Instances of reinforcing steel fracture in concrete structures damaged by the alkali-silica reaction (ASR) have been discovered recently in Japan. As long as reinforcing steels are not broken due to ASR-caused expansion, the safety of a structure is considered not to be seriously compromised. However, the safety of a structure becomes questionable when the confinement of concrete becomes degraded due to the fracture of reinforcing steel bars. Therefore, it is important to clarify the mechanism of the fracture of reinforcing steel bars and develop methods for detecting steel bar fractures and strengthening concrete structures damaged by ASR. This paper describes the fracture of reinforcing steels in the case of concrete structures damaged by ASR in the Kansai area in Japan. It then introduces the results of investigation on the fracture mechanism, nondestructive testing methods, and repair and strengthening methods for damaged concrete structures.
The X-ray diffraction (XRD)/Rietveld method was applied to analyze the hydration progress of cement paste prepared with blast furnace slag powder (BFS) and limestone powder (LSP). The Rietveld method can be used to quantify the amount of amorphous phase as well as crystalline phases. However, in cement paste containing BFS, two kinds of amorphous phases, unhydrated BFS and C-S-H, are included and the Rietveld method cannot distinguish them. In order to discriminate these two phases, a selective dissolution method was used to quantify the amount of unhydrated BFS. By combining these two methods, it became possible to quantify the amount of BFS and C-S-H in hydrated cement. The analysis results show that BFS accelerates the hydration of C3S, C3A, and especially C4AF. The strength increasing effect of LSP is more pronounced in the ordinary Portland cement (OPC) - BFS system than in the OPC system. This is attributed to the higher amount of calcium aluminate hydrates generated in the BFS contained system compared to the system without BFS. The additional calcium aluminate hydrates fill pores as calcium carboxaluminate by the reaction with LSP, increasing the compressive strength. These results show the effectiveness of the XRD/Rietveld method as an analysis tool for cement hydration relating to various properties of cement materials.
Electron probe microanalysis (EPMA) has been applied for the quantitative evaluation of Cl ingress into concrete. In order to obtain quantitative data on Cl concentration, the measurement conditions were discussed statistically in detail and sample measurement conditions were introduced. The absolute concentrations of Cl obtained through EPMA were found to be equivalent with wet analysis and the effect of matrix differences to be negligible. By using the difference in chemical composition between cement paste and aggregate measured by EPMA with a spatial resolution of 100 μm, it was possible to discriminate the paste part in concrete. Since Cl penetrates into concrete through the paste part, the Cl concentration profile obtained by EPMA is useful for the estimation of the apparent Cl diffusion coefficient, Da. The quantified value of Cl concentrations obtained with EPMA were confirmed through comparison with traditional slicing and grinding methods. Based on the measurement results, the Da value was calculated for various concrete mixtures and the results were found to be equivalent with those yielded by conventional methods.
It is well known that mineral admixtures, such as granulated blast-furnace slag and fly ash, control the ingress of chloride ions into concrete. However, the effects of the qualities and the replacement ratios of these admixtures on the diffusivity of chloride ions are not clear. Moreover, the micro-structure of concrete is changed by carbonation and change in microstructure affects mass transfer in concrete. This effect on the chloride diffusivity of concrete using mineral admixtures has not been clarified sufficiently. Therefore, in this study, the chloride ion diffusion coefficients of mortar specimens containing mineral admixtures were measured, and the influences of the kind of mineral admixture, replacement ratio, and carbonation on the chloride diffusion coefficient were estimated, and the relation between the microstructure and diffusion coefficient was studied. Furthermore, an estimation method of the diffusion coefficient using the pore volume and surface area of the pore system as parameters was proposed.
A computational system for predicting the long-term degradation of cement hydrates due to calcium leaching is presented. The leaching of calcium ions from hardened cement hydrates is simulated as the multi-phase equilibrium of calcium in solid and liquid phases and their transport is formulated on the basis of thermodynamics. The time-dependent properties of cement hydrates associated with hydration, pore-structure development, and moisture transport are evaluated by integrating calcium leaching and statistical models of chemo-physics. The proposed model delivers reasonable predictions of calcium leaching in high-performance concrete with a low water-to-cement ratio as self-curing takes place.
