Concrete Bonding Agents & Primers

A concrete bonding agent is a chemical primer applied to an existing concrete surface to ensure that freshly placed cementitious material bonds to it as a single composite structure. Without a bonding agent, new concrete, mortar or screed placed over old cured concrete has no reliable means of adhesion. The cement in a fresh mix contains no natural bonding properties of its own, so once cured, a repair or overlay placed without preparation will simply sit on top of the substrate as a separate layer, subject to movement, delamination and water ingress at the interface.

Concrete bonding agents work by penetrating and keying into the pores of the prepared surface, creating a chemically active interface that the incoming cementitious material can grip. Depending on the chemistry, they may also improve the durability and flexibility of the repair zone, reduce permeability at the bond line, and in some formulations act as a barrier to chloride migration. The result is a repair or overlay that performs as part of the original structure, not as an add-on sitting above it.

Why new concrete won't bond to old without a primer

The fundamental problem with concrete-to-concrete adhesion is that cured concrete is a dense, low-porosity substrate with very little residual chemical activity at its surface. New concrete or mortar placed directly onto this surface without preparation will not bond reliably, regardless of how well it is mixed or compacted. The two layers will cure and shrink independently, and any differential movement, moisture ingress or thermal cycling will open up the interface over time.

Surface preparation addresses part of this problem. Removing laitance, dust, oil contamination and loose material by mechanical preparation gives the new material a roughened profile to key into. But mechanical preparation alone is rarely sufficient for a durable repair. A concrete bonding agent applied to the prepared substrate provides the chemical bridge that mechanical roughening cannot, ensuring that the new cementitious material bonds at the molecular level rather than simply resting against a textured surface. Does concrete bonding agent really work? Applied correctly to a well-prepared substrate, yes, with bond strengths that in the case of epoxy formulations consistently exceed the tensile strength of the concrete itself.

SBR bonding agents

SBR, or styrene butadiene rubber, is a polymer emulsion bonding agent used widely in concrete repair, render and screed applications. Applied as a slurry primer, SBR is gauged with clean water and fresh cement and scrubbed firmly into the prepared substrate. The repair mortar, screed or render must then be applied onto the wet slurry before it has a chance to dry. SBR can also be incorporated directly into site-batched mortars and mixes to improve their cohesion, workability, durability and resistance to freeze-thaw cycling.

The key practical advantage of SBR over PVA-based bonding agents is moisture resistance. SBR does not re-emulsify on contact with water, making it suitable for external applications, renders, waterproof screeds and permanently immersed conditions. It bonds to concrete, masonry, stonework, plaster and blockwork, and can be applied to damp substrates where the surface is prepared but not saturated. Products in this range contain no chloride admixtures, which is a relevant consideration for reinforced concrete repair work.

Epoxy bonding agents for concrete

Epoxy resin bonding agents are two-part systems used where the demands of the application go beyond what a polymer emulsion primer can deliver. Where SBR is a general-purpose bonding agent across a wide range of construction and repair applications, epoxy products are specified for structurally demanding environments: loading bays, bridge decks, roads, industrial floors and any situation where the concrete will be subject to heavy trafficked loading or permanently immersed in water.

Two-part epoxy bonding agents are solvent-free and applied to dry or damp prepared surfaces, but not to surfaces with standing or running water. They provide excellent bond retention in permanently immersed conditions and act as an effective barrier to chloride migration, which makes them particularly relevant for concrete repair in aggressive or contaminated environments. The bond strength achieved by epoxy formulations consistently exceeds the tensile strength of the parent concrete, meaning that correctly specified and applied, the bond line will not be the point of failure. For horizontal and vertical surfaces where new cementitious mortars and screeds are being bonded to existing concrete, epoxy bonding agents represent the technically superior choice where conditions justify the specification.

Applying a concrete bonding agent

The single most important factor in a successful concrete bonding application is surface preparation. The existing concrete must be structurally sound, clean and free of dust, laitance, oil, grease and any loose or contaminated material. Pressure washing and mechanical preparation such as grinding or shot blasting are the standard methods. If the substrate is contaminated with oil or other penetrating materials, preparation needs to go deeper than the surface layer to reach clean concrete.

For SBR slurry primer applications, the prepared surface is typically pre-dampened but brought to a saturated surface-dry condition before the slurry is applied, with any standing water removed. The slurry is gauged to a workable consistency and scrubbed firmly into the substrate, and the repair mortar or overlay must be placed while the slurry remains wet and tacky. Allowing the slurry to dry before placing the new concrete is one of the most common causes of bond failure and should be avoided.

For epoxy bonding agent applications, the same substrate preparation standards apply. The two-part epoxy is mixed according to the manufacturer's instructions and applied to the prepared surface. There is a defined working window, which varies with temperature, within which the cementitious overlay must be placed. Working at the limits of this window in hot conditions reduces pot life significantly, so temperature and timing need to be managed on site.

Choosing the right bonding agent for concrete

Selecting the correct bonding agent depends primarily on the conditions the repair or overlay will face during its service life. For standard concrete repair mortars, renders, floor screeds and waterproof applications in normal site conditions, SBR polymer emulsions provide reliable adhesion across a broad range of substrates and can be applied to damp surfaces. They are suited to both internal and external use and to applications with permanent water contact.

Where the application involves heavy structural loading, industrial traffic, bridges or roads, or where chloride contamination of the substrate is a concern, an epoxy resin bonding agent is the more appropriate specification. The higher cost is offset by the superior bond strength, chloride barrier performance and long-term durability in aggressive environments.

In both cases, the bonding agent selected should be compatible with the repair mortar or cementitious overlay being placed. Manufacturer guidance on mix designs, overlay timing and application temperatures should always be followed, and the product datasheet reviewed before work begins.

Why choose Construction Supplies UK for concrete bonding agents and primers

Specialist Fosroc range: We stock a focused range of Fosroc concrete bonding agents and primers covering SBR polymer emulsions and two-part epoxy resin systems, chosen for their performance in demanding construction and repair environments.

Correct product for the application: SBR and epoxy bonding agents suit different conditions. Our team can help you confirm the right product for your substrate, repair system and service environment before you order.

Focused service: We supply directly to contractors, groundworkers and concrete repair specialists, with no minimum order value and free delivery on orders over £300 ex VAT.

Technical support available: Concrete repair is a specification-sensitive discipline. Call us on 03300 55 32 02 or email sales@constructionsuppliesuk.co.uk if you need guidance on product compatibility, mix designs or application conditions.