Impact of Packing Density and Paste/Mortar Film Thickness on the Rheological Properties of Self-Consolidating Concrete Containing Tuff Powder

This study investigates the physical effects of internal components of self-consolidating concrete (SCC) on its rheological properties and thixotropy, with a focus on packing density and paste/mortar film thickness (PFT/MFT) as key metrics for particle packing. It also examines SCC mixtures incorporating volcanic tuff powder (a natural pozzolanic cement substitute with limited rheology/packing data) and compares them with mixtures containing silica fume. The study consists of two phases. In phase ( i ), the effect of internal components on both fresh-state properties [including workability, static and dynamic yield stresses, plastic viscosity, thixotropy, and wet packing density (WPD)] and hardened-state properties [compressive strength and ultrasonic pulse velocity (UPV)] of SCC was analyzed through three groups of mixtures, including varying aggregate distributions (aggregate gradation curve exponent ( n ) of 0.2, 0.25, 0.3, and 0.35), cement replacement with 15% tuff powder and 8% silica fume as supplementary cementitious material (SCM), and cement content adjusted by ± 5 % . Phase ( ii ) explored relationships between workability, rheology, wet packing density, and thixotropy. It also examined the influence of PFT and MFT on yield stress, plastic viscosity, and thixotropy. Results identified a gradation exponent ( n ) of 0.3 as optimal, corresponding to reduced yield stress and enhanced packing density. Using 15% tuff as a cement substitute increased dynamic yield stress and plastic viscosity by 100% and 14%, respectively; however, it reduced thixotropy by 12% and packing density by 2%. Increasing PFT was associated with reduced plastic viscosity and increased thixotropy, whereas greater MFT exhibited the reverse trend. No statistically significant association was detected between either PFT or MFT and yield stress.