A Preservation Primer: Which Pavement Facelift is Right for Your Situation?
“Preserve” can be defined as to keep in good condition or to keep safe from harm; to prevent decay and maintain. Think about preserving or upkeeping your home’s exterior. Rather than tearing everything down and starting over, properly timed preservation techniques can help to keep your house looking like new and functioning properly and efficiently. Some of these techniques include painting or replacing the windows or the roof. Applying these treatments at the right time prevents costly repairs and even complete reconstruction.
Maintaining an asphalt pavement is like preserving your house. There are times you don’t have to do full reconstruction of the road or even mill off the surface of the pavement. There are treatments to extend the lifespan of the pavement, keeping it in good condition and safe for the driving public. Not only does this extend the life of the pavement, but it also benefits the bottom line. Roads are a capital investment. Maintaining them properly can save money and stretch a tight budget.
Simply stated, pavement preservation is applying the right treatment to the right place at the right time. An asphalt deterioration curve can help determine timing and treatment.
Surface preservation treatments are meant to delay the need for rehabilitation or milling of the structural asphalt. As the pavement moves from fair to poor condition, the treatments available to you must be more structural in nature. Figure 1 shows an asphalt deterioration curve with different treatments applied to the pavement as the condition deteriorates.
Pavement preservation treatments can be divided into four categories:
- Surface treatments
- Pre-treatments
- Recycling and reclamation
- Base treatments
Some common surface treatments are crack seal, rejuvenating fog seal, micro surfacing and thin lift HMA overlay. Let’s discuss those in some depth.
Crack Seal
Placement of material into existing cracks.
Crack filling is the placement of modified asphalt material into non-working cracks to prevent the infiltration of water and incompressible materials like gravel and sand. Routing and sawing is performed as needed to create a more uniform area for the filler allowing for better control and ensuring a proper seal to the existing pavement.
There are certain conditions that should exist for a pavement to be a good crack sealing candidate:
- New or recently rehabilitated surface
- Limited linear cracking
- Little or no secondary cracking
- Little or no raveling at crack face
- Good base support
- Limited structural deterioration
Cracks must be cleaned and dried prior to applying the filler or sealant. Having a dry road is even more important than the air or pavement temperature. It’s also important to keep the meter tank no less than ¾ full to maximize the effective melting point.
Rejuvenating Fog Seal
Process of restoring chemical properties of the aged asphalt binder.
A rejuvenating fog seal or asphalt rejuvenation does not provide any structural improvement, rather it’s an emulsion that penetrates the top 3/8” of pavement restoring the aged asphalt binder. The effectiveness of the treatment is typically determined by the reduction of the binder viscosity. It is a “top of the curve” preventative maintenance and extends a typical pavement life by 3-5 years per treatment when applied at the right time.
Asphalt rejuvenation works by re-proportioning the aged fractions (asphaltenes) to the fluid portion (maltenes) of the asphalt binder. The maltene fraction gives the asphalt binder its flexibility. Asphaltenes effects the viscosity of the asphalt; as this fraction increases with age, the viscosity also increases making the pavement less pliable and prone to cracking. Thus, by restoring the maltene to asphaltene ratio like that of a new binder, the life of the pavement is extended. This process is not unlike a bowl of spaghetti (asphaltenes) with sauce (maltenes).
Over time the water in the sauce evaporates causing the sauce to dry out or age making it difficult to move or separate the spaghetti. Refreshing (rejuvenating) the sauce brings the bowl of pasta back to life, making it easier to separate the spaghetti noodles and move them around. Rejuvenation aims to restore the pavement’s flexibility and resistance to cracking.
Micro Surfacing
Surface treatment mixture of polymer-modified asphalt emulsion, mineral aggregate, water, and additives. The mixture is made and placed on a continuous basis by mixing the ingredients using specialized equipment.
The micro surfacing formulation is based on mix design parameters: resistance to raveling, lateral displacement and resistance to moisture damage. The emulsion plays a key part in micro surfacing. The wrong emulsion or an emulsion that doesn’t work well with the other components can cause issues such as long set times, durability, and delamination.
Remember that temperature, humidity, and wind conditions all affect emulsion break and cure times. If using a micro surface in combination with crack sealing, ensure crack sealing material is not excessive where it can be picked up by the paver tires and spreader box runners. Screen the material immediately prior to loading the mobile support unit so there is no possibility of contamination when re-handling the material. Treat vegetation with herbicide early to prevent it from returning after the project is completed.
Many agencies have reported ten years or more of service life when micro surfacing is installed on newer asphalt pavements in good condition. However, when used as a “band aid” or as a stop-gap measure on an asphalt pavement in marginal condition in order to delay rehabilitation/reconstruction, surface treatments including micro surfacing will have a shorter service life. Having the proper mix design for the micro surfacing is critical to ensure all materials work together. Some emulsion chemistries do not work with certain aggregate combinations, and some react poorly to excessive mineral filler.
Thin Lift HMA Overlay
Applied thin layer (1.5 inches or less) of hot mix to the surface of the road.
Thin lift overlays can be used to extend the life of a pavement by sealing small cracks and providing a new riding surface. Overlays can be applied with or without milling, but the existing pavement must be in good condition since the overlay provides little additional structure. Thin lift overlays are available as conventional dense graded mix, open graded friction course (OGFC) and stone matrix asphalt (SMA).
Thin lift overlays improve ride quality and surface friction, correct surface defects, enhance appearance, seal the surface, and reduce splash and spray.
Regardless of the pavement preservation technique, quality control and quality assurance (QC/QA) are vital prior to, during, and after construction. Look at these three elements of the typical preservation project—the emulsion, the distributor truck, the aggregate.
Emulsion
- Certificate of Analysis (Manufacturer)
- QC Sampling and Testing
- Temperature
Distributor Truck
- Calibrate ➔ Quantity
- Proper Pressure, Spray and Coverage
- Speed
Aggregate
- Proper Certified Test Results
- Quality Control Results
- Mix Design
While the materials for HMA and pavement preservation treatments are the same (aggregates, liquid binder, emulsions), the equipment varies, and the material testing is different. In efforts to standardize the testing and material selection, the American Association of Highway and Transportation Officials (AASHTO) created an emulsion task force (ETF) in 2008. Material standards, design standards and construction guides have since been developed, and in a 2012 passage from the Moving Ahead for Progress in the 21st Century (MAP-21), state DOTs could begin to use federal funds for pavement preservation.
This high level of confidence was only one benefit to the on-going AASHTO standardization efforts. Other benefits include educating the next generation of DOT staff and preservation crews and offering a consistent way to select materials, test and design. Constant collaboration and education will only further enhance the quality of the pavement preservation techniques, ultimately extending the life of our pavements.
The team at Asphalt Testing Solutions & Engineering LLC, Jacksonville, Florida, prepared this back-to-basics article for the AsphaltPro and PreservationPro audience.