About Fosroc Conbextra MCG

Fosroc Conbextra MCG is the large-gap grout within the well-known Conbextra family of non-shrink cementitious grouts. Where Conbextra GP and GP2 are intended for the thin precision gaps typical under most structural base plates, MCG is purpose-built for situations where the void to be filled is unusually deep — anywhere from 25 mm to 125 mm. Pouring an ordinary precision grout to that depth in a single lift risks excessive heat build-up, plastic cracking and shrinkage; MCG is graded with a coarser filler fraction and a controlled expansion system specifically so that it can be placed at full thickness while still flowing freely and remaining dimensionally stable. It is supplied as a dry, pre-blended powder, so on-site batching variation is eliminated and the only site control required is accurate water dosing. The product is chloride-free and contains no metallic iron, so it will not stain or corrode embedded steel plates, anchors or reinforcement — an important consideration for permanent machine foundations and rail-sector bearings that must survive decades of service. For consultants writing specifications and contractors executing them in India, MCG occupies a clear niche: it is the answer for heavy, deep-pour grouting where a thin precision grout is simply the wrong tool. Space Arc Engineering supplies and applies Conbextra MCG as an authorised Fosroc distributor and applicator across Delhi NCR and Pan-India, with site support on substrate preparation, formwork and placement.

The technical insight

The defining technical challenge MCG solves is heat and shrinkage in a thick pour. Cementitious grouts generate heat as they hydrate; in a deep section that heat is trapped, the core stays hot while the surface cools, and the resulting thermal gradient plus drying shrinkage can crack the grout and break contact under the plate — exactly what a foundation grout must never do. Fosroc addresses this two ways. First, the filler is graded with a coarser aggregate component than thin-gap grouts like Conbextra GP; the larger aggregate reduces the cement paste fraction per unit volume, which lowers both peak hydration heat and overall shrinkage potential while still allowing the mix to self-level. Second, MCG uses the same gaseous/expansion principle as the rest of the Conbextra range — a controlled positive volume change while the grout is still plastic that compensates for plastic settlement, so the hardened grout bears tight against the underside of the plate. Because it is non-bleeding and non-segregating, the coarse aggregate stays in suspension during the pour rather than settling out and leaving a weak, water-rich layer at the top of the section. The trade-off engineers should understand is the gap window itself: below 25 mm the coarse aggregate cannot pack or flow properly, so a thinner-graded grout (GP/GP2/HF) should be used; above 125 mm the heat and shrinkage demands move beyond a grout entirely and into micro-concrete territory (Conbextra HF for high-flow large pours, or a designed flowable concrete with added aggregate). MCG is the correct chemistry for that 25-125 mm band, no wider.

Where it is used

• Grouting under heavy machine bases and equipment foundations where the bedding gap is 25-125 mm
• Bedding of structural steel base plates and stanchions in a single deep lift
• Fixing and anchoring of holding-down bolts in deep pockets and sleeves
• Filling and repairing cavities, voids and gaps in concrete elements
• Bedding of crane rails and rail support steelwork in industrial buildings
• Bedding of bridge bearing sections in the rail sector and on heavy infrastructure
• Reinstatement of deep voids beneath base plates of turbines, compressors, pumps and mills in power, cement and process plants

Key advantages

• Fills deep gaps from 25 mm up to 125 mm in a single continuous pour, avoiding multiple lifts
• Shrinkage-compensated: a controlled plastic-state expansion maintains tight contact under base plates
• Free-flowing and self-compacting, reaching beneath plates and around bolts without vibration
• Non-bleeding and non-segregating, so the coarse aggregate stays uniformly suspended
• Chloride-free with no metallic iron content, so it does not corrode or stain embedded steel
• High compressive strength for dependable, durable bearing support under heavy loads
• Good early strength for faster formwork removal and quicker return to service
• Pre-packaged dry powder eliminates on-site batching errors; only water dosing is controlled
• Suitable for both internal and external applications and for permanent structural duty

Limitations & considerations

• Designed only for the 25-125 mm gap band; below 25 mm use a thinner grout (Conbextra GP/GP2/HF), above 125 mm use a micro-concrete or aggregate-extended pour
• Cementitious chemistry, so it lacks the chemical resistance of an epoxy grout (e.g. Conbextra EP series) for acid, oil or aggressive-chemical exposure or for high-vibration dynamic loads
• Strength and flow depend heavily on accurate water dosing; excess water reduces strength and can cause bleeding
• Requires sound surface preparation and several hours of pre-soaking of the concrete substrate before grouting
• Must be poured continuously from one side; interrupted or two-sided pouring can trap air and leave voids
• Effective expansion occurs only while the grout is plastic, so it must be placed promptly within the working time
• Formwork must be leak-proof and the unrestrained surface area kept to a minimum for the expansion system to work

Selection & best practice

Specifications

Industry use cases

How we position Fosroc Conbextra MCG

Within the Fosroc Conbextra range, MCG sits at the large-gap end of the cementitious grouts: Conbextra GP and GP2 cover thin precision gaps, Conbextra HF handles high-flow grouting of large areas, and MCG is the choice when the bedding gap is a deep 25-125 mm. Where chemical resistance, high dynamic loading or aggressive exposure are involved, the right partner product is an epoxy grout from the Conbextra EP family rather than MCG. For anchoring slim starter bars or bolts in thinner gaps, pair the project with Conbextra GP or a Lokfix anchoring grout. Against the wider Indian market, MCG competes with comparable large-gap non-shrink cementitious grouts such as Sika’s SikaGrout range, MBS (MYK) Saint-Gobain grouts, Pidilite Dr. Fixit Pidigrout, and STP/BASF (Master Builders Solutions) MasterFlow grouts; specifiers typically select on verified strength, the certified gap range and proven non-shrink performance rather than brand alone. Space Arc Engineering’s value as an authorised Fosroc distributor and applicator is in matching the correct grade to the actual gap depth and load case, and supporting correct placement on site so the specified performance is actually achieved. Note: Fosroc’s public product page confirms MCG as a real shrinkage-compensated flowable cementitious grout for 25-125 mm gaps but does not publish exact compressive-strength figures; specific N/mm² values, pack sizes and yields should be confirmed against the current Fosroc India technical datasheet before specification.

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