## 6 Conclusions  
This study was conducted to develop modular buildings in the form of unit boxes using UHPFRC. Structural performance evaluation, structural analysis, and economic assessment of UHPFRC modular buildings that can replace conventional steel structures were performed. The conclusions of this study are as follows:  
1. To develop UHPFRC-based modular members, the flexural performance of NC elements and that of UHPFRC-F, UHPFRC-T, and UHPFRC-I series were evaluated. The results indicated that in UHPFRC modular structures, concrete usage could be reduced by up to 40-60% compared to that in modules with NC. However, ensuring structural safety while achieving similar performance and ductility effects as NC is crucial. The flexural strength of the N24-H20-R1 element was evaluated as 36.6 kN-m, while the S120-H12-R1 element exhibited a flexural strength of 37.2 kN-m. Their ductility indices were 6.44 and 7.08, respectively, showing similar flexural performances. Hence, the primary variables determining the optimal cross-section of the UHPFRC modular member were designated as follows: design compressive strength of 120 MPa, thickness of 120 mm for the member, and longitudinal rebar diameter of D10.
2. To fabricate a modular structure based on UHPFRC, structural assessments were conducted for a total of 6 sections, 4 stories high, with each unit module 3.37 m in width, 10.5 m in length, and 2.9 m in height. Basic structural assessments, including bending and shear, were carried out solely to verify the constructability of the unit modules. An initial module was manufactured with dimensions of 3.3 m in width, 2.9 m in height, and 1.0 m in length utilizing UHPFRC to confirm the feasibility of implementation.
3. An analysis was conducted on the construction costs of modular structures using UHPFRC compared to those of structures employing steel. The results indicated an approximately 18.7% reduction in construction costs for UHPFRC modular structures compared to steel structures (using the infill method). Recent increases in raw material prices have notably affected the economics, particularly the price of steel in steel-structure modular constructions, which was found to be a significant factor, reaching USD 3000 per ton. Considering the volatility in steel prices, economic feasibility could be ensured up to 70% of the current steel price. The application of UHPFRC in modular structures appears to be adaptable to various construction environments. However, detailed studies regarding the productivity of units of UHPFRC modular structures, connections, and seismic considerations were deemed necessary.  
Conceptualization, K.K. ([PERSON]) and K.K. ([PERSON]); methodology, K.K. ([PERSON]) and G.R.; investigation, K.L. and Y.Y.; data curation, K.L. and Y.Y.; writing--original draft preparation, K.K. ([PERSON]) and G.R.; writing--review and editing, K.K. ([PERSON]) and K.L.; supervision, K.K. ([PERSON]); funding acquisition, K.K. ([PERSON]). All authors have read and agreed to the published version of the manuscript.  
Research for this paper was carried out under the KICT Research Program (20240097-001, Research on the Establishment for Integration and Linkage of Infrastructure for the Co-prosperity of South and North Korea) funded by the Ministry of Science and ICT.  
Not applicable.  
Data Availability Statement: Data are contained within the article.  
The authors declare no conflicts of interest.