Alkaline aging effects on thermal stability and apparent interlaminar shear strength of GFRP/epoxy/graphene bars

Efeitos do envelhecimento alcalino na estabilidade térmica e na resistência aparente ao cisalhamento interlaminar de barras de GFRP/epóxi/grafeno

Abstract

Composite materials are increasingly used in civil construction as alternatives to traditional materials. Pultruded glass fiber-reinforced polymer (GFRP) bars stand out for their corrosion resistance, and the addition of graphene enhances polymer performance. This study evaluated the durability of GFRP/epoxy/graphene bars exposed to a pH 13.5 alkaline solution at 60 °C for 60 and 90 days. Microstructural, thermal, chemical, and mechanical analyses were performed using SEM, DSC, TGA, MCC, FTIR, and short-beam shear testing. After 90 days, the glass transition temperature decreased by 3.42 °C, FTIR indicated significant changes in O–H groups while C–H bands remained stable, and apparent shear strength decreased by 15.67%. These results demonstrate that the composite maintains substantial structural performance under aggressive alkaline conditions, providing valuable insights for its use in reinforced concrete applications requiring durability and corrosion resistance.

Keywords
Polymer composites; GFRP bars; Graphene; Alkaline attack

Resumo

Materiais compósitos têm sido cada vez mais utilizados na construção civil como alternativas aos materiais tradicionais. As barras pultrudadas de polímero reforçado com fibra de vidro (GFRP) se destacam pela resistência à corrosão, e a adição de grafeno melhora o desempenho da matriz polimérica. Este estudo avaliou a durabilidade de barras de GFRP/epóxi/grafeno expostas a uma solução alcalina de pH 13,5 a 60 °C por 60 e 90 dias. Foram realizadas análises microestruturais, térmicas, químicas e mecânicas utilizando MEV, DSC, TGA, MCC, FTIR e ensaios de cisalhamento aparente (short-beam). Após 90 dias, a temperatura de transição vítrea diminuiu 3,42 °C, o FTIR indicou alterações significativas nos grupos O–H enquanto as bandas C–H permaneceram estáveis, e a resistência ao cisalhamento aparente reduziu 15,67%. Esses resultados demonstram que o compósito mantém desempenho estrutural significativo em condições alcalinas agressivas, fornecendo informações relevantes para sua aplicação em concreto armado que exige durabilidade e resistência à corrosão.

Palavras-chave
Compósitos poliméricos; Barras de GFRP; Grafeno; Ataque alcalino

1 Introduction

Composite materials are developed to improve mechanical properties, combining a matrix that can be polymeric with structural reinforcements (Sharma et al., 2020). Thermoplastic and thermoset resins can be used, with thermoset resins (epoxy, polyester, phenolic, etc.) being more widely used due to their greater mechanical strength (Utekar et al., 2021). Although the properties of composites depend largely on the reinforcing fibers (Zhao et al., 2020), these fibers, such as glass, carbon, basalt, and aramid, mitigate the limitations of thermoset resins, expanding the applicability of these materials. Thus, composites have been widely used in sectors such as aerospace, automotive, and civil construction (Chaudhary; Ahmad, 2020).

Epoxy resins stand out among the thermoset resins widely used in polymer composites due to their excellent chemical and corrosion resistance, high adhesion, good thermal and electrical properties, and dimensional stability with low shrinkage after curing. They are used in coatings and insulating materials (Bajpai; Davidson; Robert, 2021). Glass fibers, widely used in composite manufacturing due to their good performance-to-cost ratio, include E- and S-types, specified by D578 (ASTM, 2011), and the AR-type, defined by C1666 (ASTM, 2015). E-glass has good electrical insulation, moisture resistance, and high mechanical strength, while S-glass has higher tensile strength and modulus of elasticity, but at a higher cost. AR glass, in turn, stands out for its resistance to alkali attack. Despite their high tensile strength, glass fibers have a low modulus of elasticity, reduced resistance to elasticity and shear, and lower adhesion to polymer resins (Cousin et al., 2019; Thomason, 2019).

Due to its great potential, graphene has been integrated into polymer composite materials, standing out in various applications due to its mechanical properties. This two-dimensional material is formed by a single atomic layer of carbon atoms in a hexagonal structure, being flexible, transparent, resistant to chemicals, bacteria, and viruses, in addition to having thermal stability (Govindaraj et al., 2021). A more economical alternative to pure graphene is graphene nanoplatelets (GNP), which consist of thin layers of graphene ranging from 2 to 10 layers, with a thickness between 0.34 nm and 100 nm, and are generally marketed with a thickness of 10 nm. These nanoplates can be produced more easily on an industrial scale, mainly by mechanical exfoliation methods, which reduces costs. GNPs have a wide range of applications, including the formulation of new composites (Yee; Ghayesh, 2023), and are capable of enhancing the properties of composites (Hashim; Jumahat; Jawaid, 2021). Many researchers use tensile, hardness, and impact tests to evaluate the mechanical properties of polymers with different forms of loading, improving their physical and mechanical characteristics (Mateab; Albozahid, 2022).

