Multi-scale design of ultra-high performance concrete (UHPC) composites with centroplasm theory

Designing ultra-high performance concrete (UHPC) composition is critical for optimal performance, but current methods have limitations in addressing chemical interactions and multi-scale characteristics. Therefore, this study introduced a novel multi-scale design method by combining the packing model and centroplasm theory. Three scales of design were integrated into this method: 1) firstly, micro centroplasm loop thickness (MCLT, water film) was incorporated into the modified Andreasen and Andersen model to optimize the mixture of UHPC paste; 2) then the optimized UHPC paste was progressed to further design UHPC mortar through sub centroplasm loop thickness (SCLT); 3) finally, steel fibers were involved into the designed UHPC mortar for the strength enhancement. The results showed that increasing MCLT promoted the degree of cement hydration. At a MCLT of 0.02 μm, the designed UHPC paste exhibited a higher density of C–S–H gel. The increased SCLT enhanced workability, compressive strength, and reduced porosity in UHPC mortar. Optimal comprehensive performance, with a workability of 230 mm and compressive strength of 168.9 MPa, was achieved with 2.0 vol% steel fibers and 30 μm SCLT. The results demonstrated the existence of optimal distances between particles at different scales in the UHPC system, emphasizing the importance of precise design. This work refined the fundamental design theory of UHPC composites by packing theory, film theory, cement hydration and centroplasm effects.