Development and Evaluation of Highly Thixotropic UHPC Mixtures for Bridge Overlay Applications

Ultra-high-performance concrete (UHPC) is an advanced construction material renowned for its exceptional strength and durability. However, conventional UHPC is typically self-consolidating, making it unsuitable for bridge deck overlay applications due to challenges posed by potential surface slopes. UHPC overlay construction requires a highly thixotropic material that not only responds well to vibration but also maintains workability over an extended period and remains stable on sloped surfaces. Despite the complex rheological properties of highly thixotropic UHPC, limited testing methods are available to effectively assess the workability and rheology of overlay mixtures. To address this gap, the present study provides a comprehensive evaluation of overlay UHPC workability and rheology by employing both existing and newly developed testing methods. Specifically, static and dynamic flow tests were complemented with two novel tests: the Patting Response (PR) test and the Vibration-Slope Stability (VSS) test. These tests were designed to assess the materials' response to external energy and their stability on sloped surfaces under vibration, simulating the rheological behavior of overlay UHPC mixtures in real-world conditions. An additional test was developed to evaluate the rheology of these mixtures when subjected to vibration. Seven groups of overlay UHPC mixtures were prepared, varying in binder content, water-to-binder ratios (w/b), fiber content, and admixture dosages. Correlations between different tests, particularly between static and dynamic flows and VSS, were examined to establish flow ranges for cases where running the VSS test in field conditions may be impractical. A lab-scale sloped slab was constructed to validate the constructability of the most promising mixtures. Results demonstrated that static and dynamic flow tests, along with the PR and VSS tests, effectively evaluated overlay UHPC from multiple perspectives, offering valuable insights into flowability, vibration response, and slope stability. Based on the findings from the constructability test, recommended criteria for static and dynamic flow ranges were proposed for overlay UHPC, providing essential guidelines for future applications. Advisor: Jiong Hu