Fosroc Renderoc UW
Anti-washout, non-shrink micro-concrete that repairs submerged and tidal-zone concrete without dewatering or cement wash-out.
What is it?
A single-pack, cement-based micro-concrete that flows and sets under water without its cement paste washing away, purpose-built for repairing submerged and tidal-zone structures.
Where is it used?
Quay walls and piling in ports and jetties, bridge piers, dam and barrage faces, sluice and valve chambers, and any concrete that sits in stationary or moving water.
Why use it?
It lets a diver or a tremie crew rebuild deteriorated concrete in place, so you avoid the cost and downtime of cofferdams and dewatering while still reaching structural strength.
How is it applied?
Mix each 25 kg bag with 5.5 litres of water, then pour or pump it through a 50 mm tube to the lowest point of leak-proof formwork so it displaces water as it rises.
About Fosroc Renderoc UW
Renderoc UW is Fosroc’s answer to a problem that stops most repair mortars cold: how do you rebuild concrete that is permanently under water? Ordinary cementitious repair materials disperse the moment they hit the water column, their cement phase clouding away before anything sets. Renderoc UW is engineered around anti-washout admixtures that thicken and bind the fresh mix so it stays cohesive as it is placed, displacing water rather than mixing with it. Supplied as a factory-controlled dry powder, it needs only clean water on site to become a free-flowing micro-concrete that can be poured or pumped into submerged voids.
For engineers working on India’s ports, inland waterways, irrigation headworks and coastal infrastructure, this changes the economics of a repair. A jetty pile, a bridge pier below the waterline, or the face of a barrage can be reinstated by divers or by tremie placement, without the expense of building a cofferdam and pumping the site dry. The material carries a gaseous expansion system that compensates for plastic shrinkage and settlement, so the cured repair stays in full contact with the parent concrete and the surrounding void. It is chloride-free and iron-free, which matters when the repair sits permanently against embedded reinforcement in an aggressive, often saline environment.
The technical insight
The whole product hinges on rheology. Conventional grout placed under water behaves like sugar in a glass, its fines leaching out and leaving a weak, porous skeleton. Renderoc UW’s admixture package raises the cohesion and viscosity of the cement paste so the mix holds together as a single mass, resisting the erosive drag of stationary or moving water and displacing it cleanly as the pour rises from the lowest point upward. Working in parallel, a gaseous expansion system produces a controlled 1 to 4 percent volume increase while the material is still plastic, offsetting the shrinkage and settlement that would otherwise pull a curing repair away from the substrate and leave voids. Because the reaction is chloride-free, high early strength develops without introducing the very ions that drive reinforcement corrosion. The submerged environment also acts as a heat sink, drawing off the heat of hydration and letting the material be placed in thicker lifts under water than would be safe in air, which is why the permissible pour thickness roughly doubles once you go below the waterline.
Where it is used
- Reinstating deteriorated quay pillars, dolphins and berthing structures in ports and harbours
- Repairing concrete piling and pile caps below the waterline in marine and jetty works
- Restoring bridge piers and columns in the submerged and tidal zones of river crossings
- Rebuilding the concrete faces of dams, weirs and barrages without dewatering
- Patching sluice gate frames, valve chambers and other water-retaining structure elements
- Repairs to canal linings, intake structures and irrigation headworks in flowing water
- Underwater void filling and reprofiling around foundations and abutments
- Section restoration in reservoirs and water treatment structures while kept in service
Key advantages
- Resists cement wash-out, so it can be placed directly in stationary or moving water without a cofferdam
- Gaseous expansion of 1 to 4 percent compensates for plastic shrinkage and keeps the repair in full contact with the substrate
- High early strength (15 N/mm2 at 1 day) allows rapid installation and early return of the structure to service
- Reaches 53 N/mm2 at 28 days for a genuinely structural, load-bearing repair
- Develops high bond strength with no separate primer required on a sound, clean substrate
- Chloride-free and iron-free composition protects embedded reinforcement in aggressive water
