Fosroc Conbextra UW
Anti-washout, non-shrink cementitious grout engineered to place cleanly in stationary or moving water without diluting the cement phase.
What is it?
A pre-bagged, single-component cementitious grout with built-in anti-washout technology, designed specifically to be placed under water or in tidal zones where ordinary grouts would simply wash away.
Where is it used?
Marine and civil works below the waterline: quay pillars, concrete piling, bridge columns, stanchion baseplates and slipways.
Why use it?
It resists loss of the cement phase in flowing water, expands slightly to compensate for settlement, and still develops high strength, so a submerged pour stays intact and load-bearing.
How is it applied?
Mix a 25 kg bag with about 5.5 litres of clean water, then place by tremie or pump within 20 minutes so the gaseous expansion works while the grout is still plastic.
About Fosroc Conbextra UW
Conbextra UW is Fosroc’s answer to a problem every marine and bridge contractor knows well: conventional grouts fall apart when they meet water. Poured into a submerged annulus or a tidal void, an ordinary cementitious grout loses its finer cement fraction to the surrounding water, leaving behind a weak, segregated, chalky mess. Conbextra UW is a blend of Portland cements, graded fillers and chemical additives that is formulated to hold the cement phase together even when placed into stationary or slowly moving water, so what leaves the pump is very close to what ends up cured in place.
For engineers and applicators across Delhi NCR and Pan-India, the practical appeal is that this is still a single-component, prepackaged powder needing only clean water on site. There are no separate anti-washout admixtures to dose in the field and no guesswork about ratios. It arrives ready to mix, displaces water effectively as it flows, and cures to a dense, durable grout that conforms to BS EN 1504-3 Class R4, the top structural-repair classification. That combination of field simplicity and specification-grade performance is why it turns up on jetty, dam and pile-grouting jobs where dewatering a section fully is either impossible or uneconomical.
The technical insight
The performance comes from two mechanisms working together. First, an anti-washout (cohesion) system raises the grout’s internal cohesion and resistance to dilution, so as the material sinks through water it stays as a coherent mass rather than dispersing; this is what stops the cement fines from clouding off and bleeding away. Second, a gaseous expansion system generates a controlled volumetric expansion of up to 1% while the grout is still plastic. That expansion pushes the grout to fill the void completely, displaces trapped water and compensates for the natural plastic settlement and shrinkage that would otherwise leave gaps at the top of a pour or the crown of an annulus. The graded aggregate keeps flow high while suppressing segregation and bleed. The net result is a fully compacted, positively contacting grout, which is exactly what you need under a baseplate or around a pile where the transfer of load depends on the grout bearing tight against both surfaces. Cured at 20°C it exceeds 40 MPa at 7 days and 60 MPa at 28 days, with an elastic modulus around 30.7 GPa, and its freeze-thaw resistance and chloride-free formulation support long service life in aggressive marine exposure.
Where it is used
- Grouting the annulus around concrete and steel piling driven or cast below the waterline
- Bedding stanchion and column baseplates for jetties, wharves and marine structures
- Grouting bridge columns and pier foundations in flowing or standing water
- Quay pillar and dock-wall grouting where dewatering is impractical
- Filling voids and cavities in submerged dam and spillway sections
- Slipway construction and repair in the tidal splash zone
- Sealing and grouting sheet-pile interlocks below water level
- General underwater void filling where a conventional grout would wash out
Key advantages
- No significant wash-out of the cement phase, even when placed in moving water
- Gaseous expansion of up to 1% compensates for plastic settlement and fills the void completely
- Displaces water effectively so the grout makes positive contact with both faces
- High early and ultimate strength: exceeds 40 MPa at 7 days and 60 MPa at 28 days
- Chloride-free formulation suited to reinforced marine and coastal structures
- Exceptional freeze-thaw resistance for long-term durability in the splash zone
- Single-component prepackaged powder needing only clean water on site
- Conforms to BS EN 1504-3 Class R4, easing specification and approval
Limitations & considerations
- Designed for still or slowly moving water; fast-flowing currents can still disrupt any underwater pour and may need diversion or bagging
- The short 20-minute placing window means mixing, transport and tremie/pump set-up must be planned tightly before batching
- Requires accurate water dosing (about 5.5 litres per 25 kg bag); over-watering to ease flow undermines strength and anti-washout behaviour
- Not a substitute for structural concrete or a load-bearing section in its own right; it is a grout for filling and bedding, not for casting large volumes
- In warm Indian site conditions the working time shortens further, so batch sizes should be kept small enough to place promptly
Selection & best practice
Plan the pour before you open a bag. Confirm the void is reasonably clean and that water movement across the section is minimal; divert or bag off any live current. Mix each 25 kg bag with approximately 5.5 litres of clean water in a forced-action or paddle mixer until uniform and lump-free, and resist the temptation to add extra water for flow. Place strictly by tremie pipe or by pump with the outlet kept below the rising grout surface, so fresh grout emerges into already-placed material rather than falling through open water. Keep the flow continuous and finish placing within 20 minutes of mixing so the expansion works while the grout is still plastic. Where a large annulus or deep pour is involved, stage the work so successive batches are ready without interruption. For very warm ambient conditions, use cool mixing water and reduce batch sizes to protect the working time.
Specifications
| Product type | Anti-washout, non-shrink cementitious grout |
| Standards compliance | BS EN 1504-3 Class R4 (DoP UK9-32, 0370-CPR-0845) |
| Composition | Blend of Portland cements, graded fillers and chemical additives; chloride free |
| Compressive strength | >40 MPa at 7 days, >60 MPa at 28 days (cured at 20°C) |
| Elastic modulus | 30.7 GPa |
| Expansion | Volumetric expansion up to 1% in the plastic state (gaseous system) |
| Mixing water | Approx. 5.5 litres of clean water per 25 kg bag |
| Yield | Approx. 14.5 litres per 25 kg bag |
| Placing time | Place within 20 minutes of mixing |
| Pack size | 25 kg bag |
Industry use cases
Ports & Marine
Grouting pile annuli, quay pillars and jetty baseplates below the waterline, where the cement phase must survive tidal movement to develop full bearing.
Bridges & Highways
Bedding bridge column and pier foundations in river or flooded excavations without the cost and delay of full dewatering.
Dams & Hydraulic Works
Filling submerged voids and cavities in spillways and dam sections where access above water is impossible.
Industrial & Power Coastal Sites
Grouting intake structures, slipways and sheet-pile interlocks in the tidal splash zone of coastal plants.
Repair & Rehabilitation
Restoring load transfer under submerged baseplates and around piling on ageing marine and dock infrastructure.
Frequently asked questions
Can Conbextra UW really be placed straight into water?
What strength does it reach?
How much water do I add and how much grout do I get?
How long do I have to place it after mixing?
How does it differ from standard Conbextra grouts like GP2?
Is it suitable for reinforced marine structures?
How we position Fosroc Conbextra UW
Reach for Conbextra UW specifically when the pour is under water or in the tidal zone; that anti-washout capability is what you are paying for. For the same non-shrink, high-strength grouting in dry, in-air conditions, the standard Fosroc precision grout Conbextra GP2 is the right and more economical choice, and Conbextra GP1 covers thinner sections. Where a machine baseplate or cracked section calls for chemical resistance and very high early strength in the dry, the epoxy grout Conbextra EP10 is the natural step up within the same Fosroc family. To complete a marine repair scope, Space Arc also supplies Fosroc’s Renderoc range for concrete patch repair and Nitocote and Dekguard protective coatings for the exposed faces, alongside comparable systems from Sika and MC-Bauchemie where a project standard calls for them.
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