Effect of intumescent paint coating on mechanical properties of FRP bars at elevated temperature

Accepted Manuscript Effect of intumescent paint coating on mechanical properties of FRP bars at elevated temperature Mohammad Houshmand Khaneghahi, Esmaeil Pournamazian Najafabadi, Parham Shoaei, Asghar Vatani Oskouei PII:

S0142-9418(18)30812-2

DOI:

10.1016/j.polymertesting.2018.08.020

Reference:

POTE 5582

To appear in:

Polymer Testing

Received Date: 21 May 2018 Revised Date:

5 August 2018

Accepted Date: 15 August 2018

Please cite this article as: M.H. Khaneghahi, E.P. Najafabadi, P. Shoaei, A.V. Oskouei, Effect of intumescent paint coating on mechanical properties of FRP bars at elevated temperature, Polymer Testing (2018), doi: 10.1016/j.polymertesting.2018.08.020. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Effect of Intumescent Paint Coating on Mechanical Properties of FRP bars at Elevated Temperature

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Mohammad Houshmand Khaneghahia, Esmaeil Pournamazian Najafabadib, Parham Shoaeib, Asghar Vatani

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Oskoueic,*

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master student, Department of Civil Engineering, Sharif University of Technology, Tehran, Iran c

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master student,Department of Civil Engineering, Shahid Beheshti University, Tehran, Iran

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Faculty of Civil Engineering, Shahid Rajaee Teacher Training University, Tehran, Iran

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E-mail: [email protected]; TEL: +98-2122970021; FAX: +98-2122970021; Address: Iran, Tehran Province, Tehran

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City, Lavizan

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10 Abstract

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This paper investigates the influence of intumescent paint on the performance of FRP bars subjected to low

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(25-450 °C) and severe (450-800 °C) elevated temperatures. In this research, glass and carbon FRP bars

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with epoxy resin and coated with nitrogen-based intumescent paint were used. In addition to the temperature

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effects, a variety of FRP bar diameters were employed to determine its effect on the tensile behavior of FRP

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bars in the presence of intumescent paint. Further, Bayesian regression methods and ANOVA (ANalysis Of

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VAriance) were applied on the results to develop a predictive model form and quantify the contribution of

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the variables in the tensile performance of FRP bars, respectively. The results revealed that the intumescent

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paint prevented the degradation of the mechanical properties of FRP bars significantly, up to 30% of GFRP

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bars tensile load capacity, at the temperature range 350 to 600°C.

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Keyword: Intumescent paint; Predictive model; Elevated temperature; FRP bars; Tensile strength.

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1. Introduction

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Since fiber reinforced polymer (FRP) composites have become attractive for use in the construction

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industry, many studies have been conducted to extend the knowledge of FRP materials [1-7]. FRP materials

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have been used in miscellaneous forms namely profiles, sheets, rods, and bars. FRP bars have been

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extensively used as an alternative to steel bars because they exhibit better performance in corrosive and

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harsh environments [1, 2, 8]. In addition, FRP bars have higher durability characteristics and a higher 1

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strength-to-weight ratio than conventional steel bars [4]. Hence, FRP bars are advantageous in many

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applications such as offshore structures, foundations, and bridges. Numerous investigations have been

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performed in recent years to evaluate the performance of FRP bars under different harsh conditions and for

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miscellaneous applications [9]. One of the major severe conditions to which FRP bars are susceptible is fire

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hazard. Consequently, it is of paramount importance to conduct thorough research on the performance of

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FRP bars at elevated temperatures in order to achieve more extensive use of FRP bars in the construction

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industry.

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Some studies have been carried out to determine the influence of elevated temperatures on FRP

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mechanical properties and specifically to discover a promising solutions for the low fire-resistance of these

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materials [10-15]. Generally, elevated temperatures have significant effects on the mechanical properties of

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FRP bars particularly when the temperature exceeds glass transition temperature (Tg). At this temperature,

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which is typically in the range of 65-120 °C for the polymer matrices, the state of resin changes from glassy

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to rubbery, leading to debonding of fiber and resin and rapid strength loss [16-19]. The next phase in FRP

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materials under elevated temperatures occurs at decomposition temperature, Td (300-400 °C for the polymer

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matrix), at which the chemical bonds and modular chains of the matrix and the bonds between fibers may

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break. At higher temperatures, the ignition state leading to combustion of the resin commences. It should be

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noted that the fibers themselves may be able to sustain some level of axial load in their longituidinal

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direction before their threshold temperature, which is about 980 °C for glass fibers [20].

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Based on the literature review, the larger portion of the studies have been dedicated to the performance

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of FRP bars exposed directly to fire. Kumahara et al. [21] have indicated that the initial tensile strength of

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glass-FRP (GFRP) and carbon-FRP (CFRP) bars was reduced by more than 20% at a temperature of 250 °C.

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In another study conducted by Robert and Benmokrane [17], they reported about 40% reduction in tensile

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strength of GFRP bars at temperature of 250 °C. Wang and Zha’s [20] experimental investigation of GFRP

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bars at elevated temperatures indicated reductions of around 22% in tensile strength of GFRP bars at 120 °C

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and 67% at 500 °C. Hajiloo et al. [16] carried out a study on GFRP bars at a temperature range of 25-500 °C

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to investigate the effects of resin contents and thermal properties on GFRP bars on their tensile behavior.

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The results of their research showed that the GFRP bars lost about 75% of their ultimate tensile strength at a

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temperature of 400 °C. Furthermore, Hamad et al. [18] investigated the influence of temperature on not only

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tensile strength but also the elastic modulus of FRP bars. According to their study, the FRP bars’ tensile

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strength and elastic modulus at a temperature of 325 °C decreased by about 55% and 30%, respectively.

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Moreover, Ashrafi et al. [19] found that the ultimate tensile strength of different CFRP and GFRP bars was

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reduced by about 50-70% as they faced a temperature of 450 °C. Another study in this field has been

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performed by Yu and Kodur [23] to evaluate the effects of high temperatures on the mechanical properties

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of CFRP rods and strips. In research, no significant loss of the mechanical properties was observed in CFRP

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rods and strips before a temperature of 200 °C since the resin properties remained intact. However, as the

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temperature exceeded 305 °C and 330 °C for strips and rods, respectively, the reduction of about 50% in

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ultimate tensile strength was reported.

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In addition, other studies have been conducted on embedded FRP bars in concrete members at elevated

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temperatures [24-28]. Because the FRP bars are embedded in concrete, mortar, or coated with shotcrete, the

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direct contact from flames is prevented, and the lack of abundant oxygen results in no combustion state of

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FRP bars. In this case, the FRP bars can endure higher temperatures, and the reduction in their tensile

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strength is less than that of bare bars, particularly at temperatures greater than Td. In these studies,

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temperature distribution models in concrete members were proposed in order to simulate standard fire

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situations [15, 29]. Although FRP bars are used as reinforcements in concrete members, they may become

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exposed as the concrete covers fall, which makes them very susceptible to fire. Consequently, structures

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reinforced with FRP bars that are prone to fire hazard must be protected in an efficacious and practical way.

