Measuring moisture in existing concrete: methods compared
Measuring moisture in existing concrete: CM test, kiln-drying method and continuous sensors compared. Use before refurbishment, waterproofing and covering.
Whether a refurbishment works, waterproofing performs or an existing floor may be covered is decided by the moisture inside the concrete — not at its surface. This article compares the three relevant measuring methods for existing concrete and shows when continuous sensing makes the decisive difference.
The problem with existing concrete
Concrete stores moisture for years and releases it extremely slowly. In existing buildings the moisture situation is rarely clear-cut: rising damp from the ground, old water damage, condensing room air and residual moisture from earlier conversions overlap. Before any refurbishment or covering decision, the question is therefore: how moist is the component — and in which direction is it developing?
The second half of the question is regularly overlooked. Yet it decides whether a measure holds: a component with a falling moisture curve is drying out; one with a constant or rising curve has an active moisture source — which must be found before any covering.
Method 1: CM test
The calcium carbide method is the established on-site proof (DIN 18560 context, analogous in the SIA area): sample extraction from the component, reaction with calcium carbide, pressure reading, conversion to CM-%.
Strengths: recognised, performable on site, immediate result. Limits: destructive (chisel/drill point), a point value at one time and place, operator-dependent. Trend questions (“is it drying?”) require repeated tests at new points — each one an intervention.
Method 2: Kiln-drying (Darr) method
The Darr test (oven-drying a sample to constant mass, determining water content in mass-%) is the most accurate reference — a laboratory procedure with sample shipping and waiting time.
Strengths: highest accuracy, court-proof as a reference. Limits: laborious, slow, destructive, unsuitable for ongoing monitoring. Sensible as a calibration and dispute reference, not as a monitoring tool.
Method 3: Continuous in-material sensing
An embedded sensor is placed into the concrete at the measuring point (drill, place, close) and permanently measures moisture and temperature there. The values travel by low-power radio through the concrete to a gateway — nothing protrudes, no cables, no open measuring point. The battery lasts well beyond the typical observation period.
Strengths: a gapless curve instead of snapshots, trend and event detection (remoistening, water events), remote access without site visits, documented history for decisions and warranty. Limits: does not replace the normative single proof — where a CM value is required, the CM test remains decisive. The sensors schedule it and provide the context before and after.
Method comparison
| Criterion | CM test | Kiln-drying | Continuous sensing |
|---|---|---|---|
| Result | point value (CM-%) | point value (mass-%) | curve (g/m³, trend) |
| Statement location | one measuring point | one sample | measuring point, permanent |
| Intervention | destructive, per test | destructive, per sample | one-time placement |
| Time horizon | snapshot | snapshot | weeks to years |
| Normative proof | yes (established) | yes (reference) | no — progress control |
| Remote monitoring | no | no | yes, with alerts |
Typical use cases in existing buildings
- Before waterproofing/injection: the curve before and after the measure shows objectively whether it works — instead of debating a new point measurement a year later.
- Before floor build-up and covering: observe the existing slab for months, take the covering decision on data. The proof runs via the CM test — scheduled by the curve.
- After water damage: track drying progress inside the component, switch drying equipment off based on data instead of flat rates, detect renewed rise early (more on remoistening).
- Unclear moisture source: the temporal curve (correlated with rainfall, use, season) narrows the source — rising, lateral, condensation-driven.
Besides screeds (CAF C25/C35, CT C20–C35, CTF C25, foam concrete), the method is tested in existing concrete — readings are displayed comparably to the CM reference in g/m³.
Specify and test
A specification template for existing-concrete moisture monitoring is available; the basics of specification wording are covered in moisture monitoring in the specification. To try the method on a concrete component, start with a pilot project.
Frequently asked questions
- Which method is the reference standard for concrete moisture?
- The kiln-drying (Darr) method — drying a sample in an oven to constant mass — is the most accurate reference, but a laboratory procedure. On site, the CM test is the established proof; continuous sensors deliver the curve between the point measurements.
- Why is a single measurement before refurbishment not enough?
- A point measurement shows the state of one day — not whether the component is drying out, stagnating or re-wetting. For refurbishment and covering decisions the trend is decisive: a component with a falling curve is judged differently from one with constant re-wetting.
- How is the sensor placed in existing concrete?
- A hole is drilled at the measuring point, the sensor is placed in the material and the point is closed. Nothing remains visible afterwards: no cables, no open measuring point — the sensor transmits by low-power radio through the concrete.
- Which decisions need concrete moisture data?
- Before waterproofing measures (does the waterproofing work?), before floor build-ups and covering (is the substrate dry enough?), after water damage (is the component dry again?) and in refurbishment planning (where is the moisture source?).