Optimisation of aggregate gradation of ultra-high-performance concrete based on the modified compressible packing model

The compressible packing model (CPM), which is a method for designing aggregate gradation, was used to calculate the packing density of quartz aggregate. The aggregate gradation of ultra-high-performance concrete (UHPC) was optimised using the CPM. The number of interfaces between the aggregates and cement was determined using Image-Pro Plus software. The CPM modified using the equation Y = 0·60073X + 0·20551 was found to calculate the packing density of quartz aggregate accurately. The compressive strength of the UHPC was found to be positively correlated with the packing density of the aggregate: an increase in the packing density led to a compressive strength increase from 97·9 MPa to 124·1 MPa, an increase of 26·7%. The flexural strengths of the material was found to be sensitive to the number of interfaces: the smaller the number of interfaces, the greater the flexural strength. The specimen with an interface length of 5·56 mm/mm² had the smallest flexural strength (21·5 MPa) and the specimen with an interface length of 3·596 mm/mm² had the largest flexural strength (25·9 MPa). It is recommended that, when designing aggregate gradation, a gradation with high density should be selected and the proportion of small-size aggregates should not be too large.