Effect of the Cyclic Crack Opening-Closure during Epoxy-Curing Period of a CFRP Strengthening System Bonded on Concrete Substrate

This article investigates the potential detrimental effects of cyclic load during the installation of externally bonded (EB) carbon fiber-reinforced polymer (CFRP) on a damaged reinforced concrete (RC) structure. Four RC specimens were tested in three point bending to study the consequences of crack cyclic opening-closure during epoxy-curing period. A first RC specimen (without bonded CFRP) was loaded monotonically up to failure to serve as undamaged control sample. The three other specimens were pre-cracked before being subjected to a fatigue loading procedure to simulate service condition of a damaged RC structure. Two of the three pre-cracked specimens were strengthened by EB CFRP. One specimen was repaired before the fatigue test while the other one was repaired during the fatigue test. Finally, remaining capacities of all three pre-cracked specimens were measured through monotonic bending tests until failure. It was found that, although bonding of CFRP reinforcement during cyclic load can induce some interesting features with regard to serviceability, cyclic crack opening and closing alters the cure process of epoxy located below the initial crack and decreases the effectiveness of the strengthening at ultimate state. Extended experimental studies are then needed to assess reliable safety factor for the design of repairing operations in which the bridge has to be maintained in service during CFRP installation.

Experience Study on Long-Life Microsurfacing with High Water Resistance Performance

Microsurfacing is a standard preventive maintenance technology developed on the basis of slurry sealing technology. However, the high temperature and rainy season in Guangdong Province affect its expanded application because of its low water resistance and short service life. So, high-performance microsurfacing, a new microsurfacing technology, has been developed. The key to this technique is an appropriate proportion of water-based epoxy resin and waterborne epoxy curing agent, which could generate a chemical reaction to form a high-performance bonding network structure of space. And indoor wet-wheel wear test shows that its antiwear ability and resistance to water damage are evidently increased (to over 50%) compared with the conventional microsurfacing. Furthermore, from the long-term road performance results, the antisliding and water resistance performance of high-performance microsurfacing is much higher than the conventional technique.

MODELING AND SIMULATION OF THE CURING PROCESS OF EPOXY RESINS AND FIBER COMPOSITES

This study discusses simulations of the curing process in epoxy and fiberreinforced polymer composites incorporating changes in the thermal and mechanical properties of epoxy during curing at various temperatures. A coupled constitutive model that includes an exothermic process from the cross-linking, heat conduction across the specimen and deformations of the specimen from the thermal expansion and shrinkage effects is formulated. The model is used to capture the curing process in the epoxy resin. The coupled constitutive model is then integrated into a micromechanics model of fiber-reinforced composites and used to study the influence of epoxy curing on the formation of residual stresses in the composites. Furthermore, the micromechanics model is also used to predict the macroscopic properties, i.e., elastic moduli, of the cured composites. The model can then be used to understand the influence of processing parameters, i.e., temperatures and pressure, on the formation of residual stresses and their consequences on the overall properties of cured composites.

A Dyciandiamine-Based Methacrylate-Epoxy Dual-Cure Blend-System for Stereolithography

In this research, an epoxy-based dual-cure system is developed and characterized for SLA additive manufacturing. Dual-cure systems consist of UV-curable acrylates and thermal active components. The second curing step offers an additional degree of freedom to design specific material properties. In this study, a blend of varying concentrations of an epoxy/curing agent mix, respectively, DGEBA, DICY and photocurable methacrylate, was used to create a material that is printable in the SLA process into a UV-cured or green part and subsequently thermally cured to achieve superior thermal and mechanical properties. Calorimetric measurements were performed to determine the reactivity of the thermal reaction at different concentrations of epoxy. The fully cured specimens were tested in mechanical and dynamic mechanical measurements, and the results showed a significant improvement in tensile stress and glass transition temperature with rising epoxy concentrations. Fractured surfaces from tensile testing were investigated to further characterize the failure of tested samples, and thermal degradation was determined in TGA measurements, which showed no significant changes with an increasing epoxy concentration.

Embedded Resistance Wire Technique for Epoxy Curing and Self-Healing of PET Thermoplastics

The aim of the present study is to apply an embedded resistance wire technique for curing of thermosetting resins as well as for the self-healing of thermoplastic polymers. The work consists of two parts. In the first part, Kanthal resistance wires embedded in a resin plate acted as heating elements when direct electrical current was flowing through them (Joule heating). During heating, the temperature was continuously monitored using a thermal camera, and accurate temperature times for fixed position diagrams as well as temperature positions for fixed time diagrams were calculated. The effects of curing with this method were evaluated by studying the three-point bending mechanical behavior of the cured resin, comparing it with the corresponding behavior of the same resin when cured using a conventional oven curing method at the same temperature. In the second part of the present work, the possibility of using the same technique for healing existing notches and flaws in a PET thermoplastic is explored. We examined whether providing energy through the resistance wires created the right amount of heat to heal the thermoplastic, or, more specifically, whether it closed the notches and eliminated the abrasions that were artificially created on the specimens. The technique using embedded resistance wires worked equally well, with interesting and promising preliminary results regarding the curing of thermoset resins and the healing of thermoplastics.

Novel Cycloaliphatic Cashew Nut Shell Liquid Derivatives and their use as Isocyanate Protective Groups

Cardanol (3-pentadecenyl-phenol), a well-known non-edible natural oil obtained as a by-product of the Cashew Industry, represents a valid alternative to petro-based derivatives, thanks to its peculiar chemical structure. When selected as polymer building block in the synthesis of epoxy curing agents or polyols and diols for polyurethane applications, cardanol can impart unique benefits, like chemical resistance, hydrolytic stability, thermal resistance and balanced mechanical properties. Once used in polyurethane prepolymers, it demonstrates various benefits, including favorable and easily tunable deblocking conditions, lower viscosity of resulting NCO-blocked prepolymers and excellent storage stability in comparison to commonly used phenolic compounds (phenol, nonylphenol, in particular). In this paper, novel cardanol-based fully cycloaliphatic derivatives (e.g. cyclohexanol, oxime, lactam-types) will be presented, investigating their use as innovative isocyanate protective groups with faster reactivity and lower deblocking temperatures than petro-derived benchmarks.

Interfacial Binding Energy between Calcium-Silicate-Hydrates and Epoxy Resin: A Molecular Dynamics Study

Microcapsules encapsulated within epoxy as a curing agent have been successfully applied in self-healing materials, in which the healing performance significantly depends on the binding behaviour of the epoxy curing agent with the cement matrix. In this paper, the binding energy was investigated by molecular dynamics simulation, which could overcome the shortcomings of traditional microscopic experimental methods. In addition to the construction of different molecular models of epoxy, curing agents, and dilutants, seven models were established to investigate the effects of chain length, curing agent, and epoxy resin chain direction on the interfacial binding energy. The results showed that an increase of chain length exhibited had limited effect on the binding energy, while the curing agent and the direction of the epoxy significantly affected the interfacial binding energy. Among different factors, the curing agent tetrethylenepentamine exhibited the highest value of interfacial binding energy by an increment of 31.03 kcal/mol, indicating a better binding ability of the microcapsule core and the cement matrix. This study provides a microscopic insight into the interface behaviour between the microcapsule core and the cement matrix.