Frequently Asked Questions

What is the difference between using Nitobond AR and Nitobond EP as a bonding agent for concrete repair and when is each appropriate?

Nitobond AR and Nitobond EP are both Fosroc bonding agents for improving the adhesion of repair mortars to existing concrete, but they are fundamentally different in chemistry, performance level, and appropriate application scope. Nitobond EP is a two-component solvent-free epoxy bonding agent that cures to a rigid, high-strength, chemically cross-linked polymer film — the epoxy chemistry provides adhesion of 2 to 4 MPa or higher on well-prepared concrete (exceeding the tensile strength of normal concrete), and it is the standard specified bonding agent for all structural concrete repairs, overhead repairs, and any repair where the bond must meet minimum pull-off adhesion requirements. Nitobond AR is a single-component acrylic-SBR polymer emulsion that forms a softer, more flexible polymer-cement film when mixed with cement as a slurry — the adhesion improvement it provides over a plain cement slurry is significant (typically 0.5 to 1.5 MPa pull-off on prepared concrete), but this is below the performance level of Nitobond EP. Nitobond AR is the appropriate choice when: the application is non-structural (rendering, plastering, floor screed, cosmetic concrete repair) where bond strength of 0.5 to 1.5 MPa is adequate; the surface preparation is limited (painted surface, lightly cleaned old concrete where abrasive blasting was not practical) and the cost and time of epoxy bonding agent application is not justified; the budget does not support epoxy-grade products for the entire repair scope; or Nitobond AR is used as a polymer admixture in the repair mortar mix (as a gauging water replacement) rather than as a surface-applied bonding agent, where it modifies the mortar properties rather than creating an adhesive interface. Structural repairs, repairs with a minimum pull-off bond specification (typically 1.5 to 2.0 MPa minimum per IS repair specifications), overhead repairs, and any repair in a water-retaining structure require Nitobond EP — Nitobond AR is not a substitute for Nitobond EP in these applications.

How is Nitobond AR mixed with cement to form a bonding slurry and what is the correct consistency for brush application?

Making a Nitobond AR bonding slurry requires mixing the liquid polymer with Portland cement powder at the correct proportion to achieve a consistency that is fluid enough to brush into the substrate surface pores but viscous enough to stay on the surface as a coating rather than running off immediately. The basic mixing procedure is: pour a measured volume of Nitobond AR (undiluted, straight from the container — do not dilute with water for the bonding slurry application) into a clean bucket; slowly add OPC (ordinary Portland cement) powder while stirring continuously with a stick or slow-speed drill mixer until a smooth, lump-free slurry is formed. The target consistency for brush application is approximately that of thick cream or thin yogurt — the slurry should coat a brush and be brushable onto the surface without running off, but should be fluid enough to penetrate into the surface texture of the prepared concrete. The typical proportion for a bonding slurry is approximately 1 part Nitobond AR (by volume) to 1 to 1.5 parts OPC cement (by volume) — however, the exact proportion depends on the absorption of the substrate and the required consistency, and should be adjusted within this range to achieve the correct application consistency. After preparing the slurry, brush it onto the prepared concrete substrate with a stiff bristle brush, working the slurry into the surface profile to ensure good contact. The repair mortar must be applied while the Nitobond AR bonding slurry is still wet and tacky on the surface — if the slurry is allowed to dry before mortar application, the dried SBR film becomes a low-adhesion layer and the purpose of the bonding slurry is negated. The working time of the Nitobond AR slurry on the surface (before it dries) depends on temperature, substrate porosity, and air movement on site, but is typically 20 to 40 minutes in normal Indian ambient conditions.

Can Nitobond AR be added to a cement-sand screed mix for a floor screed in a new residential apartment and what benefit does it provide over a plain cement-sand screed?

Yes — adding Nitobond AR to a site-batched cement-sand screed mix for a floor screed in a residential apartment (or commercial building) is a practical and cost-effective way to improve the performance of the screed over a plain cement-sand mix without the cost and complexity of a pre-bagged polymer-modified screed product. The benefits of adding Nitobond AR to a floor screed are: improved adhesion to the concrete substrate (Nitobond AR in the screed mix replaces part of the gauging water — typically 10 to 20 per cent of the total water by volume — and the polymer component improves the chemical and mechanical bond between the screed and the base concrete, reducing the risk of screed debonding and hollow areas that are a common complaint in building floors); improved flexibility and reduced cracking (the SBR polymer in the cured screed provides a degree of flexibility that a plain cement-sand screed lacks, reducing the tendency to crack from drying shrinkage and thermal movement in the screed panel — this is particularly beneficial in large rooms where the screed area is large and restraint to shrinkage is high); and slightly improved surface hardness and abrasion resistance (the polymer modification densifies the surface zone of the screed). The practical application: replace 10 to 20 per cent of the gauging water in the cement-sand screed mix with Nitobond AR (for example, if the screed mix requires 10 litres of water per 50 kg cement, replace 1 to 2 litres with Nitobond AR); mix the screed to the standard consistency and apply at the required thickness. The cement and aggregate quantities remain unchanged. The water:cement ratio should be kept as low as practical (consistent with workability for laying) to maximise the screed compressive strength and minimise drying shrinkage cracking.