Why the substrate decides the floor

A resin floor is a thin skin bonded to a thick structure. Its performance is therefore bounded by what it is bonded to. Where a resin system lifts, blisters, cracks or dusts, the cause is more often the concrete below than the resin above. Assessing the substrate is not preparation for specification: it is part of it, and it should happen before a system is named.

Moisture is the first question

Concrete releases water for a long time after placement, and a sealed resin layer traps whatever is still moving upwards. Vapour pressure then builds at the bond line and produces osmotic blisters or a clean delamination. We measure residual moisture before specifying, rather than assuming a slab is dry because it looks dry.

A relative humidity of 75 per cent measured in the slab is a common upper limit for conventional resin systems, and many manufacturers set it explicitly in their technical data. The value is measured in a sealed hole or with an insulated hood, not at the surface, because the surface dries first and misleads. Carbide bomb testing gives a moisture content by mass and is used alongside it. Where a slab is ground bearing without a functioning damp proof membrane, the reading may never fall below the threshold, and the honest conclusion is a moisture tolerant system such as PU cement or a dedicated moisture suppressing primer, not a longer wait.

Strength, soundness and contamination

Adhesion of a resin system routinely exceeds the tensile strength of weak concrete, which means the floor does not fail at the glue line: it pulls the top few millimetres of slab away with it. Pull off testing gives a surface tensile strength, and most resin systems ask for a minimum in the region of 1.5 N per square millimetre. A slab that tests lower needs to be ground back until sound material is reached, or repaired.

Laitance, curing compounds, power float polish, oils, sugars and old adhesive all sit between the resin and the concrete. Mechanical preparation removes them. Shot blasting opens a dense slab and gives a consistent profile, diamond grinding suits thinner coatings and edges, and scarifying is used to take off bulk material or a failed coating. Acid etching is not an equivalent: it does not reliably remove contamination and it leaves residue and moisture behind.

Cracks, joints and movement

Static shrinkage cracks can be resin injected and over banded. Moving cracks and structural joints cannot: a joint that moves under load will always reappear through a rigid layer. Movement joints in the structure must be carried through the finished floor, and their position should be agreed with the designer before laying begins, not negotiated afterwards. Day joints, saw cuts and column details each need a decision.

The survey checklist

• Residual moisture measured in the slab, with the method and the reading recorded.
• Surface tensile strength by pull off test, at a representative number of positions.
• Presence of a damp proof membrane, confirmed from drawings or by inspection.
• Contamination: oils, sugars, curing agents, previous coatings and their adhesion.
• Crack survey, separating static cracking from moving cracking and joints.
• Levels and falls, checked against what the drainage actually requires.
• Ambient and substrate temperature expected during installation and cure.

Where these are established in advance, system selection becomes a short conversation. Where they are not, the first day on site becomes a redesign.