Investigation to improve the workability of UHPC
Ultra-high performance concrete (UHPC) is one of the latest achievements of concrete technology in the world with excellent performance, mechanical properties, and durability. At present, UHPC has been successfully used in some structures and bridges in the world, but they are mostly small scale due to its high cost. Compared with building new structures, UHPC's high early strength and high toughness performances are more suitable for strengthening existing structures. However, there are still some problems with the workability of UHPC that need further study, such as concrete shrinkage, air bubbles moving and so on. These will sometimes greatly reduce the workability of UHPC. Therefore, the investigation to improve the workability of UHPC is very important. On the other hand, as the utilization of recycled concrete aggregates (RCAs), the recycled aggregate concrete (RAC) has been developed. So if the UHPC can also be made with high quality recycled aggregates, the workability will be improved as well, which will be investigated in this research. In conclusion, considering the increasing application of UHPC, this study has a very important significance for the economy and society. The methods in the present doctoral dissertation might include: 1) The investigation of the factors that influence the workability of UHPC, such as air bubbles behavior, flowing performance, viscosity strength, and so on, which will also involve the workability of UHPC with recycled aggregates; 2) The investigation of the elephant skin in UHPC, which is to find out the effects of elephant skin on the workability of UHPC and when the elephant skin should be prevented, and also the study about the factors that influence the formation of the elephant skin, including influences of the composition materials, mix proportioning, steel fiber, recycled aggregates and so on. 3) The investigation on the improvement of the workability of UHPC, which is to find out how to deal with the elephant skin and how to improve the workability. 4) Research on using the recycled aggregates to improve the workability of UHPC, which is to find out whether the recycled aggregates can improve the workability and how the recycled aggregates can improve the workability. Through the investigation focusing on when, why and how to prevent the elephant skin, and all the relative issues in order to improve the workability of UHPC, The research results will be beneficial to the development and extensive use of UHPC, and it is of great practical significance for the safety and efficiency of civil engineering industry with the ultra-high performance materials. Besides, the UHPC with recycled aggregates is also of good economy and environmental protection.