JOURNAL OF ADVANCED CONCRETE TECHNOLOGY (Japan Concrete Institute)
Remaining Shear Capacity of Fire-Damaged High Strength RC Beams after Moist Curing
Higuchi, K., Iwama, K., Maekawa, K.
JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.19, 2021, AUG.
(https://doi.org/10.3151/jact.19.897)
Abstract
This study presents an analytical method for estimating the remaining shear capacity of reinforced concrete (RC) beams subjected to high temperature heating and verifies it by experiments. We focused on the mechanism by which the remaining shear capacity of members self-recovers in a humid environment after slow cooling. The authors confirmed that the infiltration of water vapor in the air into concrete after heating caused rehydration of quicklime (CaO) produced by high-temperature heating and self-recovery of material strength. Based upon material experiments, the mechanical model of calcium hydroxide formation by rehydration of quicklime was incorporated into the multi-scale analysis. Using the proposed model of this study, the effect of damage on the remaining strength of damaged members was also investigated. It was confirmed that the series of high temperature heating, slow cooling, moisture absorption, self-healing, and remaining shear capacity could be reproduced by the multi-scale analysis.
Correlation between Conventional Spacing Factors of Air Voids in Concrete and Characteristic Distances Defined by Point Process Statistics
Igarashi, S., Taniguchi, M., Yamashita, S.
JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.19, 2021, AUG.
(https://doi.org/10.3151/jact.19.886)
Abstract
The most important parameter used to determine the frost resistance of concrete is the distance between air voids. The linear traverse method has been used to obtain this spacing factor as a representative distance parameter. One of the present authors has proposed a new method of evaluating the distance using point process statistics. In this study, the spacing factors independently obtained for 52 extant mixtures of concrete are compared with the characteristic distance defined by the nearest neighbor distance distribution function. For that, a procedure for obtaining the necessary parameters for the point process method from records of linear traverse measurements is proposed. There exists a strong correlation between the spacing factor and the characteristic distance. The difference between two distance parameters is at most a few tens of micrometers. Furthermore, the median distance simply defined by the nearest neighbor distance distribution function is also found to have a good correlation with the spacing factor. The characteristic distance or the median distance is representative of an air void system and is an alternate to the conventional spacing factor.
This paper is the English translation from the authors‘ previous work [Igarashi, S., Taniguchi, M. and Yamashita, S., (2021). “Derivation of characteristic distances of the point process method from records of linear traverse measurement and their correlation with spacing factors.” Cement Science and Concrete Technology, 74, 131-138. (in Japanese)].
Sustainability Evaluation of Concrete Materials Utilizing the Desirability Approach with Varying Function Shapes
Henry, M., Opon, J., Vios, N. A.
JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.19, 2021, JULY.
(https://doi.org/10.3151/jact.19.771)
Abstract
To address sustainability issues, concrete design and specification must consider a variety of evaluation criteria, necessitating analytical methods that can optimize mixes to meet performance requirements while maximizing sustainability. This paper proposes the desirability approach, a multi-response optimization technique, as a new method for the sustainability evaluation of concrete materials. A demonstration study, using four sustainability indicators and six concrete mixes with varying binder compositions and aggregate types, is presented that explores how changing the shape of the desirability function, which translates indicator values to desirability, affects the sustainability evaluation output. Treating the function shapes as a source of uncertainty, sustainability evaluation is conducted with uncertainty analysis to produce a sustainability score distribution for each mix, which is described by statistical measures. Mixes with the highest and lowest indicator values exhibited the least variance in their scores, as these values were unaffected by the function shape. Sensitivity analysis, which measures the contribution of the sources of uncertainty to the total output uncertainty, found that the interactions when varying multiple function shapes simultaneously were the most influential source of uncertainty, which may be caused by multi-collinearity among the indicators. It was also found that sustainability scores calculated by geometric aggregation were lower than those calculated by linear aggregation.
Development of Foamed Geopolymer with Addition of Municipal Solid Waste Incineration Fly Ash
Li, Z., Kondo, R., Ikeda, K.
JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.19, 2021, JULY.
(https://doi.org/10.3151/jact.19.830)
Abstract
In order to develop a safe technology for recycling municipal solid waste incineration fly ash (MSWI-FA), the authors have attempted to produce foamed geopolymers with addition of MSWI-FA, and have investigated their various properties, including bulk density, strength, thermal conductivity, and leaching concentration of heavy metals and chlorine, etc. The polymerization reaction products, crystalline compounds and microstructure were also examined through X-ray diffraction (XRD) and Scanning Electron Microscope (SEM) analyses. It was found that it is possible to produce foamed geopolymer with a bulk density of less than 0.5 and a uniform distribution of air voids, by adding metallic aluminum powder as foaming agent and using less than 20% MSWI-FA and suitable alkali activator having low NaOH mole concentration. CaCl(OH)2?Ca(OH)2, CaO and KCl, initially included in the MSWI-FA, were not found in the foamed geopolymers. The geopolymers, which used ground granulated blast furnace slag (BFS), coal fly ah and MSWI-FA as precursors, were mainly composed of C-A-S-H gels, incompletely reacted precursors and a small amount of N-A-S-H gels. The foamed geopolymer had very high immobilization capacity of heavy metals (Cd, Cr, Cu, Mn, Pb, Zn), not varying with the pH of leachate. The immobilization efficiency of As changed with the pH of leachate and BFS content. When the BFS content was not less than 60%, the leaching concentrations of all the traced heavy metals including As were low, satisfying the environmental criteria of Japan for recycled construction materials without direct contact with water. The chlorine immobilization capacity of the foamed geopolymers is expected to exceed 70% in long-term age.
