Five mechanisms attack a polyester mesh in service. They do not act at similar rates, and the order is not what people expect.
1. Ultraviolet exposure — fastest
Sunlight breaks bonds in the polymer directly, and it does so far faster than any chemical mechanism in this list.
Where it acts: only above water and only where light reaches the material. Submerged, buried, covered by fill or vegetation, or shielded by units above, the mechanism effectively stops.
What raises it: high sun, high altitude, low latitude, no cover, long exposure before covering.
What controls it: covering. The on-land service life position depends on units being installed and covered within one month, which is exactly why that condition is welded to the figure. See EN 12224 and weathering.
The practical failure: stock left in the open for years before use, and units placed above the waterline and never covered. Both are avoidable and both are common.
2. Abrasion — highly site-dependent
Mechanical wear from material moving against the mesh.
Sources: bedload moving past, ice, units working against each other and against rock, sediment in suspension, vessel contact at a berth.
Why it varies so much: a sand-bed channel with heavy transport is an aggressive abrasion environment. A cobble-bed reach with little transport is not. Two sites with identical hydraulics can differ by a lot on this alone.
What we can say: the mesh is evaluated for tensile, puncture, and tear under ASTM and ISO methods, and the raschel construction limits how far damage propagates once started. See raschel weave explained.
What we cannot say: an abrasion life. In particular, we hold no citable ice-abrasion data, and in an ice environment that is a real gap worth raising. See ice damage to bank protection.
3. Mechanical damage — episodic
Impact and cutting rather than gradual wear.
Sources: debris strike during floods, ice floes at breakup, vessel impact at a berth, dropped objects on subsea work, sharp fill loading from inside, and damage during handling and placement.
Rate: event-driven rather than continuous. A single flood carrying woody debris can do more in a day than a decade of ordinary service.
What limits the consequence: the weave. Damage that stays local is a unit with a hole rather than a unit that empties. See puncture, abrasion, ice, and debris.
4. Thermo-oxidative degradation — slow
Reaction with atmospheric oxygen, accelerated by heat, over long exposure. Screened by BS EN ISO 13438, and it sits alongside weathering in supporting the on-land service life.
Where it acts: primarily above water, and more where it is hot.
Rate: slow, and a design-life consideration rather than something you observe.
5. Hydrolysis — slowest
Chemical breakdown from prolonged water contact, screened by EN 12447.
This is the mechanism people ask about most, because the product lives in water and it sounds like the obvious threat. In practice, at ambient temperatures, it is the slowest of the five, which is why the submerged service life position is the longer of the two.
What raises it: elevated temperature and high pH. Warm water is more aggressive than cold, and strongly alkaline conditions attack polyester specifically.
What the ranking tells a specifier
The exposed-above-water case is the constrained one. If your design leaves units uncovered in sunlight indefinitely, the governing question is UV, and the published on-land figure does not describe that condition.
Ask about abrasion, not about water. On a sand-bed channel with heavy transport, or an ice-affected site, abrasion is the mechanism to interrogate. It is also the one where our data is thinnest.
Raise unusual chemistry. High pH or industrial discharge is worth naming, because it moves the slowest mechanism up the list.
Do not over-worry about submergence. The submerged case is the well-supported one.
The bigger point about service life
All five of these are about the material. An installation frequently stops protecting for reasons that have nothing to do with the mesh:
- Fines winnowed out from beneath the armor, which then settles into the void
- The toe undermined as the bed lowers
- Flow getting in at an edge or over the crest
- The channel continuing to degrade beneath the whole installation
None of those is a material failure and none is prevented by a more durable mesh. They are covered in why scour protection fails, and on most sites they are more likely to end an installation than degradation is.