Frequently Asked Questions

Must Nitobond EP be applied wet-on-wet (repair mortar placed before the Nitobond EP cures) and what happens if the Nitobond EP dries before the mortar is placed?

Yes — the wet-on-wet application principle is the most critical aspect of Nitobond EP usage and the most common source of bonding failure when Nitobond EP is misused on site. Nitobond EP functions as a fresh-to-fresh adhesive bond: the repair mortar aggregate and cement particles embed into the fresh, uncured Nitobond EP surface and are gripped by the epoxy matrix as it cures around them, creating a mechanical interlocking bond within the chemical epoxy matrix. This wet-on-wet bond is extremely strong. However, if the Nitobond EP is applied and then allowed to fully cure before the repair mortar is placed (which happens when: Nitobond EP is applied in the morning and the repair mortar placed in the afternoon after lunch; when a large area of Nitobond EP is applied and the repair mortar placement crew cannot keep up with the application front; or when unexpected site delays occur after Nitobond EP application), the cured Nitobond EP surface becomes a smooth, hard, glossy epoxy layer with very low surface roughness — this surface is actually a very poor bonding substrate for cementitious repair mortar, and the resulting mortar-to-epoxy bond is weaker than a well-prepared dry concrete-to-mortar bond would be. When Nitobond EP has fully cured before mortar placement, the correct remediation is to abrasive blast or grind the cured Nitobond EP surface to create a roughened profile with exposed aggregate and then re-apply fresh Nitobond EP immediately before re-attempting mortar placement. To manage this on site: apply Nitobond EP only to an area that can be covered with repair mortar within the pot life (30 to 45 minutes at 25 degrees Celsius) — work in small, controllable sections.

Is Nitobond EP necessary for all concrete repair jobs or can Renderoc repair mortars be applied directly to prepared concrete without a bonding agent?

Whether Nitobond EP is specified as a mandatory component of a concrete repair is a function of the criticality of the repair, the performance of the repair mortar adhesion required by the repair specification, and the nature of the repair. Many Fosroc Renderoc repair mortars (particularly polymer-modified grades such as Renderoc S and Renderoc HB) have integral polymer bonding admixtures that provide significant inherent adhesion to prepared, dry, and SSD (saturated surface dry) concrete substrates — in many routine non-structural repairs (cosmetic and protective repairs where the requirement is to restore the cover concrete profile for carbonation and chloride protection without structural load transfer), applying Renderoc S or Renderoc HB directly to well-prepared concrete without Nitobond EP is technically acceptable and is consistent with Fosroc application guidelines in non-critical conditions. Nitobond EP is mandated (not optional) in: all structural repairs where the bond interface must transfer load (shear, tension, or bending moment through the repair-to-substrate interface); overhead and vertical repairs where the dead weight of the repair mortar imposes tension on the bond interface; repairs where the pull-off bond strength of 2 MPa or greater is specified in the repair specification or contract; and in any repair where previous repairs have failed at the bond interface, indicating that the substrate preparation and inherent mortar adhesion alone were insufficient in that specific condition. In summary: for non-structural, moderate-size, vertical or horizontal repairs in good preparation conditions, direct application of Renderoc repair mortar may be acceptable per Fosroc guidance. For structural repairs, overhead repairs, or any repair where bond strength is a contractual requirement, Nitobond EP is essential.

Can Nitobond EP be used at construction joints in new concrete work (not just repair) and what are the requirements for the cured concrete substrate before application?

Yes — Nitobond EP is extensively used in new construction as a construction joint bonding agent at horizontal and inclined joints between successive concrete pours, where the bond between the first-pour concrete and the second-pour concrete must develop the full shear and tensile strength required by the structural design. In reinforced concrete construction, construction joints are inherently weaker than monolithic concrete because: the interface between the two concrete pours is a plane of potential weakness (concrete-to-concrete rather than concrete-to-aggregate-to-concrete as in monolithic concrete); the first-pour concrete has a surface laitance layer (weak cement-rich skin) that must be removed before the second pour; and the chemical shrinkage of the second-pour concrete during hydration causes interfacial stress that can open the construction joint slightly if the bond is not strong enough to resist it. To use Nitobond EP at a construction joint in new work: the first-pour concrete must cure to at least 16 to 24 hours of age (early application before adequate early strength development can cause cracking of the substrate during Nitobond EP application); the surface laitance must be removed by green-cutting (water jetting at 12 to 24 hours after placing, before the laitance fully hardens) or by wire brushing, grinding, or sand blasting after full cure; the surface must be clean, dust-free, and at surface dry (no free water standing on the surface); Nitobond EP is then applied to the prepared first-pour surface and the second-pour concrete or Renderoc mortar is placed while the Nitobond EP is still in the wet, tacky state. The resulting construction joint bond routinely achieves tensile adhesion of 3 to 5 MPa in good conditions — exceeding the tensile strength of the parent concrete.