Pre-curing Time Research Articles

Bioinspired Silicone Rubber Foams With Adjustable Gradient Structures Induced via Temperature Fields

ABSTRACTThe structure‐gradient silicone rubber foam (SRF) is an advanced bionic material with potential for applications in energy‐damping, triboelectric nanogenerators, and electromagnetic interference shielding. However, the production of SRFs with an adjustable gradient structure is still a significant challenge. Herein, a novel strategy combining in situ precuring of silicone rubber induced by gradient temperature fields with supercritical CO2 foaming was developed to prepare SRFs with biomimetic gradient cellular structures. By regulating the unilateral heat source temperature and precuring time, SR samples with gradient curing degrees from 16.13% to 51.21% were achieved. Finally, a prospective structure‐gradient SRF with a cell size range of 24.10–182.80 μm was successfully prepared. The energy loss coefficient of gradient SRF can be reduced by 41.65% compared with that of uniform cellular structure foam with average cell size of 31.25 μm after 50 cycles. Gradient SRFs show significantly enhanced mechanical performance, recoverability, and elasticity. Besides, the underlying mechanism for generating this gradient cellular structure was clarified.

Research Progress on Effects of Antifreeze Components, Nanoparticles and Pre-Curing on the Properties of Low-Temperature Curing Materials

There are long periods of winter construction in China’s eastern and western Alpine regions. The decreased construction temperature adversely affects the workability, mechanical properties, and durability of cement-based materials and alkali-activated materials. Under low-temperature curing conditions, the hydration reaction of these materials slows down, resulting in limited strength development and reduced durability. In response to this problem, researchers have summarized three measures to improve performance: the use of anti-freezing components, nanoparticles, and pre-curing. The effects of anti-freezing components on the mechanical properties and micro-mechanism changes of Portland cement, sulphoaluminate cement, magnesium phosphate cement-based materials, and alkali-activated cementitious materials are organized. Additionally, the improvement of macro-micro properties in cement-based materials through mineral admixtures, nanoparticles, and hydrated calcium silicate seeds is summarized. The influence of pre-curing on the mechanical properties of cement-based materials is analyzed, focusing on the relationship between pre-curing time and the critical strength of frost resistance. Finally, existing research challenges are summarized, and future research directions are proposed, providing valuable references for the further development of materials and engineering applications.

Effect of Chemical Foaming Process Parameters on the Performance of Epoxy Foam and Parameter Optimization Strategies

To prepare polymer foams with low‐density and high‐energy absorption efficiency, this study designs epoxy foaming experiments employing the Box–Behnken method and investigates the impact of process parameters on the microscopic geometric parameters and uniaxial compression response of foam. Finite element analysis models are created to investigate the microscale deformation mechanism. The main results are as follows: 1) The average equivalent cell diameter is significantly affected by foaming temperature and foaming agent content, while cell wall thickness is more influenced by the foaming agent content and the precuring time. 2) The compression response is most significantly affected by foaming temperature, followed by foaming agent content, with precuring time showing less significant influence. The differences in the stress–strain curves during various stages of deformation are due to the buckling of cell walls and the subsequent collapse of cells. 3) Density exhibits a highly positive correlation with strength and modulus while showing a relatively high negative correlation with energy absorption efficiency. Based on these findings, process parameters are optimized using the Hooke–Jeeves algorithm and experimentally validated, demonstrating the reliability of the optimization strategy. The experimental design and process parameter optimization strategy can be applied to other polymer foaming research.

Characterization and simulation study of rGO@epoxy strain sensor for implantation into CFRP composites

Abstract Epoxy-based strain sensors are considered an ideal choice for online monitoring due to their excellent compatibility with carbon fiber-reinforced polymer (CFRP) composites. However, the existing preparation processes for sensors are usually quite complex and have high requirements for experimental equipment and process conditions. This study proposes a novel and simple preparation method for sensors, which integrates laser reduction of graphene oxide and pre-cured epoxy film technology. By precisely controlling the pre-curing time of the epoxy film, the integrity of the sensing layer is effectively protected during the sensor curing process. In uniaxial tensile tests, the sensitivity of the sensor increases with the increase in scanning speed. The sensors fabricated at a laser scanning speed of 120 mm/s have a sensitivity of 11.6, which is approximately 2.01 times higher than the sensitivity at a 24 mm/s laser scanning speed. The sensor’s cyclic response was further tested to characterize its online monitoring capability. Finally, simulation analysis was conducted to investigate the impact of implanting sensors of different thicknesses on the stress concentration of CFRP laminates. This study provides a reference basis for the design of epoxy-based strain sensors with less invasion and better compatibility with CFRP in the future.