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As mentioned above, it is important to find a suitable solution to rectify the weakness of the FRP

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materials under fire conditions and to evaluate their performance under elevated temperatures. Several

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approaches can be found in the literature that provide solutions to minimize the impact of high temperatures

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on the mechanical and thermal properties of FRP materials by using materials such as fire retardant

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materials, macro and nanoparticle additives in the matrix, ceramic thermal barriers and intumescent paint

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coatings [30-33]. The ceramic barriers delay heat conduction, which postpones the degradation of FRP 3

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composites’ mechanical properties. Intumescent coating provides a physical barrier over FRP materials in

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order to slow of the progression of the heat to the material and to prevent the oxygen reaching the material

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that is required for combustion. In addition, nanoparticle additives can enhance the performance of FRP

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composites thermal and mechanical properties. Nonetheless, it has been proven that it is preferred to utilize

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nanoparticle additives in conjunction with fire retardant coatings.

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1.1 Scope and Objective

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Considering aforementioned literature review, the present study evaluates the performance of FRP bars

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coated with intumescent paint in order to determine the effects of fire retardant material on FRP bars’

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mechanical properties. In addition, this research contributes to enrich the existing database on the area of

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FRP material performance under elevated temperatures. Furthermore, the influence of fiber type, bar

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diameter and temperature on the ultimate strength of FRP bars coated with intumescent paints is

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investigated. Moreover, the temperature range in which the intumescent paint enhances the performance of

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FRP bars considerably is determined in this investigation. Further, statistical analyses are performed to

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quantify the contribution of variables to the results of the experiments, and probabilistic analyses to develop

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the predictive model form for intumescent paint coated GFRP and CFRP bar performance at elevated

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temperature. The outcome of this investigation will provide a comprehensive and practical database for

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guideline modification, engineering purposes and further investigations on the field of FRP materials

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performance subjected to fire hazard.

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2. Experimental Program

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This paper investigates the tensile performance of fire retardant coated GFRP and CFRP bars were under

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elevated temperatures. The effects of bar diameters and fiber materials were taken into account in the

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conditions of low and high elevated temperatures.

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2.1 Materials

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2.1.1

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FRP Bars

Sand-coated GFRP and helically wrapped CFRP reinforcement bars were utilized in this study. The

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nominal diameters of the GFRP bars were 4, 6, 8 and 10 mm, and of the CFRP bars were 4 and 5 mm (Fig.

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1). All FRP bars are made of epoxy resin through a pultrusion process. The thermal and mechanical

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properties of these bars are tabulated in Table 1 as provided by the manufacturer. Moreover, elemental

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analyses were performed on the glass fibers, carbon fibers, and epoxy resin separately in order to determine

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the chemical composition of these materials (Table 2). Hence, one can use the elemental analysis results to

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compare the composite materials used in this study with the other products in the same category.

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2.1.2

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To prevent crushing due to grip pressure and slippage during the tensile test to which FRP bars are

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vulnerable, strong anchors have to be provided at free ends of FRP bars. To overcome this issue, steel pipes

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with circular section were used at the free ends of the FRP bars. These steel pipes were filled with high

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strength adhesive with a shear strength of 36 MPa. Using this method, satisfactory confinement pressure was

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provided, which counteracts the low transverse strength of FRP bars. Note that in order to increase pipes

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inner shear lock, the inside of the pipes were grooved.

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2.1.3

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In this research, FRP bars were coated with nitrogen-based fire retardant (intumescent paint). The

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mechanism of the utilized fire retardant is to consume heat and release non-flammable gases leading to

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expansion of the intumescent paint material which precludes direct heat and oxygen reaching the FRP

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materials. This mechanism is activated at a temperature of about 350 °C. The activated intumescent paint

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coating used in FRP bars is depicted in Fig. 2, which shows considerable expansion. Detailed information

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about physical properties of the intumescent paint provided by the manufacturer are listed in Table 3. In

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addition, a scanning electronic microscopy (SEM) was performed on the activated intumescent paint to

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scrutinize its mechanism (Fig. 3). The micro-holes detected in this intumescent paint acted as physical

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barriers and caused the delay in heat reaching the FRP material surface.

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2.2 Specimens Preparation

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In this study, GFRP and CFRP bars with the length of 800 mm length were used for the tensile test.

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Three identical specimens were tested for each combination of the FRP bar diameters and the temperatures,

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resulting in a total number of 216 specimens for this investigation. The diameter and length of steel pipes

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used for anchorage varied based on the FRP bar diameter and its ultimate tensile strength. Furthermore, an

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aluminum cap was used in order to place the FRP bar at the center of the steel pipe. The scheme of the

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prepared specimens is depicted in Fig. 4.

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2.3 Specimens Conditions

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The specimens were categorized into two groups: 1) unconditioned (control), i.e. tested at ambient

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temperature (25 °C); 2) conditioned series, i.e. tested at elevated temperatures. In order to simulate short-

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term fire conditions, low elevated temperatures of 80°, 120°, 200°, 250°, 300°, and 350 °C and extreme

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elevated temperatures of 400°, 500°, 600°, 700°, and 800 °C were selected. The use of intumescent paint

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facilitates the evaluation of the effects of extreme elevated temperatures as well as the low elevated

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temperatures on the performance of the FRP bars. Normally, the GFRP and CFRP bars, with the same resin

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type, experience the combustion phase at temperatures around 500 °C; however, the FRP bars coated with

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intumescent paint could perform up to 800 °C. The conditioning of the specimens was performed until a

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steady-state regime was induced on the FRP bars.

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2.4 Mechanical Testing Procedure

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The tensile tests were conducted using a UTM SANTAM-150 servo electric testing device in which a

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three-zone split electric furnace was incorporated as shown in Fig. 5. Note that the furnace complied with

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the ASTM standards recommendations of FRP tensile properties tests [34]. The specimens were loaded at a

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rate of 1.2 mm/min in a displacement-control procedure. In addition, an extensometer was placed in the

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middle of the control specimens to determine the exact value of the elastic modulus (Fig. 6). The steady-

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state regime was achieved through keeping the specimen at the target temperature for 30 minutes. Using this

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method, assures a uniformly distributed temperature over inner and outer parts of the bars. The placement of

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the specimens in test machine is shown in Fig. 7.

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3. Test Results and Discussion

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The effect of intumescent paint coating on the ultimate tensile strength, failure modes, and physical

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properties of FRP bars at low and extreme elevated temperatures is presented and discussed in the following.

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According to the test results, the intumescent paint did not have any effects on the performance of FRP bars

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at temperatures less than 300 °C. As the temperature reaches the glass transition temperature (Tg) of the

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matrix, a significant degradation in the tensile strength was observed. This is due to the fact that the resin

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softening initiated at Tg, which resulted in the reduction in force transfer capacity between the fibers and

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resin. For temperatures above 300 °C, the intumescent paint was activated and prevented the direct heat and

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oxygen exposure to the FRP bars. The intumescent paint effectively improved the performance of the bars

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up to a temperature of 600 °C and a slight retention in the tensile strength of the FRP bars was observed. As

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the temperature increases to 700 °C, a considerable tensile strength reduction was observed. It was

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concluded that the intumescent paint lost its effectiveness at temperatures above 700 °C.

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3.1. GFRP Bars

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The tensile test results of the control and conditioned GFRP bars coated with intumescent paint is

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presented in Table 4. Generally, it is expected that the ultimate tensile strength of GFRP bars without any

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fire retardant material decreases as the ambient temperature of the bar increases. This decreasing trend was

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observed for temperatures below 350 °C; nonetheless, as the intumescent paint was activated, it precluded

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and delayed the degradation of tensile strength in the temperature range of 350 to 600 °C. The results

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indicated a sudden drop in the ultimate tensile strength of GFRP bars at temperatures of 700 and 800 °C

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since these temperatures exceeded the intumescent paint functionality temperature range, i.e. 300 °C to 600

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°C. In addition, the coefficients of variation of the results were low which indicated the low dispersion

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around the mean values expect for bar diameters of 4 and 6mm at 800 °C (Table 4). Although high

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coefficient of variation was obtained for the mentioned GFRP bars, the mean values at that temperature were

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considerably low. Hence, the observed dispersions are not an issue where ultimate tensile strengths are

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relatively low.

