Frequently Asked Questions

What is the maximum depth of a single pour of Fosroc Concrex and how are deep repairs carried out?

Fosroc Concrex, as an epoxy-based compound, generates exothermic heat during the curing reaction — the larger the volume of mixed material in a single application, the greater the heat generated, and in deep repairs this heat can cause the epoxy to cure too rapidly, leading to shrinkage cracking, thermal stress, and reduced strength. For epoxy repair compounds of the Concrex type, the maximum recommended depth of application in a single lift is typically 30 to 40 mm for a standard ambient temperature of 25 to 35 degrees Celsius — at this depth, the exothermic heat is managed within acceptable limits. For deeper repairs (holes, voids, or spalled areas deeper than 40 mm), a two-stage approach is recommended: fill the lower portion of the repair with Concrex, allow the first lift to cure to a solid state (typically 2 to 4 hours at 25 degrees Celsius), then apply the final lift of Concrex to complete the repair to the finished surface level. Alternatively, for deep repairs (50 to 150 mm depth), the extended Concrex system using clean, dry, pre-sized granite or quartz aggregate blended into the Concrex to extend the epoxy mortar and reduce the volume of resin per unit volume of fill is an option that reduces exothermic heat and material cost for deep void filling. The Fosroc technical data sheet specifies the aggregate grading and proportioning for extended Concrex mixes.

How should the concrete around a repair area be prepared before Concrex is applied?

The performance of a Fosroc Concrex repair depends critically on the quality of the substrate preparation — the epoxy mortar can only develop its full bond strength to a sound, clean, mechanical-key concrete surface. Surface preparation for Concrex follows the same principles as for all concrete repair systems but with particular emphasis on providing a rough profile for the epoxy to interlock with. The repair boundary should be saw-cut at a minimum depth of 10 mm around the perimeter of the repair area to provide a vertical or undercut edge — this prevents the thin feather edge of Concrex tapering to near-zero thickness at the repair perimeter, which would be a stress concentration point under traffic loading that could cause the repair edge to chip or crack. Within the saw-cut boundary, all weak, spalled, contaminated, and laitance-covered concrete is removed by chipping, grinding, or hydrodemolition to expose sound concrete throughout the repair footprint. The surface of the prepared concrete is then cleaned by vacuum to remove all dust and debris. Any oil, grease, or chemical contamination must be removed by solvent degreasing, mechanical grinding, or shot blasting — oil contamination is particularly damaging to epoxy adhesion and even a very thin residual oil layer will prevent the epoxy from bonding. The prepared surface should be dry for epoxy mortar application — unlike cementitious repair mortars, Concrex should not be applied to a wet substrate. Nitobond EP epoxy bonding agent may be applied to the prepared surface to further enhance the bond before Concrex is applied.

Can Fosroc Concrex be colour-matched to grey concrete for an inconspicuous floor repair?

Two-component epoxy repair compounds including Concrex are typically a natural tan, beige, or grey colour after curing, depending on the specific formulation and the hardener type. The cured colour of Concrex does not exactly match standard grey concrete — the epoxy cured colour tends to be slightly different in tone and texture from the surrounding concrete, and standard epoxy repair mortars are not manufactured in a range of concrete-matching colours. For critical appearance situations (such as architectural concrete repair in a visible public area), the colour difference between the Concrex repair and the surrounding concrete can be addressed by: applying a colour-matched cementitious skim coat or render over the cured Concrex repair (this provides the surface colour of the cement-based skim rather than the epoxy, and a skilled finisher can closely match the surrounding concrete texture and colour); or using a pigmented Renderoc micro-concrete or fine render (Renderoc LA or Renderoc FC) for the final surface layer over a Concrex structural repair base, where the Concrex provides the structural repair and the Renderoc provides the surface finish that matches the concrete appearance. For industrial concrete floor repairs where the floor will subsequently receive an epoxy coating (Nitoflor Epoxy SL or Nitoflor Hardtop), the colour of the Concrex repair under the coating is immaterial because the coating covers the entire floor surface including the repair area with a uniform colour.