Effect of steam curing regime on the mechanical properties, drying shrinkage, and microstructure of mortar in high-altitude areas with low air pressure

Low air pressure in plateau regions impacts the microstructure and macroscopic properties of cement-based materials by altering water transport. However, the efficiency of steam curing, a frequently-used curing approach, under these conditions has been inadequately studied. This work compares the effects of steam curing on the mechanical properties, drying shrinkage, and microstructure of cement-based materials at low air pressure. Results show that low air pressure decreases the compressive strength and increases the drying shrinkage of steam-cured mortar due to reduced hydration and increased total pore volume. Optimally, a steam curing temperature of 40°C to 60°C minimizes thermal damage, and a pre-curing time of 3 hours yields the best mechanical properties and drying shrinkage. Additionally, a field case confirms the effectiveness of the proposed steam curing regime; the demolding strength of precast beams meets engineering standards, and drying shrinkage fluctuates significantly with daily temperature changes. Notably, a 25.30 % difference in 90-day drying shrinkage between steam-cured and non-steam-cured specimens underscores the necessity of steam curing in high-altitude areas. These findings are crucial for optimizing steam curing practices in low-pressure environments.

Research on mechanical properties and microscopic mechanism of multi-based geopolymer concrete under combined action of pre-curing and nano-silica

In order to study the combined action of curing conditions and nano-silica (NS) on geopolymer concrete (GPC), the multi-based geopolymer concrete (FSSGC) was prepared with fly ash (FA), slag (SG) and steel fiber (SF), the experiments were carried out in three stages. Firstly, the mechanical properties of FSSGC was tested by monofactor analysis, with pre-curing temperature (20, 40, 60, 80, and 100 °C) and pre-curing time (6, 12, 18, 24, 30, and 36 h) as parameters. The compressive strength and splitting tensile strength initially increased and then decreased with the increase of parameters, and there was an optimal value for subsequent experimental research (80 °C and 18 h). Secondly, the effects of NS content (0, 0.5, 1.0, 1.5, and 2.0 wt%) on the mechanical properties of FSSGC under different curing conditions were analyzed. The results indicated that the mechanical properties of the FSSGC exhibited a trend of initially increasing and then decreasing with the increase of NS content, with the optimal mechanical performance observed under the combined action of high-temperature pre-curing conditions and the incorporation of NS (1.0 wt%). Thirdly, X-ray diffraction, scanning electron microscopy, image analysis, and microhardness tests had been conducted, and the results were consistent with the mechanical properties. The pre-curing temperature affected the rate of geopolymerization, while the pre-curing time influenced the degree of geopolymerization by maintaining this rate. The small particle size and high Young's modulus of the NS effectively filled pores and inhibited the propagation of microcracks. High-temperature pre-curing enhanced the nucleation effect and pozzolanic effect of NS, resulting in a combined action that formed a denser microstructure in the matrix and interfacial transition zone (ITZ).

Experimental study on mechanical property and pore structure of nano-modified concrete in the large temperature difference environment of cold-region tunnels

To reveal the mechanism of nanomaterials on concrete in the large temperature difference environment, three different types of nano-modified concrete were explored in this study. The mechanical properties and pore structure characteristics of concrete under these different nano-modified measures were investigated. Furthermore, mathematical models of the strength and the pore structure were established. The experimental results demonstrate that: Untimely entering the large temperature difference environment is unfavourable for the strength of concrete; The incorporation of nanomaterials is conducive to the strength and pore structure of concrete, of which 2 % Nano-SiO2 shows the best effect; When the pre-curing time ranges from 1.5 days to 3 days, the strength of 2NS group develops slowly, which is consistent to the change of pore structure; The multi-factor strength mathematical model established can express the quantitative relationship between concrete’s strength and pore structure.

