Fosroc Conbextra GP4
A dual shrinkage-compensated, free-flow cementitious grout built for heavily loaded base plates and precision machine foundations.
What is it?
A factory pre-blended, single-component cementitious precision grout that flows without vibration and compensates for both plastic and long-term shrinkage.
Where is it used?
Under heavily loaded machine bases, turbine and generator pedestals, crane and gantry rails, structural column base plates and precast connections.
Why use it?
It keeps full, positive contact between the base plate and foundation while delivering more than 55 MPa at 28 days, so dynamic loads transfer cleanly into the concrete.
How is it applied?
Mix a 25 kg bag with clean water to the required consistency, then pour or pump it into the gap from one side so it flows across and fills without trapping air.
About Fosroc Conbextra GP4
Conbextra GP4 sits at the higher-strength end of Fosroc’s general-purpose grout range. It arrives as a single-component dry powder that only needs clean, measured water on site, which removes the guesswork of batching cement, sand and additives by hand. The mix is engineered as a Type C grout to BS EN 1504-6, meaning it is intended for load-bearing structural grouting where the bond and load transfer between two elements actually matter. On a busy site, that consistency from bag to bag is often as valuable as the headline strength figure, because every pour behaves the same way.
What sets a precision grout like GP4 apart from ordinary mortar is its dual shrinkage compensation. Fresh cementitious mixes tend to lose volume twice: once as plastic shrinkage while they set, and again as drying shrinkage over the following weeks. Either movement can open a hairline gap under a base plate, and under vibrating machinery that gap grows into a rocking, hammering fault. GP4 is formulated to counter both phases, so it stays in firm, gap-free contact with the steel above and the concrete below. Combined with a free-flow rheology that reaches under and around a plate without vibration, it is a natural fit for equipment that will not tolerate a soft or partial bearing.
The technical insight
The grout works by pairing controlled expansion with a self-levelling flow. As it sets, an expansive action offsets the natural plastic shrinkage of the cement paste, and the binder chemistry is designed to limit long-term drying shrinkage as well. Because both stages are managed, the hardened grout does not pull away from the base plate or the foundation, which is exactly what keeps load transfer uniform. The free-flow consistency does the other half of the job: poured from one side, the grout travels across the full footprint under gravity and fills the space completely, so there are no voids to concentrate stress. The result is a dense, high-strength block, more than 55 MPa at 28 days, that acts as a continuous bearing rather than a patchwork of contact points.
Where it is used
- Grouting heavily loaded machine bases and reciprocating equipment
- Bedding large turbine and generator base plates
- Fixing high-load crane rails and gantry pedestals
- Grouting structural column base plates in high-rise and heavy structures
- Filling precast connection joints designed for high loads
- Grouting rail bearings and bridge shoes
- Anchoring holding-down bolts and heavy fixings in pockets
- Setting sole plates for pumps, compressors and stamping presses
Key advantages
- Dual shrinkage compensation keeps the plate in positive contact through both setting and curing
- Free-flow rheology fills under a base plate without vibration or rodding
- High 28-day strength above 55 MPa suits dynamically loaded foundations
- Factory pre-blended, so on-site mixing needs only clean water
- Can be pumped or gravity-placed to suit access and gap size
- Single 25 kg bag format simplifies dosing and stock control
- Manufactured to BS EN 1504-6 Type C for structural load-bearing grouting
- Consistent bag-to-bag performance reduces the risk of a weak pour
Limitations & considerations
- Working time shortens noticeably when ambient temperature climbs above 35 degrees C, which matters on North Indian summer sites
- Only cementitious performance is offered; for chemical or hydrocarbon exposure an epoxy grout is the correct choice
- Gaps wider than 100 mm need extension with clean, pre-wetted aggregate no larger than 10 mm, adding a site step
- Correct water dosage is critical, as over-watering cuts strength and flow control
- The 25 kg bag format can make very large single pours labour-intensive without continuous mixing
- Like all cementitious grouts it needs proper curing and protection from rapid drying to reach full strength
Selection & best practice
Start by checking that the foundation concrete is sound, clean and free of laitance, oil and loose material, then saturate it with clean water and remove any standing pools before you pour. Build a leak-tight formwork with enough headbox height on the pouring side to keep a positive head of grout moving. Measure water accurately and mix mechanically to a smooth, lump-free consistency rather than adding water by eye. Pour continuously from one side only so the grout pushes air ahead of it and fills the whole gap in a single flowing movement; never pour from opposite sides at once. In hot weather, keep bags and mixing water in shade and place early in the day to protect working time. Where the gap exceeds 100 mm, extend the mix with clean, dust-free, pre-wetted aggregate up to 10 mm. Once placed, protect exposed edges from rapid drying and cure the grout so it develops its full strength.
Specifications
| Product type | Free-flow cementitious precision grout, dual shrinkage compensated |
| Standard | BS EN 1504-6 Type C |
| Compressive strength (28 days) | Greater than 55 MPa |
| Flow (ASTM C940) | Greater than 140 mm |
| Shrinkage compensation | Dual (plastic and long-term drying) |
| Placing method | Pumpable or gravity-placed, no vibration required |
| Composition | Factory pre-blended, single-component dry powder |
| Pack size | 25 kg bag |
| Maximum aggregate extension size | 10 mm, for gaps over 100 mm |
Industry use cases
Power generation
Bedding turbine and generator base plates where full, gap-free contact is essential for vibration-free running.
Heavy manufacturing
Grouting presses, compressors and reciprocating machine bases that impose sustained dynamic loads on their foundations.
Ports and material handling
Fixing crane rails and gantry pedestals that carry concentrated wheel loads across long runs.
Infrastructure and bridges
Grouting bridge shoes and rail bearings, and filling precast connection joints designed for high loads.
Commercial construction
Setting structural column base plates in high-rise and heavily loaded frames.
Frequently asked questions
What is the difference between Conbextra GP4 and GP3?
Does Conbextra GP4 need vibration to place it?
What gap thickness can it handle?
How much water do I add per bag?
Is Conbextra GP4 suitable for chemical exposure?
Do you deliver Conbextra GP4 across India?
How we position Fosroc Conbextra GP4
Within the Fosroc grout family, Conbextra GP4 is the higher-strength general-purpose option; where a design needs a lower grade for standard machine bases, Conbextra GP3 is the natural alternative, and for very high early strength or specialist bearings other Conbextra grades apply. When the exposure is chemical or hydrocarbon rather than purely structural, step across to the epoxy Conbextra EP range instead of a cementitious grout. For repair and rehabilitation work that often accompanies foundation grouting, our Fosroc Renderoc and Nitobond ranges pair well, and where a comparable cementitious grout is preferred we also carry MC-Bauchemie and Dr. Fixit grouting products. We will help specify the right grade for the load and environment on your project.
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