Choosing Tie Rod Grade: 4.8 vs 8.8 vs 10.9 in Practice
> In brief > - The grade number is a yield strength, not a quality ranking. > - Extra strength is only worth paying for where the load demands it. > - Availability is confirmed against stock before a date is quoted.
Grade numbers are not a quality ranking. They describe minimum tensile strength and yield ratio, and the right grade follows from the load and the failure mode you want — not from a preference for "stronger".
Our own production and test data are available on request — see how our factory works and contact the engineering team for the documents behind any figure quoted here.
Tensile strength is the maximum stress that a material can withstand while being stretched or pulled before failing. Yield strength refers to the stress at which a material begins to deform plastically. Stress area means the effective cross-sectional area of the threaded portion of a fastener, used in load calculations. Fatigue allowance is a reduction in allowable load to account for repeated use and potential material degradation. Hydrogen embrittlement is the process by which certain metals become brittle and fracture due to the absorption of hydrogen.
What the numbers mean.
The first digit is one tenth of minimum tensile strength in MPa; multiplying both digits gives one tenth of minimum yield strength. Class 4.8: 400 MPa tensile, 320 MPa yield. Class 8.8: 800 MPa tensile, 640 MPa yield. Class 10.9: 1000 MPa tensile, 900 MPa yield, and it is quenched and tempered rather than plain carbon steel.
Class 8.8 and above are medium-carbon or alloy steels with controlled heat treatment. Class 4.8 is plain carbon steel, normally supplied as-rolled or normalized.
Standard basis: ISO 898-1 — Mechanical properties of fasteners made of carbon steel
From grade to allowable load.
Allowable tension is not the yield strength of the nominal diameter. It is calculated from the stress area (the effective section through the thread), multiplied by yield, then divided by a safety factor and, for repeated use, reduced by a fatigue allowance.
For a DW15 tie rod the stress area is roughly 176 mm². At class 8.8 that is about 113 kN at yield, and formwork practice applies a working load well below it — typically landing in the 90-150 kN band depending on the standard used. An Rd20 rod reaches roughly 230-260 kN on the same basis. The exact figure must come from the supplier as a certified rated capacity, not from a formula copied off a website.
When a higher grade is the wrong answer.
Higher grades are more notch-sensitive and more susceptible to hydrogen embrittlement after electroplating. Class 10.9 rods that are electro-galvanized without a proper baking cycle can crack days after installation at loads far below rated capacity. If a 10.9 rod must be coated, specify hot-dip galvanizing or mechanical plating, and require the baking record if electroplating is used.
Higher grade also costs more and machines differently: 10.9 threads are harder to roll, and re-cutting or die-chasing them on site removes the surface layer that carries much of the fatigue performance.
Selection logic that holds up.
- Take the design load from the formwork drawing, including the concrete pressure case and any dynamic allowance.
- Divide by the number of ties per unit area and by the safety factor your standard requires — commonly 1.5-2.0 on rated capacity, higher for repeated use.
- Choose the lowest class that satisfies it with margin. If class 8.8 covers the load, ordering 10.9 buys risk, not safety.
- Match nut and cone to the same class. A 10.9 rod with a 4.8 nut simply relocates the failure.
- Require the mill certificate (EN 10204 3.1 for structural work) and a batch tensile report, and check that the heat number on the rod matches the certificate.
Standard basis: EN 13670
Reuse and derating.
Reusable rods accumulate damage: nicks at the thread entry, straightened bends, corrosion pits. Most formwork standards require inspection before each reuse cycle and derate or scrap rods with visible thread damage. A rod that has been bent and straightened has lost part of its fatigue life even if it looks serviceable.
| Grade | Minimum Yield Strength (MPa) | Consequence for Load Capacity |
|---|---|---|
| 4.8 | 320 MPa | Lower capacity, suitable for light loads |
| 8.8 | 640 MPa | Medium capacity, suitable for standard formwork |
| 10.9 | 900 MPa | High capacity, but more notch-sensitive and costly |
| 8.8 | 640 MPa | Balanced cost and performance, less risk of embrittlement |
| 10.9 | 900 MPa | Higher risk of hydrogen embrittlement if electroplated |
| 4.8 | 320 MPa | Cheapest, but not suitable for heavy or dynamic loads |
References and standards.
- EN 10204:2004 — Types of inspection documents
- ISO 898-1 — Mechanical properties of fasteners made of carbon steel
- ISO 3501 — Corrosion resistance of stainless steel fasteners
- NIST material measurement standards
- EN 14399-1 — High-strength structural bolting assemblies
- ISO 16047 — Hexagon head bolts and hex nuts
About us: This article discusses the properties and selection of tie rod grades 4.8, 8.8, and 10.9, and it is checked against our production and test data. Our factory capabilities include controlled heat treatment and CNC thread rolling. Our contact page provides access to the engineering team for detailed documents, and the about page documents our manufacturing processes. This article was reviewed by the Qianheng Formwork Engineering team.