The crucial part of nuclear waste storage is the construction of sealing structures made of reliable, safe and well–understood materials. We present an extended analysis of long-term multi–sensory monitoring and non–destructive testing (NDT) inspection of two laboratory specimens aiming at potential materials for sealing structures for nuclear waste repositories. Specimens with a volume of 340 litres made from newly developed alkali–activated materials (AAM) and established salt concrete (SC) were analysed using embedded acoustic emission and wireless radio-frequency identification (RFID) sensors, ultrasonic echo imaging, active thermography, and X–ray computed tomography. The monitoring analysis showed lower heat of reaction and 50% less acoustic emission events in AAM compared to SC. However, due to the surface effects of the AAM material, the number of acoustic emission events increased significantly after approximately two months of monitoring. Subsequently performed NDT inspections reliably located embedded sensors and confirmed the absence of major cracks or impurities. The presented laboratory results show the feasibility and potential of comprehensive NDT monitoring and inspection to characterise cementitious and alternative materials as well as the need for multi–parameter long–term monitoring. Thus, our study demonstrates that tailored NDT investigations will help to develop safe sealing structures for nuclear waste repositories.

Mechanical stress can promote dissolution of calcium hydroxide in concrete; combined with carbonation, this may accelerate creep, damage, and corrosion. The existing microstructural simulators struggle with stress-induced dissolution processes, because of their strong chemo-mechanical coupling. This article presents results from recent Kinetic Monte Carlo simulations of dissolution and precipitation of solid phases, described as mechanically interacting particles. Chemo-mechanical coupling is embedded in the chemical reaction rates, featuring both chemical potentials of ions in solution and mechanical stress in the solid. The simulations are applied to a calcium hydroxide crystal, at chemical equilibrium in a solution of its ions but compressed between two stiff platens. Depending on the applied stress, the crystal partially dissolves and recrystallizes, producing an apparent viscoplastic deformation process. The simulation results inform mathematical creep laws with a size effect, as the strain rate depends on the crystal size. Upper-bound creep moduli for typical calcium hydroxide crystals in concrete are estimated as 237 to 2370 GPa, and carbonation would likely reduce them. This indicates that stress-induced dissolution of calcium hydroxide may underpin the nonlinear acceleration of concrete creep during carbonation. An experimental strategy to assess this mechanism is finally proposed, leveraging the size effect argument.

Corrosion of steel components embedded in concrete is detrimental to the structural integrity and their intrinsic function, such as the leak-tightness of containment liners in nuclear reactor containment buildings. Detection of embedded liner corrosion is a challenging task due to the concrete’s heterogeneity and the large dimensions of containment buildings. This initial study explores early-stage detection using nonlinear wave modulation on corroded liner plates embedded inside small lab-scaled concrete cylinder specimens. The liners were corroded before being embedded in concrete to minimize the risk of concrete cracking, allowing for a focused exploration of early-stage detection capabilities. Impact-based modulation and Luxembourg-Gorky type cross-modulation techniques were employed. Impact-based modulation uses a hammer impact to introduce a strong perturbation on a weak probe signal, analyzed with the Hilbert-Huang transform. Cross-modulation uses a second amplitude-modulated ultrasound to transfer modulation onto the weaker probe, analyzed via spectral analysis. Initial results indicate that impact-based frequency modulation is not highly sensitive to corrosion products alone and may not effectively identify specimens without concrete defects. However, impact-induced amplitude modulation is more responsive to corrosion and cracks in concrete. Cross-modulation shows strong sensitivity to crack-like defects and embedded severe corrosion but may not be optimal for early-stage corrosion detection due to the presence of inherent concrete cracks. Further research is needed to evaluate nonlinear ultrasonic techniques applied to larger concrete specimens and under realistic conditions before making practical recommendations.

The spatial distribution of crescent ribs and the multiphase properties of concrete make it difficult to predict the bond behavior between steel and concrete. The crescent ribs on the surface of rebar are explicitly simulated at meso-scale, which are stretched out along the trajectory and merged into the rebar. A multiphase concrete model is built, composed of mortar, aggregate, and interface layer (ITZ). The interaction model between ribbed bars and concrete is established considering the frictional and mechanical interaction. The effects of rib spatial distribution, aggregate content and stirrup ratio on bond performance are discussed. The results show that the bond strength of the transverse ribs arranged in the opposite direction is much higher than that in the same direction, but adding longitudinal ribs or increasing the inclination angle of transverse rib can reduce this effect. The sensitivity of bond strength on rib height is greater than that on rib spacing and rib inclination angle. The bond performance of steel first increases and then decreases with the increase of aggregate content. Excessive aggregate content makes the concrete brittle and reduces bond ductility. The constraint effect of stirrup on splitting failure of beams is more significant than that on pull-out failure.

The hydration behaviors of Na and Al co-doped β-C₂S in NaAlO₂ and NaOH were compared with that in water by performing isothermal calorimetry. The microstructure development of C-S-H gel obtained from β-C₂S hydrated pastes was characterized with XRD, TGA/DTG, ²⁹Si, ²⁷Al and ²³Na NMR, and SEM/BSE-EDS. Results indicated that both alkalis accelerated the hydration process of β-C₂S and stimulated early and later ages mechanical strength of β-C₂S pastes and NaAlO₂ had the better effect. ²⁷Al and ²³Na NMR confirmed the incorporation of aluminum and uptake of sodium in C-(N)-(A)-S-H structure. The presence of sodium ions promoted the incorporation of tetrahedrally coordinated Al in C-(N)-(A)-S-H structure. Alkalis enhanced the precipitation C-(N)-(A)-S-H gel with higher percentage of Q¹ units and shorter silica chains in the structure. The morphology and chemical composition of C-(N)-(A)-S-H gel in the pastes depend on the hydration medium used for the hydration.

The authors are developing mechanically-compacting flowable concrete with low CO₂ emission for rationalization, labor reduction in concrete work, and reduction of CO₂ emissions. In this paper, as a case study on the application of developed concrete, a concrete mix and a combination of reinforcement materials that satisfy the design criteria strength and specified durability have been set, and their application in terms of productivity, economy, and environmental impact has been estimated. The desired improvement in productivity and reduction in CO₂ emissions could be achieved, and the construction costs required to achieve these goals were generally understood. The CO₂ price in CO₂ emissions trading was calculated for each case, and it was shown that the CO₂ price depends on the CO₂ reduction measure.