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RISS 인기검색어
- 검색키워드
- 저자 : Lek, Sophearika (검색결과 4 건)
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Interpretation of GPR data for condition assessment of underground reinforced concrete tunnels
Lek-Udom, Sathaporn Cleveland State University 2005 해외박사(DDOD)
This research evaluates the use of one of the most advanced Ground Penetrating Radar (GPR) systems for subway tunnel inspection. The purpose is to determine whether the current state of the art hardware, including the available GPR data processing software, is ready for employment as a stand alone nondestructive testing tool for that application. The study evaluated the use of ground-coupled radar for mapping specific defects in the concrete test slabs at early ages and determined how changes of moisture affected the GPR results. For laboratory testing, 14 concrete test specimens, 48 in x 48 in by 12 in, were poured. Three common defects including air gaps, voids, and honeycombing were simulated. For quality assurance, the freshly cast concrete specimens were subjected to required laboratory testing as well as the NDT, using Impact Echo and Moisture meter at 7, 14, 21, and 28 days. For field testing, three different reinforced concrete panels around the campus area were selected. The first panel was selected such that the concrete had low moisture content and was not subjected to a severe environment. The second panel was selected underground so that it had experienced salt and water penetration problems. The third panel consisted of severely deteriorated concrete with corrosion problems. The six NDT methods: Visual Inspection, Rotary percussion, Hammer Sounding, Ultrasonic Pulse Velocity, Impact Echo, and Moisture Meter, were conducted along with the GPR method. The results showed that the system did not provide the transmitted signals sufficient energy to penetrate through the entire depth of the reinforced concrete specimen. Second, the rebar scattering phenomenon posed three common effects, which decreased the performance of the GPR. Those effects included screening, ringing, and interference effects. Although the GPR can be used to map major defects, which were present in concrete at shallow depth, the responses from those defects were superimposed by the interference and/or ringing due to the rebar scattering phenomenon. According to the results, although modern GPR systems can provide valuable data and relatively rapid screening of concrete tunnels, there are still limitations of that system, which significantly restrict its performance as a stand alone tool for tunnel inspections.
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Foundations of numerical multilinear algebra: Decomposition and approximation of tensors
Lim, Lek-Heng Stanford University 2007 해외박사(DDOD)
The subject of this thesis is best described as the study of a few new problems in multilinear algebra that are analogous to important classical problems in numerical linear algebra. Among other things, we will study the problem of finding a best low rank approximation to a tensor, a symmetric tensor, and a nonnegative tensor; define a notion of eigenvalues and eigenvectors for symmetric tensors and a notion of singular values and singular vectors for general tensors; we apply our multilinear spectral theory to hypergraphs in a way that parallels spectral graph theory, and to nonnegative tensors in a way that parallels the results of Perron-Robenius; we will also study an extension of nonnegative matrix factorization to nonnegative tensors.
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