What Should Be Noted in UHPC Construction? Ten Key Control Points to Ensure Successful Application

The construction of UHPC (Ultra-High Performance Concrete) is a precise science, where negligence at any stage can lead to substandard performance or even failure. Therefore, once you have decided to use UHPC, clarifying "what to pay attention to during construction" is more urgent and critical than understanding its properties. This article systematically outlines the ten key points that must be strictly controlled in UHPC on-site construction (including precast plant production), providing a practical technical guide for the smooth implementation of your project.

Key Point 1: Absolute precision in raw materials and mix proportions. UHPC performance is extremely sensitive to raw material quality and mix proportion accuracy. Cement, silica fume, quartz sand, superplasticizer, and fibers of specific brands and specifications that meet design requirements must be used. All raw materials should be properly stored to prevent moisture damage and deterioration. Weighing must be performed with high-precision electronic scales, with errors controlled within ±0.5%. Batching by visual estimation based on experience is strictly prohibited, as a single proportioning error may result in the entire batch being scrapped.

Key Point 2: Strict control of the mixing process. A forced-action mixer must be used. The feeding sequence should strictly follow the process requirements provided by the material supplier, typically: first add the dry materials (cement, silica fume, quartz sand, etc.) and mix slowly until uniform, then add most of the water and the superplasticizer solution and mix at high speed, and finally add the fibers evenly while mixing. The total mixing time must be sufficient to ensure even fiber dispersion, but should not be too long, to prevent excessive temperature. After mixing, workability tests (such as flow/spread) should be performed promptly.

Key Point 3: High standards for formwork. Formwork must have sufficient strength, rigidity, and tightness to withstand UHPC's relatively high lateral pressure without grout leakage. Formwork surfaces should be smooth and even, coated with a high-quality release agent (a dedicated release agent or water-based release agent is recommended). For decorative surfaces, the texture of the formwork will be directly "copied" onto the UHPC surface, so formwork quality is crucial. Before casting, thoroughly check the formwork's dimensions, tightness, and cleanliness.

Key Point 4: Prompt and moderate casting and vibration. UHPC mixtures are highly viscous, so the casting sequence should be carefully arranged to avoid the formation of cold joints. Since setting is relatively fast, the time from completion of mixing to casting into formwork should be as short as possible (usually no more than 45 minutes). Vibration must be performed with high-frequency poker vibrators, inserted quickly and withdrawn slowly, at evenly spaced points. Vibration time should be judged by the appearance of paste on the surface and no further air bubbles escaping; over-vibration, which causes fiber settling and mixture segregation, must be strictly avoided. For thin-walled members or areas with dense reinforcement, external formwork vibration can be used as a supplement.

Key Point 5: Prompt and moisture-retaining early curing. This is the key to preventing plastic shrinkage cracks! UHPC has an extremely low water-binder ratio, and surface moisture evaporates easily. After initial setting (usually within 1-2 hours after casting), exposed surfaces should immediately be tightly covered with plastic film or wet geotextile for moisture-retaining curing. Never leave the surface exposed to wind and sun. During curing, the surface should remain moist at all times.

Key Point 6: Caution in formwork removal and subsequent curing. UHPC gains strength quickly at early ages, but this does not mean formwork can be removed prematurely at will. The formwork removal time should be determined based on the strength of specimens cured under the same conditions, ensuring that the member has sufficient strength at demolding to avoid damage to edges and corners. After demolding, continuous wet curing (water spraying, immersion, or covering with wet blankets) should begin immediately, or a curing agent should be applied; the curing period should be no less than 7 days. Steam curing is a better option, as it significantly improves early strength and promotes microstructure development, but the rates of temperature rise, constant-temperature holding, and cooling must be strictly controlled.

Key Point 7: The critical importance of temperature control. UHPC's hydration heat release is concentrated, and it is sensitive to temperature. Construction should be avoided when the daily maximum temperature is above 35°C or below 5°C. In high temperatures, cooling measures must be taken (such as mixing with ice water and shading); in low temperatures, insulation and heating measures are required. The mixture's temperature at discharge should be controlled between 10-25°C. For mass UHPC casting, temperature monitoring is required to prevent thermal cracks caused by excessive temperature differences between the interior and exterior.

Key Point 8: Special considerations for reinforcement and joint treatment. The UHPC cover can be very thin, but requirements for rebar positioning accuracy are higher. Due to UHPC's high density, bonding with new or old concrete or with other materials requires special treatment. Interfaces should be roughened and cleaned thoroughly, and it is recommended to use a dedicated UHPC interface agent or mortar mixed with polymer powder for the transition.

Key Point 9: Quality testing and documentation throughout the process. During construction, specimens must be made and cured in batches as required, and tested for compressive strength, flexural strength, and other properties. At the same time, detailed records should be kept of each batch of raw materials' source, mix proportions, mixing time, casting time, ambient temperature, curing conditions, etc., to establish a traceable quality archive.

Key Point 10: Place high priority on safety protection. Fine powders such as silica fume in UHPC mixtures easily become airborne dust, and fibers may irritate the skin, so construction personnel must wear dust masks, gloves, safety goggles, and other protective equipment. Electric vibration equipment must be ensured to have good insulation.

In summary, the success of UHPC construction depends on ultimate attention to detail and strict discipline. It is recommended to prepare a detailed construction-specific plan before construction and provide targeted training for all workers. The most reliable approach is to work closely with an experienced UHPC material supplier, who can provide technical guidance and services throughout the process to implement the above precautions, thereby ensuring that UHPC's outstanding performance is perfectly realized in the project.