Two GRC Production Processes Compared: Spray-Up vs Premix Casting — Applications and Performance Differences

Two processes produce essentially all GRC components worldwide: spray-up (also called spray-dewatering or spray-suction) and premix casting (also called premix or slurry-cast). They produce materials with measurably different mechanical properties, and they fit different applications. This article unpacks the two processes, compares fibre orientation, strength, surface quality, and cost, and gives application guidance for when each process is the right answer.

Spray-Up Process

In the spray-up process, cement slurry and chopped AR glass fibre are sprayed simultaneously into the mould through a chopper gun. The fibre is cut to length (typically 25–40 mm) at the spray head and deposited along with the slurry in a continuous stream.

Key characteristics:

• Fibre orientation: fibre is oriented by the spray stream, generally parallel to the mould surface. This produces 2D random orientation with a strong in-plane fibre network.
• Fibre content: typically 4%–6% by weight of the total mix. High fibre content drives the mechanical performance.
• Layered structure: the panel is built up in multiple spray passes, each 2–5 mm thick. The layered structure is visible in cross-section.
• Surface quality: the spray texture is relatively coarse. Surface is typically finished by hand-trowelling or roller for architectural finishes.
• Cycle time: relatively slow per m² because of the multi-pass build-up. A 12 mm-thick panel takes 15–25 minutes of spray time per m².
• Labour intensity: high. Spray-up requires 2–3 operators per station (sprayer, chopper, finish).

Premix Casting Process

In the premix process, short AR glass fibre (12–25 mm) is pre-mixed with the cement slurry and quartz sand in a high-shear mixer. The mix is poured or injected into the mould and vibrated to consolidate.

Key characteristics:

• Fibre orientation: fibre is oriented randomly in 3D. No preferred orientation; the fibre network is isotropic.
• Fibre content: typically 2%–4% by weight. Lower than spray-up because higher content makes the mix hard to place.
• Bulk structure: the panel has a uniform bulk structure without visible layers.
• Surface quality: smooth as-cast surface, suitable for thin decorative elements and fine detail. Surface may need light sanding for premium finishes.
• Cycle time: faster per m². A 12 mm-thick panel takes 5–10 minutes of casting time per m².
• Labour intensity: lower. Premix requires 1–2 operators per station (mixer, caster).

Mechanical Property Comparison

The two processes produce measurably different strength:

• Compressive strength: spray-up 50–80 MPa, premix 40–60 MPa. Both well above typical structural requirements.
• Flexural strength (LOP — limit of proportionality): spray-up 7–12 MPa, premix 5–8 MPa. Spray-up is 30%–50% higher.
• Flexural strength (MOR — modulus of rupture): spray-up 18–28 MPa, premix 12–20 MPa. Spray-up is 30%–50% higher.
• Tensile strength: spray-up 5–8 MPa (proportional limit), 8–12 MPa (ultimate); premix 4–6 MPa (proportional limit), 6–10 MPa (ultimate).
• Impact resistance: spray-up 15–25 kJ/m², premix 8–15 kJ/m². Spray-up is roughly 2× higher.

Spray-up outperforms premix across every mechanical metric, typically by 30%–100%. The advantage comes from the higher fibre content and the 2D fibre orientation that resists in-plane stress (the dominant stress in most GRC applications).

Surface Quality Comparison

The two processes also differ in surface finish:

• Spray-up: coarse as-sprayed, requires hand-trowel or roller finishing for architectural surfaces. Texture variation is normal. Suitable for "stone-look," "concrete-look," and other textured finishes. Not ideal for mirror-smooth or fine-detailed surfaces.
• Premix: smooth as-cast, suitable for thin decorative elements (cornices, capitals, ornate mouldings, trim). Can be polished or sanded for premium finishes. Better for fine detail and sharp edges.

For decorative elements with fine detail (capitals, finials, ornate mouldings), premix is the right answer. For structural panels and curtain walls, spray-up is the right answer.

Cost Comparison

Per-m² production cost at 12 mm thickness (factory cost, 2026 reference):

• Spray-up: RMB 180–300/m² (factory)
• Premix: RMB 130–220/m² (factory)

Spray-up is 20%–40% more expensive per m² because of the higher fibre content, the labour intensity, and the longer cycle time. The cost premium is justified by the mechanical performance advantage — for structural applications, the premium is money well spent.

Application Guidance

Use Spray-Up When:

• The component is a structural panel (curtain wall, load-bearing cladding, large-format façade element)
• The component is exposed to high wind load, seismic load, or impact risk
• The design intent allows a textured surface (stone-look, concrete-look, sculptural finish)
• The component is large (≥1 m² per panel) and benefits from the layered strength
• The component is curved or double-curved (spray-up conforms to complex geometry better than premix)

Use Premix When:

• The component is a decorative element with fine detail (cornices, capitals, finials, mouldings, trim)
• The component is small (≤0.5 m² per piece) and benefits from fast cycle time
• The design intent requires a smooth, paint-ready surface
• The component is non-structural or lightly loaded (interior decoration, ornamental)
• Cost is the primary driver and the structural margin is comfortable

Use Both (Hybrid) When:

Many projects use both processes: spray-up for the structural panels and premix for the decorative elements. The hybrid approach matches the process to the function. For example, a museum lobby might have spray-up GRC wall panels (structural, textured) and premix GRC cornices and capitals (decorative, smooth).

Quality Control Differences

The two processes have different QC profiles:

• Spray-up QC: fibre content and orientation are the key parameters. QC checks per batch include: fibre content (burn-out test per ASTM C1224), flexural strength (4-point bend test per EN 1170), and dimensional scan. Spray-up is more sensitive to operator skill — a poor sprayer produces an inconsistent panel.
• Premix QC: fibre dispersion uniformity is the key parameter. QC checks include: fibre content, flexural strength, surface quality, and dimensional check. Premix is more sensitive to mix consistency — a bad mix produces a brittle panel.

Both processes should be sampled per batch (typically 1 in 50 to 1 in 100 panels) for full mechanical testing, with all results logged for traceability.

Equipment and Skill Requirements

The two processes need different equipment and operator skills:

• Spray-up equipment: chopper gun, spray pump, slurry mixer, often a dewatering rig (vacuum or roller). Capital investment RMB 200,000–500,000 per station.
• Premix equipment: high-shear mixer, casting table, vibration table. Capital investment RMB 50,000–150,000 per station.
• Spray-up operator skill: 6–12 months training to reach production-grade consistency. The skill is hard to acquire and is the factory's core competitive advantage.
• Premix operator skill: 2–4 weeks training to reach production-grade consistency. The skill is more accessible and the labour pool is larger.

For a factory considering both processes, the spray-up skill set is the strategic asset. It takes years to build and is the entry barrier for high-end GRC production.

Bottom Line

Spray-up and premix are the two GRC production processes, each with distinct mechanical properties, surface qualities, costs, and applications. Spray-up produces higher strength and impact resistance but at higher cost and with coarser surface. Premix produces lower strength but smoother surface, faster cycle, and lower cost. For structural panels, use spray-up. For decorative elements with fine detail, use premix. For mixed projects, use both — match the process to the function.

Guangdong Qinglong Construction runs both spray-up and premix production lines with full mechanical testing capability. Visit the official website for sample process specifications and per-m² cost benchmarks.