Modeling mixing kinetics for large-scale production of Ultra-High-Performance Concrete: effects of temperature, volume, and mixing method

The mixing of ultra-high-performance concrete (UHPC) featuring low water-to-binder ratios involves different evolution of microstructures and mixing torques from conventional concrete. This paper investigates the mixing kinetics of UHPC in the mixing process, presents a mixing kinetics model to predict the mixing torque at an arbitrary time instant, and develops a multi-batching method to reduce the mixing torque for large-volume production of UHPC. The presented mixing kinetics model considers the effects of mixing temperature, mixing volume, and mixing methods. The mixing kinetics model is calibrated using experimental data, and the calibrated model shows high prediction accuracy. The multi-batching method enables large-volume mixing of UHPC by reducing the mixing torque while retaining desired flowability and hardened properties of UHPC. Specifically, when the number of sub-batches is two, the peak mixing torque of the multi-batching method was approximately reduced to half of the peak mixing torque of the mono-batching method. Besides, the differences in workability, compressive strength, and autogenous shrinkage by using the multi-batching method and mono-batching method are within 5%. Finally, the reliability and repeatability of the presented mixing kinetics model are verified through the validation tests with different UHPC mixture designs and mixing methods. This study will advance understandings of the mixing kinetics for UHPC and promote large-volume UHPC production.