The incorporation of graphene into the epoxy matrix has proven to be an effective strategy for simultaneously improving the mechanical and thermal properties of composites, increasing their durability, especially in alkaline solutions. The addition of graphene or its derivative forms, such as graphene oxide, promotes significant gains in tensile strength, short beam bending strength, and impact strength. Studies report increases of up to 83% in tensile strength and 71% in flexural strength, even with low mass fractions of reinforcement, a result attributed to the strong interfacial interaction between graphene and the matrix and its ability to restrict deformation and bifurcate crack propagation, delaying mechanical failure (Embrey et al., 2017; Kamaraj; Dodson; Datta, 2020; Shelar; Suryawanshi; Wayzode, 2024; Yu et al., 2024; Zhao et al., 2022). The uniformity in the dispersion of nanoparticles and the surface functionalization of graphene are determining factors in enhancing these effects, ensuring efficient charge transfer and improving the overall performance of the composite (Budak et al., 2024; Meng et al., 2023; Osman et al., 2022; Wan et al., 2013).

Graphene contributes significantly to increasing the conductivity and thermal stability of epoxy matrix composites. Even at low concentrations, it can raise the glass transition temperature and thermal conductivity by up to 80%, an effect associated with its high intrinsic conductivity and the formation of efficient thermal networks (Embrey et al., 2017; Feng et al., 2018; Foad Abbaspour; Kanvisi, 2023; Pramodkumar; Budhe, 2024; Sun et al., 2020; Zhao et al., 2022). When graphene nanoplatelets have controlled alignment and orientation, these gains become even more significant (Du et al., 2022; Foad Abbaspour; Kanvisi, 2023). In addition to reinforcing the matrix-fiber interface, graphene reduces water absorption and increases resistance to chemical degradation, favoring the performance of composites in aggressive environments, such as alkaline solutions (Feng et al., 2018; Kamaraj; Dodson; Datta, 2020; Sharma et al., 2022). Depending on the configuration adopted, it can also confer additional properties, such as flame retardancy and greater electrical insulation (Feng et al., 2018; Zhao et al., 2022).

Graphene-modified fiberglass reinforced plastic (FRP) materials have attracted increasing academic and industrial interest due to their superior mechanical, electrical, and functional properties. Since the discovery of graphene in 2004, the number of publications and patents involving FRP composites containing this nanomaterial has grown exponentially, demonstrating its innovative potential and applicability in different sectors (Islam et al., 2022; Mirabedini et al., 2020). Currently, commercial graphene-modified FRP products are already available, particularly in areas such as sports, rackets, bicycles, and fishing rods, where the incorporation of new technologies is progressing more rapidly (Valorosi et al., 2019). However, the transition of these materials from laboratory development to large-scale commercial use in civil construction still faces technical and economic challenges, including proper graphene dispersion, quality control, high cost, and lack of standardization (Hamidul; Afroj; Karim, 2024; Islam et al., 2022; Wu; Qureshi, 2021).

The development of FRP (Fiber Reinforced Polymer) composites must consider not only the specific contributions of the fibers and the matrix, but mainly the system formed by the interfaces. The interface is a three-dimensional region of contact between the fibers and the matrix, involving not only a two-dimensional area of contact, but also a region of finite thickness that extends on both sides of the interface, both in the fiber and in the matrix (Ray; Rathore, 2014).

The pultrusion process, which began in the 1950s, is used to manufacture bars, tubes, and straight profiles with continuous sections in a wide variety of geometries and shapes. GFRP bars are produced by first tensioning the glass fibers and impregnating them with thermoset resin in liquid form in the impregnation tank. The material is then braided and heated, causing the polymer resin to cross-link, and finally cut to the desired standard length (Silva, 2024).

GFRP bars, even without electrolytic corrosion, can undergo chemical changes due to the alkaline solution of concrete, whose pore solution has a pH between 12.5 and 13.5 (Fergani et al., 2018; Helebrant et al., 2017). Cement hydration products release alkaline compounds, intensifying the manipulation of glass fibers and compromising the mechanical properties of the bars (Yue et al., 2024).

The reduction in glass transition temperature (T_g) is closely linked to chemical degradation in the polymer matrix, primarily caused by macromolecular chain scission, which increases segmental mobility and decreases matrix stiffness (Ernault et al., 2017a, 2017b). The absorption of water or other plasticizing species from the alkaline solution further intensifies this phenomenon by enhancing the plasticization of the polymer (Krauklis et al., 2019; Sawpan, 2016, 2024; Zhou; Lucas, 1999b). These structural changes result in a lower T_g and, consequently, a decline in mechanical performance, as evidenced by reductions in flexural strength and elastic modulus (Sawpan, 2016, 2024). Additionally, ambient moisture can also function as a plasticizer, contributing to decreases in T_g, strength, and stiffness of the resin (Li; Xian; Li, 2018; Yin et al., 2019).

However, the magnitude of these changes depends on several factors, such as the type of resin, temperature, solution pH, exposure time, and the presence of fillers or protective coatings (D’Antino et al., 2023; Sawpan; Alshahrani; Kafi, 2023; Yin et al., 2019, 2023). In general, T_g tends to decrease after alkaline attack, but in many cases, this reduction is moderate, with residual values exceeding 90% of the original T_g even after long periods of exposure, especially when water diffusion is limited (Ahmad Sawpan; Beg, 2024; Sawpan; Alshahrani; Kafi, 2023; Yin et al., 2023). The appropriate choice of resin type (vinyl ester, polyester, or epoxy) and reduction of the alkalinity of the medium are effective strategies to mitigate the effects of degradation (Jiang et al., 2025; Moura; Ribeiro; Lima, 2021; Wang et al., 2022).

