Four areas of focus for longer-lasting open-grade friction course pavements
By Dr. Grover Allen, Ph.D., P.E. and Dr. Buzz Powell, Ph.D., P.E.
Basic asphalt mix design teaches us that the following steps are required to ensure a long-lasting pavement. First, high-quality asphalt and aggregate materials are selected. Then, the aggregate structure and asphalt content are designed so that air voids are reduced to the absolute minimum amount possible without causing other issues. Reducing air voids maximizes density and generally maximizes its lifespan. This normally means targeting six percent or less air voids in the compacted mixture. The final pavement should be resistant to all major distress, including moisture intrusion and damage. That just about summarizes it.
Now, open-graded friction course (OGFC), also known as porous friction course (PFC), rapidly absorbs and drains rainwater beneath the surface, by using basically the opposite design strategy of a dense-graded mixture. OGFC mixture design aims to maximize air voids and water intrusion. In fact, OGFC mixtures are typically designed to have air voids in the range of 15-20%. Say that again!
So, let’s get this straight…if reducing air voids increases lifespan, doesn’t that mean increasing air voids to three times the normal amount decreases lifespan? Well, yes, that is exactly what it means if something isn’t done to counteract the inherent poor durability of these mixtures. Experience has shown that the typical lifespan of an OGFC is roughly about half that of a densely compacted mixture. You may have heard Department of Transportation (DOT) representatives refer to their OGFC failure rates as “an epidemic” or heard them mention that if something isn’t done to make them last longer, their already razor-thin budgets will simply not be capable of sustaining these pavements any longer. They need more durable surfaces, plain and simple.
Now, you might assume that we have stopped building OGFC pavements due to this growing problem of poor durability, but that isn’t the case at all. DOTs have increasingly made it mandatory for OGFC pavement surfaces to be constructed on all major highways and interstates. So, now that we know there’s a major durability issue with our OGFC pavements, why do we still build them? This creates quite the dilemma now, doesn’t it?
We will explain why OGFC pavements continue to be built despite their known durability issues. We’ll also share examples of poor OGFC performance in the field and provide real implementable solutions for how to make OGFC pavements last much longer. Can we make them last as long as a dense-graded mixture? It is entirely possible with the right approach. Let’s dive in and see why we should continue building these pavements and how to make them last much longer.
Why do we build OGFC pavements if they don’t last as long as standard asphalt pavements?
Well, it’s complicated. We can understand that OGFC pavements drain water more efficiently than standard pavements. They accomplish this because their high-void structure allows rainwater to quickly leave the surface where it is absorbed into the top pavement layer and drained transversely toward the pavement shoulder as shown below.
See, it was probably never a great idea to design pavements that require water to drain on the surface of the pavement in the exact location where tires must maintain contact with the surface to avoid skidding off the road. When heavy traffic loads repeatedly drive over the same location (known as the wheel paths) on the pavement surface, small indentations, or ruts, form. This creates a place for water to pond. During wet weather, most drivers will try to straddle these depressed wheel paths to obtain better traction and avoid hydroplaning, a condition in which the tires of the vehicle lose direct contact with the surface causing the vehicle to spin out of control.
Practically anyone who has ever driven a car has used this method to avoid hydroplaning. So, back to the original point. Is drainage really so important that we’d be willing to sacrifice half of the pavement surface life to improve it? It depends on how concerned the agency is about the increased potential for vehicle accidents, injuries and fatalities. Data compiled in Japan indicates that about 80% of all fatalities on roadways occur in wet weather, and surfaces that rapidly drain water reduce those fatalities by about 70%. Wow! Those are big numbers.
Now, all roadway fatalities could be eliminated if we stopped driving altogether, but no one is likely to agree to do that. Society as we know it would crumble if we did. We accept inherent risk vs reward in the basic activities of society, and roadway fatalities are no different. However, it has been deemed essential by many DOT agencies that we build safer roadways, thus increasing human life by using a design that inherently reduces pavement life. Now you know why we continue to build OGFC pavements even though they don’t typically last nearly as long as standard asphalt pavements. Let’s look at some examples of a typical OGFC lifespan.
Typical condition of OGFC pavements over its lifespan
Now, let’s first acknowledge that putting a lifespan on a pavement can be difficult, since so many variables can affect how long a pavement lasts, and different pavements will have different lifespans. However, we can consider average pavement lifespan data just to give us a benchmark. It is often cited that an asphalt pavement surface should typically last 15-20 years, and according to NCHRP Report 877, the typical lifespan of an OGFC is 6-12 years. If we take the midpoints of those two ranges, we get approximately half the lifespan of an OGFC compared to a standard asphalt pavement. That tells only half the story. What type of condition is the OGFC in prior to reaching the end of its life?