Numerical Simulations of Chloride Transport in Concrete Considering Connectivity of Chloride Migration Channels in Unsaturated Pores
Nukushina, T., Takahashi, Y., Ishida, T.
JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.19, 2021, JULY.
(https://doi.org/10.3151/jact.19.847)
Abstract
This study aimed to develop a method for the reasonable prediction of chloride ingress into concrete under unsaturated conditions. To achieve this, the connectivity of both the pore networks and the water paths in hardened cement paste were investigated and modeled in the existing numerical analytical system. Previous measurement results for the continuous porosity in hardened cement paste were reorganized, and the relationship between the total porosity and continuous porosity was clarified and formulated as the pore connectivity. In addition, the connectivity of the liquid water in unsaturated pore structures was formulated referring to previous numerical studies, suggesting that the connectivity of the liquid water decreases at a lower degree of saturation. Furthermore, by calculating the chloride transport considering the pore connectivity and the liquid water connectivity under unsaturated conditions, the chloride penetration into unsaturated concrete, including an airborne chloride environment, could be reproduced more realistically than was previously possible.
Long-term Performance Assessment of Concrete Exposed to Acid Attack and External Sulfate Attack
Qiao, D., Matsushita, T., Maenaka, T., Shimamoto, R.
JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.19, 2021, JULY.
(https://doi.org/10.3151/jact.19.796)
Abstract
This study aims to assess the degradation of below-grade concrete of nuclear power plants (NPP) in Japan, considering possible acid and sulfate attacks. A survey on the underground environments of several NPPs and residential buildings across the country was conducted, and their associated concrete performance was evaluated where concrete samples from core drillings were available. Moreover, acid and sulfate exposure tests on mortar specimens lasting for up to four years were carried out in the laboratory to simulate actual field situations. The effects of exposure conditions, such as solution concentrations, temperature, and immersion conditions, were examined. The surveyed environments were classified into non-aggressive or slightly aggressive environments. The concrete core samples investigated showed insignificant degradation and satisfactory strength after 40 years of exposure. The laboratory test results showed that the accelerated tests using highly concentrated solutions could exacerbate the extent of decalcification and even alter the degradation mechanism for magnesium sulfates. Therefore, a close-to-reality concentration is preferred for reproducing field situations. The carbonation/neutralization depth was used as an indicator to estimate the degradation extent. The measured values in the laboratory using low-concentration solutions correlated well with the field results, suggesting that the below-grade concrete‘s degradation in the NPPs investigated may be less than 10 mm after 60 years of exposure.
Cathodic Protection Range of the Embedded Sacrificial Zinc Anode for RC Member Deteriorated by Carbonation and Mixed Chloride
Yoshida, T., Watanabe, Y., Nomura, N., Aramaki, S.
JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.19, 2021, JULY.
(https://doi.org/10.3151/jact.19.785)
Abstract
Much is left unknown about applicable conditions of the method or quantification of the cathodic protection range. In particular, little has been studied about how the range or effect of cathodic protection is influenced by such factors as the thickness of reinforcing bars (rebars) to be protected, combined deterioration by carbonation and mixed chloride, amount of chloride ions and carbonation depth. This study experimentally evaluated the effect of differences in the chloride ion content in concrete, the carbonation depth or the rebar thickness on the cathodic protection range of the embedded sacrificial anode method. The results of the experiment were that the smaller the carbonation depth, the amount of chloride ions or the rebar thickness, the larger the amount of depolarization of the rebars in the concrete to be protected was. That is, for corrosion protection of rebars near patch repair areas embedded with sacrificial anodes, it would be appropriate to make the steel surface area of components smaller and the salt damage environment less corrosive.
Development of Improved Loop Joint Applied for Precast PC Deck Slab
Hatakeyama, S., Sagawa, Y., Hamada, H., Hino, S., Masaki, M., Sato, Y.
JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.19, 2021, JUNE.
(https://doi.org/10.3151/jact.19.644)
Abstract
Ordinary “gloop joint” applied to precast prestressed concrete (PC) deck slab tends to increase its thickness. By decreas-ing this thickness, the dead load of the deck slab can be reduced. Hence, we have developed an “gimproved (inclined) loop joint” that reduces the deck slab thickness. This study presents the results of static bending test of slab specimens to confirm the load-carrying behavior and wheel moving load test to examine the fatigue durability of precast PC deck slabs with improved loop joints.
The bending load-carrying behavior satisfied the requirements for highways in Japan. In addition, in the wheel moving load test, no sudden increase in vertical deflection and joint opening was confirmed at a load step of 250 kN × 100 000 times equivalent to 100 years on an actual bridge. Moreover, there was no water leakage at the bottom surface of the deck slab during the water-filling test. Based on the test results, it was inferred that the required load-carrying behavior and fatigue durability could be retained for 100 years in the improved loop joint.
Time-dependent Effect of Expansion due to Alkali-silica Reaction on Mechanical properties of Concrete
Ji, X., Joo, H. E., Yang, Z., Takahashi, Y.
JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.19, 2021, JUNE.
