What Risks Require Key Control in Mass UHPC Construction?

Mass UHPC construction (usually referring to a single element with volume ≥1m³ or continuous pouring area ≥50㎡) involves high cementitious material content, concentrated hydration heat, and significant shrinkage stress, which can easily lead to risks such as thermal cracks, shrinkage cracks, and insufficient durability. As an enterprise with extensive large-scale project construction experience, Qinglong Group has summarized four core risk control points based on project practice including the Guangxi New Media Center and the Shanghai Qingpu R&D Base, to ensure construction quality.

Concentrated hydration heat risk: temperature control and crack prevention are the core. The cementitious material content of mass UHPC reaches 800-1000kg/m³, the hydration heat peak can easily exceed 70°C, and when the temperature difference between interior and exterior is ≥25°C, thermal stress is generated, causing through cracks. Qinglong's control plan includes: first, optimizing the mix proportion by adding 30%-40% mineral admixtures (ultra-fine fly ash, mineral powder) to reduce the hydration heat peak below 55°C; second, adopting layered pouring with each layer 300-500mm thick and an 8-12 hour interval between layers, pouring the next layer only after heat dissipation; third, embedding cooling water pipes and using circulating water for cooling, controlling the interior-exterior temperature difference ≤20°C; fourth, covering with thermal insulation and moisture retention materials (geotextile + plastic film) to slow the cooling rate. In the construction of hyperbolic elements for the Guangxi New Media Center, this plan kept the interior-exterior temperature difference of the elements at 18°C, with no thermal cracks occurring.

Shrinkage crack risk: multi-dimensional compensation and curing. The superposition of autogenous shrinkage and drying shrinkage in mass UHPC easily leads to surface and internal cracks. Qinglong's control measures include: first, material design, adding 6%-8% expansion agent and 0.2% polypropylene fiber to compensate for autogenous shrinkage and inhibit crack propagation; second, extending the moisture curing time, covering for moisture retention within 24 hours after pouring, with steam curing for 7 days or natural curing for 14 days to ensure full hydration; third, setting expansion joints at 2-3 meter intervals, filled with elastic sealing materials to release shrinkage stress; fourth, controlling the pouring speed to avoid uneven internal moisture distribution caused by overly fast pouring, which triggers differential drying shrinkage. In the mass UHPC roof construction of the Shanghai Qingpu R&D Base, the above measures kept the surface crack width of the elements ≤0.02mm, meeting design requirements.

Pouring quality risk: ensuring uniformity, compactness, and freedom from defects. The high viscosity of mass UHPC (spread of 200-250mm) easily leads to defects such as incomplete compaction, air bubble accumulation, and aggregate pile-up. Qinglong's control plan includes: first, using automated mixing equipment to ensure uniform mixing of raw materials, with a mixing time ≥3 minutes; second, using immersion vibrators (poker diameter 50mm) with a vibration spacing ≤300mm and a vibration time of 15-20 seconds per point to avoid missed or excessive vibration; third, pouring from low to high and advancing from one end to the other to ensure full mold filling; fourth, preheating molds to 10-20°C before pouring to prevent reduced UHPC workability caused by cold molds. In the mass UHPC floor construction of the Yangshengtang Pharmaceutical Hangzhou Industrial Park, this plan achieved element compactness ≥99% with no defects such as air bubbles or honeycombing.

Insufficient durability risk: full-process control from materials to construction. Improper construction of mass UHPC affects densification and interfacial bonding, reducing long-term durability. Qinglong's control measures include: first, strict screening of raw materials, controlling aggregate particle size within 0.15-5mm and ensuring uniform fiber dispersion to avoid agglomeration; second, controlling the water-binder ratio ≤0.20, adjusting workability with high-efficiency water reducers and avoiding additional water; third, strengthening interface treatment—during layered pouring, roughening and thoroughly cleaning the surface of the lower layer, and applying a bonding agent before pouring the upper layer; fourth, later-stage protection, promptly applying an anti-corrosion coating after construction (especially in coastal or highly corrosive environments) to prevent the intrusion of harmful substances. In the mass UHPC curtain wall panel construction of the Panzhihua Third-Front Construction Culture and Tourism Project, full-process control achieved a 20-year strength attenuation rate ≤3%, meeting the design service life requirement.

Risk control in mass UHPC construction must run through the entire process of material design, pouring, and curing/protection. Qinglong Group's plan is based on extensive project practice, balancing scientific rigor and operability. Customers are advised that, in mass UHPC projects, they should prioritize manufacturers with large-scale project construction experience, prepare specialized construction plans in advance, clarify control measures for temperature control, crack prevention, compactness, and durability, and equip professional construction teams and testing equipment to monitor the construction process in real time, ensuring project quality and safety.