Effect of elevated temperatures on concrete strengthened with externally bonded carbon fiber reinforced polymer and various types of thermal insulation

Effect of elevated temperatures on concrete strengthened with externally bonded carbon fiber reinforced polymer and various types of thermal insulation

 Fábio Sérgio da Costa Pereira ^a, José Daniel Diniz Melo ^b, Ana Paula Cysne Barbosa ^b*, Maria Carolina Burgos Costa ^b

 ^a Department of Civil Engineering, University Center of Rio Grande do Norte-UNI-RN, Natal - RN, BRAZIL 59014-545

^b Department of Materials Engineering, Federal University of Rio Grande do Norte, Natal - RN, BRAZIL 59078-970

^* Corresponding author. Tel.: +55 (84) 8849-6326

E-mail address: anacysne@yahoo.com

 ABSTRACT

An efficient approach to repair and strengthen concrete structures that have been diagnosed with building pathologies is the use of externally bonded fiber-reinforced polymer composites.  However, a major concern regarding the application of this technique is the loss of mechanical reinforcement in case of fire.  This research aims to investigate the resistance of concrete strengthened with externally bonded carbon fiber reinforced polymer composite and thermally insulated subjected to high temperatures.  Four types of thermal insulation were evaluated: intumescent paint, vermiculite shotcrete, shotcrete with nodulated mineral fibers and ceramic fiber blanket insulation.  Specimens were exposed to high temperatures in an oven for 2 h: 400°C for specimens without thermal insulation and with intumescent paint and 1,200°C for the other types of thermal insulation.  All specimens were then tested under compression load.  According to the experimental results, specimens with thermal insulation of vermiculite shotcrete, shotcrete with nodulated mineral fibers and ceramic fiber blanket insulation were able to maintain their compressive strength after being exposed to the elevated temperature.  Intumescent paint was able to provide only partial thermal protection and the compressive strength was reduced by about 20% after the exposure to high temperature.

 Keywords: Repair, fire, concrete, carbon fiber composite, strengthening, high temperature resistance

 1. Introduction

In recent years, the construction industry has shown significant interest in the use of fiber-reinforced polymer (FRP) materials for reinforcement and strengthening of concrete structures. This can be attributed to the numerous advantages, including extremely high strength to weight ratios, versatility, and resistance to electrochemical corrosion that FRPs offer over conventional materials such as steel. However, since FRP materials are combustible and susceptible to deterioration of mechanical and bond properties at elevated temperatures, and because they are typically applied to the exterior of structural members in these strengthening applications, concerns exist regarding the behavior of such FRP strengthening systems in case of fire [[1]]. Thus, the performance of these materials during fire, and the ability of FRP-strengthened members to meet the fire endurance criteria established in building codes, must be evaluated they can be used with confidence in buildings as reinforcement and repair. Currently, information in this area is still very limited [[2],[3],[4],[5]].

Structural fire safety is a major concern in the design of buildings, and the provision of fire resistance is a requirement for structural materials and systems [[6]]. Appropriate fire resistance of structural members ensures that, when measures for preventing, extinguishing, or containing a fire fail, structural integrity is the last line of defense for building occupants and emergency personnel.

Fire-safety concerns associated with the use of FRPs as externally bonded reinforcement for concrete structural members in buildings include the potential for increased flame spread and smoke generation, loss of FRP strength and stiffness, and loss of bond between the concrete and the FRP [6,[7]]. FRP properties are known to be sensitive to elevated temperatures [7,[8],[9],[10]]. Deterioration of mechanical and/or bond properties can be expected at temperatures approaching the glass transition temperature (Tg) of the polymer matrix/adhesive [7, 8, 9, 10], which is typically less than 100^oC. Thus, there are concerns that loss of structural effectiveness of FRP reinforcements during fire could lead to sudden collapse of FRP-strengthened structures under service loads. Some authorssuggested the use of vermiculite cement based mortar thermal insulation to extend the fire endurance of CFRP strengthening systems [[11]].

