How Can UHPC Autogenous Shrinkage Be Mitigated Through Material Design?

The UHPC autogenous shrinkage problem can be effectively mitigated through three major material design directions: optimizing the cementitious system, adding modified materials, and fiber regulation. Based on 58 patented technologies, Qinglong has developed a mature autogenous shrinkage control solution, ensuring components are free of shrinkage cracks and suitable for complex shapes and long-span applications.

1. Optimizing the Cementitious Material System: Reducing the Source of Autogenous Shrinkage

Cementitious materials are the main source of autogenous shrinkage, and optimizing the mix ratio can control it at the source. Low-activity cementitious material blending: mixing Portland cement with low-calcium fly ash at a 7:3 ratio reduces hydration heat and the autogenous shrinkage rate; Qinglong's experiments show that this ratio reduces autogenous shrinkage by 30% compared with a pure cement system. Controlling total cementitious material content: keeping cementitious material usage within 800-900kg/m³ avoids shrinkage accumulation caused by excessive cementitious materials; Qinglong's standard UHPC uses 850kg/m³ of cementitious materials, balancing strength and shrinkage control. Selecting ultra-fine cementitious materials: adding 10%-15% ultra-fine silica fume fills the gaps between cement particles and reduces autogenous shrinkage caused by negative pressure in capillary pores; Qinglong's silica fume has a specific surface area of ≥20000m²/kg, improving density while suppressing shrinkage.

2. Adding Mineral Admixtures: Compensating Shrinkage and Optimizing Structure

Mineral admixtures mitigate autogenous shrinkage through physical filling and chemical reactions. Expansive agent addition: an ettringite-type expansive agent at a dosage of 5%-8% is selected, whose hydration-generated expansion compensates for autogenous shrinkage; Qinglong's expansive agent is compatible with its own cementitious system, with the expansion rate controlled at 0.02%-0.03% and no risk of excessive expansion. Inert admixture filling: adding 20%-25% quartz powder with a particle size of 0.1-0.3mm physically fills capillary pores, reduces porosity, and decreases shrinkage caused by water evaporation; Qinglong's quartz powder has a purity of ≥99%, synergizing with aggregate gradation to improve density. Composite admixture synergy: combining expansive agents with fly ash, where the expansive agent compensates for early shrinkage and fly ash suppresses later-stage shrinkage; the UHPC components of Qinglong's Yangshengtang Pharmaceutical Hangzhou Industrial Park project adopted this solution, reducing the 28-day autogenous shrinkage rate to below 200με.

3. Fiber and Admixture Regulation: Suppressing Shrinkage Crack Development

Fibers and admixtures assist in controlling autogenous shrinkage from the mechanical and workability perspectives. Fiber modification: adding 2%-3% alkali-resistant glass fiber or steel fiber, where the fiber bridging effect suppresses the propagation of shrinkage cracks—even if micro-shrinkage occurs, it will not develop into macro cracks; Qinglong's steel fiber UHPC components, with a fiber aspect ratio of 80, achieve remarkable shrinkage crack control. Admixture optimization: selecting a retarding high-range water reducer extends hydration time, lowers the hydration rate, and reduces early autogenous shrinkage; the water reducer dosage is 1.5%-2% with a water reduction rate of ≥35%, and Qinglong's customized water reducer can extend the initial setting time to 6 hours, avoiding shrinkage concentration caused by rapid hydration. Water-retaining agent addition: adding 0.1%-0.2% cellulose ether improves water retention, reduces water evaporation, and alleviates drying autogenous shrinkage; Qinglong's water-retaining agent brings the UHPC bleeding rate close to zero.