Weathering steel (WS) is known to develop higher corrosion resistance than ordinary steel under atmospheric condition due to the formation of a protective, dense rust layer. This aspect, however, has not been studied so far in cement-based materials, which are characterized by high alkalinity and limited oxygen. To address the need for durable RC structure in extreme environments, it is necessary to study the behavior of WS in concrete. Here, a basic investigation was conducted to compare the short-term behavior of a newly developed WS with 1% Ni (NT) to conventional WS with 1% Cr (CT) and carbon steel (PC). One set of steel bars was exposed to solutions with varying air and pH to simulate concrete condition under chloride-containing environment. Another set was embedded in mortar under wet-dry cycle. Corrosion degree based on mass loss, coupled with half-cell potential, and corroded area was obtained. Results indicate that alkalinity or low oxygen appreciably reduces the corrosion rate of steels regardless of composition. These conditions make the corrosion behavior of NT comparable with other conventional steels. It is recommended to explore using longer time and wider cracks in future studies to achieve clearer difference between the steels.
To assess the durability of concrete structures at nuclear power plants in Japan, plant life management technical evaluation is performed in accordance with the guidelines of the Architectural Institute of Japan for the maintenance and management of structures in nuclear facilities. Concrete carbonation is one of the degradation factors covered in the guidelines, and sampling is performed to confirm the progression of carbonation and predict future progression. Electricity providers in Japan perform sampling at locations constituting environments (temperature, relative humidity, carbon dioxide concentration) where carbonation progresses relatively fast to confirm that carbonation has not reached the rebars and to predict progression. In this study, accelerated carbonation tests were performed at high temperature, which is believed to accelerate carbonation, considering the parameters of relative humidity. Progression of carbonation and its impact on rebar corrosion were examined. The results showed that progression of carbonation in high-temperature environments can be predicted with a margin of safety using the Architectural Institute of Japan’s durability prediction equation. Moreover, because the humidity environments where carbonation progresses and the temperature environments where rebar corrosion progresses do not correspond with each other, rebars are typically unlikely to corrode even if carbonation has reached the rebars, provided they are not subjected to extreme wet-dry cycles.
Cement-based materials used at radioactive waste disposal sites are required to possess long-term stability. However, when these materials come in contact with groundwater, calcium leaching from the solid occurs, and the material becomes porous. The use of mineral admixtures is recommended to minimize porosity. However, few studies have focused on the diffusion performance of cement-based materials blended with mineral admixtures after leaching. Therefore, in this study, the diffusion performance of such materials using blended cement after leaching was evaluated. It was found that the diffusion coefficient of the blended cement increased with leaching, and when leaching progressed considerably, the diffusion coefficient of the blended cement was close to that of ordinary Portland cement. Furthermore, the diffusion coefficient after leaching demonstrated good correlation with the pore volume when the pore diameter was 50 nm or larger.
Calcium leaching from cementitious materials into bentonite is a key process for the long-term alteration of cement–clay interfaces of engineered barrier systems. Strong chemical gradients between cement and clay drive the precipitation of minerals such as calcium silicate hydrate (C–S–H) and calcite. To analyze the mineralogical and porosity evolution at the cement–clay interface, composite specimens consisting of cement paste and bentonite mixed with various amounts of sodium carbonate were subjected to immersion and chloride migrations tests and were investigated by electron probe micro-analysis (EPMA), thermogravimetry/differential thermal analysis (TG-DTA), and X-ray diffraction (XRD) after 4–20 months of immersion. The results show that adding sodium carbonate to the bentonite enhanced the formation of calcite in the form of a surface layer on the cement paste. This suggests pore clogging at the interface and implies the existence of a threshold amount of carbonate addition above which pore clogging occurs. This is the first of two papers; the accelerated evolution of the samples in the presence of an electrical field is discussed in the second paper.
