About Fosroc Conbextra HF

Conbextra HF is Fosroc’s high-flow member of the Conbextra precision-grout family, made for situations where the grout has to travel a long way under a plate yet still fill every corner. It arrives on site as a factory-blended dry powder, so the only site variable is the water you add. That matters, because the mix is designed to run at a low water demand: at a water-powder ratio of around 0.19 you still get a fluid grout that reaches into thin gaps without needing extra water that would weaken the set material or invite bleed channels. For engineers commissioning rotating equipment, transformers, or heavy structural steel, that combination of reach and strength is the whole point.

What separates a precision grout from ordinary sand-cement is the way it manages shrinkage. Cement shrinks as it hydrates and dries, and under a stiff steel base plate that shrinkage leaves voids exactly where you need bearing. Conbextra HF answers this with a dual expansion system: a gaseous, hydrogen-free expansion while the grout is still plastic to counter plastic settlement, followed by a controlled expansion in the hardened state to offset drying shrinkage. The result is a grout that stays in intimate contact with both the foundation and the underside of the plate, transferring load evenly instead of through a handful of high spots. For Delhi NCR and Pan-India projects, Space Arc supplies it as an authorised Fosroc distributor with the application backup contractors expect.

The technical insight

The engineering trick in Conbextra HF is holding two opposing goals at once: it has to flow like water yet set like structural concrete. The graded filler system is what makes that possible. By packing particles across a controlled range of sizes, the blend fills the spaces between cement grains, which lets the mix stay cohesive and pumpable at low water content instead of separating into a heavy cement layer and a watery top. That is why it resists bleed and segregation even when poured through a deep head or pumped over distance. Because the water demand stays low, the water-cement ratio of the hardened grout stays low too, and a low water-cement ratio is directly responsible for the high early strength (20 N/mm2 at one day) and the dense, low-permeability matrix that gives long-term durability. The expansion mechanism is deliberately hydrogen-free and non-metallic: older grouts used iron filings or gas-generating systems that could rust or produce hydrogen, both undesirable under a critical machine. Conbextra HF generates its plastic-state expansion through a controlled gaseous reaction and its hardened-state expansion through the chemistry of the blend, so the grout keeps positive volume change without introducing corrosion or unstable gas into the load path.

Where it is used

• Bedding heavy machine base plates, sole plates and skid-mounted equipment where full, void-free contact is critical
• Grouting bridge bearings, pot bearings and elastomeric bearing plates on flyovers and rail structures
• Fixing crane rails, gantry rails and columns subject to repeated dynamic loading
• Filling joints and connections between precast concrete panels, beams and columns
• Pumped grouting of large or repetitive base plates where hand pouring is impractical
• Precision grouting in restricted-access locations where the grout must flow rather than be placed by hand
• Anchoring holding-down bolts and fixing pockets ahead of the main under-plate pour
• Under-plate grouting for pumps, compressors, motors and transformers during plant installation

Key advantages

• Excellent initial flow and flow retention, so a single continuous pour reaches the far side of large plates
• High early strength of about 20 N/mm2 at one day allows plant to be commissioned quickly
• Dual expansion in both plastic and hardened states keeps the grout in full contact and eliminates bearing voids
• Low water demand produces a dense, low-permeability, durable grout with 28-day strength around 56 N/mm2
• Hydrogen-free, non-metallic expansion avoids the corrosion and gas risks of older iron-based grouts
• Chloride-free formulation is safe to use in contact with reinforcement and prestressing steel
• Resists bleed and segregation across flowable and fluid consistencies thanks to graded fillers
• Can be either poured or pumped, giving flexibility for both small pockets and large-volume work

Limitations & considerations

• Single-pour thickness is limited to 125 mm; deeper gaps need bulking with Conbextra Grout Aggregate or a switch to Conbextra Deep Pour
• Being cement-based, it is not the right choice for chemically aggressive or oil-contaminated environments where an epoxy grout such as Conbextra EP 150 is required
• Performance depends on accurate water measurement; over-watering to ease flow reduces strength and can cause bleed
• Formwork must be genuinely leakproof because the grout is free-flowing and will find any gap
• Cold conditions slow strength gain, so below 10 degrees C the mix should be kept flowable and formwork left in place for at least 36 hours
• Shelf life is only about 6 months, and high heat or humidity in storage shortens it further

Selection & best practice

Specifications

Industry use cases

How we position Fosroc Conbextra HF

Conbextra HF sits in the middle of Fosroc’s precision-grout range and is the one to reach for when flow and reach matter most. For routine base-plate grouting in gaps of 10-100 mm where extreme flow is not needed, Conbextra GP is the more economical general-purpose choice. When the gap is deep or the loading is severe, dynamic or chemically aggressive, step up to Fosroc Conbextra EP 150, a solvent-free epoxy grout that fills 10-150 mm and shrugs off oils and chemicals that would attack a cement-based grout. For very deep pours, pair Conbextra HF with Conbextra Grout Aggregate or specify Conbextra Deep Pour. Cure exposed surfaces with Fosroc Concure. Across our other brands, comparable non-shrink precision grouts are available from MC-Bauchemie (Emcekrete range) and Sika (SikaGrout), and Space Arc can advise on the right fit where a project standard calls for a specific manufacturer. As an authorised Fosroc distributor and applicator serving Delhi NCR and Pan-India, we can supply, estimate and support the full grouting system rather than just the bag.

Related Grouts & Anchors products

View all Grouts & Anchors products →