Experimental study and mechanism analysis on the effect of pre-curing time on the microstructure and mechanical properties of magnetorheological elastomers under compression

The effects of the pre-curing time before exposing the carbonyl iron particles (CIPs) filled liquid polydimethylsiloxane (PDMS) mixture to a preparatory magnetic field on the magnetorheological (MR) effects of the magnetorheological elastomers (MREs) were experimentally and theoretically investigated. The anisotropic MRE samples with different pre-curing time were firstly fabricated and their compressive MR effects were tested. The results showed that the longer pre-curing time induces the shorter and more irregular particle chains formed in the MREs, and the sample pre-cured for 20 min showed the maximum MR effect. Then based on the rheological characterization, a rheological model was proposed to predict the rheological property evolution of CIPs/PDMS mixture with respect to the pre-curing time, and a modified three-dimensional (3D) particle dynamics (PD) model was developed to simulate the particle structure evolution in the MRE mixtures under a preparatory magnetic field. The simulation results showed that short chains were first formed owing to the magnetic attraction between the adjacent particles. Then these short chains approached each other to form long chains and even thick chain-like clusters. The later process can be restrained by increasing the pre-curing time, thus more short and incompact particle chains are formed in the MREs. Lastly, simplified microstructure models representing the wavy chains were used to explore the effect of particle arrangement on the MR effect of the MREs under compression. It is found that the wavy chains with incompactly packed particles are beneficial for improving the MR effect owing to the strong transverse magnetic attraction between the neighboring particles. This work demonstrates that PD simulation is an efficiency method to predict the particle arrangement evolution in the MRE mixture, and the possibility to tailoring the microstructures of the MREs by adjusting the viscosity of MRE mixtures to improve their MR effects, while keeping a high zero-field modulus simultaneously.

Thermoset droplet curing performance in the microbond test

ABSTRACT Users of the microbond test assume that a microbond resin droplet’s properties are equivalent to a macroscale specimen. However, there is currently no standardised methodology for determining the cure state of droplet specimens used in the microbond test. In this paper, we present a technique for microbond test users to better understand the properties of thermoset droplet specimens. Utilising a conventional benchtop spectrometer, a novel sample preparation technique involving curing epoxy droplets on thin-steel filaments allowed for high-throughput determination of the microbond droplet cure state. The parity between steel filament and glass fibre microbond samples was confirmed by infrared microspectroscopy. It is shown that cure schedules used in manufacturing composite parts produced microbond droplets with degrees of cure lower than that of bulk matrix specimens subjected to an identical thermal history. Testable microbond droplets could only be prepared for commercial resin systems when introducing a room temperature pre-curing time of at least 2 h. It is concluded that microbond testing should be supported by some droplet cure state characterisation methods to ensure that interfacial effects are not artefacts of droplet sample preparation.

Effect of the Rehydration Method on the Physical–Mechanical Properties of CO2-Cured Magnesium-Based Fiber Cement Boards

This article analyzes the effect of the rehydration method on the physical–mechanical properties of accelerated carbonation-cured magnesium-based fiber cement boards. The rehydration process of the boards was analyzed in conjunction with the analysis of the pre-curing time before accelerated carbonation (24, 48, and 72 h before carbonation), resulting in eight different curing parameters used in this investigation. The physical–mechanical performance and microstructural characteristics of magnesium oxysulfate boards before and after carbonation were investigated by water absorption, apparent porosity, and bulk density using the four-point bending test, X-ray diffraction, thermogravimetric analysis, and scanning electron microscopy. According to the results, the accelerated carbonation process improved the mechanical properties of the boards. The samples that carbonated after 48 h showed a higher modulus of rupture. The rehydration process of the composites before carbonation led to enhancements in the pre-cured boards for 48 and 72 h, demonstrating that carbonation occurred more effectively after water rehydration. The mechanical improvements were associated with the formation of hydration products, which preferentially formed in the pores and voids of the fiber cement matrix. These carbonation products altered the physical properties of the composites, increasing the density of the boards and reducing the void volume. The decomposition of the formed carbonates was confirmed by thermogravimetric analysis, which indicated that the rehydration process favored the carbonation of the composites.

