In 2008, Nuclear and Industrial Safety Agency (NISA) (currently integrated to the Nuclear Regulatory Authority) launched a project to develop a soundness assessment method for concrete members subject to a radiation environment. Presently, the soundness of concrete members subject to radiation is evaluated based on whether the predicted fast neutron fluence and gamma-ray dose values are lower than specific reference values in Japan, which are 1×1020 n/cm2 and 2×105 kGy, respectively. These reference values were determined based on report by Hilsdorf et al. This project begins by reviewing Hilsdorf et al.’s report, and we find that the scientific evidence for the current reference values is weak. We thus conclude that new experimental research is required to assess the current reference values and to propose a new alternative soundness assessment procedure if needed. We quantitatively evaluated the influence of neutrons, gamma-rays, and the resultant heating and drying processes on the strength of concrete as well as their underlying mechanisms. The irradiation experiments confirmed the degradation mechanism of concrete due to neutron irradiation. The main reason for this degradation is the metamictization of rock-forming minerals, which, in turn, leads to aggregate expansion. Due to aggregate expansion, cracks around aggregates form, which reduce the compressive strength and Young’s modulus of concrete. Among the rock-forming minerals, α-quartz is the most sensitive to neutron radiation. 60Co gamma-ray irradiation experiments demonstrated that concrete strength increased as the gamma-ray dose and gamma-ray flux does not have a dose-rate impact on the first radiolysis of evaporable water in cement paste within the present study. The effect of gamma-ray irradiation on the properties of concrete is equivalent to that of heating and drying. Concrete strength alteration due to heating and drying is attributed to the colloidal and porous nature of hardened cement paste and crack formation around the aggregate due to a mismatch in the volume changes of the mortar and aggregate. In addition, a numerical analysis code called DEVICE (Damage EValuation for Irradiated ConcretE) is developed to harness knowledge obtained from concrete samples to predict the distribution of the physical properties in concrete members and their changes over time. From these fundamental studies, we propose a new soundness assessment procedure for concrete members subject to radiation. We also recommend a new radiation-induced strength-degradation reference value of 1×1019 n/cm2 for fast neutron.
Rheology of two grades of Self-Compacting concrete containing selected volumetric replacement levels of coarse Recycled Concrete Aggregates (SCRCA) is investigated. Fly ash, metakaolin and silica fume were used in different combinations of cementitious materials to achieve the normal- and the medium-strength SCRCAs. Irrespective of the RCA replacement level, a shear thickening response was noted in both the normal- and the medium-strength SCRCAs and their flow behaviour could be described using the Modified Bingham (MB) as well as the Herschel-Bulkley (HB) model. The decrease in the degree of shear thickening with an increase in concrete grade is attributed to the use of either silica fume or metakaolin as part of the ternary cement in the higher strength concretes with silica fume being relatively more effective. Within a given grade, shear thickening increased with increase in the recycled aggregate replacement level. The MB as well as the HB model were equally effective in representing observed flow behaviour.
Self-healing concrete can repair itself by closing micro-cracks and thus protect itself from ingress of deleterious gasses and liquids that can affect its durability. Many self-healing concepts have been developed in the recent years which target on the recovery of water tightness after cracking. Among those systems, the bio-based healing agents have shown promising results regarding the crack sealing performance. This paper studies the crack sealing efficiency of bio-based healing mortar with expanded clay particles. The investigation of sealing performance is conducted through experimental and computational approaches. Image processing and crack permeability test results are compared with results obtained by computer simulations. The study reveals that the experimental approaches might overestimate the crack closure percentage, while the computer simulation mostly underestimates the crack sealing. Finally, recommendations are given to improve the results obtained by both methodologies.
Reinforced concrete (RC) buildings are often constructed with non-structural walls. Severe damage to non-structural walls has been observed in many RC buildings after major earthquakes in and around Japan, such as the 2011 earthquake off the Pacific coast of Tohoku. Several studies have verified that non-structural walls affect the seismic per-formance of RC buildings. However, no design methodology has been proposed for considering the structural effects of non-structural walls. This study focuses on the RC non-structural walls used as exterior/partition walls in typical residential buildings in Japan. Cyclic loading tests were performed using three 1/2.5 scale, one-story, one-bay RC moment-resisting frame specimens with and without non-structural walls, which were monolithically constructed or structurally isolated by seismic slits. The isolated wall as well as the monolithic wall significantly increased the strength of the moment-resisting frame specimen. Furthermore, this study proposes analytical models to simulate the experimental results and to clarify the effects of non-structural walls on the overall performance of test specimens. The test specimens were replaced by line elements with multi-spring models while considering the interaction between the bending moment and axial force. The analytical simulations generally agreed well with the experimental results. In conclusion, the analytical models applied to the simulations in this study are effective for evaluating the seismic behavior and performance of RC moment-resisting frames with the typical non-structural walls used in Japan.
The ultra high strength fiber reinforced concrete (UFC) is an advanced cementitious material which grants the high performances in strength, durability, and ductility. Because of these high performances, in this study, it was applied into the new idea for shear strengthening to RC beams, namely the post-tensioned UFC panel. It is the prefabricated UFC panel prestressed by some prestressing force. Three parameters (i.e., the number of post-tensioned UFC panel, the amount of prestressing rods, and the prestressing level in the UFC panels) were studied from eight strengthened beams for investigating the shear strengthening performance. All specimens were subjected to four-point bending. The results of the strengthened beams and the non-strengthened beam revealed that the strengthening by post-tensioned UFC panel increases the shear capacity, stiffness, and ductility of the beams. Moreover, the prestressing level also influenced the delay of the initial cracking load and the yielding of stirrups. Finally, the shear capacity carried by the post-tensioned UFC panel was proposed by using the force equilibrium along the assumption of imaginary diagonal crack.
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