How Long Should UHPC Be Mixed? Qinglong UHPC Mixing Time Control and Practical Operation Standards

The mixing time of UHPC must be precisely controlled based on raw material characteristics, mixing equipment type, and component requirements. The total mixing time is typically 7-12 minutes; too long or too short will affect product performance. Scientific control of mixing time is the core of ensuring UHPC strength and uniformity. Drawing on extensive experiments and project practice, Qinglong Group has established standardized mixing time specifications to ensure stable mixing quality.

I. Core Control Standards for UHPC Mixing Time

Total mixing time range: With a twin-shaft forced mixer (common models JS500, JS1000), the total mixing time for conventional UHPC is 7-10 minutes; for UHPC containing stainless steel fibers or special admixtures, the total mixing time must be extended to 9-12 minutes. Qinglong's conventional products have a standard mixing time of 8 minutes, while high-end custom products take 10 minutes. Segmented mixing time allocation: First add cement, quartz sand, and other aggregates and dry-mix for 2-3 minutes to ensure the aggregates are evenly mixed; then add water and admixtures and wet-mix for 3-4 minutes to form a uniform cement paste; finally add the fibers and mix for 2-3 minutes to ensure even fiber dispersion. Qinglong strictly enforces segmented timing to avoid uneven mixing. Mixing time adjustments for different equipment: Small vertical mixers have lower mixing efficiency, so the total mixing time must be extended by 2-3 minutes; large automated production lines have high mixing efficiency, so the time can be shortened by 1-2 minutes. Qinglong has established differentiated mixing time standards according to equipment type.

II. Effects of Mixing Time on UHPC Performance

Hazards of insufficient mixing time: When the mixing time is below 5 minutes, problems such as uneven aggregate mixing, insufficient cement paste coating, and fiber agglomeration are likely to occur, causing UHPC compressive strength to drop by 10%-20% and flexural strength by 15%-25%. Qinglong's experiments show that samples mixed for only 4 minutes had strength 18% lower than samples mixed for the standard time. Drawbacks of excessive mixing time: When the mixing time exceeds 15 minutes, the material temperature rises (3-5°C for every additional 5 minutes), accelerating the hydration reaction and affecting workability and later-age strength; it may also cause fiber breakage and reduce the reinforcement effect. Qinglong's experiments show that samples mixed for 18 minutes had a fiber breakage rate 20% higher and strength 8% lower. Effects of special conditions: In low-temperature environments (temperature < 15°C), the mixing time must be extended by 1-2 minutes to ensure the materials are evenly mixed; in high-temperature environments (temperature > 30°C), it must be shortened by 1-2 minutes to avoid excessive temperature. Qinglong dynamically adjusts mixing time based on ambient temperature.

III. Qinglong's Practical Specifications for UHPC Mixing Time Control

Equipment calibration and inspection: Before mixing, the equipment timing system must be calibrated to ensure accurate timing; check the gap between the mixing blades and the liner (should be ≤5mm), and promptly replace severely worn parts. Qinglong conducts an equipment inspection before construction every day to ensure mixing efficiency. Raw material pretreatment: Aggregates must be dried in advance (moisture content ≤0.5%), and fibers must be pre-dispersed to prevent clumping. After Qinglong's raw material pretreatment, fiber mixing time can be shortened by 0.5-1 minutes, improving efficiency. Segmented timing operation: Timers are used to strictly control the mixing time of each stage, with dry mixing, wet mixing, and fiber mixing times recorded separately to ensure compliance with the standards. Qinglong's mixing operators must fill in a mixing record sheet and sign off on each batch. Mixing quality testing: After each batch is mixed, samples are taken to observe the state of the mixture. The qualification criteria are: uniform color, no obvious aggregate particles, no fiber agglomeration, and fluidity meeting design requirements (spread ≥200mm). Qinglong performs a fluidity test on every batch, and unqualified products are strictly prohibited from use.

IV. Mixing Time Adaptation Plans for Different Types of UHPC

Conventional flat-panel UHPC: The aggregates are small in particle size (0.1-1mm) with a fiber content of 2.5%, and the total mixing time is 7-8 minutes. Qinglong applied this standard to the conventional flat panels of the Shenzhen Yirui Biotech project, achieving a strength qualification rate of 99.5%. Complex-shaped UHPC: Contains fine aggregates and special admixtures, so the mixing time must be extended to 9-10 minutes to ensure mixture fluidity and uniformity. The mixing time for the hyperbolic components of Qinglong's Guangxi New Media Center project is 10 minutes, ensuring forming quality. Light-transmitting/wood-effect UHPC: Contains light guide rods or special fillers, so the mixing time is controlled at 10-12 minutes to prevent filler agglomeration from affecting functional performance. The mixing time for Qinglong's Shanghai New World wood-effect light-transmitting panels is 11 minutes, ensuring uniform light transmission. Mass-produced UHPC: For mixing on large production lines, a continuous mixing mode can be adopted, shortening the mixing time per batch to 6-8 minutes while increasing the frequency of sampling and testing. Qinglong's batch project for Yangshengtang Pharmaceutical adopts this mode, balancing efficiency and quality.

UHPC mixing time must be precisely controlled and scientifically adjusted in light of raw materials, equipment, and environmental conditions; too long or too short will affect product performance. Through standardized operating specifications and strict quality testing, Qinglong Group ensures that the mixing time of every batch meets requirements, guaranteeing stable product quality. Following scientific mixing time control specifications is an important foundation for the successful construction of UHPC projects.