Development and Application of Medium-reactivity Epoxy Infusion Resin System in Large-scale Wind Turbine Blades

In view of the requirement of cost reduction and efficiency increase for large-scale wind turbine blades, a medium-reactivity epoxy infusion resin system was developed, which contained 20% fast curing agent and 80% slow curing agent. The properties of medium-reactivity epoxy infusion resin system were evaluated via physical and chemical analysis, application process ability analysis, resin clear casting mechanical properties analysis, and laminate specimens mechanical properties analysis. The results indicated that the mechanical properties of this medium-reactivity epoxy infusion resin system can meet the requirements of blade design, and the pre-curing time of the resin was reduced from 3.0h to 2.3h. The application process ability of medium-reactivity epoxy infusion resin system in large-scale wind turbine blade was also verified. The blade parts pre-curing time was shortened by 0.5-1 hour, which meets the target of blade cost reduction and efficiency increase.

Hydrodynamic control of silicone elastomers on between porous media

Silicone elastomers, for example, polydimethylsiloxane (PDMS), have been widely used as cross-linkers for fabrication of flexible strain sensors. They not only lend strong adhesion to adjacent materials, for example, porous fabrics, but also tune their elastic property. Silicone elastomer precursors, which are typical non-Newtonian fluids, can easily penetrate into porous fabrics, driven by the capillary effects of fibers. Unfortunately, such a penetration has negative effects on both adhesion strength and elastic property of PDMS, thus limiting their applications. Here we report a facile method for preparing uniform silicone elastomer films, that is, PDMS, on between porous media via controlling the hydrodynamics of elastomer precursors. Our experiments show that the hydrodynamics of elastomer precursors can be easily controlled by modulating the pre-curing time of PDMS precursors to prevent them from penetration into porous media but keep their high adhesion. Based on this hydrodynamic modulation of PDMS precursors, we firmly adhere conductive silver nanowires (AgNWs) onto knitted fabrics, and further combine composites with common clothing from the point of view of ergonomics, showing the possibility of applying such a modulation to the fabrication of wearable strain sensors. Our findings not only present an understanding of liquid transport in porous media, but also provide a novel method of controlling the hydrodynamics of elastomer precursors in porous media for achieving the effective wearable sensors.

Effect of precuring time on the strength, microstructure, and energy consumption of direct electric curing concrete

The study assesses the role of precuring duration in facilitating the rapid curing of concrete and its associated impacts on current, resistance, and temperature variations during direct electric curing (DEC) of concrete. Additionally, the research examines the effects of precuring duration on aspects such as concrete strength development, pore structure, hydration product morphology, and the interfacial transition zone. For a comprehensive analysis, it introduced the "unit-strength energy–time product" as a measure to compare the relative benefits and drawbacks of DEC and steam curing concerning concrete productivity and environmental implications. the findings reveal that the variations in current, resistance, and temperature during DEC are affected by the precuring time duration, with both current and temperature decreasing with an increase in precuring time. A precuring duration of 90 min under DEC yields an early high strength. At this point, the 1 d compressive strength of the concrete specimen is 51.5 MPa, representing 90.51 % of the 28 d strength, without any significant reduction in the late strength. DEC also mitigates the pore-coarsening effect induced by steam curing, thereby reducing the specimen's porosity and enhancing the performance of the interfacial transition zone. The precuring time and energy required for DEC are significantly lower compared to steam curing. The optimal DEC regime, consuming a mere 450 min, contrasts with the lengthy 780 min process required for steam curing. By considering time and energy consumption, the unit-strength energy–time product of DEC ranges between 22.01% and 27.86 % (at age 1 d) and 17.93%–23.45 % (at age 28 d) of that of steam curing. In summary, the research provides evidence that an appropriately devised DEC regime can augment production and energy efficiency, possibly instigating a technological transformation in contemporary construction practices.

Research on the creep properties of cement paste after microwave pre-curing based on indentation test

This article researched the influence of microwave pre-curing on the creep properties of cement pastes. Microwave pre-curing with different durations was performed on cement paste specimens after standard cured for 3 d. Subsequently, the creep performance of the cement paste and the phase was acquired using indentation tests. The quantitative analysis of the phase was performed on the cement paste specimens after microwave pre-curing. The reasons for the variation of creep properties of the specimens affected by microwave pre-curing were also analyzed based on the pore characteristics, gel structure, and dielectric properties. The results indicated that the creep modulus of cement paste samples gradually increased with the increase of microwave pre-curing time. Microwave pre-curing significantly enhanced the contact creep modulus (CCM) of low-density (LD) C–S–H gel and high-density (HD) C–S–H gel. With the increase of microwave pre-curing time, the porosity in the samples decreased and the pore structure was significantly improved. Microwave pre-curing decreased the percentage of LD C–S–H gel and significantly increased the percentage of HD C–S–H gel, and also increased the complex structure of C–S–H gel. Microwave pre-curing enhanced the dielectric performance of paste specimens and increased their ability to store and absorb electromagnetic waves.

