Laboratory Calibration and In Situ Measurements of Moisture by Using Time-Domain Reflectometry Probes

Excessive moisture in pavement systems can cause considerable damage and can lead to early deterioration. One method for continually monitoring the moisture content of pavement systems nondestructively is the use of time-domain reflectometry (TDR) probes. Although originally developed to measure faults in electrical cables, TDR probes employ an electromagnetic wave that is transmitted along a set of metallic conducting rods (or waveguides). The velocity of the electromagnetic wave is influenced by the dielectric constant (ϵ) of the material surrounding the waveguides. The large contrast between the dielectric constants of free water and of dry soil makes this an effective nondestructive evaluation method. Soil samples with different moisture contents were prepared and the TDR output, which is a function of the dielectric properties, was compared with the measured gravimetric moisture content. Calibration equations were developed in a laboratory setting for two types of TDR probes (CS610 and CS615) embedded in the Virginia Smart Road test facility at Blacksburg, Virginia. Preliminary field data were collected for the two different probe types embedded in different pavement structures. It is shown that the two types of TDR probes yield similar data in some situations and different data in other circumstances. It appears that the composition of the pavement structure has an effect on the moisture measured in the subbase layer. Although preliminary results indicate that the use of CS615 TDR probes in pavement applications is promising, further continuous monitoring of both types of TDR probes is necessary to determine if the CS615, which can be readily connected to a data acquisition system, can be used in lieu of the CS610, which requires a time-consuming collection procedure or possible additional data collection equipment.