Limitations

The confidence rating system enables clients to make informed decisions about the fitness for purpose of REVEAL information. The rating system is based on a matrix of GPR data quality and its correlation with calibration data. Results with low confidence may be adequate where a general understanding of pavement construction is required across a network, whereas higher confidence should be sought for complex engineering situations such as scheme design. You can increase the confidence rating of uncalibrated GPR data by taking cores; this could, for example, increase a rating of 5 to a rating of 9. On the other hand, there is little that can be done to increase the rating where GPR data quality is low and you may need to rely on additional coring.



The limitations associated with the construction data are the result of three factors:

• Relocation accuracy


• Level of correlation with calibration data such as coring


• GPR data quality.

Relocation Accuracy

Relocation accuracy is a function of a number of factors, including speed of data collection. Construction information from the REVEAL database is obtained from GPR data gathered at traffic speed. Because of the greater positional mis-match between core locations and GPR readings, confidence rating of 7 or 9 can not be achieved for REVEAL information. Slow speed data collection is required to achieve a higher confidence rating.

Calibration Data

While a number of different data streams, including design records and deflection data, can be used to improve confidence in the interpretation of GPR data, the confidence rating system focuses on intrusive tests including coring and trial pitting. Poor correlation between the GPR and calibration data can be the result of a number of factors including:

• Poor relocation between GPR data and calibration data


• Misinterpretation/measurement of materials in cores/trial pits. This is often related to deterioration of the pavement materials. Stripping or delamination of internal asphalt layers can, for example, be misinterpreted as the transition from asphalt to subbase when coring.

GPR Data Quality

The quality of GPR data is primarily a function of the electrical properties of the pavement. Whilst most pavement construction materials can be resolved, GPR is more effective through some materials than others: stripped asphalt or wet, clay-rich subbase, for example can scatter and absorb radio energy which can prevent resolution of deeper layer boundaries. Reinforcement bars are highly reflective and can also prevent resolution of deeper layers. Where subbase and subgrade layers are intermixed GPR will not resolve a boundary. This is because there is a gradual change in materials rather than a clear boundary.

Whilst the multi frequency survey approach and advanced processing software used by Aperio can minimise the adverse affects of problem materials they cannot be totally overcome, as they are a result of fundamental physics principles.