Granite Radius Kerb Dimensions: Blocks, Chords and Joint Gaps
A granite radius kerb is not a straight length bent round a curve. The radius comes from the number of blocks, the chord of each one and the gap that opens at the back edge. At R 6 m with a 0.5 m chord, a 150 mm kerb opens a 12.5 mm joint at the back.
How are granite radius kerb dimensions actually set?
Fix the radius and the block count first, then work back to chord and edge lengths, not the other way round. Stone does not bend: forcing a straight length round a curve cracks it. The curve is built from short tapered blocks laid end to end, each with a longer outer edge than inner, and the radius is what that run of blocks adds up to rather than a shape cut on the saw. That leaves three numbers to pin on the order: how many pieces go round the arc, what chord each piece takes, and how far the joint opens at the back. Once those are fixed, every length, taper and piece number follows. When price and lead time will not settle, one of the three is usually still only verbal. Sesame Grey G602 is the grade most often specified for radius work, and one run of curved pieces should come off a single slab batch: mixed batches show as banding along the arc, which matters more on a curve than on a straight run.
There is no radius on the drawing, so how do I measure R on site?
Measure the chord and the sagitta, then use R = chord² ÷ (8 × sagitta) + sagitta ÷ 2. The chord is the straight distance between the two ends of the arc; the sagitta is the rise from the middle of that chord to the crown, taken square to the chord. With a 2 m chord and a 0.17 m sagitta: 4 ÷ 1.36 ≈ 2.94, plus 0.085, gives R ≈ 3.03 m. Three cautions. Take the longest chord you can, because a short one magnifies the error. Keep the sagitta square to the chord at the crown. And measure both ends on the same face, not one on the inner edge and one on the outer. Once R is known, add and subtract half the width, 75 mm each way for a 150 mm kerb, and all three face radii are set.
How many pieces does a 90° bend need, and which face do I measure?
Settle the face first, because the three faces are not the same length. Arc = angle in radians × radius, and 90° is π/2 ≈ 1.5708. Blocks = arc ÷ chord, rounded up, never down, since half a block short is one open joint. Take a 90° bend at 6 m centreline radius, 150 mm wide, 0.5 m chord: the three faces give two answers, 19 and 20. Counting on the outer face is the safer habit. It is the longest, so if the outer arc is covered the inner edge still has material to spare inside the taper; count on the inner face instead and the last gap has to be filled with a straight piece, right where it shows most.
| Face measured | Radius | 90° arc | ÷ 0.5 m chord | Blocks, rounded up |
|---|---|---|---|---|
| Inner | 5.925 m | 9.31 m | 18.6 | 19 |
| Centreline | 6.000 m | 9.42 m | 18.9 | 19 |
| Outer | 6.075 m | 9.54 m | 19.1 | 20 |
Same radius, so why does the joint open a dozen millimetres at the back?
Because each block is a trapezoid: the joint faces point at the centre, so the two long sides are not parallel. Included angle = chord ÷ radius in radians; back gap = that angle × kerb width. At R 6 m, chord 0.5 m, width 150 mm: 0.5 ÷ 6 ≈ 0.0833 rad, about 4.8°, and 0.0833 × 150 ≈ 12.5 mm. Front edge here means the longer outer edge, back edge the shorter edge facing the centre; which one faces the carriageway flips with the side the centre falls on, so the cutting list should say outer/inner, not front/back. The 12.5 mm is both the difference between the long and short sides of one block and the gap that opens at the back when straight lengths are laid round the curve, the same number, not two separate sums. Nineteen blocks add up to about 0.24 m (19 × 12.5 = 237.5 mm), which matches the difference between the outer and inner arcs (0.2356 m before rounding); the 2 mm left over is the 19 blocks running slightly past 90°. Either cut the taper, or leave 3 to 5 mm in each joint for a flexible sealant.
What must be on the cutting list, or the pieces will not fit?
Six items: inner and outer radius; the centre setting-out, with datum and offset direction; block count and chord per block; whether the joints point at the centre; front and back edge lengths given separately; and piece numbering with the direction of laying. The first two decide whether the curve lands in the right place, the middle two whether the joints stay even, the last two whether the site can match piece to position. The centre point is the item most often left off: write R = 6 m alone and countless centres will draw that arc, so one end lines up and the other is tens of millimetres out. Next comes a single edge length: the works cut to the outer edge, the inner edge arrives long and gets ground on site, and every joint ends up a different width.
| Item on the drawing | What happens without it |
|---|---|
| Inner and outer radius | Only the centreline radius is known, edges are 75 mm out each way, and the bend will not meet the straight run |
| Centre setting-out (datum, offset) | Arc length right, position wrong, the end is tens of millimetres off and gets cut on site |
| Block count and chord per block | Each side counts for itself and the last piece is either long or short |
| Joints pointing at the centre | Joints drift off radial, and after a dozen pieces the face steps visibly |
| Front and back edge lengths separately | Cut to one length, the inner edge runs long and is ground on site, joint width out of control |
| Numbering and laying direction | Nobody knows which end to start from, and wrong pieces have to come back off |
Do curved pieces waste more stone, and can I load them at 346 ㎡ a box?
Every block has a longer outer edge than inner, and the offcut from one cannot become the next, so yield is below straight kerb of the same section: a slab packs solid with straight lengths, while radius work loses area to taper and direction. Piece count climbs too, 19 or 20 pieces for one 90° bend at half a metre each, so bundling is counted per piece in timber and strapping, and the container carries more air than it would with long lengths. 346 ㎡ is a flat-stacked 30 mm slab figure and does not transfer to curved work; weigh the pieces and compare with the weight ceiling, which is set out in our price-per-metre guide. We cut 30 mm, take orders from 100 ㎡, ship in 5 days on 20 large saws, and turn out 500,000 ㎡ a year from a quarry and yard inside the Cenxi plant.