Method to fabricate round cross-sectional channel using thermal expansion of air for passive flow regulators.

We present a novel method for fabricating round cross-sectional channels for autoregulatory pressure regulators. The application of previously reported methods in multilayered soft lithography using three-dimensional printed molds is challenging. Herein, we used a thermal expansion technique to create round cross-sectional channels in replica layers using air in the cavity space of a master mold. The width and height of the round channel in the replica could be adjusted in the range of 80-300 and 3-57µm, respectively, by varying the precuring time of the replica in the gel state and adjusting the cavity size of the master mold. We successfully fabricated a pressure regulator with a round cross-sectional channel, achieving a constant output pressure at a low threshold input pressure. Our device exhibited superior performance, with a constant output pressure at a threshold input pressure of less than 77%, compared to a device with a rectangular cross-sectional channel. Our method has significant potential for application in the fabrication of integrated microfluidic systems with round cross-sectional channels.

Utilization of carbide slag in autoclaved aerated concrete (CS-AAC) and optimization: Foaming, hydration process, and physic-mechanical properties

Autoclaved aerated concrete (AAC), a lightweight porous material, has been widely used in the building field because of its energy savings. A solid waste called carbide slag with a high Ca(OH)2 content is a potential substitution for CaO to prepare AAC. In this study, carbide slag was used to replace quicklime to prepare carbide slag-based autoclaved aerated concrete (CS-AAC). The temperature, foaming rate, and rheological properties of the slurry during the foaming stage, the strength of the rough body, and the physical and mechanical properties of the autoclaved products were investigated. Hydration products, microstructure, and pore structure were further analyzed to investigate the mechanisms of carbide slag on the properties of AAC, and the use of sodium carbonate (SC) to improve the performance of CS-AAC was explored. The results show that the addition of carbide slag causes a degradation of the properties, including foaming rate and physic-mechanical properties. The “water locking” effect due to the irregular surface of the carbide slag results in an increase in slurry consistency. The increase in slurry consistency, as well as the decrease in slurry temperature is the main reason for the decrease in slurry foaming rate and roundness of bubbles, which are unfavorable for the strength development of AAC. Due to the alkali excitation characteristics of sodium carbonate. The addition of SC can react with Ca(OH)2 in the slurry to produce OH-, to promote cement hydration, which can accelerate the development of rough body strength, and save pre-curing time. The carbide slag-based AAC with 0.25% SC has a significant improvement in foaming properties and facilitates the generation of more C-S-H gels to form tobermorite under hydrothermal conditions, which improves the AAC properties. However, too much SC can consume a large amount of Ca(OH)2 and affects the production of tobermorite, which is not beneficial to the mechanical properties.

Experimental evaluation of the frost resistance durability of RCC layer subjected to early-age freezing

Freeze–thaw cycle, shear and nuclear magnetic resonance tests were conducted to investigate the effect of early-age freezing on the frost resistance durability of roller compacted concrete (RCC) layers during their service period. The effects of interval time and pre-curing time on the frost durability of early-age frozen RCCs were discussed. The appearance damage, shear failure mode, shear strength and pore structure of the early-age frozen RCC layers after freeze–thaw cycles were evaluated. Results showed that the appearance damage of the early-age frozen RCC layers after the freeze–thaw cycles began with microcracks at the layer joints, which then extended to the upper and lower layers. The shear failure mode was dominated by slip failure. Early-age freezing caused initial damage to the layer and increased the rate of loss of peak shear strength and the volume of harmful pores during the freeze–thaw cycles. The degree of shear strength loss due to interval time was in a descending order as follows: 24, 12, 4 and 0 h. The loss in shear strength was the largest in the specimens pre-cured for 12 and 24 h, followed by those pre-cured for 48 and 72 h. The loss in shear strength was the smallest for the specimens pre-cured for 7 days, similar to the result for the unfrozen control group. In addition, the corresponding relationship between shear strength and pore structure at the layer of the early-age frozen RCCs during the freeze–thaw cycles was quantitatively described based on fractal dimension theory. A freeze–thaw damage model was developed to consider the effects of early-age freezing on the surface of RCC, and the calculated results of the model were in good agreement with the test results.

