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D15 or D20? Choosing Tie Rod Diameter Without Oversizing
Load capacity, wall thickness constraints, panel compatibility and reuse cost — the four factors that decide between the two standard sizes.
Read the comparisonComparison Guides
Direct answers to the either/or questions that decide a formwork order.
Most asked comparisons
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Five-Section Water Stop Tie Rod vs One-Piece Tie
Both keep a bar permanently inside a water-retaining wall. They differ in what you throw away every pour, how the seal is made, and what a repair costs later.
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Reusable Through-Tie vs Lost-End Tie: Real Cost
A through-tie is recovered and re-used; a lost-end tie stays in the wall. Break-even is set by reuse cycles, hole-filling labour and what the spec allows.
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Cast Iron Water Stop Nut vs Welded Water Stop Plate
A welded plate is made on the bar; a cast iron water stop nut is fitted on site. Same job, different tolerance for weld quality, wall thickness and site error.
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Choosing Tie Rod Grade: 4.8 vs 8.8 vs 10.9 in Practice
What property classes actually guarantee, how allowable tension is derived, and when a higher grade is wasted money or an active risk.
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Single-Sided Embedded Bolt vs Through-Tie
When you can only reach one face of the wall, the tie cannot pass through. What changes is not just the hardware but where the load goes.
Read the comparisonSizing and strength
Which diameter, which property class, and what the choice actually changes on site.
- D15 or D20? Choosing Tie Rod Diameter Without OversizingLoad capacity, wall thickness constraints, panel compatibility and reuse cost — the four factors that decide between the two standard sizes.
- Choosing Tie Rod Grade: 4.8 vs 8.8 vs 10.9 in PracticeWhat property classes actually guarantee, how allowable tension is derived, and when a higher grade is wasted money or an active risk.
- Concrete Lateral Pressure and How It Sets Tie Rod SpacingFresh concrete pressure as a function of pour rate, temperature and height, and the sequence that turns that pressure into tie spacing.
Waterproofing and water-retaining walls
How a penetration is made watertight, and what each detail costs in consumables and labour.
- Five-Section Water Stop Tie Rod vs One-Piece TieBoth keep a bar permanently inside a water-retaining wall. They differ in what you throw away every pour, how the seal is made, and what a repair costs later.
- Cast Iron Water Stop Nut vs Welded Water Stop PlateA welded plate is made on the bar; a cast iron water stop nut is fitted on site. Same job, different tolerance for weld quality, wall thickness and site error.
- How a Water Stop Tie Rod Actually Seals a WallThe water path through a tie hole, why a welded plate interrupts it, and what must be true for the detail to pass a leak test.
System choice and cost
Recoverable or embedded, two faces or one — the decisions that drive cost per pour.
- Reusable Through-Tie vs Lost-End Tie: Real CostA through-tie is recovered and re-used; a lost-end tie stays in the wall. Break-even is set by reuse cycles, hole-filling labour and what the spec allows.
- Single-Sided Embedded Bolt vs Through-TieWhen you can only reach one face of the wall, the tie cannot pass through. What changes is not just the hardware but where the load goes.
- Where the Money Goes in Formwork HardwareThe cost structure of a fastener order, which levers actually move price, and why a small hardware change can cost more than the hardware.
Material and surface
Coating, stainless grade and thread manufacturing, compared by environment and failure mode.
- Coating Choice: Black Oxide, Zinc, Hot-Dip and DacrometCoating thickness, salt-spray life, thread fit and hydrogen embrittlement risk compared, with selection rules per environment.
- Stainless Steel Fasteners: 304 vs 316 vs DuplexWhich grade survives which environment, why stainless still corrodes, and the galling problem that catches out first-time buyers.
- Rolled vs Cut Threads: Why It Matters for FatigueGrain flow, fatigue strength, surface finish and cost — and how to tell a rolled thread from a cut one without a metallurgist.
- Wing Nuts, Disc Nuts and Washers: Selection and TighteningHow nut geometry affects grip, why the washer matters more than the nut, and how tight a formwork tie should actually be.
Specification comparison tables
The parameters behind the choices, side by side. Every figure here appears elsewhere on this site — in the product pages and in the articles linked above — so the table can be checked against them rather than taken on trust.
Tie rod diameter: D15, D20 or larger
One D20 replaces roughly two D15 ties in capacity, so the diameter changes how many tie positions you need — not just the size of each one. Compare total cost per square metre of wall formed, never price per rod.
| Parameter | D15 | D20 (Rd20) | D25 / D26.5 |
|---|---|---|---|
| Rated tensile capacity | 150 kN | 230 kN | 330-430 kN |
| Nominal stress area | approx. 176 mm² | approx. 314 mm² | approx. 1.8x the D20 section |
| Unit weight of the bar | 1.5 kg/m | 2.6 kg/m | 4.1 kg/m (D25) · 4.8 kg/m (D26.5) |
| Practical wall and pour range | Walls up to about 3.5-4 m at normal pour rates | Thick walls, fast pours, cold concrete | Heavy slab, bridge and mass concrete formwork |
| Tie spacing that works on site | 0.6-0.8 m | Wider spacing, fewer tie positions | Set by the formwork drawing, not by habit |
| Where it usually loses | Panel hardware and cones are diameter-specific, so switching mid-project means changing the fleet | Needs a larger hole and a bigger cone envelope in a thin wall | Heavier to handle; above about 4 m it is a two-person lift |
Capacity is never the diameter alone: it comes from the thread form, the stress area and the material class together. A D15 in a higher class can outperform a D20 in a lower one, so the two are always specified as a pair.
