Moisture in concrete: assessment before resin flooring
Moisture in a concrete slab is a common reason to pause before installing a resin floor. The key question is not whether the surface looks dry; it is whether the slab condition, moisture source and chosen coating system have been assessed together. A damp-tolerant primer, a purpose-designed topical moisture mitigation system and an under-slab vapour retarder do different jobs. This guide explains the testing and decision process before a project team selects one of them.
Where is the moisture coming from?
Recent placement or wet curing, rain during construction, wash-down, plumbing leaks, water at slab edges and moisture movement from below can produce different site conditions. Review drawings for the slab, any vapour retarder and its continuity, drainage and earlier repairs. Ask whether the slab is on grade, below grade or suspended. Existing coatings and levelling compounds can obscure what is happening at the concrete surface. A dark patch after cleaning is a clue to investigate, not a quantitative test result.
The distinction between residual construction moisture and a continuing source matters. Extra drying time may help the former when the site environment allows it; it does not repair a leak or compensate for water entering from below. Record the location of wet areas, dates, recent weather and operating conditions. Where water pressure, seepage or persistent rising damp is suspected, obtain project-specific investigation before choosing a coating.
ACI PRC-302.2-22 includes epoxy coatings among moisture-sensitive floor materials and addresses moisture movement, testing, drying and vapour-retarder design. Its subject is the whole slab and floor system, not a single primer.
Choose a test for the question being asked
Agree the method and testing locations with the specifier and manufacturer. Measure more than one representative zone where the slab has different exposure, thickness, age or history. Document the condition of the surface and the building environment at the time. A number without its method and location cannot be compared reliably with a product requirement.
| Method | What it helps establish | Decision limit |
|---|---|---|
| Plastic sheet, ASTM D4263 | Indication of capillary moisture at the tested surface. | Qualitative indication, not a quantitative acceptance result. |
| Electronic meter, ASTM F2659 | Comparative survey to locate potentially wetter and drier zones. | Preliminary comparison; it does not itself approve a moisture-sensitive floor. |
| In-situ RH, ASTM F2170 | Quantitative relative humidity within the slab at specified test locations. | Results describe the time and locations tested; compare with the approved system requirement. |
| Calcium chloride, ASTM F1869 | Quantitative vapour emission from a bare concrete surface. | Not a measurement through an existing coating, patch or leveller. |
ASTM methods define how a measurement is made; they do not give ALKA products a universal acceptance threshold. The surface may also need assessment for soundness, contamination and previous coatings. If a slab changes use, is flooded or develops a leak after testing, reassess it. Readings taken before environmental conditions stabilise may not represent the planned installation condition.
What does “moisture barrier” mean in this project?
An under-slab vapour retarder is a construction component intended to limit moisture movement from below the slab. A topical moisture mitigation system is applied to appropriately prepared concrete as a specified component beneath a compatible floor build. A wet-area waterproofing membrane is selected and detailed to manage liquid water in its intended assembly. These terms should not be used interchangeably.
ASTM F3010-24 addresses certain two-component, resin-based membrane-forming mitigation systems for high-moisture concrete under resilient floor coverings. Its scope is useful for understanding what a documented mitigation system involves, but it does not certify an ALKA epoxy primer for a resin floor or override any manufacturer’s installation instructions. The project specification must establish the named product, substrate preparation, required film build, compatible subsequent layers, test method and permitted moisture condition.
A barrier layer will not make unsound or contaminated concrete sound. It also should not be assumed to manage active seepage, significant vapour pressure or hydrostatic pressure without express written approval for those conditions. Joints, cracks, drains, penetrations, terminations and edges need detail as part of the complete system.
What ALKA documentation currently supports
The ALKA 100 product page describes an epoxy primer and binder suitable for dry and damp substrates. Its linked Revision 4 TDS should be checked for the current substrate and application limits. The ALKA 110 product information does not establish ALKA 100 as a moisture mitigation barrier. “Suitable for damp concrete” therefore does not mean approved for every wet slab or every finish.
The published ALKA 112 TDS dated October 2022 describes low permeability and adhesion to damp concrete, but also prohibits application where significant vapour pressure may occur and gives no substrate-moisture acceptance value. On those documents, ALKA 112 should not be presented as a proven topical moisture barrier. Ask ALKA for the current approved TDS and a written recommendation if ALKA 112 or another product is being considered for a moisture-affected project.
For a general dry-substrate pathway, the published ALKA-902 example lists prepared concrete, ALKA 100 primer and ALKA 110 coats. That layer list does not establish moisture mitigation. The final system must follow the measured slab condition and approved project specification.
A practical decision sequence
- Survey: map leaks, edges, damp patches, slab construction, previous coatings and service conditions.
- Measure: use agreed methods, representative locations and recorded environmental conditions.
- Compare: obtain the current approved TDS and written limits for all components of the proposed build.
- Resolve: allow drying where appropriate, rectify ongoing water sources, or obtain a documented mitigation specification.
- Prepare and verify: remove laitance and contamination, establish the nominated profile, detail joints, and agree any trial and adhesion testing.
- Release the work: record the approved primer or mitigation layer, compatible overcoat sequence and the conditions that allow coating to begin.
Where the choice remains uncertain, send ALKA the slab history, moisture test records, photographs, floor area, existing build, intended exposure and installation programme. Request technical project support before committing to the coating schedule.
Frequently asked questions
Does a negative plastic-sheet check mean the slab is ready?
No. It is a useful indication of capillary moisture at that location, but it does not quantify in-slab RH or establish acceptance for a specified coating.
Can a damp-tolerant epoxy act as a moisture barrier?
Only if the named product and complete installation are expressly approved for that role and the measured site condition. Damp adhesion and mitigation performance are different claims.
Will an extra coat fix a moisture problem?
Additional ordinary coating does not resolve the source of moisture or establish the preparation, continuity and compatibility required of a documented mitigation system.
Technical references
- ACI PRC-302.2-22, concrete slabs receiving moisture-sensitive flooring materials.
- ASTM D4263-24, plastic-sheet indication; F2659-23, comparative meter survey; F2170-19a, in-situ RH; F1869-23, surface vapour emission.
- ASTM F3010-24, resin-based moisture mitigation under resilient floor coverings; scope and manufacturer instructions must be respected.
- ALKA 110 product information, ALKA 100 Revision 4 TDS and ALKA 112 2022 TDS; request current approved issue and project advice.
Need a project-specific build?
Send the substrate condition, exposure, photos and programme to ALKA for a technical review of the complete system.
Planning guidance only. Confirm the current technically approved ALKA TDS/SDS, compatibility, testing and application limits before specifying or installing a system.