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Volume 4, Number 4, August 2007

A method based on the lower-bound theorem of limit analysis is presented for the capacity assessment of nodal zones in strut-and-tie models. The idealized geometry of the nodal zones is formed by the intersection of effective widths of the framing struts and ties. The stress distribution is estimated by dividing the nodal zones into constant stress triangles separated by lines of stress discontinuity. The strength adequacy is verified by comparing the biaxial stress field in each triangle with the corresponding failure criteria. The approach has been implemented in a computer-based strut-and-tie tool called CAST (Computer-Aided Strut-and-Tie). An application example is also presented to illustrate the approach.

Key Words
computer aided design; interactive graphics; concrete; struts; ties; limit analysis.

Tjen N. Tjhin; Buckland & Taylor Ltd., North Vancouver, British Columbia, Canada
Daniel A. Kuchma; Department of Civil and Environmental Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois, USA

The paper considers a theoretical model for the study of the process of transfer of sulfate ions in saturated porous media - mineral composites. In its turn, the model treats diffusion of sulfate ions into cement based composites, accounting for simultaneous effects such as filling of micro-capillaries with ions and chemical products and liquid push out of them. The proposed numerical algorithm enables one to account for those simultaneous effects, as well as to model the diffusive behavior of separate sections of the considered volume, such as inert fillers. The cases studied illustrate the capabilities of the proposed model and those of the algorithm developed to study diffusion, considering the specimen complex configuration. Computations show that the theoretical assumptions enable one to qualitatively estimate the experimental evidence and the capabilities of the studied composite. The results found can be used to both assess the sulfate corrosion in saturated systems and predict and estimate damage of structures built of cement-based mineral composites.

Key Words
diffusion; sulfate attack; cement paste; corrosion; mathematcal model; numerical analysis.

P. Gospodinov; Institute of Mechanics, Bulgarian Academy of Sciences,1113 Sofuia, G. Bonchev str., Bl. 4, Bulgaria
M. Mironova; Central Laboratory of Physico-Chemical Mechanics, Bulgarian Academy of Sciences,
1040 Sofia, Bulgaria
R. Kazandjiev; Institute of Mechanics, Bulgarian Academy of Sciences, 1113 Sofuia, G. Bonchev str., Bl. 4, Bulgaria

Corrosion of Reinforcement (rebar) is nondestructively estimated by the half-cell potential measurement. As is the case with other nondestructive testings (NDT), understanding of the underlying principles should be clarified in order to obtain meaningful results. Therefore, the measurement of potentials in concrete is analytically investigated. The effect of internal defects on the potentials measured is clarified numerically by the boundary element method (BEM). Thus, a simplified inversion by BEM is applied to convert the potentials on concrete surface to those on rebars, taking into account the concrete resistivity. Because the potentials measured on concrete surface are so sensitive to moisture content, concrete resistivity and surface condition, an inverse procedure to convert the potentials on concrete surface into those on rebars is developed on the basis of BEM. It is found that ASTM criterion is practically applicable to estimate corrosion from the potential values converted. In experiments, an applicability of the procedure is examined by accelerated corrosion tests of reinforced concrete (RC) slabs. For practical use, the procedure is developed where results of IBEM are visualized by VRML (Virtual Reality modeling Language) in three-dimensional space.

Key Words
NDE; inversion by boundary element method (IBEM); half-cell potential; corrosion; concrete resistivity; VRML (Virtual Reality modeling Language).

Je-Woon Kyung; Department of Architectural Environmental Engineering, Hanyang University,
7 Haengdang-dong, Seongdong-gu, Seoul, 133-791, Korea
Sung-Ho Tae and Han-Seung Lee; School of Architecture & Architectural Engineering, Hanyang University,
1271, Sa-1dong, SangNokgu, Ansan, Gyunggido, 425-791, Korea
Yalcin Alver; Graduate School of Science and Technology, Kumamoto University, Kumamoto, Japan
Jo-Hyeong Yoo; Department of Architectural Environmental Engineering, Hanyang University, Ansan, Korea

The compressive strength of concrete is a commonly used criterion in producing concrete. However, the test on the compressive strength is complicated and time-consuming. More importantly, since the test is usually performed 28 days after the placement of the concrete at the construction site, it is too late to make improvements if unsatisfactory test results are incurred. Therefore, an accurate and practical strength estimation method that can be used before the placement of concrete is highly desirable. In this study, the estimation of the concrete strength is performed using support vector regression (SVR) based on the mix proportion data from two ready-mixed concrete companies. The estimation performance of the SVR is then compared with that of neural network (NN). The SVR method has been found to be very efficient in estimation accuracy as well as computation time, and very practical in terms of training rather than the explicit regression analyses and the NN techniques.

Key Words
concrete strength; strength prediction; support vector regression (SVR); concrete mix proportion data; kernel function.

Jong Jae Lee; Department of Civil and Environmental Engineering, Sejong University,
Seoul 143-747, Korea
Doo Kie Kim and Seong Kyu Chang; Department of Civil and Environmental Engineering, Kunsan National University, Kunsan, Jeonbuk 573-701, Korea
Jang-Ho Lee; Department of Mechanical Engineering, Kunsan National University, Kunsan, Jeonbuk 573-701, Korea

Steel reinforced concrete is perhaps the most versatile and widely used construction material. The versatility is attributed to mouldability of concrete to any conceivable shape. The inherent property of cracking of concrete is the reason for its low tensile strength and hence the design approach of RCC sections in flexure adopts the cracked section theory where in concrete in tension zone is ignored. Means, modes and methods of exploitation of concrete strength by conceiving shapes other than rectangular whereby ineffective concrete in tension zone is reduced and incorporated in compression zone where it is effective needs consideration. Shape optimization of beams is attempted in this analytical investigation employing Sequential Unconstrained Minimization Technique (SUMT). The results clearly show that trapezoidal beams happen to be less costlier than their rectangular counterparts, their usage needs serious reconsideration by the designers.

Key Words
reinforced concrete; simply supported beams; uniformly distributed load; limit state design; shape; optimization

Department of Civil Engineering, National Institute of Technology Karnataka, Surathkal, PO: Srinivasnagar - 575 025, Mangalore, India

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