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The tensile load versus displacement curves are illustrated for a set of selected GFRP bars in Fig. 8. In

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this figure, all the specimens showed the linear deformation succeeded by a short nonlinear deformation.

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Nevertheless, slight distortion was observed in the linear deformation phase for specimens tested at

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temperatures of 700 and 800 °C. The reason is that at these high temperatures, fibers reach their melting

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temperature [20]. Further, according to the extensometer results used in this study, the average elastic

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modulus of the unconditioned GFRP bars was calculated to be 47.9 GPa with a CoV of 2.1%. Noted that as

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the used extensometer was susceptible to temperature and could encounter damages during the test of

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conditioned specimens, the stain and elastic modulus of only control specimens were determined in this

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study.

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In order to interpret the tensile strength and temperature relations in details, the ultimate tensile strength

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and its retention versus temperature are illustrated in Fig. 9. According to this figure, the GFRP performance

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trend at elevated temperature can be categorized into five temperature zones. The first zone is the behavior

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of GFRP bars at the temperature range of 25 to 80 °C, i.e. ambient temperature to glass transition

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temperature. In this zone, the ultimate tensile strength of GFRP bars decreased slightly as the temperature

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increased, e.g., 4.5% reduction was observed for GFRP with the diameter of 4mm. This phenomenon can be

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justified with the fact that the mobility of the matrix chains may not be influenced significantly by the

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temperatures below the transition temperature. The second zone consists of temperatures between 80 and

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120 °C at which the transition temperature was reached and the state of the resin was changed from glassy to

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rubbery, and a sudden drop in the ultimate tensile strength was observed. The third zone includes the

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temperatures from 120 to 300 °C, i.e. below the decomposition temperature of the resin and also the

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intumescent paint functionality temperature. In this temperature range, no significant reduction was

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observed and the tensile strength retention remained approximately constant. Furthermore, the color of

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intumescent paint coating changed from white to gray despite the fact that it did not affect the results. The

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fourth zone lies in the range of 300 to 600 °C at which the intumescent paint was activated. Generally, at the

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temperatures more than the decomposition temperature, the considerable degradation in mechanical

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properties of FRP bars is expected due to combustion and oxidation of the polymer. However, the presence

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of intumescent paint coating in this study changed this trend. The physical barrier produced by the expanded

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intumescent paint prevented direct heat and oxygen to reach the surfaces of the FRP bars. As a result, the

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combustion and oxidation phases did not occur and the ultimate tensile strength of GFRP bars experienced

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only a slight decrease. Note that an increase in ultimate tensile strength at 350 °C was detected which was

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contrary to the general trend. The reason for this phenomenon is that the intumescent paint activation

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initiated immediately as the temperature reached 350 °C precluding more heat transfer to the FRP bar

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surface. Hence, a slight increase in the ultimate tensile strength was observed between temperature 300 and

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350 °C. This phenomenon is promising in a case of real fire. In a fire condition, as the concrete cover is

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detached suddenly from the FRP bar surface and the concrete face is exposed to temperatures higher than

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350 °C, the intumescent paint activates immediately and prevent direct heat and oxygen reaching the bar.

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The fifth and the last zone includes temperatures of 600 to 800 °C, where the temperatures were so high that

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the intumescent paint could not prevent the heat transfer reaching bar surface. Typically, the test cannot be

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performed at this temperature range in bare FRP materials due to combustion phase which can cause severe

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damages to the test devices and furnace due to toxic fumes and a great amount of heat production. However,

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the intumescent paint was still capable of avoiding the abundant oxygen to reach the surfaces of FRP bars

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and as a results no combustion occurred and the test continued at this temperature range. In this zone, the

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temperatures went toward the melting point of glass fibers which resulted in a great loss in the fibers

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longitudinal load capacity and subsequently in the ultimate tensile strength of GFRP bars.

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Moreover, the effect of bar diameter on the tensile test results is demonstrated in Fig. 9. It is expected

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that the bar with greater diameter, shows a lower ultimate tensile strength. As it can be seen from Fig. 9 (a),

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the FRP bars with lower diameters indicated higher tensile strength than the one with larger bar diameters in

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temperature before 120 °C, i.e. glass transition temperature of the matrix. The reason is that the resins could

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not transfer the loads to the inner fibers in the bars having large diameters. On the other hand, the resin/fiber

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load transfer occurred more easily as the bar diameters were low. As a result, the FRP bars with lower

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diameters expressed higher values of tensile strength.

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Contrarily, the FRP bars with larger diameters are less affected by the elevated temperatures. The reason is

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that fewer detachment of fiber/matrix occurred at the core of the larger bars and also there was a higher

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confinement force for inner fibers in such bars. Consequently, the inner materials of bars with larger

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diameters stay immune to chemical reactions and lower degradation is experienced. It is why the FRP bars

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with greater diameters showed higher ultimate tensile strength retention as can be seen in Fig. 9 (b).

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Although the decrease rate of tensile strength was lower in FRP bars having greater diameters, these bars

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had lower tensile strength. Hence, the differences of the tensile strength between bars with different

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diameters decreased as the temperature increased (the lines of Fig. 9 (a) approaching each other). In the case

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of bars with 8 and 10 cm diameter and the temperatures above 120 °C, the tensile strength retention of bars

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with 10 cm diameter were so much higher than the bars with 8 cm diameter caused the phenomenon that the

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bars with 10 cm diameter showed higher values of tensile strength than the ones having 8 cm diameters. For

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instance, the tensile strength retention of the bars with 10 cm diameter was about 5% greater than the one

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with 8 cm diameter at temperature 120 °C and became around 12% in temperature 200 °C.

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It is observed that the difference between the ultimate tensile retention of 10 mm and the other GFRP

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bars was higher. On the other hand, the differences between the retention of other GFRP bars was not

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considerable. Further, it is concluded that there were slight differences between the ultimate tensile strength

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retention of GFRP bars at the temperatures below 80 °C, i.e., below the glass transition temperature and also

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above 700 °C, which is close to the melting point of glass fibers.

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3.2. CFRP Bars

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The test results of CFRP bars coated with intumescent paint at elevated temperatures are tabulated in

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Table 5. In general, the bare CFRP bars, similar to the GFRP bars, experience a degradation in their

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mechanical properties as the temperature increases [19]. Table 5 indicates the same trend for CFRP bars

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with intumescent paint coating subjected to the temperatures below 350 °C. As the intumescent paint was

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not activated at this temperature range, it did not influence the ultimate tensile strengths of CFRP bars. At

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temperatures higher than 350 °C, the intumescent paint swelling activated that caused the delay and

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obstruction in heat and oxygen transfer to the surface of the FRP bars. Consequently, a slight decrease in the 10

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ultimate tensile strength was observed at the intumescent paint functionality temperature range, i.e., 350 to

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600 °C. Finally, a sudden decrease in the mechanical properties of CFRP bars was observed at the

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temperatures above 700 °C. Moreover, the low coefficient of variation indicates the reliability of the test

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results.

