The mould filling characteristics of fresh ultra-high performance concrete (UHPC) in a precast shell of large capping beams: a numerical study

The flow behaviour of Ultra-High Performance Concrete (UHPC) plays a critical role in determining its workability at the fresh stage, as well as its mechanical performance and durability after hardening. A three-dimensional computational model is established to simulate UHPC flow, in which the Herschel–Bulkley constitutive law is adopted to represent the nonlinear fresh state rheology of UHPC. The model is validated against L-box experiments, yielding an average relative error not exceeding 7.2%. Building upon this validated framework, this study investigates mould filling evolution in a large precast capping beam shell, with particular attention to the effects of different boundary conditions and related mechanisms. The obtained results indicate a transition in the flow regime from an initial shear-dominated phase to a confined mode governed by hydrostatic pressure accumulation and yield stress resistance. Moreover, both the number of filling locations and the total filling area exhibit strong positive correlations with filling efficiency. From a rheological perspective, the findings elucidate the dynamic mechanisms that govern UHPC flow in confined environments and provide a theoretical foundation for optimising filling processes in large precast UHPC components.