By Dwight Walker
Today’s asphalt paving mixtures are a diverse and complex group of materials. Pavement engineers can select from dense-graded mixes, open-graded (permeable) mixes, stone matrix asphalts (SMAs) and mixes with various reclaimed components and numerous modifiers.
Asphalt mixes can be produced by a hot or warm process. Then there are specialty mixes for very targeted applications. One common element of all of these asphalt mixes is that they have to be designed in order to meet performance expectations.
Purpose of mix design
The purpose of a mix design is to find an economic combination of asphalt binder and aggregates that will provide long-lasting performance. The mix design process uses a series of lab procedures to select an appropriate blend of aggregate sources and sizes and determine the type and amount of asphalt binder.
It should be recognized that a mix design is just the starting point for achieving the desired asphalt pavement performance. Other factors, such as structural design, construction practices, and maintenance operations significantly influence pavement performance.
A poor or inappropriate mix design can contribute to poor pavement performance. So it is important that a mix design be done properly. Good design procedures are based on sound research and many years of observing the performance of asphalt pavements.
A good mix design procedure closely simulates actual field conditions. The goal is to closely model the performance of the actual mix that will be produced, including binder absorption, compaction during construction and under future traffic, moisture damage sensitivity, and rutting and fatigue properties.
Mix design basics
Designing a mix generally consists of the following steps:
• choosing the aggregate types, sizes and combined gradation;
• selecting the type and grade of asphalt binder (if not already specified by the owner);
• preparing and testing the test specimens; and
• determining the binder content.
A good mix design has to achieve a balance of desired properties, which can include stability, durability, impermeability (or, in some cases permeability), workability, flexibility, fatigue resistance and skid resistance. The design gradation and binder content are selected to optimize properties for each specific application.
Balancing the mix properties can be somewhat challenging. For example, a mix must have sufficient asphalt to be durable, but too much asphalt contributes to rutting. Similarly, there must be enough air voids (in the compacted pavement) to accommodate some additional compaction under traffic, but too many air voids allow air and water to enter the pavement and contribute to damage. And a mix must be compactible during placement but not become unstable under repeated loads.
For dense-graded mixes, the design is generally based on optimizing air voids and VMA. Field performance has shown that dense-graded mixtures designed with low air voids (generally less than two percent) can be susceptible to rutting and shoving. Similarly, experience has shown that mixes designed with more than 5 percent air voids are susceptible to durability concerns such as oxidation and raveling.
Mix designs with RAP
With the growing interest in using higher amounts of Reclaimed Asphalt Pavement (RAP) this mixture component must be carefully considered in the mix design. These materials may be able to be used in well-performing mixtures, but some adjustments may be needed.
Consistency of the recycled materials is a very real concern with these materials, particularly at high usage rates. Both the quality and quantity of the old asphalt binder is subject to differ if the source of these materials changes. Many agencies have specific rules on how these components can be used; check before developing the mix design.
Bailey Method
Experienced mix design practitioners occasionally encounter mixes which seem very sensitive to really small gradation changes. Gradation variations that are within the allowable job mix formula tolerances may result in very different air voids and VMA properties from previous results. For these sensitive mixes, small changes can make big differences. The Bailey Method provides a tool for dealing with these mixes.
The Bailey Method evaluates the aggregate “packing” characteristics (or how the aggregate particles fit together). Having this information allows the mix technician to make educated adjustments to aggregate structure.
According to an Asphalt Institute training presentation, “It was originally developed as a method for combining aggregates to optimize aggregate interlock and provide the proper volumetric properties. The procedures have been refined to a systematic approach to aggregate blending that is applicable to all dense-graded aggregate mixtures, regardless of the maximum size aggregate in the mixture.”
Bailey works for coarse and fine graded mixes, as well as for SMAs.
Performance testing
After a mix design is developed, performance testing can be done to estimate performance prior to use. The two primary distresses evaluated are rutting and cracking.
Rutting occurs at high pavement temperatures under loaded conditions. As the temperature increases, the mix softens and is more susceptible to movement under loading. Rutting (or permanent deformation) develops when the mix deforms under load and then does not recover to its original position. Rutting tests are conducted at high temperatures to represent the in-service temperature experienced by an asphalt mix in hot weather.
The Marshall stability and Hveem stabilometer tests have historically been used to provide an indication of rutting resistance. More recently, other rutting tests have been introduced, including Loaded Wheel Testers (LWTs), the Hamburg Wheel-Tracking Test (HWT), and the Asphalt Mixture Performance Tester (AMPT).
Loaded wheel testing can be used to approximate the rutting susceptibility of an asphalt mix. The LWT runs a wheel over a mix specimen at an elevated temperature. After a specified number of loading cycles, the amount of rutting is determined and compared to established criteria. The Asphalt Pavement Analyzer is a commonly used version of an LWT.
Another commonly used loaded wheel test is the Hamburg wheel-tracking test (HWT). The HWT is used to evaluate rutting and stripping susceptibility. A loaded steel wheel tracks back and forth over test specimens to induce rutting. Samples can be tested dry or while submerged in water. Rutting tests should be performed under dry conditions.
