A Glass Bead Specification Checklist for Road Marking Projects

The most expensive sentence in a glass bead purchase order is “glass beads, 25 kg bags”. It looks like a specification. It is not. Between that line and a marking that passes acceptance sit about a dozen parameters, and most disputes in this trade can be traced to one of them being left implicit.

Here is the checklist we would want to see filled in before a batch is booked. It is organised in three groups: what goes in the bag, how much gets used, and how it gets applied.

Part A — What is in the bag

1. Bead type and application

Drop-on or premix. These are graded differently and cannot substitute for each other.

Under GB/T 24722-2020: Type 1 is drop-on for thermoplastic, two-component and waterborne coatings; Type 2 is premix for thermoplastic and two-component coatings; Type 3 is drop-on for solvent-based coatings; Type 4 is wet-night beads used in combination with non-wet-night beads as drop-on. Under US practice, TxDOT’s Type I beads are the smallest and are used only as intermix beads for thermoplastic, Type II is the standard drop-on grade, and Type III is a coarser drop-on bead marketed for wet-night visibility and coated to aid embedment.

Specify: type, and the coating system it will go into.

2. Refractive index class

Not a single number — a class, because it drives both performance and cost.

Class	RI range	Typical use
Low	1.50 ≤ RI < 1.70	General markings; the mainstream product
Medium	1.70 ≤ RI < 1.90	Coated beads; better wet performance
High	RI ≥ 1.90	Wet-night and all-weather markings; optimum around 1.93

Chinese practice for pavement markings generally treats RI ≥ 1.70 as sufficient, with performance improving as RI approaches 1.93. Note the cost-durability trade-off documented in US practice: 1.50 beads are made from recycled glass and are the most widely used, while 1.65 and 1.90 grades are made from virgin glass, cost more and may be slightly less durable — which is why some agencies blend high- and low-index beads.

Specify: the class, and where relevant the test method (in China, refractive index is measured by the 20°C immersion method).

3. Sieve distribution

The single most under-specified parameter in our experience. Do not write “20/40” without saying which table it comes from.

GB/T 24722-2020 prescribes distributions for Type 1 and Type 2 beads. For Type 1 (drop-on): no material above 850 µm; 15%–30% in 850–600 µm; 30%–75% in 600–300 µm; 10%–40% in 300–106 µm; 0–5% below 106 µm. For Type 2 (premix): nothing above 600 µm; 50%–90% in 600–300 µm; 5%–50% in 300–150 µm; 0–5% below 150 µm.

Note that these differ meaningfully from the sieve tables in AASHTO M247, EN 1423 and other national standards. A Chinese study of international specifications identified three common grading bands used across foreign standards — approximately 0.3–0.6 mm, 0.425–0.85 mm and 1.0–1.4 mm — which is a useful cross-check when a quotation gives only a mesh number.

Specify: the standard and table the distribution must meet, not just a mesh range.

4. Roundness

Roundness is the property that makes a bead a lens rather than a pebble. Only a near-spherical bead returns light along the incoming path; a poorly formed bead scatters it inside itself and sends the ray somewhere other than the driver’s eye.

Under GB/T 24722-2020, low-RI Type 1 and Type 2 beads must achieve roundness ≥80%, with Type 1 additionally requiring ≥70% in the 850–600 µm band. For medium- and high-RI beads the standard uses a defect measure: ≤20% defective beads.

Specify: the roundness figure or defect percentage, plus the test method.

5. Density

GB/T 24722-2020 requires density between 2.4 and 4.6 g/cm³. Low-index beads sit at the lower end, around 2.4–2.6 g/cm³. Density is a quick sanity check on RI class: if a supplier claims a high-index bead at 2.5 g/cm³, something does not line up.

6. Water resistance and coating

Water resistance is a proxy for chemical durability. For Type 1 and Type 2 beads, the standard requires that the 0.01 mol/L hydrochloric acid solution used to neutralise a boiled sample be no more than 10 mL; for Type 3, no more than 15 mL.

Separately, where a moisture-proof coating is specified — commonly applied to beads intended for wet conditions, and standard on TxDOT’s Type III beads to aid embedment — the coated beads must pass through a funnel without stalling.

