Fosroc Conbextra GP3
High-strength, non-shrink cementitious precision grout for turbine bases, heavy machinery and bridge bearings — ASTM C1107 compliant, reaching 68 N/mm² at 28 days.
What is it?
A ready-to-use, dual-expansion cementitious precision grout that mixes with clean water alone into a free-flowing, non-shrink grout for gap thicknesses up to 100 mm.
Where is it used?
Under turbine, compressor and boiler feed pump base plates, bridge bearing pedestals, stanchion bases and anchor bolt pockets carrying static and dynamic loads.
Why use it?
Reaches 26 N/mm² within 24 hours and 68 N/mm² at 28 days with chloride-free chemistry, so critical plant can be aligned and loaded early without shrinkage voids.
How is it applied?
Poured or pumped into presoaked, leakproof formwork within 20 minutes of mixing, using 4.125-4.5 litres of water per 25 kg bag depending on consistency.
About Fosroc Conbextra GP3
Conbextra GP3 is Fosroc’s high-strength precision grout for installations where a base plate simply cannot rock, settle or lose contact: turbines, compressors, boiler feed pumps and bridge bearings. Supplied as a ready-to-use dry powder, it blends Portland cement with graded fillers and chemical additives that cut water demand and drive controlled expansion while the grout is still plastic. Mixed with a measured dose of clean water, it flows freely into gaps up to 100 mm and hardens into a chloride-free, iron-free bearing medium conforming to ASTM C1107.
Space Arc Engineering supplies Conbextra GP3 across Delhi NCR and pan-India as an authorised Fosroc distributor, with applicator crews available for machinery grouting where alignment tolerances are tight. Project teams choose GP3 over site-batched grout because the numbers arrive pre-verified: 26 N/mm² within a day, 68 N/mm² at 28 days, and dynamic testing to two million load cycles — figures a consultant can write straight into a performance specification.
The technical insight
Ordinary cement grout shrinks as it hydrates, and even a hairline gap under a base plate turns smooth bearing into point loading. Conbextra GP3 counters this with a gaseous expansion system: from about 20 minutes after mixing until roughly the two-hour mark, the plastic grout swells by up to 4% unrestrained, pressing gently against the plate at exactly the stage when shrinkage would otherwise open a gap — only around 0.004 N/mm² of restraint is needed to contain it. A second compensation mechanism works in the hardened state. Meanwhile, graded fillers hold the water-powder ratio down to 0.165-0.18, which is what delivers high early strength without any chloride accelerators.
Where it is used
- Turbine, compressor and boiler feed pump base plate grouting
- Bridge bearing pedestal grouting
- Stanchion and column base plates on steel structures
- Anchor bolt pockets and holding-down bolt grouting
- Anchoring masts, fence posts and general fixings
- Machinery foundations subject to dynamic and cyclic loading
- Underplate gaps up to 100 mm in a single pour; deeper pockets filled out with 10 mm aggregate
- Large-volume pours placed by diaphragm pump or hopper
Key advantages
- Dual shrinkage compensation in plastic and hardened states keeps full bearing contact
- 26 N/mm² in 24 hours without chlorides, so early loading carries no corrosion risk
- High ultimate strength: up to 68 N/mm² at 28 days at pourable consistency
- Free flow travels 3 m or more through a 50 mm gap under a 250 mm head, filling congested bolt clusters
- Survived two million alternating load cycles (5-25 N/mm² at 500 cycles/minute) undamaged
- No metallic iron content, so no rust staining at exposed edges
- Pre-packed single component with water-only mixing removes site batching variation
- Conforms to ASTM C1107 and carries the CII GreenPro Ecolabel
Limitations & considerations
- Must be placed within about 20 minutes of mixing at 30°C to capture the expansion phase, so batching and labour need planning
- Single-pour thickness is capped at 100 mm; deeper sections need 50-100% clean 10 mm aggregate by weight to control heat build-up
- Below 10°C, strength gain slows: warm mixing water (30-40°C) and 36-hour formwork retention are required; above 40°C, use cool water below 20°C
- Rapid chloride penetration rating is moderate, so oil- or chemical-exposed bases are better served by an epoxy grout
- Results depend on presoaked substrate and leakproof formwork; poor preparation, not the grout, causes most grouting failures
- Shelf life is 6 months in dry storage and shortens in hot, humid godowns
Selection & best practice
Cut back defective concrete or laitance, blow bolt pockets clean, and saturate the substrate with fresh water for several hours before the pour, removing all free water just before grouting. Build leakproof formwork with no more than 150 mm gap on the pouring side and about 50 mm opposite, and grease any shims you intend to recover. Measure the water first — 4.125 litres per 25 kg bag for pourable consistency, 4.5 litres for flowable — then add the full bag and mix for five minutes with a slow-speed drill and paddle (400-500 rpm), or a mechanical grout mixer for larger volumes. Pour from one side only, across the shortest flow path, keeping a continuous grout head so air and presoak water are driven ahead of the front. Cure exposed areas with Concure WB, continuous water or wet hessian.
Specifications
| Compressive strength (pourable, W/P 0.165) | 26 N/mm² at 1 day; 51 N/mm² at 3 days; 68 N/mm² at 28 days (ASTM C109 / IS 4031) |
| Compressive strength (flowable, W/P 0.18) | 24 N/mm² at 1 day; 65 N/mm² at 28 days |
| Flexural strength | 10 N/mm² at 28 days (EN 196) |
| Pull-out bond strength | 17 N/mm² at 7 days; 20 N/mm² at 28 days |
| Fresh wet density | Approx. 2,220 kg/m³ |
| Expansion window | Starts 20 minutes after mixing, completes by 120 minutes; 0-4% unrestrained plastic expansion |
| Gap thickness | Up to 100 mm in a single pour; deeper with 50-100% 10 mm aggregate by weight |
| Water addition per 25 kg bag | 4.125 litres (pourable); 4.5 litres (flowable) |
| Yield and packaging | 12.5 L (pourable) / 13.3 L (flowable) per 25 kg bag; 6-month shelf life |
Industry use cases
Power generation
Turbine, generator and boiler feed pump base plates where alignment tolerance and cyclic loading demand full, permanent bearing contact.
Bridges and flyovers
Bearing pedestal grouting on highway and metro viaduct structures, where dual expansion keeps bearings seated under traffic loads.
Manufacturing and process plants
Compressor and pump skid grouting subject to vibration — verified through two million alternating load cycles without damage.
Steel and PEB structures
Stanchion base plates and holding-down bolt pockets, where free flow fills congested anchor clusters from a single-side pour.
Utilities and telecom
Anchoring masts, poles and fence posts where a pre-packed grout beats site batching for consistency on dispersed works.
Frequently asked questions
How is Conbextra GP3 different from Conbextra GP1 and GP2?
How much water does one 25 kg bag need?
What is the maximum thickness in one pour?
How soon can equipment be loaded after grouting?
Is it suitable under vibrating machinery?
How many bags do I need per cubic metre of grout?
How we position Fosroc Conbextra GP3
Within the Fosroc family, Conbextra GP3 sits at the top of the general-purpose cementitious grout range. Where the drawing calls for routine non-shrink base plate grouting, Conbextra GP1 or Conbextra GP2 will usually do the job at lower cost; where the programme hinges on loading within hours, look at Conbextra HES; and where machine bases face oil, chemical spillage or severe dynamic loading, step up to the epoxy-based Conbextra EP10. Space Arc stocks all these grades alongside the wider Fosroc anchoring and concrete repair range, so we can match the grade to the load case rather than upsell by default.
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Authorised distribution, technical advice & professional application across Delhi NCR and Pan-India.