This experimental and numerical study aims to evaluate the penetration depth of contaminated water in the concrete structures involved in the Fukushima Daiichi nuclear powerplant. The influence of the mortar mixture on water absorption was investigated by varying the composition: mortar containing aggregates from river sand and crushed limestone sand was compared, and 15% of the cement in the mixture was substituted with fly ash. The effect of temperature in nuclear conditions is also significant; therefore, water uptake at temperatures of 20 and 60°C was considered. Finally, pre-drying conditions were studied by drying the sample at two different conditions: at 105°C and at 40% RH (relative humidity) and 20°C. Water uptake was monitored using x-ray computed radiography in combination with mass measurements. In all cases, anomalous sorption, or a nonlinear relationship between penetration depth and the square root of exposure time was observed, with the sorption curves showing bimodal behavior. The aggregate type had no significant effect on the water uptake results. However, the samples containing fly ash clearly had lower water uptake rates, which can be explained by the differences in the calcium silicate hydrate (C-S-H) structures. With increasing temperature, the penetration was slightly accelerated at the beginning of the experiment, with the rate of penetration then decreasing rapidly. The densification of C-S-H at higher temperatures could contribute to this phenomenon. Microstructural rearrangements can also explain why the highest uptake rates occurred for samples that were exposed to severe drying conditions (105°C). The experimental results were consistent when the microstructural rearrangement was considered, further confirming these conclusions.
A detailed inspection of a dam in Thailand is reported for its time-dependent gradual expansion towards upstream, contrary to the expected downstream creep deflection owing to hydrostatic loads. In this study, based on petrographic analysis and SEM, a sample of cored concrete from the dam was found to undergo a low to moderate level of ASR. The potential for future expansion was verified using an accelerated laboratory test. The experimental data were used to evaluate the dam performance by conducting an FEM analysis. The numerical model was calibrated with the observed deflection, and the mechanical stresses owing to the combined ASR and hydrostatic loads were estimated for various ages of the dam. In addition, stress and deflection were predicted using probabilistic methods. A sensitivity analysis was also performed to monitor the behaviour of the dam under various environmental conditions and input parameters. It was found that the gradual deterioration by ASR does not pose a high risk to the dam under normal loading conditions.
The resistivity of fresh concrete was obtained for the period from casting to the age of 72 h by a non-contact electrical resistivity measurement. Early hydration process of three types of concrete with the ratio of water to binder in mass of 0.4, including ordinary Portland cement concrete (OPC), concrete with fly ash of 30% (FAC) and concrete with silica fume of 5% (SFC), were analysed and compared. Based on resistivity data and Krstulovic-Dabic model, parameters of kinetics model were obtained, and hydration kinetics process was characterized. Results show that early hydration process of concrete can be characterized by the development of resistivity. Early hydration process is divided into five stages, which are ion dissolution, induction, acceleration, transition and deceleration. Incorporation of fly ash reduces the peak of hydration rate and delays the second hydration acceleration, whereas addition of 5% of silica fume has little effect on the early hydration of concrete. The compressive strength and electrical resistivity of three types of concrete show a good linear correlation at early age. The parameters of Krstulovic-Dabic kinetic model can be fitted well by using resistivity data of concrete.
Degradation of reinforced concrete (RC) structures can occur through the carbonation-induced corrosion of reinforcing bars, and this process is a major concern for the durability of RC buildings. Structures located in relatively humid inland environments are especially vulnerable. Therefore, it is important to clarify how relative humidity (RH) affects steel corrosion rates in carbonated concrete. In this study, a novel accelerated test method is presented, which shortens the experimental duration and simplifies the experimental method. A miniaturized specimen was created with 20 × 20 × 40 mm3 dimensions and an effective carbonation depth of only 5 mm. The corrosion rate of rebar in the small mortar specimens was studied at different equilibrium RH conditions, which were controlled using saturated salt solutions. The accelerated carbonation process was found to be much faster than in traditional concrete experiments. Finally, the relationship between water content (as a function of RH) and corrosion rate showed that the corrosion rate of rebar in carbonated mortar has a strong dependency on RH. The relationship between the mortar resistance and the corrosion rate indicated that the corrosion process of rebar in carbonated mortar is under resistive control when RH above 80%, and under anodic control when RH below 80%.
