Journal of Advanced Concrete Technology

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Volume 14 (2016) Issue 4 Pages 144-162

A Numerical Model for Concrete Strength Change under Neutron and Gamma-ray Irradiation

Ippei Maruyama, Kazumoto Haba, Osam Sato, Shunsuke Ishikawa, Osamu Kontani, Masayuki Takizawa

For performance evaluation of existing reinforced concrete members under irradiation conditions, a numerical code called “DEVICE” (Damage EValuation for Irradiated ConcretE), which takes into account the heat, moisture, and radiation transport coupled with cement hydration, is proposed. This code is composed of the established computational cement-based material (CCBM) model and the one-dimensional deterministic transport Sn code “ANISN”. In the proposed model, temperature-dependent irradiation-induced expansion of aggregate minerals and resultant strength deterioration of concrete are introduced. Currently, the knowledge and modeling of irradiation-induced expansion of aggregate mineral is limited only for α-quartz. DEVICE was used for evaluating the strength distribution of the decommissioned plant Japan Power Demonstration Reactor (JPDR). Compressive strength distribution in a concrete biological shielding (CBS) wall of the JPDR was obtained by core sampling, and the compressive loading test results were compared with the calculation results. This comparison proved the practicality potential of DEVICE to predict the concrete strength distribution in a CBS. In addition, concrete strength change and its distribution in a CBS of an anonymous two-loop pressurized water reactor was simulated by DEVICE. The contributing factors for the change in the distribution of concrete strength at the inner surface of the CBS are discussed. Furthermore, the ways of integrity evaluation other than the existing allowable fast neutron fluence method are proposed and discussed as follows: 1) mineral composition-based allowable fast neutron fluence; 2) strength prediction at the inner surface based on the expansion of mineral composition of aggregates and the lower limit curve of the ratio of compressive strength of the specimen after irradiation (Fc) to that of the reference specimen (Fco) as a function of concrete expansion; and 3) direct numerical calculation for seismic performance by considering irradiation-induced volume expansion and degradation of concrete.

Volume 14 (2016) Issue 5 Pages 183-204

Mechanism of Long-Term Excessive Deformation and Delayed Shear Failure of Underground RC Box Culverts

Koichi Maekawa, Xiaoxu Zhu, Nobuhiro Chijiwa, Shigeru Tanabe

The aim of this study is to clarify the mechanism of the progressive excessive deformation observed in real underground RC box culverts of about 30 years of age. It was found by the site-inspection, monitoring and the destructive testing that the excessive deflection of top slabs for the culverts, which is almost 10 times the design estimated value, accompanies the out-of-plane shear failure. It is also computationally investigated that the coupling of subsidence of the backfill soil and the combined creep and shrinkage of concrete after cracking is closely associated with the delayed shear failure found in the culvert in service. In order to prove the delayed shear failure under higher sustained loads, the time-dependent shear crack propagation was reproduced in the laboratory test and the computational approach used in this study was examined.

Volume 14 (2016) Issue 6 Pages 311-323

Action Mechanisms of Shrinkage Reducing Admixture in Hardened Cement Paste

Ippei Maruyama, Katsutoshi Beppu, Ryo Kurihara, Akihiro Furuta

Here we investigate the mechanism by which shrinkage reducing admixture (SRA) affects hardened cement paste (hcp). The first desorption process for hcp is always accompanied by irreversible shrinkage. Initially we demonstrate the well-known mechanism of SRA acting on capillary force using Vycor glass. Additionally, sorption isotherms and length-change isotherms are measured for both saturated hcp as well as hcp aged at 11% RH for two years. SRA was concluded to be present on the surface of the concave meniscus of the pore solution in Vycor glass, and that the inclusion of SRA reduces the surface tension of the pore solution, the equilibrium Kelvin radius, and the shrinkage due to capillary force. However, a comparison of long-term and short-term length-change isotherms and water vapor sorption isotherms of hcp suggests the possibility of partial evaporation of SRA molecules during the 2-year drying process, as well as the presence of immobile SRA in cement paste. Moreover, the immobile SRA is still active, and is found to reduce the amount of water sorption and shrinkage strain. It is thought that the secondary role of the SRA, which is related to immobile SRA in the cement paste, becomes active at room temperature, at below 80％ RH, and only occurs in the irreversible shrinkage component of hcp produced by the initial desorption process.

Volume 14 (2016) Issue 11 Pages 664-678

Modeling of Corrosion-Induced Damage in Reinforced Concrete Considering Electro-Mechanical Coupling

Di Qiao, Hikaru Nakamura, Yoshihito Yamamoto, Taito Miura

This paper presents an electro-mechanical model for evaluating the corrosion-induced damage in reinforced concrete under accelerated conditions. The model consists of structural analysis of concrete and the rebar using the Rigid Body Spring Method and corrosion current analysis with the truss networks model. The electric corrosion process is coupled with concrete cracking conditions by relating current efficiency to local crack width; the predicted radius losses of rebars are used to evaluate concrete crack propagation and residual tensile performance of corroded rebars. The model is validated with the results of accelerated corrosion tests using impressed current and a sodium chloride pond on the concrete cover. Good agreement with the test data is obtained with the proposed model, in terms of corrosion degree and profile, concrete crack pattern, and tensile behavior of corroded rebars. The model offers a corroborative tool with the accelerated corrosion technique to study the mechanical behavior of corroded concrete structures.

Volume 13 (2015) Issue 10 Pages 438-448

Rapid Degradation of Concrete Anchorage Performance by Liquid Water

Nobuhiro Chijiwa, Thi Mai Hong, Mitsuyasu Iwanami, Tomohisa Saito, Atsushi Yamaya, Hiroyuki Motegi, Hiroo Shinozaki

Sludge ejection from the foundation of a wind turbine tower fixed by the anchor-ring method and the resulting sludge buildup have been confirmed to cause the undesirable phenomenon of lifting of the tower. Using specimen that is a model part of the wind turbine foundation, this study investigates the influence of liquid water, differences in W/C, and differences in loading speed, to analyze developing factors on the concrete damage . The results shows that penetration of liquid water from rainfall and snowfall produces a wedge effect, breakdown of the concrete pore skeleton structure, grinding by sludge particle, cavitation and other effects, and leads to rapid deterioration of the anchorage performance of concrete foundation. Based on the knowledge on the deterioration process, rational countermeasures on design and maintenance are proposed.

View the past featured articles

2018 Volume 16 Issue 7

The Effect of Alkali Concentration and Sodium Silicate Modulus on the Properties of Alkali-Activated Slag Concrete Hamed Taghvayi, Kiachehr Behfarnia, Mohammadbagher Khalili
The Top-Bar Effect in Specimens with a Single Casting Point at One Edge in High-Performance Self-Compacting Concrete Piotr Dybeł, Daniel Wałach, Krzysztof Ostrowski

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