To address the global challenges of waste valorization and climate change mitigation, alkali-activated slag (AAS) is used as a sustainable alternative binder to produce low-carbon concrete. However, the low hydraulic reactivity and slow early-age strength development of AAS limit its practical use in marine civil engineering. This research examined the impact of synergy between carbonation curing and marine exposure on the mechanical properties and durability of AAS pastes. The findings indicated a synergistic enhancement and resistance to chloride-ion erosion in carbonated AAS pastes, with a 24-hour carbonated specimen achieving a strength of 42 MPa compared to the 6-hour specimen (12.7 MPa) after 28 days of corrosion. This was attributed to the combined effects of carbonation and chloride-ion penetration, which not only reduced porosity, free chloride ions, and pH but also refined the pore structure. The reduced porosity facilitated the progressive development of calcite in the paste matrix, thereby encapsulating non-carbonated particles within the densified matrix, enhancing the stability of C-S-H gels and promoting the formation of Friedel's salt. Overall, carbonated AAS pastes exhibited stable CO₂ sequestration capacity, remarkable stability, and corrosion resistance in marine environments, indicating that they are well-suited for marine civil engineering applications.

We investigated eight hardened cement paste (HCP) mixtures with different water-to-cement ratios (W/C) and cement types (C₃S/C₂S ratio and Blaine fineness). We systematically measured pore structure evolution, C–S–H surface properties, mass and length changes, bending strength, and Young's modulus during the first drying. The strength and Young’s modulus exhibited a two-stage response, decreasing over 95%–40% relative humidity (RH) and increasing again over 40%–11% RH. Based on the Griffith–Irwin energy balance, the strength decrease in the 95%–40% RH range was accurately reproduced using only the changes in surface free energy and porosity. In contrast, in the 40%–11% RH range, C₃S-rich, high–W/C systems showed pronounced hardening of the HCP skeleton, accompanied by a corresponding increase in strength. For drying shrinkage, we quantitatively demonstrated that different mechanisms dominate in the 100%–40% RH and 40%–11% RH ranges using a model that incorporates the low-density/high-density C–S–H volume fractions and the water-vapor BET specific surface area per unit HCP volume. These results provide a unified microstructural basis for understanding the coupled evolution of volume stability and mechanical properties of HCP during the first drying process.

Loads applied to concrete structures or members, such as prestressing load, connection load, and restraint load, affect the deterioration caused by frost damage. In particular, frozen concrete exhibits expansion. The freezing expansion of deteriorated concrete accumulates gradually, and previous research has suggested that it is related to frost damage. However, the relationship between loading and freeze–thaw resistance has not been clearly identified yet. In this study, restraint jigs were designed for freeze–thaw tests using restrained concrete specimens. The restraint jigs applied compressive load to specimens by tightening the bolt at the top of the jig. Four methods to monitor the bolt loads were investigated, and the strain gage method was most appropriate. The design of the restraint jig was reconsidered based on the test results, and the accuracy of the strain gage method was confirmed. The bolt loads and the specimen loads were also monitored during freeze–thaw cycles in air and water. Due to the complex behavior of unfrozen water in concrete, the bolt load should be managed with the bolt strain at the maximum concrete temperature.

The depletion of natural high-quality aggregates has hindered the sustainable and stable supply of high-strength concrete recently. This study investigates the effect of electric arc furnace slag fine aggregate (ES) on the strength of high-strength mortar, the fundamental matrix of high-strength concrete (water binder ratio of 0.20 and 0.30) to make effective use of resources. The compressive strength and elastic modulus of mortar using the ES were improved compared to those using normal fine aggregate. Three critical aspects, interfacial properties of aggregate, surface characteristics of aggregate, and intrinsic strength of aggregate, were investigated to elucidate the strengthening mechanism of ES. These results indicated that the critical feature is that ES has higher strength and elastic modulus than normal fine aggregate. Considering mortar as a two-phase material consisting of fine aggregate phase and cement paste phase, it can be concluded that the improvement in the strength of high-strength mortar using ES is achieved by the positive effect of distributing the stress induced on cement paste phase in mortar to rigid ES phase. These findings highlight that ES not only contributes to sustainable resource circulation but also has the potential to realize the cementitious materials with superior mechanical performance.

This study proposes a new evaluation method for shear capacity of RC beams with FRP stirrups based on beam and arch actions. The method introduces an integrated relationship between the applied shear force, the shear resistances and the stress conditions of concrete and stirrups within shear span, which enables systematic evaluation corresponding to several shear failure modes. In this paper, after introducing the concept of the new evaluation method, analytical programs regarding the beam and arch actions in RC beams with FRP stirrups are presented. Based on the results, theoretical and empirical equations are formulated for each component of the integrated relationship. The relationship is incorporated into an iteration process to determine shear capacity, defined as the shear force at which stress of either concrete or stirrups reaches the material strength. From comparisons to experimental and analytical results, the proposed method demonstrates improved accuracy to estimate the shear capacity of RC beams with FRP stirrups and provides capability to estimate the shear failure mode.