A Study on the Characteristics of Compressive Strength and Compressible Packing Density of Ultra-High-Performance Concrete according to Sand Combination

This study aims to assess the feasibility of a mix design for ultra-high-performance concrete (UHPC) using the compressible packing model (CPM). To this end, the packing densities of binary combinations of Nos. 2, 4, 6, and 7 fine aggregates were analyzed. The particle interaction coefficients of the CPM were calibrated based on the experimental packing densities of fine aggregate mixtures. The optimal fine aggregate combination, exhibiting the maximum packing density, was derived at 70/30% of No. 2/No. 7 sand. In the subsequent UHPC compressive strength tests, the maximum strength of 150 MPa was achieved at an 80/20% ratio of No. 2/No. 7 sand. The CPM-based UHPC packing density was also highest at the 80/20% and 70/30% combinations. The CPM-based packing density and compressive strength of UHPC exhibited a quadratic relationship with a coefficient of determination of R<sup>2</sup> = 0.92.