Development and performance of pouring self-luminescent asphalt mixture

The application of long afterglow-based materials to the self-luminescent pavement could significantly reduce the power consumption of lighting at night and improve the traffic safety. This study designs a pouring self-luminescent asphalt mixture (PSAM) through pouring a grouting material including the long afterglow luminescent material (SrAl2O4:Eu, Dy) and epoxy resin into the porous pavement void configuration. First, the mechanical and afterglow properties of the epoxy resin-based long afterglow grouting material were systematically tested to determine the optimal composition ratio of raw materials. Then, the influence of time and viscosity on the grouting was studied by obtaining the luminous images of the cross and longitudinal sections of self-luminous asphalt mixture specimens. According to the volume characteristics of the grouting material and the mixture, the relationship between the theoretical grouting consumption and the actual grouting consumption was analyzed. Finally, the influence of porous mixture air void on afterglow characteristics and road performance of the PSAM was analyzed. From the acquired results, it was demonstrated that the optimum proportion of each component of the epoxy resin long afterglow grouting material was the following: epoxy resin: curing agent: toughener: long afterglow material = 100:18:15:14. Moreover, it was found that the temperature and pre-curing time have remarkable effects on the viscosity of grouting materials. In order to ensure a good grouting effect, the viscosity of the grouting material should be controlled within 1.7 Pa s. There was a good linear relationship between the actual grouting amount and the connected air void, although the actual grouting consumption was less than the theoretical grouting consumption. Furthermore, PSAM exhibited good afterglow performance and road performance. The afterglow brightness and water stability had no obvious relationship with the air void. In contrast, with the further increase of the air void, both the low-temperature crack resistance and the skid resistance were improved, and the high-temperature stability was reduced.

Highly flexible, stretchable, durable conductive electrode for human-body-attachable wearable sensor application

Flexible/stretchable electrodes based on conductive polymers are used in various fields, including wearable devices and human body sensors. However, the durability of flexible/stretchable electrodes is insufficient. In this study, a carbon nanotube (CNT) electrode layer was formed on a polydimethylsiloxane (PDMS) surface with a controlled precuring time using a simple spray-coating method. The electrical properties and durability of the CNT-based conductive polymers were evaluated. To evaluate the durability of the electrodes, bending, stretching, tribo, and scratch tests were performed. The degree of embedding of the CNT electrode layer on the PDMS surface differed according to the PDMS precuring time, which showed a difference in durability. The case in which the CNTs were coated at a precuring time of approximately 8–12 h had the best durability. It was verified that CNT-embedded PDMS electrodes fabricated under optimal conditions can be attached to the skin of human joints and used as human-body-attachable wearable sensors that measure changes in electrical resistance according to joint movement, resulting in excellent elasticity, durability, and performance.

INFLUENCE OF PRE-CURING PERIOD AT SUB-ZERO TEMPERATURE （-20℃） ON THE COMPRESSIVE STRENGTH OF CONCRETE

Average temperatures over most of Mongolia are below freezing from November through March. The lowest temperatures in January are as low as -36℃ to -40℃. However, in the extremely cold climate of Mongolia, it is often executed the concrete works even outside temperature is below -20℃. Therefore, in this experimental work, we investigated the required time span of pre-curing in order to achieve the potential strength at 28 days before being exposed to extremely low temperature（-20℃）, simulating Mongolian weather conditions. Moreover, internal temperature changes and microstructure analyses have been executed. We have found that concrete freezing temperature decreases remarkably from 0℃ to -15.6℃ with increasing pre-curing time, however, depending on the water-cement （W/C） ratio. Our experiments have shown that there was no freezable water after 14 days when the W/C ratio is below 0.39. Our research concluded that concrete should at least be cured at 20℃ for 14 days before being exposed to sub-zero temperatures, in order to reach in fair to middling potential strength at 28 days.