This paper describes the validity of a new parameter for the content and grading of fine aggregate used in the quantitative rheological formula of fresh mortar when the mortar is assumed as a Bingham body. The rheological constants of fresh mortar, i.e. the yield value and the plastic viscosity were measured at 20±2°C with a parallel-plate plastometer using the mortar specimens of 30cm in diameter and about 3cm in height. All the measurements of these rheological constants were carried out by the same procedure within six minutes after mixing. The consistency of each mortar mix ranged from 110 to 250 in terms of JIS flow value.
The rheological constants of fresh mortar using coarser fine aggregate were found to be smaller than those of mortar using finer one at the same mix proportion. By assuming that the particle of fine aggregate is a sphere, the effective volume fraction was obtained from the fine aggregate volume fraction of mortar and the thin paste (10μm) adhered over the surface of the particles. A good correlation was obtained between the fine aggregate effective volume fraction and the yield value or the plastic viscosity. So this parameter is considered suitable for the evaluation of the effects of grading and content of fine aggregate on the rheological constants of fresh mortar.