What Are the UHPC Casting Methods? Choose the Right Process Based on Site Conditions to Ensure Forming Quality

UHPC (Ultra-High Performance Concrete) mixes are characterized by high viscosity and low flowability. Their casting methods differ significantly from those of conventional concrete and must be carefully selected and finely executed according to component shape, dimensions, reinforcement conditions, and construction conditions. Incorrect casting methods can cause problems such as uneven fiber distribution, internal defects, and cold joints, severely affecting final performance. UHPC casting methods fall into two main categories: precast casting and cast-in-place casting, each with different specific processes. This article systematically introduces these methods along with their applicable scenarios and key operation points.

Category One: Precast Casting (Factory Production)
This is currently the most widely used method and the easiest for quality control. It is carried out on fixed casting beds or production lines in the factory, where the environment is stable and processes are controllable.

1. Conventional casting method: Suitable for most standard components such as slabs, beams, and columns. The mixed UHPC is discharged from the mixer into a hopper, then lifted by crane or pumped above the mold, and fed into the mold through a hose or chute. Operation points: - Layered casting: For deep components, cast in layers, with each layer no thicker than 300mm. - Control drop height: Avoid excessive free fall of the mix, which causes segregation; use drop chutes or pipes if necessary. - Timely vibration: Casting and vibration must be closely coordinated; compact the lower layer by vibration before casting the upper layer.

2. Pumped casting method: For large precast components or long-distance conveying, a plunger pump can be used for pumping. The UHPC mix must have good pumpability (typically requiring a spread of over 650mm), and pump lines should be laid as straight as possible with fewer bends. The pipeline must be lubricated with mortar before pumping. This method is highly efficient and suitable for large-scale production.

3. Spraying method (or sprayed UHPC): This is a special process in which a wet-spray machine mixes the UHPC with an accelerator at the nozzle and sprays it at high speed onto the mold or substrate surface. Its advantages are coverage of any shape and high bond strength. It is particularly suitable for: - Strengthening works: reinforcing and strengthening existing structures. - Components with complex shapes: such as sculptures and irregular shells. - Thin-layer repairs. This method demands high operator skill; spraying angle, distance, and thickness must be well controlled.

Category Two: Cast-in-Place Casting
Cast-in-place conditions are complex and quality control is difficult. It is usually used where precasting is impossible or where transport and installation after precasting would be difficult, such as bridge wet joints, structural strengthening, and on-site integrated forming.

1. On-site mixing and casting method: Set up a mixing station on site, mix UHPC strictly according to the mix proportion, and then cast. This method requires that the site has qualified forced-action mixers, accurate metering equipment, and experienced workers. Environmental factors (temperature, wind) greatly affect workability, so contingency plans are needed. Suitable for locations with relatively concentrated usage and extremely high monolithic integrity requirements.

2. Premix on-site casting method: Use factory-premixed UHPC dry powder (with binders and aggregates already blended); on site, only the specified amount of water and fibers need to be added for mixing. This method reduces on-site weighing errors, but still places high demands on the dispersion capacity of the mixing equipment. It is a common method for small-scale on-site repairs or casting.

3. Self-compacting UHPC casting method: Uses specially designed self-compacting UHPC (SCC-UHPC), which has extremely high flowability and the ability to pass through reinforcement, filling every corner of the mold under its own weight without vibration. This places extremely high demands on mix design and costs more. Suitable for structural locations with extremely dense reinforcement, complex shapes, or where effective vibration is not possible.

Whichever method is used, the following general points must be observed:

Before casting: 1. Inspect and clean the molds to ensure they are clean and coated with release agent. 2. Check reinforcement positions and cover thickness. 3. Confirm smooth coordination between the mixing station and casting points to avoid excessive waiting time.

During casting: 1. Continuous operation: Plan properly to ensure casting proceeds continuously and prevent cold joints. If interruption is unavoidable, the interval should be shorter than the initial setting time of UHPC. 2. Proper material placement: Start from one end of the component, or from the middle toward both ends, to avoid asymmetric casting that creates lateral pressure differences. 3. Close coordination with vibration: vibrate while casting, using high-frequency internal vibrators inserted quickly and withdrawn slowly, with evenly spaced vibration points to prevent under-vibration and over-vibration. Stop when the surface shows paste rising and air bubbles are essentially released.

After casting: Immediately trowel the surface smooth (if required) and cover it to retain moisture.

Selection recommendations: For most new construction projects, precast casting is the preferred choice, as it is completed in a controllable factory environment with the best quality. Cast-in-place casting should only be used as a last-resort supplement, and an extremely strict dedicated construction plan must be prepared with strengthened on-site monitoring. Self-compacting UHPC offers a better solution for cast-in-place work, but its performance must be verified and the higher material cost accepted. In short, UHPC casting is a fine technique; the choice of method and the quality of execution directly determine the final performance of this remarkable material.