Fosroc Conbextra Cable Grout HS
High-strength, non-bleed cementitious grout for post-tensioned cable ducts — exceeds 70 MPa at 28 days and meets EN 447 and MORTH/IRC requirements for critical bridge and metro tendon grouting.
What is it?
A pre-bagged, all-fines cementitious grout from Fosroc’s Conbextra range, formulated for grouting post-tensioned cable ducts at a very low water-powder ratio of 0.28-0.30 with negligible bleed and controlled plastic-state expansion.
Where is it used?
PT tendon ducts in bridge box girders, PSC girders, metro viaduct segments, prestressed silos and tanks, and ground anchors with small annular gaps.
Why use it?
It combines pumpable fluidity (efflux under 25 seconds) with wick-induced bleed below 0.3% and compressive strength above 70 MPa at 28 days — the numbers that high-specification PT jobs actually ask for, backed by EN 447 and MORTH/IRC compliance.
How is it applied?
Mixed with a maximum of 7.5 litres of water per 25 kg bag in a high-shear colloidal mixer or 1500 RPM paddle mixer for 5 minutes, then pumped into flushed, leak-plugged ducts through standard PT grouting equipment.
About Fosroc Conbextra Cable Grout HS
Conbextra Cable Grout HS is the high-strength grade in Fosroc’s post-tensioning grout family. It arrives on site as a single pre-weighed 25 kg bag of dry powder — an all-fines blend of Portland cements and additives — so the only site variable is the water you add. Mixed at the recommended water-powder ratio of 0.28 to 0.30, it produces a free-flowing, non-shrink grout with an efflux time under 25 seconds, wick-induced bleed below 0.3% and controlled expansion of up to 2% while the grout is still plastic. The product complies with EN 447 guidelines when tested to EN 445, and with MORTH/IRC requirements for fluidity, bleeding and strength, which allows direct acceptance on NHAI, metro and railway contracts.
The HS designation matters on paper and in the duct. Where a standard cable grout satisfies routine tendon-grouting specifications, Cable Grout HS delivers over 55 MPa at 7 days and over 70 MPa at 28 days on 50 mm cubes tested to ASTM C109 — headroom that designers of long-span and heavily stressed structures increasingly write into their specifications. Space Arc Engineering stocks and supplies Conbextra Cable Grout HS across Delhi NCR and pan-India, with technical support for grout trials, cube sampling protocols and mixer selection.
The technical insight
The engineering problem in tendon grouting is contradictory: the grout must be fluid enough to travel tens of metres through a congested duct, yet hold its mix water so tightly that none of it bleeds and collects at high points or anchorages — the classic origin of strand corrosion. Cable Grout HS resolves this with additive chemistry rather than extra water. Dispersing admixtures in the all-fines blend deflocculate the cement at a water-powder ratio of just 0.28-0.30, giving sub-25-second efflux at a fresh density of 2.07 g/cc. Water-retention agents then suppress wick-induced bleed to under 0.3%, while a controlled gas-expansion mechanism generates up to +2% volume growth in the plastic state, compensating settlement before initial set (over 3 hours) so the hardened grout stays in intimate, void-free contact with the strand. The low water content is also what drives the dense, low-permeability matrix behind the 70 MPa-plus strength.