Specify: water resistance limit, and whether a coating is required.

7. Magnetic particle content

Limited to ≤0.1% under GB/T 24722-2020. Metallic contamination partly reflects the raw material stream and partly the processing line; it matters both for optical performance and for equipment wear.

8. Heavy metals

New in the 2020 edition and increasingly requested by buyers: lead ≤200 mg/kg, arsenic ≤200 mg/kg, antimony ≤200 mg/kg. If your market has any tightening material restrictions, get these on the certificate now rather than requesting them later.

Part B — How much gets used

9. Premix content

GB/T 16311-2024 requires thermoplastic reflective markings to contain at least 30% premix glass beads by mass. Real projects vary: a municipal procurement specification set the internal bead minimum at 20%, a Ganzhou highway scheme required bead usage of at least 30% of the coating, and a high-performance long-life expressway design specified premix content of 48%, split between 212–850 µm at 30% and 850–1700 µm at 18% with higher roundness requirements on the coarse fraction.

Commonly cited practical ranges for thermoplastic are 20%–30% by weight overall, with premix types blended in at 18%–25%.

Specify: the required premix percentage, and the marking grade that drives it.

10. Drop-on dosage

Government technical schemes and procurement specifications commonly require a total drop-on dosage of not less than 0.4 kg/m² using a double-drop method — coarse first, then fine. Academic work cites an optimum of 0.37–0.45 kg/m²; general practice sits at 0.3–0.4 kg/m²; one manufacturer recommends 280–320 g/m² for flat-line markings. High-performance long-life designs may go to 600 g/m².

Both tails are defects. Too few beads and incoming light cannot form the concentrated return beam; too many and the beads stack, embed poorly and re-refract light among themselves, slashing effective reflection while also trapping dust.

Specify: a dosage band with an upper limit, not just a minimum.

Part C — How it gets applied

These are not bead properties, but they determine whether good beads produce a good line. If your supplier offers technical support, these are the variables worth asking them about.

11. Embedment depth

The target is 50%–60% of bead diameter embedded, with the remainder exposed. A government technical scheme states it as a mandatory requirement at 40%–60%, with a trial application required before full production if the range is not met. Expressway design documents specify 55%–60%. US practice cites 60% as optimal.

The failure modes are asymmetric: under-embedded beads both reflect poorly and wear away quickly, while over-embedded beads reflect weakly but last. TxDOT’s guidance is that if an error must be made, too deep beats too shallow.

Application controls

Control	Target	Source basis
Drop timing	Within 10–30 seconds of coating application, while semi-molten; some guidance says simultaneous with no lag	Government technical scheme; field guidance
Coating temperature	Approx. 180–220°C; ~200–205°C spring/autumn, ~200°C summer, 210–215°C winter; expressway design 200°C ± 10°C	Field guidance; design documents
Coating thickness	2.0–2.5 mm expressway; 2.0 mm general; thinner films need finer beads	Design documents; government scheme
Ambient temperature	Not below 10°C	Government scheme; procurement spec
Surface condition	Dry, clean, free of debris and dust	Government scheme
Weather hold	Suspend in strong wind, fog, high humidity, heavy dust	Field guidance; procurement spec
Curing before traffic	≥6 hours normal, ≥8 hours cold; no forced cooling	Field guidance
Traffic speed during thin-film application	Keep below approx. 10 mph (~16 km/h) so beads do not roll and get coated	TxDOT

Two verification steps worth building into the job

Run a trial section. Both the government scheme and field guidance recommend calibrating drop timing and temperature on a trial strip before full production. It is the cheapest possible way to find out whether your bead grade embeds properly in this coating, on this road, at this temperature.

Measure at the right time. Under GB/T 16311-2024, the initial retroreflective luminance coefficient is a measurement taken between 48 hours and 30 days after striping. Measuring on day one, or after three months, produces numbers that mean something else. And clean the surface before measuring — contaminant films drop the reading artificially; wiping with alcohol rather than water is the recommended approach.

If you want the checklist applied to your job

Send us the marking type, the coating system, the target retroreflectivity class, the coating thickness and the destination country. We will return a bead specification covering all eleven points above, mapped to whichever standard your project references — and a sample bag if you would rather run a trial section before committing to a container.