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Understanding the time-dependent behaviour of calcium silicate hydrates (C-S-H) under stresses is crucial for unraveling the microscopic creep behavior of concrete and its consequences on the evolution of nanostructure of C-S-H. In this study, the structural evolution of C-S-H nanostructure under stress was investigated using reactive molecular dynamics and stress perturbation technique. Under sustained stress, C-S-H nanostructure undergoes a relaxation process leading to a creep deformation and transitioning the system towards a more energetically stable state. The process was comprehensively examined under shear stress through a detailed analysis of the system's potential energy. A hardening phenomenon was observed within calcium silicate layers throughout this relaxation process, categorizing in three distinct stages. in Stage I, structural reorganization occurred in the silicate chains, progressing towards a stable state by reducing non-bonded interactions. The calcium-oxygen ionic bonds within the intralayer were strengthened in Stage II, leading to a decrease in bonding energy. Finally, the interface between calcium silicate layers and the interlayer contributed to overall structural stability in StageIII. • A hardening phenomenon was observed in calcium silicate hydrates nanostructure within intralayers during creep deformation. • The microscopic mechanism underlying this hardening process was elucidated and categorized into three distinct stages. • Stage 1: Structural reorganization in silicate chains, progressing toward a stable state by reducing non-bonded interactions. • Stage 2: The ionic bonds between calcium and oxygen within the intralayer were reinforced. • Stage 3: The interface between calcium silicate layers and the interlayer contributed to the overall stability of C-S-H. [ABSTRACT FROM AUTHOR]

Modelling of early-age behaviour of cement-based materials is still a challenging task. The challenge is implied by the extent of the knowledge on the subject which results in a variety of different models used for simulation of cement-based materials. That is why a numerical benchmark program has been launched within the COST Action TU1404 aiming at improvement and harmonisation of computational prediction of early-age behaviour of cement-based materials as well as its behaviour on structural level. This paper presents the result of the proof-of-concept stage of the benchmark. The goal of this stage of benchmark was to compare the performance of currently used models for simulation of early-age behaviour of concrete. The participants were requested to simulate thermo-chemo-mechanical behaviour of simple concrete elements covering adiabatic and real evolution of temperature, shrinkage, stiffness and stresses accounting for early-age creep. The tasks were formulated based on the experimental measurements. This stage of benchmark allowed to evaluate the influence of different phenomena occurring in early-age concrete on the behaviour of early-age concrete structures, define the discrepancies between experimental results and numerical simulations, as well as to indicate the weak points in the models. [ABSTRACT FROM AUTHOR]

Delayed ettringite formation (DEF) is a sulfate attack affecting civil engineering structures. This chemical reaction takes place within the concrete matrix of structures and causes damage in concrete and tension in reinforcements. For managers and owners, the ability to predict and reassess the mechanical behaviour of such structures is a major challenge. The influence of both reinforcements and prestress on DEF expansion were studied in the present work. Several tests were performed in laboratory: expansion under both uni and tri-axial restraint due to reinforcements and expansion under prestress (stress level of 14.5 MPa). Uniaxial restraint led to decreased strain in the restrained direction. For prestressed concrete, the loaded direction exhibited creep strain. In both cases, expansions were not impacted in transversal free directions. Therefore, DEF expansion under uniaxial stress is anisotropic. Cracks were observed parallel to the restrained direction. The final volumetric expansion was lower than in stress-free conditions (decrease of 27% for uni-axially restrained condition). For triaxially restrained tests and prestressed specimens, the volumetric decreases were 56% and 34% respectively. Data provided by these results will be used for numerical reassessment of DEF damaged structures. [ABSTRACT FROM AUTHOR]