The purpose of this study is to clarify the effect of silica fume and fine fibers on the freeze-thaw resistance of porous concrete. The crack-inhibiting effect of fibers was also investigated by microscopically observing and quantitatively evaluating microcracks along and near interfaces between the matrix and aggregate, which cause low durability. As a result, the length and width of microcracks in fiber-reinforced porous concrete were found to widely differ from those of cracks in unreinforced porous concrete.
In order to evaluate the effect of the water supply curing condition on the physical properties of concrete, pore size distribution of concrete cured under various moisture conditions has been determined. As a result, it became clear that pore volume decreases with extension of water curing and compressive strength has high correlation with pore volume. Moreover, it became clear that good correlation was seen between the pore volume and the volume of hydration products calculated in consideration of the influence of water retaining condition on the hydration velocities of cement. Furthermore, we investigated the estimate method of the parameters of pore size distribution function from the calculated volume of hydration products and the possibility of prediction of the pore size distribution of the concrete cured under various conditions was shown.
This study was conducted to identify the mechanism of steel corrosion in the re-deterioration of reinforced concrete that was earlier deteriorated due to salt attack; and repaired by partially restoring the cross section. In the test, a series of specimens were arranged to simulate a part of a reinforced concrete repaired in the deck of road bridge. The depth and area of cross section to be repaired were varied in cases with and without surface coating. The specimens were then exposed in an oceanic environment. Steel was taken from the specimens at one, three, and ten years after the start of exposure to investigate the state of corrosion. As a result, it was revealed that the rate of corrosion of steel in concrete was reduced with time as long as the surface coating remained sound. It was also verified experimentally that the corrosion of reinforcement in the area plus and minus 10 mm from the interface between the base concrete and the repaired area of cross section progressed further than in other parts owing to macro-cell corrosion in the case where the partial restoration method was applied for cross section. The rate of corrosion of reinforcement near the boundary between repaired and non-repaired areas was also presented.
It is essential for the mix design of fresh concrete to provide adequate segregation resistance. The segregation resistance should be considered based on the amount of bleeding after placement of concrete. This paper discusses the mechanism of bleeding and proposes the prediction model of bleeding content under a given mix proportion of concrete. Since the bleeding content is an amount of extra water which is not retained by both cement particles and fine aggregate, the model consists of water retaining mechanism on both cement particles and fine aggregates. The characteristics of the model are as follows; (1) the influence of flock of cement particles is taken by water cement ratio, and (2) the amount of retained water by fine aggregates is estimated using the relationships between the relative flow area ratio and the water-cement volume ratio determined by a mortar-flow test. The effectiveness of the model is proved sufficient by experimental test results varying the mix proportion of 45 cases.
A simulation model to estimate the drying shrinkage of mortal and concrete based on the pore structure of composition materials by curing in arbitrary relative humidity is presented. This paper describes procedures for predicting drying shrinkage of cement paste and aggregates based on the pore size distribution measured by mercury porosimetry, the water content in pore structure under arbitrary humidity and change of surface energy inside of cylinder type pore. And the drying shrinkage of mortar or concrete was evaluated using the theory of composite material. To evaluate the effectiveness of this model, simulation results were compared with experimental results. As a result, it was found that the experimental and simulated results were in close agreement, and the effectiveness of simulated model based on the pore structures of composition materials was verified.
We investigated the relationship between the spalling behavior and internal vapor pressure of high strength concrete (HSC) with natural fiber when heated. The concrete specimens had a water-cement ratio of 0.30 (fc ≈ 70 MPa). The fibers used were natural jute fiber and PP fiber. The length of the jute and was 12 mm and its addition rates were 0.075% and 0.15% by volume, respectively. The HSC specimens were heated and their internal temperature and vapor pressure were then measured. Explosive spalling did not occur in the jute specimens.