Kentaro Yasui

This study investigates the applicability of the miniature compression test (MCT) for evaluating the yield strength of existing steel bridges. Preliminary tests were conducted to compare standard tensile tests with MCTs, confirming that the latter can reproduce the characteristic yield behavior, including upper and lower yield points. Material tests using MCT specimens extracted from existing bridges revealed that the measured yield strength was approximately 1.1–1.2 times higher than the specified minimum yield strength. Based on these results, a preliminary empirical correction factor of 1.084 was derived for the investigated material to convert the MCT results into equivalent tensile yield strength. Furthermore, structural performance was evaluated by applying the corrected yield strength to load-carrying capacity calculations. The results indicate that the influence of yield strength on load-carrying capacity differs between positive and negative bending regions, with a more pronounced effect observed in the negative bending region. These findings demonstrate that the MCT provides a practical and reliable method for strength evaluation of existing steel bridges.

Three-dimensional printing (3DP) has gained increasing attention in construction as a means of addressing labor shortages and improving efficiency. Various studies have investigated fiber-reinforced mortars for 3DP. However, only a few studies have examined mixture design strategies aimed at controlling early structural build-up, and the relationships between early structural build-up, printability, and interlayer stability remain largely unexplored. This study aimed to establish a practical method for evaluating the static yield stress and early buildability of 3DP mortars under construction-site conditions. Vane shear and 15-stroke flow tests were conducted to assess the static and dynamic behavior of mortars incorporating polyvinyl alcohol (PVA) fibers, and their compressive and flexural strengths were also evaluated. According to the results, the vane shear test sensitively captured the rheological changes associated with variations in fiber content and superplasticizer dosage. The addition of PVA fibers increased the maximum shear stress of the mortar, resulting in atypical static yield stress development compared to fiber-free mortars. While the 15-stroke flow test further elucidated flowability, the vane shear test revealed a stronger correlation between mechanical properties and overall buildability. Thus, vane shear testing can be reliably used to assess early-age structural build-up and interlayer stability in 3DP mortars for optimizing print performance.

This study aimed to establish fabrication methods for two types of functional microcapsules (MCs) and to verify their fundamental performance in cementitious materials. The alkaline-supplying and CO₂-absorbing MCs were evaluated independently. It was hypothesized that alkaline-supplying MCs could suppress carbonation while limiting strength loss to ≤20%, whereas CO₂-absorbing MCs could increase long-term CO₂ uptake by ≥30% relative to plain mortar. The alkaline-supplying MCs reduced the 24-week carbonation depth by approximately 35%, with a ~20% reduction in compressive strength at a 5% addition level. In contrast, CO₂-absorbing MCs resulted in a ~30% strength reduction at 5% addition but increased CO₂ uptake by 1.4 times that of plain mortar over 140 d. In conclusion, the results demonstrate that MCs can effectively impart carbonation-resistance or CO₂-absorption functionality; however, a clear trade-off exists between functional enhancement and mechanical degradation due to capsule stiffness and density limitations. Because the two types of MCs rely on different mechanisms, they were tested in separate mortar systems. As this study focused on MC fabrication and fundamental functional verification, future work should optimize shell design, density control, and amine selection to balance structural performance with durability enhancement and CO₂-sequestration capability.

Three-dimensional printing (3DP) has attracted growing attention in the construction industry due to labor shortages and the need for greater efficiency. However, there have been only a few previous studies focused on mixture design strategies that address the thixotropy of fiber-reinforced mortars for 3DP. In addition, the relationships among thixotropy, printability, and interlayer stability have not been sufficiently verified. This study aims to establish a quantitative method for evaluating the thixotropic properties of mortars used in construction 3DP, specifically on their practical applicability at construction sites. Vane shear and 15-stroke flow tests are conducted on mortars incorporating polyvinyl alcohol (PVA) fibers to assess their thixotropic behavior under static and dynamic conditions. Fiber-reinforced mortar mixtures are prepared, and their compressive and flexural strength developments over time are examined. The results indicate that the vane shear test is a sensitive and effective method for detecting changes in mortar rheology, particularly in response to variations in fiber content and admixture dosage. The inclusion of PVA fibers increased the maximum shear stress owing to fiber aggregation, resulting in atypical thixotropic behavior compared to that of fiber-free mortars. While the 15-stroke flow test provided supplementary information on flowability, the vane shear test exhibited a stronger correlation with mechanical properties and printed build quality. These findings suggest that vane shear testing offers a practical and reliable means of evaluating and managing the thixotropic properties of mortars for 3DP, thereby enhancing quality control in additive construction.

Carbonation degrades reinforced concrete structures and reduces their service life, and it raises environmental concerns. Moreover, the cement industry faces increasing pressure to mitigate CO2 emissions. Therefore, this study developed a functional mortar incorporating microcapsule (MC) technology to suppress carbonation and enhance CO2 absorption. Two types of MCs were prepared: alkali-supplying MCs for carbonation resistance and CO2-absorbing MCs for carbon fixation. Mortars incorporating these MCs were evaluated using compressive strength tests, accelerated carbonation tests, and CO2 absorption measurements. The alkali-supplying MCs effectively suppressed carbonation, particularly at a dosage of 10%. However, the compressive strength decreased owing to the low strength and density of the MCs. The CO2-absorbing MCs significantly increased the CO2 uptake, particularly when impregnated with amines such as trioctylamine. However, they did not suppress carbonation, and their absorption capacities decreased over time. MCs without amine impregnation absorbed CO2, although they did not affect the carbonation depth owing to the absence of reactive components. These findings demonstrate the potential of MC-based mortars to improve the durability of concrete and promote carbon management. The use of cementitious materials with CO2-absorbing and protective functions is a promising approach for sustainable construction and long-term infrastructure resilience.

