Roads deteriorate over time, due to climate (water, freeze and thaw, sunshine, heat, etc.) and traffic. From the moment the road is constructed, it begins to age.
"Most pavements will deteriorate through the phases listed in the [PASER (Pavement Surface Evaluation and Rating)] rating scale. The time it takes to go from excellent condition (10) to complete failure (1) depends largely on the quality of the original construction and the amount of heavy traffic loading. Once significant deterioration begins, it is common to see pavement decline rapidly. This is usually due to a combination of loading and the effects of additional moisture. As a pavement ages and additional cracking develops, more moisture can enter the pavement and accelerate the rate of deterioration." PASER Manual - Asphalt Roads, page 14.
Water is deemed the roads' worst enemy. Most of pavement preservation is designed to prevent the damage that water causes. Once a pavement cracks, whether it be concrete or asphalt, water in the cracks freezes and thaws with enormous force, causing further damage to the pavement. Water in the cracks washes out particles of pavement as traffic hits the cracks and the cracks become potholes. Water infiltrating the surface into the subsurface or "base" begins to undermine the surface causing the surface to sag. Water infiltrating from the sides through improper drainage causes similar damage. So, when we are talking about pavement preservation, and particularly so in the early stages of the pavements' life, we are talking about preventing these water damages.
However, asphalt naturally aging is also something we need to be concerned about. Asphalt is composed of asphaltenes and maltenes. The maltenes are the volatile components that escape into the atmosphere over time, drying out the asphalt which causes the asphalt to shrink and ultimately develop cracks. Thus, some of the more recent pavement preservation methods involve replacing some of the maltenes to “rejuvenate” the asphalt. These relatively low cost treatments are usually applied very early in the asphalt’s life, to prevent the cracks from even starting. (So now you know one of the reasons road agencies are doing work on roads that look in perfect condition, while other roads in worse shape are left alone.) (For an excellent introduction to asphalt aging and the use of rejuvenators, see Bob Boyer’s presentation.)
The PASER pavement condition ratings are generally related to the maintenance or repair that is needed
Rating 9 & 10 No maintenance required
Rating 8 Little or no maintenance
Rating 7 Routine maintenance, crack sealing and minor patching
Rating 5 & 6 Preservative treatments (sealcoating)
Rating 3 & 4 Structural improvement and leveling (overlay or recycling)
Rating 1 & 2 Reconstruction
However, the potential treatments vary widely and must be matched to the age of the pavements, the specific defects in the pavements, the climatic conditions, and the traffic, as well as the budget.
Typical defects in an asphalt pavement include:
- Surface defects: Raveling, flushing, polishing.
- Surface deformation: Rutting, distortion—rippling and shoving, settling, frost heave.
- Cracks: Transverse, reflection, slippage, longitudinal, block, and alligator cracks.
- Patches and potholes
See PASER Manual - Asphalt Roads, pages 3-13 for excellent descriptions and photos of these distresses for asphalt.
Source:Asphalt Pavement Rejuvenation, presentation by Robert E. Boyer, PhD, PE, Consultant Engineer-Asphalt Pavements, Lynn Haven, Florida, at the August, 2012 National Pavement Preservation Conference.