Effects of hygrothermal conditioning on epoxy adhesives used in FRP composites
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Epoxy adhesive
Vinyl ester
The curing speed of a wood adhesive directly determines the required clamping time. Therefore, the curing time has a considerable influence on the production time and consequently on the efficiency / productivity of wood gluing factories. Conventionally, the curing speed is laboriously determined via lap shear tests according to EN 302-6 and EN 15416-5 for phenolic and aminoplastic and one-component polyurethane (1K-PU) adhesives, respectively. Due to moisture exchange between adhesive systems and wood adherends, only plate rheometers that are equipped with wooden plates offer the possibility to monitor the visco-dynamic curing process of a wooden bond realistically. This paper compares the results of rheological measurements that were performed during the curing of four different adhesive types with shear strength results. Epoxy and – with limitations – moisture curing 1KPU adhesives corresponded well with the theoretical curing behaviour, with the occurrence of a gel point at the crossing of the loss modulus G’’ and storage modulus G’. In contrast, Polycondensation and EPI adhesives displayed a completely different curing behaviour, most notably that G’ was larger than G’’ throughout the entire curing time. The rheological curing data showed a good correlation with regard to the shear strengths for the Epoxy, 1K-PU and MUF adhesives but not for the EPI adhesive. The difference in behaviour of the EPI adhesive could potentially be a result of its special chemistry.
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The essence of common concrete durability design is to determine the w/c ratio and the minmal,cement consumption that can meet the demand of concrete durability,This paper adopts the formula method to directly design the mixture ration of concrete by analyzing durability while considering both strength and durability.Based on the deduction of the mathematical relationships of w/c ratio,unit cement consumption and concrete strength.f cu ,the formula of w/c ratio is established that meets durability index.Then ,comparing w/c ratio that meets durability with w/c ratio that meets strength,the lower one is chosen to calculate unit cement dosage by formula(4).In this paper concrete durability is divided into frost resistance durability,carbonization durability,acid radical diffusion durability and grinding durability.
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Isothermal process
Epoxy adhesive
Vitrification
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Alfa Laval has been using its unique adhesive mixture for over 50 years for bonding of rubber gaskets to the corrugated plates used in their heat exchangers. The adhesive mix is designated GC6 and is composed out of an epoxy component and a phenolic component. GC6 has throughout the years shown exceptionally good performance in both adhesion to the rubber gasket and to the plate with minimal pre-treatments of the surfaces required. The manufacturer of the phenolic component recently announced that they would discontinue this product. Thus, a replacement with a similar composition was identified. This new component showed to have a faster cure rate which as a result meant that the same curing temperature and time could be used as for the old component. However, data on the minimum curing conditions with respect to time and temperature to obtain a well performing adhesive bond had not yet been established. Thus, the aim of this project was to investigate this. The adhesive showed sufficiently good performance when samples were cured to a residual enthalpy of <2.3J/g, both in initial bond strength and chemical resistance. This required a curing of at least 1h at 120°C and showed no decrease in bond strength after ageing for one week in a humidity chamber. For curing temperatures under 110 °C, in addition to reduced performance also a separation of the individual components. These findings suggest that the current minimum curing recommendations of 120°C for 3h are well above the findings in this report. This also opens the possibility for Alfa Laval to optimize their curing conditions to lower the energy consumption and environmental impact of each cycle. (Less)
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Gasket
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The purpose of this study is to develop a new technique for strengthening and repairing existing concrete structures with sprayed fiber-reinforced polymers (FRP) by mixing chopped carbon or glass fibers with epoxy or vinyl ester resins in open air and randomly spraying the mixture onto the concrete surface with compressed air. The use of sprayed FRP for repair and strengthening purposes using epoxy or vinyl ester resins has never been fully investigated. In this study, tensile testing was conducted on material specimens to determine the optimum length of chopped carbon or glass fibers and the mixture ratio of fiber, epoxy, and vinyl ester resin for sprayed FRP. These variables were adjusted to produce a material strength equivalent to that of one FRP sheet. The optimal length of glass and carbon chopped fibers was determined to be 38 mm, and the optimal mixture ratio of chopped fiber to resin was found to be 1:2. The thickness of sprayed FRP required to provide the same strengthening effect as one FRP sheet was also calculated. During this study, experiments were conducted to evaluate the strengthening/repair effects of the sprayed FRP on flexural beams, shear beams, and damaged beams. The results showed that the strengthening effect of sprayed FRP on the flexural and shear specimens was similar to those of one FRP sheet. The maximum strength of the damaged beams reinforced by sprayed FRP was approximately the same as that of the reinforced flexural and shear beams. Moreover, existing design equations for FRP sheets were found to be applicable to flexural beams reinforced with sprayed FRP. The shear beam specimens could be safely designed using the coefficient of shear strength reduction α= 0.18, determined to result in computed values that most closely approximate the experimental values. Overall, the sprayed FRP technique was found to be suitable for strengthening existing reinforced concrete buildings.
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Shear Strength
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The curing reaction of Epoxy Vinyl Ester Resin was studied by using of DSC.Effects of rate of temperature increasing on the characteristics of DSC curves were investigated.The curing reaction equation of the Epoxy Vinyl Ester Resin was ascertained.Gelling temperature,curing temperature and post-treating temperature were obtained by extrapolating T—βcurves.Based on the theoretical curing temperature,optimum cure processing parameters were attained by means of experiments.
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Structural cold-curing adhesives are widely used to strengthen Reinforced Concrete (RC) structures with Fibre Reinforced Polymers (FRPs). The performance of these adhesives, and therefore the performance of the strengthening system, may be affected by temperature, as ambient-cured structural adhesives usually have low glass transition temperature (Tg). This paper presents a comprehensive experimental investigation on the influence of temperature on mechanical properties and Tg of a structural epoxy adhesive. The experimental program was divided in four groups of specimens. In Group 1, the effect of curing and post-curing temperature on Tg of the epoxy adhesive was investigated. In Groups 2–4, the effects of testing temperature, curing temperature and post-curing temperature, respectively, on adhesive mechanical properties were studied. Experimental results confirm that curing and post-curing temperature affected Tg differently depending whether the applied temperature was below or beyond the epoxy Tg. Similar behavior was observed in the mechanical properties of the epoxy, as they showed improvements when curing process (curing and post-curing) temperature was below Tg and they were negatively affected when curing process temperature was beyond Tg. Besides, tensile and compressive mechanical properties were negatively affected by testing temperatures beyond 20 °C.
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Externally bonded fiber-reinforced polymer (FRP) reinforcement offers an attractive method for the strengthening of structures constructed of various materials. In such strengthened structures, the characteristics of FRP-to-parent material bonded joints play an important role. While extensive research has been carried out on the characteristics of FRP-to-concrete bonded joints, existing work on FRP-to-steel bonded joints is much more limited. In an FRP-to-steel bonded joint, the weak link is the epoxy adhesive, while in an FRP-to-concrete bonded joint, the concrete is the weak link. This paper examines, through a series of pull-off tests in which the FRP-to-steel interface is subjected to direct shear, the parameters that affect the behaviour of FRP-to-steel bonded joints. The test results are first presented and discussed. Based on these test results, the bond-slip relationship relating the interfacial shear stress to the interfacial slip is then investigated, leading to the development of the first ever bond-slip model for FRP-to-steel interfaces.
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Some problems of durability have been existing in concrete. This paper analyses factors affecting durability of concrete, summarizes the corresponding technical measures of impr oving durability of concrete, and points out the necessity and feas ibility of using High-Performance Concrete of excellent durability.
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