Observed damages in reinforced concrete wall buildings following some recent earthquakes raised concerns about the seismic performance of rectangular RC walls. Damages in RC walls included spalling and crushing of concrete and longitudinal reinforcement buckling at boundaries as well as global buckling. Preliminary studies attributed these damages to the lack of adequate confinement and detailing in wall boundary regions and high axial load level. Prism specimens representing wall boundaries were tested to study the influence of reinforcement detailing, cross-section slenderness, and loading type on the damages, failure modes, and compressive capacity of isolated confined boundary regions of RC rectangular walls. It was found that the tensile strain prior to compressive strain affected the performance of thin wall boundaries and may lead to different failure modes when subjected to cyclic loading. It was also found that dense transverse reinforcement detailing in thin confined boundaries did not improve their compressive capacity. Design and detailing rules to prevent global buckling and reinforcement bar buckling were also evaluated. A Numerical model that takes into account buckling of reinforcement was proposed to simulate response curves of cyclically tested specimens. The model showed the influence of reinforcement buckling behavior on reducing the compressive capacity for elements with buckling of reinforcement failure.
Recently, bacterial concrete is gaining popularity due to the advantages such as self healing property, increased durability, enhanced strength etc. When living organisms are introduced into cementitious materials, many factors are expected to influence their activity. A conducive environment such as temperature, nutrients, pH etc. is necessary for the survival and intended activity of bacteria in concrete. Among all the influential factors, nutrients for the survival of bacteria in cement mortar is considered in this study. In order to supply nutrients for the bacterial growth, three different curing media such as normal water, Luria Bertania broth (controlled nutrients) and wastewater (uncontrolled nutrients) are chosen. Compressive strength are determined at various stages of curing to study the influence of bacterial activity in cement mortar. Further, X ray diffraction and thermo gravimetric analysis are carried out to understand the transformation in hydration phases with the incorporation of bacteria. From the strength study conducted, cubes cured in waste water showed more strength which indicates that the bacterial activity is more in waste water.
Water re-curing of fire-damaged concrete may reduce the environmental and economic impacts of repair operations by re-using the existing concrete. To clarify the rehydration mechanism, the effects of water re-curing on the microstructure and chemical properties of fire-damaged cement paste were examined. Analysis by X-ray CT showed that heating led to radial cracks that propagated horizontally and vertically in the cement paste specimen. Water supply led to a growth in the cracked space through an increase in the connectivity of the crack network. This growth may be due to expansion caused by the rehydration of CaO into Ca(OH)2. Chemical analyses suggest that the rehydration reaction differs from the initial hydration in that C2S plays a greater role in generating C-S-H gel during the earlier period of water re-curing.
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.
The use of “Energy CO2 Minimum (ECM)” cement incorporating high amount of blast-furnace slag contributes to reduction of energy consumption and CO2 emission. In this paper, results of experiments which were conducted in the purpose of applying the concrete using the ECM cement for civil engineering structures are presented. The Results of experiments show that the fundamental properties of fresh and hardened ECM concrete with water-cement ratio of 50% are equivalent to those of concrete using blast-furnace cement type B (BB) with water-cement ratio of 55%. In addition, the excellent performance of durability was verified by the tests using large-scale wall members.
The evolution of steel bar and corrosive-crack propagation of reinforced concrete in three different solutions including MgSO4, Mg2+-SO42--Cl-, and seawater were examined using electrochemical impedance spectroscopy. Results showed that seawater had the most serious corrosion effect on steel bar in concrete. MgSO4 combined with chloride ions could accelerate the damage process of passive film, whereas Mg(OH)2 and gypsum particles could be produced and deposited on the steel-bar surface to block the diffusion of sulfate and chloride ions into the passive film. The crack propagation of concrete in single chloride environment increased with time linearly, but the existence of MgSO4 in chloride solution could change the crack-propagation behavior of reinforced concrete from linear to nonlinear. The existence of Ca(OH)2 in MgSO4 and MgSO4-NaCl solutions could not delay the initial corrosion time but reduce the corrosion rate of steel bar by 60%–85%.