Strength Vs Absorption

How does epoxy flooring bond to cement-based substrates?

Resafloor HB can bond successfully to concrete, sand-and-cement screed and suitable cementitious flow-applied screeds. However, these substrates do not all have the same strength, density or absorbency.

Reliable adhesion depends on more than simply establishing that a floor contains cement. The substrate must also be:

• Correctly identified

• Strong and securely bonded

• Sufficiently dry

• Free from contamination

• Mechanically prepared

• Open enough for the epoxy to wet the surface effectively

The important balance is between surface strength and absorbency. A strong, moderately absorbent surface generally provides the most dependable base. Very dense concrete may resist wetting, while very absorbent concrete may be too weak to support the coating.

What are the three main cement-based substrates?

Concrete

Concrete is normally the structural floor slab. It contains cement, sand and coarse aggregate and is generally stronger and thicker than a screed.

Depending on its mix, finishing method and condition, its surface may be:

• Soft and highly absorbent

• Strong with normal absorbency

• Very hard, dense and minimally absorbent

Coarse aggregate may become visible when concrete is mechanically prepared, although appearance alone should not be used to judge its strength.

Concrete is the preferred substrate for Resafloor HB when it is sound, dry and properly prepared.

Sand-and-cement screed

A sand-and-cement screed is normally laid over a structural slab to create the required level or finished floor height. It has a finer and more consistently granular appearance than concrete because it does not contain the same coarse aggregate.

Sand-and-cement screeds are often more absorbent than structural concrete. This can assist wetting and penetration, but only if the screed has adequate cohesive strength.

A weak or poorly compacted screed may:

• Scratch or crumble easily

• Produce loose sand or dust

• Contain weak, friable surface material

• Separate from the structural slab beneath it

• Break down within its own thickness

Resafloor HB may bond firmly to the top of a weak screed while the screed itself fails immediately below the bond line. The strength and attachment of the screed must therefore be assessed before coating.

Cementitious flow-applied screed

A flow-applied screed is pumped in a fluid state and spreads to produce a smooth, level surface. Cementitious versions can provide a suitable substrate for Resafloor HB after the necessary checks and preparation.

Typical characteristics include:

• A very flat and uniform surface

• A fine, smooth texture

• Few visible laying or trowel marks

• A possible weak layer of surface laitance

• Product-specific drying and preparation requirements

A smooth appearance does not confirm that a flowing screed is cement-based. Calcium sulphate, anhydrite and latex-containing smoothing materials require different assessments and should not automatically be treated as suitable cementitious substrates.

The original product specification should be checked wherever possible. If the material cannot be identified confidently, it should be investigated before applying Resafloor HB.

How does concrete density affect epoxy bonding?

Concrete varies considerably. Two slabs may both appear strong but interact very differently with a liquid epoxy coating.

For practical assessment, concrete can be divided into three common surface conditions.

Soft and highly absorbent concrete

Soft concrete normally has an open, porous surface that absorbs liquid quickly. Water placed on the floor may darken and disappear rapidly rather than remaining as a bead.

Other warning signs can include:

• Dust forming under abrasion

• Easy scratching or scoring

• Loose grains becoming detached

• Rapid and uneven darkening when wetted

• A friable surface after mechanical preparation

Its high absorbency may initially appear helpful because the resin can penetrate readily. However, adhesion is only as reliable as the concrete supporting it.

If the surface has low cohesive strength, Resafloor HB may bond to the uppermost concrete particles while that weak layer separates from the stronger material below. This is surface delamination within the substrate, rather than failure of the epoxy bond itself.

Soft concrete must therefore be assessed for strength. Weak surface material should not simply be coated because it absorbs resin well.

Sound concrete with good absorbency

The most favourable concrete is strong enough to resist scratching and abrasion but sufficiently open to allow the resin to wet the mechanically prepared surface.

It will often:

• Darken reasonably evenly when wetted

• Absorb moisture without immediately becoming saturated

• Resist crumbling or excessive dusting

• Remain firm after mechanical preparation

• Produce a clean, textured surface

This provides the best balance between strength and absorption. The prepared concrete is strong enough to support the coating while its open texture helps the epoxy establish intimate contact and a mechanical key.

For a conventional cement-based floor, this is the ideal substrate condition for Resafloor HB.

Dense, low-absorption concrete

Dense concrete can be very strong but have a hard, tightly closed surface. Power-floating, polishing or a low-porosity concrete mix can reduce its ability to absorb liquid.

Common indications include:

• Water remaining beaded on the surface

• Very slow or uneven darkening

• A smooth, hard or polished finish

• Limited visible texture

• Greater resistance to scratching and abrasion

Low absorption does not necessarily mean the concrete is unsuitable. It means that surface preparation becomes particularly important.

A dense surface gives a liquid coating less opportunity to penetrate or mechanically key into the concrete. Thorough mechanical preparation is needed to remove the closed surface and produce a clean, textured profile before Resafloor HB is applied.

Does greater absorption mean a stronger bond?

Only up to a point.

A more open surface can allow better wetting and penetration than dense, closed concrete. However, increasing absorption often corresponds with decreasing surface strength.

The relationship is therefore:

• Too dense: Strong substrate, but limited wetting and mechanical key

• Balanced: Strong substrate with useful absorbency and texture

• Too soft: Good penetration, but insufficient strength beneath the bond

The most absorbent concrete is not automatically the best substrate. The objective is a surface that is both strong enough to carry the coating and open enough to receive it.

How can concrete absorbency be checked?

Observing how a clean area reacts to water can provide a useful initial indication:

• Immediate absorption and rapid darkening may indicate a very open or potentially weak surface.

• Controlled, even darkening may indicate a sound surface with useful absorbency.

• Persistent beading or very slow darkening may indicate dense or contaminated concrete.

This is a practical screening observation, not a complete suitability test. Surface contamination, sealers, curing compounds and previous treatments can also prevent water absorption.

The floor’s strength, moisture condition and surface integrity must still be assessed independently.

Why mechanical preparation remains essential

Neither concrete type nor apparent absorbency removes the need for preparation.

Mechanical preparation helps to:

• Remove laitance and weak surface material

• Eliminate suitable levels of surface contamination

• Open dense or tightly finished concrete

• Reveal defects and weak areas

• Create a clean, consistent surface profile

• Improve contact between the epoxy and substrate

Preparation must expose sound material. Coating over dust, laitance or a weak surface merely transfers the bond to a layer that may later detach.

Choosing the right cement substrate for Resafloor HB

Concrete, sound sand-and-cement screed and confirmed cementitious flow-applied screed can all provide suitable bases for Resafloor HB.

The ideal substrate is not defined by its name alone. It must combine:

• Adequate cohesive strength

• Structural stability

• Suitable moisture condition

• A clean surface

• Useful absorbency

• An appropriate mechanically prepared profile

Resafloor HB performs best on a strong cement-based floor with an open, properly prepared surface. Where the concrete is dense, preparation creates the required key. Where it is soft and absorbent, the priority is confirming that the substrate itself is strong enough to support the coating.

Once those conditions are established, Resafloor HB provides a substantial, straightforward high-build finish over the cement-based substrates found in most industrial and commercial floors.