What happens when the base of a road fails? You can expect to see cracks spreading rapidly, for one. These cracks will often compound into a variety of other distresses, resulting in severe raveling and the formation of potholes. Soon the roadway conditions become undrivable and even dangerous. Expensive reconstruction is the only option.
Deflection testing is the process by which the structural strength of a road’s base is measured. By understanding the strength of a pavement base, a municipality can have a better idea of the future rate of deterioration on that street. This leads to better condition and budget projections as well as more accurate rehabilitation plans.
The surface of a street can often tell you a lot about the condition of its base; as well. Poor drainage and alligator cracking may represent structural failings that stem from flaws beneath the surface, while sporadic transverse cracking, bleeding or raveling may not provide any information on a street’s underlying weaknesses. Surface based analysis on pavement is very reliable, so why do so many cities also go through the process of conducting structural testing with Dynaflect or Falling Weight Detectors (FWD)?
Through the use of oscillating or falling weights the unit applies a nondestructive 1,000lb load to the pavement and measures the load’s deflection through a series of sensors. The data is typically collected every 300-500 feet along the outside lanes of a roadway. A structural analysis is then performed using the data collected from the sensors and expressed as a 0-100 score. A score greater than 75 indicates a structurally adequate roadway, while a score in the 50-75 range reflects a pavement that requires additional surface thickness. A score below 50 typically means that a road segment will require reconstruction as well as an increase in surface thickness.
There are many instances where a deflection test is precisely what the city needs to accurately determine their pavement condition. Here are a few all-too-common scenarios:
Saving Time and Money
Preforming a deflection test before investing huge amounts of money in rehabilitation efforts often saves municipality’s money as well as time. Any roadwork done to the surface of a pavement segment with a structural score less than 50 will need to be completely redone in the near future. For example:
A rural community has seen rapid growth and is now contending with an Average Daily Traffic (ADT) far greater than their streets were originally designed to support. In this case, it is common for the city to make the mistake of simply widening the road, without reconstructing the full base. As traffic persists the base will weaken until the entire road, including the newly widened areas, need to be completely reconstructed. A deflection test would have revealed this outcome, saving the community time, and potentially millions of dollars.
The Pavement Management Prophet
Another value of deflection testing is that it enables the city to make more accurate predictions for pavement condition and required budgets. For example:
A busy metropolitan district performs their own pavement condition assessment. They create a rehabilitation plan based on the findings of a manual survey of their network, and petition the city council for a budget. Five years down the road they begin to see a rapid decline in many of the areas that had recently been given overlays and other surface based rehabs.
When the city hires a contractor to perform a deflection test it is revealed that the base of many of the city streets are failing rapidly, and several require a full reconstruction. Their projections and budget analysis were totally off, because they were unable to detect how many streets had begun to experience structural failure. Instead of preemptive rehabilitations, the city must now focus their budget on digging themselves out of a hole. This situation can often lead to Major Pavement Backlog Problems within the community.
Short Term Savings, Long Term Benefits
Sometimes the pavement is in better condition than it looks. Performing a structural test may allow a city to defer some pavements that were otherwise scheduled to receive treatment. This can open up more funds for rehabilitation on segments that are in more desperate need of repair. For example:
An older city is facing budget shortcomings and looking to save as much money as possible by deferring a lot of their pavement rehabilitation needs. By conducting a deflection test on many of their higher traffic collector and arterial streets, they can make more accurate predictions about the deterioration rates of these streets. The city discovers that many of the roads with previously planned rehabs could be deferred several more years before they need repairs. This saves the city money and allows them to focus repairs on streets in more desperate need of work.
Each of the examples above is a common representation of a municipality that benefits through the added information that can only be gained through deflection testing. In practice, nearly every pavement network could save money and time through the more accurate pavement condition data provided by deflection testing. If your city is planning a pavement survey, be sure to consider performing a deflection test on your high-traffic streets. The benefits will continue to show for years to come.
Take a look at our Pavement Assessment Page to learn more about performing a Pavement Condition Assessment in your area.