Concrete reads as permanent, and on the duties where its properties are required — a trafficable surface, a designed hydraulic profile, a founded-on-rock installation — it is the right answer and it will outlast everything around it. Rock bags vs concrete revetment sets out those duties.
These are the costs that sit outside the concrete line item.
1. The dry working area
The largest, and the one most often left as a provisional sum or a client risk.
Placing concrete in water generally requires a coffer dam, a diversion, or a substantial pumping arrangement. That package includes:
- Design of the temporary works
- Materials and installation
- Pumping and its running cost for the duration
- Dewatering discharge management, usually with its own authorisation and treatment
- Fish salvage and exclusion where required
- Removal and site reinstatement
- The schedule to build and remove it, inside the work window
What dewatering really adds covers this in full. On some in-water sites the temporary works rivals the permanent works, and a comparison that omits it is not a comparison.
2. Formwork and finishing
For cast-in-place work: formwork design, materials, erection, striking, and reuse limits. For a curved or profiled surface, formwork is a significant proportion of the cost.
Precast avoids formwork on site and substitutes manufacture, storage, transport of heavy finished units, and lifting plant.
3. Cure time
Concrete has a critical path that does not care about your programme.
On a short in-water window this is decisive rather than inconvenient: the sequence of installing a coffer dam, dewatering, placing, curing, and removing has to fit inside the window. On a tidal site with a few workable hours a day, it frequently does not. See in-water work windows.
4. Subgrade preparation
Concrete bears on what is beneath it, so the bed or slope has to be prepared: excavation, grading, compaction, and often a granular bedding layer.
On an irregular or already-scoured bed, preparing a regular bearing surface is itself a construction operation, and sometimes a larger one than the armoring.
5. Drainage and pressure relief
An impermeable face on a bank with groundwater behind it builds hydrostatic pressure. Relief is required: weep holes, drainage layers, filters behind the face.
Omitted or under-designed, this produces its own failure — pressure behind the wall, or seepage finding a path around it. It is a real cost and it is a design item, not a detail.
6. Joints
Rigid systems crack unless they are allowed to move. Joints have to be designed, formed, sealed, and eventually maintained.
Joints are also the route by which water gets behind a face, so their detailing matters disproportionately.
7. The undermining repair
The recurring one, and the mechanism that ends most rigid revetments.
The bed at the edge lowers, a void forms beneath, the slab spans it, and eventually cracks or drops. The whole sequence is invisible until the last step. See why scour protection fails.
The repair is not a patch. By the time it is visible, the void is large, the bed is well below design, and the work is a section of structure — usually requiring dewatering again.
The single change that most reduces this cost is armoring the toe at construction, which is a small addition to the original package and removes the dominant failure mode. See toe protection.
8. Demolition at end of life
Rarely in the comparison and genuinely different between systems.
Removing a concrete revetment is a demolition operation with material to dispose of. Removing a bagged installation is a recovery operation. On an asset with a decommissioning obligation, or where a shoreline may need to be restored, that difference is real.
9. The permitting profile
Concrete generally carries a heavier regulatory load on in-water work:
- Dewatering and diversion are separately regulated activities
- Discharge from dewatering usually needs authorisation
- Fish salvage may be required
- A larger, impermeable footprint reads worse against the minimisation test a reviewer applies. See what an agency reviewer wants to see
- In coastal programs, new hard impermeable structures face policy resistance. See state coastal zone approvals
Each of those is elapsed time as well as cost.
Where concrete wins on cost
To be fair to it:
- Founded on rock, where the undermining mechanism does not operate
- Above water, where no dewatering is needed
- Where the site is already dry for other reasons
- Where a trafficable or precisely shaped surface is required, in which case nothing else is competing
- Large, repetitive precast work with the manufacture amortised
- Very long design life on a stable foundation, where it may genuinely outlast alternatives