If you write or review tenders for manhole covers, catch basin frames, or grates, AASHTO M306 is the specification that turns "traffic rated" from a marketing phrase into a testable requirement. Here is what it actually asks of a casting.
The three things M306 requires
- Scope — frames, grates, rings, and covers for inlets, manholes, and similar structures in traffic service.
- Material — gray iron per AASHTO M105 / ASTM A48 Class 35B as the baseline.
- Proof load — 40,000 lb for H-20 service or 50,000 lb for H-25, applied through a 9" bearing area: a 2.5× factor over the 16,000 / 20,000 lb design wheel loads. No cracks, no permanent deformation beyond the limits, and the cover must still seat in its frame after the test.
H-20 vs H-25 in practice
H-20 covers the standard highway wheel load most municipal streets are designed around; H-25 adds margin for heavier axle loads at ports, industrial yards, and truck routes. Your municipal standard usually names one — carrying it into the casting line items keeps the whole assembly consistent.
H-20 vs H-25 at a glance
| H-20 | H-25 | |
|---|---|---|
| Design wheel load | 16,000 lb | 20,000 lb |
| M306 proof load (2.5×) | 40,000 lb | 50,000 lb |
| Bearing area | 9" × 9" block | 9" × 9" block |
| Typical service | Municipal streets, highways | Ports, industrial yards, truck routes |
How the proof-load test works
The casting is seated in its frame exactly as it would sit in the road. The proof load is applied through a 9-inch by 9-inch bearing block centred on the cover (or on the grate's weakest orientation) and held. To pass, the casting must show no cracks, no permanent deformation beyond the specified limit, and the cover must still seat properly in its frame afterward. It is a pass/fail structural test of the finished casting — not a material certificate.
Related standards you'll see in tenders
| Standard | What it covers |
|---|---|
| AASHTO M306 | Proof-load requirement for drainage, sewer & utility castings |
| AASHTO M105 / ASTM A48 | Gray iron castings — Class 35B is M306's baseline material |
| ASTM A536 | Ductile iron castings, where the municipal standard calls for ductile |
| EN 124 | European load classes (A15–F900); D400 is the rough analogue of heavy traffic service |
| OPSD / OPSS (Ontario) | Provincial standard drawings & specs that name the exact casting patterns Ontario tenders use |
Sample tender wording
"Castings shall conform to AASHTO M306 with a proof load of 40,000 lb (H-20). Gray iron shall meet AASHTO M105 / ASTM A48 Class 35B. Manufacturer's proof-load test documentation shall be submitted with shop drawings."
Swap in 50,000 lb / H-25 where the road authority requires it. Certification is confirmed per SKU — ask for documentation with your quote.
Frequently asked questions
What is AASHTO M306?
AASHTO M306 is the standard specification for drainage, sewer, utility, and related castings — the frames, grates, rings, and covers used on manholes and inlets. It sets the material requirement and a proof-load test the casting must pass.
What proof load does M306 require?
For H-20 traffic service, a 40,000 lb proof load; for H-25, 50,000 lb — each applied through a 9-inch by 9-inch bearing block. That is a 2.5x safety factor over the 16,000 lb and 20,000 lb design wheel loads. The casting must show no cracks and must still seat properly afterward.
What material does M306 call for?
Gray iron conforming to AASHTO M105 / ASTM A48 Class 35B is the baseline material requirement for castings under M306.
How do I specify M306 in a tender?
Cite "AASHTO M306" together with the traffic rating (H-20 or H-25) for each casting item, and require test documentation. Certification is confirmed per SKU — ask for it with your quote.
Further reading: Municipal Castings Association on AASHTO standards · NPCA on load-bearing definitions. Certification status is confirmed per SKU at quote time.