Table 3 Overview of a selection of moisture measurement techniques used for rock, stone and other porous building materials (for a more comprehensive list see [60, 61])

Gravimetric

Oven-drying of samples

High (chipping off or drillhole)

Reference method

Low temporal resolution

[7]

Borehole humidity

Equilibrium between air humidity and rock moisture

Moderate (drillhole needed)

Monitoring possible, good calibration via sorption isotherms

Not suitable for high saturation, values are stuck at 100% for long periods

[60]

Infrared thermography

Modification of temperature by attenuation and evaporative cooling

None

Quick spatial overview at high resolution

Only qualitative surface information; reflectivity at different angles

[62]

Electrical resistivity

Electrical current flow between two (or four) electrodes

Moderate (drillholes needed)

Simple, relatively cheap, strong response to water content

Strongly affected by pore water salinity; individual calibration required

[58]

2D/3D-electrical resistivity (ERT)

Combining automatically switched measurements to a 2D or 3D section

High with drilled electrodes; none when adhesive electrodes are used

Producing clear and instructive sectional views

[55]

Microwaves

Attenuation or reflectance of microwaves (1–10 GHz)

None when surface sensors are used

Unaffected by pore water salinity

Calibration required, limited depth information, susceptible to surface roughness

[63]

Ground-penetrating radar

Attenuation of electromagnetic waves (10–1000 MHz)

None

Measurement along profile lines or 2D arrays

Affected by coupling conditions at uneven surfaces

[64, 65]

Time-domain reflectometry

Moderate (drillholes needed)

Less error sources than electrical resistivity, only slightly influenced by salts

Physically difficult to install, edge effects along sensor rods

[66]