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Fig. 10 displays the load-displacement curves of a set of CFRP bars. As seen, the load-displacement

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curves experienced a slight distortion at temperatures of 700 and 800 °C. In addition, the average modulus

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of elasticity for the control CFRP specimens was determined equal to 152.3 GPa with a 3.4% CoV.

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Furthermore, the variations of the ultimate tensile strength of the CFRP bars with elevated temperature

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are presented in Fig. 11 together with the tensile strength retention versus temperature curves. As seen, the

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performance of CFRP bars coated with intumescent paint at elevated temperatures can be divided into five

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different zones. In the first zone, which is between 25 and 80 °C, the tensile strength remained almost

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unchanged with increasing values of the temperature. In this zone, the maximum reduction in the tensile

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strength was 2.1% for CFRP bar with the diameter of 4 mm. The second zone ranges from 80 to 200 °C,

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where a great decrease was observed in the tensile strength of CFRP bars, e.g., up to 30.9% reduction was

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observed for CFRP bar with 4 mm diameter. Since in this zone the temperature exceeded the glass transition

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temperature of the matrix, fiber/matrix bond reduction occurred due to the plastic state of the matrix. The

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third zone covers the temperatures between 200 and 350 °C. These temperatures are below the

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decomposition temperature of the resin and also the activation temperature of the intumescent paint. The rate

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of the tensile strength degradation of this zone was lower than the second zone. Similar to GFRP bars, the

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ultimate tensile strength increased slightly at 350 °C. This event can be explained as the intumescent paint

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activated promptly when temperature reached 350 °C and thus, the heat could not reach the FRP bar surface.

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As a result, the aforementioned increase in the ultimate tensile strength was detected between temperature

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300 and 350 °C. In the fourth zone, i.e., between 350 and 600 °C, the intumescent paint swelling activated

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and the degradation trend was interrupted due to the presence of the physical barrier. It acknowledged the

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fact that intumescent paint used in this study, can effectively prevent the mechanical properties degradation

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of both GFRP and CFRP bars. The fifth and the final zone includes the temperatures between 600 °C and

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800 °C. As the temperatures are severe at this zone, the intumescent paint coating could not prevent heat

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transfer. As a consequence, the tensile strength of the CFRP bars decreased notably. Although intumescent

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paint coating did not have the significant effect on the heat transfer, it was able to prevent the oxygen to

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reach the CFRP bar surface. Therefore, the combustion phase did not occur in the absence of abundant

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oxygen. By comparing the results of CFRP and GFRP bars at the fifth zone, it can be concluded that the

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GFRP bars almost lost their entire tensile strength, however, the CFRP bars maintained about 20% of their

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tensile strength. It is owing to the fact that the melting point of the carbon fibers is about twice the glass

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fibers [20]; hence, the carbon fibers could still carry load in their longitudinal direction. Note that an

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increase was observed in the tensile strength of CFRP bars at the temperature of 350 °C similar to the GFRP

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bars. This finding can be attributed to the fact that the intumescent paint initiates expansion process as the

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temperature reaches 350 °C (Fig. 2).

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In addition, the influence of bar diameter on the ultimate tensile strength is observed through Fig. 11.

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According to this figure, higher tensile strength retentions were observed for CFRP bars with a larger

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diameter in the fourth temperature zone, where the intumescent paint is activated. On the other hand, the

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results were not affected by the bar size in other temperature regions. It can be concluded that the bar size is

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the only influential parameter in CFRP bars which intumescent paint is used and activated.

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3.3. Failure Modes

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Based on the test observations, the failure modes of the GFRP bars can be divided into six types with

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respect to different temperatures applied to the specimens (Fig. 12). The failure modes that are associated

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with specific test temperature are presented in Table 4. The first failure mode was detected in the GFRP bars

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at the temperature range 25 to 120 °C at which the fibers were not separated completely and the failure was

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brittle. The second failure mode occurs in the temperature range 120 to 300 °C, in which the separation of

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fibers and matrix is visible (Fig. 12). In addition, the intumescent paint color changed from white to gray at

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this failure mode, indicating the fact that the fire retardant material is activated at higher temperatures. Next

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failure mode, type 3, was observed at temperatures between 300 and 500 °C, where the intumescent paint

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was activated. In this stage, the color of fibers changed from white to brown due to fiber slight oxidation that

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can be seen in Fig. 12. It is clearly obvious that the intumescent paint color turned black upon its activation.

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The failure mode type 4 occurred at the temperature range of 500 to 600 °C at which the intumescent paint

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lost its functionality to some extent. The reason is that as the temperature increases to extreme values, i.e.,

311

above 600 °C, the physical barriers produced by intumescent paint were not capable of preventing heat

312

transfer. On that account, the fibers were more prone to temperature, further oxidation of fibers took place,

313

and the color of the fibers changed to dark brown. In the fifth failure mode, which occurred at temperatures

314

600 to 700 °C, the color of the fibers completely turned black due to the high decomposition phase. The

315

final failure mode was experienced at temperatures of 700 to 800 °C under which the fibers lost their entire

316

longitudinal capacity and melted. The white color of the fibers at the failed section was due to melting.

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On the other hand, the failure modes for CFRP bars varied from the ones observed for GFRP bars.

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According to this investigation, the CFRP bar failure modes were categorized into four types, which are

319

demonstrated in Fig. 13. These failure modes and their associated temperatures are presented in Table 5. The

320

first failure mode of the CFRP bars was similar to the GFRP bars, which was detected in the temperature

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range of 25 to 120 °C and occurred due to brittle failure. In failure mode type 2, between temperatures 120

322

and 300 °C, the carbon fibers were separated while the helical fibers remained almost intact (Fig. 13). The

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next failure mode, type 3, occurred at temperatures of 300 to 600 °C, where the separation of fibers was

324

more pronounced and also the helical fibers failed. In this failure mode, the intumescent paint color changed

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from white to black as the intumescent paint was activated and its expansion was initiated. The failure mode

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type 4 includes the temperature range of 600 °C to 800 °C at which CFRP bars’ longitudinal and helical

327

fibers were entirely failed and separated. Noted that the mechanism of failure for CFRP bars was in a brittle

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manner that most of the expanded intumescent paint materials fell off after failure. Hence, only the trace of

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the intumescent paint can be seen in Fig. 13.

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In addition, the test observations indicated that the intumescent paint produced a bulky cylindrical

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barrier on the specimens. No part of this barrier did not detach from the specimens during the test and thus

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the intumescent paint continued to be functional until the failure of the specimens occurred (see Fig. 2). As

333

the failure of specimens occurred, some portion of the barrier fell off because of the impact induced to the 13

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specimens due to sudden failure. Despite the sudden failure, many pieces of the intumescent paint bulky

335

barrier still remained attached to the specimens. The remained segments of the intumescent paint can be

336

seen in the failure mode figure, i.e. Fig. 12. Although the aforementioned observations were achieved from

337

the test performed vertically, the barrier produced by the intumescent paint can demonstrate sufficient

338

adhesion to the specimens even they are placed in a horizontal direction. This can be acknowledged by the

339

fact that the produced barrier has low weight and the gravity force do not cause this barrier to detach from

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the specimens.

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4. Probabilistic and Statistical Studies

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4.1.1. Bayesian Regression Analysis

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Predicting the behavior of material properties under different conditions has been the aim of several

344

studies [35-37]. To develop the model predicting ultimate tensile strength of FRP bars coated with

345

intumescent paint under elevated temperatures, Bayesian regression is employed. Using this approach, the

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model randomness is taken into account explicitly [38, 39]. The model form using this method can be

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presented as follows:

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σ ut = θ1 ⋅ g1 (x) + θ2 ⋅ g2 (x) + L + θm ⋅ gm (x) + ε σu

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Eq. 1

In the above equation, σut/σu is the ratio of ultimate tensile strength at the target temperature to ultimate

350

tensile strength at ambient temperature. In addition, gm and θm are mth explanatory function and regression

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coefficient, x is the predictor variable vector and ε is the model error that is a random variable with zero

352

mean and Normal probability distribution.

