Owing to the shortage of space for land reclamation in Hong Kong, it is difficult to dispose of tons of masonry waste generated daily from construction activities. Adoption of recycled aggregate from concrete waste thus becomes a burning issue. The Hong Kong SAR Government has set up a recycling plant in Tuen Mun Area 38 aiming at turning concrete waste into recycled aggregate with a practice note and specifications issued for controlling the quality of recycled aggregate. However, the use of recycled aggregate concrete to high grade applications is rarely reported because of its poorer compressive strength and high variability in mechanical behavior. This paper proposes a new approach in mixing concrete, namely "two-stage mixing approach (TSMA)", intended to improve the compressive strength for recycled aggregate concrete and hence lower its strength variability. Based upon experimental works, improvements in strength to recycled aggregate concrete were achieved. The effect can be attributable to the porous nature of the recycled aggregate and hence the pre-mix process can fill up some pores and cracks, resulting in a denser concrete, an improved interfacial zone around recycled aggregate and thus a higher strength when compared with the traditional mixing approach.

With the increase in the use of recycled aggregate concrete, the demand on recycled aggregate (RA) is escalating. As such, the behaviour and characteristics of RA need to be clearly understood. In practice, the testing procedures of aggregates in Hong Kong follow those laid down in British Standard Institution (BSI) (BS: 812), which provide a good foundation for assessing properties of natural aggregates. As RA may have cement paste attached that may detach from the mass during sample preparation when repetitive soaking in water and drying are employed. Thus, the traditional testing approach for water absorption cannot give accurate results for RA, based upon which, errors in concrete mix designs may result. This paper proposes an innovative method for testing the water absorption of RA named Real-Time Assessment of Water Absorption (RAWA). The detailed testing procedure of the new method is illustrated with examples.

While multiple studies have explored the mechanism for DC and AC microscale gas breakdown, few have assessed the mechanism for pulsed voltage gas breakdown at the microscale. This study experimentally and analytically investigates gas breakdown for gap widths from 1 μm to 25 μm. Using an electrical-optical measurement system with a spatial resolution of 1 μm and a temporal resolution of 2 ns, we measure the breakdown voltages and determine breakdown morphology as a function of the gap width. An empirical fit shows that the breakdown voltage varies linearly with the gap distance at smaller gaps, agreeing with an analytical theory for DC microscale gas breakdown coupling field emission and Townsend avalanche that shows that the slope is a function of field emission properties. Furthermore, the curved breakdown paths captured between 5 μm and 10 μm demonstrate a similar effective length (∼11.7 μm) independent of the gap width, which is consistent with a “plateau” in breakdown voltage. This indicates that Townsend avalanche alone is insufficient to drive breakdown for these gaps and that ion enhanced field emission must contribute, in agreement with theory. The overall agreement of measured breakdown voltage with theoretical predictions from 1 μm to 25 μm indicates the applicability of DC microscale gas breakdown theory to pulsed breakdown, demonstrating that pulsed voltages induce a similar transition from Townsend avalanche to field emission as DC and AC voltages at the microscale.

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Density functional theory (DFT) calculations were carried out to explore the adsorptions of reactive species and the reaction mechanisms on Pd−Cu bimetallic catalysts during CO 2 hydrogenation to methanol. All the possible preferred adsorption sites, geometries, and adsorption energies of the relative intermediates on pure Cu(111) and three PdCu(111) surfaces were determined, revealing that both the adsorption configuration and corresponding adsorption energy are changed by doping with Pd atoms. The strengthened COOH* adsorption and the greatly weakened OH* adsorption change the rate-limiting step from CO 2 hydrogenation forming trans-COOH* on Cu(111), Pd 3 Cu 6 (111), and Pd 6 Cu 3 (111) surfaces to cis-COOH* decomposition forming CO* and OH* on Pd ML surface. Additionally, the highest activation barriers for the overall reaction pathway are reduced in the following trend: Cu(111) > Pd 6 Cu 3 (111) > Pd 3 Cu 6 (111) > Pd ML (monolayer). Compared to the reaction on clean Cu(111) surface, the complete reaction pathways for CH 3 OH synthesis on PdCu(111) surfaces, especially on Pd ML, were facilitated and the yields of byproducts CO and CH 4 are suppressed, which corroborates well with experimental reports showing that Pd−Cu bimetallic catalysts have a strong synergistic effect on CO 2 hydrogenation to methanol. The present insights are helpful for the design and optimization of highly efficient Pd−Cu bimetallic catalysts used in CH 3 OH formation from CO 2 hydrogenation.

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Due to the miniaturization of microelectromechanical systems (MEMS), nanoelectromechanical systems (NEMS) and molecular devices, the problem of vacuum insulation becomes more and more prominent. The nanoscale thermal effects caused by electron emission and electric current Joule heat under high electric field lead to gasification and migration of material in the device. In this work, a coupled molecular dynamicselectrodynamics method is used to simulate the thermal evaporation of nanotips under high electric field. Moreover, Cu nanotips with different initial geometries and different macroscopic electric fields are modelled. The deformation and damage mechanisms of nanotips under high electric field are discussed. Our simulations show that the aspect ratio of nanotips has a significant influence on the thermal evaporation of nanotips. The thermal runaway occurring in picosecond time-scale plays an important role for the initiation of the vacuum breakdown. An empirical relationship is obtained between the on-set breakdown time and the macroscopic electric field and the geometry of nanotips by analysing the numerical results.

Exhaustion of landfill areas coupled with the extensive redevelopment programme in Hong Kong has prompted the use of recycled aggregate. However, the inferior quality of recycled aggregate (RA) has restricted its use to low grade applications such as roadwork sub-base and pavements, while its adoption for higher grade concrete is rare because of the lower compressive strength and higher variability in mechanical performance of RA. A new concrete mixing method: two-stage mixing approach was advocated to improve the quality of recycled aggregate concrete by splitting the mixing process into two. This paper describes two modified mixing methods with some alteration to the two-stage mixing approach by proportioning ingredients of cement and water with the percentage of RA added in the first mix, named as two-stage mixing approach (proportional 1) (TSMA p1 ) and proportioning the cement content (without water) with the percentage of RA used in the first mix, named as two-stage mixing approach (proportional-2) (TSMA p2 ). Based on experimental works and results, improvements in strength to RAC were achieved with both TSMA p1 and TSMA p2 . This can be attributable to the porous nature of RA and the pre-mixing process that fills up some of its pores and cracks, resulting in a denser aggregate and concrete. An improved interfacial zone around RA with lower water/cement ratio generated from TSMA p2 without changing the ultimate water/cement ratio gives a higher strength than the normal mixing approach (NMA) and TSMA p1 .