By Mike Anderson, P.E.
I read somewhere that sales at fast food restaurants are the worst in January. Makes sense; people are trying to start the year off by focusing on their health. I also heard that the best day for fast food sales is the Monday after the Super Bowl. I get that too. My diet is officially shot after eating throughout the day at our friend’s Super Bowl Party. I’m tired of cold/snow (check local weather for conditions in your area). I just want some fast comfort food.
Maybe it’s coincidental that Valentine’s Day is so close to January. Or maybe the powers that be knew that with our focus on healthy eating reduced we’d need to have something else, like love, fill that void. So why not focus on the balance between food and love? Which is why I’ve been busy working on a new app…TenderTinder. It’s an app that shows nearby restaurants where people who love food can get chicken tenders. Swipe right and the app gives you directions and an e-coupon. Chick-fil-A and I have a thing going on.
I think we all need balance in our lives – in food, love or asphalt mixtures.
The concept of “balanced mix design” isn’t really new, but it is seeing a rebirth. From a materials perspective, we know that we need a mix to have good strength to resist rutting, but we also need it to have good flexibility to resist cracking. The properties of the asphalt binder are important in that they contribute to the asphalt mix strength at high temperatures (stiffer is better). But the asphalt binder also has an impact on the flexibility of the asphalt mix at intermediate and low temperatures (stiffer is generally worse). Add in the fact that the volume of asphalt binder and air voids also impact performance and it’s easy to see how balance, while necessary, takes effort.
The accepted definition of balanced mix design is an “asphalt mix design using performance tests on appropriately conditioned specimens that address multiple modes of distress taking into consideration mix aging, traffic, climate and location within the pavement structure.” Whew. What it means is that you should test the mix under conditions that you expect in service to address cracking and rutting at least.
The developers of the Superpave mix design system during SHRP (Strategic Highway Research Program) believed that for roads with lower traffic volume, basic mix design predicated on achieving specified volumetric properties would be enough to ensure a reasonable level of performance. For roads with higher traffic volume performance-based testing was needed. This performance-based testing concept was known as Superpave Level 2 and Level 3 mix design. The problem was that the equipment needed for Level 2 and 3 mix design was sophisticated and expensive. Couple that with questions about data interpretation and modeling and the system never really went far.
The idea behind the balanced mix design system today is that volumetric properties can’t tell us everything – particularly considering the variety of materials (reclaimed asphalt pavement, recycled asphalt shingles, warm mix additives, fibers, recycling agents) available for use in an asphalt mix – so some form of performance testing is needed.
There are three types of balanced mix design that are in use today. “Volumetric Design with Performance Verification” means that performance testing is used to validate a mix designed using basic volumetric properties. No adjustments are made in the design using the performance test results. “Performance Modified Design” uses volumetric design as a starting point and then tweaks the design asphalt content based on one or more performance tests. “Performance Design” means that the design asphalt content is established based on performance test results, regardless of the volumetric properties. The further up the scale to true performance design the more innovation available to the contractor to make a mix that will perform in service.
Which performance tests should you use? It’s really up to you, depending on the distresses you need to address. At the least, we would expect a rutting test, like the Hamburg Wheel Tracking test, and a cracking test of some kind, like the Semicircular Bend (SCB) test. Users may choose different tests and criteria for their specific combinations of climate and traffic.
The AAPT-AI webinar series in 2017 is a good starting point to find out more about balanced mix design.
Feeling that I’ve accomplished my goal of sharing a little about balanced mix design concepts, I’m ready to return to my latest project, eHamburger, which uses 29 dimensions of toppings to find your best hamburger match. I am getting a little hungry. For information on Asphalt Institute’s laboratory training, research, and testing services please contact Mike Anderson (firstname.lastname@example.org) or Gary Irvine (email@example.com).
Mike Anderson is the Director of Research and Laboratory Services at the Asphalt Institute.