(https://doi.org/10.3151/jact.19.714)
Abstract
This study investigates the time-dependent mechanical properties of concrete deteriorated by the alkali?silica reaction (ASR). Previous analytical and experimental studies have indicated the positive impact of ASR gel in the cracks against mechanical damage in concrete. To study the effects of ASR gel on cracked concrete, groups of cylinder specimens with different expansion levels were prepared and tested at different material ages. The compression test results showed that the deteriorated elastic modulus of the specimens could be recovered over time. Mechanical property data from the other ASR studies were collected and assessed to observe similar trends across the literature. It was observed that the recovery of the elastic modulus also occurred in previously reported experiments. The recovery of the elastic modulus is assumed to be due to the time-dependent chemical and physical properties of ASR gel, which fills the cracks. Moreover, the data indicated that parameters other than material age and expansion could be attributed to the time-dependent mechanical properties of concrete affected by ASR.
Analytical Study on the Effect of Different Shear Reinforcement Shapes on Shear Failure Behavior and Shear Resistance Mechanism of RC Beams
Kawamura, K., Nakamura, H., Takemura, M., Miura, T.
JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.19, 2021, JUNE.
(https://doi.org/10.3151/jact.19.571)
Abstract
The authors have reported from experimental investigation that the use of closed stirrups, U-shaped stirrups and rod-shaped reinforcements as shear reinforcements in reinforced concrete (RC) beams results in clear differences in internal crack patterns and maximum strength. In this study, simulations of the experimental beams were performed using a three-dimensional rigid-body spring model, demonstrating that the behavior observed in the experiments is a mechanical phenomenon that can be reproduced numerically. Using the stress distribution over the beam cross section obtained from the analysis, the beam and arch action components of shear resistance are decoupled and considered from the perspective of the shear resistance mechanism. Further, through analysis of additional cases, the role of the upper and lower horizontal legs of the shear reinforcement, the effect of end anchorages on rod-shaped reinforcements were clarified.
The Relative Humidity Range for the Development of Irreversible Shrinkage in Hardened Cement Paste
Maruyama, I., Kishi, N., Aili, A.
JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.19, 2021, JUNE.
(https://doi.org/10.3151/jact.19.585)
Abstract
For better understanding of irreversible shrinkage, nine hardened cement paste (hcp) samples with three different cement types and three different water to cement ratio were prepared. Four different relative humidity conditioning histories containing the first desorption, re-humidification and the second desorption are investigated for all the specimens to obtain the length change and water sorption isotherms. The irreversible shrinkage strain was developed when the specimen was dried up to less than 80% relative humidity (RH), while other previous experiments in literatures showed that the shrinkage strain between 40% RH and 11% RH is reversible. It is concluded that the irreversible shrinkage strain is developed between 80% RH and 40% RH, which is also supported by the change in water vapor BET surface area of hardened cement paste after long-term drying.
Structural Behavior of Brick Wall Specimens Reinforced on the Surface with RC Walls under Horizontal Loading
Mishra, C., Yamaguchi, K., Araki, K., Ninakawa, T., Hanazato, T.
JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.19, 2021, JUNE.
(https://doi.org/10.3151/jact.19.593)
Abstract
This study aims to develop and propose a seismic retrofitting method for unreinforced brick masonry for improving the seismic safety of the 1st and 3rd headquarters buildings of Kyushu University. Five brick wall specimens representing parts of the walls that were adjacent to the openings in the headquarters buildings were prepared on a 3/4th scale of the actual structure and they reinforced on either one or sides of their surfaces with reinforced concrete (RC) walls. Horizontal loading experiment was conducted to confirm the reinforcing effect. Results indicated that the maximum load of the specimens (CS01, CS02) reinforced on one side with RC walls was 6.1 to 6.2 times higher than that of the unreinforced specimen. Similarly, the maximum load of the specimens (CD01, CD02) reinforced on both sides with RC walls was 12.6 to 14.2 times higher than that of the unreinforced specimen. The specimen CS02 reached its maximum load at a smaller deformation angle compared to the specimen CS01 because of the effect of twisting. Additionally, the authors also derived and verified a horizontal strength evaluation formula for brick wall specimens reinforced with RC walls. The values calculated using horizontal strength evaluation formulas were close to the experimental values.
Phase Assemblage, Microstructure and Shrinkage of Cement Paste during Carbonation at Different Relative Humidities
Suda, Y., Tomiyama, J., Saito, T., Saeki, T.
JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.19, 2021, JUNE.
(https://doi.org/10.3151/jact.19.687)
Abstract
This study investigates the effect of relative humidity (RH) on the carbonation of hydration products. The samples of ordinary Portland cement (OPC) and OPC/blast furnace slag paste were exposed to accelerated carbonation under 3% CO2 concentration at different RHs from 11% to 85%. Thermogravimetric/differential thermal analysis and X-ray diffraction/Rietveld analysis were performed to quantify the amount of portlandite and calcium carbonate polymorphs. The Ca/Si ratio of the carbonated C-S-H phase was determined using a phase equilibrium calculation. To evaluate the microstructural changes in carbonated sample, water vapor sorption and nitrogen sorption measurements were conducted before and after carbonation. Results show that the carbonation shrinkage progressively increased at intermediate RHs (43% and 66% RH). The relation between carbonation shrinkage and the Ca/Si ratio of C-S-H agrees with the change of C-S-H mean chain length. There were significant changes in the carbonation rates of C-S-H and portlandite at different RHs. The coarsening of the meso scale pore likely derives from the polymerization and aggregation of C-S-H due to decalcification. This paper is an extended and enhanced version of an earlier study [Suda, Y., Tomiyama, J., Saito, T. and Saeki, T., (2020). “Impact of relative humidity on carbonation shrinkage and microstructure of hardened cement paste.” In: Proc. 6th International Conference on Construction Materials (ConMat'20), Fukuoka, Japan 27-29 August 2020. Tokyo: Japan Concrete Institute].
