When the time comes to plan a pavement condition survey for your city, there are a lot of factors to keep in mind. Which software do you use? What functional classes will you survey? Do you want to include structural condition data? At IMS, one of the most common questions we are asked is how conducting a deflection test will affect the overall analysis results, and how that might impact budgeting decisions. Let’s examine that question further, using a real world example.
First, understand that the purpose of a deflection test is to measure the structural strength underneath the pavement. The condition data gained from this new information can dramatically affect the overall assessment of a street, leading to much more accurate deterioration predictions and more optimized budget allocations throughout the network.
In order to recommend an appropriate rehabilitation treatment, a street’s condition is summarized by a number referred to as a PCI (Pavement Condition Index) score. This number takes into consideration surface distress measurements, Roughness Index score, and occasionally structural condition data from a deflection test. The simplest way to see how much of an impact deflection data has on treatment recommendations is to calculate a street segments PCI score with and without deflection data.
Here is a real example of a town that used structural condition data to save hundreds of thousands of dollars:
Lynnwood is a beautiful, small town located in Snohomish County, Washington just north of Seattle. When the town decided to conduct a pavement survey back in 2016, town officials sought after the most reliable pavement condition data possible. Officials in Lynnwood elected to perform deflection tests on all their arterial and collector roadways. IMS considers deflection testing on arterial and collector streets to be, “best practice” as these streets see a majority of the traffic and are more vulnerable to base failure from compounding load associated distresses.
The most common trend that emerges from reviewing the analysis of Lynnwood is the increase in PCI resulting from the implementation of deflection data. On most roadway segments, PCI is higher when it includes the structural condition data. This simply means that the streets of Lynnwood are built on a strong base, but suffer from surface distresses. From a pavement management prospective this is very good news for Lynnwood, because surface treatments are far cheaper than reconstruction jobs. Let’s take a look at one of the most common situations in the Lynnwood analysis.
On one street designated by Lynnwood as a Minor Arterial the initial PCI score was calculated to be a 58. From this score an Edge Mill + Moderate Overlay was suggested at $29.00/yd2 .
The analysis was then calculated to include the structural condition data from Lynwood’s deflection tests.
With the structural data included the PCI was calculated to be a 66 with the recommended rehabilitation to be an Edge Mill + Thin Overlay at $20.00/yd2 .
With the structural data implemented the newly recommended treatment results in $9.00/yd2 cost reduction. To put this is perspective; this results in Lynnwood saving approximately $306,000 per mile on this road without compromising the quality and longevity of their repairs.
In this small town network, this is just 1 of 170 similar rehab adjustments analysts identified as a result of deflection testing.
If deflection testing saved the small town of Lynnwood, Washington over $300,000 per mile, can you imagine the impact it would have in your home town?