The degradation of GFRP bars results from a complex physical-chemical process that compromises the polymer matrix and leads to the dissolution of glass fibers, altering their physical, chemical, thermal, and mechanical properties. In this process, hydroxyl ions (OH⁻), present in the concrete pore solution due to the dissolution of hydroxides from the cementitious matrix (NaOH, Ca(OH)₂, and KOH), diffuse through the polymer matrix of the bars, promoting the hydrolysis of ester groups, especially in polyester and vinyl ester matrices. This progressive degradation of the matrix exposes the glass fibers to the alkaline solution, intensifying the deterioration of the composite (Khennane; Melchers, 2003; Micelli; Corradi, 2017).

The scientific literature presents experimental evidence that water diffusion in composites varies according to the type of material, the shape of the test specimens, and the immersion conditions. In many cases, this phenomenon follows Fickian diffusion (Avena; Bunsell, 1988; Gellert; Turley, 1999; Grammatikos et al., 2015), although there are also reports of non-Fickian behavior (Alzamora Guzman; Brøndsted, 2015; El Yagoubi et al., 2012). Furthermore, while some authors point out that composites reach a point of water saturation (Gellert; Turley, 1999), others observe a continuous mass gain (Alzamora Guzman; Brøndsted, 2015) or even a mass loss associated with the leaching of low molecular weight components (Dewimille; Bunsell, 1983; Grammatikos et al., 2015; Kootsookos; Mouritz, 2004). In composites reinforced with inorganic fibers, such as glass or carbon, water molecules can lodge in voids, at the interfaces between the matrix and the fibers (Gellert; Turley, 1999), in the free volumes present in the polymer matrix (Apicella; Tessieri; de Cataldis, 1984) or even chemically bond to the matrix itself (Zhou; Lucas, 1999a).

Epoxy matrix composites are particularly susceptible to hygrothermal aging when exposed to high temperature and humidity environments. In such cases, moisture absorption can degrade the mechanical properties of the material, either reversibly or irreversibly (Bullions; Loos; McGrath, 2003; Selzer; Friedrich, 1993; Silva, 2007; Tsenoglou; Pavlidou; Papaspyrides, 2006; Zrida et al., 2017), compromising, for example, interlaminar shear strength (Bian et al., 2012; Gellert; Turley, 1999).

The attack of OH⁻ ions leads to corrosion of the glass fiber surface, forming soluble silicates and promoting the loss of adhesion at the fiber–matrix interface (Deng et al., 2024; Lan et al., 2023; Peng et al., 2025; Quan et al., 2021). This process compromises stress transfer, resulting in a significant reduction in tensile strength, shear strength, and composite stiffness. The penetration of alkaline solutions, combined with stress concentrations in curved regions, accelerates the formation of microcracks and delamination, intensifying chemical and mechanical degradation (Ghabezi, 2022; Peng et al., 2025). Consequently, the integrity of the interface is impaired, reducing the load transfer capacity between the composite and the concrete, which compromises bond strength and the durability of structural elements (Peng et al., 2025; Quan et al., 2021). The loss of cohesion in the matrix and interface also decreases the modulus of elasticity and increases deformability, making the material more susceptible to permanent deformation and cracking (Deng et al., 2024; Lan et al., 2023; Peng et al., 2025). The propagation of these microcracks facilitates the ingress of aggressive agents, accelerating deterioration and reducing the service life of the composite in structural applications (Ghabezi, 2022; Peng et al., 2025; Quan et al., 2021).

When FRP bars are used as reinforcement in concrete structures, they must meet the performance and characterization requirements defined by specific technical standards. The American standard D7957 (ASTM, 2022a) establishes minimum criteria for physical, thermal, mechanical, and durability properties. Among its requirements, it specifies that the bars must retain at least 80% of their initial tensile strength after 90 days of immersion at 60 °C in an alkaline solution. However, the standard does not define acceptance criteria for shear strength under the same conditions. Complementarily, ACI 440.1R (ACI, 2015) also does not provide experimental acceptance limits, although it acknowledges the potential degradation of the bars in aggressive environments. Instead, it recommends the use of environmental reduction factors (C_E) in structural design, typically 0.7 for indoor environments and 0.5 for humid or alkaline solutions, to account for the performance loss resulting from exposure. In the Brazilian context, NBR 17201-05 (ABNT, 2025a) adopts a methodology like the American standard for testing and characterization. However, as with international standards, it still does not provide numerical acceptance values related to degradation in alkaline solutions.

Benmokrane et al. (2017) analyzed the behavior of GFRP bars with polyester, vinyl ester, and epoxy matrices, and the study revealed that interlaminar shear strength was significantly affected by alkaline attack. The bars with polyester matrix showed a 21% reduction after 5000 h, while those with vinyl ester and epoxy matrices suffered a less pronounced drop of 13% each in the same period. The fiber-resin interface plays a crucial role in controlling degradation. Delamination between the fibers and the matrix was more pronounced in the polyester bars, resulting in greater moisture absorption at saturation and affecting the mechanical properties more intensely compared to the vinyl ester and epoxy bars.

