How to Mitigate UHPC Autogenous Shrinkage Through Material Design: Core Technical Solutions for Material Optimization

Autogenous shrinkage of UHPC is a volume shrinkage phenomenon caused by moisture consumption during its hardening process, which can easily lead to component cracking and affect structural safety and durability. Through scientific material design optimization, autogenous shrinkage can be effectively suppressed—this is one of the core technical challenges Qinglong has overcome in its 28 years of UHPC R&D and production.

I. Precise Proportioning Optimization of Active Mineral Admixtures

Rational selection of active admixtures is key to mitigating autogenous shrinkage, and the synergistic use of silica fume and fly ash is remarkably effective. Silica fume can fill gaps between cement particles and improve compactness, but excessive amounts tend to aggravate autogenous shrinkage; through testing, Qinglong has determined that silica fume content should be controlled at 15%-20% of the total cementitious materials. Class I low-calcium fly ash should be selected at a proportion of 20%-30%; its spherical particles improve paste flowability, delay the hydration rate, and reduce rapid moisture consumption. In Qinglong's UHPC project for the Yangshengtang Pharmaceutical Hangzhou Industrial Park, a combined silica fume and fly ash blending scheme was adopted, reducing the autogenous shrinkage rate by more than 35%. In addition, the addition of ground slag powder can further regulate the hydration process, forming a ternary composite system with silica fume and fly ash to jointly suppress autogenous shrinkage.

II. Adapted Application of High-Performance Admixtures

The selection and dosage of admixtures directly affect the degree of autogenous shrinkage, with the core lying in the balance between water retention and retardation. Polycarboxylate high-range water reducers should be water-retentive products with a water reduction rate controlled at 25%-30%, which both lowers the water-binder ratio to reduce shrinkage and prevents moisture from evaporating too quickly; Qinglong commonly uses a dosage of 1.0%-1.2% of the cementitious material mass. Sodium gluconate or citric acid should be selected as retarders at a dosage of 0.05%-0.1%, extending the hydration induction period so that moisture fully participates in the reaction and reducing internal drying shrinkage. For the super-large UHPC components of the Century Plaza on Nanjing East Road in Shanghai, Qinglong additionally added a special water-retaining agent that locks in free moisture, further reducing the risk of autogenous shrinkage and ensuring crack-free components after forming.

III. Aggregate Gradation and Particle Morphology Optimization

The gradation and morphology of aggregates have a significant regulatory effect on autogenous shrinkage; tight packing must be achieved to reduce paste consumption. Qinglong selects quartz sand with particle sizes of 0.15-1.2mm and, through a three-stage gradation design, maximizes packing density and reduces void volume, thereby decreasing the space available for paste shrinkage. Round-particle aggregates are preferred, as their small specific surface area and low water demand reduce cement consumption, indirectly suppressing autogenous shrinkage. In the production of UHPC perforated panels for the Ouargla Hotel in Algeria, optimizing the aggregate gradation reduced paste consumption by 15%, with the autogenous shrinkage rate correspondingly falling by 20%. In addition, incorporating an appropriate amount of lightweight aggregates such as ceramsite, which store and release water through their internal pores, enables internal curing and further mitigates autogenous shrinkage.

IV. Fiber Addition and Precise Water-Binder Ratio Control

Adding fibers can suppress the propagation of shrinkage cracks, and combining steel fibers with polypropylene fibers works even better. Steel fiber content is controlled at 2%-3% of the cementitious material mass, preventing microcrack development through bridging action; polypropylene fiber content is 0.1%-0.2%, and its elastic modulus matches the UHPC matrix, dispersing shrinkage stresses. Qinglong adopted this hybrid fiber scheme in the UHPC sculpture project for the Shanghai Astronomy Museum, effectively keeping shrinkage crack width below 0.1mm. The water-binder ratio is a core factor influencing autogenous shrinkage and must be strictly controlled between 0.20 and 0.25—too low easily leads to insufficient moisture that aggravates shrinkage, while too high reduces strength. Through automated metering equipment, Qinglong controls the water-binder ratio error within ±0.01 and, combined with high-range water reducers, achieves performance balance and suppresses autogenous shrinkage at the source.