Asphalt ain’t rocket science — but there are several things that directly affect pavement performance. Paying attention to these critical details is important to achieving the expected performance. Over the years, through experience I have identified these factors as some that matter.
Appropriate materials
You must use the right materials. The asphalt binder must be suited for the climate and the traffic. It must be the right binder to contribute to rutting resistance and to avoid cracking. The aggregate has to be clean and consistent. The appropriate mix must be applied. There are a wide range of mixes available today and using the one most suited for the particular application goes a long way to providing a long lasting pavement.
VMA
The voids-in-mineral aggregate content controls the durability of a mix. In my opinion, it is the most critical part of a mix design. There must be sufficient space within the aggregate structure to accommodate an adequate asphalt film thickness for durability and to still retain room for void space to allow some consolidation/compaction under traffic, etc.
Adequate structure/pavement thickness
A pavement cannot perform if it is not strong enough to carry the loads. The foundation, either sub-grade (for a full-depth pavement) or the aggregate base for a conventional pavement, must be stable and strong. The pavement structure must be able to carry the design traffic loads. It is important to recognize, and design for, future traffic increases. It is not a premature pavement failure if the anticipated traffic level is reached long before the number of years in the design life is reached. Remember “Build it and they will come” holds true for roads, too. The traffic on our pavements is incredible.
Managing segregation
Segregation probably cannot be completely eliminated, but it can be managed. Managing segregation is really about being careful of how materials are moved and handled. Paying attention to stockpiling, feeding the bins, proper flights alignment, maintaining appropriate materials levels, applying good loading and unloading procedures, etc., all make a real difference in how consistent the mass of material remains. Even having a well-designed mix makes a difference.
Compaction
The strength and durability of a pavement is greatly dependent on achieving interlock of the aggregate particles. Compaction is dependent on the mix properties, the environmental conditions, and the placement conditions. Obviously, mixes with rough-textured surfaces and stiff binders take more effort to compact. Cool air and paving surface temperatures and thin lifts reduce the time for compaction. The stiffness of the underlying materials also influences compaction.
Joint construction
Too many pavements fail because the joints open up, allowing air, water and incompressible materials to enter the joint. The problem typically is the result of segregation or poor density. Good construction technique is critical.
Drainage
Water is the enemy of pavements. It erodes the base support beneath the pavement and can break the bond between the aggregate and the asphalt coating, leading to stripping, raveling, potholes, etc. Both surface and subsurface water must be managed. And the drainage systems must be maintained. If the drainage is blocked, water will find a way out and it probably will not be where we want it to come out.
Timely maintenance
Although many pavement owners may be struggling with funding, delaying timely maintenance and repairs does not pay. Delaying preventative work simply allows deterioration to accelerate and worsen and to result in higher repair costs, ultimately. Pavement owners should look at the Pavement Management concept and plan their pavement maintenance activities accordingly. Maintenance work to slow the rate of deterioration of a pavement keeps your investment in a better condition and delays more expensive rehabilitation work.
Commitment to doing it right
Countless amounts of money have been spent on research and yet we often routinely ignore well-established good practices. I still see diesel fuel being sprayed in truck beds, or we pave over roadkill or trash. Come on, folks; we know better than that. Getting a long lasting pavement takes a lot of good decisions and we cannot afford to make any bad ones. Many of our pavement performance problems are the result of simply not following through with what we know should be done. Yes, we need to continue performing research to find better materials and processes. But, it may be more important to apply what we already know to be the right way to do things.
There are many resources available to teach us or remind us of good practices. There are training sessions available; there are publications and webinars, for those who need them. Take advantage of these opportunities, as needed.
These are some of my hot-button points. I will cover some of them in more detail in future columns. What do you think influences asphalt pavement performance? Let me know.