Papers by Keyword: Curing

Abstract: Thermoset adhesives convert from liquid to solid due to chemical reactions. Once cured, these adhesives carry the potential to create strong load-bearing joints, resisting even severe detrimental service conditions. In the progress of curing of a thermoset adhesive the viscoelastic properties of the resin and hardener formulation change as the chemical reaction proceeds. Gelation occurs once a continuous 3-dimensional network of polymer chains has been created. After gelation, the microstructure of the resin is fixed and further cure is affected by diffusion limitations [1]. Mastering of the curing kinetics and the physicochemical changes in the transition from the liquid to the solid-state is essential to reliably process adhesives in industrial applications. Rheological experiments in parallel plate configuration have become a well-established practice in investigating viscoelastic properties in the progress of curing. In practice, it has shown to be challenging to access the full range of viscoelastic parameters of thermoset resins with a low initial viscosity from the very beginning of the curing reaction to the post-cure consolidation of the vitrified polymer. This paper will discuss experimental methods and criteria for the viscoelastic analysis of curing thermoset adhesives and present experimental data of the time-, temperature-, and frequency-dependent viscoelastic properties of a curing thermoset epoxy in relation to the features of its time-temperature-transformation-diagram.

Abstract: The process of producing a monolithic concrete structure on site is constructed out under different climatic conditions, which can often be unsuitable for setting and hardening of concrete. The necessary conditions for setting and hardening of concrete are ensured by various ways of its curing. In practice, concrete curing is carried out in most cases by water spraying. It is used mostly in reinforced concrete ceiling slabs, which are further discussed in the work. A common procedure is to cure the upper surface of reinforced concrete ceiling slabs. This work therefore deals with the effect of curing of a reinforced concrete slab, on its strength properties. Long-term curing would yield higher values of compressive strength, but it is also necessary to consider how effective it is. As a pilot research in this work is investigating the properties of concrete cubes in various curing. It further develops theoretical possibilities for continuing research.

Abstract: This study also investigates the effectiveness of multiple surface modification of recycled concrete aggregate (RCA) with respect to mechanical properties of the resultant recycled aggregate concrete (RAC) when exposed to various curing conditions, namely, normal water (N_w) and seawater (S_w). The properties included compressive and flexural strength and impact resistance. Findings show that effect inclusion of the treated coarse RCA is significant in enhancing the mechanical properties of RAC. The exposure to S_w appears to affect the degradation of the compressive strength of RAC remarkably in a marine environment. In this case, however, using of treated RCA is found beneficial in decreasing the deterioration in the compressive strength of RAC.

Abstract: The values of the system energy change caused by the structure formation of coatings based on paints using mineral binders are given. It is established that the process of curing coatings can be described using the first law of thermodynamics and thermodynamic potential - enthalpy. The main regularities of changes in thermodynamic parameters in the process of curing coatings on cement concretes depending on the curing time, the porosity of the substrate is revealed. It has been established that, depending on the type of coatings, there is an unequal change in the energy of structure formation, which allows predetermining the different operational durability of coatings. It is shown that the smallest change in entropy during the curing process is characteristic of coatings on substrates characterized by lower surface porosity. The increase in entropy in the process of curing coatings on substrates with high porosity suggests that the coatings are characterized by a greater defectiveness and, consequently, a greater probability of their destruction during operation. The increase in the surface porosity of the cement substrate reduces the adhesion strength of the coatings and reduces its durability during exposure to the environment.

Abstract: In the case of exposure of reinforced concrete structure to accidental fire, an assessment of its residual capacity is needed. Bond strength of concrete was observed under elevated temperatures (150°, 250°, 350° and 500°C) in this study. Cylindrical specimens were prepared for pull-out tests to find out the bond behavior and to observe the mechanical properties of concrete. All the specimens were 100 mm diameter and 200 mm height. The pull-out specimens contain a 10 mm steel bar at its center. The specimens were tested at 52 days age following a 28 days water curing. Samples were preheated for 3 hours at 100°C temperature and then put into the furnace for 1 hour at the target temperature. Samples were tested before preheating as controlled specimens. In case of mechanical properties and the bond strength of concrete, there were no remarkable changes due to elevated temperature up to 150°C. However, the mechanical properties and bond strength were decreased gradually after 150°C temperature. Maximum reduction of bond strength observed was 52.13% and 49.8% at 500°C for testing within 1 hour and after 24 hours of heating respectively when compared to the controlled specimens. Bond strength was found to reduce at a greater rate than compressive strength due to the elevated temperature.

