Frequently Asked Questions

What is the difference between Conbextra GP2, Conbextra HES, and Conbextra HS and how should the correct grade be selected for industrial grouting?

The three main grades in the Fosroc Conbextra cementitious precision grout range are designed for different combinations of strength level, early strength development rate, and application requirements. Conbextra GP2 is the general-purpose grade providing approximately 40 to 50 MPa compressive strength at 28 days and is suitable for a wide range of standard machinery, pump, motor, and equipment grouting applications. It is the most commonly specified and stocked grade, appropriate for the majority of industrial and commercial grouting applications where the machinery manufacturer specifies a grout compressive strength of 30 to 50 MPa. Conbextra HES (High Early Strength) provides similar or slightly lower 28-day strength to GP2, but achieves a much higher proportion of its final strength at 1 to 3 days — achieving 20 to 25 MPa within 24 hours — making it the appropriate selection where plant shutdown time is very limited and the machinery must be recommissioned quickly after grouting. Conbextra HS is specified when the 28-day compressive strength requirement from the machinery manufacturer or foundation engineer exceeds 50 MPa — typically for heavy rotating machinery with very high dynamic loads, bridge bearings with high bearing pressure, or other applications where a standard general-purpose grout does not meet the specified strength class. For a given project, the grout selection should be based on: the compressive strength specified by the machinery manufacturer or structural engineer; the time available from grouting to recommissioning; and any special requirements for chemical resistance, pour depth, or construction temperature.

What is the maximum depth of pour for Conbextra HS in a single application under a large baseplate?

Conbextra HS and the Conbextra cementitious grout range are designed for grouting applications where the grout pour depth (the clearance between the concrete foundation surface and the underside of the baseplate) is in the range of 25 to 100 mm for typical machinery grouting applications. For baseplates with a clearance of less than 25 mm (very thin underplate), the grout must be sufficiently fluid to flow into the narrow gap without segregation — Conbextra HS at the correct water-to-powder ratio (following the Fosroc technical data sheet recommendation) is typically suitable for clearances down to approximately 15 to 25 mm, depending on the baseplate width. For baseplate clearances greater than 100 mm (deep foundation pockets in industrial floors, bridge bearing recesses), the grout volume is large and the exothermic heat of cement hydration in a large pour volume may cause temperature rise and potential cracking — for deep pours above 100 mm, the use of approved sized aggregate blended into the Conbextra HS (a practice called extended grout with aggregate, following Fosroc guidelines for the specific product) reduces the grout temperature and volume of powder needed without sacrificing the strength or non-shrink properties. The Fosroc technical data sheet for Conbextra HS provides specific guidance on the maximum single-layer pour depth and on the aggregate extension procedure for deep pours.

How does the non-shrink mechanism in Conbextra HS work and does the non-shrink behaviour persist throughout the service life?

The non-shrink mechanism in Conbextra HS (and in the Conbextra precision grout range generally) is a controlled expansion system that compensates for the drying shrinkage that occurs in all cement-based products as the mix water evaporates from the hardened grout during curing. The expansion mechanism is typically a gas-generating chemical reaction (such as the oxidation of aluminium powder in an alkaline cement environment, releasing hydrogen gas) that produces micro-bubbles in the fresh grout during the early plastic phase of curing. These micro-bubbles cause a small, controlled volumetric expansion of the grout while it is still plastic and is in contact with the underside of the baseplate — the expansion pushes the grout upward against the plate, maintaining positive contact at the grout-to-plate interface as the initial cement hydration proceeds. The expansion is calibrated to approximately offset the subsequent drying shrinkage as the grout cures and dries, so that the net volumetric change of the cured grout is close to zero or marginally expansive. This expansion mechanism is a chemical process that occurs during the plastic and early hydration phase only — once the grout has hardened, the gas-generating reaction is complete and there is no further expansion in service. The long-term dimensional stability of the cured grout is then determined by the creep and thermal properties of the hardened high-strength cementitious matrix, which has similar long-term behaviour to high-strength concrete.