Concrete RH Risk Calculator

Formula Used

This tool uses a weighted risk score (0–100) based on practical moisture drivers. The core driver is the difference between measured in-situ RH and the finish limit (ΔRH). Additional points are added for conditions that slow drying or increase vapor drive.

• Base score from ΔRH: ≤−5% → 10, −5% to 0% → 30, 0% to +5% → 60, >+5% → 85.
• Adjustments: ambient RH, temperature, slab thickness, w/c ratio, curing days, vapor retarder, finish sensitivity, and wet exposure add or subtract points.
• Risk bands: 0–24 Low, 25–49 Moderate, 50–74 High, 75–100 Critical.
• Extra drying estimate: if measured RH exceeds the limit, additional days ≈ (measured−limit) × k, where k depends on slab thickness (2–4 days per 1% RH).

How to Use This Calculator

1. Run in-situ RH tests at the correct depth and record the highest reading.
2. Enter the finish RH limit from the adhesive or floor system data.
3. Fill in site conditions: temperature, ambient RH, slab thickness, w/c ratio, and curing days.
4. Indicate whether a vapor retarder exists and if the slab was re-wetted.
5. Click Calculate Risk to see the score and recommended next actions.
6. Download CSV or PDF for submittals, meetings, or tracking.

Example Data Table

Moisture Acceptance Targets and Finish Tolerances

In-situ relative humidity (RH) is a widely used readiness metric for concrete slabs that will receive adhesives, coatings, or floor coverings. Product limits vary by system, and many specifications fall in the 75%–90% range. Lower limits generally mean lower failure probability, especially for wood, rubber, and sheet goods.

Interpreting In-Situ RH and Safety Margin

A pass/fail decision should not rely on an average. Use the highest valid reading from the test set, then compare it to the finish limit. The difference (ΔRH = measured RH − limit) is a quick indicator of urgency. A small “pass” can still be risky if ambient RH is high or the slab has been re-wetted. Many teams target an extra 3%–5% margin for sensitive finishes.

Drying Drivers: Thickness, Mix, and Environment

Drying time rises quickly with slab thickness because moisture must migrate from the interior to the surface. Higher water–cement ratios typically increase initial free water and can keep internal RH elevated longer. Temperature and ambient RH control the drying gradient: cool interiors and ambient RH above 70% can slow evaporation dramatically. Air movement matters too; stagnant zones near walls and in corners often lag behind open areas.

Mitigation Options and Schedule Impacts

If RH remains above the limit, options include delaying installation, increasing dehumidification, switching to higher-tolerance systems, or installing a moisture mitigation layer. Coating selection should match the substrate condition and the adhesive system. Surface preparation, crack treatment, and primer compatibility are critical; poor prep can fail even when a coating is rated for high RH. Planning for grinding, cure time, and re-testing can shift milestones by days or weeks.

Documentation and Retesting Strategy

Good records reduce disputes. Log test locations, depth convention (one-side versus two-side drying), equilibration times, and ambient conditions at the time of testing. Use multiple tests per area to limit uncertainty and repeat measurements to confirm trends. Retest after leaks, washdowns, or HVAC changes, and archive reports for closeout and warranty files.

FAQs

1) What does the risk score represent?

It is a practical 0–100 index that combines measured RH versus the finish limit with site factors like ambient RH, thickness, curing, and barrier presence. Higher scores indicate a greater chance of flooring or coating problems.

2) Which RH value should I enter if I have many tests?

Enter the highest valid in-situ RH reading from the area that will receive the finish. Using the maximum reading helps avoid installing over a localized wet zone that can trigger adhesive failure.

3) Does ambient humidity really affect slab drying?

Yes. High ambient RH reduces the moisture gradient that drives evaporation from the slab surface. Without dehumidification and air movement, internal RH can plateau even when the slab is weeks old.

4) How accurate is the extra drying days estimate?

It is a rough planning figure based on percent RH above the limit and slab thickness. Actual drying depends on HVAC stability, airflow, surface condition, and moisture sources. Always confirm progress by retesting.

5) What if there is no vapor retarder under the slab?

Risk increases because ground moisture can migrate upward for the life of the building. Consider mitigation systems designed for ongoing moisture drive, and investigate perimeter drainage, capillary breaks, and slab edge details.

6) Should I install a mitigation coating whenever RH is high?

Not always. If schedule allows, improving drying conditions may be cheaper and simpler. Use mitigation when RH remains above the finish limit, when moisture sources persist, or when the finish system is highly sensitive.