This paper evaluates the efficiency of polypropylene fibers in improving the tensile and flexural characteristics of fiber-reinforced alkali-activated slag composites with strain-hardening behavior. Several different composites were produced and the mechanical properties of the composites were investigated during compressive and flexural tests. Furthermore, the mid-span curvature of the beams was measured by applying the digital image correlation technique to the flexural tests, and the curvature-deflection diagrams of the composites were analyzed and demarcated by structural mechanics. Finally, an analytical model considering strain-hardening behavior was proposed by fitting the calculated moment-curvature diagrams to the experimental results to recognize the tensile behavior of composites more precisely than the existing indirect methods. The results showed that an ultra-high tensile strain capacity was obtained for composites cured under laboratory conditions. Also, it was found that the fiber interfacial bond with the matrix was improved by using slag with lower Al₂O₃ content. Moreover, weakening the interfacial transition zone between the fine aggregate and the paste, the heat-treatment curing reduced the fracture energy of the composites which is favored to make strain-hardening cementitious composites.

Carbonated wollastonite-based clinker is proposed as a potential novel supplementary cementitious material. The clinker can be synthesized from limestone and a silica source, and exhibits a CO₂-footprint after carbonation, which is approximately 60-70% less than the one of Portland cement. When carbonated, a silica-rich amorphous phase forms besides various polymorphs of calcium carbonate. When the carbonated clinker is blended with Portland cement, hydration is accelerated, and a significant pozzolanic reaction of the silica-rich amorphous phase takes place. This leads to a positive contribution to compressive strength. In a blend with 30% replacement of Portland cement by the carbonated wollastonite-based clinker almost the same mortar compressive strength is reached after 28 days compared to the plain Portland cement. Further investigations, especially regarding the production of the carbonated clinker, the durability of concrete incorporating this novel material, and life cycle analyses as well as economical assessments are needed.

Moisture transport is the key phenomenon indicating the deterioration of the durability and structural performance of concrete structures. Although various studies have attempted to evaluate moisture transport in concrete, an anomalous behavior, which does not follow the root-t law compared to other porous material, was not explicitly taken into account. To quantitatively evaluate anomalous moisture transport, this study developed a couple of numerical methods between the truss-network model (TNM) and the rigid-body-spring model (RBSM) for this purpose. The colloidal behavior of calcium-silicate-hydrate (C-S-H), which is the major phase of cement-based material, was introduced to consider the anomalous behavior and mechanical response regarding the microstructural change of cement paste as well as cracks that significantly accelerate the moisture transport in concrete. The numerical results indicated that both microstructural change of cement paste and rapid absorption through cracks cause anomalous behavior. In addition, the numerical results suggest that volumetric change of cement paste should rely on water content related to the colloidal behavior of C-S-H in order to reproduce the realistic expansion and the closure of cracks during a rewetting process that affects structural performance and durability of concrete.

This study investigates the wet carbonation of concrete fines with CO₂ and natural air gas bubbling in a carbonation system at low temperatures. After the air- and CO₂-wet carbonations, the properties of a solution and hydrated cement paste powder are determined. In the air and CO₂-wet carbonations, more Ca is extracted into the solution at a low temperature of 5°C. This high Ca concentration in the solution through air-wet carbonation primarily originates from the portlandite and unhydrated phases of the cement paste. Even in solutions with high pH values, the rehydration process and C–S–H decomposition occur simultaneously in air-wet carbonation. Moreover, CO₂-wet carbonation indicates that the decalcification of C–S–H occurs rapidly, even in the presence of portlandite. Air-wet carbonation presents a potential method for the direct air capture of CO₂ using concrete waste fines in a short period.

The coefficient of thermal expansion (CTE) of cement paste is an essential parameter for estimating cracks of cement-based structures, including under normal operating conditions. The CTE of low-heat Portland cement pastes dried for a long term at various relative humidities were measured by applying trapezoidal temperature history. The measured CTE was a convex function when displayed versus relative humidity and was highest at the relative humidity of 58%. At the relative humidity of 11%, the CTE was similar to the one of the fully dried sample. Based on a drying shrinkage model in the literature that classifies pore water as free liquid water and adsorbed water, we computed pore pressure change and corresponding strain, from which the CTEs were estimated. The microstructural rearrangements of cement paste due to long-term drying were taken into account by obtaining pore size distributions from water vapor sorption isotherm. The CTEs predicted with the model agree well with the measured ones.

The tension stiffening behavior of reinforced concrete (RC) prisms, affected by the aggregate volume expansion induced by neutron irradiation, were numerically investigated using a rigid body spring network model. First, the model was validated by comparison with the uniaxial tension test results of wet- and dry-cured (with volume contraction of concrete) RC prisms. Subsequently, different degrees of expansion strain were applied to the aggregate elements in the RC prism model and the uniaxial tension loading was simulated again. Tension stiffening decreased under larger radiation-induced volume expansion of the aggregate owing to the corresponding decrease in the concrete tensile strength with increasing damage, this behavior changed considerably according to the restraint condition. Indeed, the Young’s modulus of the restrained concrete after aggregate expansion was larger than that of the unrestrained concrete after aggregate expansion. However, the compressive stress in the concrete after aggregate expansion was effectively transmitted to the rebar during uniaxial tension loading; this behavior indicated that RC could maintain its integrity under uniaxial tension even after 0.5% aggregate linear expansion.

