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.
It is important to quantify the permeability of chloride ions in steam-cured concrete to evaluate the durability of precast concrete. However, until now, there has been little research on this issue. In this study, the diffusion of chloride ions was determined by an exposure test on steam cured concrete specimens and seal cured concrete specimens. The permeability of chloride ions was estimated based on the diffusion of chloride ions. The results indicated that exposure of steam cured specimens that contain a large amount of non-reactive cement to an environment that supplies ample moisture reduces the diffusion of chloride ions. Moreover, the chloride ion diffusion coefficient of concrete was found to decline with exposure time.
The purpose of this study is to calculate the bridging law based on the slip-out characteristics of aramid single fiber and polypropylene (PP) single fiber and to investigate its adaptability to tensile performance evaluation of fiber-reinforced cementitious composites (FRCC). The pullout load-slippage relationship, which is the most basic characteristic of bridging performance, was obtained by the pullout test of a single fiber embedded in a matrix. The bond behavior, snubbing effect and tensile strength degradation were confirmed by the experiment results. The pullout load-slippage relationship was modeled based on the experimental results. The bridging law was calculated by using the model. The calculated results were found to express well the results of the uniaxial tension test that was conducted for comparison purposes.
Progress of carbonation of actual structures is greatly affected by material factors such as kind of cement, water cement ratio, mix proportions and environmental factors such as temperature, humidity, carbon dioxide concentration and exposure to rainwater. In this study, we investigated carbonation depth in real concrete structures constructed using blast furnace cement about 50 years ago under various rainwater exposure conditions. In addition, the mechanism of carbonation progression in blast furnace concrete was examined using chemical analysis. The results of this investigation revealed that exposure to rainwater has a remarkable influence on the progress of carbonation, that there is a relationship between pH level and calcium carbonate production, and that the mechanism of carbonation progression thus differs according to the environment.
Ductile Fiber-Reinforced Cementitious Composites (DFRCC) are cementitious materials mixed with short fibers. It is well known that tensile and bending characteristics of DFRCC are affected by fiber orientation and dispersion. In this study, the relationships between tensile stress and crack width (bridging law) are evaluated considering fiber dispersion in order to investigate the variation in tensile characteristic of DFRCC. The fiber dispersion is evaluated by Poisson distribution from the results of visualization simulations using sodium silicate solution (water grass) and introduced in the calculation of the bridging law. The variation of maximum tensile stress (bridging strength) can be confirmed by Monte Carlo simulation, in which the fiber dispersion following Poisson distribution is considered. The calculated bridging law is modeled by trilinear model, and section analysis is conducted to compare with test results. The possibility to assume the variation of bending strength can be found out by considering fiber dispersion.
Air-void structures of concretes were evaluated by two methods. One is the traditional procedure of ASTM C457, the linear-traverse method. The other is a method in which a set of air voids is treated as a spatial point process. The sizes of the air voids identified and the assumed spatial arrangements differ between these two evaluation methods. As a result, the characteristic distances defined by the nearest neighbor distance function for air voids are different from the traditional spacing factors. However, random properties and distances between the voids that were evaluated by the point process procedure did not contradict the properties evaluated by the linear-traverse method. The method based on the point process statistics is promising as an easy method for evaluating air-void systems in concretes.