Despite significant progress in the incorporation of nano-additives into epoxy resins, few studies have examined the performance of glass fiber-reinforced epoxy composites containing graphene when exposed to alkaline solutions. Such conditions, typical of cementitious systems, can markedly compromise the chemical and mechanical integrity of polymeric materials. It is important to note that the present study is restricted to evaluating the composite in a controlled alkaline solution, without addressing its direct interaction with a cementitious matrix, which represents a more complex degradation scenario. Although numerous investigations have focused on fiber-reinforced polymer composites, no integrated analysis combining glass fiber, epoxy resin, and graphene has been identified in the literature, underscoring the originality and relevance of this work. Moreover, current standards do not include provisions for graphene-modified epoxy systems, revealing a regulatory gap. In this context, the present study contributes by experimentally assessing the alkaline durability of GFRP bars produced with graphene-modified epoxy, examining their thermal, chemical, and mechanical properties and comparing them with those of unconditioned reference samples.

2 Materials and methods

This study presents the characterization of the durability of GFRP bars with an epoxy/graphene matrix, manufactured using the pultrusion process and obtained through donation. The bars are composed of an epoxy resin matrix modified with the addition of graphene and reinforced with glass fibers, as shown in Figure 1. The type of glass fiber and the amount of graphene used in the production of the material were not disclosed by the donating company. The surface of the bars exhibits a helical wrapped conformation, with fiber filaments wound in a helix along their length, resulting in a nominal diameter of 3.40 mm.

Figure 1
Tested GFRP reinforcement bars

2.1 Alkali resistance procedure

To simulate the concrete pore solution, the recommendations of D7957 (ASTM, 2022a) were followed, which specify conditioning FRP bars in an alkaline solution at 60 °C for 90 days. This solution was prepared according to Procedure A of D7705 (ASTM, 2019), using 118.5 g of Ca(OH)₂ (calcium hydroxide), 0.9 g of NaOH (sodium hydroxide), and 4.2 g of KOH (potassium hydroxide), dissolved in 1 L of tap water. In this study, exposure periods of 60 and 90 days were adopted in comparison with the reference sample. The solution had a pH of 13.5, as verified using indicator tape. The samples were immersed in the alkaline solution in a polypropylene (PP) container equipped with two heaters and temperature controllers, dual voltage (127/220 V), and 1000 W of power. For this test, five GFRP bar samples were used, each 605 mm in length.

2.2 Scanning Electron Microscopy (SEM)

The morphology of the GFRP/epoxy/graphene bars was analyzed, both on the lateral surface and in the core, before and after exposure to the alkaline solution, to identify possible changes in the fiber-matrix interface. For this purpose, the samples were cut transversely to the axis, with a length of 10 mm, and metallized with a thin layer of gold. Micrographs were then obtained using a Zeiss EVO 10 scanning electron microscope, operating at voltages of 0.2 to 30 kV, equipped with a tungsten filament.

2.3 Moisture diffusion

The moisture absorption test of GFRP/epoxy/graphene bars was conducted in accordance with D570 (ASTM, 2022b). Three GFRP bar samples, 30 mm in length, were used, with their ends sealed with epoxy resin to prevent solution infiltration through the cut surfaces. The samples were initially weighed and then immersed in an aqueous medium (distilled water) and in an alkaline solution, the latter prepared according to D7705 (ASTM, 2019), as described in Section 2.1. In both the aqueous medium and the alkaline solution, the samples were analyzed at two temperatures, 23 °C and 50 °C. For each condition, samples were removed from the solutions after 4, 25, 49, 100, 169, 361, 841, 961, and 1024 hours; samples were cooled in a desiccator before each weighing. The moisture absorption percentage (M%) was determined using Equation 1.

Eq. 1 $M\%=\frac{Wt-Wo}{Wo}x100$

Where:

M% is the percentage of moisture absorption;

Wt is the mass of the sample after immersion; and

W0 is the mass of the sample before immersion.

The diffusion coefficient is obtained from the linear section of the absorption curve, based on the evaluation of the relationship between M% and time h^1/2. The moisture diffusion coefficient (D) can be determined in mm²/min using Equation 2 (Won et al., 2012):

Eq. 2 $D=\frac{\pi }{16}{\left[\frac{h.(M_2-M_1)}{M_m.({\sqrt{t}}_2-{\sqrt{t}}_1)}\right]}^2$

Where:

M_m is the percentage of moisture at the end of linear behavior;

M₁ is the percentage of moisture after time t₁;

M₂ is the percentage of moisture after time t₂; and

h is the diameter of the GFRP/epoxy/graphene bar.

2.4 Fourier-transform Infrared Spectroscopy (FTIR)

The chemical properties were investigated by FTIR to identify possible changes in the chemical composition of the polymer matrix before and after conditioning. For this purpose, solid fragments of the material were used in the analysis. The equipment used was the PerkinElmer Spectrum Two, and the spectra were obtained by the attenuated total reflectance (ATR) method, in the range of 4000 to 500 cm⁻¹, with 8 scans and a resolution of 4 cm⁻¹.

