Evaluation of mechanisms governing the stabilization of unbound aggregate bases by geogrids in flexible pavements

The benefits of using geosynthetics to stabilize the unbound aggregate layers in flexible pavements, in terms of improvement to pavement life or potential decrease in layer thickness, have been well documented. Early research focused on developing ratios such as the traffic benefit ratio (TBR) or the layer coefficient ratio (LCR) to quantify (empirically) the benefits of including geogrids, and for their use in the design of flexible pavements with those geogrids. However, the lack of a mechanistic understanding of geogrid-stabilization and the empirical nature of this data limited their use to the geogrids and pavement materials that were used to develop them. With the proliferation of different geogrid products and pavement materials, the scope of the early research has become limited and attempts to correlate the properties of the geogrids and pavement materials to the improvement in performance of the pavement have not been successful. This study aims to further the understanding of the mechanisms involved in geogrid-stabilization and to identify the mechanistically relevant properties that contribute to this stabilization. Accelerated pavement tests are conducted, using the model mobile load simulator (MLS11), on reduced-scale pavement test sections with and without geogrids for stabilization under controlled, laboratory conditions. The performance of the pavement sections is evaluated by monitoring the deformation of the surface, the internal particle displacements (within the base), and the dynamic increase in stress (within the pavement structure) for increasing traffic volume. The deformation of the surface is obtained by profiling the surface of the pavement at regular traffic intervals using a laser profilometer, designed and built in-house for this study. A unique cost-effective displacement measuring technique is developed and implemented to obtain the horizontal displacement data of particles in the base. The data from the dense array of the particle tracking sensors is used to generate the horizontal displacement field, horizontal normal strain field and shear transfer efficiency plots in the base. The vertical stress distribution is obtained from the earth pressure cells installed within the pavement structure that are monitored for dynamic stress responses with applied traffic. The inclusion of the stabilizing geogrid resulted in reduced rut development, reduced particle displacements within the base and a wider distribution of the applied load for similar traffic volumes in the control and stabilized sections. The improvement in pavement life due to the stabilization of the base by the geogrid is quantified as the traffic benefit ratio (TBR). The TBRs are determined for seven different geogrids from the accelerated pavement tests, and correlated with the most commonly used in-isolation properties of the geogrid (geometric and tensile properties), and interaction properties of the geogrid-base aggregate composite. The TBRs are found to be best correlated to the coefficients of soil-geosynthetic composite stiffness (KSGC) obtained from the soil-geosynthetic interaction tests.Civil, Architectural, and Environmental Engineerin

Tube-enhanced multi-stage model predictive control for flexible robust control of constrained linear systems with additive and parametric uncertainties

The trade-off between optimality and complexity has been one of the most important challenges in the field of robust model predictive control (MPC). To address the challenge, we propose a flexible robust MPC scheme by synergizing the multi-stage and tube-based MPC approaches. The key idea is to exploit the nonconservatism of the multi-stage MPC and the simplicity of the tube-based MPC. The proposed scheme provides two options for the user to determine the trade-off depending on the application: the choice of the robust horizon and the classification of the uncertainties. Beyond the robust horizon, the branching of the scenario-tree employed in multi-stage MPC is avoided with the help of tubes. The growth of the problem size with respect to the number of uncertainties is reduced by handling small uncertainties via an invariant tube that can be computed offline. This results in linear growth of the problem size beyond the robust horizon and no growth of the problem size concerning small magnitude uncertainties. The proposed approach helps to achieve a desired trade-off between optimality and complexity compared to existing robust MPC approaches. We show that the proposed approach is robustly asymptotically stable. Its advantages are demonstrated for a CSTR example

Heterogeneous hierarchical workflow composition

Workflow systems promise scientists an automated end-to-end path from hypothesis to discovery. However, expecting any single workflow system to deliver such a wide range of capabilities is impractical. A more practical solution is to compose the end-to-end workflow from more than one system. With this goal in mind, the integration of task-based and in situ workflows is explored, where the result is a hierarchical heterogeneous workflow composed of subworkflows, with different levels of the hierarchy using different programming, execution, and data models. Materials science use cases demonstrate the advantages of such heterogeneous hierarchical workflow composition.This work is a collaboration between Argonne National Laboratory and the Barcelona Supercomputing Center within the Joint Laboratory for Extreme-Scale Computing. This research is supported by the U.S. Department of Energy, Office of Science, Office of Advanced Scientific Computing Research, under contract number DE-AC02- 06CH11357, program manager Laura Biven, and by the Spanish Government (SEV2015-0493), by the Spanish Ministry of Science and Innovation (contract TIN2015-65316-P), by Generalitat de Catalunya (contract 2014-SGR-1051).Peer ReviewedPostprint (author's final draft

Evolutionary Search and Theoretical Study of Silicene Grain Boundaries' Mechanical Properties

Defects such as grain boundaries (GBs) are almost inevitable during the synthesis process of 2D materials. To take advantage of the fascinating properties of 2D materials, understanding the nature and impact of various GB structures on the pristine 2D sheet is crucial. In this work, using an evolutionary algorithm search, we predict a wide variety of silicene GB structures with very different atomic structures compared to those found in graphene or hexagonal boron-nitride. Twenty-one GBs with the lowest energy were validated by density functional theory (DFT) - a majority of which were previously unreported to our best knowledge. Based on the diversity of the GB predictions, we found that the formation energy and mechanical properties can be dramatically altered by adatoms positions within a GB and certain types of atomic structures, such as four-atom rings. To study the mechanical behavior of these GBs, we apply strain to the GB structures stepwise and use DFT calculations to investigate the mechanical properties of 9 representative structures. It is observed that GB structures based on pentagon-heptagon pairs are likely to have similar or higher in-plane stiffness and strength compared with the zigzag orientation of pristine silicene. However, an adatom located at the hollow site of a heptagon ring can significantly deteriorate the mechanical strength. For all the structures, the in-plane stiffness and strength were found to decrease with increasing formation energy. For the failure behavior of GB structures, it was found that GB structures based on pentagon-heptagon pairs have failure behavior similar to graphene. We also found that the GB structures with atoms positioned outside of the 2D plane tend to experience phase transitions before failure. Utilizing the evolutionary algorithm, we locate diverse silicene GBs and obtain useful information for their mechanical properties.Comment: 25 pages, 9 figure