An experimental study on creep behavior in concrete at early age, which much influenced to thermal stress due to hydration heat has been done as a part of the study on the thermal stress. The important factors on this creep behavior in concrete at early ages are loaded age, loaded stress level, and temperature effects so on comparing to matured concrete. Several creep tests with several parameters mentioned above have been done in the laboratory, and investigation has been done to the results. Quantitative assessment was carried out and useful formula was presented on each parameter. Following conclusion could be obtained. 1) The effect of stress level was much significant rather than loaded age. 2) Effective age could be applied for taking the influence of temperature during curing and loading into account. The creep model with 5 Vogit elements was applied for the experimental results, which was obtained by varied load creep experiment. Good fitting results was obtained between analysis and experimental results for varied loading state.

View full abstract

As a fundamental study on applicability of fiber reinforced plastic rods (FRP rods) to prestressing tendons, fatigue and creep tests were performed for FRP rods using glass, aramid and carbon fibers, and the effect of mean stress, stress amplitude and applied stress on the fatigue and creep properties were investigated. A number of fatigue cycles to be ruptured was decreased with an increase in mean stress and stress amplitude for GFRP rods, and could be estimated using these parameters. Failure time due to creep is decreased with an increase in applied stress. The ratio of applied stress to tensile strength is in proportion to sustained time in logarithm, and the threshold value of applied stress in a service life could be estimated by the relationship.

View full abstract

Over the years, it was observed the changes of roughened surfaces, mass loss and decrease of relative dynamic modulus of elasticity. After more than 10 years, the bending performance of conventional concrete specimens immersed without lining decreased in comparison to specimens with lining. Based on microstructure and chemical analyses, the surface samples had more obvious changes than interior ones, with the degree of change depending on the concrete mixture and whether or not the surface had been coated with lining.

View full abstract

In maintenance of RC structures, the deterioration level is judged by visual inspection based on maintenance manual. And after then, detail inspection is carried out in order to determine whether repair or reinforcing is needed. Decision making of repair and reinforcing is carried out by administration maintenance manual based on the criterion of JSCE, JCI and soon.
This study is to seek the boundary values of repair and reinforcing using the survey data in situ by considering the records of past decision making of repair and reinforcing on thermal power plant's RC structures. Crack width and area of peeling off are chosen as parameters of chloride induced damage. Allowable boundary values on repair are obtained by inverse analysis based on reliability theory.

View full abstract

A crack is occured by many causes in the concrete structure. Epoxy resin in organic materials is usually used in repairing of a crack. Since its particle was large, properties of injection was poor in inorganic materials, and hence was not frequently used for injecting. But we have begun to use it for injecting to make its particle smaller in these days.
This paper investigates properties of inorganic materials and supposes injection time from its properties of injecting by using crack model.

View full abstract

To apply the high strength fibers in woven sheet shape to sewerage concrete structures for repair and strengthening, their long term durability in acid environment should be made clear as well as their load carrying behaviors. This paper describes the acid resistance of several kinds of fiber sheets, resins and FRP plates of their combination. Flexural strengthening of concrete prisms using the fiber sheets and the strengthening durability in acid solution were also discussed. Finally, the external loading test of hume pipes strengthened with the fiber sheets was conducted. The results indicate that appropriate combination of fiber sheet and resin can be adopted as one of the repair and strengthening methods for sewerage concrete structures.

View full abstract

The deterioration phenomena of concrete members, such as AAR, carbonation and chloride attack are closely related to ion migration in and through concrete. Also, electrically controlling the movement of ions in and through concrete is expected to be preventing or rehabilitating concrete structures damaged by chloride attack or carbonation (desalination, re-alkalization, etc.). However, the mechanism of electro-migration in and through concrete is still unclear.
In this paper, as one step to clarify the complete mechanism of electro-migration, the authors intended to clarify that the interfacial transition zones between paste and aggregates are the main passage ways of ions in electro-migration through the investigation of the influence of aggregate and curing conditions on the mobility of ion through mortar or concrete specimens.

View full abstract

The concept of the effective cover thickness was applied to predict the deformation and ultimate strength of reinforced concrete members subjected to pure torsion. The principal compressive strain was applied as an index of the effective cover thickness in order to evaluate the effect of cover thickness on the torsional strength. The predicted behavior using the reinforced concrete plate model showed good agreement with experimental data. When the concrete cover is spalled out at ultimate state, the shear flow should flow inner to the reinforcement cage. Therefore, the ultimate strength assuming the center line of lateral reinforcement as the perimeter of shear flow was investigated. The space-truss model was applied for calculating the ultimate torsional strength. It was found the proposed method was applicable for predicting the ultimate torsional strength of reinforced concrete members.