Where it is used
- Grouting of PT cable ducts in highway bridge box girders and PSC I-girders
- Tendon grouting in precast segmental metro viaduct spans
- Railway ROB/RUB prestressed girder duct grouting to MORTH/IRC requirements
- Vertical and inclined tendon ducts in prestressed concrete silos and water tanks
- External post-tensioning ducts on strengthening and rehabilitation projects
- Ground anchor grouting where annular gaps are small
- High-specification PT works where the consultant demands grout strength above 70 MPa
Key advantages
- Compressive strength above 55 MPa at 7 days and 70 MPa at 28 days (ASTM C109, 50 mm cubes) — the strongest grade in the Conbextra cable grout line
- Wick-induced bleed under 0.3% to EN 445, protecting strand at duct high points and anchorages
- Efflux time below 25 seconds allows pumping through long, congested tendon profiles
- Controlled expansion up to +2% in the plastic state compensates settlement and eliminates shrinkage voids
- Factory pre-bagged single component removes site batching variation — only water is added
- Complies with EN 447 (tested per EN 445) and MORTH/IRC fluidity, bleed and strength requirements
- Cementitious, non-flammable formulation with low permeability for long-term strand protection
- Doubles as a grout for small-gap ground anchors, reducing product count on site
Limitations & considerations
- Cold weather slows strength gain: below 10°C keep formwork in place at least 36 hours, and at 5°C or below mix with warm water (30-40°C)
- Above 35°C ambient, chilled water and shaded storage are needed to hold grout temperature near 25°C
- Demands a high-shear colloidal mixer or paddle mixer of minimum 1500 RPM with 5 minutes of mixing — hand or drill mixing will not disperse it properly
- Water is strictly capped at 7.5 litres per 25 kg bag; any excess destroys the non-bleed performance and strength
- Not intended for baseplate, machine-foundation or large-void grouting — Conbextra GP2 or HF grades suit those better
- Shelf life is 6 months in sealed bags in dry storage, and drops in hot, humid godowns
Selection & best practice
Preparation decides the result. Flush every duct with lime water before grouting and plug any leaks — a leaking duct loses grout pressure and leaves voids. Measure the water first (never more than 7.5 litres per 25 kg bag; reduce towards a 0.28 ratio in warm conditions), then add the full bag slowly into a high-shear colloidal mixer or a paddle mixer running at minimum 1500 RPM, and mix for a full 5 minutes to a smooth, even consistency. Run the grout plant as a train — mixer, agitated collector tank, pump — so mixed grout is kept moving until it enters the duct. In summer, store bags in shade, use chilled water and aim to grout outside the hottest hours, keeping grout temperature around 25°C. For quality control, cast 50 mm cubes in two 25 mm lifts within 15 minutes of mixing, tap rather than tamp, cure at 27°C per the datasheet protocol and crush three cubes per age. Cure exposed grout surfaces with a Concure curing membrane and wash tools with clean water immediately after use.
Specifications
| Water-powder ratio | 0.28 – 0.30 by weight (max 7.5 litres per 25 kg bag) |
| Fresh grout density | 2.07 g/cc |
| Fluidity (EN 445 efflux cone) | Efflux time < 25 seconds |
| Bleed (EN 445 wick-induced test) | < 0.3% |
| Volume change (EN 445) | Up to +2% expansion in plastic state |
| Setting time (EN 445) | Initial > 3 hours; final < 24 hours |
| Compressive strength (ASTM C109, 50 mm cube) | > 55 MPa at 7 days; > 70 MPa at 28 days (at W/P 0.30) |
| Standards compliance | EN 447 guidelines (tested per EN 445); MORTH/IRC for fluidity, bleed and strength |
| Supply, yield and shelf life | 25 kg bag; approx. 15.5 litres per bag; 6 months in dry, sealed storage |
Industry use cases
Highways & Expressways
Tendon duct grouting in cast-in-situ and precast box-girder superstructures on NHAI and state expressway packages, where MORTH compliance and 70 MPa grout strength are written into the QAP.
Metro Rail
Grouting of longitudinal and transverse tendons in precast segmental viaduct spans, where fast pumping through congested ducts and nil bleed at anchorages are non-negotiable.
Railways
PSC girder grouting for ROBs, RUBs and rail flyovers, where cube results above 55 MPa at 7 days keep launching and stressing cycles on programme.
Industrial & Storage Structures
Vertical and circumferential tendon ducts in prestressed silos, water reservoirs and containment structures, where the dense low-permeability grout matrix protects strand for the design life.
Geotechnical Works
Ground anchor grouting where the annular gap is small and a pumpable, non-shrink, high-strength grout is needed around the tendon bond length.
Frequently asked questions
What is the difference between Conbextra Cable Grout and Conbextra Cable Grout HS?
How much water is added per 25 kg bag?
Can I mix it with a hand drill or ordinary mortar mixer?
Is it acceptable under MORTH and IRC specifications?
How many bags do I need for a duct?
Can I use Cable Grout HS under machine baseplates?
How we position Fosroc Conbextra Cable Grout HS
Conbextra Cable Grout HS sits at the top of Fosroc’s post-tensioning grout family. Where the specification calls for standard EN 447/MORTH-compliant tendon grouting without the 70 MPa strength requirement, the standard Conbextra Cable Grout does the same job more economically. For machine foundations, baseplates and larger structural voids, Conbextra GP2 or Conbextra HF are the right Fosroc grouts. Pair duct grouting with Concure curing membrane for exposed grout surfaces. For clients who prefer site-batched cable grout, we also supply bleed-control grouting admixtures from Sika as an alternative route — though pre-bagged HS grout removes the batching risk entirely. Space Arc Engineering supplies the full Conbextra range ex-stock in Delhi NCR with pan-India dispatch, and can support grout trials and mixer selection on request.
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