In a previous study published in the literature, results of full-scale fire resistance experiments on three insulated FRP-strengthened reinforced concrete (RC) columns were presented [1]. A comparison was made between the fire performances of FRP-strengthened RC columns and conventional unstrengthened RC columns. The experimental part consisted of fire endurance tests on five RC columns: one unstrengthened circular RC column, two circular FRP-wrapped and insulated RC columns, one unstrengthened square RC column, and one square FRP wrapped and insulated RC column. Unlike conventional square RC columns, FRP-strengthened square RC columns require suitable fire protection, in most cases, to achieve the required fire endurance ratings under increased (strengthened) service loads. The performance of fire-protected FRP-strengthened square RC columns at high temperatures was shown to be similar to, or better than, that of conventional RC columns. Clearly, the superior fire resistance of the strengthened columns, as compared to the conventional column, can be attributed to the presence of the fire protection system. Thus, fire resistance requirements can be met for FRP-wrapped concrete columns through the incorporation of proper fire protection measures into the overall FRP-strengthened structural system [1].

Other authors conducted experimental and numerical investigations on the fire behavior of reinforced concrete (RC) beams strengthened with carbon fiber reinforced polymer (CFRP) laminates [[12]]. In this case, the main objective was to assess the efficacy of different fire protection systems and to evaluate the viability of their use in buildings floors. Fire resistance tests were conducted using an oven to investigate the behavior under fire of loaded CFRP-strengthened RC beams (ISO 834). The fire protection systems comprised calcium silicate boards and layers of vermiculite/perlite cement based mortar, with thicknesses of 25 mm and 40 mm, respectively, applied along the bottom soffit of the beams that was directly exposed to fire. In addition, the anchorage zones of the CFRP laminates were thermally insulated in order to evaluate the benefits of this particular constructive detail. Mid-span deflections and temperatures were measured and recorded during the tests. The fire resistance test of the unprotected beam confirmed the susceptibility of externally bonded CFRP strengthening systems when exposed to high temperatures; although the anchorage zones of the laminate were thermally insulated over a length of 0.20 m, the CFRP laminate debonded after only 23 min of fire exposure. This result points out to the need of insulating the strengthening system not only over the anchorage zones but also along the beam span. Both protection materials, used together with the thermal insulation of the anchorage zones, allowed the CFRP strengthening system to be effective during a considerably longer period of fire exposure. Such extension of fire endurance, which increased for thicker protections, was obtained due to a considerable temperature reduction, particularly at the concrete-CFRP interface.

Due to the well-known degradation of FRP materials at high temperatures, guidelines for design of FRP strengthened structures [[13], [14]] specify that the interaction between the concrete member and the FRP strengthening should be ignored unless a fire-protection system able to maintain the FRP temperature below its critical temperature (defined as the lowest Tg of its components) is used. Therefore, the load carrying capacity of FRP strengthened structural members under fire exposure is influenced by thermo-mechanical properties of the polymer matrix and adhesive.

This work aims to evaluate the behavior of concrete strengthened with externally bonded carbon fiber reinforced polymer composite and thermally insulated subjected to high temperatures. The effect of passive protection of intumescent paint, vermiculite shotcrete, shotcrete with nodulated mineral fibers and ceramic fiber blanket insulation were evaluated. The capability of the FRP strengthened structural members to maintain the load carrying capacity under high temperatures is assessed.

2. Experimental

 2.1. Materials and specimen preparation

The cement employed was CP II-Z-32-RS from Nassau Company.  Cement, sand and gravel were mixed using a ratio of 1:2:4 by volume, with a water to cement ratio of 0.7. Samples for compression tests were molded cylinders with dimensions of 5 cm x 10 cm (diameter x length). All samples were kept submerged under water for the first 7 days of curing.

After 28 days of curing, carbon/epoxy reinforcement was applied by hand lay-up according to the following sequence: application of epoxy primer (Viapol Carbon Primer), one layer of structural epoxy (Viapol Carbon Saturante), application of carbon fiber cloth (Viapol Carbon CFW 300) and application of a second layer of structural epoxy (Viapol Carbon Saturante). Following the carbon/epoxy composite reinforcement, the thermal insulation was applied.