Advancing Concrete Durability Research through X-ray Computed Tomography
Sugiyama, T., Promentilla, M. A. B.
JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.19, 2021, JUNE.
(https://doi.org/10.3151/jact.19.730)
Abstract
Radiation technology such as X-ray computed tomography (X-ray CT) is a powerful tool for materials research. Such non-destructive imaging technology allows us to visualize in three dimensions (3D) the internal structure of materials without damaging the specimen. This paper thus presents our studies on the application of microfocus X-ray CT, synchrotron radiation X-ray CT, and the integrated computed microtomography and X-ray diffraction (CT-XRD) method to advance cement and concrete research. CT images coupled with 3D image analyses allow us to identify and measure the air voids, pore scale microstructure and crack geometry. Image-based computational simulations provide us to estimate transport parameters such as diffusion tortuosity and water permeability in the digitized pore space. With the introduction of novel experimental techniques, deterioration or microstructure changes in hydrated cement systems, for example, attributed to calcium leaching, freeze-thaw cycles, elevated temperature, and steel reinforcement under tension are further elucidated. This review paper then ends with speculation of the future directions of concrete durability research via X-ray CT.
Maintenance Scenario of Concrete Structures Damaged by Reinforcement Corrosion Based on Corrosion Propagation Mechanism of Steel and Moisture Behavior in Concrete
Takaya, T., Saito, R., Satoh, S., Yamamoto, T.
JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.19, 2021, JUNE.
(https://doi.org/10.3151/jact.19.614)
Abstract
It can be said that corrosion of reinforcing steel is a serious problem for concrete structures. However, the corrosion mechanism of steel is still unclear, and it is difficult to prevent corrosion perfectly. In this study, in order to make clear the corrosion process in alkaline environment, the corrosion products formed in solutions of various OH-?and Cl-?concentrations were evaluated by Raman spectroscopy. In addition, the influence of dissolved oxygen on corrosion products was investigated. Furthermore, the corrosion mechanism of steel in concrete was investigated based on field study. These investigations made it clear that the behavior of moisture in concrete has large influence on corrosion propagation. Therefore, the influence of W/C, chloride ions and cracks on the behavior of moisture in concrete was also investigated. Based on the the results obtained, the authors propose a maintenance scenario for concrete structures damaged by corrosion of the reinforcement.
Experimental Study Investigating the Effects of Concrete Conditions on the Penetration Behaviors of Cs and Sr at Low Concentration Ranges
Yamada, K., Igarashi, G., Osawa, N., Kiran, R., Haga, K., Tomita, S., Maruyama, I.
JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.19, 2021, JUNE.
(https://doi.org/10.3151/jact.19.756)
Abstract
To decommission of the Fukushima-Daiichi nuclear power plant after the reactor accident, it is important to estimate the distribution of radionuclide contamination in the concrete for key elements such as Cs and Sr. A reaction transport model will be developed for these calculations. However, for a realistic model, the behaviors of Cs and Sr penetration in concrete must be experimentally investigated. A part of the results of a MEXT project called “The Analysis of Radionuclide Contamination Mechanisms of Concrete and the Estimation of Contamination Distribution at the Fukushima Daiichi Nuclear Power Station” are presented in this technical report. From our penetration analyses, the behaviors of neither Cs nor Sr were affected by each other. Additionally, the apparent diffusion coefficients of Cs and Sr were not significantly affected by the concentration or the presence of clay in the mortars. The penetration depth of Sr was smaller than that of Cs, and fly ash blended cement increased the resistance to penetration compared with ordinary Portland cement. Carbonation in the mortar samples increased the adsorption of Cs especially. Sr interacted with cement hydrates more than with clays. In oven-dried mortars, under the condition of water suction, the presence of clay retarded Cs penetration but had no effect on Sr. When the mortars were carbonated and oven-dried, the interactions between the solid phase and Cs or Sr took hours at least to complete.
Behavior of High-Nickel Type Weathering Steel Bars in Simulated Pore Solution and Mortar under Chloride-Containing Environment
Benito, E. K. D., Ueno, A., Fukuyama, T.
JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.19, 2021, MAY.
(https://doi.org/10.3151/jact.19.370)
Abstract
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.
Soundness Evaluation Method Using S-N Curve for Equipment Supports Subjected to Machine Vibrations
Kontani, O., Koge, M., Shimamoto, R.
JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.19, 2021, MAY.
(https://doi.org/10.3151/jact.19.414)
Abstract
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.
Effect of Calcium Leaching on Diffusivity of Blended Cement Paste and Mortar
Kurumisawa, K., Haga, K., Shibata, M., Hayashi, D.
JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.19, 2021, MAY.
(https://doi.org/10.3151/jact.19.426)
Abstract
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.
Exploring the Experimental Design and Statistical Modeling of Cementitious Composite Systems Using Various Sampling Methods
Ma, S., Henry, M., Opon, J.
JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.19, 2021, MAY.
(https://doi.org/10.3151/jact.19.501)
Abstract
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.
