A variety of factors go into determining the deterioration rate of a street. When managing a network of roads, an important step is identifying which of these factors influence the deterioration rates on your roads.
The deterioration rate of a road is an average rate at which a pavement condition is projected to decay. This is instrumental to developing a pavement management plan that optimizes rehabilitation activities in conjunction with the network’s budget requirements. Among the factors that should be considered are impacts from surface distresses, drainage, ADT & ESAL (Average Daily Traffic and Equivalent Single Axis Load), and the conditions of the pavement base.
Let’s examine each of these factors individually:
The extent and severity of surface distresses along a pavement segment can reveal a lot about the future deterioration of the road. In some cases, a surface distress such as alligator cracking may even reveal failures in the pavement base. Certain pavement distresses may spread quickly if untreated, and some can appear as symptoms of other problems along the roadway (i.e. edge cracking spreads quickly as a result of poor drainage).
Tracking surface based distresses of a pavement network is vitally important for municipalities that seek to optimize their budget. This is because the distresses are often the most heavily-weighted component in determining the best rehabilitation strategy for a road. Experts recommend that pavement condition surveys are scheduled on a regular 1-3 year basis to track surface distresses and ensure that public works departments get the most out of their pavement budget.
Another important factor to consider when assessing the deterioration rate of a pavement surface is the condition of the drainage systems along the roadway. The drainage along the side of a roadway significantly impacts the deterioration of the base, as well as the surface conditions. When a roadway has inadequate drainage, water may tend to pool along the edge or in the cracks of pavement. This is particularly worrisome in colder climates where the freeze/thaw cycle of water can accelerate the rate of surface and base deterioration.
On a roadway with poor drainage, pavement distresses such as edge cracking tend to spread much more quickly. More severe distresses, like alligator cracks, may fill with water and quickly end up becoming potholes. These failures accelerate the deterioration rate of a roadway segment, but are usually easily addressed through the addition of drainage ditches and proper curb sides.
ADT/ESAL and Traffic Loads
If you have experience in pavement management than you will certainly be familiar with these two terms. Average Daily Traffic, or ADT, is an obvious general indicator of the lifespan of a road. In most cases, however, Equivalent Single Axle Load, or ESAL, is a much more reliable indicator. The reason mostly boils down to the impact of heavy vehicles on the pavement surface.
If you are examining a quickly deteriorating pavement segment with a low overall ADT, it may be practical to take a look at the ESAL. Heavy truck traffic by garbage, recycling, and semi-trucks have approximately 1000x the impact on a pavement surface than their smaller, lighter counterparts – personal vehicles.
“The ESAL statistic represents a mix of different axle loads and axle configurations predicted over the design or analysis period and then converted into an equivalent number of 18,000-lb. single axle loads summed over that period.” (Texas DOT)
Since the measurement references 18,000-lb increments, heavier traffic contributes much more significantly into the final results. This is not calculated in the standard ADT figures and means that ESAL is a much more reliable measurement for the rate of deterioration on a road.
The final, and most telling, component that forecasts the deterioration rate of a pavement segment is the condition of the pavement base. Though surface based distresses can often provide fair insight into the base conditions, it is often best practice to perform a deflection test on roads from which the base condition is an important consideration.
Structural testing is performed using a device such as a Dynaflect or Falling Weight Deflectometer. The purpose of the structural testing is to identify the relationships and interactions between pavement quality, life expectancy and rehabilitation planning. The structural analysis facilitates a much more in-depth understanding of the City’s transportation corridors. Structural analysis also assist in rehabilitation selection by constraining inadequate pavement sections from receiving too light of a rehabilitation and conversely, identifying segments suitable for lighter weight treatment.
The overall deterioration rate of a roadway segment considers a variety of factors. ADT/ESAL, Base Conditions, Drainage, and Surface Distresses all play a valuable role to pavement managers that are peering into the tea leaves to determine the deterioration rate of their roads. Luckily all of these factors are measurable through a pavement condition survey and deflection test. Forward thinking public works departments should consider the benefits of conducting a survey in their home town. It could save the city millions.