Fosroc Conbextra Cable Grout
Single-component, non-shrink cementitious grout engineered for post-tensioned cable ducts — EN 447 and MORTH/IRC compliant, with negligible bleed and controlled plastic expansion.
What is it?
A pre-bagged, all-fines cementitious grout based on Portland cement and selected additives. Add measured clean water on site (w/p 0.33-0.35) and it yields a free-flowing, non-shrink grout built for pumping through cable ducts.
Where is it used?
Grouting of post-tensioned and prestressed cable ducts in bridges, flyovers, metro viaducts and PT building slabs; also suited to ground anchors with small annular gaps.
Why use it?
Tested to EN 445 protocols and compliant with EN 447 and MORTH/IRC limits for fluidity, bleed and strength — efflux under 25 seconds, bleed below 0.3%, expansion up to +2%, and over 30 MPa at 28 days.
How is it applied?
Mixed in a grout plant using a high-shear colloidal mixer or 1500 rpm paddle mixer for 5 minutes, held in an agitated collector, then pumped into the duct from one end until uniform grout discharges at every vent.
About Fosroc Conbextra Cable Grout
Fosroc Conbextra Cable Grout is a factory-blended, single-component grout developed for one demanding job: filling post-tensioned cable ducts completely, so the tendons inside stay protected for the life of the structure. Supplied as a dry powder in 25 kg bags, it needs only a controlled addition of clean water — 0.33 to 0.35 by weight of powder, never more than 8.75 litres per bag — to produce a grout fluid enough to travel long, draped tendon profiles and slip between individual strands where ordinary sand-cement mixes stall.
For Indian bridge and metro packages, the numbers that matter to the independent engineer are all documented. The grout is tested to EN 445 protocols and complies with EN 447 as well as MORTH/IRC requirements for fluidity, bleeding and strength: efflux time under 25 seconds, wick-induced bleed below 0.3 percent, plastic expansion controlled at up to 2 percent, and 50 mm cube strength above 30 MPa at 28 days (ASTM C109). Space Arc Engineering supplies Conbextra Cable Grout ex-stock across Delhi NCR and pan-India, with batch documentation and application guidance for grouting crews.
The technical insight
The enemy in duct grouting is bleed. When mixing water separates from cement paste inside a sealed duct, it migrates to the high points of the tendon profile and sits there as a water lens. Once that water is reabsorbed or evaporates, it leaves a void — and a void at a duct crest means bare prestressing steel, which is exactly where tendon corrosion failures begin. Conbextra Cable Grout attacks this at the mix-design level: an all-fines formulation keeps water demand low to begin with, while the additive system holds the remaining water within the paste, driving wick-induced bleed below 0.3 percent. A controlled expansion of up to 2 percent in the plastic state then compensates for settlement before set, so the duct stays full. The hardened grout is chloride-free and highly alkaline, maintaining the passive layer on the strand.
