The Science of Pavement Preservation

The Science of Pavement Preservation

By Chris Evers / Published March 2017

The Villages, 2016, before and after rejuvenation

 

Have you ever looked at the roads in your community and wondered why they went from that attractive, smooth condition to the tired, worn, and bleached-out mess they are now?  I mean, the roads only see an occasional trash truck and a few cars driving to and from their homes, right?  Yet your gated community’s roads look rougher than the surrounding city streets and have not lasted nearly as long as intended. 

Join the club!  This is a common experience for private community residents, board members, and managers.  Let’s examine the causes of asphalt deterioration and how a pavement preservation program can extend the life of your road to eliminate those concerns and ultimately save you money!

The Cause of Road Failure

In order to adequately explain what happens to your road as it ages, let’s take a peek at the road starting at birth.  Your road was born under extreme conditions: the asphalt plant mixes the aggregate and asphalt binder together under high temperatures, between 300F and 330F.  High temperatures are necessary to blend the materials together. Later, when the paving contractor ships the material to the project and places the asphalt mix, the higher the temperature, the easier it is for the paving contractor to pave and compact the asphalt to the desired density.

Roads with PCI performance grades of 80 or higher, such as this one in The Villages, are good candidates for pavement preservation.

Exposure to high temperatures is unavoidable, but the higher the temperature of the asphalt mix, the more short-term aging occurs to the asphalt binder.  The light ends in the asphalt binder, called maltenes (more on them later), begin to burn off at around 275F to 285F.  It is the loss of maltenes that causes aging. In fact, on average, your newly paved road has already aged two years on day one of its application!

As the years go by, the asphalt binder gets progressively more brittle as evidenced by lab measurements produced by an instrument called the dynamic shear rheometer, which measures the viscosity of the binder.  This increase in viscosity (the higher the viscosity, the more brittle the asphalt) is one of the major causes of asphalt road deterioration.

My Road Needs a Plan

The loss of maltenes is a natural and regrettable by-product of hot mix asphalt production and aging, especially in the tenacious Florida sun.  That’s why pavement preservation is as important to your road as an oil change is to your car.  “Changing the oil” in your road at the right time can extend the life of your road by four or five years.  Unfortunately, while the public sector generally does a decent job of preserving its roadways, few private communities start the preservation process early enough, if they maintain them at all. 

Predictably, roads that aren’t properly cared for will underperform.  Such underperformance is evidenced by cracking and the loss of aggregate or raveling, which refers to the loss of fines that occurs once the binder has become brittle.  So, now that you understand the need for a plan, how do you get star-ted creating one?

Step One: What’s the Grade?

All roads fit somewhere on a 100-point curve called the Pavement Condition Index (PCI).  Think of it as your road’s performance grade.  At different PCIs your road will require a different plan.  For instance, roads that are from 100 to 80 PCI are in good condition and will benefit from “top of the curve” preservation methods, such as asphalt rejuvenation.  Roads between 70 and 80 PCI require a solution a bit more extensive, such as an asphalt surface treatment.   Roads that rate in the 50s or 60s are a better fit for mill and overlay.  Once you get into the 40s and below, the road likely has some structural deficiency that will require rehabilitation rather than preservation. 

Roads with PCI performance grades between 70 and 80, such as this one in a private community in Vero Beach, are good candidates for an asphalt emulsion seal coat.

Therefore, the key to extending the life of your roadways is to grade your road to determine what it needs.  Even if your road didn’t have a plan from day one, you can still establish a plan at any time, as long as you know what grade your road is. 

Step Two: Get in the game

For this first example, let’s use a road with a pavement grade of 89 PCI. The sample road was paved four years ago and still looks good with no obvious signs of distress, such as cracking or raveling.  An 89 PCI is a good grade, and you want to keep it as such for as long as possible.  This road is a good candidate for a maltene-based asphalt rejuvenator, as previously explained. 

Now that you know what type of solution is needed, you need to find a quality contractor to perform the work.  One resource worth using is your friendly, local Public Works Director. If you share with them the PCI grade of your road, they will likely be comfortable recommending a few contractors for you.

Roads with PCI performance grades between 70 and 80, such as this one in a private community in Vero Beach, are good candidates for an asphalt emulsion seal coat.

When interviewing potential contractors for your road work, you need to talk to them about more than just the price.  Ask them to review the road and to explain how their product will extend the road’s life. Find out how long it will be before the road can be driven on and how long the treatment will last.  Ask the contractor to supply a list of references and take the time to contact those references with questions about workmanship and long-term performance.  In addition, you will want to ask the contractor to supply quality control testing from previous projects that demonstrate their product works. 

As a rule of thumb, if the contractor is using a true rejuvenation product, it can be expected to reduce the viscosity of the asphalt binder by at least 40 percent. Equally important, the contractor must assure you that this reduction is not achieved through the use of a solvent (bad!), because you do not want to damage the binder. 

Step Three: Stay in the Game

Now that you have completed your first pavement preservation treatment, the next step is to monitor the grade of the road.  By year four after the first rejuvenation, it’s time to evaluate how the road is performing and prepare to retreat the pavement.  Since you treated the road in the first four years of its life, it will be easier to extend its life and stave off more extensive repairs. 

A beautifully preserved public road, Winter Haven, Polk County.

But eventually all roads get down into the 70s and will require the second most common type of preservation treatment, which is either an asphalt emulsion seal coat or a process called micro-surfacing, which is a mixture of emulsion and aggregate applied on the road with a type of paver. 

One important thing to remember for roads with PCI grades between 70 and 80 is to always choose asphalt-emulsion-based products, rather than coal tar products.  Coal tar is rarely used on public works projects, but private communities do not always understand its hazards.  The health and environmental issues related to coal tar are due to the presence of a human carcinogen called PAH (polycyclic aromatic hydrocarbon), which is 1000 times higher in coal tar than in asphalt-based products.PAH is also toxic to aquatic life, adversely impacting nearby lakes and rivers. 

FAQs

Question: What’s the right time to preserve my road? 

Answer: The earlier the better; recommended best practice is to rejuvenate roads in the first one to five years.

Question: What if I missed the boat and my road is too far gone (below 80 PCI)?

Answer: If it’s between 70 and 80 PCI, it’s time for an asphalt-emulsion seal coat or micro-surfacing.

Question: Oops! We waited too long for that too…now what? 

Answer: Sometimes it’s best to defer preservation treatment until you resurface your road.  If that’s the case, make sure you get in the preservation game once the road has been resurfaced.

Conclusion

Now you know why your roads look the way they do and how to evaluate preservation strategies for extending the life of your pavement.  Pavement preservation applied at the right time can yield a 200 to 300 percent return on investment.  As long as your roads are good candidates, as described in this article, the science supports using pavement preservation to extend the life of private community asphalt roadways.Now, get in the game!

Chris Evers

Technical Representative for Pavement Technology, Inc.

Chris Evers has served as a technical representative for Pavement Technology, Inc. since 2011 and has been involved in the road building business since 1996. He is a past president of the APWA Florida Chapter, and an active participant on its Legislative Affairs committee. He founded and continues to moderate the annual APWA International Public Works Director Roundtable and was also instrumental in founding the Florida Pavement Preservation Council. Chris is a popular pavement preservation educator/speaker throughout Florida and resides in Vero Beach. For more information on Pavement Technology, visit www.pavetechinc.com or contact Chris via email hidden; JavaScript is required.