Accelerated Concrete Carbonation and Resulting Rebar Corrosion Under a High Temperature Condition in Nuclear Power Plants
Mitsugi,?S., Owaki, E. Masuda,?H., Shimamoto, R.
JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.19, 2021, MAY.
(https://doi.org/10.3151/jact.19.382)
Abstract
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.
Calcite Precipitation at Cement?Bentonite Interface. Part 1: Effect of Carbonate Admixture in Bentonite
Nakarai, K., Shibata, M., Sakamoto, H., Owada, H., Kosakowski, G.
JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.19, 2021, MAY.
(https://doi.org/10.3151/jact.19.433)
Abstract
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.
Calcite Precipitation at Cement?Bentonite Interface. Part 2: Acceleration of Transport by an Electrical Gradient
Nakarai, K., Watanabe, M., Koibuchi, K., Kosakowski, G.
JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.19, 2021, MAY.
(https://doi.org/10.3151/jact.19.447)
Abstract
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)]
Response of Reinforced Concrete Dapped-End Beams Exhibiting Bond Deterioration Subjected to Static and Cyclic Loading
Quadri, A. I., Fujiyama, C.
JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.19, 2021, MAY.
(https://doi.org/10.3151/jact.19.536)
Abstract
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.
Modal Analysis of Rock Forming Minerals: Contribution of XRD/Rietveld Analysis Compared to the Classic Point Counting Method
Samouh, H., Nishimoto, S., Yoshida, H., Sawada, S., Kontani, O., Suzuki, K., Maruyama, I.
JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.19, 2021, MAY.
(https://doi.org/10.3151/jact.19.395)
Abstract
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.
Structural Performance Evaluation and Monitoring of Reinforced Concrete Shear Walls Affected by Alkali-Silica Reactions
Sawada, S., Takaine, Y., Okayasu, T., Nimura, A., Shimamoto, R.
JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.19, 2021, MAY.
(https://doi.org/10.3151/jact.19.477)
Abstract
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.
Modeling and simulation on static and fatigue behaviors of intact and frost damaged concrete with ice-strengthening effects
Benito, E.K.D., Ueno, A., Fukuyama, T.
JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.19, 2021, APR.
(https://doi.org/10.3151/JACT.19.346)
Abstract
Concrete structures serving in cold and wet regions usually suffer frost damage and thus have server deterioration. Many researches have been conducted to reveal the damaging mechanism and damaged mechanical properties of concrete under the effect of frost action. It has been widely known that the strength and stiffness of frost damaged concrete without using air-entraining agent decrease under room temperature. However, there will be a different story if the frost-damaged concrete is saturated and loaded under freezing temperature. Water existing in pores and cracks will freeze into ice, which provides additional strengthening effects. This paper presents a multi-scale modeling and simulation work on the static and fatigue behaviors of frost damaged concrete with consideration of such ice-strengthening effects. The micro-mesoscale damaging and strengthening effects induced by ice formation are modeled and integrated into the mesoscale analytical approach - Rigid Body Spring Model, and the macroscale static and fatigue behaviors are simulated. It is found that the freezing temperature has a positive (strengthening) effect on the static strength, while it has a negative effect on the fatigue life for both intact and frost-damaged concrete. Test is also conducted with available experimental evidence to validate the developed approach. Satisfactory correlation is found through the comparison between simulation and experiment.
Kinematic Model for Shear Assessment of RC Short Columns Subjected to Frost Damage
Kanazawa, T; Ushiwatari, Y
JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.19, pp.335-345, 2021, APR.
(https://doi.org/10.3151/jact.19.335)
Abstract
Few reports have described practice-based models assessing ultimate strengths of existing reinforced concrete (RC) members subjected to frost damage. This paper presents a kinematic model for shear assessment of damaged RC short columns based on the upper bound theorem. Without regressive functions, the developed model predicts the shear strength contribution of damaged concrete when the displacement field is divided into undamaged and damaged zones based on damage depths obtained from core sampling. The model accuracy is verified by comparison of its predictions with those of earlier test results of 14 RC columns presenting shear failure after freeze-thaw exposure. The analytical predictions show good agreement with experimentally obtained results within error of 20%. Shear strength predictions for different damage depths are presented for an existing RC bridge pier with severe frost damage. Rational shear assessment was achieved because the kinematic analysis directly correlates the damage depth with shear strength reduction.
Benchmark Finite Element Calculations for ASCET Phase III on a Reinforced-Concrete Shear Wall Affected by Alkali-Aggregate Reaction
Kojima, M; Kodama, T; Jin, CR; Maruyama, I
JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.19, pp.280-300, 2021, APR.
(https://doi.org/10.3151/jact.19.280)
Abstract
In this study, finite element (FE) analyses were conducted on a reinforced-concrete (RC) shear wall that is affected by an alkali-aggregate reaction (AAR), which were then applied for a benchmark studies in OECD/NEA/CNSI/ASCET (Organization for Economic Co-operation and Development/Nuclear Energy Agency/Committee on Safety of Nuclear Installations/Assessment of Structures subjected to Concrete Pathologies) Phases II and III assessments. A commercial software has been modified to account for this AAR expansion, which is affected by the stress field and change in physical properties of the concrete. The impacts of boundary conditions, modeling in two and three dimensions, and material properties on the load-displacement curve and crack patterns were carefully evaluated. Finally, although similar load-displacement curves and crack patterns were obtained, the peak load due to brittle failure of an RC shear wall affected by AAR could not be reproduced. Consequently, it was found that the rotation of the loading stub and anchoring procedure of the base stub were critical conditions for load-displacement relationship of RC shear wall, and meshing capturing the arrangement of reinforcement bars is crucial for FE analysis with two-dimensional (2D) condition, and finally, the occurrence of initial cracks and the loading capacity could not be clearly reproduced. This suggests that consideration of the placement of rebars and covering concrete in the mash setting in three-dimensional (3D) model affected the failure mode of the concrete. It is necessary to consider the possible failure mechanism and to reflect such features in numerical modeling.
