Highly thixotropic ultra-high-performance concrete (UHPC) as an overlay

Ultra-high-performance concrete (UHPC) is an emerging overlay material for the bridge deck rehabilitation. As a durable material, UHPC overlay can effectively extend the service life of the bridge. In this study, the highly thixotropic UHPC overlay was designed and developed to facilitate sloped overlay construction with minimum formworks. A new type of water-based and well-dispersed nanoclay suspension was utilized to enhance the thixotropy. Two indices, including initial fluctuation of yield stress (tau floc) and thixotropy index (IA), were pro-posed to evaluate the thixotropy of fresh UHPC. It was found that, as nanoclay contents increased from 0 to 0.20 % (by mass of binders), the initial fluctuation of yield stress (tau floc) increased from 392.7 Pa to 3290.1 Pa and the thixotropy index (IA) increased from 9.7 Pa/min to 16.4 Pa/min, representing the significant enhancement on thixotropy. The highly thixotropic UHPC overlay showed good shape stability after the placement, but showing plenty of defects on the interface. It was shown that the bond strengths between thixotropic UHPC overlay and substrate are reduced by over 30 %. To address this issue, the application of proper vibration index on highly thixotropic UHPC overlay was investigated. Results showed that the bond strength between thixotropic UHPC overlay and substrate was increased by 45 % after the optimal vibration index. The SEM observation of the interface confirmed the increased air voids for highly thixotropic UHPC overlay and the reduced air voids after optimal vibration index. This study will promote the acceptance of highly thixotropic UHPC as an overlay.