In this study, a new precast concrete (PC) beam-column joint of moment resisting frame applicable for moderate seismic regions is proposed. A semi PC beam-column connection with U-shaped strands is developed in an attempt to improve workability and provide effective stress transfer mechanism at the joint. The structural system consists of PC beams with U-shaped strands, PC columns, PC slabs, and topping concrete. A series of three interior and three exterior semi PC joint specimens was tested to investigate the structural behavior of the system subjected to the lateral cyclic load. Key test variables are the number of strands placed in the PC beam and the presence or absence of the transverse reinforcements at the connection. The experiment and performance evaluation of the system were conducted in accordance with ACI T1.1–01 (2001). According to the test results, the proposed structural system with transverse reinforcements at the joint is sufficient to use in moderate seismic regions.

Evaluation of the ultimate strength for the UHPFRC (ultra-high-performance fiber-reinforced concrete) flexural members was conducted. In this study, an experimental program about UHPFRC beams was conducted with the effect of fiber volume fraction, shear span to depth ratio, and compressive strength of matrix as the main variables. Among them, it was found that fiber volume fraction was the variable that had the greatest influence on the ultimate strength. The inclusion of 2% volume fraction steel fiber increases the shear and flexural strength of UHPFRC beams significantly. In particular, steel fiber inclusion changed the mode of failure of beams from diagonal shear failure into flexural failure. For the classification of failure patterns, the ultimate flexural strength and shear strength of UHPFRC members were evaluated using the current design code and UHPC guidelines. Flexural ultimate strength was affected by the size and shape of the stress block and consideration of the matrix’s tensile strength. For the accurate shear strength prediction of UHPFRC beams, the tensile strength of the high strength matrix and the effect of steel fiber should be considered.

Previous studies have shown that the drying shrinkage of recycled aggregate concrete (RAC) is greater than that of natural aggregate concrete (NAC). Drying shrinkage is the fundamental reason for the cracking of concrete, and tensile creep caused by the restraint of drying shrinkage plays a significant role in the cracking because it can relieve the tensile stress and results in the delay of cracking occurrence. However, up till now, all research has been focusing on the compressive creep of RAC. Therefore, in this study, a uniaxial restrained shrinkage cracking test was executed to investigate the tensile creep properties caused by the restraint of drying shrinkage of RAC. The mechanical properties, such as compressive strength, tensile splitting strength, and Young’s modulus of RAC were also investigated in this study. The results confirmed that the tensile creep of RAC caused by the restraint of shrinkage was about 20–30 % larger than that of NAC.

In this research, flexural performance was evaluated using macro-synthetic fiber-reinforced concrete (MFRC) in structural deck plates. Material tests were performed to evaluate the mechanical properties of the MFRC, and the flexural strength evaluation was conducted in two experiments, positive and negative moment tests. In the material test results, compressive strength and modulus of elasticity of the MFRC were increased compared with normal concrete. Flexural tensile tests showed that, after achieving maximum strength, the deck plates had sufficient residual strength until fracture. Structural tests showed that flexural strength and cracking load of all specimens increased according to macro synthetic fiber dosage. According to the experimental results, we proposed a flexural strength model of a steel deck plate containing macro synthetic fiber. The model showed greater accuracy than the current standard compared with the experimental results. In addition, since it was confirmed that the MFRC steel decks had greater flexural stiffness until yielding, it will be necessary to quantitatively evaluate the effect of MFRC on the effective flexural stiffness of steel decking in future studies.

Structures experience damage and deterioration over their life cycle. The recent increase in interest in sustainable development of cities is re-evaluating the long-term use of structures. Therefore, researches on the retrofitting method for existing structures is receiving high attention again. However, there are very few cases where the direct comparison between the repair and retrofitting effects of structures has been experimentally verified. In this study, retrofitting methods and repair methods with carbon fiber sheets for reinforced concrete columns under shear failure were investigated. 10 reinforced concrete columns were tested under reversed cyclic loading. As a result of the experiment, it was found that, when reinforced with CFS strips, the increase in the width of the strip serves to enhance the strength and ductility of the reinforced member by providing confinement against shear cracks occurring in concrete. It was confirmed that as the number of overlaps of CFS increased, a greater strength-enhancing effect and ductility-enhancing effect appeared. However, the two-way reinforcement of CFS did not show a great effect in improving strength and ductility. As a result of evaluating the repair performance of damaged members using CFS, it was found that the strength and ductility of the repaired specimen exceeded the original strength and ductility of the base material as well as the strength and ductility of the reinforced specimen.