This paper reports on the frictional properties of smooth rubber substrates sliding against rigid surfaces covered with various densities of colloidal nano-particles (average diameter 77 nm). Friction experiments were carried out using a transparent Poly(dimethyl siloxane) (PDMS) rubber contacting a silica lens with silica nano-particles sintered onto its surface. Using a previously described methodology (Nguyen et al., J. of Adhesion 87 (2011) 235-250 ), surface shear stress and contactpressure distribution within the contact were determined from a measurement of the displacement field at the surface of the PDMS elastomer. Addition of silica nano-particles results in a strong, pressure-independent enhancement of the frictional shear stress as compared to the smooth lens. The contribution of viscoelastic losses to these increased frictional properties is analyzed in the light of a numerical model that solves the contact problem between the rubber and the rough surface. An order-of-magnitude agreement is obtained between experimental and theoretical results, the latter showing that the calculation of viscoelastic dissipation within the contact is very sensitive to the details of the topography of the rigid asperities.
We report on measurements of the local friction law at a multi-contact interface formed between a smooth rubber and statistically rough glass lenses, under steady state friction. Using contact imaging, surface displacements are measured, and inverted to extract both distributions of frictional shear stress and contact pressure with a spatial resolution of about 10 µm. For a glass surface whose topography is self-affine with a Gaussian height asperity distribution, the local frictional shear stress is found to vary strongly sub-linearly with the local contact pressure over the whole investigated pressure range. Such sub-linear behavior is also evidenced for a surface with a non Gaussian height asperity distribution, demonstrating that, for such multi-contact interfaces, Amontons-Coulomb's friction law does not prevail at the local scale.
The environmental impact assessment of the construction phase is often not fully considered compared to other phases of the project life cycle. Previous studies on environmental impact reduction have often focused on technical aspects rather than organisational aspects. The value stream mapping (VSM) method has been extended to capture and improve environmental performance by systematically adopting lean methods in the manufacturing process. However, in the construction field, this approach encounters difficulties establishing state maps and considering the interrelationships between different processes in an uncertain and dynamic environment. This study proposes a hybrid approach combining Multi-Agent Systems (MAS) and System Dynamics (SD) based on process patterns to overcome these obstacles. First, process patterns, including activity packages, are developed to assist the VSM in creating state maps and identifying environmental impact sources. Then, construction operations with their state maps and needed resources are modelled as autonomous agents containing causal-effect loops (SD modules) in a MAS model. These agents interact with each other to describe the construction operating mechanism. Finally, different lean methods are analysed to find opportunities to improve environmental performance.
Construction activities account for a significant amount of greenhouse gases (GHG) and fine particulate matter (PM10) emissions, particularly in developing countries with expeditious urbanization. To achieve sustainability in the construction phase, researchers have made considerable efforts to estimate the emissions accurately. Although several building-level emission databases and related calculation systems have been set up in developed countries, there unluckily remains a vacancy in Vietnam. This study aims to integrate Agent-based simulation (ABS) and process-based life cycle assessment (pLCA) based on construction norms and environmental data available in Vietnam to evaluate the GHG and PM10 emissions under the uncertain and dynamic conditions of on-site construction and supply chains. This method is developed and applied in the case study of a 20-storey building in Hanoi. The results indicate that, among all of the construction equipment, the off-site and on-site transport equipment are the dominant cause for GHG and PM10 emissions. The formwork related-material contributed the most to GHG among the auxiliary materials.
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