Where it is used
- Post-tensioned cable duct grouting in road and railway bridges
- Segmental box girders and viaduct spans on metro and elevated corridors
- PT flat slabs, transfer girders and beams in buildings
- Prestressed cable ducts in silos, tanks and other PT structures
- Ground anchors and tie-backs with small annular gaps
- Inflating flat jacks in precast and bridge-jacking operations
- Filling fine cracks, voids and fissures; consolidation of voided concrete or strata
Key advantages
- Single-component and pre-bagged — removes on-site batching variation from a safety-critical operation
- High fluidity (efflux time under 25 seconds) penetrates congested multi-strand ducts and long tendon profiles
- Wick-induced bleed below 0.3% — no water lenses or voids at duct high points
- Shrinkage compensated with controlled plastic expansion of up to +2%
- Chloride-free, high-alkalinity mix keeps prestressing steel passivated
- Complies with EN 447 (tested per EN 445) and MORTH/IRC limits for fluidity, bleed and strength
- Dependable strength gain: over 27 MPa at 7 days and over 30 MPa at 28 days on 50 mm cubes
- Economical coverage — roughly 16.5 litres of mixed grout per 25 kg bag
Limitations & considerations
- Needs a proper grout plant — high-shear colloidal mixer or a paddle mixer running at minimum 1500 rpm with an agitated holding tank; slow hand mixing will not achieve the tested fluidity
- Water is capped at w/p 0.35 (8.75 litres per 25 kg bag); overdosing water to “improve flow” causes bleed and segregation
- Temperature-sensitive at the extremes: warm water (30-40°C) is needed below 5°C, and chilled water plus shaded storage above 35°C
- PT duct grouting must be executed by qualified, experienced grouting crews — this is not general mason work
- Shelf life is 6 months in sealed bags in dry storage, and shortens in hot, humid godowns
- Not intended for large-clearance baseplate or machine-foundation grouting — the Conbextra GP range is the right tool there
Selection & best practice
Preparation decides the result. Flush ducts with lime water before grouting and plug every leak — grout escaping under pressure means an incomplete duct. Charge the measured water into the mixer first, then add the full bag steadily and mix continuously for 5 minutes until the grout is smooth and even; transfer to the agitated collector so pumping is never interrupted. Pump from one end (typically the low point) in a single direction and keep going until grout of uniform, undiluted consistency discharges from each vent before closing it in sequence. In Indian summer conditions use cold water and keep the grout near 25°C; above 35°C, grout during cooler hours and shade the equipment. For QC, sample only in 50 mm cube moulds following the laboratory procedure on the datasheet, and cure any exposed grout — Fosroc recommends Concure curing membrane.
Specifications
| Water:powder ratio | 0.33 – 0.35 (max 8.75 litres water per 25 kg bag) |
| Fresh wet density | 2.05 g/cc |
| Fluidity (EN 445 efflux cone) | Efflux time < 25 seconds |
| Bleed (EN 445, wick-induced test) | < 0.3% |
| Volume change, plastic state (EN 445) | Up to +2% |
| Setting time (EN 445) | Initial > 3 hours; final < 24 hours |
| Compressive strength (ASTM C109, 50 mm cubes) | > 27 MPa at 7 days; > 30 MPa at 28 days |
| Yield | Approx. 16.5 litres per 25 kg bag |
| Supply and shelf life | 25 kg bags; 6 months in dry, sealed storage |
Industry use cases
Highways & Expressways
Duct grouting of post-tensioned girders and box sections on NHAI and state expressway packages, where MORTH compliance for fluidity, bleed and strength is checked before every grouting permit.
Metro Rail
Segmental viaduct spans and precast yard operations on metro corridors — high fluidity suits long external and internal tendon profiles grouted against tight launch-cycle schedules.
High-Rise & Commercial Buildings
PT flat slabs, podium transfer girders and band beams, where complete duct filling protects tendons embedded in occupied structures for decades.
Geotechnical & Anchoring
Ground anchors and tie-backs with small annular gaps in basements, slopes and retaining systems, where a pumpable non-shrink grout must fully encapsulate the tendon.
Industrial & Storage Structures
Prestressed silos, water-retaining tanks and containment structures where chloride-free, low-permeability grout is specified around stressing cables.
Frequently asked questions
How much water is added per 25 kg bag?
Can we mix it with a hand drill and paddle?
Is it accepted on MORTH/IRC bridge projects?
What strength does it achieve, and how is it tested?
Why not just use Conbextra GP2 or a site-batched cement grout in the ducts?
How do we grout in peak summer, when it is above 35°C?
How we position Fosroc Conbextra Cable Grout
Within the Fosroc grout family, Conbextra Cable Grout is the standard specification-grade choice for PT duct work; where the contract demands a higher strength class, step up to Conbextra Cable Grout HS. For machine foundations, baseplates and pocket grouting — jobs with larger clearances and different flow demands — the Conbextra GP1/GP2 range is the correct product, and Lokfix resin anchors cover post-fixed rebar and bolt anchoring. If your project has standardised on another manufacturer, Space Arc also supplies comparable precision and structural grouts from Master Builders Solutions (MasterFlow range) and Sika, so the grouting method statement can be matched to whichever system your consultant has approved.
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