Why Curved Edging Behaves Differently
A weathering steel strip is stiff in its own plane. Bending it into a radius stretches the outer face and compresses the inner face, and if the radius is tight relative to the thickness, the outer fibres yield and the surface cracks. As a working rule, the inside radius should not be smaller than about two to three times the plate thickness for a smooth, controlled bend; a 2 mm strip therefore needs an inside radius of at least 5 to 6 mm, and much larger radii are needed if the material is to be hand-formed without ripples.
Weathering grades such as SPA-H to JIS G3125, Q355NH to GB/T 4171 and S355J2W to EN 10025-5 combine a yield strength of 355 MPa or more with good ductility, so they bend well when the technique is right. The failures that appear on site almost always come from trying to force a small radius along a short length of strip instead of distributing the deformation.
Setting Out Radii and Ordering Panels
Mark the curve centre with a stake and set out the radius with a string line or a flexible batten, or use a spare panel held against the ground to trace the true curve.
Establish the run, the radii at each change of curvature and the number of tangent points, then draw the plan to scale.
For radii below about 3 m, order factory pre-bent panels to a stated radius; this is the only reliable way to hold a smooth, uniform curve over a long run.
For radii above about 5 m, gentle site bending of straight panels is practical, provided the bend is worked along the full length.
Allow extra length for the radius development and 5 per cent cutting waste, and specify the fold direction so that the finished face is consistent.
Panel lengths and joints on a curve
Shorter panels of 300-600 mm follow a radius more closely than 2 m sections, because each panel subtends a smaller angle and the joint becomes a natural tangent point. Where joints must fall on the curve, overlap them in the direction of the curve so water cannot enter and the joint line stays optically smooth.
The Relief-Cut Bending Technique
The standard method for site-formed curves is to make a series of shallow relief cuts on the inside face of the bend, then close them as the strip is formed.
Mark equally spaced cut positions along the inside face, at 40-60 mm centres for a moderate curve and closer for a tight one.
Cut to a depth of about one third to one half of the strip height, leaving the lower part of the web continuous.
Bend the strip progressively by hand or with a light clamp, working outward from the centre of the curve.
Close the gaps by tapping the strip with a rubber mallet against a former, checking the radius against a template.
Weld or overlap the closed cuts, or leave them as designed drainage slots, and dress the face so the cut edges patinate evenly.
The alternative, and the better option for public schemes, is a pre-bent panel produced in the works by rolling or press forming. Radius accuracy is far higher, there are no field cuts to corrode, and the delivered panel goes straight into the trench.
Fixing and Backfilling a Curved Run
Excavate the trench along the curve to a depth of 100-150 mm and a uniform width, and compact the base. Curved runs need more fixings than straight ones because lateral soil pressure acts across the bend and tries to straighten the strip: pin curved panels at 400-500 mm centres, and always pin at both sides of a joint and at each tangent point.
Set the panel vertically, then check the radius with a template at intervals along the run.
On soft ground, place a 100-150 mm concrete collar or granular bed behind the curve so the panel cannot bow outward.
Backfill in 50 mm layers, compacting both sides evenly; compacting one side only will push the curve out of line.
Water each layer to settle it, then recheck the line before the next layer.
Acceptance limits for landscape work are usually a centre-line deviation of about 5 mm from the drawn curve over a 2 m length and a top-edge level difference under 3 mm between adjacent panels.
Curves in Service: Patina, Drainage and Care
Handled and installed correctly, a curved run develops the same uniform oxide as a straight one within roughly 6 to 18 months of natural wet and dry cycling, reaching a thickness of about 50 to 100 micrometres and oxidising at a rate far below plain carbon steel. Two points deserve attention on curves. First, the concave side collects water and debris, so a weep gap or a slightly lower section should be provided at the low point to let water escape. Second, relief cuts must be dressed and, where exposed, sealed with a matching weathering weld or left fully open so that no crevice holds moisture. Aftercare is limited to brushing off soil and cut grass, and re-tamping any section disturbed by the first winter frost.
Frequently Asked Questions
Q: Can Corten edging be bent on site?
A: Yes, for gentle curves using progressive bending and shallow relief cuts on the inside face. Radii below roughly 3 m are better supplied as factory pre-bent panels, which hold a uniform curve and cost less in site labour.
Q: How deep should the relief cuts be?
A: About one third to one half of the strip height, at 40-60 mm centres, leaving the lower web continuous. Deeper cuts weaken the section and should be welded back after forming.
Q: What is the minimum bend radius?
A: As a working rule the inside radius should be at least two to three times the plate thickness, and considerably larger for hand forming. Pre-bent panels are specified to a drawn radius rather than to this geometric minimum.
Q: How are curved runs kept from straightening after backfilling?
A: Fix the panels at 400-500 mm centres, pin both sides of every joint and at tangent points, support the back of the curve with a concrete collar or compacted granular bed, and compact the backfill evenly on both sides.
Q: Do the cut edges of relief cuts need treatment?
A: They should be dressed and either welded closed or left fully open so that no narrow crevice retains water. Open slots that drain freely will oxidise the same way as the surrounding surface.
Q: How long before a curved edging run looks finished?
A: A visible rust tone appears within weeks, and the run settles into a stable oxide within 6 to 18 months of natural exposure. Pre-weathered supply gives a uniform colour from the day of installation.