The images below provide some insight into a typical OGFC pavement condition over time. As shown, the OGFC pavement may begin to exhibit raveling as early as three years into its service life and would potentially have very noticeable roughness and raveling at about five years of service life. Of course, some OGFC pavements may perform better than this and some worse, hence the broad 6-12-year lifespan range. This seems unacceptable to build a very expensive pavement that only remains in good condition for just a few years. We could get into a lot more here, but this should at least highlight the primary reason there is a high level of concern among DOTs about their OGFC pavements and why we should be focused on making these pavements last much longer than they currently do.
How do we make OGFC pavements more durable and long-lasting?
There are four distinct areas in which an OGFC pavement can be altered to extend its lifespan quite significantly, and those are (1) materials selection, (2) design, (3) construction and (4) preservation. The following approaches indicate substantial OGFC durability improvements if multiple of these strategies are implemented simultaneously.
1. Material selection
Selecting a binder that is more age-resistant than a typical binder should be a requirement for building OGFC pavements. Since we are selecting a mix type with high air voids, which leads to much higher rates of oxidation, we must counter that effect during material selection.
There are a couple of categories of materials that have shown the ability to slow the aging rate of asphalt. These include highly polymer-modified asphalts (HiMA, or HP) as well as a category of asphalts simply referred to as “age-resistant asphalts”, which have been researched since 2019 under FHWA-PROJ-19-0011. Report BE287 prepared for the Florida DOT, (Arámbula-Mercado et al., 2019) predicted that HP may increase OGFC life by as much as 7-8 years.
The graphs above show the change in chemical and physical aging properties of two control binders (indicated by the dotted red line and normalized to a value of 1.0) after those same binders are treated with different aging-resistant additives. Any value below the dotted line represents a slower aging rate compared to the control, and any value above the dotted line represents a higher aging rate compared to the control. As shown, some of these additives have the potential to reduce the aging rate by as much as 70-90 percent. We should require better binder aging performance for our OGFC mixtures.
2. Mix design
Now, OGFC mixtures are required to have high air voids, but that shouldn’t stop us from pulling the different levers available to us to increase the durability of our OGFC mixtures. This simply means altering the way aggregates and binder are proportioned. The aim of mix design for improving OGFC durability should be to:
• increase binder film thickness (thicker films age slower)
• add baghouse fines (a mastic created from dust is stronger and more durable than just a binder)
• reduce air voids (less voids means slower aging).
Now, the first two are fairly straightforward, but I know what you’re thinking about the latter suggestion. Isn’t the objective to maximize air voids? Well, the objective is to create interconnected void space sufficient to permit rapid drainage of rainwater. Maximizing total air voids can certainly help in that regard, but so can making the voids that are there more interconnected. Reducing overall voids within the allowed range from 20% to 15%, for example, while maintaining the required permeability, or rapid drainage of the OGFC mixture, can lead to a substantial improvement in the final OGFC mix durability. That should be the goal.
Skilled mix designers will have a few tricks available on how to achieve this. A common way of measuring OGFC durability is using the Cantabro mass loss test. This test is widely used and strongly correlated to OGFC mix durability. Critically aging OGFC mix before Cantabro testing can be used to simulate aging and make results even more meaningful. Research has shown that loose mix aging for four hours at 275°F before specimen preparation makes Cantabro testing a more rigorous screening tool. A balanced mix design (BMD) for OGFC can be achieved when production binder content is high enough to provide good, aged durability while not so high the binder drains down during production. Abrasive aggregates preferred by some agencies in OGFC mixes can also be more absorptive, which makes critical aging even more important. Agencies can also consider using 3/8” nominal maximum aggregate size (NMAS) OGFC (proven at the NCAT Pavement Test Track) for higher AC, aggregate supply sustainability, improved long-term permeability, reduced noise and lower rolling resistance.
The data sets above show the Cantabro mass loss improvement associated with each of these strategies while still maintaining the minimum permeability required. According to these data, adding 2% baghouse fines, increasing binder from 5% to 7%, and decreasing air voids from 20% to 15% reduces Cantabro mass loss by 30%, 40% and more than 80% respectively! Now, results will obviously vary with different materials and mixtures, but these experiments show just how big of an impact relatively minor changes in mix proportioning can have on optimizing OGFC durability.
