This study aims to systematically analyze the effects of different curing temperatures, unit binder content, and the mixture ratios of ground granulated blast-furnace slag and fly ash based on ordinary Portland cement in binders on the development of concrete compressive strength. Particularly, the study evaluates strength characteristics by calculating the maturity equivalent to 28 days of curing at 20 °C. A model based on the relationship between maturity and strength was applied to predict the compressive strength, and the experimental data were analyzed to derive strength coefficients for each variable. The results showed that at a low temperature of 5 °C, the actual strength was lower than the predicted strength, leading to higher error rates. In contrast, at temperatures of 20 °C and 40 °C, the coefficient of determination (R² > 0.90) for the predictive equation was high, and the error rates were reduced to within 10%. The study demonstrates that by combining the maturity method with the strength–maturity relationship, the concrete compressive strength can be effectively predicted under specific curing and binder design conditions.

In this study, accelerated chloride diffusion tests are performed on ordinary Portland cement (OPC), ground granulated blast furnace slag (GGBFS), and fly ash (FA) concretes aged 4–6 years. Passed charge is evaluated according to ASTM-C-1202 for 12 mixtures, considering water–binder (W/B) ratios (0.37, 0.42, and 0.47), GGBFS replacement rates (0%, 30%, 50%), and FA replacement rates (0% and 30%). The effects of aged days on passed charge reduction behavior are quantified through repetitive regression analysis. Among existing machine learning (ML) models, linear, lasso, and ridge models are used to analyze the correlation of aged days and mix properties with passed charge. Passed charge analysis considering long-term age shows a significant variability decrease of passed charge by W/B ratio with increasing age and added admixtures (GGBFS and FA). Furthermore, the higher the water–binder ratio in GGBFS and FA concretes, the greater the decrease in passed charge due to aged days. The ML model-based regression analysis shows high correlation when compressive strength and independent variables are considered together. Future work includes a correlational analysis between mixture properties and chloride ingress durability performance using deep learning models based on the time series properties of evaluation data.

This study analyzed the chloride ingress repair costs for RC girders by considering cold joints and external force conditions. The diffusion behavior of chloride ions, which varies with the stress conditions, was quantified. A two-span RC continuous girder was assumed to evaluate the magnitude of stress in each section under cracking load, and these results were comprehensively considered along with the quantification of chloride ion diffusion behavior. Then, the change in chloride ion diffusion behavior owing to external forces on the RC girders was analyzed to evaluate the service life under chloride ingress. At the locations with cold joints, the service life decreased by up to 33.9% owing to the higher baseline diffusion coefficient in the cold joint areas than that in sound concrete and significant increase in the diffusion coefficient with increased stress. Furthermore, this study analyzed the chloride ingress repair costs required to achieve the target service life of the RC structure with consideration of the external forces and cold joints using deterministic and probabilistic analyses methods. Probabilistic analysis offers the advantage of deriving the repair frequency and cost results in the form of continuous functions. Considering the four main points of the structure under analysis, the maintenance costs were evaluated at 2777.6 $/m² in the deterministic analysis and 2337.6$/m² in the probabilistic analysis. A repair cost analysis flowchart that integrates various repair processes can facilitate the analysis of repair processes that have a dominant influence on the overall maintenance cost estimation of RC girders.

Mechanochromic sensors are promising for structural health monitoring as they can visually monitor the deformation caused by discoloration. Most studies have focused on the large deformation problems over 100% strain; however, it is necessary to investigate the discoloration characteristics in a small deformation range to apply it to engineering structures, such as reinforced concrete. In this study, a photonic crystal-based discoloration sensor was investigated to determine the discoloration characteristics of the red, green, and blue (RGB) as well as hue, saturation, and value (HSV) color spaces according to displacement levels. B and S showed the highest sensitivity and linear discoloration at displacements < 1 mm, whereas R and H showed significant discoloration characteristics at displacements > 1 mm. The Vision Transformer model based on RGB and HSV channels was linearly predictable up to 4 mm displacement with an accuracy of R² 0.89, but errors were found at the initial displacement within 2 mm.

Concrete structures are considered as durable construction material, but corrosion of the embedded steel reinforcement occurs under chloride exposure as concrete has porous properties. Herein, a cyclic drying–wetting test was performed for two years using saltwater to accelerate steel corrosion in a reinforced concrete (RC) member. The open-circuit potential (OCP) was measured using a newly developed and replaceable agar sensor. The corrosion potential was measured considering the chloride concentration, water-to-cement (w/c) ratio, and cover depth at three levels. Furthermore, its relationships with influential parameters were evaluated using averaged OCP results. The measured OCP showed a linear relationship with the cover depth, and this tendency was more distinct with increasing retention period and higher chloride concentration. For the highest w/c ratio (0.6), values below −100 mV were monitored after only six months regardless of the cover depth, and values below the critical potential level (−450 mV) were evaluated at lower cover depths (30 and 45 mm). The results of regression analysis considering the exposure environment showed a clear relationship in the case of high chloride concentration (7.0%). A linear relationship between cover depth and OCP was derived with a reasonable determination coefficient ranging from 0.614 to 0.771.

As the modular industry expands, the most widely used building materials are primarily concrete, steel, and wood. However, the use of wood and steel is severely limited compared to concrete for reasons such as durability and economy. To overcome these shortcomings, we aimed to apply ultra-high-performance fiber-reinforced concrete (UHPFRC), which has excellent compressive strength and tensile strength, high durability, and minimal reinforcement with steel fibers. In this study, research was conducted on the development of unit box-type architectural modules using UHPFRC with a compressive strength of 120 MPa and a tensile strength of greater than 7 MPa. Various amounts of steel fibers (Vf = 1.0, 1.5, and 2.0%) were evaluated to determine the optimal mixing ratio of UHPFRC, in which both the durability and mechanical performance were assessed. The compressive strength and tensile strength of UHPFRC were found to be 132 MPa and 10.1 MPa, respectively, while its resistance to chloride penetration averaged 14.47 coulombs, indicating superior durability compared to conventional concrete. To reduce the weight of the unit components of the architectural modular system, both normal concrete (NC) components and UHPFRC were applied. The main variables in the flexural tests were the cross-sectional thickness, steel fiber content, and presence of an insulation material, comprising a total of three variables for evaluating the flexural performance. The application of UHPFRC with a compressive strength of 120 MPa, a cross-sectional thickness of 120 mm, and a 10 mm diameter reinforcement provided a similar performance to that of NC components while reducing concrete usage by 60% compared to NC components. Additionally, structural analysis was performed to prototype the unit box-type modular structure using UHPFRC. The modular structural system developed in this study was found to reduce construction costs by 18.7% compared to traditional steel structural systems. Further research is necessary to address issues such as floor slab vibration and noise, connections, and expansion to multistory buildings for commercialization of modular structures using UHPFRC.