1H NMR relaxation time of fluids confined in porous materials gives us information on pore structures. The purpose of this study is to establish the methodology for the estimation of time-dependent pore structure development in cement paste using 1H NMR relaxation times. We carried out 1H NMR relaxation time measurements of cement paste at 20 min curing intervals up to 72 h. Nitrogen adsorption and XRD measurements of cements prepared under the same conditions as those used for NMR measurements were performed to estimate pore size distribution and composition, respectively. By comparing the NMR results and pore size distribution, we found that 1H NMR relaxation time can identify the proton species in medium (10 to 50 nm) and large (50 nm to 1 μm) capillary pores in the cement. 1H NMR relaxation time measurements of cement paste should be a powerful tool for non-destructive and continuous monitoring of pore structure during cement solidification.
Taking into consideration the climatic conditions in the Tohoku region, a thermal cracking hazard map and a frost damage hazard map were made using GIS. Then, a data map based on the inspections of 2600 bridges administered by the Tohoku Regional Bureau of the Ministry of Land, Infrastructure, Transport and Tourism, and a data map on deicing salt application volumes administered by the same, were made. The analysis of the deterioration factors for RC bridge decks, RC bridge piers and abutments revealed that these members undergo deterioration under the influence of multiple factors consisting of the number of years in service, large vehicle traffic volume, deicing salt application volume, and average ambient temperature in winter. The tool created as detailed above was found to be useful for bridge management by local governments in the Tohoku region.
It is known that the compressive strength of concrete will increase as the powder is mixed into the concrete at constant cement content, even in the case of non-hydraulic powder. Despite this, the understanding of the mechanisms is very limited, and this has prevented the establishment of methods for estimating the compressive strength from the mix proportion. In this paper, the authors focus on the phenomenon of the pore size becoming small even if the total pore volume is constant by mixing in various types of powder, and the volume of pores of 50 nm or larger, which have a negative effect on the compressive strength, decreasing. A method for estimating compressive strength, applicable to concrete mixed with various types of powder at constant cement content, was evolved.
Methods to evaluate the strength and deformation capacity of reinforced concrete walls with openings were studied using a wide range of experimental data. Firstly, a method to multiply the shear strength of a wall by a reduction factor due to opening proposed by AIJ in 2010 was examined. Next, a method adding up the strengths of columns with wing walls besides an opening was examined. In these examinations, the currently used empirical method and the theoretical method based on the truss-arch analogy were considered. Finally, an evaluation method for obtaining the deformation capacity by using an effective factor of concrete strength to calculate the shear strength based on the theoretical method was examined.
Recently, to minimize thermal cracks that occur due to excessive temperature differences, several slits have been provided as a crack-prevention measure in the concrete guard walls of bridges. However, this practice may affect the amount of stress in steel girders due to insufficient stiffness of these bridges. This paper presents an investigation of the effect of crack-prevention slits on steel girders through the use of FEM analysis. A RC deck bridge with 4 steel plate girders was used as the model bridge. Our analysis revealed that the stress in the lower flange of exterior girders exceeds that of bridges that have no concrete guard wall when the slits are provided at a long distance from each other.
Recently, a number of concrete structures constructed several decades earlier are showing significant degradation, especially in coastal areas. For the effective repair and reinforcement of such structures, the development of high performance repairing material is required. Meanwhile, ion exchange resin has traditionally been used to remove impurities such as chloride in the process of cleaning industrial wastewater and manufacturing pure water. In this study, for the purpose of removing the chloride in deteriorated concrete structures, the application of ion exchange resin to repairing material is investigated. Firstly, the effect of ion exchange resin on the adsorption of chloride ion in mortar is examined by chemical analysis. The diffusion of chloride in mortar with ion exchange resin is experimentally and numerically studied. Finally, it is experimentally verified that the chloride in mortar is removed by inverse diffusion due to the ion exchange resin.
