Visual condition surveys document aspects of both functional and structural pavement conditions, but generally serve as a qualitative indicator of overall condition. Specialized equipment is used to quantify both functional and structural properties of the pavement structure. Ideally, for any given section of highway, two or more evaluators would arrive at the same assessment of the section’s current condition. However, there are still many aspects of pavement evaluation that are highly subjective. For example, in visual condition surveys, the percent of surface area affected by alligator cracking is highly dependent upon the visual acuity of the evaluator. Progress continues in automating the mapping of common surface distress to eventually eliminate this subjectivity.

NDT is the collective term for evaluations conducted on an existing pavement structure that do not require subsequent maintenance work to return the pavement to its pretesting state. This is generally desirable to minimize disruption to traffic, and is essential as a screening tool to determine locations where selective material sampling should be conducted to evaluate other material properties in the laboratory. As such, its focus is to assess in situ properties that can be used to evaluate the need for further “destructive” testing (i.e., coring, boring, trenching), location of that destructive testing, and the current structural capacity of the highway as related to layer stiffness and strength. Non-destructive testing methods can assess either functional or structural condition. For NDT, data collected in the field is generally objective in nature, but engineer’s data analysis and interpretation are subjective.

Destructive testing opens the door to characterization of the pavement’s constituent materials not possible to obtain through non-destructive testing alone. Material characterization includes: