Hyun Oh Shin

Ultrasonic monitoring of setting and strength development of ultra-high-performance concrete

In this study, the setting and tensile strength development of ultra-high-performance concrete (UHPC) at a very early age was investigated by performing the penetration resistance test (ASTM C403), as well as the direct tensile test using the newly developed test apparatus, and taking ultrasonic pulse velocity (UPV) measurements. In order to determine the optimum surface treatment method for preventing rapid surface drying of UHPC, four different methods were examined: plastic sheet, curing cover, membrane-forming compound, and paraffin oil. Based on the test results, the use of paraffin oil was found to be the best choice for measuring the penetration resistance and the UPV, and attaching the plastic sheet to the exposed surface was considered to be a simple method for preventing the rapid surface drying of UHPC elements. An S-shaped tensile strength development at a very early age (before 24 h) was experimentally obtained, and it was predicted by a power function of UPV. Lastly, the addition of shrinkage-reducing and expansive admixtures resulted in more rapid development of penetration resistance and UPV of UHPC.

Behavior of High-Strength Concrete Beams Reinforced with Different Types of Flexural Reinforcement and Fiber

Six high-strength concrete beam specimens reinforced with different types of flexural reinforcement were constructed and tested. All beam specimens consisted of two layers of flexural reinforcements. Three types of conventional steel bars, CFRP bars, and GFRP bars were used as a flexural reinforcement. While three beam specimens were reinforced with single type of flexural reinforcement, the other three beams were reinforced with combination of two types of flexural reinforcement. In addition, four more high-strength concrete beam specimens were constructed and tested to investigate the influence of fibers on the behavior of FRP bar-reinforced concrete beams. Two types of hooked steel fiber and crimped synthetic fiber were used.