Modeling and Simulation on Static and Fatigue Behaviors of Intact and Frost Damaged Concrete with Ice-strengthening Effects
Wang, Z; Gong, FY; Ueda, T
JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.19, pp.346-358, 2021, APR.
(https://doi.org/10.3151/jact.19.346)
Abstract
Concrete structures serving in cold and wet regions usually suffer frost damage and thus have server deterioration. Many researches have been conducted to reveal the damaging mechanism and damaged mechanical properties of concrete under the effect of frost action. It has been widely known that the strength and stiffness of frost damaged concrete without using air-entraining agent decrease under room temperature. However, there will be a different story if the frost-damaged concrete is saturated and loaded under freezing temperature. Water existing in pores and cracks will freeze into ice, which provides additional strengthening effects. This paper presents a multi-scale modeling and simulation work on the static and fatigue behaviors of frost damaged concrete with consideration of such ice-strengthening effects. The micro-mesoscale damaging and strengthening effects induced by ice formation are modeled and integrated into the mesoscale analytical approach - Rigid Body Spring Model, and the macroscale static and fatigue behaviors are simulated. It is found that the freezing temperature has a positive (strengthening) effect on the static strength, while it has a negative effect on the fatigue life for both intact and frost-damaged concrete. Test is also conducted with available experimental evidence to validate the developed approach. Satisfactory correlation is found through the comparison between simulation and experiment.
Novel Accelerated Test Method for RH Dependency of Steel Corrosion in Carbonated Mortar
Cheng, LG; Maruyama, I; Ren, YQ
JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.19, pp.207-215, 2021, MAR.
(https://doi.org/10.3151/jact.19.207)
Abstract
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 x 20 x 40 mm(3) 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%.
41 Year Long-Term Durability of High Volume Blast-Furnace Slag Cement Concrete
Hashimoto, M; Kurata, K; Ohtsuka, Y; Dan, Y
JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.19, pp.248-258, 2021, MAR.
(https://doi.org/10.3151/jact.19.248)
Abstract
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)].
Time-Dependent Deformation of a Concrete Arch Dam in Thailand - Numerical Study on Effect of Alkali Silica Reaction on Deflection of Arch
Joshi, NR; Sriprasong, T; Asamoto, S; Sancharoen, P
JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.19, pp.181-195, 2021, MAR.
(https://doi.org/10.3151/jact.19.181)
Abstract
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.
Water Uptake in OPC and FAC Mortars under Different Temperature Conditions
Kiran, R; Samouh, H; Matsuda, A; Igarashi, G; Tomita, S; Yamada, K; Maruyama, I
JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.19, pp.168-180, 2021, MAR.
(https://doi.org/10.3151/jact.19.168)
Abstract
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: mortars containing aggregates from river sand and crushed limestone sand were 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 degrees C was considered. Finally, pre-drying conditions were studied by drying the sample at two different conditions: at 105 degrees C and at 40% RH (relative humidity) and 20 degrees 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 degrees C). The experimental results were consistent when the microstructural rearrangement was considered, further confirming these conclusions.
Improvement of Concrete Properties using Granulated Blast Furnace Slag Sand
Ayano, T; Fujii, T
JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.19, pp.118-132, 2021, FEB.
(https://doi.org/10.3151/jact.19.118)
Abstract
A high volume of ground granulated blast furnace slag (GGBS) or granulated blast furnace slag (BFS) can enhance the resistance of concrete to freezing and thawing without the use of air-entraining (AE) agents. Furthermore, it can also enhance the resistance of concrete to chloride ion penetration and sulfuric acid attack, although the mechanism of improvement differs. In particular, BFS can reduce time-dependent strains, such as drying shrinkage strain and creep strain. The use of granulated blast furnace slag, either GGBS or BFS, promotes the durability of concrete structures by improving the mechanical properties of cementitious materials. Some of the concrete properties that are improved by the incorporation of BFS are presented in this paper. The detailed improvement mechanism of BFS has not yet been clarified. However, it is clear that it depends on the chemical reactions involving BFS and thus a critical time is required for BFS to hydrate in order to improve concrete properties. It takes four weeks to achieve high resistance to freezing and thawing by using BFS without the addition of an AE agent; use of a thickening agent can further shorten this curing period to one week. This paper is an English translation from a previous work by the authors [Ayano et al., (2014). Resistance to freezing and thawing attack of concrete with blast furnace slag fine aggregate. Journal of Japan Society of Civil Engineers, Ser. E2 (Materials and Concrete Structures), 70(4), 417-427 (in Japanese)] and [Jariyathitipong et al., (2013). Improvement of resistance to sulfuric acid attack of concrete by use of blast furnace slag sand. Journal of Japan Society of Civil Engineers, Ser. E2 (Materials and Concrete Structures), 69(4), 337-347 (in Japanese)].
