Yongjae Kim

Fundamental properties and mechanical characteristics of high performance cement composite with steel fibres under high temperature

Abstract High-performance fibre-reinforced cementitious composite (HPFRCC) is characterized by multiple functioning composite materials comprising fibre and cement matrix. HPFRCC is able to withstand tensile stress forming distributed micro-cracks since embedded fibres in concrete are to improve energy-absorption capacity and apparent ductility. This characteristic can be applied to protect structures under an extreme state of loading, such as blast or impact. In order to determine a mix proportion, this study considered various variables in the mix. Following experimental results, this study suggests an optimum mix proportion of HPFRCC and investigated fundamental properties of the considered mixes. Flexural performance is one of the representing characteristics of the high-fracture energy-absorbing materials. This study investigated the flexural performance of HPFRCC, which used high-strength steel fibres with volume fraction 8%. Under ASTM 1609, load–deflection curves were obtained. The major test variables include silica fume replacement ratio 0 and 15% and exposure temperature, ambient and 400 °C. The flexural strength was similar to the compressive strength and the absorbed fracture energy was relatively greater than other typical HPFRCC. This may be a composite reaction of the strong bonding between fibre and matrix and high-compressive strength of the matrix itself.

Investigation of Potential Fire Hazard Resources of Bridges on National Routes by Field and Web-based Satellite

PURPOSES : The occurrence of unexpected disasters, including fire events, increases as the road network becomes complicated and traffic volume increases. When a fire event occurs on and under bridges, the damage extensively influences direct damage to structures, vehicles, and human life and secondary socioeconomic issues owing to traffic blockage. This study investigated potential fire-hazard risks on bridges of the Korean national route road. METHODS : The investigation was conducted using field investigation and analysis with satellite pictures and road views from commercial websites and the Bridge Management System (BMS). From the filed investigation, various potential fire resources were identified. The satellite pictures and road views were helpful in measuring and recognizing conditions underneath bridges, stowage areas, etc. RESULTS : There are various potential fire resources underneath bridges such as piled agricultural products, parked petroleum tanks, construction equipment, and attached high-voltage cables. A total of 94.6% of bridges have underneath clearances of less than 15 m. A bridge underneath volume that can stow a potential fire hazard resource was 7,332 m on average, and most bridges have about 4,000 m of space. Based on the BMS data, the amounts of PSC and steel girders were 29% and 25%, respectively. CONCLUSIONS : It was found that the amount of stowed potential fire hazard resources was proportional to the underneath space of bridges. Most bridges have less than 15 m of vertical clearance that can be considered as a critical value for a bridge fire. The fire risk investigation results should be helpful for developing bridge fire-protection tools.

Compressive Strength Properties of Concrete Using High Early Strength Cement and Recycled Aggregate with Steam Curing Conditions

Recycled aggregate is a valuable resource in Korea in lack of natural aggregate. Government recognizes the importance and suggests various policies enhancing its use for higher value-added application. Most of recycled aggregate produced currently in Korea, however, is applied for low value-added uses such as embankment, reclamation, etc. Its higher valued application such as for structural concrete is very limited. Although domestic manufacturing technology of recycled aggregate is at the world level, recycled aggregate is not applied for structural concrete. Primary reasons for the limited use of the recycled aggregate include bonded mortar and cracks occurred during crushing and hence it is very difficult to predict and control the quality of recycled aggregate concrete. This research intended to grasp combined characteristics of recycled aggregate, high early strength cement, maximum temperature and time duration of steam curing and then, analyze the effects of factors. Also, it suggested the method to improve field applicability of recycled aggregate concrete.