Workability Characteristics of Underwater Anti-washout Self-compacting Concrete and its Mix Design Method Based on Rheological Threshold Theory

Existing mix design methods for underwater anti-washout self-compacting concrete lack reliability due to unclear underwater flow behaviors. This study investigates the workability evolution of this concrete in shallow-water environments. The effects of water-to-binder ratio, superplasticizer, and underwater protective agent dosage on flowability and segregation were examined through systematic experiments. Results indicate that underwater slump flow is significantly lower and flow time markedly longer than in air, while increasing the protective agent dosage improves anti-dispersion stability. Water depth variations within the shallow range showed negligible influence. To interpret these mechanisms, the traditional rheological threshold theory was modified by introducing a mortar film retention coefficient to account for dispersion inhibition and an effective density difference to incorporate buoyancy effects. Based on these improvements, a rheological threshold model and an enhanced mix design method were established. Experimental validation confirms that the proposed model accurately predicts qualified mix proportion regions, demonstrating improved design precision within the tested shallow-water environmental parameters compared to traditional methods. This study provides a systematic methodology for the rational mix design and quality control of underwater concrete.