Based on a review from a view point of multi-scale perspective, it is concluded that possible mechanisms of natural frequency change of concrete structures under an ordinary drying condition are reduction of Young’s modulus of concrete and stiffness reduction of reinforced concrete member due to shrinkage-induced cracking. Both two mechanisms can be provoked by drying, which is natural environmental condition for all the building structures. This natural frequency change is important for the integrity analysis of nuclear power plants because seismic response change and sympathetic vibration between structure and installed implementations are expected. In addition, to understand the mechanism this mechanism should contributed to post-earthquake integrity evaluation which is necessary to efficient and continuous operation of nuclear power plants. The lost parts of scientific knowledge and needs of experimental data for whole picture of this natural frequency change have been pointed out.
This paper presents an electro-mechanical model for evaluating the corrosion-induced damage in reinforced concrete under accelerated conditions. The model consists of structural analysis of concrete and the rebar using the Rigid Body Spring Method and corrosion current analysis with the truss networks model. The electric corrosion process is coupled with concrete cracking conditions by relating current efficiency to local crack width; the predicted radius losses of rebars are used to evaluate concrete crack propagation and residual tensile performance of corroded rebars. The model is validated with the results of accelerated corrosion tests using impressed current and a sodium chloride pond on the concrete cover. Good agreement with the test data is obtained with the proposed model, in terms of corrosion degree and profile, concrete crack pattern, and tensile behavior of corroded rebars. The model offers a corroborative tool with the accelerated corrosion technique to study the mechanical behavior of corroded concrete structures.
The evolution and spatial distribution of internal random pore structure are key factors in damage mechanism and macro-mechanical properties of concrete material under freeze-thaw (F-T) environment. This paper was presented to discover the deterioration mechanism of concrete under F-T actions. Through the experiment, visual examination was employed to evaluate the surface damage; the degradation considering the mass loss and uniaxial compressive strength of concrete was statistically analyzed under F-T environment. Moreover, X-ray tomography was adopted to characterize the concrete internal structure subjected to F-T cycles. The specimens exposed to F-T environment were scanned at regular intervals. Coupled with CT test, the pore space was characterized in terms of porosity and pore distribution by image analysis. And the relationship between the pore structure and the F-T cycles was described quantitatively by the fractal theory. The results indicated that the fractal dimension of the pore structure presented “down—up” trend with the number of F-T cycles increasing. The pore structure evolution changed from chaos to order and then to a process of disorder. The results demonstrated that F-T cycles accelerated the generation of meso-damage in concrete, which leads to more severe damage under F-T cycles.
We investigated changes in the density of natural rock minerals following high-energy electron irradiation, using the plasmon peak shift of electron energy-loss spectra and transmission electron microscopy. The target materials were the natural rock minerals α-quartz, orthoclase, anorthite, albite, biotite, muscovite, and chlorite. These crystalline minerals can be classified into three groups based on their Si-network geometries: 3-dimensional 6-member ring; 4-member ring + 6-member ring; and planar 6-member ring. The metamictization rates and changes in relative density are discussed using a phenomenological model, which we used to identify the physical parameters that describe the metamictization process as a function of the volume density of Si and Al atoms, or Si atoms alone, in the crystal structures. The relative densities following metamictization all decreased by more than a few percent, except for albite, which became denser. These results suggest that radiolysis damage causes initial compaction, then metamictization, characterized by the expansion of the Si- and Al-polyhedra in the aggregate. The stability of concrete containing α-quartz, orthoclase, and anorthite should be further investigated in the light of the present results.
In order to limit the release of CO2 emissions produced by cement manufacturing, clinker, the major cement component, is often partially replaced by mineral additions such as Ground Granulated Blast-Furnace Slag (GGBFS). Civil-engineering structures made with GGBFS cement can however present cracking at early age (< 28 days) due to re-strained shrinkage that significantly affects their durability and their material transport properties. Self-healing may limit these phenomena. In order to study and quantify self-healing kinetics, X-ray tomography tests for mortars with different GGBFS contents are performed. Results show that X-ray tomography provides valuable information inside the specimens: crack openings and healing products distribution. It is also shown that self-healing evolves rapidly during the first weeks of water curing and it is more important for cementitious materials containing GGBFS. This is due to their lower early age hydration degree allowing an ongoing hydration after cracking and the formation of supplementary C-S-H along the crack.
This study proposes reasonable models to evaluate chloride ion ingress in cementitious materials with dense micropore structures. Salt water immersion tests with mortar specimens, including low water-to-cement (W/C) ratio materials, were conducted. The results show that chloride ion ingress is so slow in specimens with low W/C ratios that existing models cannot follow experimental trends, and pores in the nanometer range may have significant effects on the total extent of chloride ion transport. Two phenomena related to chloride ion ingress in nanopores were considered and installed in the existing system: a threshold radius regarding the chloride ion movement, and a reacting friction force along the pores against the water movement pressure. Measured chloride ion distributions with salt water immersion tests were used to verify the proposed models. By conducting additional numerical simulations, the possible mechanisms of high resistivity against chloride ingress were studied.
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