View full abstract

Exposure tests of concrete in marine environments were conducted over a period ten years to establish the rational design considered durability of concrete structures. In this tests, penetration of chloride ions, corrosion of reinforcing bar and quality change of concrete were investigated. The following result were obtained. (1) Chloride ion distribution can be predicted by using the chloride ion diffusion coefficient (Dc) and surface chloride ion density (Co) obtained from exposure tests. (2) The rate of chloride ion penetration in marine environments is fastest in the splash zone, followed by submerged and atmospheric zones. (3) In marine environments, properly treated construction joints and cracks under 0.05mm to 0.1mm in width do not significantly expedite the reinforcement corrosion in this experiment up to 10 years of exposure.

View full abstract

An experimental study on compressive and tensile creep for concrete and mortar has been carried out for early aged concrete in order to define the difference of both creep mechanisms. Tensile creep strain was slightly bigger than compressive creep train in the mortar, which had no coarse aggregate and tensile creep coefficient was equal to compressive creep coefficient. Change of pore size distribution supported the experimental results. However on both strain and coefficient, compressive creep was bigger than tensile creep in concrete which had the same mix proportion in part of mortar, and this tendency particularly observed in early age and low stress strength ratio below 20%. Since tensile creep of host rock of coarse aggregate was observed much smaller than compressive creep in creep test, it was concluded that the difference was influenced by creep behavior of coarse aggregates in early age and low stress strength ratio.

View full abstract

Recently, reinforcing the outside of concrete columns with high-performance fibers such as carbon or aramid have gained increasing interest. In order to clearly understand the structural effects of these reinforcements, the stress-strain relationship of the concrete must be determined. High-performance fibers demonstrates an increase in stress that is proportional to the strain until the material fails. For this reason, results on the compressive behavior of concrete confined with steel could not apply to the concrete confined with the fibers. The present study describes the effects of fiber type, shape, fiber ratio and compressive strength of the concrete on the compressive behavior. Based on the experiments, we propose an estimation method to determine the stress-strain relationship of the concrete.

View full abstract

We investigated experimentally placing of fresh concrete for diaphragm wall by visualization technique using model diaphragm wall. As a result, it was found that this experimental metod was suitable for the simulation of placing of fresh concrete for diaphragm wall. Furthermore, it was also found that casting velocity of the concrete was influenced by the viscosity of the concrete, and casting form of the concrete was influenced by the consistency of the concrete and execution conditions.

View full abstract

Bridge deck movements induced from traffic loading affect a buried joint as repetitive dynamic outer force. This kind of movement is not very long, but acting rate is very quick. In current structural design procedure, buried joint type and detail specifications are determined by longer thermal movement. It means that dynamic deck movement is not taken into account. In this study, dynamic movement influence toward the grid panel type buried joint was discussed. Actual bridge deck movement was quantitatively evaluated from field measurements, and fatigue failure envelope of asphalt layer was derived from cyclic bending test. Based on these investigations, fatigue life of buried joint layer was predicted in accordance with Miner's low. From the results of case studies applied for regional national highway, it was found that movement influence toward the grid panel type buried joint is so little as to be able to ignore, since influence of horizontal deck movement is hardly propagated to upper layer.

View full abstract

A micromechanical constitutive relationship is presented for the deformational behavior of steel fiber reinforced concrete under uniaxial tension. Its properties are characterized by a nonlinear interface between concrete matrix and fiber. The interface debonding is examined by the stress criterion expressed in terms of the interfacial shear stress. The present model can take into account the shear stiffness and the shear strength of the fiber-matrix interface, the frictional bond stress, the constraint condition of fiber end, and the effect of the inclination of fiber. The validity of the model is confirmed through simulation for the experimental data.

View full abstract

A new technique to modify the lattice model is described by the authors. This new method significantly depends on the calculation of the minimum total potential energy of the structure starting from the elastic stage up to the failure stage inside each increment of the calculation. Adoption of the minimum total potential energy for the structure is studied. Angle of inclination of the diagonals and the appropriate discretization method for the truss member are very important parameters affecting the results of the lattice model and are studied in this paper. The applicability of the Modified Lattice Model is examined by proposed shear strength equations and existing experimental data.

View full abstract

The moment tensor analysis of acoustic emission (AE) is reviewed, on the basis of the damage mechanics. The relation between the damage variable and the moment tensor is clarified. In bending tests of notched beams, the crack kinematics are determined. Tensile cracks are nucleated extensively at the outer region of the fracture process zone, while shear cracks are observed inside the zone and close to the final crack surface. The damage evolution process under the nucleation of the fracture process zone is estimated. It is found that the evolution process in cementitious materials is not necessarily dependent on the modes of final cracks, but associated substantially with the mechanisms of microcracking.

View full abstract