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First, several candidate model forms were defined to determine the convenient explanatory functions

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based on the initial model diagnosis results. The model diagnosis includes consideration of non-collinearity,

355

errors normality, regression coefficients correlation, homoscedasticity, and model prediction quality. The

356

results led to a model form presented in Eq. 2. As high correlation coefficients were observed between the

357

model coefficients (Table 6), the model coefficient reduction is required to revise the model form. For this

14

358

purpose, a stepwise reduction procedure is proposed by Gardoni et al. [39, 40]. According to this approach,

359

the parameters with high correlation coefficients (higher than 0.5) are combined as described in Eq. 3.

360

σut 1 1 =θ1 +θ2 ⋅ +θ3 ⋅ 2 +ε σu D ln(T)

361

θˆi = µ θ i + ρ θ iθ j

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Eq. 2

σ θi (θ − µ θ j ) σθ j j

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Eq. 3

Where ρθiθj is the correlation coefficient of model parameter θi and θj and the posterior mean and

363

standard deviation of the θi are denoted by µθi and σθi. This procedure is applied to the model form until no

364

correlation coefficients higher than 0.5 is detected. The model form developed in this research comprised

365

only one single model coefficient after reduction procedure (Eq. 4).

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 σut 1 1 1 = θ2  β + + β′ ⋅ 2  +α +α′ ⋅ 2 + ε σu D  D  ln(T)

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Eq. 4

In the above equation, coefficients α, α’, β, and β’ are constant parameters which were added to the

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model form during model parameter reduction process and are presented in Table 7. The information

369

regarding model form probabilistic characteristics is illustrated in Table 8.

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Finally, the model diagnosis results of the reduced model form are presented in Fig. 14 and 15. Each

371

figure contains four plots presenting the quality and accuracy of the proposed model. The propriety of the

372

model is investigated by model prediction versus observation plot as the points are on/close to the 45° line.

373

Further, prediction to observation ratio versus predicted values plot checks the same matter. As one of the

374

basic assumptions of Bayesian linear regression is that model error follows a Normal distribution, this

375

assumption is checked through model error normality plot. In addition, the model homoscedasticity or

376

heteroscedasticity is investigated using residuals versus regressor plot. As the pattern of this plot is

377

homogeneously distributed, the model form is considered as homoscedastic, otherwise, it is called

378

heteroscedastic. The developed model form in this research met all the aforementioned criteria; hence, it is

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considered as an apt predictive model.

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4.1.2. Analysis of Variance (ANOVA)

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One of the essential parts of experimental investigation is to quantify the influence of input parameters,

382

individually and together, on the results. Several statistical and probabilistic methods can be found in the

383

literature for this purpose [41-44]. Analysis-of-Variance (ANOVA) is regarded as an effective approach to

384

calculating the contribution of variables to the response [45]. As two variables, temperature and bar diameter

385

were involved in this research, two-way ANOVA was performed on the tensile strength retention of GFRP

386

and CFRP bars. The results of ANOVA analyses are shown in Table 9 to 12. In these tables, SS, df, and MS

387

are referring to the sum of square, degrees of freedom, and mean square, respectively. As the P-values are

388

less than 0.05, it can be concluded that the test variables have substantial effects on the response. In addition,

389

the F-values are greater than F-critical-values indicating the same interpretation achieved by P-values. In

390

order to consider the effect of intumescent painting on the tensile strength retention of FRP bars, ANOVA

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analysis was utilized separately for the whole temperature range (25-800 °C) and the functionality

392

temperature range of intumescent paint (350-600 °C). The results indicated that the temperature has the most

393

effect on the tensile strength retention of both GFRP and CFRP bars in the whole test temperature range,

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95.51 and 98.95% for GFRP and CFRP bars respectively. It was observed that the temperature contribution

395

percentage for CFRP is slightly higher than GFRP bars.

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Furthermore, it was realized that bar diameter is not a substantial factor in tensile strength retention in

397

the aforementioned range in comparison with temperature. On the other hand, the ANOVA results are

398

different as only the data in the range of intumescent paint functionality temperature are taken into

399

consideration. It was noticed that the bar diameter turned to be more influential than the temperature factor

400

in the range of 350-600 °C for GFRP bars. Although the temperature is more effective than bar diameter for

401

CFRP bars in the functionality range of intumescent paint, the contribution percentage of bar diameter

402

increased from 3.25 to 27.93% in this range. In all the ANOVA analysis results, P-values are less than 0.05

403

except for interaction in Table 12, that acknowledge the fact that the variables interaction was not influential

404

in CFRP bars tested at 350-600 °C. In addition, one can conclude that interactions of bar diameter and

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temperature have the considerable impact on the tensile strength retention despite the fact that their P-values

406

are extremely low.

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5. Comparison with previous studies

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In this section, the ultimate tensile strength of GFRP and CFRP bars coated with fire retardant material

409

at elevated temperature is compared to the results of the study conducted by Ashrafi et al. [19] and also with

410

predictive models developed by Nadjai et al. [26] and Wang et al. [46]. As the FRP bars tested in the study

411

by Ashrafi et al. [19] and the ones utilized in this study have the same mechanical and thermal properties,

412

the comparison of these studies is of great help to determine the effect of intumescent paint coating on the

413

thermal performance of FRP bars. The results of predictive models proposed by previous researchers and the

414

experimental results of this study are presented in Fig. 16 and 17.

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The model proposed by Nadjai et al. [26] predicted the tensile strength retention of FRP materials at

416

elevated temperature. They proposed separate models for GFRP and CFRP which are shown in Eq. 5 and 6,

417

respectively. 0 ≤ T p 400 400 ≤ T

 1,  = 1.267 − 0.00267T , fu   0,

f u ,t

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1 − 0.0025T , = f u  0,

f u ,t

Eq. 5

0 ≤ T p 100

100 ≤ T p 475 475 ≤ T

Eq. 6

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In above equations, fu,t and fu are the tensile strength at temperature T and at ambient temperature,

421

respectively. In addition, the model developed by Wang et al. [46] indicated three temperature ranges for

422

FRP materials to relate the residual tensile strength to the temperature (Eq. 7).

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Eq.

420 ≤ T p 706

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0.9  T − 22 ) ( ,  1− 200  0.7 f u ,t  T − 150 ) ( , =  0.59 − 490 fu  1.8  T − 420 ) ( 0.48 − , 76000 

According to Fig. 16, which shows a comparison of GFRP bar studies, the results for all the bar diameters

426

by Ashrafi et al. [19] and from this study are almost the same for temperatures below 300 °C, as expected.

427

Since the intumescent paint is activated at temperatures higher than 300 °C, no significant differences were

428

expected in this temperature range. In the temperature range of 350 to 600 °C, the intumescent paint

429

function is initiated. This can be explained by the tensile strength retention, obtained by Ashrafi et al. [19]

430

that tended to decrease considerably. However, the results of this study show almost no significant decrease.

431

Although the effect of intumescent paint coating on GFRP bars varies for different bar diameters, it caused

432

an increase of approximately 20 to 30% in the tensile strength retention compared to bare bars. It was

433

observed that the intumescent paint coating is most effective for GFRP bars with a diameter of 4 mm.

