Fosroc Conbextra HS
Dual shrinkage-compensated, ultra high-strength cementitious precision grout for heavy machinery baseplates and structural gaps from 10 to 125 mm.
What is it?
A pre-blended, non-shrink cementitious precision grout built around ultra-high compressive strength and a non-metallic dual expansion system.
Where is it used?
Under baseplates and soleplates of large machines, crane rail soleplates, columns and structural members, and bearing pockets carrying heavy static and moderate dynamic loads.
Why use it?
It combines rapid strength gain with very high ultimate strength and controlled expansion, so plant can be returned to service quickly without voids or settlement under the base.
How is it applied?
Mixed with a measured quantity of clean water using a mechanical paddle mixer, then poured or pumped continuously into the gap from one side to displace air.
About Fosroc Conbextra HS
Fosroc Conbextra HS is the heavy-duty member of the Conbextra cementitious grout family, formulated for situations where ordinary general-purpose grouts simply do not carry enough load. It is a factory-blended powder of Portland cements, graded fillers and chemical additives that, once gauged with a controlled amount of clean water, becomes a free-flowing, self-consolidating grout able to fill gaps from 10 mm up to 125 mm in a single pour. The low water demand is deliberate: it drives high early strength and long-term durability while keeping bleed and segregation in check across a range of flow consistencies. For engineers commissioning turbines, compressors, presses or crane rails, that translates into a bedding layer that transfers load cleanly from the machine into the foundation with full contact and no soft spots.
What distinguishes Conbextra HS from a standard grout is its dual shrinkage-compensation system. A non-metallic, hydrogen-free gaseous expansion counters plastic settlement while the mix is still fresh, and a separate hardened-state mechanism offsets drying shrinkage as the grout cures. The result is a grout that stays in intimate contact with both the baseplate underside and the concrete below throughout its service life. It is chloride free, which makes it safe around embedded steel and reinforcement, and its low permeability protects the hardened mass against the aggressive site conditions common on Indian industrial and infrastructure projects.
The technical insight
The performance of Conbextra HS rests on two independent expansion mechanisms working at different stages. In the plastic phase, a controlled gaseous reaction generates an expansion of up to around 2 percent, which lifts the grout against the underside of the baseplate and overcomes the settlement and bleed water channels that would otherwise leave voids at the very interface where load transfer matters most. Once the grout stiffens, a second hardened-state system compensates for the drying shrinkage that all cementitious materials undergo, so the cured grout does not pull away from its edges. Because the two systems act in sequence rather than together, the grout achieves effectively neutral or slightly positive volume change from placement through to full cure. Layered on top of this is a carefully graded filler system that minimises segregation and lets the same product be gauged anywhere from a flowable to a fluid consistency without losing its non-shrink behaviour. The low water-to-powder ratio then delivers the dense, low-permeability matrix responsible for both the high ultimate strength and the durability of the hardened grout.
Where it is used
- Grouting baseplates and soleplates of large machinery subject to moderate dynamic loads
- Bedding turbine, compressor and pump foundations where rapid return to service is required
- Crane rail soleplates carrying heavy repetitive loads
- Structural steel column baseplates and building members
- Grouting beneath bridge bearings, parapet posts and flanged lighting columns
- Fixing holding-down bolts and anchors in pockets requiring high strength
- Deep-gap grouting from 10 mm up to 125 mm in a single controlled pour
- High-load industrial presses, grinding and rolling equipment bases
Key advantages
- Ultra-high ultimate compressive strength for heavily loaded and dynamically stressed bases
- Rapid early strength gain that allows plant to be commissioned sooner
- Dual shrinkage compensation eliminates voids in both plastic and hardened states
- Free-flowing and self-levelling, so it fills under baseplates without vibration
- Can be poured or pumped over a wide range of consistencies
- Chloride free, making it safe around reinforcement and structural steel
- Low permeability gives long-term durability in aggressive environments