3. Construction
There are two primary areas to highlight in which construction can “make or break” the OGFC mix durability. One is the bond between layers, and the other is mixture cooling rate. If a strong bond is not achieved between the underlying layer and the OGFC layer, premature raveling of the surface is inevitable. One of the best examples of poor tack coat application and subsequently, poor interlayer bonding occurring is shown below and was published in Report No. FHWA/TX-12-0-5836-2 “Performance and Cost Effectiveness of Permeable Friction Course (PFC) Pavements.” This OGFC pavement on I-35 began failing about mid-way through its intended life, which led to a forensics investigation and the cause of the premature failure was identified as poor bonding.
Regarding mixture cooling rates, it may surprise many to know that the thickness of the lift has a dramatic effect on the cooling rate of that mixture. For example, if a 300 °F mixture is being constructed at 50 °F ambient temperature, the construction crew has 44 minutes to complete compaction operations for a 3-inch lift; whereas, a 1-inch lift (which is a common thickness for OGFC) only allows for seven minutes to complete compaction operations under the same conditions. If seven minutes is not enough time to complete compaction or seating of the OGFC, the minimum allowable temperature to place that mixture may need to be increased. A warm mix additive can also be used while maintaining the same production temperature (typical hot mix temperature) to increase the compaction window for OGFC mix. An added benefit of a warm mix additive is enhanced stripping protection. Another key consideration when using a warm mix additive is that if the production temperature is reduced, this can reduce asphalt absorption, which increases effective asphalt content and durability. These considerations relative to how they impact OGFC durability would be an agency decision. However, the important thing to be aware of is that failing to properly construct the mixture will ensure it will fail prematurely.
Several other practical steps can be taken to improve the construction quality of OGFC mixes. For example, cleanup with diesel fuel at the end of the work shift should be avoided. At shift startup, it may be ideal to run a whole waste load of mix through the paving train to preheat and remove all release agent residue. Milling should be deep enough to avoid all scabbing in the top of the underlying layer that can facilitate delamination. Some agencies are now using a stone mastic asphalt (SMA) with a 4.75 mm (#4) nominal maximum aggregate size to minimize leveling and control project costs when deeper milling is necessary. Because OGFC is rock heavy, dense-graded thickness tapering may facilitate raveling. Saw cutting and keying may be more effective.
4. Preservation
We all seem to be aware that very expensive assets, such as the vehicles we drive, must receive routine maintenance; otherwise, their lifespan will be very short. However, we often ignore this fact when it comes to pavements. We can’t put an asphalt pavement into service and just ignore it until it has reached a critical distress level if the goal is to increase the pavement’s durability and longevity. This is especially true for an OGFC mixture, which ages at a higher rate. All types of emulsified asphalt treatments can be applied to standard asphalt pavements to extend their lifespan, but OGFCs require more caution because many of these treatments contain asphalt that can clog the voids meant to allow rapid drainage of rainwater. One maintenance strategy that will not clog the voids and has a good track record of success in slowing the aging rate is the rejuvenating fog seal. Any agency interested in getting the most out of their OGFC life should consider this treatment in conjunction with known methods of maintaining the required friction of the OGFC surface. It shows the effects of rejuvenating fog seal on in-service asphalt stiffness, measured before application and four weeks after application. Asphalt stiffness is strongly correlated to common asphalt distresses, such as cracking and raveling. Research is ongoing at the NCAT Pavement Test Track to capture this effect in OGFC pavement, which is expected to be even more pronounced.
Summary
We have a major OGFC durability problem, and the agencies are primarily the ones facing difficult decisions in managing it. Due to the much higher driver safety profile of OGFC pavements compared to standard asphalt pavements, agencies are now specifying more and not less OGFC pavements – many states require OGFC on all major highways.
Therefore, the OGFC durability problem will only grow if it is not addressed. If our efforts are focused on known methods published in various literature to enhance these mixtures, their lifespan will increase, and in many cases very substantially. We can achieve these improvements of OGFC during material selection, design, construction and preservation. If we optimize each of these areas, it is entirely feasible to make OGFC pavements last at least as long as standard asphalt pavements, if not longer, and at a much lower life-cycle cost than what is currently expected of our rapidly deteriorating OGFCs. Research into OGFC durability issues is constantly underway. Several states are presently investigating these principles at the NCAT Pavement Test Track.
Allen is an Asphalt Institute Regional Engineer. Powell is the Technical Director for the Asphalt Pavement Alliance.