434

Further, the model developed by Wang et al. [46] presents the same trend as that observed in this study, with

435

only a vertical shift in the results of fire retardant-coated materials. Contrarily, the model proposed by

436

Nadjai et al. [26] is incapable of predicting the performance of FRP bars at elevated temperature accurately,

437

particularly at temperatures above 200 °C. Further, the aforementioned model leads to highly conservative

438

estimates.

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In addition, Fig. 17 presents the comparison of previous models and the results of present research for

440

CFRP bars. It can be observed that, below the intumescent paint activation temperature (350 °C), the results

441

of Ashrafi et al. [19] and this study are relatively similar to each other. In the range of 350 to 600 °C, the

442

intumescent paint functionality range, the tensile strength retention of CFRP bars with intumescent paint

443

coating is about 15 to 25% higher than what was observed by Ashrafi et al. Noted that CFRP bars with only

444

one diameter were used by Ashrafi et al. [19]; hence, no conclusions can be drawn on the effect of the bar 18

445

diameter in the effectiveness of intumescence paint coating on CFRP bars. Moreover, it is realized that the

446

trend of the model presented by Wang et al. [46] is in agreement with the trend calculated in the present

447

study. On the other hand, the model developed by Nadjai et al. [26] is only valid at temperatures below 250

448

°C and it does not properly predict the trend of bars with or without fire retardant.

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To put the issue into perspective, as the mechanical and thermal properties of FRP bars used in the

450

literature are varied extensively, slight or significant differences can be observed in the comparison of their

451

results. Furthermore, the effects of different parameters such as bar diameter, mechanical and thermal

452

properties have not been considered in previous studies and they mostly emphasize on the influence of

453

temperature only. Consequently, more extensive studies have to be conducted in order to provide a

454

comprehensive database required to update and revise the existing predictive models. Despite the

455

aforementioned problematic issues, the comparison of the results presented in this section provided us with

456

insights and advantageous conclusions of the results.

457

6. Conclusions

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This study deals with the tensile performance of FRP bars coated with intumescent paint subjected to

459

low and extreme temperature conditions. This research focused on several problematic issues, namely, the

460

influence of the bar diameter and the temperature on the tensile behavior of FRP bars, the efficiency of

461

nitrogen-based intumescent paint, and the development of a predictive model for FRP bars’ performance

462

during a fire hazard. Based on the results of this research, the conclusions can be summarized as follows:

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The mechanical properties of FRP bars remained unchanged at the temperatures ranging between

464

ambient and glass transition, i.e., 25 and 80 °C. Hence, no major concern about the behavior of FRP

465

bars exists at temperatures below the glass transition temperature.

466

At temperatures ranging from 80 to 120 °C, considerable reductions were observed in the tensile

467

strength of FRP bars due to the debonding of fiber/resin. It is concluded that as the temperature

468

exceeds the glass transition temperature of the FRP bar, the line of actions such as applying

469

reduction factor to the FRP bar tensile strength has to be performed. 19

470

At temperatures between 120 and 350 °C, where intumescent paint was not activated yet, a slight

471

reduction in the tensile performance of FRP bars was detected.

472

As the intumescent paint activation was initiated at 350 °C, the degradation progress of the FRP bar

473

tensile strength was precluded. The mechanism of nitrogen-based intumescent paint is to expand and

474

produce physical barriers, causing a delay in heat and oxygen transfer. It is observed that the

475

intumescent paint performs at a temperature ranging from 350 °C to 600 °C, which causes a 20% to

476

30% and 15% to 25% increase in the tensile strength retention of GFRP and CFRP bars,

477

respectively. In addition, it was concluded that at the aforementioned temperature range, the bar

478

diameter influence on the tensile strength increases considerably, as the bar diameter contributed to

479

a variance in results, which increased from 3.25% to 60.31% and 0.42% to 27.93% for GFRP and

480

CFRP bars, respectively.

481

As the temperature exceeds 600 °C, the efficiency of intumescent paint reduced significantly, since

482

the temperatures were extremely high and the delay in heat transfer could not be obtained. It is

483

despite the fact that the produced physical barrier could prevent the oxygen reaching FRP bar

484

surface. Hence, the combustion does not occur at this temperature range and FRP bars do not

485

experience the extra heat of combustion phase.

486

In essence, this study presents an effective solution to increase the fire resistance of FRP bars; it also

487

proposes predictive models that can be used for code calibration, guideline revision, and design

488

procedure development. Although this investigation focused on comprehensively studying the tensile

489

behavior of FRP bars subjected to low and extreme elevated temperatures, additional experimental

490

studies are required to increase the database and understanding of the performance of FRP materials

491

subjected to fire hazard.

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Acknowledgements

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The support of Vatan Composite Company in supplying materials are greatly acknowledged.

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[44] Saltelli, A., Ratto, M., Andres, T., Campolongo, F., Cariboni, J., Gatelli, D., ... & Tarantola, S., Global ACCEPTED MANUSCRIPT sensitivity analysis: the primer, John Wiley & Sons, 2008.

599

[45] Tabachnick, B. G., & Fidell, L. S., Experimental designs using ANOVA, Thomson/Brooks/Cole, 2007.

600 601

[46] Wang, K., Young, B., & Smith, S. T., Mechanical properties of pultruded carbon fibre-reinforced polymer (CFRP) plates at elevated temperatures, Engineering structures (2011) 33(7), 2154-2161.

602

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603 604 605

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608 609 610 611

613 614 615

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TE D

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List of Figures:

617

Fig. 1 FRP bars utilized in this research with and without intumescent paint coating

618

Fig. 2 Intumescent paint activation and expansion on FRP bars: (a) shown in test rig, (b) magnified view

619 620

Fig. 3 Micrograph of the activated intumescent paint coating used on the FRP bars of this study: (a) 200x SEM magnitude; (b) 600x SEM magnitude

621

Fig. 4 Specimen configuration in detailed

622

Fig. 5 UTM SANTAM-150 device, three-zone split furnace and thermocouple

623

Fig. 6 Extensometer used in this study

624

Fig. 7 Specimen placement in the test rig

AC C

616

24

Fig. 8 Tensile force versus displacement curves of selected GFRP bars coated with intumescent paint at low ACCEPTED MANUSCRIPT and high elevated temperatures

627 628

Fig. 9 Results of GFRP bars coated with intumescent paint: (a) ultimate tensile stress versus temperature, (b) ultimate tensile stress retention percentage versus temperature

629 630

Fig. 10 Tensile force versus displacement curves of selected CFRP bars coated with intumescent paint at low and high elevated temperatures

631 632

Fig. 11 Results of CFRP bars coated with intumescent paint: (a) ultimate tensile stress versus temperature, (b) ultimate tensile stress retention percentage versus temperature

633

Fig. 12 Failure modes of GFRP bars coated with intumescent paint at low and high elevated temperatures

634

Fig. 13 Failure modes of CFRP bars coated with intumescent paint at low and high elevated temperatures

635 636

Fig. 14 Model diagnosis for GFRP bars: (a) model prediction versus observation, (b) model error normality, (c) model predication to observation ratio, (d) residual versus regressor

637 638

Fig. 15 Model diagnosis for CFRP bars: (a) model prediction versus observation, (b) model error normality, (c) model predication to observation ratio, (d) residual versus regressor

639 640

Fig. 16 Comparison of the results obtained in this study with previous results and predictive models for GFRP bars

