Investigation on an effective bond arrangement of insulated CFRP-strengthened flexural members for improved thermo-mechanical performance

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scopus – master journals – JCR

ایمپکت فاکتور

۴٫۲۷۶ در سال ۲۰۲۰

شاخص H_index

۲۶ در سال ۲۰۲۱

شاخص SJR

۰٫۹۰۱ در سال ۲۰۲۰

شاخص Quartile (چارک)

Q1 در سال ۲۰۲۰

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Investigation on an effective bond arrangement of insulated CFRP-strengthened flexural members for improved thermo-mechanical performance

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Investigation on an effective bond arrangement of insulated CFRP-strengthened flexural members for improved thermo-mechanical performance


Application of insulation on Carbon Fiber Reinforced Polymer (CFRP) strengthened concrete members in buildings is essential to preserve the structural integrity during the fire. Investigation on suitable bonding techniques to minimize the usage of insulation without affecting the structural performance may lead to a cost optimum solution for retrofitting projects. An experimental program was conducted to compare the thermal and structural performance of grooved and external bonding techniques. A total of 15 CFRP- Concrete members were exposed to different fire scenarios with and without insulation. A numerical model was also developed to predict the effects of sensitive parameters on the thermo-mechanical performance of CFRP/Concrete composites under standard fire. The results indicated that the application of CFRP laminates in a groove within the nominal cover of the member can save up to 36% of the insulation than the external bonding technique. This study provides design recommendations in the form of charts for easy guidance to select a suitable bonding technique, insulation type and application depending on the feasibility of the project.

Keywords: Buildings, CFRP-Concrete composites, Design charts, Fire rating, Insulation

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  1. Introduction

In recent years, Fiber Reinforced Polymer (FRP) applications in the Civil engineering industry has been significantly increased due to their superior properties. The FRP retrofitting technique has become an efficient method for retrofitting the concrete structures especially in buildings and bridges [1–۳]. Though FRP has superior mechanical properties, the fire performance remains an obstacle for practical applications [4]. Degradation of mechanical and bond properties of FRP composites was found when it was subjected to elevated temperatures since the glass transient temperature of the adhesive component is around 70°C [5–۸]. The fire performance of the CFRP/Concrete depends on the glass transition temperature (Tg) of the bond between CFRP and Concrete which is the temperature where the polymer substrate changes from rigid to viscous [9]. At high temperatures, all polymer resins will soften and eventually ignite, resulting the resin matrix to weaken, and hence a potential concern raises regarding the structural integrity of FRP-concrete composite structures [5,10].

Several investigations have been carried out to study the bond behavior of CFRP strengthened concrete and steel members at elevated temperature using single-lap shear tests, double-lap shear tests, and beam test [8,9,11,12]. Gamage et al. have performed a single-lap shear test on externally CFRP bonded concrete members and observed a sudden strength reduction up to 80% when the bond line reaches the temperature between 50ºC – 70 ºC. A combination of bond failure and concrete rupture was noted in bond line temperature less than 50ºC and peeling off of CFRP was noted in bond line temperature above 60ºC [8]. Palmieri et al. [13] have also explored the Near-surface mounted (NSM)-CFRP bonded concrete members in the temperature range between 20ºC – 100 ºC and pointed out a bond strength reduction above its glass transition temperature (Tg). Building regulations require that a fire resistance period of 2-4 hours must be provided when designing buildings [14]. In general, strengthening takes place to enhance the capacity to support additional dead and live loads [15]. Though the live loads acting on the members reduce during the fire, the optimum structural performance of the system should be ensured within the evacuation period. In this regard, it is important to maintain the bond line temperature below Tg within the fire resistance period [16]. Even though several techniques have been investigated to control the temperature of the CFRP composite structures [9] during a fire event, controlling the temperature is impossible in large scale civil engineering applications. Therefore, using insulation materials for CFRP-Concrete composites have become an effective solution [17,18]. The suitability of several insulation materials; calcium silicate board, thick coating, ultra-thin coating, vermiculate-cement blend, Perlite has been investigated for CFRP-concrete composites and fire endurance of 2 hours was able to achieve with relatively high thickness of insulation [10,11,14,19]. From past studies, it was found that the insulation thickness affects the fire resistance period of both EBR and NSM- CFRP/Concrete composites. Gamage et al. and Firmo et al. [14,20,21] have pointed out that, the fire performance of the CFRP/Concrete composites increases with larger insulation thickness. In most of the investigations, fire endurance of 2 hours was achieved using a 40-50 mm thickness of insulation which is an unlikely modification to the structure, considering the high cost, added dead load to the structure, and also the possibility of spalling of the insulation [4,10,22].

The three base materials, CFRP, epoxy and concrete, a combination of which would provide better strength and service performance in a CFRP/Concrete composite system.  CFRP material itself is inherent to fire even up to 500 ºC to 800 ºC [23,24]. However, the concrete substrate itself is sensitive to fire and spalling of concrete may occur at temperatures beyond 300 ºC depending on the concrete grade [25,26]. Nevertheless, the epoxy adhesive is more susceptible in an event of a fire and the composite would eventually lose its load transferability as the Tg is between 70 ºC and 90 ºC in construction adhesive [8,9,27]. Therefore, if the bond line temperature can be maintained below the Tg of the epoxy, all three elements will successfully contribute to share the loads up to a certain time in the fire. Hence, it is important to make a balance between the bond performance, the type of insulation, and the quality to have an economical and safe construction project. Accordingly, the main objective of this research was to provide the most suitable bond arrangements which can preserve the bond and material from excessive temperature during the fire. This study provides guideline charts to select a proper application technique and material to ensure fire performance while improving structural integrity and safety in economical way.

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