To clarify the compressive static and fatigue strength of concrete immersed in various liquids, experimental studies were conducted. The influence of the surface tension of immersion liquid on static and fatigue strength of concrete were examined based on the knowledge that (1) the fracture process of concrete at the macro scale is the result of generation and propagation of internal microcracks, (2) strain energy is partially released as surface energy when microcracks are formed, and (3) the magnitude of the surface energy between a solid and a liquid is affected by the surface tension of the liquid. The results indicate that the larger the surface tension of the immersion liquid, the smaller the static and fatigue strength of concrete, and that these are characterized by a linear relationship. It also became clear that the fatigue of concrete in air hardly matters whereas special attention should be given to the fatigue of concrete in the case of concrete structures that are frequently immersed in liquid with a larger surface tension than water, such as marine structures.
Because most shrinkage-reducing admixtures (SRAs) significantly reduce the surface tension of a cement paste pore solution, they will naturally influence all physical properties and processes that are dependent in some way on surface tension. Such properties include internal relative humidity, capillary stresses, and freezing point depression, all via the Kelvin equation and its variants (Kelvin-Laplace, Gibbs-Thomson). Processes that will thus be strongly influenced by the presence of SRAs include drying, autogenous stress and strain development, and freezing. In this paper, experimental measurements of these processes in cement pastes and mortars with and without SRA additions will be presented in light of the Kelvin equation. The experimental measurements that are applied to early-age specimens include X-ray absorption measurements to quantify drying profiles, bulk mass loss measurements, measurements of internal relative humidity, assessments of autogenous deformation under sealed curing conditions, and low temperature calorimetry scans to quantify freezable water content. The results indicate that SRAs can provide benefits in several new applications beyond their conventional usage to reduce drying shrinkage.
Seismic rehabilitation of rectangular columns with corner deformed bars lap-spliced at floor level through RC jacketing or CFRP wrapping, is experimentally studied. In three unretrofitted columns, deformation capacity and energy dissipation drop fast with lap length below 45-bar diameter. Three columns were cyclically tested after concrete jacketing of their full length and another nine after CFRP wrapping of the lap splice and plastic hinge region. Five CFRP layers were found to be more effective than two, but the improvement was not major. The positive effect of FRP wrapping on flexural resistance, ultimate deformation and energy dissipation declined with decreasing lap length. There is a limit to the improvement that can be obtained through FRP wrapping: if lapping is as short as 15-bar diameter, its adverse effects on force capacity and energy dissipation cannot be sufficiently removed by FRP wrapping. Overall, RC jacketing is more effective than FRP wrapping for the improvement of the deformation capacity of columns with 15-bar diameter lapping.
A direct path-integral scheme with fatigue constitutive models for concrete tension, compression and rough crack shear is used to predict the life-cycle of RC slabs. The three-dimensional fatigue analysis successfully predicts the characteristic mode of failure under moving loads as well as in the case of fixed-point pulsation in shear. Importantly, drastically shortened fatigue life under traveling wheel-type loads is mechanically demonstrated by implementing a constitutive model of cracked concrete using a direct path-integral method of fatigue damage simulation. A sensitivity study is carried out to clarify the influence of shear transfer decay and compression fatigue on RC slab performance. The effect of boundary conditions on fatigue life is also investigated.
This paper presents an investigation on the flexural behaviour of reinforced concrete beams produced from oil palm shell (OPS) aggregates. Utilising OPS in concrete production not only solves the problem of disposing this solid waste but also helps conserve natural resources. A total of 6 under-reinforced beams with varying reinforcement ratios (0.52% to 3.90%) were fabricated and tested. Data presented include the deflection characteristics, cracking behaviour, ductility indices and end-rotations. The investigation revealed that the flexural behaviour of reinforced OPS concrete beams was comparable to that of other lightweight concretes and the experimental results compare reasonably well with the current Codes of Practice. It was observed that beams with low reinforcement ratios satisfied all the serviceability requirements as per BS 8110.
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