Frequently Asked Questions

What is the key difference between Conbextra GP2 and Conbextra PM and how should a structural engineer decide which to specify for a given project?

Conbextra GP2 and Conbextra PM are both factory-produced, pre-bagged, non-shrink cementitious precision grouts from Fosroc, and both are suitable for the core precision grouting application — machine base plates, equipment foundations, anchor bolts, and column base plates. The principal difference is the polymer modification in Conbextra PM and the resulting performance enhancements this provides. Conbextra GP2 is the standard precision grout, optimised for the core performance requirements of machine base grouting: high compressive strength (typically 50 to 60 MPa at 28 days), non-shrink expansion, excellent flowability at grout consistency, and good adhesion to clean prepared substrates. For standard precision machinery grouting applications — turbines, pumps, compressors, CNC machines, industrial gear boxes — Conbextra GP2 is the standard specification and is the product most widely tested and documented for precision grouting performance. Conbextra PM is specified where the additional polymer modification attributes are required or beneficial: when the grout is exposed to chemical or water in service (chemical plant, water treatment, coastal industrial facility) and the enhanced chemical resistance and lower permeability of the polymer-modified grout will extend the service life of the grouted connection; when the grout is placed in a structural connection that is subject to dynamic loading, vibration, or impact where the crack resistance and toughness of the polymer-modified matrix better withstands cyclic stress; and when the grouted connection must bond strongly to both the concrete substrate and a steel element (such as a column base plate that must develop tensile capacity through the grout layer), and the enhanced adhesion of the polymer modification is required to develop this bond. In many standard grouting applications, either product would perform well — the specification of Conbextra PM over GP2 is justified by the specific aggressive condition or performance requirement that polymer modification addresses.

Can Conbextra PM be used for grouting precast concrete connection pockets and sleeves and what is the mixing and placement procedure?

Yes — Conbextra PM is well-suited to grouting precast concrete connection pockets, sleeves, and keyways because the combination of non-shrink performance (ensuring the grout fills the full void volume without settlement voids) and polymer modification (improving bond to both the precast concrete pocket walls and the steel bar or dowel being grouted) provides higher confidence in the structural performance of the precast connection than a plain cementitious grout without polymer modification. For grouting precast concrete connections: the pocket or sleeve must be clean, free of dust and oil, and the concrete surfaces must be dampened before grouting (SSD condition — damp but no free water) to prevent the dry concrete from absorbing the gauging water from the grout and causing rapid premature stiffening before the grout has consolidated around the bar; the Conbextra PM is mixed in a clean drum by adding the dry powder to the measured gauging water (not water to powder) and mixing with a slow-speed drill and paddle mixer to a lump-free, homogeneous consistency at the appropriate consistency for the application — grout consistency (high water addition end of the range) for poured or gravity-filled pockets, tighter consistency (lower water addition) for side-form or pressure-grouted sleeves. For vertical pockets (column base to foundation pocket connections), the grout is poured from the top; for horizontal sleeves (wall panel to floor connection), the grout is usually pressure-grouted through an injection port at the bottom of the sleeve until grout emerges from the vent port at the top. Conbextra PM at grout consistency is typically self-levelling and self-compacting in pockets up to 50 to 75 mm across; for larger pockets or deep pockets, external vibration or rodding may be needed to ensure full compaction without air voids.

What is the minimum ambient temperature for placing Conbextra PM grout in a North Indian winter and what precautions are needed for cold-weather grouting?

Conbextra PM (and indeed all cementitious grouting products) requires a minimum ambient and substrate temperature of 5 to 7 degrees Celsius for cement hydration to proceed at a sufficient rate for the grout to achieve adequate early strength development. Below approximately 5 degrees Celsius, cement hydration slows to a very low rate and the grout may not achieve sufficient strength to support any load within the expected 24 to 48 hours — at temperatures approaching 0 degrees Celsius, cement hydration effectively stops and the grout remains in a soft, weak state until temperatures rise again. In North Indian winter conditions (Delhi NCR, Ghaziabad, UP), ambient temperatures from December to February can fall to 4 to 8 degrees Celsius overnight and remain below 10 degrees Celsius for extended periods during cold spells. For Conbextra PM grouting during cold weather: mixing water should be warm (not hot) water — using warm water at 25 to 35 degrees Celsius to gauge the grout raises the mix temperature sufficiently to initiate cement hydration properly even when ambient temperature is cold; the machine base, base plate, or pocket being grouted should be warmed before grouting by covering with tarpaulins and heating the air underneath, or by using a heat gun to warm the steel base plate before installation — cold steel bases conduct heat rapidly from the fresh grout, chilling the grout interface and slowing cure; after grouting, the grouted area should be insulated against cold by covering with hessian and polythene sheeting to retain the heat of hydration within the grout for the critical first 24 to 48 hours; and grouting should not be carried out when the temperature is expected to fall below 5 degrees Celsius during the 24-hour period after placement. In very severe cold (below 4 degrees Celsius), grouting should be postponed or a heated enclosure erected over the grouting area.