Fosroc Conbextra AT
Non-shrink, free-flow, extra-high-strength cementitious grout for turbo generators, crane rails, rolling mill beds and heavily loaded base plates — around 40 N/mm² within a day, 80 N/mm² at 28 days.
What is it?
A single-pack, extra-high-strength cementitious grout from Fosroc (formerly sold as Conbextra Spl). Add 4 litres of water to a 25 kg bag and it becomes a free-flowing, non-shrink grout with no bleed or segregation.
Where is it used?
Under base plates and in bolt pockets of turbo generators, DG sets, compressors, pump sets, crane and transporter rails, rolling mill beds and similar plant carrying high static and dynamic loads.
Why use it?
Roughly 40 N/mm² in 24 hours and about 80 N/mm² at 28 days, with controlled expansion that keeps the grout in full contact with the plate — so operational loads transfer cleanly into the foundation.
How is it applied?
Mixed mechanically at a 0.16 water-powder ratio, then poured or pumped from one side into clean, pre-wetted, leak-proof shuttering within 30 minutes of mixing. Wet cure for at least 7 days.
About Fosroc Conbextra AT
Conbextra AT is the heavy-duty member of Fosroc’s cementitious grout range, engineered for the space between a machine base plate and its foundation — the few centimetres that carry every tonne of static weight and every cycle of vibration the equipment produces. It is supplied as a ready-to-use dry powder: a factory blend of specially processed cement, pre-graded fillers and additives. With only clean water added, it produces a free-flowing grout that fills bolt pockets and plate voids completely, without rodding, poking or chaining, and then holds that filled volume as it hardens.
Two things separate it from general-purpose grouts. First, strength: at a water-powder ratio of 0.16 it reaches around 40 N/mm² within a day and about 80 N/mm² at 28 days, so turbines, compressors and mill drives can be aligned and re-commissioned quickly. Second, cleanliness of chemistry: the formulation is iron-free and chloride-free, so there is no rust expansion, no staining, and no corrosion risk to anchor bolts or machined surfaces in contact with the grout. Space Arc Engineering supplies Conbextra AT across Delhi NCR and Pan-India, with trained applicators for plant and infrastructure grouting works.
The technical insight
The performance comes from two mechanisms working in sequence. The very low water demand — just 4 litres per 25 kg bag — keeps the water-powder ratio at 0.16, which is what drives both the early strength gain and the dense, low-permeability hardened matrix. Then, about 20 minutes after mixing, a controlled expansion phase begins in the still-plastic grout and completes over roughly the next 150 minutes. Confined under the base plate, this expansion (up to 4% unrestrained, needing only about 0.004 N/mm² to restrain) counteracts plastic settlement and shrinkage, pressing the grout against the underside of the plate so the effective bearing area stays high. Because the expansion system contains no metallic iron, there is no delayed rust-jacking. The set grout has a microcellular structure with useful frost, fire and oil resistance — relevant under machinery where lubricant spillage is routine.
