Pavement Structural Testing

The substructure of a road, which cannot be seen, is the most significant determining factor in the long-term performance of pavement.

• Every preservation treatment and minor rehabilitation is a potential waste of money if the structure of the road is not adequate to support the traffic loads.
• Alligator cracking may not always indicate structural failure! Pavements that have age-related alligator cracking that is a result of the binder having oxidized and become brittle may be structurally adequate and only require resurfacing, as opposed to rehabilitation. This can be identified with FWD testing and further confirmed from the rutting results and substantially reduce the treatment costs.
• The structural strength of a road is especially important for roads with higher and heavier (truck) traffic. Inadequate structure on roads with more traffic may deteriorate much faster than expected.

The interpretation of FWD data is greatly enhanced by reliable traffic data, in particular the heavy vehicle composition and traffic volumes (i.e. AADT).

FWD testing, together with surface condition data collection, ensures that treatments are targeting the actual pavement needs, maximizing budget efficiency.

How Structural Data Enhances PMS Decisions
The chart below shows how structural data greatly expands the number of possible treatment recommendations. Without incorporating structural analysis, we’re only using the first column. With structural data, we can access the whole table. For example, if both surface condition and structural condition suggest the need for reconstruction (RC), then reconstruction is indeed necessary. If surface condition suggests preventative maintenance (PM), but structural condition suggests reconstruction (RC), then the PM should not be performed and instead we should do nothing (DN). Relying solely on surface condition (e.g. PCI) is insufficient for this determination. Studies have shown that using PCI can lead to biased recommendations when structural indices are not considered.

Credit: Brian Diefenderfer (VDOT, VTTI)

Further Reading
Studies indicate that deflection data can be used not only to assess existing damage but also to inform preventive measures for future damage. Furthermore, Pereira et al. (2022) highlighted TSDD’s ability to detect subtle changes in structural integrity, which can support more accurate pavement health indices when integrated with traditional pavement condition measures.

In the current state of practice, most departments of transportation (DOTs) and city municipalities assume visual pavement condition ratings, windshield surveys, or automated distress collection provide adequate information about the overall in-situ condition of the pavement. Therefore, structural indicators have generally been left for project-level evaluations and designs (‎Bryce et al., 2013). However, research studies have shown that the structural condition of the pavement should be considered for an unbiased and correct assessment of the road network (‎‎Bryce et al., 2013).

Another research study by New Jersey DOT suggested that network-level decisions made based only on surface condition were significantly different from those made with deflection testing (Zaghloul et al., 1998). In addition, for some agencies where surface treatments such as microsurfacing, seal coating, and slurry seal are often used to cover the distresses at the surface for aesthetics purposes or preserving the road, the visual pavement evaluation could be misleading in the decision-making process since cracks and distresses are concealed. These results and studies suggest that the visual pavement indicators such as Pavement Condition Index (PCI) could be independent from underlying structural conditions. Therefore, pavement management informed with structural evaluation provides a unique opportunity for proactive pavement management which may close the gap between budget allocations and project needs due to better and more cost-effective pavement rehabilitation options (Zaghloul et al., 1998).