ลูกแก้วสะท้อนแสง AASHTO M247

One standard, six sizes — and a version trap

The current M247 grades beads purely by size, from fine to very coarse. Type 1 is the global workhorse; Type 3 and up are the “big beads” engineered for wet-night visibility:

The trap: the original M 247-81 had only two types, and its “Type 2” was a fine uniform bead — what the current standard calls Type 0. Old tenders in Southeast Asia and the Middle East still copy the 1981 wording, so “M247 Type 2” can mean a 180–600 μm fine bead or a 0.6–1.4 mm coarse bead depending on the vintage of the document. Whenever Type 2 appears in a tender, we exchange full gradation tables and lock the numbers into the contract before production.

Type 1 gradation — M 247-13, Table 1

The working range is 150–850 μm with the bulk at 300–600 μm — essentially the same size class as BS 6088 Class B (180–850 μm): they are the American and British write-ups of the same drop-on bead, and one grading line covers both. One reading note: M247 states cumulative passing per sieve, while BS 6088 states retained per band — the two formats give completely different numbers for the same product, so check which convention a datasheet uses before comparing.

Type 2 gradation — one step coarser

Type 2 sits between the standard bead and the big beads — mostly 0.85–1.18 mm. It buys extra dry-night brightness and a first step of wet-film performance on medium-build markings, or serves as the transition size in a double-drop system. Remember this is the current Type 2: if your tender dates from the M 247-81 era, its “Type 2” means the fine Type 0 bead instead — see the version trap above.

Type 3 gradation — the wet-night big bead

On a rainy night a film of water drowns standard beads and the line goes dark. Type 3 beads — mostly 1.0–1.4 mm — are big enough to stand proud of that water film and keep reflecting. The typical use is a double-drop: a first pass of Type 3 for wet-night performance, a second pass of Type 1 for dry initial brightness. Big beads need a thick, high-build marking to hold them — which is why US state specs pair Type 3+ with adherence coatings and thick thermoplastic or structured lines. As working figures from industry practice (not clauses of M247): hot-applied thermoplastic takes a drop-on rate around 400–500 g/m², with the bead embedded 50–60% of its diameter for best return.

Bead quality — roundness, glass, refractive index

Two practical notes. First, although the standard’s floor is 70% true spheres, US state qualified-product lists and most buyers work to ≥ 80% — specify the roundness you need and it goes on the COA. Second, many State DOTs add total heavy-metal limits on top of the TCLP clause — arsenic, lead and antimony each ≤ 200 ppm is the common ceiling (New Jersey holds arsenic to ≤ 100 ppm), tested by EPA 3052 digestion + 6010 ICP. If your project cites these, ask for the As / Pb / Sb totals with your COA.

Coatings, and how to order

M247 treats surface treatments as buyer options, alone or combined, and requires that a specified coating be positively detectable on the delivered beads:

A clean contract clause names the version, type and coatings:

“Glass beads shall conform to AASHTO M 247-13 (2022), Type 1, with moisture-resistant and adherence coating.”

For new 2026 contracts referencing the revision, write “AASHTO M 247-26, Type 1” — either way, name the year.

M247, BS 6088B, EN 1423 — which to cite

M247 Type 1 and BS 6088 Class B overlap so closely that the same grading line supplies both — the difference is paperwork, not product. What M247 alone offers is the built-in big-bead series: if your project needs wet-night performance, Type 3 is the entry point. Tell us which code and which type your tender cites and we quote against that text.