During my nearly forty years of asphalt work, there have been some challenges. Two of the biggest have been rutting and reflective cracking. Reflective cracking is still a tough issue. But, I believe we’ve got a pretty good handle on rutting. Yet, some problems still linger.
One rutting problem that I still see is at intersections. And, that bothers me – it’s not that we don’t know how to deal with rutting at intersections – it’s that we don’t routinely use what we know. More on that later.
We can build well-performing asphalt intersections and similar site-specific, high-stress pavements, such as truck-climbing lanes and weigh stations. There are several resource documents available to help those interested in doing so. AI publishes one, “PR-2, High Performance HMA Intersections.” The Asphalt Pavement Alliance (APA) also has one, “High-Performance Intersections.”
The general approach can be summarized as follows:
• Assess the situation.
• Ensure structural adequacy.
• Select and verify high-performance materials.
• Use proper construction techniques.
Assess the situation
This step typically consists of doing a functional and a structural evaluation to evaluate what’s in place. The functional evaluation looks at cracking, smoothness and frictional properties. The structural evaluation looks at sub-grade properties, pavement thickness, traffic and how the existing pavement reacts to loading.
Ensure structural adequacy
For new installations, this step is a pavement design that is adequate for the actual conditions (sub-grade, traffic, materials properties and pavement thickness). For existing intersections, this means an evaluation of the load-carrying capability of the in-place materials. Any failed or weak layer or material must be removed and replaced.
Select and verify high-performance materials
This is the mix design phase and then verifying the rutting resistance of the designed mix. Typically the asphalt binder is selected by “grade-bumping” two grades higher than the normally selected PG binder. Premium aggregate is needed (no rounded, flat or weak aggregate) with a high degree of particle interlock. High-gyration designs (typically more than 100) are needed. After the mix design is done, some type of mixture performance test (Asphalt Mixture Performance Tester, Hamburg Wheel Tester, etc.) should be performed to assure the selected combination of materials is rut resistant.
Use proper construction techniques
Three points of emphasis during construction are critical to the desired high level of performance: proper compaction, avoiding segregation and constructing good joints. Compaction greatly contributes to the strength and durability of the installation. Segregation must be avoided to eliminate weak and likely-to-fail areas. The same applies for poorly constructed joints.
I said earlier that we do not routinely install high performance intersections. One of the reasons that I hear given for this is the high cost. And, yes, done one at a time, the cost for a single intersection can be relatively expensive – but maybe not so much, if you consider the total lifetime costs. Many urban areas have a series of poorly performing intersections, and I would suggest that these intersections be grouped into a single, larger project to rehab them and bring down the unit costs.
Building well-performing intersections is not a mystery – it’s just doing the work the way it should be done. Let’s build better intersections.
Email Dwight at talkingasphalt@asphaltinstitute.org to talk asphalt.