PSI - Issue 66

Mansi Gupta et al. / Procedia Structural Integrity 66 (2024) 122–134

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Mansi Gupta et al./ Structural Integrity Procedia 00 (2025) 000 – 000

1. Introduction Concrete is a widely used construction material throughout the world with cement being the major constituent in it. Concrete is heterogeneous by nature and can be classified as multiscale and multiphase material. The various phases interact across multiple length scales ranging from macro- to nano- scale; this interaction between different constituting particles is responsible for the concrete’s behaviour. Hence, a complete understanding of the structural response of concrete requires the analysis of different scales and their contribution in overall properties. At macroscale, concrete is characterised as homogeneous material and plays an important role in long term durability properties. At mesoscale, concrete is a three phase material consisting of mortar, coarse aggregate and interfacial transition zone. The interactions at mesoscale determines the load response of concrete structures under different loading and environmental conditions. At nanoscale, the hydration of cement results in format ion of various Bogue’s compounds which are essential for strength and durability of concrete mix. Among the various hydration products, Calcium Silicate Hydrate (CSH) is primary binding phase attributing to 60% of total volume and is mainly responsible for structural performance. It does not have a well-defined crystalline structure. Instead, it has a highly disordered, amorphous nature, which contributes to its ability to fill pores and bind other components together [Taylor (1986)]. The chemical composition of CSH can vary, but it generally contains calcium, silicon, and water. The Ca/Si ratio typically ranges between 1.5 to 2.0, although this can vary based on the hydration circumstances and the specific cement used. The experimental studies reveal the stoichiometry of CSH to be (CaO) 1.7 (SiO 2 )(H 2 O) 1.8 [Pellenq et al. (2009)]. A comprehensive understanding of the response of CSH gel helps in improving the properties of cement matrix. The domain of high performance sustainable material by improving the qualities at nanoscale attracts a lot of attention and is highly dynamic. However, addition of different materials can alter the concrete’s chemistry and needs to be analysed carefully. Hence, a thorough understanding of properties of CSH at nanoscale is imperative to facilitate the creation of lightweight and environmentally friendly infrastructures with reduced material requirements. Molecular Dynamics (MD) is a robust technique used to computationally mimic the physical movements of atoms and molecules over time. By determining Newton's equations of motion for a system of particles, MD provides comprehensive insights into the behaviour of materials at the atomic and molecular scales. By capturing the interactions between atoms, MD can envisage the material response to various conditions, offering a microscopic view of processes that are often challenging to observe experimentally. Fracture and crack propagation in materials is primarily caused by the breakage of atomic bonds at nanoscale. Hence, atomistic modelling can provide substantial information on the fracture characterisation in view of bond breakage. In recent years, MD has been effectively used for simulating the fracture in various metals and composites [Dorr et al. (2019), Molaei (2022), Abhiram and Ghosh (2023)]. In context of concrete, the MD has been extensively used to determine the mechanical and chemical characterisitics of cement [Redondo et al. (2022), Bahraq et al. (2022), Barbhuiya et al. (2023)]. It enables the investigation of hydration processes, binding mechanisms and provides valuable insights by modelling the motion of atoms and molecules inside the concrete matrix. Molecular dynamics (MD) simulations offer a vigorous framework for investigating the processes of defect accumulation and deterioration in concrete. Computational modelling (MD) simulations allow researchers to examine many categories of flaws, including as vacancies, dislocations, and grain boundaries, that greatly influence the mechanical and chemical characteristics of the material. However, it is crucial to acknowledge that molecular dynamics simulations are subjected to certain limits, including restrictions on size and timeframe. The intricate nature of concrete at larger dimensions and extended time periods continues to present difficulties for molecular dynamics simulations. Despite length and time scale restrictions, MD studies on concrete show a great potential for exploring the intricate failure mechanisms in concrete.

Nomenclature CSH

Calcium Silicate Hydrate Molecular Dynamics

MD

ps

Picoseconds