Frequently Asked Questions

How is rebar prepared before Ferroseal application and what standard of steel preparation is required?

The quality of steel preparation before Ferroseal application is critical to the performance of both the Ferroseal coating and the overall concrete repair — a poor preparation standard leaves corrosion products and chloride contamination on the steel surface that the Ferroseal coating cannot passivate through. The minimum steel preparation standard required before Ferroseal application is removal of all loose rust scale, active corrosion products, and chloride-contaminated rust layers to a clean, rust-free steel surface. The preferred method of preparation for structural repair is grit blasting to Sa 2.5 (near-white metal) standard as per ISO 8501-1, which removes all visible mill scale, rust, paint, and contamination, leaving the steel surface uniformly grey-white with only slight staining in pits. Where grit blasting equipment is not practical (confined areas, inaccessible locations), mechanical cleaning by wire brushing using an angle grinder-mounted wire cup or disc brush to St 3 standard (thorough mechanical wire brushing removing all loose rust and scale until the surface has a metallic sheen) is acceptable as a minimum. Hand wire brushing alone (St 2 standard) is not adequate for structural repair where chloride-induced corrosion is the cause of deterioration — the tightly adherent, chloride-rich rust layers cannot be removed by hand wire brushing and must be mechanically removed by power tools or grit blasting. After preparation, Ferroseal should be applied to the cleaned steel without delay to prevent re-rusting — typically within 4 hours in normal conditions, or immediately if humidity is high or the steel surface is likely to re-rust quickly.

Can Ferroseal be used on heavily corroded rebar where the cross-section has been significantly reduced, or is structural replacement needed?

Fosroc Ferroseal is a protective and passivating coating — it is not a structural repair material that can compensate for or restore lost reinforcement cross-section. Before applying Ferroseal to corroded rebar, the residual cross-section of the steel must be assessed and compared to the original design cross-section and the structural capacity required. The assessment involves measuring the remaining diameter of the rebar at the point of maximum corrosion (typically the cross-section loss can be estimated by visual inspection and measurement with calipers after cleaning, or by calculation from the depth of section loss measured from the bar surface to the deepest pitting). As a general structural guideline, rebar with cross-section loss up to approximately 15 to 20 per cent can typically be retained with protective coating and the design capacity can be maintained if the bar is continuous — but this threshold depends on the specific structural design and the engineer of record should confirm acceptability. Rebar with cross-section loss exceeding 25 to 30 per cent typically requires structural intervention: either supplementary new bars lapped or welded alongside the corroded bar to restore capacity, or complete replacement of the corroded length by cutting out and splicing in new bar of equivalent specification. Ferroseal is applied after the structural assessment has confirmed that the retained bars are adequate for the design loads, and after structural replacement (where required) is complete.

What is the typical sequence of operations in a full structural concrete repair using Fosroc products including Ferroseal?

The typical full concrete repair sequence using Fosroc products is a multi-step procedure that follows a logical progression from diagnosis through to final protective coating. First, the deteriorated or spalled concrete is mapped and the repair boundaries marked out — the repair area should be saw-cut at a minimum depth of 10 to 15 mm around the perimeter to avoid featheredging of the repair mortar and to provide a mechanical key. Second, the concrete within the repair boundary is broken out by chipping or hydrodemolition to expose the corroded rebar and to remove all contaminated and carbonated concrete to a minimum 15 mm behind the rebar (full breakout depth typically 30 to 50 mm for shallow spalling repairs). Third, the exposed rebar is cleaned by power wire brushing or grit blasting to the specified preparation standard (Sa 2.5 preferred, St 3 minimum). Fourth, Ferroseal is applied by brush to the cleaned rebar and adjacent concrete surfaces in two coats, each coat allowed to dry to a firm touch before the next. Fifth, a bonding bridge coat (Nitobond SBR or Nitobond AR diluted with cement slurry) is applied to the prepared concrete repair surface and the repair mortar (Renderoc S, Renderoc DS, or other appropriate grade) is applied while the bonding bridge is still wet and tacky — the repair mortar is packed, trowelled, and finished to match the surrounding concrete profile. Sixth, after the repair mortar has cured, a protective anti-carbonation coating (Dekguard S or Dekguard W) is applied to the whole structure including repair patches to provide a unified long-term protective system.