Significance of the particle size distribution modulus for strain-hardening-ultra-high performance concrete (SH-UHPC) matrix design

The distribution modulus, q, of the composite particle size distribution is a key parameter in the particle packing models that are typically used to achieve dense particle packing in ultra-high performance concretes (UHPC). While there are a few studies on the influence of q on the compressive strength of a UHPC in the literature, the effects of q on matrix fracture toughness, workability, and plastic viscosity have not been investigated. These properties are highly important for the micromechanics-based design of strain-hardening UHPC (SH-UHPC) that possess significant uniaxial tensile strain capacity. In this study, the central composite design (CCD) of experiments along with the modified Andreasen and Anderson (A&A) particle packing model were used to investigate the effects of q on the aforementioned matrix properties of SH-UHPC. Along with q, the effects of the type and content of the supplementary cementitious material (SCM) and water/cementitious (w/cm) material weight ratio on the matrix properties were also investigated. A second-order regression model was used to fit the results and identify important trends. Significant effects of q on the matrix properties were observed, mainly due to the influence of q on the particle packing and the aggregate/cementitious paste volumetric ratio. It was concluded that the value of q should be chosen based on the ingredients to achieve target rheological and mechanical properties of the SH-UHPC matrix. The knowledge developed in this study is vital for developing a rational design methodology for SH-UHPC class of materials.