2.5 Thermal Properties

The GFRP/epoxy/graphene bars were analyzed by Differential Scanning Calorimetry (DSC) to determine the glass transition temperature (T_g), in accordance with E1356 (ASTM, 2008). Approximately 5 mg of solid material fragments were used for the test. The analyses were performed using a TA Instruments Q50 calorimeter. The DSC curves were obtained by heating the samples from –80 °C to 300 °C at a rate of 10 °C/min, under a nitrogen atmosphere.

Thermal stability was evaluated by Thermogravimetric Analysis (TGA), following D6370 (ASTM, 1999). Approximately 15 mg of solid material fragments were analyzed using a TA Instruments Q50. The samples were heated from 20 °C to 750 °C at a heating rate of 20 °C/min.

The heat release rate of the GFRP bars was determined using Microscale Combustion Calorimetry (MCC). For this test, approximately 5 mg of solid fragments were analyzed in an FAA microcalorimeter (FTT model), according to the procedures established in D7309 (ASTM, 2021a). The samples analyzed had an average mass of approximately 6.13 mg. The furnace was heated to 900 °C at a heating rate of 60 °C·min⁻¹, in an inert atmosphere. The gaseous products generated during pyrolysis were conducted to the combustion zone, where they were oxidized in the presence of a mixture of nitrogen and oxygen gases, with flow rates of 80 and 20 cm³·min⁻¹, respectively. Based on oxygen consumption, the heat release rate (HRR) and total heat release (THR) were calculated according to Huggett (1980) equation. The THR and peak heat release rate (pHRR) values were obtained from the HRR graphs as a function of temperature by calculating the area under the curves and the maximum HRR value, respectively.

2.6 Short-beam test

As specified by D4475 (ASTM, 2021b), four samples with a nominal diameter of 3.40 mm were used in the short-beam test. After alkaline conditioning, the samples were washed with distilled water, dried, and kept in a desiccator for 24 hours, until they reached room temperature and hygroscopic equilibrium. The test was performed on an Emic/Instron universal testing machine, model EMIC 23-5D, equipped with a 5 kN load cell. The displacement speed adopted was 1.3 mm/min, and the span was defined as 6 times the sample diameter (6×d). The apparent interlaminar shear strength was determined by Equation 3:

Eq. 3 $S=0.849\frac{P}{d^2}$

Where:

S is the apparent shear strength (N/mm² or MPa);

P is the breaking load (N); and

d is the diameter of specimen (mm).

To evaluate the effects of different exposure times on GFRP bars, analyses of variance (ANOVA) were performed, including single-factor ANOVA, based on the means of apparent horizontal shear strength. A significant level of 5% (p < 0.05) was adopted. Multiple comparisons between groups were performed using Tukey's post-hoc test to identify possible significant differences between exposure times.

3 Results and discussions

The pultruded GFRP/epoxy/graphene bars with added graphene had a density of 1.96 g/cm³, an average effective diameter of 3.45 mm, and an effective cross-sectional area of 9.35 mm², as shown in Table 1. The results presented in Table 1 were calculated experimentally, following the respective standards listed in the "Regulatory standard" column. In addition, the composite under study had an average fiber content of 72.60%, meeting the requirements established by D7957 (ASTM, 2022a), which establishes a minimum content of 70%. Regarding the void volume, the material presented 1.21%, which demonstrates good efficiency, considering that, according to D2734 (ASTM, 2016), the ideal is to keep this value below 1%. However, values of up to 4% can still be considered acceptable, according to the study by Kiam et al. (2017). Above this limit, the mechanical properties and durability of the material can be significantly compromised (Thomason, 2023).

Table 1
Physical properties of GFRP bars

Properties	GFRP - Reference	Regulatory standard
Composite density (g/cm3)	1.96 ± 58.94	D792 (ASTM, 2020)
Effective composite diameter (mm)	3.45 ± 0.103	D7205 (ASTM, 2021c)
Fiber content by weight (%)	72.60 ± 0.744	D3171 (ASTM, 2022c)
Resin content by weight (%)	27.40 ± 0.744
Reinforcement volume (%)	52.60 ± 0.539
Matrix volume (%)	46.19 ± 1.254
Voids volume (%)	1.21 ± 0.715

3.1 Moisture absorption and diffusion mechanisms

The moisture absorption curves of the GFRP/epoxy/graphene bars subjected to alkaline solution and distilled water at temperatures of 23 °C and 50 °C are shown in Figure 2. It can be observed that, up to 10 h¹ᐟ², diffusion followed a linear behavior at both temperatures, characterizing a typical Fickian diffusion profile. However, after this period, the curves began to show distinct behaviors.

Figure 2
Moisture diffusion in alkaline and aqueous media

It was observed that the first signs of non-Fickian diffusion behavior were identified in moisture absorption of 0.48% and 0.68% for samples immersed in water at 23 °C and 50 °C, respectively. Under conditions of immersion in alkaline solution, the corresponding values were 0.68% and 0.64% at 13 h^1/2. This non-Fickian response can be attributed to structural changes in the polymer matrix caused by high temperature and humidity conditions, as suggested by Almeida, Carlos and Cardoso (2024). According to Rocha et al. (2017), this type of behavior is often linked to matrix relaxation phenomena or the occurrence of hydrolytic reactions. In hygrothermal aging environments, mass gain tends to reflect a dynamic equilibrium between water absorption and the loss of material constituents caused by degradation processes, as pointed out by Correia, Rodrigues and Branco (2012). Benmokrane et al. (2002) add that the presence of porosity, whether pre-existing or formed during exposure to the environment, can also significantly influence this deviation from the classic Fickian model.

