Abstract
This study developed a high-performance composite mortar with a low water-to-binder (W/B) ratio to improve the mechanical strength and volumetric stability of preplaced aggregate concrete-filled steel tubes (PACFST). Silica fume was incorporated to optimize the interfacial transition zone (ITZ) between the matrix and coarse aggregates. The effects of the sand-to-binder (S/B) ratio, water-to-binder (W/B) ratio, and expansive agent content on the flowability, compressive strength, and volume stability of the composite mortar were systematically analyzed. Experimental tests were conducted using vibration-free molded specimens, and the influence of different S/B ratios (0.8-1.4), W/B ratios (0.26-0.32), and expansive agent dosages (0-8%) on mortar properties was evaluated. The results indicate that an optimal S/B ratio of 1.2 significantly enhances flowability and strength, whereas further increases offer limited improvement. Reducing the W/B ratio enhances strength, with a decrease from 0.32 to 0.28 leading to a 23.4% increase in 28-day compressive strength. Additionally, a 6% expansive agent dosage reduces 90-day shrinkage by 13.1% while maintaining high compressive strength. The optimized PAC achieved a 28-day compressive strength of 115.9 MPa, with an 11.6% increase in 7-day strength and a 51.2% reduction in 90-day shrinkage compared to conventional C100 concrete. These findings provide theoretical guidance for designing high-strength, low-shrinkage PAC, offering insights for bridge, tunnel, and high-rise building applications.