The Fukushima-Daiichi nuclear power station accident in East Japan in 2011 released radioactive cesium (r-Cs) into the surrounding environment. In addition to the major contamination of soil and vegetation, surrounding concrete structures have also been contaminated. Understanding the contamination characteristics is important for the decontamination or disposal of offsite and onsite concrete structures. However, r-Cs contamination of actual concrete has not been extensively investigated. In this paper, the r-Cs contamination characteristics of actual concrete are introduced on the basis of several field surveys and analysis of concrete samples taken from actual contaminated structures in the Fukushima prefecture. As no dissolution to water was detected, the surface contamination was safely removed by water-jet abrasion. However, in KCl solution, significant dissolution of r-Cs was detected. A contamination depth analysis was performed by quantitative r-Cs concentration mapping based on β-ray radiography using imaging plates. In many cases, such as roadside gutter cover plates, r-Cs was located on the surface without significant penetration into the concrete. However, in cases such as degraded concrete, deeper penetration up to several millimeters deep was observed. In cracked concrete, the r-Cs penetrated at least 10 cm, with the contamination level decreasing one order of magnitude in the first 1 cm.
Hardened cement pastes (HCP) with different water contents were irradiated with gamma rays under different temperatures and irradiation dose rates. The relationship between the quantity of hydrogen gas produced and the water content as well as the stability of HCP under gamma irradiation was evaluated. It is experimentally confirmed that hydrogen gas was mainly produced from the evaporable water. The G value of the hydrogen production assuming the radiation energy absorbed by the total water composed of chemically bound water (CBW) and evaporable water was ranging from 0.03 to 0.42. The G value of the hydrogen production for CBW was ranging from 0.03 to 0.07, which were an order of magnitude smaller than that of the bulk water (0.45).
Assuming that the radiation energy on evaporable water is used for the formation of hydrogen, it is experimentally confirmed that, in case of low dose rate, the G value tended to converge to a constant value when the evaporable water exceeded a certain value, while, in case of high dose rate, the G value increased as evaporable water increased. However, the G values of all cases grew with increasing evaporable water content and exceeded the G value of the bulk water (0.45). The CBW was not susceptible to gamma irradiation. Only 2 to 3% of the CBW was estimated to be decomposed by 200 MGy of gamma irradiation.
A new method aiming to improve bond performance between rebar and concrete is proposed in present paper. In this method, the rebar embedded in self-compacting steel fiber reinforced concrete (SSFRC) was magnetized to attract surrounding steel fibers. The special combination of rebar and the attracted steel fiber was expected to improve bond strength of rebar and concrete. Six groups of specimens were made to investigate the effect of rebar magnetization, steel fiber volume fraction and compacting method on bond strength between rebar and concrete. Results show that magnetization of rebar can greatly improve bond strength with the highest increase of 52%. The steel fiber volume fraction can be largely reduced under rebar magnetization condition and the highest bond strength 23.78 MPa was found on specimen with rebar magnetization of 30 mT and 0.25% steel fiber. Moreover, SSFRC was better than vibrated steel fiber reinforced concrete (VSFRC) when using this new method. A bond stress-slip model reflecting the effect of rebar magnetization and steel fiber volume fraction was also developed and verified by experimental results.
Recent studies in the field of concrete materials show that the early cracking criteria in micro-size can occur as soon as the cement matrix becomes hardened. In many ways, these cracks can become macro-size and opened cracks resulting in significant issues for the durability and appearance of concrete structures as water leakage and corrosion. The technique of self-healing using bacteria has recently received attention for its potential applications. However, the effectiveness and the repeatability of this method over a long period have not been clarified. The information on both the survival and the number of bacteria after healing is limited. This paper aims to improve the self-healing ability and repeatability of concrete when using Bacillus subtilis natto. The experimental studies evaluate the effect of biomineralization with lightweight aggregate as the protecting-carrying vehicle, which can control the release of healing fluid through four cracking-healing cycles. The urease activity and the biomineralization of the bacteria with urea as the main carbon source were assessed and the effect of cracking age on the self-healing capacity, associated with the compressive strength improvement was studied. The results obtained from the optical microscope and SEM/EDS analysis indicated the existence of bacteria CaCO3 forming in concrete after four healing cycles. During long duration, bacterial concentration in concrete was determined by microscopic counting method. Based on experimental results, the restoration of the compressive strength confirmed the high self-healing ability of concrete when using bacteria in lightweight aggregate.
Multiple factors, regarding mechanical properties, load levels, and environmental conditions, affect the fatigue lifetime of reinforced concrete (RC) bridge decks. For the mechanical behavior, previous research predicts the fatigue lifetime of RC decks as a function of their punching shear capacity, where they give less attention to other modes of failure due to experimental limitations of fatigue loading and the restriction of girders spacing in the past design practice. Nowadays, multi-scale simulation can deal with fatigue loading problems, which secures examining complex situations that cannot be easily reproduced in the laboratories of fatigue tests. In this study, various RC decks with wide range of dimensions, material properties, reinforcement ratio, and load levels are analyzed by the validated multi-scale simulation. Then, artificial neural network (ANN) based model is also proposed based on wide-range of studied cases, which estimates the fatigue life of newly constructed RC decks, where it can be the basis of performance-based design. After that, the impact of deck’s properties on fatigue life is evaluated based on the built ANN model, which matches the conceptual design of RC decks. Finally, coupling of an empirical equation and ANN model is proposed, which may support conceptual decision-making.
This study was aimed at developing novel and sustainable alkali-activated binder of silico-manganese fume (SMF) and ground blast furnace slag (BFS) at room temperature. The level of BFS substitution was varied from 0 - 50% and activation was done using 10 M NaOH and Na2SiO3 of an initial silica modulus of 3.3. The Na2SiO3/10MNaOH ratio was kept constant at 2.5. The setting time, flow and compressive strength were evaluated and complemented with the microstructural and bond characteristics of the product. A 28-day strength of 45 MPa was achieved with the base materials ratio (BFS/(BFS+SMF)) of 0.3. The activation of SMF resulted in the formation of glaucochroite and nchwaningite minerals (C-Mn-S-H) while the inclusion of BFS influenced the formation of potassium feldspar (K-A-S-H), gehlenite hydrate (C-A-S-H) and additional C-S-H due to later-age contribution of hydration of BFS. The Manganese in SMF contributed significantly to the consistency of the mixture while lime in BFS improved the microstructure thereby enhancing strength of the developed alkali-activated binder.