What distinguishes structural distress from functional distress in pavement evaluation?

Structural distress refers to pavement damage that indicates a weakness or failure in the pavement's load-bearing capacity. This means the pavement layers are no longer able to adequately distribute traffic loads, leading to potential failure of the pavement structure. Examples of structural distress include fatigue cracking (also known as alligator cracking), rutting (permanent deformation in the wheel path), and faulting (vertical displacement between adjacent pavement slabs). These distresses are directly related to the pavement's ability to withstand traffic loads and are often indicators of underlying problems within the pavement structure, such as inadequate layer thickness, poor material quality, or insufficient subgrade support. Functional distress, on the other hand, refers to pavement surface irregularities that affect ride quality, safety, and skid resistance but do not necessarily indicate a structural weakness. Examples of functional distress include roughness (unevenness of the pavement surface), raveling (loss of surface aggregate), bleeding (excess asphalt on the pavement surface), and polishing (reduction in aggregate texture). While functional distresses can be annoying to drivers and can reduce safety, they don't usually indicate a failure of the pavement's structural integrity. Instead, they are related to surface characteristics and the pavement's ability to provide a smooth, safe riding surface. Therefore, structural distress indicates a problem with the pavement's ability to carry loads, while functional distress indicates a problem with the pavement's surface characteristics and its ability to provide a comfortable and safe ride.