How Can UHPC Autogenous Shrinkage Be Mitigated Through Material Design?

The autogenous shrinkage of UHPC is an inherent challenge arising from its low water-binder ratio and high cementitious material content. It mainly manifests as volume shrinkage in the early stage caused by the self-desiccation effect and hydration reactions; if not properly controlled, it can easily induce internal microcracks and compromise long-term durability. Through a systematic material design strategy, Qinglong Group carries out comprehensive control at four levels—adjusting the cementitious system, optimizing aggregate gradation, introducing internal curing and shrinkage-reducing components, and scientifically designing fibers—effectively mitigating autogenous shrinkage and ensuring the dimensional stability and high durability of its products.

First, optimizing the cementitious material system is fundamental. While ensuring strength, Qinglong UHPC adjusts the hydration process and shrinkage performance by blending active and inert micro powders. For example, adding an appropriate amount of relatively inert supplementary cementitious materials such as fly ash and limestone powder can partially replace the highly active cement and silica fume, reducing the driving force of chemical shrinkage at an early stage. Meanwhile, we strictly control the silica fume content, as its enormous specific surface area would intensify the self-desiccation effect, and find the balance between strength and shrinkage through testing.

Second, introducing internal curing materials is an effective means of actively countering self-desiccation. A small amount of pre-wetted superabsorbent polymer (SAP) or saturated lightweight porous aggregate is added to the mix. When the internal humidity of UHPC drops due to self-desiccation, the water stored inside these materials is released slowly, continuously supplying moisture for the later-stage hydration of cement, thereby significantly reducing self-desiccation shrinkage. Qinglong has adopted this technique in some ultra-thin components that are extremely sensitive to early-age cracking.

Furthermore, a high-performance shrinkage-reducing admixture (SRA) is used and the aggregate system is optimized. SRA can lower the surface tension of capillary pore water, thereby effectively reducing shrinkage stress. At the same time, the gradation of quartz sand is optimized and the aggregate volume fraction is appropriately increased, making use of the skeleton effect of rigid aggregates to suppress overall shrinkage. Finally, although scientifically proportioned steel fibers cannot reduce the shrinkage value, they can effectively restrain shrinkage deformation, prevent micro shrinkage stresses from developing into macro cracks, and improve the crack resistance of the material. Through this “combination” of material design measures, Qinglong keeps the autogenous shrinkage of UHPC at a manageable low level, laying a solid foundation for engineering applications.