The samples immersed in water at 50 °C showed the highest final absorption value (0.95%), followed by those exposed to the alkaline solution at the same temperature (0.85%). Despite this, under all conditions, they showed saturation with absorption below 1%, in accordance with the criteria established by D7957 (ASTM, 2022a) and NBR 17201-11 (ABNT, 2025b), which consider samples to be saturated if they show three consecutive weighings with a variation of less than the greater value between 1% of the total mass gain or 5 mg. Furthermore, all absorption curves exhibited an abrupt variation approximately at 30 h^1/2, a behavior generally associated with the initial saturation of larger pores, followed by slower diffusion of water to smaller or less accessible regions, or even internal redistribution of moisture in the material (Alderete; Zaccardi; Belie, 2019).

The experimental values of moisture saturation concentration (M_m) and diffusion coefficients (D), calculated using Equation 2, are presented in Table 2.

Table 2
Moisture saturation concentration and diffusion coefficient of GFRP bars immersed in water and alkaline solution at 23 °C and 50 °C

Parameter	Alkaline solution 23 °C	Alkaline solution 50 °C	Water 23 °C	Water 50 °C
Mm (%)	0.68 ± 0.17	0.64 ± 0.17	0.48 ± 0.11	0.68 ± 0.13
D (10-4 mm2/min)	2.05	2.67	1.13	1.85

The comparison of the diffusion coefficients calculated for the different aging conditions reveals distinct behaviors among the evaluated media. In the aqueous medium, an increase of approximately 63.72% in the diffusion coefficient was observed with the temperature increase from 23 °C to 50 °C, from 1.13 × 10⁻⁴ mm²/min to 1.85 × 10⁻⁴ mm²/min. In samples immersed in alkaline solution, the increase was 30.24%, with values ranging from 2.05 × 10⁻⁴ mm²/min to 2.67 × 10⁻⁴ mm²/min. In general, diffusion in epoxy resins is more intense in distilled water than in alkaline solution, increases with temperature, and predominantly follows the Fickian model (Chin; Nguyen; Aouadi, 1998; Escobar et al., 2016; Fan et al., 2019).

3.2 Functional, thermal, and microstructural properties

The SEM micrographs of the core and lateral surfaces of the GFRP bars, shown in Figures 3, 4, and 5, were used to evaluate microstructural changes in the reference samples and after exposure to alkaline solutions for 60 and 90 days. It was observed that degradation was predominantly restricted to the epoxy matrix, with no clear evidence of direct chemical attack on the glass fibers. Although the presence of graphene in the polymer matrix cannot be directly identified by SEM due to its size, its incorporation is assumed to be based on the dark and uniform coloration of the matrix. Furthermore, the presence of voids in the core of the material can be seen in Figure 5(a), along with the degradation of the side surface illustrated in Figure 5(b), where the polymer matrix is compromised.

Figure 3
SEM micrographs of GFRP bars in the reference condition: (a) core; (b) lateral surface

Figure 4
SEM micrographs of GFRP bars after 60 days: (a) core; (b) lateral surface

Figure 5
SEM micrographs of GFRP bars after 90 days: (a) core; (b) lateral surface

The FTIR spectra, shown in Figure 6, reveal several characteristic bands. Initially, a broad band observed between 3700 cm⁻¹ and 3100 cm⁻¹ is attributed to O–H stretching vibrations, indicating the presence of hydroxyl groups and possible water absorption because of alkaline aging. Typical bands of the epoxy matrix are identified by C–H stretching vibrations of CH₂ and CH₃ groups in the range of 3050–2800 cm⁻¹. Absorption bands associated with carbonyl groups (C=O) appear between 1730 cm⁻¹ and 1650 cm⁻¹, while aromatic C=C stretching vibrations are observed in the region of 1600–1500 cm⁻¹. Additionally, bands related to carboxylate groups (COO⁻), detected between 1550 cm⁻¹ and 1400 cm⁻¹, are evident in the samples exposed to the alkaline solution. The characteristic C–O–C stretching vibrations of the epoxy network are also observed in the range of 1250–1000 cm⁻¹.

Figure 6
FTIR spectrum of GFRP bars

Alkaline aging induced notable changes. In the spectra of the aged samples, the emergence of a new band at 960 cm⁻¹ stands out, absent in the reference condition. This band is classically attributed to Si–O stretching of less ordered silicate structures. This change suggests partial leaching of the coating and the formation of amorphous products, derived from the alkaline reaction with the silica-rich surface of the fibers, a phenomenon corroborated by Figure 5(b). Additionally, a new peak emerged at 875.05 cm⁻¹ in samples subjected to 60 and 90 days of aging. The presence of this peak suggests a structural alteration, possibly due to mild oxidation or molecular rearrangement. As widely described in the literature (Garg et al., 2025; Praspaliauskas et al., 2021), this range may be related to out-of-plane deformations of aromatic groups or C–H bonds of aromatic rings.