Abstract: The aim of this work is development of method to determine the conversion of epoxy groups in the epoxy amine resins based on the epoxy oligomer ED-20 and hardener DDM. The studies were carried out in the NIR range. The mathematical division of the spectrum of the resins into individual peaks was used and concentrations of the functional groups were calculated on them. The dependence of the maximum conversion of epoxy groups in the resins after curing at 140 ^оС was obtained.

Abstract: The environmental burden due to the production and use of Ordinary Portland Cement(OPC) has become significant in terms of resource use and atmospheric emissions. Alkali-activatedbinders have gained significant attention in recent years as a possible alternative to OPC. Concreteundergoes physical and chemical changes through its lifetime. These changes affect propertiesresulting in either increase or decrease of performance and serviceability. Concrete contracts due tothese changes leading to shrinkage. Shrinkage induces tensile stresses within the members, whichleads to a tendency of cracking, resulting in compromised durability. Shrinkage behavior analysis ofAlkali-Activated Concrete (AAC), without any external loading, is crucial to assess as there is limiteddata available. To further test for shrinkage under restraint conditions, mix development of AAC isnecessary. Mechanical properties and drying shrinkage potential of AAC are evaluated and comparedwith OPC concrete. Basic properties assessed to understand the performance of AAC are compressivestrength, splitting tensile strength and free drying shrinkage. Drying shrinkage test is performed as perASTM C157 for 90 days. Furthermore, the effect of different curing regimes (dry curing, three daysmoist curing, and seven days moist curing) on the mechanical properties and shrinkage is evaluated.The results show that drying shrinkage of AAC exhibit similar behavior to that of OPC concrete whilehaving better early-age strength under moist curing regimes.

Abstract: Concrete use is fundamental to most infrastructural development plan of humanity today. This underscores the need to understand the strength characteristics of concrete made with crushed glass aggregate as partial replacement for fine aggregate and mixed with magnetic field treated water (MFTW). This study investigates the mechanical properties of concrete mixes prepared and cast into cubes of varying constituents such as concrete mixed with normal water, concrete mixed with MFTW, concrete with varying degree of crushed glass as replacement of fine aggregate from 15 to 45% with or without MFTW. The cubes were thereafter crushed after 7, 14, 28 and 56 days of curing to determine their compressive and tensile strengths. From the results obtained, it was observed that the optimum percentage partial replacement of sand with crushed glass aggregate is 15% to attain a suitable using MFTW in the mixing of the concrete. At this percentage replacement, it was observed that both the compressive and tensile strengths of the concrete mixed with MFTW improved by 25-30% relative to the conventional concrete. The study therefore recommend the use of MFTW in place of normal water in concrete production and use especially when partially replacing fine aggregate with crushed glass aggregate.

Abstract: The aim of this study was to investigate the effect of replacing carbon black (CB) with inexpensive and environmentally friendly fillers – bentonite (BNT) and modified bentonite (M-BNT), on the curing properties of natural rubber (NR) composites. A control sample (unfilled NR) and thirteen NR composites filled with varied proportions of CB (x₁), M-BNT (x₂), and BNT (x₃) based on a third degree – simplex lattice mixture design of experiment (DOE) were prepared in this study. Rheometric results showed that 33% substitution of CB with M-BNT provides the highest elastic torque values. Mixture of 10phr CB and 5phr M-BNT (CB/M-BNT/BNT 10/5/0) produces synergistic effect on curing. The presence of CB increases vulcanization rate due to its high basicity and low oxygen content while M-BNT serves as vulcanizing accelerator due to the present amine groups. Coefficients of reduced hierarchical models showed that the main factors contributed mainly on the curing parameters: β₁ for the torque values, β₂ for the scorch and curing time, and β₃ for CRI. High values of coefficient of determination (r²) were computed particularly for M_H (98.20%), ΔS (99.13%), t_s2(95.68%), t_c90(95.70%) and CRI (95.97%) establishing best fit between the model and experimental values.

Abstract: Information is provided about the stability of polysilicates solutions, obtained by mixing liquid glass and silica sol. It was found, that at increases the amount of silicic acid sol added, the pH of the solutions decreases with an unchanged alkali concentration. Information on the structure of polysilicate solutions are given. It has been found, that addition of the sol (increasing the silicate module) contribute to prolonging fraction of high-polymer fractions of silicic anion. The dependence of the silica content of the monomeric form of α-SiO₂ in the early stages of interaction between the sol and liquid glass is extreme. The maximum content of α-SiO2 depends on the type of liquid glass and the amount of sol. It is shown, that films based on polysilicates solutions, are characterized by faster curing. The manifestation of the scale factor for coatings based on polysilicate solutions was revealed.