix could lead to
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rapid losses of interfacial shear strength (Ferrier et al. 2016; Ashbee and Wyatt, 1969).
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Unfortunately, limited research attention has been paid to the effect of the resin-system type on
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the physical and mechanical properties as well as the durability characteristics of GFRP bars.
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Most of the glass-fiber-reinforced polymer (GFRP) bars available are manufactured with
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E or ECR glass fibers that are normally wetted with a thermosetting resin such as epoxy or
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vinyl ester. Numerous studies have investigated FRP bars made with vinyl-ester resin to
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determine the effect of environmental conditions (water, salts, alkalis) on their physical and
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mechanical properties (Mouritz et al. 2004; Wang 2005; Zou et al. 2008; Robert et al. 2009;
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Robert and Benmokrane 2013; Benmokrane et al. 2014, 2015, 2016b, 2016c). Similarly, Soles et
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al. (1998); Amaro et al. 2013; Ali et al. 2015; and Benmokrane et al. (2016a) are some of the
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numerous researchers who have investigated the durability performance of FRP bars made with
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epoxy resins. GFRP bars made with these resin systems are the most commonly used as
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reinforcement for concrete structures given their high performance and very good durability
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characteristics. Studies into the behavior of fiber-reinforced isophthalic polyester-resin
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composites have primarily addressed industrial and nonstructural products such as natural-fiber
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composites (Manalo et al. 2015). GFRP bars manufactured with isophthalic polyester resin are
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normally used for temporary structures such as soft-eyes in underground excavations and
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tunneling works (Schurch and Jost 2006). In these proprietary applications, GFRP-bar durability
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is not a concern. The key advantage of GFRP bars is the low cost of polyester resin and the fact
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that GFRP bars can be cut without damaging the drilling equipment’s cutter heads. Comparisons
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performed by some researchers [Ashbee et al, 1967; Ashbee and Wyatt, 1969; Abeysinghe et al.,
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1982] have indicated that the matrix formed by vinyl ester, which contains many fewer ester
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units compared to polyester, experiences very little deterioration caused by hydroxyl ions
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compared to a polyester matrix. As a result, CSA S807 (2010) classifies isophthalic polyester-
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based GFRP bars as having moderate durability (D2), while classifying epoxy- and vinyl-ester-
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based GFRP bars as having high durability (D1). Obviously, these classifications were
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established based on the results obtained by different researchers on GFRP bars manufactured
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with a specific resin system, i.e., either vinyl esters or epoxies (with very few studies on
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isophthalic polyesters). Consequently, no sound generalizations can be made. Clearly, a single
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