Accelerated Moisture Transport through Local Weakness of High-Strength Concrete Exposed to High Temperature
Iwama, K; Kato, Y; Baba, S; Higuchi, K; Maekawa, K
JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.19, pp.106-117, 2021, FEB.
(https://doi.org/10.3151/jact.19.106)
Abstract
Spilling out of condensed liquid water from needle-like holes in high-strength concrete was experimentally observed under fire attack. The presence of these holes was found to prevent explosive spalling effectively in the vicinity of the holes during fire exposure tests. This spilling out occurred at about 10 to 30 minutes after the start of high temperature heating. These needle-like holes are defined herein as local weaknesses that may act as rapid paths of water permeation to reduce the risk of explosive spalling of cover concrete. The phase change of moisture from CSH solids to condensed liquid as well as free water in micro-pores was simulated by a multi-phase chemo-physics analysis of ultra-high-strength concrete. The prediction of the high-rate phenomena was experimentally proved by using embedded moisture sensor, and the high-rate discharge of condensed water though local weaknesses was analytically simulated.
Effect of Chloride and Sulfate in the Immobilization of Cs-137 in C-S-H Gel
Duque-Redondo, E; Yamada, K; Manzano, H
JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.19, pp.95-105, 2021, JAN.
(https://doi.org/10.3151/jact.19.95)
Abstract
Cementitious materials are commonly used in nuclear repository sites to immobilize intermediate-level radioactive wastes. This is due to the large surface area of the calcium silicate hydrate (C-S-H) gel, the main hydration product of ordinary Portland cement, which provides many sorption sites in which the contaminants can be adsorbed. The retention capacity of these materials is strongly dependent on the composition, the water content, the pH or the presence of additives. Likewise, it is also known that the durability and performance of cement and concrete are adversely affected in chloride and/or sulfate environments. In this work, atomistic simulations have been employed to analyze the effect of the presence of chlorides and sulfates in the retention and transport of Cs-137, one of the most hazardous radioisotopes, in calcium silicate hydrate. The simulations suggest that the presence of a moderate amount of chlorides does not alter significantly the Cs uptake in C-S-H gel, while a moderate content of sulfates enhances substantially the retention of Cs ions and reduces their migration throughout the pore. This behavior is attributed to the ability of the sulfates to pull Ca out the high-affinity sites from the C-S-H surface, allowing Cs ions to occupy them.
Experimental and Numerical Study of Structural Effects of Anisotropic Frost Damage on Reinforced Concrete Beams
Kanazawa, T; Sato, Y; Takahashi, R
JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.19, pp.14-25, 2021, JAN.
(https://doi.org/10.3151/jact.19.14)
Abstract
Unlike plain concrete, frost damaged reinforced concrete (RC) exhibits anisotropy because of the presence of reinforcing bars. The resultant mechanical responses are influenced strongly by the loading direction. Therefore, to ascertain the mechanical behavior up to failure of RC beams subjected to freeze-thaw action and subsequent mechanical loading, anisotropic damage models of frost-damaged RC elements were assessed in this study using three-dimensional nonlinear finite element analysis (3D-NLFEA), which revealed that the anisotropic damage affected load-deflection responses and played a key role in failure modes that differed from those of an undamaged RC beam. This paper is the English translation from the authors' previous work [Kanazawa, T., Sato, Y. and Takahashi, R., (2019). Frost damage of reinforced concrete beams and analytical evaluation of its static failure behavior. Journal of Japan Society of Civil Engineers. Ser. E2 (Materials and Concrete Structures), 75(4), 293-307. (in Japanese)].
Experimental Study on the Effect of Different Shear Reinforcement Shapes on Shear Failure Behavior and Internal Crack Pattern of RC Beams
Kawamura, K; Takemura, M; Nakamura, H; Miura, T
JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.19, pp.82-94, 2021, JAN.
(https://doi.org/10.3151/jact.19.82)
Abstract
Various types of shear reinforcement are used for example general closed stirrup and reinforcement bar with mechanical anchor. However, most standards and specifications take only the cross-sectional area of the vertical components of the reinforcement components into account when determining their effect, such as on shear crack development and shear strength. Consequently, the full effect of different shear reinforcement shapes on the shear failure behavior of reinforced concrete (RC) beams is not clear. In this study, differences in shear failure behavior of RC beams using three types of shear reinforcement (closed stirrups, U-shaped stirrups, and rod-shaped reinforcements with mechanical anchor) were investigated by carrying out loading experiments. The three-dimensional displacement distribution on the side faces of each beam and the internal crack patterns were obtained. It was clarified that there is a clear difference in internal crack pattern and spreading deformation behavior according to shear reinforcement shape, and this influences the shear strength of the RC beam.
Pull-Out Performance of Eccentrically Spliced Longitudinal Headed Bars for Precast Beam-Footing Connections
Ousalem, H; Takatsu, H
JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.19, pp.1-13, 2021, JAN.
(https://doi.org/10.3151/jact.19.1)
Abstract
For common detailing of footings in steel or precast concrete structures, longitudinal reinforcement of foundation beams is bent horizontally and spliced with reinforcement of cast-in-place footings to insure an adequate juncture for load transfer. In this study, instead of bending longitudinal reinforcement bars of both, beams, and footings, headed reinforcement bars are adopted. By doing so, a discontinuity region is created where longitudinal bars of footings become eccentric to those of beams that are embedded in the footings. To allow developing forces in longitudinal bars of beams flow to longitudinal bars of footings, a set of reinforcing ties is provided between them. As such setting of headed reinforcement bars is not common, thorough investigations have been carried out. In this paper, the pull-out performance of eccentrically spliced longitudinal headed bars with different detailing of transverse reinforcement, proposed for precast beam-cast-in-place footing connection, is discussed based on an experimental investigation. A method, based on the friction shear theory, for the strength evaluation of such arrangement is suggested.