641 642

Fig. 17 Comparison of the results obtained in this study with previous results and predictive models for CFRP bars

TE D

M AN U

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625 626

643

644

EP

645 List of Tables:

647

Table 1 Mechanical and thermal properties of FRP bars used in this study

648 649

Table 2 Chemical composition of glass fibers, carbon fibers, and epoxy resin obtained due to elemental analysis test

650

Table 3 Nitrogen-based intumescent paint properties

651

Table 4 Tensile test results of GFRP bars coated with intumescent paint at low and high temperatures

652

Table 5 Tensile test results of CFRP bars coated with intumescent paint at low and high temperatures

653

Table 6 Initial explanatory functions for model response (σut/σu)

654

Table 7 Model form parameters

AC C

646

25

655

Table 8 Second-moment characteristics of the developed model form

656

Table 9 ANOVA results for GFRP bars ultimate tensile strength retention at 25-800 °C

657

Table 10 ANOVA results for GFRP bars ultimate tensile strength retention at 350-600 °C

658

Table 11 ANOVA results for CFRP bars ultimate tensile strength retention at 25-800 °C

659

Table 12 ANOVA results for CFRP bars ultimate tensile strength retention at 350-600 °C

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668

672

673

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Table 1 Mechanical and thermal properties of FRP bars used in this study

AC C

670

TE D

669

Property

Bar Type Sand-Coated GFRP

CFRP

Resin type

Epoxy

Epoxy

Fiber volume (%)

75

75

Characteristic tensile strength (MPa)

1200

2100

Modulus of elasticity (GPa)

50

150

Strain at failure (%)

2.4

1.4

Density (g/cm3)

2.2

1.65

Longitudinal coefficient of thermal expansion (1/°C)

9×10-6

-8×10-6

26

Transverse coefficient of thermal expansion (1/°C)

22×10-6

85×10-6

Longitudinal thermal conductivity (W/m°C)

3.12

70

Transverse thermal conductivity (W/m°C)

0.25

0.20

Cure Ratio (%)

97.5

98

Glass transition temperature (Tg)

110

110

ACCEPTED MANUSCRIPT

674

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675 676 677

SC

678 679

M AN U

680 681 682 683

687 688 689 690

EP

686

Table 2 Chemical composition of glass fibers, carbon fibers, and epoxy resin obtained due to elemental analysis test

AC C

685

TE D

684

Chemical Composition (wt %) Glass Fibers Carbon Fibers Epoxy Resin 93.80 72.23 SiO2 51.56 C C 1.95 8.83 CaO 19.46 H H 2.92 3.73 Al2O3 15.98 N N 7.82 1.26 14.86 Ba2O3 O O 3.09 MgO Na2O 0.44 0.37 K2O Fe2O3 0.36 27

0.16 TiO2 ACCEPTED MANUSCRIPT Others <0.01

691

692

693

694

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695

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697

SC

698

699

M AN U

700

701

702

706

707

708

709

EP

705

AC C

704

TE D

703

Table 3 Nitrogen-based intumescent paint properties Adhesion to Surface (Psi)

≥290

Tensile Strength (Psi)

290

Specular Reflection (400-900 nm)

3%

Specular Reflection (900-2300 nm)

40%

Diffuse Reflection (400-1000 nm)

90%

Diffuse Reflection (1000-2250 nm)

80%

Heat Conductivity (W/mK)

0.03

28

Water Absorption (%)

9

Density (gr/cm3)

0.81

Glass Transition Temperature (°C)

-35

ACCEPTED MANUSCRIPT

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725

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726

728

Table 4 Tensile test results of GFRP bars coated with intumescent paint at low and high temperatures

100 97.6

1059 0.52 100 1049 0.43 99.1

1003 0.45 100 977 0.41 97.4

Failure Mode

0.7 0.51

Retention (%)

1296 1265

COV (%)

100 95.5

Average Ultimate Stress (MPa)

Retention (%)

1.3 0.75

FR-G10 Retention (%)

COV (%)

1454 1388

COV (%)

Average Ultimate Stress (MPa)

25 80

Average Ultimate Stress (MPa)

Retention (%)

FR-G8

COV (%)

FR-G6

Average Ultimate Stress (MPa)

FR*-G4**

Temperature (˚C)

729

AC C

727

1 1 29

120 200 250 300 350 400

1078 930 886 877 919 873

0.81 1.28 1.35 0.97 1.49 0.39

74.1 64 60.9 60.3 63.2 60

979 1.53 75.5 893 1.04 ACCEPTED MANUSCRIPT 904 1.16 69.8 739 0.66 828 1.99 63.9 689 1.21 812 0.39 62.7 665 1.51 854 0.58 65.9 710 0.55 830 1.42 64 692 0.89

500

854

2.15

58.7

763

1.02 0.77 1.03 1.26 0.47 1.25

89 79.6 78.1 76 81.9 76.6

58.9

666

1.63 62.9

725

1.06 72.3

3.47 56.2 4.79 33.8 15.29 4.4

641 359 52

1.1 60.5 4.47 33.9 7.38 4.9

649 383 112

1.39 64.7 3.77 38.2 4.82 11.2

730

1 2 2 2 3 3 34 4 5 6

SC

731 732

M AN U

733 734 735 736

TE D

737 738 739

EP

740

742

AC C

741

Table 5 Tensile test results of CFRP bars coated with intumescent paint at low and high temperatures

COV (%)

Retention (%)

Average Ultimate Stress (MPa)

COV (%)

Retention (%)

Failure Mode

25 80 120

FR-C5

Average Ultimate Stress (MPa)

FR*-C4**

Temperature (˚C)

743

893 798 783 762 821 768

RI PT

693 5.06 47.7 728 600 479 5.15 32.9 438 700 29 19.09 2 57 800 * FR: Fire retardant coated bar ** G4: GFRP with diameter 4mm

1.45

84.3 69.8 65.1 62.8 67 65.3

2140 2096 1886

0.74 1.28 1.13

100 97.9 88.1

2081 2073 1786

0.68 0.88 0.92

100 99.6 85.8

1 1 1 30

744

745

747

749

750

751

757 758

EP

756

AC C

755

TE D

752

754

2 2 2 3 3 3 3-4 4 4

M AN U

748

753

71.3 62.9 60.4 67.5 62.3 60.8 55.3 35.8 29.6

SC

746

1.9 1.8 1.74 2.2 1.2 0.96 3.14 2.49 7.03

RI PT

1478 0.81 69.1 1484 200 ACCEPTED MANUSCRIPT 1349 1.64 63 1308 250 1214 1.73 56.7 1257 300 1321 1.97 61.7 1404 350 1241 0.98 58 1297 400 1163 2.46 54.3 1266 500 1011 3.28 47.2 1151 600 757 5.62 35.4 744 700 574 7.36 26.8 616 800 *FR: Fire retardant coated bar ** C4: CFRP bar with diameter 4mm

Table 6 Initial explanatory functions for model response (σut/σu) Explanatory Function g1 g2 g3

Description

Symbol

Unite

Intercept Temperature Bar Diameter

T D

Kelvin Mm

759 760 761 31

762

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765

766

RI PT

767

768

769

SC

770

771

M AN U

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774

775

779

780

781

782

EP

778

AC C

777

TE D

776

Table 7 Model form parameters

Coefficient α β α’ β’

