On-panel characterization of early-age dynamic contact stress in fresh dry-mix shotcrete and its relation to rebound

Dry-mix shotcrete rebound depends not only on spray kinematics but also on the early-age response of the receiving-surface during placement. This paper presents a full-scale on-panel experimental approach to characterize this response and relate it to rebound within one controlled dry-mix shotcrete material-equipment system. In this study, the term fluid zone denotes a hypothesized transient, localized, low-resistance near-surface region beneath the spray axis, identified from a wet/glossy appearance and lower on-panel dynamic contact stress ( $p_d$ ) measured by a dynamic bead-impact test. A robot-mounted, 3D-printed magnetic catapult and high-speed imaging were used to measure $p_d$ on the panel at 0.5 s after the nozzle passage and again at 3 and 6 min. Full-scale dry-mix spraying tests with controlled nozzle motion and real-time mass/flow measurements were conducted using a reference mixture and two air-entrained mixtures over a range of placement consistencies represented by ${\left(w,/,b\right)}_{\text{calc}}$ . Under the tested setup, $p_d$ at 0.5 s was consistently lower than later values, providing indirect mechanical evidence consistent with a rapidly evolving low-resistance near-surface response. Lower early-age $p_d$ was associated with lower rebound. Air entrainment shifted both $p_d$ and rebound downward, while higher ${\left(w,/,b\right)}_{\text{calc}}$ toward the upper end of the accepted placement range were associated with lower rebound. The proposed on-panel test provides a comparative indicator of early-age receiving-surface response during dry-mix shotcrete placement. Although broader validation is required (equipment, mixture design, etc.), this measurement method appears to be a significant improvement of our understanding of rebound mechanisms.