Synergetic Effect of Expansive Agent (KEA) and Superabsorbent Polymers (SAP) on the Shrinkage, Strength and Pore Structures of Mortars
Zhang, M; Aba, M; Sakoi, Y; Tsukinaga, Y; Shimomukai, K; Kuang, YH
JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.19, pp.26-39, 2021, JAN.
(https://doi.org/10.3151/jact.19.26)
Abstract
In this work, the effects of the individual or hybrid addition of superabsorbent polymers (SAP) with varying dosages (0.1%, 0.2%, 0.3%, and 0.6%) and the lime-type expansive agent (KEA) on the length and mass change, compressive strength, and pore structures (MIP) of mortars were investigated. The results showed that the incorporation of SAP can effectively mitigate its autogenous shrinkage and the length change value of the mortar with SAP smaller than reference until 49 days, regardless of the presence of KEA. The hybrid addition of SAP and KEA increases the initial expansion of the specimens as compared with individual addition of SAP, which is a beneficial effect on compensating for the shrinkage of the mortar under drying conditions. Moreover, the addition of SAP seems to delay cement hydration and increase the volume of macropores (greater than 100 nm), thereby reducing the compressive strength of the mortars. The introduction of KEA slightly promoted the formation of micropores, resulting in a slight increase in compressive strength compared with the samples without KEA. Furthermore, in our view, it promotes pore refinement, so as to reduce moisture evaporation.
Undersampling Strategy for Machine-learned Deterioration Regression Model in Concrete Bridges
Okazaki, Y; Okazaki, S; Asamoto, S; Chun, P
JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.18, pp.753-766, 2020, DEC.
(https://doi.org/10.3151/jact.18.753)
Abstract
Inspection data of actual concrete structures should be analyzed to elucidate the deterioration mechanism and construct a regression model. Although machine learning can be applied to this problem, inspection data are not suitable because machine learning targets big data with a uniform density and a balanced distribution. This study applies machine learning to a regression model of the crack damage grade in concrete bridges, using imbalanced inspection data. The model performance is improved by analyzing the influence of undersampling. Undersampling is conducted step-wise, and the models are constructed by learning all the undersampled data. The cross-validation of these models yielded the regression errors on each crack damage grade to evaluate the model performance considering the bias of data imbalance. Based on the results, the effect of undersampling on the model performance is analyzed, and the appropriate model is selected. Additionally, the influence of the model difference on the evaluation is investigated via historical change or factor analysis to confirm the effect of undersampling. This article not only presents a case study of a regression task for crack damage grades in concrete bridges, but also describes a strategy to maximize the use of imbalanced data for regression problems.
Drop-Weight Impact Loading of Polypropylene Fiber Reinforced Concrete Wall after One-Year Drying Shrinkage
Sato, Y; Naganuma, K; Ko, H; Kaneko, Y
JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.18, pp.794-807, 2020, DEC.
(https://doi.org/10.3151/jact.18.794)
Abstract
Recent progress in finite element analysis aids the simulation of seismic vibration of an entire reinforced concrete (RC) building structure and indicates that drying shrinkage cracks affect seismic resistance performance. Polypropylene fiber-reinforced concrete (PFRC) is a promising material since the fibers will reduce the cracks and strains under drying shrinkage. This paper attempts to quantify the vibration characteristics of PFRC walls by means of a drop-weight test and finite element analyses. Four wall specimens having the same geometry and bar arrangement are prepared. After a one-year drying shrinkage period, the walls are subjected to impact loading of a constant collision velocity of 5 m/s, using a steel drop weight of 398.8 kg. Shear cracks are observed in the restrained wall made of plain concrete, while cracks are insignificant in the PFRC wall. Three-dimensional (3D) nonlinear finite element analyses are conducted to simulate all behaviors from drying shrinkage cracking up to the time of impact loading, and to estimate the vibration characteristics. The analysis results indicate that the polypropylene fiber content reduces the elongation of the natural period by an average of 13.7%.
Shear Bifurcation and Gravelization of Low-Strength Concrete
Yamanoi, Y; Maekawa, K
JOURNAL OF ADVANCED CONCRETE TECHNOLOGY, Vol.18, pp.767-777, 2020, DEC.
(https://doi.org/10.3151/jact.18.767)
Abstract
Shear failure experiments of concrete beams containing a weak layer were conducted with a focus on the bifurcation of shear localization appearing at the boundary between structure and soil foundation. Low-strength concrete, which is analogous to artificial soft rocks and strengthened foundation, was used to create a weak layer that caused dispersal and bifurcation of the shear localization area, resulting in ductile fracturing of members. Pulverization of hardened cement paste and gravelization (the loss of aggregate particle's cementation) were observed in shear planes appearing in the weak layer. This confirmed the difficulty of simulating bifurcating shear localization solely by the constitutive law of concrete, which assumes firm cementation by hardened cement paste. In reference to the simulation of the disintegrated concrete slabs for bridge decks under fatigue loads, the transient model from hardened concrete to gravelized assembly was proposed, and it was successfully applied to the bifurcating shear localization of weak layers of low-strength concrete.