Fosroc Conbextra GP2
Free-flow, non-shrink cementitious precision grout that reaches 65 N/mm² at 28 days — the workhorse specification for machine base plates, stanchion bases and anchor bolt grouting across Indian industry.
What is it?
A ready-to-use, chloride-free cementitious precision grout from Fosroc. Mixed with a measured dose of clean water, it becomes a free-flowing, non-shrink grout for gap thicknesses up to 100 mm.
Where is it used?
Under base plates of turbines, compressors and boiler feed pumps; stanchion and column bases; anchor bolt pockets; and for fixing masts, posts and holding-down assemblies.
Why use it?
A gaseous expansion system offsets plastic shrinkage and settlement, so the hardened grout keeps full bearing contact under the plate — with 24 N/mm² available at just one day.
How is it applied?
Mix a 25 kg bag with 4.125–4.5 litres of water for 5 minutes, pour continuously from one side into presoaked, leakproof formwork within 20 minutes of mixing, then wet-cure exposed edges.
About Fosroc Conbextra GP2
Conbextra GP2 is Fosroc’s general-purpose precision grout — a factory-blended dry powder of Portland cement, graded fillers and chemical additives, supplied in 25 kg moisture-resistant bags. Only water is added at site, which removes the batching variation that plagues site-mixed grouts. Two working consistencies are possible from the same bag: pourable at 4.125 litres of water and flowable at 4.5 litres, letting the applicator match the mix to the gap geometry rather than adjusting water by guesswork.
On projects we support across Delhi NCR and pan-India, GP2 is the grout most often written into machine foundation and structural steel specifications, and for good reason. It gives 24 N/mm² within a day and 65 N/mm² at 28 days in flowable consistency, is completely chloride-free, and has been cycle-tested under alternating loads of 5 and 25 N/mm² for two million cycles without damage — genuine credentials for equipment that vibrates in service.
The technical insight
Ordinary cement grout shrinks as it sets, pulling away from the underside of a base plate exactly where bearing is needed. Conbextra GP2 counters this with a gas-generating expansion system that works while the grout is still plastic: expansion begins about 20 minutes after mixing and completes by 120 minutes, producing 2–4% unrestrained growth against a restraining pressure of only about 0.004 N/mm². Because the expansion happens before set, it compensates for both plastic shrinkage and settlement without stressing the hardened grout. The blend is also engineered for low water demand — a water/powder ratio of just 0.18 in flowable form — which is what drives the high early strength and a dense fresh wet density of roughly 2220 kg/m³. This is why placing within 20 minutes of mixing matters: pour late and the expansion is spent in the mixer, not under the plate.
Where it is used
- Precision grouting under turbine, compressor and boiler feed pump base plates
- Stanchion and column base plate grouting in steel structures and PEB buildings
- Anchor bolt, bolt pocket and sleeve grouting for static and dynamic loads
- Bearing plate and pedestal grouting on bridges and heavy structures
- Crane rail and equipment holding-down bolt grouting
- Fixing of masts, fence posts and cast-in fixings
- Underplate void filling up to 100 mm in a single pour
- Deep-section grouting when extended with 10 mm graded aggregate
Key advantages
- Gaseous expansion in the plastic state compensates for shrinkage and settlement — no voids under the plate
- Free-flowing mix achieves high contact with the load-bearing area without vibration
- High early strength without chlorides: 24 N/mm² at 1 day, safe alongside embedded steel
- High ultimate strength — 65 N/mm² flowable, 68 N/mm² pourable at 28 days
- Proven dynamic endurance: undamaged after 2 million alternating load cycles of 5–25 N/mm²
- No metallic iron content, so no rust staining on exposed grout shoulders
- Prepacked single-component supply eliminates on-site batching errors
- Pumpable with a diaphragm pump for large-volume pours
Limitations & considerations
- Maximum 100 mm thickness per pour neat; deeper sections must be extended with 50–100% of 10 mm silt-free aggregate to control heat build-up
- Must be placed within about 20 minutes of mixing at 30°C, or the expansion benefit is lost
- Being cementitious, it is not the right choice where aggressive chemical exposure or severe impact loading demands an epoxy grout
- Performance depends on accurate water dosing and a presoaked substrate — excess water causes bleed and segregation
- Below 10°C ambient, strength gain slows: warm water (30–40°C) is needed and formwork must stay in place at least 36 hours
- Shelf life is 6 months in sealed bags, and less in hot, humid storage
Selection & best practice
Preparation decides the result. Cut back defective concrete and laitance, blow out bolt pockets, and saturate the substrate with fresh water several hours before grouting — then remove all free water, especially from pockets. Formwork must be leakproof (foam strip or mastic under the shutters), with the gap on the pouring side kept within 150 mm and 50 mm opposite. Measure the water accurately — 4.5 litres per bag flowable, 4.125 litres pourable — add powder to water and mix 5 minutes with a slow-speed drill and paddle (400–500 rpm) for quantities up to 50 kg. Grout bolt pockets first, then pour continuously from one side only so air and presoak water are not trapped, maintaining a constant grout head. In heat above 40°C mix with water below 20°C. Cure exposed areas with Concure WB, ponded water or wet hessian.