The Asphalt Mixture Performance Tester (AMPT) can perform uniaxial testing, and the flow number from this test can be used to evaluate rutting potential. The flow number test is a repeated-load creep test that is performed at a temperature similar to that experienced at the placement location of the mix. A commonly used practice is to choose the average 7-day maximum pavement temperature at a depth of 20 millimeters.
Cracking performance
Asphalt cracking is generally caused by repeated traffic loading (load-associated) or by temperature changes (non-load-associated). Load-associated cracking occurs at all pavement temperatures when the loading of the mix causes tensile strains to develop that exceed the tensile strength of the mix. Load-associated cracking is often referred to as “fatigue cracking.” This type of cracking occurs as the mix becomes stiffer and cannot resist the repeated load deformations.
In recent years, cracking has been observed where the cracks begin at the surface of the mix, usually on the outside edges of the wheel path, and work down. This type of distress is called top-down cracking. Classic, bottom-up, fatigue cracking is usually found in thinner pavements constructed on a granular base (or other base layer). The top-down cracking is most often observed in thicker pavements, or asphalt pavements constructed over a rigid base (like an asphalt overlay of a concrete pavement). Top-down cracking may also be durability cracking caused by increasing stiffness of the mix as it ages in-service.
Load-associated cracking tests are usually conducted at intermediate temperatures to represent the temperature experienced by the mix throughout the year. Some of the tests to evaluate load-associated cracking include the flexural beam fatigue test, the resilient modulus, (Mr), test, and several procedures developed by various universities.
The properties of the asphalt mix, and specifically, the asphalt binder properties, affect the susceptibility to non-load-associated (or thermal) cracking. The properties of the aggregates, the amount of asphalt in the mix and the degree of compaction all influence the mix performance. But, for thermal cracking, the asphalt binder properties are much more important. So, the mix designer usually relies on selecting the proper binder grade to address low temperature cracking.
If the asphalt binder contains large particles (250 microns or larger), like some ground tire modified binders, or if the mix contains fibers, mix testing to evaluate low temperature performance may be needed. Indirect tensile creep and indirect tensile strength are used for this purpose.
Mix design resources
This article is intended to provide an introduction to asphalt mix design. In order to actually perform mix designs, much more detailed information is needed. There are numerous resources available from the Asphalt Institute (classes, webinars, manuals) and other sources.
Tuesday, September 30, 2014
Monday, August 25, 2014
UAPA President's Message - August 2014
As members of UAPA this time of year brings all sorts of challenges. Our people have been going strong for a few months and yet we notice that the days are getting a little shorter and the air is a little cooler in the mornings. Deadlines loom and we feel ourselves getting anxious about completing the work for the season. Welcome to the road construction industry, it's what we do!
It was nice having a break this week and spending time as an association at our annual Golf Classic at Eaglewood Golf Course. Thanks to Reed and his great planning and execution, the tournament was a great success. We had more than a complete field of participants (it slowed things down a bit) and we had great support from many sponsors. Other than a little scare from Mother Nature as we started, it was a great day. I think the best part was our contribution to the Boys and Girls Club of Sandy, giving back to charities like this should make us all feel better.
Going into the fall we have many things to look forward to as an Association. Plans are in the works for our first Southern Utah event to spread the message to members and non-members alike in the southern part of the state. That along with our continued Lunch and Learns and the UAPA presentation at the APWA Luncheon we will continue to keep preaching the gospel of Asphalt.
Thanks for all you do,
Craig Fabrizio
Staker Parson Companies
Looking Towards Fall and the Southern Utah Seminar
A big 'THANK YOU' to everyone who helped to make this year's UAPA Golf Classic a resounding success. From sponsors and participants to volunteers and vendors, please accept my gratitude for helping us raise money for the Sandy Club. The best part of working with the Sandy Club and helping them raise funds this year is the fact that 100% of our donation will be match by the Larry H. and Gail Miller Family Foundation! The kindness and goodness of this industry continues to amaze me - thank you for making a difference in the lives of children and families across the Wasatch Front.
As I stood on the 8th Hole selling Hole-In-Ones for the tournament, it was not hard to feel the chill in the air, especially with that wind blowing in morning! Feeling that chill, I realized that fall is just around the corner and that means so is the UAPA Southern Utah Seminar. Open your calendars and mark the 18th of November as a historic and important date for UAPA, for the 18th is the date for the very first UAPA Southern Utah Seminar at the Dixie Center in St. George. The Southern Utah Seminar promises to deliver a full day's worth of education, networking, and training for our industry similar to our Lunch and Learn Series and the Utah Asphalt Conference that takes place each spring.
I hope you will join us on the 18th in St. George. As an organization, we are excited to increase the depth and reach of the association by providing the same quality of presentations and presenters to other areas of the state because this truly is the Utah Asphalt Pavement Association and we need to make sure that we reach all areas of the state. Registration for the Southern Utah Seminar will open soon. There will be opportunities to sponsor, exhibit, and present at the seminar. Please do not hesitate to shoot me an email if you have an interest. More information will follow soon!
-W. Reed Ryan
Executive Director
Utah Asphalt Pavement Association
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