The glass transition temperature is strongly influenced by chemical changes within the polymer matrix. Degradation processes such as thermal oxidation can lead to chain scission and a consequent reduction in the degree of cross-linking, which typically results in a lower T_g. Conversely, certain aging mechanisms may promote additional cross-linking reactions, increasing this parameter (Chen et al., 2004; Ernault et al., 2017b). Thus, T_g is widely used as an indicator of the structural integrity and thermal stability of epoxy resins and polymer composites (Ernault et al., 2017a; Michel; Ferrier, 2020; Suter et al., 2025). Figure 7 presents the glass transition temperature curves, which show that the GFRP-Reference sample exhibits a more defined T_g compared with the samples exposed for 60 and 90 days to the alkaline medium. This behavior indicates a progressive degradation of the polymer matrix as the exposure time to the aggressive environment increases, reflected in the gradual reduction of the glass transition temperature. (Feng et al., 2024; Sun et al., 2023).

Figure 7
Heat flow curves

As shown in Table 4, the T_g of the samples conditioned for 60 and 90 days decreased to 98.69 °C and 96.49 °C, respectively. This decrease was expected, since alkaline solutions promote the degradation of the polymeric matrix, affecting its structure and segmental mobility, as already reported in the literature (Benmokrane et al., 2017; Montaigu et al., 2013).

Table 4
Property obtained by DSC

Conditions	Tg (°C)
GFRP - Reference	99.91
GFRP - 60 days	98.69
GFRP - 90 days	96.49

The reduction in T_g indicates a loss of rigidity in the polymer matrix, which can compromise the mechanical performance and durability of the composite over time. The main cause of this reduction is associated with the plasticization of the matrix due to water absorption, in addition to the occurrence of hydrolysis reactions of the ester bonds. These processes can be accelerated by high temperature and alkaline pH conditions, favoring the degradation of the polymer matrix (Moura; Ribeiro; Lima, 2021; Sun et al., 2023; Wang et al., 2023a, 2023b).

Figure 8 presents the TG and DTG curves of the GFRP bars under reference conditions and after 60 and 90 days of aging, highlighting the thermal degradation behavior of the polymer matrix. It can be observed that the reference sample shows a greater mass loss at 750 °C, attributed to the higher organic material content, resulting in a residue of 60.79%, as shown in Table 5. In contrast, the bars aged for 60 and 90 days, partially degraded by the alkaline solution, as illustrated in Figures 4(a) and 5(b), retain a larger fraction of inert glass, which explains the lower mass loss and the reduction in combustibility (Liao et al., 2024; Yiğit, 2025).

Figure 8
TG and DTG thermograms of GFRP bars

Table 5 presents the peak degradation temperature (DTG) values of the GFRP samples under the different conditions evaluated. A slight variation is observed between the samples, with the reference condition exhibiting a DTG of 422.68 °C and reductions of 0.67% and 0.78% after 60 and 90 days of aging, respectively. This slight decrease indicates a subtle acceleration of the thermal degradation process, reflecting the partial degradation of the epoxy matrix, probably associated with polymer chain breakage, oxidation, and the formation of degradation products (Dou et al., 2024; Thomason, 2023).

Table 5
Thermal degradation temperature

Conditions	DTG (°C)	Residual (%)
		750 °C
GFRP - Reference	422.68	60.79
GFRP - 60 days	419.84	63.77
GFRP - 90 days	419.37	64.44

In addition to the TGA technique, the GFRP bars were also analyzed during the combustion reaction using the MCC technique, considering that heat release by combustion is one of the most relevant parameters for evaluating the thermal behavior of materials during burning (Lyon; Walters, 2004; Marques et al., 2021). Figure 9 shows the heat release rate (HRR) curves as a function of temperature. Table 06 shows the maximum peak heat release (pHRR), the temperature corresponding to each peak (T_pHRR), the time to maximum peak (Time_pHRR), and the total heat release (THR). The HRR values as a function of temperature were obtained from a calculation based on oxygen consumption, which reacts with the gases from the combustion of the samples. The pHRR values are considered critical parameters, as they indicate the maximum heat released during the combustion of the material and are directly related to the propagation of the flame (Cardoso et al., 2020).

Figure 9
HRR curves as a function of temperature for GFRP bars before and after alkaline aging, obtained by Microscale Combustion Microcalorimetry

The reduction in HRR peak intensity with increasing exposure time in the alkaline medium indicates a progressive degradation of the polymeric matrix. This occurs because less combustible organic material remains available, reducing the heat release during thermal decomposition. This behavior is consistent with the microstructural analyses shown in Figures 3, 4 and 5, where longer immersion times resulted in more evident surface damage. Figure 5 shows a fully compromised surface, with exposed fibers and residues from the alkaline solution, confirming that the decrease in organic fraction observed by MCC is directly associated with the matrix degradation visible microscopically.

Table 6
Thermal parameters obtained from the Micro-scale Combustion Microcalorimetry curve

Sample	pHRR (W/g)	TpHRR (°C)	TimepHRR (min)*	THR (kJ.g-1)
GFRP - Reference	273.11	433.03	7.22	62.07
GFRP - 60 days	161.88	432.31	7.21	20.86
GFRP - 90 days	103.04	435.38	7.26	14.19

• Note:
• ^*
  based on the heating rate of the MCC test, which is 60 °C/min.