Calcium leaching from cementitious materials in contact with bentonite in nuclear waste repositories can alter the functionality of an engineered barrier system. In this study, we contribute to the fundamental understanding of calcite precipitation at cement–bentonite interfaces by adding carbonate to bentonite. In addition, we accelerate the transport of charged reactants towards the interface using an electrochemical migration method. The carbonate admixture successfully promotes calcite precipitation at the surface of cement paste. The analysis also revealed that the amount of precipitated calcite is not simply correlated to the amount of added carbonate or the applied electrical potential. Experiments in which bentonite pore water contains high initial contents of carbonate exhibit rapid calcite precipitation in a very narrow region at the cement–bentonite interface, resulting in pore clogging. This is the second of two papers; the system evolution without an electrical gradient was discussed in the first paper. This paper is the extended version in English from the authors’ previous works [Watanabe and Nakarai, (2008). “Effect of NaHCO3 in bentonite on calcium leaching from cementitious material.” Proc. of the JCI, 30(1), 717-722. Nakarai, et al., (2010). “Effect of carbonate mixing into bentonite on calcium leaching of cementitious material.” Proc. of the JCI, 32(1), 713-718. (in Japanese)]
Lateral loading tests using reduced reinforced concrete (RC) walls affected by alkali-silica reactions (ASR) and their simulation analyses were performed in order to evaluate the influence of ASR on the structural performance of shear walls that act as seismic resistant members in nuclear power facilities. The state of RC walls was also measured by several techniques to assess applicable monitoring methods during ASR expansion. The transition of the elastic wave velocity of the walls by the ultrasonic method has a good co-relationship with the transition of the static elastic modulus of the cylindrical specimens, indicating that non-destructive testing could capture the degradation trend of concrete due to ASR expansion. The initial stiffness of the ASR specimens became slightly smaller than that without ASR, but the maximum strength remained at the same level with or without ASR. Based on the results of experiments and analyses, a practically appropriate structural performance evaluation method and a monitoring method of ASR affected RC members were proposed.
Advances in the development of concrete materials have led to increasingly complicated composite systems. Investigation of such complex systems by traditional sampling methods requires a large sample size to understand the influence and interactions of all important factors, and conducting full size experiments with a large number of samples could be onerous and expensive. To devise more efficient experimental design and statistical modeling, this study explores the use of alternative sampling methods, and two space filling techniques, Latin hypercube and space packing, are adopted to investigate a cementfly ash-silica fume ternary paste system. The applicability of these space filling sampling methods was tested by modeling the material performance – evaluated here using microhardness – of the ternary paste system using Response Surface Methodology. It was found that the space filling design methods and microhardness test were non-ideal and provided poor results in their corresponding models due to relatively large noise caused by the proximity between sample points. However, Response Surface Methodology was confirmed to be not only a convenient tool for modeling the performance of composite systems but also a means for comparing sampling methods by their model accuracy.
The decarbonization of concrete structures sector is a priority due to the world climate. Since Portland cement is responsible for approximately 7% of the anthropogenic emissions worldwide, it is decisive to partially replace it by products with lower environmental impact. Fly ash has become highly demanded, due to its proven advantages, however, has announced end by the developed countries. The purpose of this research is to develop and characterize low cement concrete (LCC) with natural and ecological pozzolan additions as an alternative to fly ash. The main objectives are: define several structural concrete mixtures with low cement dosage, ranging from 125 to 175 kg/m3; characterize the workability and the mechanical properties, beyond the durability performance of those concretes; study the influence of pozzolans additions on LCC performance, as an alternative to fly ash; assess the service life of structures produced with the developed LCC. It is possible to produce LCC with natural pozzolan additions, instead of fly ash, with improved mechanical properties and enhanced durability performances. Results also show that the structural LCC produced with pozzolan from Cape Verde provide a great protection against steel corrosion, achieving values 50% higher than concrete with fly ash, and even higher margins when compared with the minimum concrete covers recommended by standards, which highlights the benefits of its use from the sustainability point of view.
In existing bridge structures, reinforced concrete dapped-end beams/girders (RCDEB) are frequently subjected to service loadings that exceed their design capacity, due to increasing economic and population growth. Dapped-end beams are prone to the accumulation of water due to improper drainage and sealing of the joint, providing favorable conditions for corrosion due to the stagnation of chloride rich water from de-icing salts used on roads. The situation can get even worse when freezing and thawing due to extreme weather conditions is present. Due to difficulties in maintenance of the dapped-end regions, this often leads to the deterioration of the concrete around the recess and bond deterioration, which ultimately results in durability issues. Unfortunately, the performance of RCDEB exhibiting corrosion induced bond deterioration is still not well understood. Hence, experimental, and numerical investigations were conducted to understand better the response of RCDEB subjected to static and cyclic loading, with the presence of bond deterioration. Furthermore, the static capacity of the beam was adopted to consider the variable amplitude cyclic loading at an increment of 15% static capacity for every 20 cycles. The reliability of the numerical examination under the direct-path constitutive models of concrete was extended to the moving load scenario by applying a lower load amplitude than the static capacity. It is shown that the prominent failure mechanisms observed in both static and cyclic tests were diagonal tension and shear. Through numerical investigations, it is also shown that from the damage level developed in the RCDEB under the moving load at a relatively low magnitude, high stresses were found within a relatively short number of cycles, which raises a serious cause for concern.