This study addresses the problem of pumice deposits in the southern Kyushu region, which can cause landslides during heavy rainfall. To reduce this hazard, it is important to expand pumice applications and promote its use before disaster events occur. Among construction materials, this study explores the possibility of using pumice as a concrete aggregate, considering the global shortage of natural aggregates. Because of the low strength and difficulty of use, pumice must be fired to improve its properties. In our experiment, it was fired at 1000 or 1100 °C, and the performance of the resulting concretes was compared. Concrete incorporating pumice fired at 1100 °C achieved a maximum compressive strength of 54.6 N/mm² with an increase in the amount of cement, whereas concrete with pumice fired at 1000 °C remained within the 20–24 N/mm² range even when the amount of cement was increased. This difference arises because pumice has a lower strength than the cement paste, leading to material failure. Furthermore, freeze–thaw tests showed that concrete made with pumice fired at 1100 °C was resistant to frost damage. These results suggest that pumice fired at 1100 °C has an excellent potential as a sustainable building material.

This study explores the use of volcanic glass powder (VG) derived from Shirasu volcanic deposits as a substitute for silica fume (SF) in producing high-strength precast concrete piles with a compressive strength of 123 MPa. Initially, mortar specimens with varying VG replacement ratios and curing temperatures were prepared to assess their compressive strength. After identifying the optimal mix ratios and curing conditions for high-strength mortars, concrete specimens incorporating VG were produced. Subsequent testing revealed that a VG replacement ratio of 20% by cement volume and a curing temperature of 70 °C were optimal for achieving the target compressive strength. Although the Young’s modulus of VG-incorporated concrete was slightly lower than that of pure cement and SF concrete, its performance remained satisfactory. These findings suggest that VG is a viable alternative to SF in high-strength concrete applications, providing a sustainable method to enhance concrete properties using locally available volcanic deposits.

In seawater, cement-treated soil undergoes accelerated deterioration owing to enhanced calcium leaching caused by magnesium salts. The deterioration of cement-treated soil progresses gradually from the surface in contact with seawater, necessitating extended periods for investigating the soil properties after deterioration in laboratory tests. However, an accelerated deterioration method for cement-treated soil has not been developed. This study examines the effects of Mg concentration in the immersion water (0.94–23.45 g/l) and specimen dimensions (2.0–5.0 cm in diameter) on the deterioration rate. The aim is to accelerate the production of deteriorated cement-treated soil and characterise soil properties in a short period. The results indicated that the deterioration rate of the cement-treated soil increased with increasing Mg concentration in the immersion water, and the Mg concentration of 23.45 g/l was more than five times faster than that of 0.94 g/l. Furthermore, the smaller the specimen size, the shorter the period required for deterioration. The strength of the deteriorated cement-treated soil varied depending on the size of the specimen; however, the difference was within 16% based on a diameter of 5.0 cm.

Glass cullet (GC) generated from the disposal of photovoltaic (PV) panels are typically landfilled, and effective GC utilization methods must be established for PV generation. In this study, alkali-activated material (AAM) mortars were prepared from the paste of fine blast-furnace slag powder, fly ash, and sodium orthosilicate (SO) and mixed with crushed sand and GC to investigate the potential use of GC as a fine aggregate in AAM. The replacement of crushed sand with GC did not considerably affect the flowability of the mortar, whereas the compressive strength decreased with the increasing GC replacement rates. Although expansion due to the alkali–silica reaction (ASR) was observed in mortars wherein GC replaced crushed sand, the expansion can be controlled by reducing the amount of mixed SO, autoclaving the GC, performing preleaching to remove the Si that causes the ASR, and replacing the blast-furnace slag with fly ash. By enforcing measures against the expansion, the possibility of using GC as fine aggregate is enhanced considerably, thus increasing the feasibility of continuous PV production.

Bora is a granular volcanic pumice with a certain degree of hardness. In this study, we investigated the possibility of using fired Bora as a substitute for fine sand in mortar. The objective was to develop a lightweight pavement block with extremely high permeability using fired Bora. Initially, the changes in hardness and density of Bora particles fired at different temperatures were examined. The bending strength of mortar containing fired Bora as a fine aggregate was also evaluated. Subsequently, we fabricated a permeable pavement block with a hybrid structure, comprising a mortar containing the fired Bora and a porous material with large-sized Bora particles bonded using cement paste. We examined its bending strength and permeability and performed a quantitative analysis of the bending stress distribution on the block using the finite element method. The hardness of Bora significantly increased when fired at temperatures exceeding 900 °C; hence, Bora fired at 1100 °C can be used as an effective substitute for crushed sand in mortar. Furthermore, the results confirmed that the use of fired Bora enabled the production of lightweight pavement blocks with extremely high permeability that satisfied the bending strength criterion.