Four types of passive thermal insulation materials were evaluated: intumescent paint, vermiculite shotcrete, shotcrete with nodulated mineral fibers and ceramic fiber blanket insulation.  Five specimens were manufactured for each type of thermal insulation.  For specimens protected with intumescent paint, two layers of Sikaunitherm from Sika Company were applied. This is an acrylate-based paint containing xylene, naphtha and ethylbenzene. The vermiculite shotcrete employed was composed of expanded vermiculite, Portland cement and quartz sand and was applied on the samples with a shotcrete machine in a dry process, where a 30 mm thick layer was produced. The expanded vermiculite was from Refratil Refratarios Company. It is an industrialized product composed of aluminum and magnesium hydrated silicates. The shotcrete with nodulated mineral fibers was composed of nodulated mineral fibers (T_m = 1482 °C), Portland cement and quartz sand. A 30 mm thick layer was applied by using a shotcrete machine. The employed ceramic fiber blanket is 25.4 mm thick, 7.6 cm long and 60 cm wide and is made of entangled flexible fibers composed of high purity alumina, zirconia and silica. It was applied on a layer of cement, which was applied on the concrete, providing thermal insulation to the material.

Forty concrete specimens were produced for the compression tests and thirty of them were strengthened with externally bonded carbon fiber reinforced polymer composite. Ten of the strengthened specimens were tested without any passive protection, while the other twenty received one the four types of passive protection investigated. Five of the non-strengthened specimens and five of the strengthened specimens without passive protection were tested at room temperature.  All other specimens were tested after exposure to high temperature.

2.2. Testing procedure

Seven days after the specimens were strengthened with externally bonded CFRP (35 days after molding), 30 specimens were exposed to high temperature in an oven for 2 h: 400°C for specimens without thermal insulation (C and CR) and with intumescent paint (CRIP) and 1,200°C for the other types of thermal insulation (CRV, CRS and CRB). Heating up and cooling down rates employed in both cases was 100 °C/min. The temperature of 1,200ºC is regarded as the common temperature reached at the seat of fire and 2 h was the time duration defined for the fire resistance [[i]]. Such testing conditions were selected in order to allow a comparison of each type of passive protection used and the reference unprotected samples.

After exposure to high temperature, samples were capped and tested for compression strength. Compression tests were carried out after 37 days from molding. The tests were performed using a hydraulic press type Shimadzu AG-I.

3. Results and Discussion

Compression resistance of the non-reinforced concrete was about 20 MPa. After being exposed to 400°C for 2 h, the samples lost all resistance to compression. For the samples strengthened with externally bonded carbon fiber reinforced polymer composite and without thermal insulation (CR), the epoxy resin was degraded at 400ºC and only the carbon fiber was left (Figure 3 A). Compression strength of these samples was therefore significantly reduced down to less than 50% of the original value. Samples protected with intumescent paint (samples CRIP) showed as well a reduction in strength, but the reduction was less significant (about 20% of the original value). Although these samples showed significant degradation of the epoxy resin (Figure 3 B), the thermal insulation provided by the intumescent paint probably protected at least partially the polymer composite reinforcement, so that strength reduction was less pronounced as compared to the samples without any insulation.

On the other hand, samples insulated with vermiculite shotcrete (samples CRV), shotcrete with nodulated mineral fibers (samples CRS) and ceramic fiber blanket insulation (samples CRB) showed no reduction in compressive strength. For these samples, the polymer composite reinforcement was probably not affected by the high temperature, which indicates the thermal protection of the insulations. In this case, structural integrity of these materials would be assured for the period of time required.

Some of the drawbacks of employing shotcrete passive protections are the increase in cross-section (about 3 cm), increase in applied load for the insulated structure, and the need for special equipment for their application. On the other hand, the ceramic fiber blanket are very light, easier to apply and do not require any additional equipment for its application.

 4. Conclusions

Concrete structures strengthened with externally bonded carbon fiber reinforced polymer composite without thermal insulation and thermally insulated with intumescent paint showed significant vulnerability to high temperatures. The polymer matrix of the composite reinforcement was mostly degraded at high temperatures and, as a consequence, the compressive strength of the samples was reduced significantly. Concrete structures strengthened with the carbon fiber polymer composite and insulated with vermiculite shotcrete, shotcrete with nodulated mineral fibers and ceramic fiber blanket insulation showed no reduction in compressive strength after 2 h at 1,200°C, which indicates that these passive materials are efficient in providing high temperature protection to the CFRP strengthened concrete. In this case, structural integrity of these materials would be assured for the period of time required in case of exposure to high temperature. The results presented in this work are subjected to the type of concrete employed, the type of carbon fiber composite, the type of passive protections evaluated and sample geometry. Thus, investigations of full-scale samples in a fire simulation chamber are suggested. 

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