## 1 Introduction  
Concrete is a type of material that is the most closely connected to human life and has been widely used in social infrastructure and the construction industry for the past 100 years. In recent years, technology exchange with fields other than engineering has been made to satisfy the various human requirements owing to societal development and changes. Especially in accordance with the desire to create pleasant and beautiful indoor spaces, light transparent concrete has been developed. Light transparent concrete is a material in which a large amount of optical fibers are arranged on concrete in a straight line to give light transmittance through the concrete, and its use has expanded to include that of a new type of interior and exterior material not only for building residents, but also for architects seeking changes through the continuity of discontinuous space by allowing the passage of light in addition to the separation of physical spaces, which is the original function of concrete [1,2]. Light transparent concrete was developed by Hungarian architect [PERSON], and it was selected as one of the \"36 Best Inventions of the Year\" by TIME Magazine. Later, [PERSON] founded a company called \"LiTraCon\", short for \"Light-transmitting Concrete\". Since then, light transparent concrete has been widely used in the United States and Europe as it gives spaces a new type of functionality and improves the user satisfaction despite its high price. As light transparent concrete, however, has limitations as an architectural member using optical fibers in that its production requires labor-intensive work as the tens of thousands of thin fibers should be arranged in a straight line by human hands, and the work related to the fabrication and transport of the test specimens is ineffective due to the heavy weight of concrete (2.3 ton/m\({}^{3}\)), it is produced and sold using the pre-cast method [3]. To overcome the restrictions on the field applications of light transparent concrete due to its practical limitations in terms of economic efficiency (material and labor costs) and workability (mass production), researches are continuously being conducted to overcome the existing limitations of light transparent concrete, and thus to increase its utilization rate in the field [4]. The critical areas for improvement can largely be divided into the construction method and the material. For the construction method, a relatively low-cost plastic bar was used instead of an expensive optical fiber to greatly reduce the material cost, and a method of inserting a transparent bar of a certain size from the outside was applied to reduce the labor cost and to enable mass production [4]. In the case of the material, an artificial ultra-lightweight aggregate with a 0.6 specific gravity was used, and high-temperature and high-pressure curing within an autoclave was applied in the production with quicklime and sand with high silica contents as the main materials to ensure excellent adhesion to the concrete matrix due to its surface with an open-cell structure [5; 6; 7]. To reduce the weight using foams, a method of providing lightweight and heat insulation to concrete by making a foamed slurry mixed with cement, sand, and admixture materials, and forming small and fine foams (0.1-1 mm) inside the concrete matrix, was used [8]. Lastly, a basic experiment was conducted to overcome the limitations due to the heavy weight and labor-intensive work through the application of organic fibers, which demonstrated excellent ductility without lowering the consistency of concrete, to prevent a reduction in the flexural performance and ductility when a transparent bar made of plastic is placed [4]. In this study, experiments were conducted to improve the mechanical properties due to the lightweight of light transparent concrete by gradually optimizing several methods applied to overcome the existing problems.