Specifications
| Compressive strength, flowable (W/P 0.18, BS 1881-116) | 24 N/mm² @ 1 day; 55 N/mm² @ 7 days; 65 N/mm² @ 28 days |
| Compressive strength, pourable (W/P 0.165) | 26 N/mm² @ 1 day; 68 N/mm² @ 28 days |
| Flexural strength (BS 4551) | 10 N/mm² @ 28 days |
| Pull-out bond strength | 17 N/mm² @ 7 days; 20 N/mm² @ 28 days |
| Unrestrained plastic expansion | 2–4%, starting 20 minutes and complete 120 minutes after mixing |
| Fresh wet density | Approx. 2220 kg/m³ |
| Young’s modulus (ASTM D469) | 28 kN/mm² |
| Water addition per 25 kg bag | 4.5 litres flowable; 4.125 litres pourable |
| Yield per 25 kg bag / shelf life | 13.3 litres flowable, 12.5 litres pourable; 6 months in sealed dry storage |
Industry use cases
Power & Energy
Grouting turbine, generator and boiler feed pump base plates in thermal and captive power plants, where two-million-cycle dynamic load resistance is directly relevant.
Steel Structures & PEB
Stanchion base plates and holding-down bolts on pre-engineered buildings and industrial sheds, poured flowable for full plate contact without vibration.
Manufacturing & Process Plants
Compressors, mills, presses and rotating equipment foundations in cement, steel and automotive plants — 24 N/mm² at 1 day keeps alignment and commissioning schedules tight.
Infrastructure & Metro
Bearing plates, pedestals, precast connections and anchor fixings where a chloride-free, non-shrink grout is mandated in the specification.
Telecom & Utilities
Anchoring masts, towers and fence posts — small-volume pours where prepacked consistency beats site-batched mortar.
Frequently asked questions
What is the difference between the flowable and pourable mixes?
What is the maximum thickness in one pour?
How many bags do I need per cubic metre?
Is it safe with embedded reinforcement and anchor bolts?
When should I specify an epoxy grout instead?
How should it be handled in extreme site temperatures?
How we position Fosroc Conbextra GP2
Conbextra GP2 sits at the centre of Fosroc’s grouting range, and most projects pair it with its siblings rather than replace it. Specify Conbextra GP1 where a standard-duty non-shrink grout is sufficient, and step up to the Conbextra EP epoxy range for chemical exposure, oil-contaminated environments or the most punishing dynamic equipment. Fosroc’s Lokfix resin anchors cover high-load rebar and bolt fixings, while Concure WB is the TDS-recommended curing membrane for exposed grout shoulders. Where a project has standardised on another house, we also supply equivalent non-shrink precision grouts from Sika and MC-Bauchemie — our team can map the specified grade to the correct alternative. As authorised Fosroc distributors and applicators, Space Arc supplies Conbextra GP2 ex-stock across Delhi NCR and pan-India, with application support for critical pours.
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Authorised distribution, technical advice & professional application across Delhi NCR and Pan-India.