Compaction degree is one of the key indicators for quality control in asphalt pavement construction. Insufficient compaction may lead to premature permanent deformation of the pavement, excessive aging, and water damage. Therefore, compaction degree must be tested and evaluated during the construction process. Currently, the commonly used compaction testing methods in construction quality control are mainly the core sampling method and the nuclear-free density gauge, etc. However, these methods are all point measurement methods, which have shortcomings such as low efficiency, randomness, and poor representativeness, and cannot meet the requirements for rapid compaction testing in construction quality.

Ground penetrating radar (GPR) using the amplitude reflection method can rapidly measure the dielectric constant of asphalt pavement. Its basic principle is: high-frequency electromagnetic waves are transmitted toward the ground, and the reflected wave signals from the pavement are acquired through a receiving antenna. By measuring the incident wave amplitude Ai and the reflected wave amplitude A0, the pavement dielectric constant can be calculated using the following formula. The incident wave amplitude Ai can be obtained by placing a metal plate on the pavement and measuring the reflected wave amplitude, because the metal plate fully reflects electromagnetic waves; it can also be obtained directly by calibrating the antenna before delivery, forming a calibration file that can be called directly. This GPR method achieves continuous measurement of the pavement dielectric constant.

GPR serves as a continuous method for detecting the dielectric constant of pavement, and there is a good correlation between the compaction degree of asphalt pavement and the dielectric constant of the material. When the pavement compaction degree increases, the air void content decreases and the pavement dielectric constant increases; conversely, when the compaction degree decreases, the air void content increases and the pavement dielectric constant decreases. By measuring the pavement dielectric constant and establishing the relationship between compaction degree and dielectric constant, GPR can achieve rapid, reliable, safe, and non-destructive testing of asphalt pavement compaction degree.

The GSSI Rolling Density Meter (RDM) is a compaction degree (dielectric constant) measurement device based on the GPR pavement reflected wave amplitude ratio method, and can perform continuous dielectric constant measurement on the pavement. Depending on the number of high-frequency antennas configured (1-3), 1-3 dielectric constant survey lines can be obtained simultaneously.

Kontur adopts multi-channel antenna array technology to scan the full coverage of the pavement, achieving dielectric constant measurement of the entire pavement and greatly improving detection efficiency and accuracy.

GSSI is a line measurement device

while Kontur is an area measurement device

At the 2023 Global Distributor Annual Meeting, Kontur shared their development achievements and application cases. The following are screenshots of the manufacturer's presentation.

Dielectric constant measurement: from line to area

The difference between line measurement and area measurement

Dielectric constant line measurement:

● One or several high-frequency antennas, with channel spacing of 12+ inches

● High requirements for antenna support stability and travel speed;

● One-dimensional data cross-section;

● Greater difficulty relative to "actual" conditions;

Dielectric constant area measurement:

● High-density data acquisition

● Can cover the entire lane

● Measured at normal traffic speed

● Two-dimensional high-resolution data

● More intuitive results

Longitudinal joints

● Using 3D radar dielectric constant mapping, longitudinal joints can be easily identified

● Transverse cross-sections are no longer needed

● High dielectric constant means high compaction degree

Improved signal-to-noise ratio (S/N)

The dielectric constant distribution map of 3D radar can clearly distinguish joints and cracks in asphalt pavement, and the high signal-to-noise ratio can reveal subtle changes.

Transition from old asphalt pavement to new asphalt pavement

On 25 channels with 7.5 cm spacing, the sharp changes in dielectric constant measured by the 3D radar system can reveal the transition between asphalt pavements of different types/ages/compaction degrees.

Comparison of GSSI measurement curve and Kontur measurement curve (before average value calibration)

Comparison of GSSI measurement curve and Kontur measurement curve (after average value calibration)