Concrete Research and Technology Vol. 28(2017)

This paper describes a study on the fundamental physical properties of concrete with rice husk ash, which a number of other researchers have already investigated. The study used Southeast Asian non-crushed rice husk ash (primary components: SiO₂, density: 2.41g/cm³, average size of aggregate: 80.1μm, specific surface area: 321,000 cm²/g). The purpose of this study was to examine suitable utilization forms of rice husk ash blended concrete. The test results show that rice husk ash can be blended in concrete as binder in the range of 25 to 80 kg/m³ and as mineral admixture in the range of 50 to 75 kg/m³.

To understand the conditions of frost damage in concrete, a freezing and thawing test was executed on microchannels. During the test, the straight channels stayed intact while the channels with ink bottle geometries suffered frost damage. The channels with ink bottle geometries avoided frost damage when air remained in their cavities. After water permeation, air remained in the channels having a large ratio of cavity to neck volume. Usually, air is entrained to fresh concrete to improve frost damage resistance. However, the above results indicate the important role of the air trapped during water permeation. On the basis of the results obtained for microchannels, cement paste was cured under reduced pressure to reduce frost damage. Freezing and thawing of hardened cement paste with AE water reducing agent caused surface deterioration. On the other hand, vacuumed cement paste suffered almost no damage. The above results indicate, under our experimental conditions, that vacuum curing is effective to reduce frost damage, and they validate the observed results for microchannels.

Cathodic protection controls the corrosion current of steel in concrete by polarizing the steel potential in the less noble direction. Furthermore, cathodic protection is accompanied by spin-off reactions such as increase in pH by the generation of OH^- ion and decrease in chloride ion concentration near the steel surface. In this study, a cathodic protection test was conducted to maintain depolarization values of 25 mV, 50 mV, and 100 mV for the steel in concrete specimens with different levels of chloride ion concentration. The protection effect was examined on the basis of the corrosion rate of the steel bars in concrete. The corrosion rate of the steel bars under cathodic protection was observed to decrease to less than 1.0 mA/m², indicating a passive condition. The results also showed that the corrosion rate of steel decreases under cathodic protection even for depolarization values of less than 100 mV, which is the standard value under cathodic protection conditions.

The purpose of this study was to investigate a method to estimate the deterioration area of fire-damaged concrete by the impact elastic wave method, performing measurement from the surface of the concrete. Based on the findings of this study, it is considered that the measured contact time between the surface of the concrete and the hitting hammer can be used to estimate the area of deterioration on the surface, and that multipoint measurement of the propagation time of the elastic wave on the surface of the concrete can estimate the depth of deterioration. However, the estimation accuracy was found to be lower in the case of slight deterioration.

This paper investigated the durability and hydration reaction of a 52-year-old RC structure to clarify the long-term durability of concrete using Portland blast-furnace slag (BFS) cement type C. The concrete was found to have undergone long-term strength development and to still retain calcium hydroxide and unhydrated BFS. Although it was observed that vaterite was produced and pore volume was increased by the carbonation reaction, C-S-H still remained and remarkable decomposition of hydration products, which is often seen in accelerated carbonation tests of concrete, was not observed. From these results, it is concluded that Portland blast-furnace slag cement type C can be applied to concrete structures that require long-term durability.

Full-scale shielding containers were fabricated with heavyweight concrete containing high-density (at least 4.0 g/cm³) metal slag-type aggregate at an actual plant. In consideration of the significant segregation of heavyweight aggregate during placing, the concrete was produced as segregation-free medium-fluidity concrete. The effects of the addition of an expansive additive and the vibratory consolidation time on the physical properties, surface permeability, and pore volume of the concrete were investigated. Vibration times 2 to 10 times longer than usual were found to have little effect on compressive strength, but substantially affected the elastic modulus, causing reduction of the surface water-shielding performance on the placing surface side. Compared with normal concrete, however, the pore volume was significantly smaller, and the surface permeability was also low even with such excessive vibration, thanks to the effects of a low W/P ratio and the addition of an expansive additive.

The objective of this study was to clarify the effects of angle of measurement on Surface Water Absorption Test (SWAT). There were several factors that could disturb accurate measurement of SWAT, such as moisture content of concrete, the difference between the temperature of the water used for SWAT and that of SWAT devices, and creep deformation of the rubber sponge attached to the water cup. The effects of these factors on SWAT were investigated and ways to eliminate the effects of those factors were considered. The effects of angle of measurement on SWAT were investigated using 3 different angles, 0°, 90°, 167°. It was verified by paired t-test under the conditions of this research that angle of measurement has no effect on SWAT results such as the water absorption rate at 10 minutes (p₆₀₀) and the total absorption amount in 10 minutes.

In this study, the stress transfer mechanism of steel fiber reinforced concrete (SFRC) interior beam-column joints with beam longitudinal headed steel bars was examined by non-linear 3-dimensional FEM analysis. The characteristics of the SFRC material were investigated by flexural testing and FEM analysis. Further, the behavior of headed anchors embedded in SFRC was investigated by pull-out testing and FEM analysis. Next, the behavior of SFRC interior beam-column joints with beam longitudinal headed steel bars was examined analytically using the calibrated results of these FEM analyses. Based on the results obtained, the stress transfer mechanism was identified as a double strut model involving the development of two struts in the beam-column joint under lateral loading. The first strut to be formed connects the compressive parts at the opposite beam ends. The other, subsequently formed, strut connects the compressive part at the beam end to the headed anchors of the beam longitudinal bars.