GFRP -4.81 0.1587 -130.3 7.32

CFRP -5.92 0.1574 -124.7 5.86

783

784

32

785

ACCEPTED MANUSCRIPT

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787

788

789

RI PT

790

791

792

SC

793

794

M AN U

795

796

797

798

802

803

804 805

EP

801

AC C

800

TE D

799

Table 8 Second-moment characteristics of the developed model form Model From

GFRP bars CFRP bars

Mean of θ2

17.54

21.07

CoV of θ2 (%)

0.08

0.06

Mean of σε

0.0743

0.0580

CoV of σε (%)

7.76

10.78

R-Factor

0.9996

0.9991

806 33

807

ACCEPTED MANUSCRIPT

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809

810

811

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813

814

SC

815

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M AN U

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820

TE D

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822

823

EP

824

825

827

AC C

826

Table 9 ANOVA results for GFRP bars ultimate tensile strength retention at 25-800 °C Source of Variation

SS

df

Bar Diameter

0.192973

3

MS

P-value

F critical

Contribution (%)

0.064324 702.0662

1.24592E-39

2.798061

3.25

Temperature

5.667794 11 0.515254 5623.713

1.11732E-70

1.99458

95.51

Interaction

0.069332 33 0.002101 22.93108

6.59148E-20

1.677796

1.17

Error

0.004398 48 9.16E-05

Total

5.934498 95

F

0.07

828 34

829

ACCEPTED MANUSCRIPT

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831

832

833

RI PT

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836

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M AN U

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845

EP

846

848

849

AC C

847

Table 10 ANOVA results for GFRP bars ultimate tensile strength retention at 350-600 °C Source of Variation

SS

df

MS

F

P-value

F crit

Contribution (%)

Bar Diameter

0.118298

3

0.039433

345.2355

9.51E-15

3.238872

60.31

Temperature

0.065931

3

0.021977

192.4091

9.24E-13

3.238872

33.61

Interaction

0.010109

9

0.001123

9.833578

5.29E-05

2.537667

5.15

Error

0.001828

16

0.000114

Total

0.196165

31

0.93

850 35

851

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853

854

855

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856

857

858

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860

M AN U

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TE D

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866

867

EP

868

870

871

AC C

869

Table 11 ANOVA results for CFRP bars ultimate tensile strength retention at 25-800 °C Source of Variation

SS

df

MS

F

P-value

F crit

Contribution (%)

Bar Diameter

0.009119

1

0.009119

59.60974

5.88E-08

4.259677

0.42

Temperature

2.162254

11

0.196569

1284.992

1.91E-30

2.216309

98.95

Interaction

0.010247

11

0.000932

6.089895

0.000111

2.216309

0.47

Error

0.003671

24

0.000153

Total

2.185292

47

0.17

872 36

873

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876

877

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EP

890

892

893

AC C

891

Table 12 ANOVA results for CFRP bars ultimate tensile strength retention at 350-600 °C Source of Variation

SS

df

MS

F

P-value

F crit

Contribution (%)

Bar Diameter

0.015167

1

0.015167

99.55383

8.63E-06

5.317655

27.93

Temperature

0.037207

3

0.012402

81.40561

2.46E-06

4.066181

68.52

Interaction

0.000704

3

0.000235

1.54113

0.277303

4.066181

1.3

Error

0.001219

8

0.000152

Total

0.054297

15

2.24

894 37

RI PT SC M AN U

895 896

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Fig. 1 FRP bars utilized in this research with and without intumescent paint coating

900

901

902

903

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AC C

898

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904

905

906

907 38

908 909

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Fig. 2 Intumescent paint activation and expansion on FRP bars: (a) shown in test rig, (b) magnified view

910

914 915 916 917

EP

913

AC C

912

TE D

911

918 919 920 921 39

Fig. 3 Micrograph of the activated intumescent paint coating used on the FRP bars of this study: (a) 200x SEM magnitude; (b) 600x SEM magnitude

M AN U

922 923 924

SC

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925 926

930 931 932 933

EP

929

AC C

928

TE D

927

934 935 936 937 40

938

Fig. 4 Specimen configuration in detailed

RI PT

939 940

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AC C

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TE D

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41

958

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Fig. 5 UTM SANTAM-150 device, three-zone split furnace and thermocouple

961

965

966

967

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AC C

963

TE D

962

970

971

972

42

973

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Fig. 6 Extensometer used in this study

976

977

981

982

983

984

EP

980

AC C

979

TE D

978

985

986

987

988 43

989

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Fig. 7 Specimen placement in the test rig

993

994

995

996 44

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Fig. 8 Tensile force versus displacement curves of selected GFRP bars coated with intumescent paint at low

999

and high elevated temperatures

002

003

004

EP

001

AC C

000

TE D

997 998

005

006

007

008 45

RI PT

Fig. 9 Results of GFRP bars coated with intumescent paint: (a) ultimate tensile stress versus temperature, (b) ultimate tensile stress retention percentage versus temperature

SC

009 010 011

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Fig. 10 Tensile force versus displacement curves of selected CFRP bars coated with intumescent paint at

030

low and high elevated temperatures

M AN U

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032

033

037

038

039

040

EP

036

AC C

035

TE D

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041

042

043

044 47

045

RI PT Fig. 11 Results of CFRP bars coated with intumescent paint: (a) ultimate tensile stress versus temperature, (b) ultimate tensile stress retention percentage versus temperature

SC

046 047 048

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AC C

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Fig. 12 Failure modes of GFRP bars coated with intumescent paint at low and high elevated temperatures

067

49

072

073

074

075

RI PT SC M AN U TE D

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EP

070

Fig. 13 Failure modes of CFRP bars coated with intumescent paint at low and high elevated temperatures

AC C

068 069

ACCEPTED MANUSCRIPT

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077

078

50

083

084

085

RI PT SC M AN U TE D

EP

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Fig. 14 Model diagnosis for GFRP bars: (a) model prediction versus observation, (b) model error normality, (c) model predication to observation ratio, (d) residual versus regressor

AC C

079 080 081

ACCEPTED MANUSCRIPT

086

087

088

51

089

092

093

094

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Fig. 15 Model diagnosis for CFRP bars: (a) model prediction versus observation, (b) model error normality,

AC C

090 091

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(c) model predication to observation ratio, (d) residual versus regressor

095

096

097

52

098

ACCEPTED MANUSCRIPT

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Fig. 16 Comparison of the results obtained in this study with previous results and predictive models for

102

GFRP bars

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AC C

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EP

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106

107

108

109 53

110

ACCEPTED MANUSCRIPT

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Fig. 17 Comparison of the results obtained in this study with previous results and predictive models for

114

CFRP bars

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ACCEPTED MANUSCRIPT

Highlights: The influence of intumescent paint on FRP bars performance under elevated temperatures were investigated. Further, the effects of bar diameter and temperature was considered in this study.

RI PT

It is observed that the intumescent paint functions at the temperature between 350 to 600 °C.

FRP bars coated with intumescent paint showed higher tensile strength retention than raw FRP bars, about 20% to 30% and 15% to 25% in GFRP and CFRP bars respectively.

SC

According to ANOVA, the influence of bar diameter on the tensile strength of FRP bars increased considerably in temperature ranging from 350 to 600 °C.

M AN U

The Bayesian predictive model was developed for tensile strength of GFRP and CFRP

AC C

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bars.

ACCEPTED MANUSCRIPT Conflict of Interest: None declared. Ethical Statement: Authors state that the research was conducted according to ethical standards.

RI PT

Acknowledgements: The support of Vatan Composite Company in supplying materials are greatly acknowledged.

Funding Body:

AC C

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This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.