- Factory-blended single powder, so only clean water and correct mixing are needed on site
Limitations & considerations
- Single-pour gap thickness is limited to roughly 10-125 mm; deeper or larger-volume pours need an approved aggregate-extension procedure
- Water addition must be strictly controlled, as excess water sharply reduces strength and expansion performance
- As a cementitious grout it is not intended for chemically aggressive exposure or oil-contaminated bases where an epoxy grout is more appropriate
- Continuous, uninterrupted placement from one side is essential; cold joints and trapped air compromise contact
- Very high and very low ambient temperatures affect flow, working time and strength gain and call for hot- or cold-weather precautions
- Formwork must be leak-tight and the substrate saturated surface-dry, or flow and bond suffer
Selection & best practice
Success with Conbextra HS is largely a matter of preparation and discipline. Chip the concrete surface to sound, clean substrate, remove all laitance, oil and debris, and keep the concrete saturated for several hours before grouting so it is saturated surface-dry at the time of the pour. Build leak-tight formwork with enough head and a pour box on one side, and provide vents or an escape path on the far side so displaced air can leave. Gauge only the specified quantity of clean water and mix mechanically with a slow-speed paddle to a uniform, lump-free consistency; never re-temper with extra water to restore flow. Pour or pump continuously from one side only, letting the grout advance across the gap and displace air rather than trapping it, and maintain a constant hydrostatic head until the far edge is reached. Once placed, cure the exposed shoulders promptly with wet hessian or a curing membrane to protect against rapid drying. In Indian summer conditions, keep materials and mixing water cool and grout during the cooler part of the day to preserve flow and control strength gain.
Specifications
| Product type | Dual shrinkage-compensated, ultra high-strength cementitious precision grout |
| Composition | Blend of Portland cements, graded fillers and chemical additives |
| Gap thickness (single pour) | 10 mm to 125 mm |
| Compressive strength (flowable, ~30 C) | Approx. 26 MPa at 8 hrs, 60 MPa at 1 day, 90 MPa at 7 days, ~100 MPa at 28 days |
| Expansion system | Non-metallic, hydrogen-free gaseous expansion up to ~2% in plastic state, plus hardened-state compensation |
| Non-shrink compliance | Non-shrink to ASTM C1107 |
| Chloride content | Chloride free |
| Setting time | Approx. 2.5 hrs initial, 5.0 hrs final |
| Placement | Poured or pumped; maximum aggregate size for pumping 1.0 mm |
Industry use cases
Power & Process Plants
Beds turbines, compressors and heavy pumps where high ultimate strength and quick recommissioning are non-negotiable.
Heavy Manufacturing
Grouts presses, rolling mills and grinding equipment baseplates that carry sustained dynamic loads.
Ports & Material Handling
Fixes crane rail soleplates and gantry bases subjected to heavy, repetitive wheel loads.
Bridges & Infrastructure
Grouts beneath bridge bearings, parapet posts and flanged lighting columns needing durable, non-shrink support.
Structural Steel Fabrication
Provides full-contact bedding under column baseplates and holding-down bolt pockets in steel-framed buildings.
Frequently asked questions
What compressive strength does Conbextra HS achieve?
How is Conbextra HS different from Conbextra GP2?
What gap thickness can it fill in one pour?
Can it be pumped, or only poured?
Is it safe to use around reinforcement and steel baseplates?
When should I choose an epoxy grout instead?
How we position Fosroc Conbextra HS
Within the Fosroc Conbextra grout family, Conbextra HS sits at the high-strength end. For routine machine bases, anchor fixing and everyday non-shrink grouting, our Fosroc Conbextra GP2 (fosroc-conbextra-gp2) reaching around 65 N/mm2 is the workhorse and usually the more economical choice. Step up to Conbextra HS when the base carries heavy or dynamic loads or when rapid recommissioning demands very high early and ultimate strength. Where the exposure is chemically aggressive, oil-contaminated, or subject to severe impact and vibration, move off cementitious grouts entirely to our Fosroc Conbextra EP 150 (fosroc-conbextra-ep) epoxy grout, which handles deep base-plate gaps under the harshest conditions. Space Arc Engineering supplies the full Conbextra range along with complementary Fosroc solutions such as Nitobond bonding agents and Renderoc repair mortars, so we can match the exact grade to your loading, gap and site environment rather than over- or under-specifying.
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