The effects of gamma irradiation on concrete properties during early hardening were studied towards radioactive waste storage or accelerated processing at precast plants. Concrete mixtures containing different mineral aggregates (baryte, magnetite, amphibolite) were investigated. During initial 16 hours of hardening the mixes were irradiated using 60Co gamma source at the rate of 3.5 kGy/h. The mechanical properties and microstructural features of irradiated early-age concrete were tested: the secant elastic modulus, the compressive strength, the porosity and pore size distribution. XRD and SEM analysis were also performed. The results indicate both the stiffening and pore refinement in concrete due to early gamma irradiation. Effects of early irradiation on microstructural features of cement matrix were found in the subsurface layer up to the depth of 2 mm. The influence of different mineral aggregates in concrete on the radiation-induced changes of early age properties is discussed.
The determination of the mineral composition of aggregates which constitute an important component of concrete is essential to understand and estimate the durability of structures. The modal (quantitative mineralogical) analysis of rocks is generally determined by point-counting method on thin sections or slabs. However, there is a difficulty of mineral identification depending not only on the experience and capability of the operator but also on the size of observable grains. In this study, XRD (X-Ray Diffraction)/Rietveld analysis is proposed as an alternative and a comparison study is performed for eight different rocks. The results show equivalent proportions to those of the point counting method for the major phases (minerals), although discrepancies are observed for the minor minerals. Complementary tests as XRF (X-Ray Fluorescence) and density measurements are also performed to pre-characterize and confirm the obtained modal analysis. For instance, the density calculations based on XRD/Rietveld analysis provide close values to the measured densities. Overall, this method can be an excellent alternative to the point counting method especially in the context of construction materials laboratory.
The degradation factors considered in the evaluation of long-term soundness of reinforced concrete structures in nuclear facilities in Japan are heat (high temperature), irradiation, carbonation, chloride penetration, alkali silica reaction, and machine vibration. As described in the “Guidelines for Maintenance and Management of Structures in Nuclear Facilities,” the soundness evaluation of equipment supports against machine vibration is very rudimentary compared with the evaluation methods against other factors since the degradation state due to machine vibration cannot be identified until cracks are visually detected as degradation phenomena. The phenomenon of fatigue of equipment supports can be expressed using an S-N curve that shows the relation between the maximum stress ratio and the fatigue life under repeated loading. During normal operation of nuclear facilities, the maximum stress ratio of repeated load is very low and less than the long-term allowable stress level, and the fatigue life is very long. Since the number of load repetition can be known from the operation period of the facility, soundness against fatigue can be evaluated using an S-N curve. In this paper, a soundness evaluation method using an S-N curve for equipment supports subjected to machine vibrations is proposed.
Cr3+ and Cu2+ can be cured using basic magnesium sulfate cement (BMSC). The influence of compressive strength, water resistance, and leaching characteristics of BMSC-solidified bodies was studied to evaluate the applicability of BMSC cement in industrial solid waste treatment. XRD, SEM, MIP, TG and FTIR were used to analyze the hydration products and microstructures of the solidified BMSC. The results show that when the content of Cr3+ or Cu2+ accounts for 0.2% to 2% of the molar mass of MgO, the addition of Cr3+ and Cu2+ inhibits the hydration reaction of BMSC, which causes the compressive strength to decrease, but the lowest strength is still have 27.6 MPa. The Cr3+ or Cu2+ BMSC-solidified body has good water resistance. The highest leaching concentrations in the solidified product are just 0.221 mg/L and 0.508 mg/L. XRD and microscopic analyses show that four new phases [4Cr(OH)3•Cr2H2(SO4)4•2H2O, Cr(OH)3•3H2O, Cu(OH)2•2H2O and 3Cu(OH)2•CuSO4•2H2O] are formed after adding Cr3+ and Cu2+, which shows that BMSC cement can cure heavy metals through chemical reactions. During the solidification process, the interplanar spacing of the 5•1•7 phases changed with varying amounts of copper and chromium ions. It was confirmed that copper and chromium ions entered the 5•1•7 phase crystal structure by atom substitution.