The effect of elevated temperatures on the compressive section capacity of pultruded GFRP profiles

Highlights

• The influence of cross-sectional, slenderness, elevated temperatures on the compression behavior of the GFRP pultruded profiles were studied.

• It was observed that the capacity of GFRP pultruded profiles had been lost considerably as the temperature exceeded 120 °C.

• It was concluded that the degradation trend of all the profiles capacity at elevated temperature was the same, a sudden drop near transition temperature, i.e., 90 °C, and a constant part at the temperature higher than 120 °C.

• The specimens with the greater cross-sectional area to external perimeter showed higher compressive strength at elevated temperatures.

• According to the results of ANOVA, the cross-section area to external perimeter ratio had the most impact on the performance of GFRP pultruded profiles at temperatures ranging from 25 to 90 °C and also from 120 to 400 °C.

• The model form was proposed to predict the capacity degradation factor of GFRP pultruded profiles at elevated temperatures.

Abstract

The compressive performance of glass fiber reinforced polymer (GFRP) profiles subjected to elevated temperature was investigated through a large number of (3 0 0) tests. The effects of profile cross-sectional area, slenderness, and temperature on the behavior of the GFRP profiles at temperatures ranging from 25 to 400 °C were determined. It was observed that the compressive strength of the GFRP profiles has been decreased by half as the temperature exceeded 90 °C, i.e., close to the glass transition temperature of the matrix. The temperature and ratio of cross-sectional area to external perimeter were determined as two major parameters affecting section capacity.

Introduction

Recently, fiber-reinforced polymer (FRP) profiles are widely used for several structural applications, including piping, pedestrian bridge decks, panel walls, trail decks, aircraft, marine crafts, waste treatment plants, trains, off-shore rigs, high-performance automobiles, thermo-electrical plants, etc. [1], [2], [3], [4]. Although the glass fiber-reinforced polymer (GFRP) profiles are used to perform as non-structural elements, these profiles can also be used as structural components in structures such as bridges, building bearing beams, platforms, and aisles. Due to the material characteristics, pultruded sections would overcome potential problems, namely corrosion and the thermal bridges provided by the steel elements, which strongly affect the design of steel studs [5].

Among the various type of fiber-reinforced polymer composites such as carbon and basalt FRPs, GFRPs are commonly used due to their relatively low cost and high strength [6], [7], [8]. Despite their advantages, such as high strength-to-weight ratio, superior corrosion resistance, low thermal conductivity, electric insulation, and ease in fabrication, which plays a vital role in structural strengthening applications, many challenges are still unsolved. Low shear strength, low ductility, and inappropriate mechanical properties of the resin matrix are known as some of the most critical FRP’s lack points, which should be taken into account when using such materials for construction applications [9]. Besides, the durability (short-term/long-term performance) of FRP materials, such as exposure to harsh environments including seawater, alkali, industrial chemicals, fire hazard, and elevated temperatures, is still an open issue [10]. Some investigations can be found in the literature on the aging of pultruded GFRP profiles subjected to harsh environmental conditions, namely hydrothermal, alkali, acidic, seawater, and ultraviolet radiation [11], [12], [13], [14]. Regarding the FRP composites subjected to elevated temperatures, the studies are mostly concentrated to FRP bars [15], [16], [17]. However, there are limited studies conducted on the performance of FRP sections and panels under elevated temperatures and short-term fire conditions [5], [18], [19]. Therefore, for having a broader range of acceptance for FRP profiles, their performance under elevated temperatures must be considered and also incorporated in design codes and guidelines.