Property class: 4.8, 8.8 or 10.9
Grade numbers are not a quality ranking. The first digit is one tenth of the minimum tensile strength in MPa, and the two digits together give one tenth of the minimum yield strength. Choose the lowest class that satisfies the load with margin.
| Parameter | Class 4.8 | Class 8.8 | Class 10.9 |
|---|---|---|---|
| Minimum tensile strength | 400 MPa | 800 MPa | 1000 MPa |
| Minimum yield strength | 320 MPa | 640 MPa | 900 MPa |
| Typical material and treatment | Plain carbon steel, as-rolled or normalised | Medium-carbon or alloy steel, quenched and tempered | Alloy steel, quenched and tempered, tighter process control |
| Behaviour under electroplating | Low risk | Risk rises with strength | High risk: hydrogen embrittlement without a documented baking cycle |
| Notch and bend tolerance | Most tolerant of site handling | Good for repeated reuse | Most sensitive; a damaged thread loses more of its section |
| Where it fits | Light and medium formwork loads | The default for reusable formwork tie rods | Where the load genuinely needs it, and the nut and cone match the same class |
If class 8.8 covers the load, ordering 10.9 buys risk rather than safety. A 10.9 rod with a 4.8 nut only relocates the failure. Class 10.9 in an electroplated finish must come with the baking record, or the finish should be changed.
Surface finish by site exposure
Finishes are chosen on three axes: how long the part must last, whether the thread must still assemble, and whether the steel class is sensitive to hydrogen. Optimising one at the expense of the others is the usual cause of coating disputes.
| Parameter | Black oxide | Electro-galvanizing | Hot-dip galvanizing (HDG) | Zinc-lamellar (Dacromet type) |
|---|---|---|---|---|
| Coating thickness | 1-2 µm | 5-25 µm | 45-85 µm | 8-12 µm |
| Salt-spray life | Measured in hours | Roughly 100-300 hours | Above 1000 hours in rural and urban atmospheres | Around 500-1000 hours |
| Effect on thread fit | None | Predictable, stays thread-friendly | Builds on threads: needs tapped-oversize nuts or re-tapping | None, does not block threads |
| Hydrogen embrittlement risk | Low | Present on acid-cleaned high-strength parts | Low | None: the process is not electrolytic |
| Best match | Indoor and dry storage, zero dimensional change | Sheltered outdoor use and parts that must assemble freely | Permanent embedment, coastal and high-humidity sites | Class 10.9 and above where HDG is impractical |
Write the finish as a standard plus a measured range: "galvanized to ISO 1461, minimum local thickness 45 µm" is auditable, while "galvanized" is not. One coating system per joint — a galvanized rod with a bare steel nut corrodes at the nut and loosens.
Water-retaining walls: which tie system
A plain through-tie leaves a continuous hole from face to face, which is a leak path. For tanks, basements, tunnels and reservoirs the tie has to carry a welded water-stop plate, or the penetration has to be sealed by a system specified for that exposure.
| Parameter | Through-tie (reusable) | Five-section water stop rod | One-piece embedded rod | Single-sided embedded bolt |
|---|---|---|---|---|
| What stays in the wall | Nothing: the whole rod is recovered | Only the short middle bar is consumed | The embedded section stays permanently | The bolt and plate stay in the first pour |
| Sealing mechanism | Not watertight on its own; the hole must be plugged | Welded plate across the bar breaks the capillary path | Welded plate across the bar breaks the capillary path | Embedment in the previous pour carries the load |
| Reuse | Highest: rod works hundreds of cycles | High: the two outer bars rotate through cycles | Low: consumed once | Medium: the outer tie rod is recovered |
| Cost behaviour | Cheapest per unit, highest labour to plug | Usually cheapest on repetitive pours | Cheapest on one-off structures with few tie positions | The practical answer when the far face is unreachable |
| Typical use | Walls that will not retain water, or are sealed separately | Basements, tanks, tunnels, reservoirs | Water-retaining walls where a middle bar cannot be recovered | Diaphragm and core walls against an excavation face |
Neither the five-section nor the one-piece design changes the sealing mechanism: both rely on a plate welded across the bar. What changes is how much steel is consumed and how much of the assembly can be reused.
FAQ
Comparison questions buyers ask
D15 suits walls up to about 3.5-4 m with normal pour rates and 0.6-0.8 m tie spacing. D20 earns its cost on thick walls, fast pours, cold concrete and single-sided work, where fewer and stronger ties reduce congestion. Compare the total cost per square metre of wall formed, not the price per rod, and always check the result against the formwork pressure calculation for the specific pour height and rate.
Not sure which one fits your pour? Send the wall thickness and pour rate and we will size it with you.
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