Yoshio Kosaka

Lateral confining stresses due to steel fibres in concrete under compression

The effect of various factors is examined on the inelastic deformation of steel fibre reinforced concrete under uniaxial compression and that of plain concrete under triaxial compression. By comparing the complete stress-strain curves obtained from the two series of experiments, the lateral confining effect of steel fibres on the inelastic deformational behaviour of concrete is discussed. It is found that the confining effect of steel fibres is similar to that of the lateral pressure of less than about 10 kg/cm2, and the effect of the orientation of steel fibre is reasonably evaluated by using the stress-strain curve of a specimen subjected to ill-balanced lateral stresses.

Quality evaluation of concrete exposed to high temperature by ultrasonic spectroscopy.

In recent years, it became possible to detect feeble electric signals supersensitively owing to the development of electric measuring techniques. Now, an advanced technique using ultrasonic pulses is being applied in the concrete field too. The waveform of ultrasonic pulses through concrete contains various informations related to the internal structure of material. So, it may be possible to clarify the internal structure of material by analyzing the measured waves in detail. Thus, ultrasonic spectroscopy is useful to evaluate the quality of material by measuring the frequency characteristics of ultrasonic pulse through material.In this study, the ultrasonic spectroscopy was applied to evaluate the quality of concrete exposed to high temperature conditions, as the first step to establish a new non-destructive testing technique for concrete and mortar.The main results obtained in this study are summarized as follows:1) Compressive and flexural strengths of concrete decreased according to temperature rise. Especially the flexural strength of mortar was sensitively affected by temperature rise.2) The ultrasonic pulse velocity, maximum amplitude and energy of measured ultrasonic pulse waves decreased according to temperature rise.3) The maximum amplitude and energy of frequency transfer function of concrete decreased with increasing temperature, and these decreasing rates were not affected very much by the water-cement ratio and heating duration.4) The local maximum amplitude and energy of the frequency transfer function in the frequency ranges of 50-100kHz and 200-250kHz were closely related to the change of internal structure of concrete exposed to high temperature.

Damping characteristics of acoustic emission propagating through concrete.

The damping characteristics of acoustic emission propagating through concrete were examined experimentally in order to establish the method of acoustic emission source wave analysis. The items chosen in the investigation were the effect of holding technique of sensor (Experiment-I), the effects of shape and size of specimen (Experiment-II), the effect of degree of heterogeneity of material (Experiment-III), and the effect of the degree of accumulative damage of specimen (Experiment-IV), respectively.The main results obtained in the present study are summarized as follows:(1) The frequency characteristics of acoustic emission picked up by the sensor were remarkably affected by its holding technique and the passing time after the sensor was held to the specimen, independently of the frequency characteristics of sensor or the degree of source acoustic emission.(2) The acoustic emission propagating through concrete diminished with increasing the length and the sectional area of specimen. The damping of the frequency components higher than 200kHz and 100kHz became more significant with increasing the propagating distance of acoustic emission and the sectional area of specimen, respectively.(3) Cement paste and mortar had the similar frequency characteristics of acoustic emission in the frequency range lower than 300kHz. In mortar, however, the frequency components higher than 300kHz diminished more than those of cement paste. On the other hand, the acoustic emission propagating through concrete diminished more remarkably in the whole frequency range than that of cement paste or mortar.(4) The frequency components of acoustic emission in the range of 100∼200kHz were prominent in undamaged concrete. But the frequency components in this range diminished gradually with the progress of accumulative damage of concrete by loading, and the low frequency components became prominent and the power of acoustic emission propagating through concrete decreased.