It is determined that reaching the temperature to the glass transition temperature of the composites, which is 50 °C to 120 °C based on the resin matrix and the profile curing process, is the initiation of reduction in the mechanical properties of FRP composites. The reason is that the matrix capacity to transfer the shear force between the fibers and resin reduces because of the change in resin state from glassy to rubbery [20], [21]. At temperatures adjacant to the decomposition temperature, T_d, i.e., about 300 °C to 400 °C, the degradation becomes more severe due to the breaking of the chemical bonds between the resin and its modular chains and breaking of the bonds between fibers [22]. At the final stage, at temperatures higher than T_d, due to the resin ignition, the resin may completely lose its load transfer capacity and produce extra heat due to ignition. However, the fibers still may sustain some load in the longitudinal direction until reaching their melting temperatures (around 1000–1200 °C for carbon fibers) [17]. Considering the behaviour of FRPs under elevated temperature, several researchers have proposed models to predict the mechanical properties of various FRP composites under elevated temperatures using mechanical test results [23]. Among these models, many are proposed based on the primary model of Gibson et al. [24], taking into account only the effect of temperature as the only effective factor [25], [26], [23]. However, some other researchers proposed regression models considering other factors, such as the sample thickness and exposure time [27], [28], [29], [30]. It is worth mentioning that the predicting models are mainly related to the tensile and flexural behaviour of different FRPs. However, the compressive performance of these materials under elevated temperature has gathered very limited attentions, and thus a comprehensive set of experimental data is needed to propose predicting models to evaluate the compressive mechanical properties of FRPs under elevated temperatures. In order to evaluate the mechanical properties of FRP composites under elevated temperatures, there are two basic test types conducted by researchers: (1) loading the specimen up to the failure while the temperature is constant at a predefined target, which is known as the steady state condition and (2) heating the specimen up to the failure while the load is constant at a predefined target (specified ratio of its ambient temperature strength), which is known as the transient state condition [31].

Mouritz and Mathys [32] conducted an experimental test on the mechanical properties of glass-reinforced resole phenolic composites subjected to intense radiant heat and fire, and the results showed that the post-fire tensile and flexural properties decreased rapidly (up to 70%) with an increase in heat flux and exposure time. Chemical degradation of the phenolic resin matrix was reported as the main reason for this reduction. Mouritz [1] also studied the effect of fire damage on the flexural properties of FRP composites with different fiber and resin type and reported that the post-fire fiber mechanical properties of all composite types, even fiber-reinforced phenolic materials, which have low flammability and excellent fire resistance, were significantly degraded. Wang and Wong [5] conducted compressive tests on pultruded GRP channel columns, at temperatures of 60 °C, 90 °C, and 120 °C and the results showed that at temperatures below 120 °C, the glass reinforced plastic columns can retain their substantial proportions at ambient temperature, while at temperature higher than the resin heat distortion temperature, i.e., 120 °C in their study, the compressive strength was about 16% of that at ambient temperature. Wong et al. [19] studied the effect of elevated temperatures up to 250 °C on compression strength of short glass reinforced plastic C-Shaped channels. The results revealed that at lower temperatures (up to 90 °C), the channels failed by crushing with a loud sound, while having a substantial amount of residual strength and at higher temperatures (more than 90 °C and up to 250 °C) failed by crushing due to resin softening, while a rapid reduction (up to 85% at 250 °C) in glass reinforced plastic strength occurred.

In general, limited research and experimental data about the mechanical properties and failure mechanisms of different GFRP sections under compression subjected to elevated temperature are available, and an obvious lacking in the literature is seen. It is critical to understand the performance of GFRP profiles in compression since the governing failure scenario of the pultruded profiles is mainly the compressive, particularly when they are subjected to temperature. In addition, the local buckling is another failure scenario observed in the GFRP pultruded profiles. Therefore, it is essential to consider the effect of cross-sectional slenderness on the section capacity of the profiles. With these in mind, this study investigates the effects of FRP slenderness and geometry (cross-sectional area) of the section on the compression mechanical performance and failure mechanisms of FRP pultruded profiles subjected to elevated temperatures. Compression tests using steady-state conditions were conducted to investigate the compressive strength of GFRP pultruded sections at elevated temperatures. Moreover, to find out the reliability of the results and contribution of each variable, a statistical-probabilistic analysis using ANOVA and linear Bayesian regression was used to develop probability models for the compressive strength of GFRP pultruded sections. Furthermore, to have a better understanding of the possible damages after exposure and failure mechanisms after compression tests, a Scanning Electron Microscopy (SEM) was also conducted. This research is part of an ongoing study conducted on the fire resistance of FRP composites, and the results will contribute to finding the effective parameters that contribute to the behavior of FRP sections under elevated temperatures to modify the reduction factors of current standards and guidelines.