Effects of applied environmental conditions on the pull-out strengths of CFRP-concrete bond
Introduction
Fibre Reinforced Polymer (FRP) composites possesses advantageous properties such as high strength to weight ratio, high corrosion resistance and easy application process. These properties have made it a popular choice for rehabilitation of reinforced concrete structures lately. Due to debonding being a premature mode of failure of rehabilitated RC structures, extensive research on the FRP-concrete bond system under short term loads can be found in literature. However, limited studies can be noticed on the long term performance of FRP-concrete bond subjected to environmental conditions.
Different approaches of research can be observed where some research dealt with durability of concrete beams strengthened with FRP, aiming at observations related to change in ultimate beam strength and stiffness after various environmental exposures. Other research studied the changing nature of bond strength under aggressive environment. Chajes et al. [1], Toutanji and Gómez [2], Myers et al. [3] and Li et al. [4] exposed FRP strengthened concrete beams to various environmental conditions such as freeze-thaw cycles, wet-dry cycles, combined environmental cycles, boiling water and UV radiation and studied the degradation of ultimate strength and stiffness of beams. Homam et al. [5], Dai et al. [6], Benzarti et al. [7] and Yun and Wu [8] investigated FRP-concrete bond degradation under freeze-thaw cycles, temperature cycles, alkali solutions, moisture ingression, hydrothermal ageing. They used various test set-ups such as pull-off, bend tests and single-lap-joint shear tests for their investigation. Tuakta and Büyüköztürk [9] applied peel and shear fracture tests to investigate the effect of moisture on FRP-concrete bond system by tri-layer fracture mechanics. As a variety of test methods and environmental conditions were applied in the research, comparison of the findings of these research studies is difficult. In regard to the suitable test set-up, Benzarti et al. [7] showed more sensitivity of single shear test (pull-out test) to environmental conditions in terms of changes in failure modes and bond strength, and suggested to use the set-up for the study of adhesive bonded joint. In addition, this test set-up simulates the intermediate flexural crack-induced debonding which is an important failure mode [10] of RC beams strengthened with FRP for flexure. Therefore, more research with similar test set-ups can be conducive to create a large database of FRP-concrete bond behaviour under various environmental conditions.
Studies by Litherland et al. [11], Phani and Bose [12], Dejke and Tepfers [13] and Chen et al. [14] mainly focused on durability of FRP and FRP in simulated concrete environment and proposed long term prediction models based on the acceleration of degradation rate applying high temperature. Regarding the use of high temperature as an accelerating factor, Robert et al. [15] stated that high temperature may cause amplification of the reduction of properties, leading to conservative prediction of long-term properties. The conservativeness involved in the application of very high temperature for acceleration requires further investigation separating high temperature from an intended degradation mechanism.
The lack in research findings of natural ageing of FRP-concrete bond is another aspect in the available literature. In the research by Nishizaki and Kato [16] the durability of CFRP-concrete bond exposed to outdoor environment of Tsukuba Japan (moderate climate) was studied for 14 years since 1992 by means of pull-off and peel tests. Although insignificant reduction of pull-off strength was observed after 14 years of exposure, the peel test showed much reduction of strength due to natural ageing. However, the results of peel tests were not conclusive since the unexposed specimens of this series were fabricated much later (in 2006) and not from exactly the same materials used in 1992. Al-Tamimi et al. [17] investigated the effect of dry exposure to sun as well as saline water coupled with sun exposure on CFRP bonded concrete specimens for more than 150 days. In addition, they applied sustained loads of 15% and 25% for both conditions. Summer environment of United Arab Emirates (UAE) was chosen for outdoor exposure (temperature stays within the range between 38 and 55 °C at least for three months). Single shear tests revealed that aggressive environment increased the bond strengths and the reason was attributed to greater polymer cross-linking due to elevated temperature. The interesting findings of the available two studies clearly impose the need for further research on natural ageing of FRP-concrete bond in different climate zones.
The purpose of this research was to investigate effects of three separate exposure conditions, namely, temperature cycles, wet-dry cycles and outdoor environment on FRP-concrete bond (both CFRP and GFRP) using single shear test (pull-out test) for up to 18 months. In addition, the effect of environmental conditions on the material properties of CFRP and concrete was another objective of this investigation [18].This paper only presents the experimental results of CFRP bonded specimens. The characterisation of concrete compressive strength under same environmental conditions is also discussed to understand the effect of material properties on the pull-out strength of CFRP-concrete bond.
Section snippets
Experimental program
Long term performance of CFRP-concrete bond was studied by exposing CFRP-concrete bond specimens to three different environmental conditions (temperature cycles, wet-dry cycles and outdoor environment) for durations up to 18 months and testing them using single shear test (referred to as pull-out test herein). In addition to the FRP-concrete bond specimens, FRP coupons and concrete cylinders were also exposed to the same environmental conditions and the material properties of FRP and concrete
Test results and discussions
The behaviour of CFRP-concrete bond for control and exposed series are discussed by means of observations from pull-out strengths, failure modes and strain profiles.
Conclusions
The experimental investigation on the CFRP-concrete bond exposed to cyclic temperature, wet-dry cycles and outdoor environment revealed some interesting findings based on the analysis of pull-out strength, failure modes and strain profiles. The research findings on the effects of temperature cycles, wet-dry cycles and outdoor environment are as follows:
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Temperature cycles:
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Temperature cycles caused negligible deterioration (by 1% after three months) of CFRP-concrete bond provided the maximum
Acknowledgments
This paper acknowledges the financial and technical support provided by Centre for Built Infrastructure Research (CBIR) at University of Technology, Sydney for conducting the research.
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