Minoru Kunieda

Tension softening diagrams and evaluation of properties of steel fiber reinforced concrete

Abstract For a better understanding of the fracture behavior of concrete structures, knowledge of the post-cracking behavior of concrete material is essential. The tension softening diagram can describe the post-cracking behavior of concrete in tension. In this paper, the properties of various kinds of steel fiber reinforced concrete (SFRC) were evaluated by means of the tension softening diagrams, after discussion of the applicability of the fictitious crack model to SFRC. The fracture energy of conventional SFRC was independent of the specimen size. For the evaluation of the properties of SFRC with different matrix strength, the shape of the tension softening diagram and the fracture energy were superior to the flexural strength. The fracture energy of SFRC with high strength matrix was dependent on the tensile strength of the steel fiber.

Application of tension softening diagrams to evaluation of bond properties at concrete interfaces

Abstract The bond properties at joint interfaces in concrete structures were evaluated through tension softening diagrams. For the joint surfaces between old and new concrete, the conditions to obtain good bond properties were clarified. The size effect on the flexural bond strength was well predicted through numerical analysis with tension softening diagrams. For the interfaces between repair materials and substrate concrete, the properties of repair materials and the conditions of interfaces were characterized by the shape of the tension softening diagrams and fracture energy. The shrinking and cracking behavior of repair materials was also investigated through experiments and numerical analysis.

Evaluation and Observation of Autogenous Healing Ability of Bond Cracks along Rebar

Micro cracks occurring in concrete around tensile rebar is well known latent damage phenomenon. These micro cracks develop, and can be detected after reaching the surface of the concrete. Detection of these cracks before they are fully formed is preferable, but observing the whole crack structure is difficult. Another problem is repairing micro cracks under the concrete surface. The autogenous ability of bond cracks along rebar was evaluated using the air permeability test. Air permeability coefficients were measured before and after tensile loading, and experimental air permeability coefficients became larger near cracks along rebar as a result of tensile loading. Recuring for 28 days after tensile loading made the air permeability coefficients smaller, but this restriction only occurred during water recuring. Observation of crack patterns helped the understanding of change in the air permeability coefficients. Several small cracks along rebar were observed after tensile loading, and most cracks along rebar were not found after water recuring. On the other hand, the crack pattern did not change after air recuring. These results indicate that bond cracks along rebar can be closed by autogenous healing, and cause the air permeability coefficients.

Rapid Jacketing Technique by Using UHP- SHCC for Damaged RC Column under Seismic Loading

This paper introduces a rapid jacketing technique for RC column, which involves spraying UHP-SHCC. Any formwork and/or additional reinforcement are not needed. The technique may be helpful in reducing construction time and cost. Cyclic loading tests were conducted for the column specimens repaired by UHP-SHCC and ordinary polymer cement mortar. It was confirmed experimentally that the developed technique using UHP-SHCC improve not only ultimate load but also ductility of recovered specimen. It seems that the prevention of buckling of longitudinal reinforcement, which was constrained by UHP-SHCC, imparts the mechanical improvement to the column specimens.

Analysis for Flexural Failure Behavior of PET Fiber Reinforced Cementitious Composites by Means of 3-D Meso-scale Analysis

This paper presents flexural analysis of PET (PolyEthylene Terephthalate) fiber reinforced concrete using 3-D meso-scale analysis to verify reinforcement mechanisms in PET fiber. In this proposed model, fibers with a specific length and diameter are distributed as discrete entities within a specimen. In this work, it has been clarified that the proposed model can adequately simulate load-displacement relations obtained from flexural tests of PET fiber reinforced concrete. This paper also presents numerically investigations in order to assess the effects of fiber length and strength on flexural toughness.

