PBA stands for polyurethane bonded aggregate: mineral particles — usually rock armour — glued together with polyurethane into a continuous but porous revetment. In hydraulic engineering it is used as a facing on dykes, embankments and structures exposed to wave attack.
What makes it different
Unlike a revetment of grouted asphalt or concrete, PBA leaves the space between the stones largely open. The adhesive sits only at the contact points. That gives several properties:
- Wave energy is dissipated — water enters the open structure and loses energy there instead of running up a closed face. Wave run-up is therefore lower than on a closed revetment.
- Coherence — individual stones can no longer be plucked out; the revetment acts as one coherent slab and can therefore be lighter and thinner than loose rock of equivalent resistance.
- Permeability — water pressure from within the embankment can escape, so no uplift pressure builds beneath the revetment.
- Ecology — the open voids provide attachment and shelter for algae, shellfish and small aquatic life, which a closed revetment does not.
Applications
- Dyke and bank revetments in the upper part of the slope, where wave loading is greatest
- Transition and connection details, where a closed and an open revetment meet
- Repair of existing rock revetments, fixing a loosened armour layer without replacing the whole slope
- Structures and harbour works around culverts, outfalls and berths
Points to watch
- Working conditions — polyurethane cures depending on moisture and temperature. Weather and the tidal window largely dictate when work can be done.
- Clean stone — the adhesive only bonds to a clean, sufficiently dry stone surface. Silt and fines must be washed out.
- Service life and management — above the waterline the bonding is sensitive to ultraviolet light; inspections should look for embrittled adhesive bridges and displaced stones.
- Design is bespoke — grading, layer thickness and adhesive quantity follow from the governing wave loading. This is a design calculation for a coastal or hydraulic engineer, not a standard detail.
Related terms
- Rock armour
- Revetment
- Slope
- Grouted revetment
- Wave run-up
- Hydraulic engineering
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