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Development of modified adhesives for bond between CFRP and concrete subjected to harsh environmental conditions
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posted on 09.02.2017by Al-Safy, Rawaa
The attractive properties of carbon fibre reinforced polymers (CFRPs) have made such composites effective materials to use for the rehabilitation and strengthening of existing concrete structures. The wet-lay up method is the most commonly used installation technique for CFRP applications due to its many advantages, including flexibility in coping with different geometries. CF fabrics are adhesively saturated and glued to the concrete structure. The adhesive in the strengthening system transfers the stresses from the concrete member to the CFRP. Two-part thermosetting-based adhesives are commonly used for bonding CFRP to concrete elements and such adhesives are designed for ambient temperature curing. The performance of CFRP/concrete systems is affected by the operating temperature, and the adhesive is the element in the strengthened system most sensitive to the temperature effect, since the majority of bonding adhesives are of low glass transition temperature (Tg) materials. As this temperature is approached or exceeded, the integrity of the interfacial bond in the CFRP/concrete system diminishes rapidly due to deterioration in the adhesive properties caused by the loss of the ability to transfer stresses.
Improvements in adhesive properties have been documented in the research literature in the aerospace and materials fields using many methods. These improvements are based on the fact that the modulus and Tg of polymers can be improved by increasing the cross-links (chemical bonds between the epoxy chains) in the polymer network, reducing the mobility of the polymer molecule segments, reducing the amount of un-reacted epoxy resin, and using nanomaterials as reinforcement. The incorporation of such modifiers to improve the properties of the adhesive used for infrastructure applications, namely for externally-bonded CF fabric to concrete members, needed to be investigated. In addition, the effect of applying such modified adhesives on the interfacial bond efficiency of CFRP/concrete system needed to be explored.
The main focus of the work reported in the present study is the improvement of the thermo-mechanical properties of the adhesive used to bond CF fabric to concrete members. Reinforcement with two different types of nanomaterials was used to modify a commercially-available epoxy adhesive commonly used for externally bonding CF fabric to concrete substrates. Nanoclay (NC) and carbon nanofibres (VGCF) were chosen to modify the epoxy adhesive used in civil engineering applications because of their unique properties, including low cost, ready availability and high aspect ratio. Other types of modification were carried out by the use of highly cross-linked epoxy-based resin (DGOA) to replace partially and completely the monomer of the commercially-available epoxy adhesive for CFRP applications. Post-curing at high temperature and at moderately elevated temperatures was adopted to study the effect of such techniques on the adhesive properties, as well as on the interfacial bond of CFRP/concrete systems under different environmental conditions. The practicality of post-curing techniques was also studied by exploring the application of such techniques after different ambient curing periods.
The influence of modification on pure adhesive properties was studied. The glass transition temperature of the epoxy adhesive with and without modification, measured by the Dynamic Mechanical Thermal Analysis (DMTA) and Differential Scanning Calorimetry (DSC) techniques, was addressed, as well as storage modulus using DMTA. Tensile properties at various temperature levels were evaluated by testing adhesive coupons at these temperatures. Characterisation of the structure of the adhesive with and without modification was investigated by different techniques including X-Ray Diffraction (XRD), Transmission Electron Microscopy (TEM), Differential Scanning Calorimetry (DSC) and Near-Infrared spectroscopy (NIR).
The efficiency of the bond in the fabricated CFRP/concrete specimens using modified adhesives was explored by means of adhesion (pull-off) tests and single–lap shear tests at elevated temperatures and the failure loads and modes were analyzed. The integrity of the interfacial bond in loaded CFRP/concrete systems using modified adhesives was explored under harsh environmental conditions based on the combined effects of temperature and humidity. The practicality of post-curing applications was investigated by studying the properties of the adhesives and the bond performance of CFRP/concrete systems by post-curing at high temperatures and moderately elevated temperatures after different curing periods at ambient conditions.
It was found that the chemical structure of the epoxy resin is an important parameter in the enhancement of the adhesive properties (Tg , modulus and tensile properties), hence the interfacial bond in CFRP/concrete systems. Nanomaterials were also found to enhance the modulus of the epoxy adhesive. Adequate bond performance in CFRP/concrete systems under different service conditions was achieved, even in the absence of the primer layer when a highly cross-linked epoxy adhesive was used for bonding. The application of post-curing at moderately elevated temperatures is recommended to improve the integrity of CFRP/concrete systems under severe environmental conditions.
The outcomes of the experimental program provide a new understanding of the thermal behaviour of the adhesives used in civil engineering applications and allow recommendations for further applications at different environmental temperatures. In addition, the experimental work on the bond-slip relationships using post-curing techniques at moderately elevated temperatures provide guidelines for numerical simulations to be investigated in future studies of bond performance in the CFRP/concrete system.