Which Risks Require Key Control in Mass UHPC Construction?

Mass UHPC construction requires focused control of three core risks: thermal cracks, shrinkage cracks, and construction consistency. Through its integrated "material optimization + process control + monitoring and early warning" approach, Qinglong has successfully completed multiple mass UHPC projects (single element volume ≥5m³), ensuring construction quality and structural safety.

1. Thermal Crack Risk: Controlling the Internal-External Temperature Difference and Hydration Heat

Mass UHPC generates concentrated hydration heat, which can easily cause thermal cracks. Risk causes: high cementitious material content (800-900kg/m³) with hydration heat peaks reaching 70-80℃; when the internal-external temperature difference exceeds 25℃, thermal stress cracks are likely to occur. Control measures: optimize the material mix by adding 20%-25% fly ash and slag powder to reduce cement content and hydration heat—Qinglong has reduced the cement content of its mass UHPC to below 600kg/m³; adopt low-heat mixing with pre-cooled aggregates (≤25℃) and ice added to mixing water (water temperature ≤15℃) to lower the placement temperature (≤30℃); install cooling water pipes using circulating water cooling, spaced 500-800mm apart, to keep the internal-external temperature difference at ≤20℃—in a Qinglong bridge pier project (volume 8m³), the temperature difference was controlled at 18℃ through cooling water pipes; apply thermal insulation curing by covering with insulation blankets or geotextiles after placement to slow the cooling rate and prevent cracks caused by sudden temperature drops.

2. Shrinkage Crack Risk: Suppressing Autogenous Shrinkage and Drying Shrinkage

Mass UHPC undergoes large shrinkage and is prone to through cracks. Risk causes: a low water-binder ratio (0.2-0.25) leads to significant autogenous shrinkage (200-300με); combined with drying shrinkage, total shrinkage can exceed 400με, surpassing the ultimate shrinkage corresponding to the tensile strength of UHPC. Control measures: at the material level, add 5%-8% expansive agent to compensate for autogenous shrinkage—Qinglong uses ettringite-type expansive agents with a compensation rate above 80%; add 2.5%-3% steel fibers to bridge shrinkage cracks and inhibit crack propagation; at the process level, place in layers, each 300-500mm thick with intervals of ≥4 hours, to release part of the shrinkage stress—Qinglong mass elements are placed in 4 layers, each 400mm thick; apply moisture curing by extending the curing period to 28 days and keeping the surface moist for the first 7 days to prevent excessively rapid drying shrinkage—Qinglong's intelligent curing system can automatically control humidity at ≥90%.

3. Construction Consistency Risk: Ensuring Homogeneity and Compaction

The mass UHPC construction process is complex and prone to uneven quality. Risk causes: insufficient mixing leads to uneven fiber dispersion and aggregate segregation; overly rapid placement causes entrapped air bubbles and inadequate vibration compaction; formwork deformation causes dimensional deviations in elements. Control measures: mixing process—use forced-action mixers with a mixing time of ≥5 minutes to ensure uniform fiber dispersion, and Qinglong's mixing equipment is equipped with a fiber dispersion monitoring system; placement process—place by pumping or placing boom, controlling the placement rate at 0.5-1m³/h, and vibrate with internal vibrators (frequency ≥12000 times/min) to ensure compaction and avoid missed vibration; formwork engineering—select high-strength steel formwork with a support system of adequate stiffness, and monitor formwork deformation in real time during placement, keeping deformation ≤3mm—in a Qinglong mass foundation project, formwork deformation was controlled within 2mm; quality inspection—after placement, use ultrasonic testing to check internal compaction and promptly repair defective areas by grouting, with Qinglong's ultrasonic testing coverage reaching 100%.