Minimum Curvature Radius for Complex-Shaped GRC: Process Limits and Design Recommendations

The minimum curvature radius a GRC component can achieve is set by the material, the mould, and the demoulding process — not by the architect's sketch. Designs that push below the limit end up with moulds that can't release, panels that crack during demoulding, or visual quality that fails inspection. This article gives the practical limits (inner radius, outer radius, sharp corners), explains why those limits exist, and gives design recommendations that stay safely within them.

The Three Curvature Configurations

When a curved surface meets a mould, three configurations arise:

• Inner radius: the curve is concave (the panel curves inward, the mould curves outward)
• Outer radius: the curve is convex (the panel curves outward, the mould curves inward)
• Sharp corner: the design has a true edge, not a curve

Each has a different minimum limit. The limit is set by the demoulding step — pulling the cured GRC panel off the mould without breaking the panel or the mould.

Inner Radius Limit: 150–200 mm

The inner radius limit is set by the panel's structural integrity at the curve. As the radius decreases:

• The GRC layer at the curve apex thins out (spray application is harder to control at tight radii)
• The fibre orientation at the curve becomes less effective at distributing stress
• Demoulding stress at the curve increases

Practical limit: 150–200 mm inner radius for standard spray-up GRC. Below 150 mm, the failure rate during demoulding rises sharply.

Workaround: at inner radii of 80–150 mm, the panel can be cast in two pieces and joined at the curve apex. The joint adds labour and creates a visible seam, but it allows the design intent to be met.

Hard limit: inner radius below 50 mm is not feasible with standard GRC. UHPC can achieve smaller radii (down to 20–30 mm) because of its finer aggregate and higher fibre content.

Outer Radius Limit: 100–150 mm

The outer radius limit is set by mould fabrication and demoulding clearance. The mould at a tight outer radius has a narrow projection that:

• Is hard to machine accurately (tool access is limited)
• Chips or wears during repeated production cycles
• Creates a tight clearance during demoulding — the panel has to slide past the projection without binding

Practical limit: 100–150 mm outer radius for standard GRC. Below 100 mm, mould life drops dramatically (50–100 pieces vs 200+ for standard moulds), and demoulding damage becomes common.

Workaround: split the mould at the tight radius into multiple pieces. The mould is fabricated in segments and assembled before casting. The segment joints are faired out before casting to leave a smooth panel surface.

Hard limit: outer radius below 30 mm is not feasible with standard GRC moulds.

Sharp Corners: 3–5 mm Minimum Fillet

A truly sharp corner (zero radius) on a GRC panel is structurally and practically infeasible:

• The GRC at a sharp corner is thin and weak — stress concentration leads to cracking
• The mould at a sharp corner is fragile — chipping during fabrication or use
• The coating at a sharp corner is hard to apply uniformly

Design recommendation: 3–5 mm minimum fillet at any "sharp" corner. The fillet is small enough to read as a sharp corner visually but large enough to be structurally and practically feasible.

For "soft" corners (where the design intent is a gentle curve, not a sharp edge), the design should specify the radius explicitly rather than relying on a default.

Why These Limits Exist: The Three Mechanisms

Mechanism 1: Demoulding Clearance

Every mould has a demoulding draft (typically 1°–2°) that allows the cured panel to release. At a tight inner or outer radius, the geometric clearance needed for demoulding may exceed the design intent.

For example, a 50 mm inner radius requires the mould surface to slope outward by 1° on each side of the curve. At a 50 mm radius, this means the mould wall at the apex is 0.5 mm thinner than the design spec — which may be below the GRC's minimum wall thickness.

Mechanism 2: Spray-up Process Limits

The spray-up process deposits GRC in a stream of slurry + chopped fibre. At a tight radius, the spray stream either:

• Misses the surface (the spray cones outward and the curve is in the shadow of the spray)
• Bounces back from the surface (high-velocity spray hitting a tight curve causes rebound and uneven deposition)

Both effects produce thin or missing GRC at the curve apex. The result is a weak point that cracks during demoulding or in service.

Mechanism 3: Fibre Orientation

Chopped fibre in spray-up GRC orients roughly parallel to the mould surface. At a tight curve, the fibre at the curve apex is oriented perpendicular to the principal stress direction, providing minimal reinforcement. The result is a weak curve apex that fails under load.

Design Recommendations

Rule 1: Stay Well Above the Limit

The 150 mm inner radius limit is the floor, not the target. Designs should target 300+ mm inner radius and 250+ mm outer radius to leave margin for process variation. A 200 mm inner radius is feasible; a 300 mm inner radius is robust.

Rule 2: Specify Radii Explicitly

The architectural model often shows "curved" features without specifying the radius. The deepening designer has to assume a radius, and the assumption may not match the design intent. Specifying the radius explicitly (e.g., "200 mm inner radius, 150 mm outer radius") avoids misinterpretation.

Rule 3: Verify at Design Freeze

Before the architectural design is frozen, every curved feature should be checked against the radius limits. Any feature below the limit triggers a design review with the architect, the structural engineer, and the GRC factory. The outcome is either a redesign, a special process agreement, or a material change (e.g., switch to UHPC for the tightest curves).

Rule 4: Document Special Process Features

If a design requires radii below the standard limits, the special process features must be documented: split mould, two-piece panel, hand lay-up at the curve apex, additional reinforcement at the curve. The documentation is the basis for the factory's quotation and the installation team's handling protocol.

Material Trade-Offs at Tight Radii

When the design requires radii below GRC's standard limits, three options exist:

• UHPC: finer aggregate, higher fibre content, can achieve 20–30 mm radii. UHPC price is 2–5× GRC's.
• GRP (fibreglass-reinforced plastic): can achieve any radius, but is combustible (B-grade fire rating) and not suitable for exterior façades in most jurisdictions.
• Cast metal (aluminium, bronze): can achieve any radius, but at 5–10× GRC's price and with significantly longer fabrication lead time.

The material trade-off should be made at design freeze, not during fabrication. Late material switches can add 30%–50% to the project cost and 2–3 months to the schedule.

Bottom Line

The practical limits for complex-shaped GRC are 150–200 mm inner radius, 100–150 mm outer radius, and 3–5 mm minimum fillet at corners. The limits come from demoulding clearance, spray-up process physics, and fibre orientation. Designs should target well above the limits (300+ mm / 250+ mm), specify radii explicitly, verify at design freeze, and document any special process features. When the design requires radii below the limits, the material trade-off (UHPC, GRP, cast metal) should be made early, not late.

Guangdong Qinglong Construction's deepening team verifies all curved features against the radius limits at design intake. Visit the official website for sample minimum-radius drawings and special-process agreements.