The cement paste adhering to recycled aggregate increases the unit weight of water and reduces the confinement effect of aggregate. Therefore, the authors presented a method to estimate the increase in unit weight of water and the reduction of the confinement effect of aggregate by means of regression analyses using data in references. Next, a formula to predict dry shrinkage for concrete made with recycled coarse aggregate, based on the JSCE prediction formula for normal concrete, was developed by taking into account the increase in unit weight of water and the reduction of the confinement effect of aggregate. Since the proposed method does not require any tests to estimate the characteristics of the cement paste adhering to recycled aggregate, it allows easy calculation of shrinkage strain. As a result of verification using prior experimental results, it was concluded that the predicted value of dry shrinkage of recycled concrete has sufficient precision comparable to that of the prediction of dry shrinkage of normal concrete using the JSCE prediction formula.
As one method of improving the freeze-thaw resistance of concrete using moderate heat cement mainly used in concrete dams, we examined the effects of air stability entrained by AE agent. The amount of bleeding was changed from 0.15 to 0.50cm3/cm2 by mixing two types of fine aggregate, and the air content of fresh concrete was further adjusted from 4.5 to 8.0% by using three types of AE agent with different components. In each case using AE agent, the value of the air-void spacing factor tended to become larger owing to the increase of the amount of bleeding in fresh concrete with an air amount of less than 6.0%. However, freeze-thaw resistance was found to be highly dependent on the components of the AE agent. In the case of AE agents with durability factor of less than 60, the number of air voids less than 0.15mm in diameter was observed to be lower in the hardened concrete. These experiments confirmed that, although freeze-thaw resistance can be reduced because of the coalescence of bubbles in fresh concrete and defoaming due to bleeding, which reduces the number of fine air voids, some components of the AE agent were capable of suppressing this effect.
This paper discusses the effects of reinforcing steel corrosion on the shear resisting mechanism of RC box culverts and the applicability of the material degradation model in a finite element method. First, in FEM analyses, loss of reinforcement section area and initial tension strain due to reinforcement corrosion, and deteriorated bond characteristics between reinforcement and concrete, were considered. Second, cyclic loading tests using full-scale corroded specimens were numerically analyzed. The analyzed crack patterns and load-displacement relationships up to the maximum load were observed to be in close agreement with the experiment results within the average corrosion ratio of 10% of primary reinforcement. The fact that corrosion cracks can importantly affect the progression of shear cracks and shear strength of RC beams was also found. On the other hand, we established that RC box culverts being statically indeterminate structures, sectional forces are redistributed after cracking damage, and local material deterioration has a minor effect on shear capacity. Furthermore, a parametric study was carried out for corroded RC box culverts using parameters such as size, steel corrosion location, and corrosion level.
The aim of this paper is to examine the shear resistant mechanism of r/c columns with an outer shell structure consisting of a divided thin steel plate with ribs and polyethylene fiber sheets when subjected to shear force. The steel plates do not require any welding on site. This paper mainly describes the shear effectiveness achieved through the use of ribs on steel plates in extremely short columns. The test results found that the shear resistance achieved through the elements of lateral ribs, web plates and polyethylene fiber sheets, is cumulative. Based on these results, the study indicates that shear capacity can be estimated by using the design guideline for ultimate shear strength issued by AIJ in 1990, which is based on the truss and arch model.
Capillary pore structures evaluated by mercury intrusion porosimetry for cement pastes were compared with ones characterized by SEM image analysis. Characteristic values defined in the Katz-Thompson equation were related to coarse capillary porosity of the image analysis. If a sample-spanning path of pores is formed using only the clusters of coarse pores, the formation factor increases with the coarse capillary porosity. Taking account of little changes in the formation factor below a certain coarse porosity, which is almost the same as a threshold value in the percolation theory, the coarse pores detected in the image analysis play the role of the backbone or pivotal paths to determine the entire conductance of cement paste.