Concrete having high resistance to chloride attack has been demanded for coastal structures. The authors have developed a pozzolanic admixture that confers excellent resistance to chloride attack. The admixture can be used even in precast concrete factories without ample storage capacity such as extra silos. The study, which focuses on the properties of steam-cured concrete, considers the use of the admixture in precast concrete production. Concrete incorporating the admixture was investigated for chloride permeation resistance and its mechanism. The test results confirmed that the admixture (4% to 12% mass ratio mixture) improves chloride resistance of concrete. In addition, the results suggested that the admixture contributes to the increase in total volume of pores with diameter of 0.01 μm or less, and that chloride ions are immobilized by the generation of Friedel's salt.

One of the construction methods to improve the quality of concrete is re-vibration that gives vibration again after a certain time after initial vibration compaction. However, there is very little information on the specific implementation timing and method of re-vibration, and the actual situation is that re-vibration is performed based on sense and experience of the workers at the site. In this study, the concrete resistance value obtained from the N-type penetration depth was used as an index of the timing of re-vibration, and the effects of re-vibration timing and duration were examined. As a result, it was found that re-vibration was most effective when the concrete resistance value reached 17 × 10^-3N / mm². In addition, when the re-vibration duration was 5 to 10 seconds, the quality of the concrete was most improved. On the other hand, if the re-vibration timing was late or the re-vibration duration was long, improvement of concrete quality due to re-vibration could not be expected.

Past research on aging open channels made of reinforced concrete in cold regions has shown that the exposed parts of the channel sidewalls contain air voids and cracks filled with white precipitates. In this study, we took core samples from the exposed parts of open channel sidewalls, and performed microscopic observations to identify the precipitates filling the air voids and cracks. We used an electron probe micro analyzer to identify surface elements, and confirmed that precipitates had filled air voids and cracks. Scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray diffraction revealed that the precipitates were composed of ettringite. We surmise that the formation of the ettringite in the air voids and cracks is related to the elution of calcium ions around the cracks and the movement of sulfide ions from carbonated regions.

The purpose of this study is to clarify the effects of various influencing factors on bleeding test results of fresh concrete using small containers. We investigated the effects of container material and size，environmental temperature, filling method of sample, time interval of measurement, raw materials and mix proportions of concrete. The main conclusions are summarized as follows: 1) The bleeding amount measured by using a container with diameter (φ) of 125 mm and height (h) of 215.5 mm or φ150 mm-h255 mm closely approximated the test result of JIS A 1123 (container size: φ250 mm-h285 mm). 2) The test result for the bleeding rate using a container of φ125 mm-h 212.5 mm was slightly larger than that of JIS A 1123.