Frequently Asked Questions

What is the difference between using Renderoc BS and using Nitobond SBR as the bonding agent before Renderoc repair mortar?

Both Renderoc BS and Nitobond SBR are bonding agents specified for use before Renderoc cementitious repair mortars, but they differ in composition and the level of bond strength and chemical resistance they provide. Renderoc BS is a polymer-modified cementitious bonding slurry — it is primarily cement-based, with a polymer modification to improve adhesion, and provides a bond strength appropriate for standard concrete repair applications. Nitobond SBR is a styrene-butadiene rubber latex used either diluted with cement slurry (as a bonding coat) or added to the gauging water of the repair mortar itself (as an admixture to improve mortar properties). Nitobond SBR typically provides marginally higher bond strength than Renderoc BS (because the SBR polymer content is higher), and it is more commonly specified for structural concrete repair where the highest bond is required and where the repair mortar must resist tensile and shear forces at the interface. In practice, for standard routine concrete repair (non-structural, decorative, or cosmetic repair of surface defects), Renderoc BS as the bonding agent is perfectly adequate and is the system recommended by Fosroc for use as part of the standard Renderoc repair system. For structural repair of beams, columns, bridge decks, and other load-carrying elements, Nitobond SBR (or Nitobond EP for the highest structural bond requirement) is often preferred by structural engineers. The two products can also be used in combination in demanding repair specifications.

How long can Renderoc BS remain on the substrate before the repair mortar must be applied?

Renderoc BS must be applied as a wet-on-wet bonding coat — the Renderoc repair mortar must be applied while the Renderoc BS slurry is still in the wet, tacky state. This is the critical application constraint that must be understood and managed on site: if the Renderoc BS is applied and then allowed to fully dry before the repair mortar is applied, the dried bonding slurry no longer provides the adhesion function — it becomes a thin, dry, brittle cementitious layer that can reduce bond rather than improve it, and in a worst case can act as a debonding layer between the substrate and the repair mortar. The working time of Renderoc BS (the period during which the slurry is still in the wet, adhesion-providing state) is approximately 15 to 30 minutes at 25 degrees Celsius — shorter in hot Indian summer conditions (10 to 20 minutes at 35 degrees Celsius) and longer in cool or humid conditions. The practical implication is that the area of substrate primed with Renderoc BS in a single batch must be limited to what can be covered with repair mortar within the available working time — a common site mistake is to apply Renderoc BS over a large area and then fail to keep up with the mortar application, leaving part of the bonding coat to dry before mortar is applied. Work in smaller sections of 2 to 5 square metres to ensure wet-on-wet application discipline is maintained throughout the repair.

Can Renderoc BS be used as a bonding agent for a cementitious self-levelling topping screed over an existing concrete floor?

Yes — Renderoc BS is suitable as a bonding primer for cementitious repair and levelling mortar applications on floor substrates, not only for vertical and overhead concrete repair. For a cementitious self-levelling topping or screed overlay on an existing concrete floor, the procedure is: prepare the concrete surface by shot blasting or mechanical grinding to remove laitance, contamination, and weak concrete, and to open the surface pores to provide a mechanical key for the bonding slurry; vacuum clean the prepared surface; apply Renderoc BS slurry by brush or squeegee to the prepared surface, working it into the surface texture; and immediately apply the Renderoc repair mortar or cementitious self-levelling screed while the Renderoc BS remains wet and tacky. This wet-on-wet application protocol is particularly important for floor applications where the self-levelling screed is poured and flows across a large area — the pour must be organised so that the Renderoc BS is applied and the screed is poured within the available working time, working in strips across the floor if needed to manage the timing. For floor topping applications requiring a higher-strength or more chemically resistant bonding agent than Renderoc BS provides — such as an epoxy floor topping over a heavily trafficked industrial floor — the two-component epoxy Nitobond EP is the preferred specification for maximum bond strength and chemical resistance at the overlay interface.