Pavement Condition
Street Lifecycle
Streets are designed to last about 20 years, but the pavement begins to deteriorate much earlier.
Studies have shown that pavement health worsens at an increasing rate as the pavement gets older.
A street's condition deteriorates 40% in the first 15 years of its life, while its pavement
condition is still good. Regular pavement maintenance (overlays on busy streets; slurry
seals on less-used streets) can keep a street in good condition for a long time. Once
pavement condition falls below the good level, deterioration accelerates.
Preventive maintenance using cost-effective slurry seals ($1.50 / sq. yd) or 2 to 3-inch
overlays ($15 / sq. yd) during the first 10 to 15 years can extend a pavement life to
30 years and more. Without these surface treatments, costly reconstruction is
required ($35 to $120/sq. yd.).
Current Conditions in Tigard
The City completed a Pavement Management Analysis report re-rating all City streets,
and providing a PCI rating for each street.
Today, Tigard's streets are in fairly good condition. The network average is an
overall condition rating of 68 and the backlog of preventative maintenance is at 10%.
However, there is cause for concern with 45% of the streets in the fair category.
This means many streets will become reconstruction
candidates in the next five to ten years. A preventative maintenance
approach is needed to stop this trend.
Pavement Rating System
Pavement health is measured by a Pavement Condition Index (PCI). The PCI indicates
the extent and severity of pavement distress such as cracking, rutting, raveling,
etc. It is expressed as a number from 0 (very bad, essentially gravel) to
100 (essentially perfect). New streets start with pavement conditions in the high
nineties. For ease of understanding, pavement condition is often classified as follows:
- Very Good (85 to 100)
- Good (70 to 85)
- Fair (55 to 70)
- Poor (40 to 55)
- Very Poor (Less Than 40)
Condition Examples
To give you an idea of what a street in very good condition versus fair condition
actually looks like, look at these examples
of pavement condition throughout the City.
Factors Affecting Pavement Condition
The primary factors causing pavement deterioration are the vehicles that
travel over the pavement. These factors include:
- Traffic volume;
- Volume of trucks and other heavy vehicles - the pavement deterioration caused by a vehicle increases exponentially with the amount of weight on each axle; and,
- Vehicles accelerating, braking, and turning which exerts more force on the pavement and accelerates pavement deterioration. This is why pavement deteriorates faster near intersections and in sharp curves.
Other factors include:
- Weather (which is the primary cause of decay on streets with very little traffic volume) - especially rain and freeze/thaw action;
- Settling of the ground beneath the pavement - especially soils with high clay content; and,
- Construction and or utility work that necessitates cutting into the pavement to access a utility line.
Examples of pavement deterioration include:
- Rutting - When pavement surface becomes depressed along the wheel paths
- Longitudinal Cracking - cracking along the roadway, parallel to the direction of travel
- Transverse Cracking - cracking across the roadway, perpendicular to the direction of travel
- Alligator Cracking - a combination of longitudinal and transverse cracking that has become so dense it resembles alligator scales
- Loss of Fines - when the cohesive material near the top of the pavement wears away, often due to weather or traffic loading
- Raveling - (perhaps better called unraveling) - When pieces of aggregate come out of the pavement as it continues to lose its fines
- Pumping - when liquids (such as water or liquid asphalt) are drawn to the surface (so it looks like the road is pumping out the liquid)
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