- Factory prepacked and single-pack — only measured water is added on site, for consistent quality
- Placeable by pour, pump or tremie, giving flexibility for diver and confined-access work
Limitations & considerations
- It is a repair micro-concrete, not a general-purpose grout — for wide gaps and annulus filling under water a flowable grout such as Conbextra UW is the correct choice
- Because it is water-resisting, tools and pumps are harder to clean; hot water is needed for cleaning (but never for mixing)
- The water-powder ratio of 0.22 is fixed — over-watering to improve flow will weaken the repair, so gauging must be accurate
- Placement must begin within about 20 minutes of mixing to gain the full benefit of the expansion process
- Formwork must be genuinely leak-proof, as the material’s fluid nature will find any gap
- For potable-water contact, suitability must be confirmed with Fosroc before specification
Selection & best practice
Success under water is mostly about preparation and placement discipline. Clean the substrate back to sound concrete — permanently immersed surfaces should be sand-blasted or high-pressure water-jetted, and loose material fully removed. Build leak-proof shuttering, sealing edges with foam rubber strips, because Renderoc UW is fluid enough to escape any gap. Measure exactly 5.5 litres of water per 25 kg bag, add the powder slowly into the running mechanical mixer, and mix for at least five minutes to a smooth consistency; fluidity increases with mixing, so resist the urge to add water. Pass the mix through a 5 mm sieve, then pour or pump it through a flexible tube of at least 50 mm diameter to the lowest point of the form, keeping the tube tip below the rising material to avoid water entrapment. Place within 20 minutes. In a single pour, keep lifts to 80 mm above water and up to 150 mm under water; for thicker sections, extend the mix with clean, rounded, well-graded 2 to 10 mm aggregate at no more than a 1:1 ratio by weight. Submerged and tidal repairs cure themselves; only surfaces cast above water need conventional curing.
Specifications
| Material type | Anti-washout, non-shrink cementitious micro-concrete (underwater) |
| Compressive strength (cast under water, 70.6 mm cube, 30°C) | 15 N/mm2 @ 1 day, 30 @ 3 days, 44 @ 7 days, 53 @ 28 days |
| Flexural strength | 6.5 N/mm2 @ 7 days; 7.5 N/mm2 @ 28 days |
| Tensile / pull-out bond | Tensile >3 N/mm2 (submerged); pull-out bond 6 N/mm2 |
| Water : powder ratio | 0.22 — 5.5 litres of water per 25 kg bag |
| Plastic expansion | Controlled 1 to 4% gaseous expansion while plastic |
| Density of hardened material | 2100 kg/m3 |
| Application thickness (single pour) | Up to 80 mm above water; up to 150 mm under water |
| Pack size & yield | 25 kg bag; yield approx. 14.5 litres. Shelf life 6 months |
Industry use cases
Ports & Marine
Reinstating spalled quay pillars, berthing dolphins and submerged piling in tidal and fully immersed zones where dewatering is impossible.
Bridges & Highways
Structural repair of pier and column concrete in the splash and submerged zones of river and estuary crossings, restoring cover and section.
Dams & Irrigation
Rebuilding eroded concrete on dam faces, weirs, barrages and canal intake structures while water continues to flow past the repair.
Water & Wastewater
In-service repair of sluice frames, valve chambers and reservoir walls in water-retaining structures without taking the asset fully offline.
Frequently asked questions
Does Renderoc UW really set under water without washing away?
How strong is the finished repair?
How much water do I add, and can I add more for easier flow?
What thickness can I place in one pour?
Does it need curing and a primer?
Is it safe against reinforcement and in water-retaining structures?
How we position Fosroc Renderoc UW
Renderoc UW is the underwater specialist within Fosroc’s Renderoc concrete-repair family. Where the repair is above the waterline, its siblings take over: Renderoc SP40 for single-pack flowable micro-concrete in formwork, and the hand-applied Renderoc GP or HB range for patch repairs in air. When the job is filling a wide gap, an annulus or a base-plate void under water rather than reprofiling a concrete section, the correct partner is Conbextra UW, Fosroc’s anti-washout underwater grout. Pair any of these with Fosroc’s Nitobond bonding aids and Dekguard or Nitocote protective coatings for a full repair-and-protect system. Space Arc also supplies equivalent underwater and concrete-repair solutions from Sika, Dr. Fixit and MC-Bauchemie where a project’s approved-vendor list calls for a different brand — our technical team can cross-match specifications on request.
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