• Structural Engineering and Mechanics
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• v.64 no.4
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• pp.427-436
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• 2017

Concrete technologists have used ultrasonic pulse velocity test for decades to evaluate the properties of concrete. However, the presented research work focuses on the use of ultrasonic pulse velocity test to study the degradation in steel-concrete bond subjected to increasing loading. A detailed experimental investigation was conducted by testing five identical beam specimens under increasing loading. The loading was increased from zero till failure in equal increments. From the experimentation, it was found that as the reinforced concrete beams were stressed from control unloaded condition till complete failure, the propagating ultrasonic wave velocity reduced. This reduction in wave velocity is attributed to the initiation, development, and propagation of internal cracking in the concrete surrounding the steel reinforcement. Using both direct and semidirect methods of testing, results of reduction in wave velocity with evidence of internal cracking at steel-concrete interface are presented. From the presented results and discussion, it can be concluded that the UPV test method can be successfully employed to identify zones of poor bonding along the length of reinforced concrete beam. The information gathered by such testing can be used by engineers for localizing repairs thereby leading to saving of time, labor and cost of repairs. Furthermore, the implementation strategy along with real-world challenges associated with the application of the proposed technique and area of future development have also been presented.

• Structural Engineering and Mechanics
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• v.40 no.6
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• pp.847-866
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• 2011

The objective of this study was to experimentally investigate the bond-related tension stiffening behavior of flexural reinforced concrete (RC) beams made with lightweight aggregate concrete (LWAC) under various high-cycle fatigue loading conditions. Based on strain measurements of tensile steel in the RC beams, fatigue-induced degradation of tension stiffening effects was evaluated and was, compared to reinforced normal weight concrete (NWC) beams with equal concrete compressive strengths (40 MPa). According to applied load-mean steel strain relationships, the mean steel strain that developed under loading cycles was divided into elastic and plastic strain components. The experimental results showed that, in the high-cycle fatigue regime, the tension stiffening behavior of LWAC beams was different from that of NWC beams; LWAC beams had a lesser reduction in tension stiffening due to a better bond between steel and concrete. This was reflected in the stability of the elastic mean steel strains and in the higher degree of local plasticity that developed at the primary flexural cracks.

• Structural Engineering and Mechanics
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• v.37 no.1
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• pp.111-129
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• 2011

This paper presents a nonlinear analysis of reinforced concrete column with lap splice confined by FRP wrapping in the critical hinging zone. The steel stress-slip model derived from the tri-uniform bond stress model presented in the companion paper is included in the nonlinear frame analysis to simulate the response of reinforced concrete columns subjected to cyclic displacement reversals. The nonlinear modeling is based on a fiber discretization of an RC column section. Each fiber is modeled as either nonlinear concrete or steel spring, whose load-deformation characteristics are calculated from the section of fiber and material properties. The steel spring that models the reinforcing bars consists of three sub-springs, i.e., steel bar sub-spring, lap splice spring, and anchorage bond-slip spring connected in series from top to bottom. By combining the steel stress versus slip of the lap splice, the stress-deformation of steel bar and the steel stress-slip of bars anchored into the footing, the nonlinear steel spring model is derived. The analytical responses are found to be close to experimental ones. The analysis without lap splice springs included may result in an erroneous overestimation in the strength and ductility of columns.

• Structural Engineering and Mechanics
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• v.41 no.3
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• pp.379-393
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• 2012

Of the various methods of splicing reinforcing bar in reinforced concrete structure, mortar-filled sleeve reinforcement splice offers diverse benefits, not only in terms of structural performance but also for the construction process. Consequently, after the mortar-filled sleeve splices have been developed in recent years, research and development on these splices has been actively carried out, in order to evaluate its macro structural performance, such as its strength and stiffness, with the aim of enabling this system to be applied to construction in the field as early as possible. However, to make a proper evaluation on the overall structural performance of the mortar-filled sleeve reinforcing bar splice, it is of critical importance to understand the lateral confining action of the sleeve, which is known to affect the bond strength between the embedded bar and mortar in the sleeve. Accordingly, in this study, an experiment of monotonic loading and cyclic loading was conducted with a full-sized mortar-filled sleeve splice attaching strain gauges on the sleeve surface with experimental variables such as development length of bar, etc. Based on the test results, the effect of the lateral confining action of the sleeve was analyzed and considered in terms of the bond strength between the bar and mortar in this splice.

• Bulletin of the Korean Chemical Society
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• v.24 no.6
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• pp.797-801
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• 2003

One of the largest challenges for quantum chemistry today is to obtain accurate results for large complex molecular systems, and a variety of approaches have been proposed recently toward this goal. We have developed the ONIOM method, an onion skin-like multi-level method, combining different levels of quantum chemical methods as well as molecular mechanics method. We have been applying the method to many different large systems, including thermochemistry, homogeneous catalysis, stereoselectivity in organic synthesis, solution chemistry, fullerenes and nanochemistry, and biomolecular systems. The method has recently been combined with the polarizable continuum model (ONIOM-PCM), and was also extended for molecular dynamics simulation of solution (ONIOM-XS). In the present article the recent progress in various applications of ONIOM and other electronic structure methods to problems of homogeneous catalyses and nanochemistry is reviewed. Topics include 1. bond energies in large molecular systems, 2. organometallic reactions and homogeneous catalysis, 3. structure, reactivity and bond energies of large organic molecules including fullerenes and nanotubes, and 4. biomolecular structure and enzymatic reaction mechanisms.