Fibre Reinforced Polymer (FRP) bars are effective alternatives to steel bars. This paper performs the shear evaluation of geopolymer concrete beams reinforced with Glass (G) / Basalt (B) FRP bars with Glass/ Basalt stirrups. Totally nine beams of GFRP/BFRP/Steel bars of size 100 × 160 × 1700 mm were cast in geopolymer/conventional concrete and tested by varying the ratio of shear span to an effective depth such as 3.6, 3.9, and 4.3 with a four-point static bending test. The deflection behaviour, moment-curvature, crack pattern, propagation spacing, and the number of cracks was studied. The results are compared with steel reinforced conventional concrete. The prediction equation of the shear strength equation is also proposed and compared with existing models.
In this study, we investigated the durability of high-volume ground granulated blast furnace slag (GGBS) blended cement concrete containing over 70% of GGBS for possible general structural applications. The concrete specimens used were exposed to natural outdoor conditions for 41 years on a building rooftop. The following is found. The exposed top surface of concrete with 88.5% GGBS 4000 replacement, the exposed top surface and the corners of sulfated slag cement showed peel failure of the paste, but the specimens of concrete with 68.5% GGBS 4000 and GGBS 2000 replacement were in sound condition. The compressive strength of all mix proportions did not decrease significantly over 41 years. The carbonation depth of concrete specimens containing 70% GGBS was about 7 to 9 mm, and about 15 mm for specimens containing 90% GGBS. Despite the high volume of GGBS content (70%) in the concrete specimens, traces of Ca(OH)2, which is involved in the chemical reaction of GGBS, were found in parts that remained uncarbonated. Ca(OH)2 increases the alkalinity of the specimen and is thus considered to have a rebar corrosion-inhibiting effect. This paper is the English translation from the authors’ previous work [Hashimoto, M., et al., (2019). “A study on the long-term durability of high-volume bast-furnace slag cement concrete for 41 years.” Concrete Research and Technology, Vol.30, pp.77-84. (in Japanese)].
In order to realize the utilization of cement-based materials in the special extreme environment, the deep sea, the authors have launched a project targeted at creating a technology platform with in-situ methods and systems for monitoring and evaluating cement-based materials located at deep ocean bottom sites. The first in-situ test in the world with a view to investigating the time-dependence of the volumetric stability and microstructure of Portland cement mortar following its long-term exposure to deep-sea conditions is currently underway at a 3515-m depth in the Nankai Trough. This paper reviews previous studies about the influences of deep-sea hydraulic pressure on cement-based materials, verifies the action of short-term hydraulic pressure using Portland cement mortars on a laboratory scale, and introduces the ongoing progress of in-situ deep-sea tests. Results from laboratory tests indicate that dimensional changes were provoked by liquid water infiltration and confinement while under short-term hydraulic pressure, however, time-dependent behavior under stresses such as creep has not appeared. Weight gain, changes in pore-size distribution, compressive strength and bending strength of the cement mortar were monitored after pressurization and depressurization processes.
High-strength and lightweight are two of the most important parameters for composites in the construction field. Here, we developed a novel foam concrete structure with sandwich porous structure by using in-situ polymerized polyacrylamide and ultra-stable foam, which can obtain higher mechanical strength in comparison to the normal porous concrete with the same density. The ratio of stiffness to weight was maximized to achieve the optimal sandwich porous structure size. The SEM images showed that the interface bond between the foam concrete and the polymer modified cement paste was tight and robust. The flexural strength of the novel structure was 65.6% higher than that of the foamed concrete at the same density. The series model was established to calculate the composite thermal conductivity of the novel foam concrete structure, indicating that the heat insulation was slightly improved compared with the normal foam concrete. Moreover, water resistance displayed a slight increase by constructing this sandwich porous structure. Hopefully, the novel composite with sandwich porous structure can put a new way for designing the lightweight and high strength insulation thermal structure.