Ultra high strength fiber reinforced concrete has the excellent properties, such as high strength, high flowability, and high ductility with short fiber reinforcement. To realize actual structures, it is necessary to clarify the bond stress transmission mechanism in the interface between UFC and reinforcing bars. In this study, the relationship between localized bond stress and slip, and the average strength as the bond stress transmission mechanism are investigated. To clarify the relationship between localized bond stress and slippage, the pull-out test is carried out. Furthermore, to clarify the average bond strength, bending tests with RC beams having the lap splice are carried out.

View full abstract

In order to elucidate mechanisms of explosive spalling of concrete due to heat, high-strength concrete specimens with moisture contents of furnace-dried, air-dried and wet were tested. In the test, acoustic emission (AE) technique was applied to monitor micro-cracking process. Internal temperature and vapor pressure were also measured to evaluate the effect of moisture contents on explosive process. Under high temperature, AE events were effectively detected in accordance with the explosive failure process and their locations were in good agreement with spalled areas. In contrast to air-dried and wet specimens, no explosive failure was observed in the furnace-dried. Thus, the effect of the vapor pressure on the explosive failure is confirmed.

View full abstract

When we retrofit the reinforced concrete pier located in the ground, usually it is necessary earthretaining and excavation etc. for the construction work. It causes the increment of cost and length of construction term. So we developed the seismic retrofitting method which retrofit the existing reinforcement concrete pier by diagonal reinforcement after constructed from the over ground. And we experimented by reversal cyclic loading test. We verified the seismic retrofitting effects by this method and relation between this effects, damage pattern, and shear capacity ratio after retrofitting etc. by the test. This paper mainly reports these results of examination and study by the test.

View full abstract

Nondestructive evaluation of corrosion in reinforced concrete is investigated. Employing the thermal on concrete surface by the thermography, heat stored in reinforcing steel-bar (rebar) due to electro-magnetic heating is applied to estimate qualitatively the corrosion degree. Results show a great promise for estimation of corroded rebars.
The thickness of corroded layer in rebar is identified by the inverse analysis, which has been developed on the basis of the history of temperature distribution on concrete surface. An applicability of the analysis is demonstarated by the comparison between corrosion thickness analyzed and those measured.

View full abstract

Although the quality of fly ash is classified by JIS, the effective use of fly ash as a mineral admixture of concrete has been hindered because there is a concern that the quality of concrete with fly ash is largely fluctuated depending on the quality of fly ash. In addition, although there have been many studies on concrete with fly ash as a part of fine aggregate, the method of quality control of the concrete with fly ash has not yet been sufficiently established. In this study, an easy method to evaluate the quality of fly ash was developed, and then the influence of fly ash quality on the fresh property of concrete with fly ash was experimentally investigated. As a result, a simplified method of measuring the quantity of un-burnt carbon, the most important factor in fly ash, was proposed. The quality evaluating method of concrete with fly ash by a simplified mortar test was also proposed.

View full abstract

The influence of chloride ion concentration and prestress on the corrosion of prestressing steel reinforcement embedded in grout was investigated in this study. The novel point of this study is the investigation of the chloride corrosion threshold under prestress. Examined levels of chloride concentrations were from 0.02 to 3.6 mass% of cement. Applied stresses for prestressing steels were 0.7 and 0.8 of yield strength. The corrosion behavior of the prestressing steels under prestress was evaluated using electrochemical measurements such as half-cell potential, polarization resistance, and polarization curve and dismantlement corrosion investigations. Based on the experimental results in this study, two major conclusions were obtained. 1) 0.09 mass% of chloride is acceptable in grout with low risk of corrosion regardless of the practical prestressing level. 2) The prestress shows no effect on corrosion initiation and propagation if the prestress level is controlled in a practical stress level in usual constructions less than 0.8 of tensile strength.

View full abstract

This paper presents a three-dimensional RC constitutive model under multi-directional and non-orthogonal cracking. Active crack approach for handling complex cracking state is strictly adopted for spatially inclined cracks with multi-directional cracks in three dimensional space. Verification of the proposed model is examined with a numerical simulation of six-way cracking and comparison with RC panel experiments with multi-directional cracks up to 4 directions under uniform stress field. Further examination for applicability to RC structure are conducted with comparing rectangular and cylindrical wall tests with multi-directional loadings and RC column tests under combined flexural moment and torsion, and good estimation is shown.

View full abstract

We conducted a three-dimensional non-linear dynamic analysis on a multispan-connected viaduct with RC piers designed after the Hyogo-ken Nanbu Earthquake using as reference several different types of earthquake motion including a long-period ground motion represented in the recent Tokachi-Oki earthquake and Tokai earthquake which is assumed to occur in the future. We made observations on the relationship between ground motion characteristics and the behavior of RC piers in the connected bridge. Findings were that depending on a support's condition and the size of the bridge, connected viaducts' characteristic period of vibration can become longer and because of the vibration's long-period constituent, ground motion and the connected bridge could produce a resonance phenomenon that could lead to the larger response value from RC piers. We also discovered that 3D fiber model is effective to do a detailed evaluation on the a seismic capacity of RC piers.

View full abstract

The purpose of this research is to study the durability of porous aggregate concrete against chloride attack by examining the influence of porous aggregates on chloride ion (Cl^-) diffusivity of concrete and Interfacial Transition Zone (ITZ) properties, and to investigate the influence of curing conditions; which are curing temperature and curing period; on Cl^- diffusivity and ITZ properties of concrete using four types of aggregate. Three types of porous aggregates; Japanese artificial lightweight aggregate, coral aggregate and crushed clay brick aggregate; are employed to compare with crushed limestone aggregate. Three different water-cement (W/C) ratios are used in this study. The properties of ITZ around coarse aggregates, such as the thickness and the Cl^- diffusivity of ITZ, are measured and used to explain the influence of porous aggregates and curing conditions on the Cl^- diffusivity of concrete. The results show that Japanese artificial lightweight aggregate concrete has lower Cl^- diffusivity compared with normal aggregate concrete because the Japanese artificial lightweight aggregate has a dense outer crust, thus Cl^- can not penetrate through aggregate and ITZ phase. Coral and brick aggregates, which are poor quality aggregates, have very high water content when they are in saturated surface-dry condition causing a thicker water film around aggregates and Cl^- can easily penetrate through them. Therefore, concrete made from coral and brick aggregates have higher Cl^- diffusivity and show expectedly poor performance under marine environment. Finally, the improvement for the poor quality aggregates by using dry aggregates for mixing concrete is successfully proposed in this study.

View full abstract

This paper describes the results of an experimental study and nonlinear finite element method in order to examine the shear failure mechanism of segmental prestressed concrete beams by varying the length of segment, prestressing force and loading position. It is observed that after the opening of the segmental joints, the stiffness of such beams considerably decreases and the tendon stress is caused to increase significantly. The simplified truss model and other existing prediction methods for shear carrying capacity have been applied in this study in order to examine their applicability and accuracy. The simplified truss model can provide reasonable accuracy under the limitation of applied prestressing force.

View full abstract