• Proceedings of the Computational Structural Engineering Institute Conference
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• 1992.04a
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• pp.65-70
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• 1992

This study deals with the finite element analysis of the monotonic behavior of reinforced concrete beams and beam-column joint subassemblages. It is assumed that the behavior of these members can be discribed by a plane stress field. Concrete and reinforcing steel are represented by separate material models which are combined together with a model of the interaction between reinforcing bar and concrete through bond-slip to discribe the behavior of the composite reinforced concrete material. To discribe the concrete behavior, a nonlinear orthotropic model is adopted and the crack is discribed by a system of orthogonal cracks, which are rotating as the principal strain directions are changed. A smeared finite element model based on the fracture mechanics principles are used to overcome the numerical defect according to the finite element mesh size. Finally, correlation studies between analytical and experimental results and several parameter studies are conducted with the objective to estabilish the validity of the proposed model and identify the significance of various effects on the local and global response of reinforced concrete members.

• Structural Engineering and Mechanics
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• v.50 no.2
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• pp.163-180
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• 2014

Inaccurate predictions of effective stiffness for reinforced concrete (RC) columns having plain (undeformed) longitudinal rebars may lead to unsafe performance assessment and strengthening of existing deficient frames. Currently utilized effective stiffness models cover RC columns reinforced with deformed longitudinal rebars. A database of 47 RC columns (33 columns had continuous rebars and the remaining had spliced reinforcement) that were longitudinally reinforced with plain rebars was compiled from literature. The existing effective stiffness equations were found to overestimate the effective stiffness of columns with plain rebars for all levels of axial loads. A new approach that considers the contributions of flexure, shear and bond slip to column deflections prior to yielding was proposed. The new effective stiffness formulations were simplified without loss of generality for columns with and without lap-spliced plain rebars. In addition, the existing stiffness models for the columns with deformed rebars were improved while taking poor bond characteristics of plain rebars into account.

• Proceedings of the Korea Concrete Institute Conference
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• 2003.05a
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• pp.902-907
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• 2003

Substantial experimental and theoretical work exists on the bond characteristics of FRP-concrete adhesive joints. Analytic solutions based on fracture mechanics are most commonly accepted for theoretical work on joint. The solutions may be derived for the simple form of the shear strees-slip curve. And it is difficult to determine the model parameters consisting the curve. In this study, the bilinear curve with softening branch is introduced. The model parameters are determined by the method described by the companion paper with comparison of test results. There are many uncertainties in the test results of CFRP sheet adhesive joints, so that test results used for the construction of the regression problem should be reasonably selected.

• Structural Engineering and Mechanics
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• v.47 no.6
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• pp.773-790
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• 2013

Bending behaviors of corroded reinforced concrete (RC) beams under repeated loading were investigated experimentally. A total of twenty test specimens, including four non-corrosion and sixteen corrosion reinforced concrete beams, were prepared and tested. A numerical model for flexural and cracking behaviors of the beam under repeated loading was also developed. Effects of steel corrosion on reinforced concrete beams regarding cracking, mid-span deflection, stiffness and bearing capacity of corroded beams were studied. The impact of corrosion on bond strength as the key factor was investigated to develop the computational model of flexural capacity. It was shown from the experimental results that the bond strength between reinforcement and concrete had increased for specimen of low corrosion levels, while this effect was changed when the corrosion level was higher. It was indicated that the bearing capacity of corrosion beam increased even at a corrosion level of about 5%.

• Bulletin of the Korean Chemical Society
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• v.13 no.2
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• pp.173-176
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• 1992

The spectroscopic studies of the 5,7-dimethoxycoumarin(5,7-DMC) and 4,5',8-trimethylpsoralen(TMP) and the conformational stability of 5,7-DMC-thymidine, 4,5',8-TMP-thymine were carried out by the CNDO/S and molecular mechanics calculation. Theoretical transition energies and direction of polarizations calculated by the CNDO/S method have been used for the interpretation of the observed results. The calculated absorption spectra of 5,7-DMC are qualitatively similar to experimental ones with their characteristic visible bands. MM2 force field calculation on the possible $C_4-cyclophotoadducts$ formed between 5,7-DMC and thymidine through a cycloaddition. of $C_3$, $C_4$ bond of 5,6-DMC to $C_5$, $C_6$ bond of thymidine showed the most stable photocycloadduct to have the anti-head to tail configuration. The major photoadduct of 4,5',8-TMP-thymine has the cis-anti configuration.