Development and Evaluation of Thixotropic UHPC Overlay Mixtures for Bridge Deck and Low-Slope Roof Slab Repair

Ultra-high-performance concrete (UHPC) is a sophisticated construction material known for its exceptional strength and durability. Conventional UHPC generally self-consolidates, which makes it unsuitable for roof and bridge deck rehabilitation applications due to its thin layers and inclined surfaces. UHPC overlay construction generally requires a highly thixotropic material that responds well to vibration and remains stable on slopes. Despite the complex rheological properties of thixotropic UHPC, there are limited testing methods for effectively assessing the workability of overlay mixes. Therefore, this paper provides a comprehensive evaluation of the workability of overlay UHPC using existing and newly developed tests. Besides the commonly used static and dynamic flow tests, this study introduces Patting Response (PR) and Vibration-Slope Stability (VSS) tests, designed to evaluate different qualities of UHPC overlay mixtures. Seven groups of mixtures with varying binder content, water-to-binder ratio (w/b), fiber reinforcement, and admixture dosages were prepared and tested. A lab-scale sloped slab was constructed to validate the buildability of the most promising mixtures. These tests and mixtures support effective overlay solutions for roof slab and bridge deck repairs, providing protection against infrastructure deterioration and improving overall performance by introducing a dense, durable UHPC overlay. Results indicate that mixtures with static flow below 6 in. and dynamic flow between 7 and 8 in. consistently passed both PR and VSS tests, demonstrating stable vibration response and slope retention. The constructability evaluation confirmed the effectiveness of the new testing methods. Additionally, the correlation between different tests, particularly flow and VSS, was examined. Recommendations for appropriate ranges for various workability tests were established based on the performance of the developed mixtures. The proposed static and dynamic flow ranges are performance-based and are expected to be broadly applicable to thixotropic UHPC overlay systems exhibiting comparable workability and rheological behavior under vibration and sloped placement conditions. Overall, these tests and thixotropic UHPC mixtures facilitate effective repair of roof slabs and bridge decks, providing overlay protection against deterioration and potentially enhancing structural capacity through composite behavior.