Search

If it is desired to experimentally verify the regularity in the effects of air-voids on the properties of concrete, methods of altering voids must be relied on. Consequently, this research explored a method that reduced the curing air pressure after mixing concrete so as to modify air-void characteristics. In addition, in order to avoid introducing variables besides air-voids, a limit was placed on the duration of low-pressure maintenance, which was restoring the air pressure to atmospheric pressure after the formation of the concrete's air pressure transient resistant strength. The results showed that if the reduction in air pressure was insufficient, the differential pressure between the inside and outside of the original bubbles was not enough to cause them to break through energy barriers resulting in expansion. Only bubble regeneration due to the decrease in gas solubility was observed. When the differential pressure exceeded the energy barriers, bubble expansion could be achieved. In addition, it was possible to achieve the air pressure transient resistant strength in short time, which was achieved in only 7 h for the concrete used in this paper. Water loss calculation, X-ray diffraction, thermogravimetry-derivate thermogravimetry, scanning electron microscope, and mercury intrusion porosimetry were used to evaluate the hydration of the concrete confirming that the low-air pressure curing of 7 h had almost no effect on hydration. The method proposed can provide an effective means of specimen preparation for scientific research involving the effects of air-voids on the properties of concrete.

Establishing an effective method to enhance the adhesion of mortar on recycled concrete aggregate (RCA) is key to the utilization of RCA. In this paper, a novel and simple approach for strengthening the mortar layer of RCA by treatment with a crystallizing agent (CA) was proposed, and the effects of different treatment regime on the density, water absorption, mineral composition, and microstructure of RCA were determined. Moreover, the performance of CA-treated RCA in recycled aggregate concrete (RAC) was investigated, and its mechanism of the treatment was explored at both the microscopic and macroscopic levels. The results showed that CA treatment significantly improved the overall performance of RCA by increasing the density and calcium silicate hydrate (C–S–H) content, which enhanced the properties of the new and old interfacial transition zones and enhanced the mechanical properties of the RAC. The improvements were attributed to the reaction of CA with calcium-rich hydration products in the mortar layer of RCA, which formed C–S–H precipitates. The newly-formed C–S–H filled the pores of RCA and blocked cracks, which improved the compactness of the RCA.

In this study, the Al–Ni–Cu–Fe alloy was fabricated using laser powder bed fusion (LPBF), with controlled Fe content, to investigate its metallurgical effects. Furthermore, the microstructural differences between the coarse-grained (CG), fine-grained (FG) regions, and the melt pool boundary were compared. The results showed that the FG region exhibited grain-boundary and precipitation strengthening effects. The grain orientation in the FG region was randomly distributed, leading to the randomly oriented columnar grains with a small ratio of low-angle misorientations in the CG region. This prevented the CG region from developing a temperature gradient texture and provided a solid foundation for the high strength. Both high (Al–4Ni–2Cu–1.4Fe) and low (Al–4Ni–2Cu–0.7Fe) Fe-addition alloys exhibited strengths of over 600 MPa. The FG region of the 1.4Fe contained more strengthening phases, contributing to its higher strength. However, the presence of more incoherent interfaces between Al9FeNi and the α-Al lattice resulted in reduced ductility. On the other hand, the 0.7Fe alloy exhibited lower brittleness and improved ductility. Overall, the LPBF Al–Ni–Cu–Fe alloy shows promising potential for aerospace applications, and a comprehensive understanding of the strengthening mechanisms and microstructure can expand its application scope.

In this study, Laser Powder Bed Fusion (LPBF) Inconel 718 is subjected to various heat treatments, namely double aging, solid solution + double aging, and homogenization + solid-solution + double aging, to investigate the effect of heat treatment on room- and high-temperature tensile properties. The results show that all three heat treatments increase hardness and room-temperature tensile strength, but greatly reduce ductility. With an increase in tensile test temperature, the stress-induced Portevin-Le Chatelier (PLC) effect can effectively prevent the occurrence of oxidation at grain boundaries and maintain a certain ductility in the range of room temperature to 600 °C. However, when the temperature is 650 °C, the PLC effect disappears, resulting in a high-temperature embrittlement effect.