Where it is used
- Base-plate grouting under turbo generators and diesel generating sets
- Crane and transporter rail grouting where wheel loads are high
- Bolt pockets and holding-down bolts for compressors, pump sets and other reciprocating machinery
- Steel rolling mill beds and metal shearing or processing machines
- Pulverising mills and plant machinery in cement, textile, granite and paper industries
- Load transfer beneath stanchion and equipment base plates subject to combined static and dynamic forces
- Bridge bearing and parapet post base-plate grouting
Key advantages
- Extra-high strength — around 40 N/mm² at 1 day and 80 N/mm² at 28 days (W/P 0.16), so machinery downtime stays short
- Controlled expansion keeps full contact with the base plate and fills voids without rodding, poking or chaining
- Genuine free-flow behaviour with no bleed or segregation — flows up to 3 m through a 50 mm gap under a 250 mm head
- Iron-free formulation — no rust expansion, corrosion or staining of the grout over time
- Chloride-free — will not corrode anchor bolts, machine parts or reinforcement in contact with it
- Factory pre-packed, so site batching and blending variations are eliminated
- Microcellular set structure with high frost, fire and oil resistance for plant environments
Limitations & considerations
- Water dosing is unforgiving: exactly 4 litres per 25 kg bag. Extra water causes bleed and a sharp strength loss
- Short working window — mixed grout must be placed within 30 minutes, and expansion starts about 20 minutes after mixing
- Nothing may be blended in on site: no cement, sand, fibres or admixtures, and no vibrators in the placed grout
- Needs properly designed leak-proof shuttering, single-side pouring and at least 7 days of curing to deliver rated performance
- Being cementitious, it is not the right choice where the plinth sees aggressive chemical attack or the most severe vibration — an epoxy grout such as Conbextra EP10 suits those cases better
Selection & best practice
Preparation decides the result. Clean the concrete and steel surfaces thoroughly — free of oil, laitance and debris — and build strong, leak-proof shuttering, since a free-flow grout will find any gap the carpentry leaves. Measure the 4 litres of water per bag precisely and mix mechanically; hand mixing cannot develop the flow without tempting the crew to add water. Pour from one side only so air escapes ahead of the grout front — pouring from two sides traps voids under the plate, which defeats the purpose of a bearing grout. Never pour from a height and never vibrate the grout. Place within 30 minutes of mixing so the expansion phase happens inside the formwork, where it does its work. Once the grout is touch-hard — typically 4 to 6 hours — begin wet curing and continue for at least 7 days. For deep pockets or unusual plate geometry, ask us for Fosroc’s detailed application procedure before pouring.
Specifications
| Compressive strength, 1 day | ~40 N/mm² (flowable, W/P 0.16) |
| Compressive strength, 28 days | ~80 N/mm² (restrained 70.6 mm cubes, 27 °C) |
| Tensile strength, 28 days | 3.5 N/mm² typical |
| Young’s modulus | 28 kN/mm² |
| Water addition | 4 litres per 25 kg bag (W/P 0.16, flowable) |
| Yield | 12.5 litres per 25 kg bag (approx. 80 bags/m³) |
| Fresh wet density | Approx. 2200 kg/m³ at flowable consistency |
| Expansion | Starts ~20 min after mixing, completes over next 150 min; up to 4% unrestrained |
| Pack size / shelf life | 25 kg bag; 6 months unopened in cool, dry storage |
Industry use cases
Power & Energy
TG decks and DG set foundations, where the ~40 N/mm² one-day strength shortens shutdown and erection windows and the chloride-free mix protects holding-down bolts.
Steel & Heavy Engineering
Rolling mill beds, shearing and processing machines — repetitive impact and dynamic loads demand the full-contact bearing that controlled expansion provides.
Cement & Process Plants
Pulverising mills, mill drives and machinery bases in cement, paper, textile and granite plants, where oil resistance and factory-controlled consistency matter.
Ports & Material Handling
Crane and transporter rail grouting under high wheel loads, using the grout’s long flow distances for continuous rail runs.
Bridges & Infrastructure
Bridge bearing plates and parapet post bases, where a void-free, non-shrink bed is essential for even load transfer.
Frequently asked questions
How much water goes into one bag of Conbextra AT?
How soon can the machine be re-commissioned after grouting?
How many bags do I need?
When should I pick Conbextra AT over Conbextra GP2?
What gap widths and flow distances can it handle?
Can I bulk it with sand or aggregate for deep pours?
How we position Fosroc Conbextra AT
Conbextra AT sits at the top of Fosroc’s cementitious precision-grout line. For everyday column bases and light equipment where 80 N/mm² is more than the job needs, Conbextra GP1 and Conbextra GP2 do the work at a lower cost per bag. Where the plinth faces chemical spillage or vibration severe enough to fatigue a cementitious grout, the epoxy-based Conbextra EP10 is the correct escalation. For site-batched duct and cable grouting, Fosroc’s Cebex 100 admixture is the usual specification rather than a pre-packed grout. Contractors standardised on other brands can source SikaGrout 214-11 from us as a comparable non-shrink option. Space Arc stocks the full range in Delhi NCR and can pair supply with an experienced grouting crew.
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