Pseudo Strain Hardening Behavior of Reinforced UHPFRC Member Under Uniaxial Tension

Strain hardening and multiple fine cracking behaviors are novel material properties of Strain Hardening Cementitious Composites (SHCC), and these can also impart novel function to structural members. Ultra High Performance Fiber Reinforced Composite (UHPFRC) has ultra-high strength over 180 MPa and significant dense matrix giving higher durability. In general, UHPFRC gives strain softening behavior in tension because of ultra-high strength matrix comparing to fiber bridging stress. Since ordinary applications of UHPFRC do not allow a crack, ordinary RC system with a reinforcing bar is not used.

Study on Effect of Heat Elimination by Pipe Cooling System in Beam Using High Strength Engineered Cementitious Composites

In the field of construction, the innovative technique using the coupling beam making use of characteristics of the Engineered Cementitious Composites (ECC) has been proposed, for instance the energy absorption by concentrating the seismic force in the core wall is implemented in high-rise R/C structures (Kanda 2006). Moreover, study with the aim of high strengthening of the coupling beam has been carried out. However, in the high strengthening coupling beam, possible temperature increase in a relatively massive element section exceeding 100 °C due to the heat of cement hydration has been concerned. When temperature in the member exceeds 100 °C due to the heat of cement hydration, the performance of organic fibers (the high-strength polyethylene fibers) is significantly reduced. Hence, in order to control the excessive temperature rise due to the heat of cement hydration, the analytical study was carried out for a pipe cooling system adopted as a countermeasure. On the basis of results of temperature measurement for the ECC coupling beam, design of the pipe layout to control the maximum temperature in the member lower than 90 °C was carried out and thermal properties were also identified. In this report, the experiments for evaluating the effectiveness of pipe cooling were described and the results were discussed.

Experimental Approach to Evaluation of Fiber Contribution in FRC

Short fibers such as steel, Polyvinyl Alcohol (PVA), Polypropylene (PP) and Polyethylene (PE) can contribute to the increase of not only tensile capacity but also shear capacity in Fiber Reinforced Concrete (FRC). There are many design guideline for FRC to estimate the fiber contribution. At a crack surface in FRC subjected to shear load, short fibers can resist shear deformation in addition to interlocking of aggregate and reinforcement. For reasonable design, exact fiber contribution should be evaluated experimentally. In this paper, fiber contribution was extracted experimentally by using melting method. In the experiment, FRC with PP fiber was used, and bending or shear crack was induced in the specimens. After that, cracked specimens were exposed to high temperature over 500 degrees, and only PP fiber bridging a crack was melted. It was clarified that the evaluated fiber contribution through this manner was much higher than that of ordinary calculation method.

Meso-Scale Analysis Considering Effect of Fiber Inclination in Fiber Reinforced Cementitious Composites

This paper introduces meso-scale modeling of fiber reinforced cementitious composites, which can consider the effect of fiber inclination. Fibers and matrix are modeled separately in this model, and each fiber is randomly arranged within the specimen models. The feature of this analytical model is to evaluate fiber resistance by calculating the pullout angle and the pullout displacement of each fiber. In order to investigate the effect of the pullout angle of the fibers on the load carrying capacity, fiber pullout analysis and bending analysis was conducted. It was confirmed that bending tests were more adequately represented by the modeling taking into consideration the pullout angle than by one without considering the pullout angle.

Impact Fracture Behavior of Ceramics and PE-Fiber-Reinforced Mortars

Ceramics will be used for power generating systems in the next generation. When they are used in this system, damage due to foreign object is inevitable. However, few systematic and comprehensive investigations have been reported on this subject. Various ceramics including fiber-reinforced mortars were investigated to understand their behavior when impacted by a spherical projectile. The volume of the cone cracks was large in ceramics which underwent transgranular fracture, while it was small in which underwent an intergranular one. Even though the energy consuming ability by the formation of surfaces was low up to 3.5% of the kinetic energy of a projectile, this ability increased with the ratio of the intergranular fracture to the transgranular one. Boron carbide showed a lower pressure as compared to the other ceramics. Fiber reinforcing increased the ballistic limits, but no clear advantage was suggested when absorbing the kinetic energy of a projectile far over its limits.