Table 6 shows that the temperatures corresponding to each T_pHRR peak occur at temperatures close to those recorded for the main stage of thermal degradation in the TGA, confirming that combustion is directly associated with this decomposition event of the polymer matrix. This coincidence indicates that the material releases the greatest amount of energy precisely at the moment when the highest decomposition rate occurs, evidencing the correlation between thermal stability and combustion behavior (Li et al., 2025; Xu et al., 2022). The pHRR peaks with the main degradation stage suggest that MCC can be used to predict the combustion performance and energy efficiency of polymeric materials, in addition to contributing to the assessment of risks of exposure to high temperatures (Hamciuc et al., 2022; Xu et al., 2022). In epoxy matrices, modifications with flame retardant additives or inorganic fillers tend to alter both the heat release profile and the temperature of the main events, changes that can be monitored complementarily by TGA and MCC (Asante; Modiba; Mwakikunga, 2016; Hamciuc et al., 2022; Zhou et al., 2018).

3.3 Apparent shear strength Short-Beam

The average apparent horizontal shear strength results and respective standard deviations of GFRP/epoxy/graphene bars under reference conditions and after 60 days and 90 days are presented in Table 7. The average apparent horizontal shear strength was 45.30 MPa. After 60 days of exposure to the alkaline solution, there was a reduction of approximately 3.07 %, while after 90 days, the drop was more pronounced, representing a reduction of about 15.67% compared to the initial condition. The data indicate that the alkaline solution, combined with temperature and humidity, causes physical-chemical degradation both on the surface and inside the glass/epoxy composite, in addition to compromising the fiber/matrix interface, resulting in a clear reduction in interlaminar shear strength (Lv et al., 2006).

Table 7
Experimental results of short-beam tests on GFRP bars

Conditions	Apparent Horizontal Shear Strength (MPa)
GFRP - Reference	45.30 ± 2.25
GFRP - 60 days	43.91 ± 3.00
GFRP - 90 days	38.20 ± 3.71

In Figure 10, the analysis of variance showed that there is a statistically significant difference between the groups evaluated (p-value = 0.02102). This indicates that the aging time in alkaline solution directly influences the horizontal shear strength of the GFRP/epoxy/graphene composite.

Figure 10
Results of the short-beam test on GFRP bars before and after aging

The Tukey test showed that the GFRP – 90 days group had significantly lower resistance than the GFRP – Reference group, with a difference in means of -7.11 MPa (p = 0.02267). In the graph, these two groups were identified with different letters, with the mean of the GFRP – Reference group (45.30 MPa) represented by the letter “a” and that of the GFRP – 90 days group (38.20 MPa) by the letter “b”. This difference shows that 90 days of exposure to the alkaline medium caused significant degradation in the composite.

The GFRP - 60 days group (43.91 MPa) received the letter “ab,” indicating that there was no statistically significant difference between the GFRP - Reference group (p = 0.79812) and the GFRP - 90 days group (p = 0.06213).

4 Conclusions

This study investigated the effects of alkaline aging on the thermal, chemical, and mechanical properties of pultruded GFRP/epoxy/graphene bars. Exposure to an alkaline solution at 60 °C for 60 and 90 days resulted in measurable degradation of the polymer matrix, evidenced by a reduction in T_g of 3.42 °C. Combustion microcalorimetry indicated significant changes in heat release parameters, suggesting alterations in the degree of resin crosslinking. From a mechanical standpoint, the reduction of up to 15.67% in interlaminar shear strength was statistically significant and consistent with values reported in the literature for GFRP composites subjected to alkaline environments.

Although these results confirm that the alkaline environment, combined with temperature and humidity, progressively compromises the integrity of the matrix and the fiber/matrix interface, there are no normative parameters that define acceptable limits of variation for T_g or short-beam strength after aging. Therefore, it is not possible to classify the observed degradation as acceptable or unacceptable in normative terms. However, the magnitude of the observed changes can be considered moderate when compared to literature, not indicating, within the studied interval, immediate inadequacy of the bars for use in concrete, although it reveals the sensitivity of the epoxy resin to the alkaline environment.

Prospectively, strategies such as modification of the epoxy resin, the use of coupling agents or functionalized graphene, the application of protective coatings, and the development of polymeric systems with greater resistance to hydrolysis can mitigate the observed effects. Furthermore, the absence of specific normative criteria reinforces the need for coupled, hygrothermal-alkaline, and long-term aging tests, capable of supporting service life prediction models and guiding future normative revisions. These results contribute to the understanding of the durability of GFRP bars and offer important subsidies for improving their performance in structural applications.

• Oliveira, M. de S.; Bueno, G. M.; Ferreira, C. A. Alkaline aging effects on thermal stability and apparent interlaminar shear strength of GFRP/epoxy/graphene bars. Ambiente Construído, Porto Alegre, v. 26, e150077, jan./dez. 2026. ISSN 1678-8621 Associação Nacional de Tecnologia do Ambiente Construído. http://dx.doi.org/10.1590/s1678-86212026000100956

Data Availability Statement

Research data is only available upon request.

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Edited by

• Editor:
  Marcelo Henrique Farias de Medeiros

Publication Dates

• Publication in this collection
  16 Mar 2026
• Date of issue
  2026

History

• Received
  06 Sept 2025
• Reviewed
  05 Nov 2025
• Accepted
  20 Nov 2025