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Stewart, Lauren K.

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    DEM analysis on the role of aggregates on concrete strength
    (Georgia Institute of Technology, 2019-10-02) Wang, Pei ; Gao, Nan ; Ji, Koochul ; Stewart, Lauren K. ; Arson, Chloé
    This study aims to understand the micro-mechanisms that drive fracture propagation in concrete and to assess the roles of the strength of aggregates and of the aggregate/mortar interfacial transition zone (ITZ) on concrete strength. We use the Discrete Element Method (DEM) to model concrete samples. Mortar is represented by a volume of bonded spherical elements. Bonds are governed by a new displacement-softening law. Aggregate centroids are randomly placed in the DEM sample. We use CT scan images of real aggregates to plot 3D aggregate contours. The spherical elements that are contained in 3D contours around the randomly placed centroids are replaced by clusters with aggregate properties. The number and the size of the clusters are determined from the experimental Particle Size Distribution. The DEM concrete model is calibrated against uniaxial compression tests and Brazilian tests of both mortar and concrete. It is found that: At same aggregate volume fraction, a concrete sample with randomly placed aggregates and ITZ bonds is stronger; Concrete strength is linearly related to aggregate tensile strength; A linear relationship exists between the contact ratio in the mortar/aggregate ITZ and concrete strength; The ITZ has more influence on concrete strength than aggregate tensile strength.
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    The Risks of Hydraulic Fracturing and the Responsibilities of Engineers
    ( 2017-03) Kirkman, Robert ; Arson, Chloé ; Stewart, Lauren K. ; Harris, Rebecca ; Francis, Amanda
    One third of U.S. natural gas is extracted by injecting fluid at high pressure into shale formations, a process associated with a number of possible hazards and risks that have become the subject of intense public controversy. We develop a three-part schema to make sense of risks of hydraulic fracturing and the responsibilities of engineers: the lab, the field, and the forum. In the lab, researchers seek to answer basic questions about, for example, the behavior of shale under particular conditions; there uncertainty seems to arise at every turn. In the field, engineers and others work to implement technological processes, such as hydraulic fracturing and the subsequent extraction of oil and gas; hazards may arise as natural and social systems respond in sometimes surprising ways. In the forum, the public and their representatives deliberate about risk and acceptable risk, questions that are framed in ethical as well as technical terms. The difficulty of characterizing – and in living up to – the responsibilities of engineers lie in part in the apparent distance between the lab and the forum. We examine in turn uncertainties in the lab, hazards in the field, and deliberation in the forum, leading to the conclusion that scientists and engineers can and should help to inform public deliberation but that their research cannot, on its own, resolve all controversies. Scientists and engineers who seek to inform deliberation should be mindful of the scope and limits of their authority, clear and modest in communicating research findings to the public, and careful to avoid even apparent conflicts of interest wherever possible. We close by drawing from the lab-field-forum schema to suggest a direction for pedagogical innovations aimed at the formation of responsible engineers in the context of college-level degree programs.
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    Workshop Report: "Risks and Policies of Hydraulic Fracturing: Assessment and Deliberation", Georgia Tech, November 13-14, 2014
    (Georgia Institute of Technology, 2014-11) Arson, Chloé ; Kirkman, Robert ; Stewart, Lauren K.
    Report of the workshop: Risks and Policies of Hydraulic Fracturing: Assessment and Deliberation, held at Georgia Tech, November 2014. Report includes the following: 1. Workshop program 2. Communication: \The Fracking Debate in Europe", Dr. Terry Engelder, Professor, Department of Geosciences, Pennsylvania State University 3. Presentation:\State Review of Oil & Natural Gas Environmental Regulations, Inc." Mr. Ryan Steadley, executive director, Stronger Inc. 4. Presentation:\Conveying the Science of Risk - Evolving Perceptions Within Non-Technical Audiences", Mr. Thomas Murphy, Director of Penn State Marcellus Center for Outreach and Research 5. Presentation: \Hydraulic Fracturing: An engineering perspective on modeling issues", Mr. Hao Xu (Ph.D. student), Ms. Amanda Francis (undergraduate research assistant), Dr. Chloé Arson (Assistant Professor), School of Civil & Environmental Engineering, Georgia Institute of Technology 6. Presentation: \Acceptable Risk and the Policy Context of Hydraulic Fracturing", Dr. Robert Kirkman, Associate Professor, School of Public Policy, Georgia Institute of Technology 7. Abstract presented at the 2015 Annual International Conference of the Association of Practical and Professional Ethics: \An Integrative Approach to the Ethics of Hydraulic Fracturing: A Report on Work in Progress", Dr. Robert Kirkman, Ms. Rebecca Harris, Ms. Amanda Francis, Mr. Hao Xu, Dr. Chloé Arson, Dr. Lauren Stewart, Georgia Institute of Technology