Frequently Asked Questions

How is Renderoc FC applied to repair a badly corroded RC column section with tightly spaced stirrups and what is the formwork requirement?

Repairing a heavily corroded reinforced concrete column section with tightly spaced stirrups using Renderoc FC requires a specific application approach that takes advantage of the flowing consistency to achieve the void-free fill that hand-applied mortars cannot achieve in congested sections. The application procedure is: concrete removal — all deteriorated concrete in the repair zone is chipped away using a chipping hammer, hydro-jetting, or angle grinder, breaking out all spalled, delaminated, and carbonated concrete and exposing all corroded reinforcement; the removal depth must extend at least 10 to 15 mm behind each stirrup and main bar on all sides to provide the annular space needed for the Renderoc FC to flow around the bars; reinforcement cleaning — all exposed bars are wire-brushed or abrasive-blasted to remove all corrosion products, then coated with Fosroc Nitoprime Zincrich (zinc-rich epoxy primer) in two coats to arrest corrosion; formwork — lightweight plywood or steel shuttering is fixed to the column faces on all sides (typically three sides with one face left temporarily open for inspection and Nitobond EP application), supported from the floor and tied to the sound concrete above and below the repair zone; all form joints are foam-taped to prevent Renderoc FC leakage; a small inspection hole is left at the top of the form on one face; Nitobond EP — the prepared concrete substrate inside the repair cavity is coated with Nitobond EP epoxy bonding agent applied by brush, ensuring all surfaces including around the reinforcement bars are coated; placement of Renderoc FC — the Renderoc FC is mixed at the flowing consistency (maximum water per bag within the specified range) and poured continuously into the open top of the form, filling the cavity from the bottom up; the material is observed rising at the inspection port to confirm filling is complete; a wooden striker rod (not a vibrator) can be gently pushed around the form to assist flow around bars without segregating the mix. Forms are stripped after 24 to 48 hours.

What water:powder ratio should be used for Renderoc FC for the best combination of flowing consistency and 28-day compressive strength?

The water:powder ratio for Renderoc FC involves an inherent trade-off between flowability and compressive strength — adding more water increases the fluidity and improves the self-compacting flow around reinforcement, but reduces the 28-day compressive strength and increases drying shrinkage. The practical target is to use the minimum water addition that provides sufficient flow to fill the specific repair cavity without voids — not the maximum water addition for maximum fluidity, which sacrifices strength unnecessarily. Renderoc FC is typically specified with a water addition range of approximately 4.0 to 5.0 litres per 25 kg bag, depending on the desired consistency: at the low end of the range (4.0 litres per 25 kg bag), the consistency is a thick, trowellable mortar suitable for hand application in standard vertical and overhead repairs where the flowing characteristic of FC is not the primary requirement; at the middle of the range (4.3 to 4.5 litres per 25 kg bag), the consistency is a flowing, pourable mortar that fills congested reinforcement cavities of moderate complexity under gravity; at the high end of the range (4.8 to 5.0 litres per 25 kg bag), the consistency is very fluid — suitable for the most congested reinforcement sections or for pressure-injected closed formwork applications where maximum flow is needed. At the flowing to very fluid consistency (4.5 to 5.0 litres per bag), the 28-day compressive strength is typically 40 to 45 MPa — lower than the 45 to 50 MPa achieved at the stiffer consistency, but still above most repair specification minimum strength requirements of 30 to 40 MPa. The correct approach on site is to carry out a trial mix at the intended water addition before the repair pour, checking the flow table value or slump flow diameter to confirm the consistency is suitable for the repair configuration, and to cast three cubes from each trial mix for 28-day strength verification.

Can Renderoc FC be used as a substitute for Conbextra HF high-flow precision grout for machine base grouting and does it meet the precision grouting performance requirements?

Renderoc FC and Conbextra HF serve different primary applications and have meaningfully different performance characteristics — Renderoc FC is a flowing repair mortar for structural concrete repair, while Conbextra HF is a precision non-shrink grout specifically formulated and tested for machine base and equipment foundation grouting — and they should not be interchangeably substituted without understanding the performance differences. The critical difference for machine base grouting is the non-shrink performance: Conbextra HF contains a specific expansive agent (similar to Cebex 200) that provides a controlled pre-set expansion to compensate for bleed and settlement, ensuring the grout fills the full gap under the machine base plate at zero net shrinkage. This non-shrink characteristic is fundamental to precision grouting — if the grout settles or shrinks before setting, a void forms between the top of the grout and the underside of the base plate, preventing full bearing contact and uniform load distribution under the machine base. Renderoc FC does not have a specific non-shrink expanding agent in its formulation — it is a flowing repair mortar with the primary objective of flowing around congested reinforcement to fill repair cavities, and its formulation is optimised for this objective rather than for the zero-settlement grouting requirement of precision machinery foundation work. Using Renderoc FC for machine base grouting may result in settlement voids under the base plate from normal bleed and shrinkage of the cementitious mortar, which is precisely the defect that precision grouting is designed to prevent. For genuine precision machinery grouting applications, Conbextra GP2 (standard flow precision grout) or Conbextra HF (high flow precision grout) are the correct Fosroc products — Renderoc FC is not a substitute in this application.