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Chloride anion attack is a major factor limiting the durability of concrete structures. To clarify the mechanisms by which chloride salts degrade concrete, nanoscale molecular dynamics (MD) simulations were used to study chloride attack on calcium silicate hydrate (CSH), the main component of cement. In MD simulations, a relaxation process is generally required to allow the system to reach equilibrium. However, relaxation is computationally expensive when performing MD simulations of large structural systems. This expense could potentially be avoided by using deep learning techniques. This paper describes the creation of a multi-fidelity physics-informed neural network model of a CSH gel pore containing an aqueous NaCl solution. The neural network's input variables are the ambient temperature and the NaCl concentration and its output variables are the system's energy, the Na-O radial distribution function, and the Na+ and Cl- ion density distributions. After training the model using the results of low-fidelity MD simulations without relaxation and a smaller number of high-fidelity simulations with relaxation, highly accurate outputs were obtained with prediction errors below 3%. Deep learning can thus greatly reduce the computational cost of MD studies of large and complex systems with no appreciable loss of accuracy. [Display omitted] • MPINN models can accurately predict the complex MD simulations (error < 3%). • The MPINN can reduce the computational costs by at least 50% and up to 90%. • The CSH surface adsorbs Na ions strongly, thus leading to adsorption of Cl ions. • Raising the temperature increases the rate at which ions reach the CSH surface. [ABSTRACT FROM AUTHOR]

This research studies the influences of pre-curing time, the number and environment of early-age freeze–thaw damage (E-FTD) and later-age curing methods on later-age performance of concrete subjected to E-FTD. The concrete specimens with pre-curing time of 18 h, 24 h, 48 h and 72 h were subjected to four different E-FTD systems. Subsequently, three later-age curing methods were performed to the time of 28 days respectively and finally the later-age compressive strength, resistance to chloride penetration and freeze–thaw of concrete were evaluated. The results show that all three later-age performance indexes improve gradually as the pre-curing time prolongs. The damage of the salt freeze–thaw environment to early-age concrete is severer than that of the water freeze–thaw environment, resulting in the unsatisfactory later-age performance even with restorative curing. Among all later-age curing methods, the restoration effect of water curing on later-age performance is the best. Compared to the standard curing, the restoration effect of natural air curing on later-age compressive strength and resistance to chloride penetration of concrete is better, but that on resistance to freeze–thaw is worse. For the concrete with shorter pre-curing time, the recovery percentage of durability indexes, especially the resistance to freeze–thaw, is always lower than that of compressive strength. The pre-curing time should be prolonged appropriately for the early-age freeze–thaw damaged concrete with the requirement of durability. [ABSTRACT FROM AUTHOR]