Electro-deposition repairing concrete crack technique is of vital importance to prolong the service life of the concrete structure in the marine environment. In this paper, the repairing effect using seawater as an electrolyte was compared with ZnSO₄ solutions, and the mechanism of electro-deposition repairing concrete cracks under seawater environments was investigated. Besides, various parameters such as electrode materials, electrode distances and current density were studied in seawater. The experimental results indicated that electro-deposition repair using seawater is highly effective compared to using ZnSO₄ solutions. The Cl^- removal efficiency can reach to 16.25% with titanium mesh as the anode and seawater as the electrolyte. The main crystal mineral composition using seawater is brucite, calcite and aragonite, and the percentages of Mg(OH)₂, Ca(OH)₂ and CaCO₃ are approximately 68%, 7% and 23%, respectively. Rather than Na⁺, Mg²⁺ and Ca²⁺ in seawater were the primary ions promoting the repair of concrete cracks. When the seawater and titanium mode was used, the reaction products were the densest and the repairing effect was the best. Furthermore, when the titanium electrode distance was 0 mm and the current density was 0.25 to 0.5 A/m², the electro-deposition repairing effect was the best. This paper presents a new method for the research of electro-deposition repairing concrete cracks in the submerged zone of marine environment.

The theory of ionic diffusion in water-saturated porous solids with surface electric charges has been constructed by using the general theory of diffusion, Gauss’s law, and the condition of electrical neutrality. The theory derives the diffusion rate of ions not from the gradient of the ionic concentration but from the gradient of the chemical potential of ions. The chemical potential is obtained by rigorously solving the Poisson-Boltzmann equation that is derived by connecting the general theory of diffusion and the condition of electrical neutrality with Gauss’s law. Application of the theory to the ionic diffusion from an outer solution into a pore solution surrounded by two parallel charged plates indicates that the maximum concentration of ions penetrable into the pore solution is lower than that of the outer solution, though the penetration rate is not much affected by the surface charge. A simple approximation method of calculating the diffusion rate without solving the Poisson-Boltzmann equation is presented.

The Fukushima Daiichi Nuclear Power Plant lost its core cooling function due to the massive tsunami generated by the 2011 off the Pacific coast of Tohoku Earthquake, which caused core meltdown, resulting in high temperature inside the containment vessel and exposing the RPV pedestal, a reinforced concrete structure, to an unusually high temperature environment. After the earthquake, water was poured into the containment vessel to cool the molten core, and the con-crete structure was gradually cooled in the process. Since it will take at least 40 years to remove the fuel from the core, the integrity of the RPV pedestal is a major concern for the decommissioning of the Fukushima Daiichi Nuclear Power Plant. In order to assess the long-term integrity of the RPV pedestal, a horizontal loading test was conducted using a 1/6 scaled model of the RPV pedestal of Unit 1 considering the effect of the high temperature heating and subsequent wet conditions. And then, the static stress analysis of the RPV pedestal was performed considering the degradation phenomena revealed by the experiments. As a result, it was confirmed that the RPV pedestal of Unit 1 would be structurally sound for 40 years against the current design basis earthquake even if the material degradation due to severe accident and aging was considered.

Countermeasures against carbon dioxide emissions are a concern in the construction field as well as in society. To solve this problem, a concept for new calcium carbonate concrete is proposed, and this concept is validated experimentally. In the proposed concept, calcium carbonate comprising Ca originating from demolished concrete or other Ca-containing industrial wastes and HCO₃^- from CO₂ gas collected from the air or emitted by industrial plants acts as a binder for aggregates, which can be natural rocks or crushed demolished concrete. This short paper describes the details of the process of making calcium carbonate concrete and discusses future perspectives.

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.

As the nuclear fleet in the United States ages and subsequent license renewal applications grow, the prediction of concrete durability at extended operation becomes more important. To address this issue, a Fast-Fourier Transform (FFT) method is utilized to simulate aging-related degradation of concrete within the Microstructure Oriented Scientific Analysis of Irradiated Concrete (MOSAIC) software. MOSAIC utilizes compositional phase maps to simulate damage from radiation-induced volumetric expansion (RIVE), applied force, creep, and thermal expansion. This compositional detail allows each mineral in the microstructure to be assigned specific material properties, allowing the simulation to be as accurate and representative as possible. The principal goal of MOSAIC is to simulate the effects of nonlinear aging mechanisms occurring in nuclear concrete on the macroscopic mechanical properties, using only the aggregate microstructure compositional information as a starting point. Several realistic example simulations are shown to demonstrate the utility and uniqueness of the MOSAIC software.