About Fosroc Conbextra GP

Fosroc Conbextra GP is the general-purpose member of Fosroc’s well-known Conbextra grout family, used across India for the routine but critical job of filling the gap between a steel or equipment baseplate and its concrete foundation. It is supplied as a factory-blended dry powder — a mix of Portland cement, carefully graded fillers and chemical additives — that needs nothing on site except clean water and a mixer. That pre-packaging is the whole point: it removes the batching guesswork and quality variation that creep in when a site tries to make its own sand-cement grout.\n\nThe defining feature of Conbextra GP is shrinkage compensation. An ordinary cementitious mix settles and shrinks as it sets, pulling away from the underside of the plate and leaving voids exactly where you need solid bearing. Conbextra GP uses a controlled gaseous expansion system that produces a slight positive expansion while the grout is still plastic, so the hardened grout stays in firm, full contact with both the steel above and the concrete below. The graded filler system also lets the grout flow and self-level without bleeding or segregating, which matters when you are trying to fill the far corners of a baseplate through a single pour point.\n\nFor Indian projects this is a workhorse product. It suits gap thicknesses from 10 mm up to 75 mm in a single placement, is chloride-free so it will not promote corrosion of the embedded steel or anchor bolts, and develops strength quickly enough to keep a construction programme moving. It is the natural choice when you need dependable, code-compliant grouting but the loads and conditions do not justify stepping up to a high-flow precision grout (Conbextra GP2) or an epoxy grout (Conbextra EP system).

The technical insight

Conbextra GP is a single-component, cement-based system that is classed as shrinkage-compensated rather than truly expansive: the gaseous expansion acts only in the plastic, pre-set state to offset plastic settlement and drying shrinkage, so you should not rely on it to generate post-set prestress the way a metallic or modern expansive precision grout might. Strength is consistency-dependent and inversely related to water content — the more water you add to make it flow, the lower the strength. Engineers should specify the consistency, not just the product: a trowelable mix delivers higher strength than a fluid mix from the same bag. Conbextra GP conforms to BS EN 1504-3 as a Class R4 structural repair mortar and meets BS EN 1504-6 for anchoring of reinforcing steel, and the grout’s non-shrink behaviour is assessed to ASTM C1107. Because it is a neat (unaggregated) grout, large-volume or deep pours generate more heat and shrinkage risk; for placements toward the upper thickness limit, Fosroc permits adding well-graded, clean aggregate to extend yield and control exotherm, tested in India to IS 516. The product is supplied free of chloride and free of metallic iron, so there is no risk of chloride-induced corrosion or rust staining at the grout face.

Where it is used

• Grouting beneath structural steel column and stanchion baseplates
• Bedding machine bases, pumps, motors and light-to-medium static equipment
• Anchoring holding-down bolts, anchor bolts, masts and railings into preformed pockets
• Installing starter bars and anchoring reinforcement steel (per BS EN 1504-6)
• Filling and sealing joints between precast concrete panels and segments
• Fixing tie bars, dowels and fixings into concrete
• General void-filling and gap-grouting from 10 mm to 75 mm in a single placement

Key advantages

• Shrinkage-compensated: gaseous expansion in the plastic state ensures full, void-free contact under baseplates
• Free-flowing and self-levelling, so it fills awkward gaps from a single pour point without bleeding or segregating
• Factory pre-blended powder removes on-site batching variation — just add measured clean water
• High early and ultimate strength achievable without chlorides
• Chloride-free and free of metallic iron, so no corrosion of embedded steel and no rust staining
• Versatile consistency: can be mixed trowelable, flowable or fluid to suit the gap and method
• Can be placed by pouring or pumping, and dry-packed or rammed where appropriate
• Conforms to BS EN 1504-3 Class R4 and BS EN 1504-6, easing specification and approval

Limitations & considerations

• Strength drops as water is increased — over-watering to ease flow can significantly reduce compressive strength
• Single placement is limited to roughly 10-75 mm; deeper or larger pours need aggregate extension or staged lifts
• As a neat cementitious grout, deep/large-volume pours generate heat and need exotherm control
• Not a high-modulus or chemical-resistant grout — for dynamic machinery, heavy impact or chemical exposure an epoxy grout (Conbextra EP) is more appropriate
• Requires correct substrate preparation: clean, sound, saturated-surface-dry concrete and pre-soaked bolt pockets
• Like all cementitious grouts it must be protected from rapid drying and cured properly, especially in hot, dry or windy Indian site conditions
• Has a limited working time once mixed; placement must be continuous to avoid cold joints

Selection & best practice

Specifications

Industry use cases

How we position Fosroc Conbextra GP

Within the Fosroc range, Conbextra GP is the entry, general-purpose grade of the Conbextra family. Step up to Conbextra GP2 (and GP2N) when you need a higher-strength, free-flow precision grout for heavier baseplates, larger gaps or more demanding machinery; move to the Conbextra EP epoxy grout system when loads are dynamic, impact-prone or chemically aggressive. It pairs naturally with Fosroc’s Nitobond bonding agents for substrate preparation, Nitoproof / Brushbond where the grouted element also needs waterproofing, and Conbextra anchor grouts or the Lokfix epoxy/resin system for chemical anchoring of bolts and rebar. Against the wider Indian market, credible cementitious non-shrink alternatives include Sika’s SikaGrout range, MBS Grout, Dr. Fixit Pidicrete / Roff cementitious grouts, and STP’s grout products; for epoxy anchoring duties, Sika Anchorfix and Hilti resin systems are common comparisons. As an authorised Fosroc distributor and applicator serving Delhi NCR and Pan-India, Space Arc Engineering supplies genuine Conbextra GP and can advise on the right grade — GP, GP2 or EP — for your specific baseplate, anchoring or precast-jointing application.

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