President's Message - UAPA Newsletter - November 2014

Time flies when you are paving into November and making sure we complete as much work as we can before the snow arrives.  At this time of the year, it’s not hard to realize that we are in a very seasonal business.  That realization comes with certain challenges for resources in terms of both equipment and people.  

Part of our charge as an association is to help our members better tell our story to the next generation of our workforce and future leaders of our industry.  I had the opportunity last month to talk “Asphalt” with some Utah Valley University Construction Management students.  It was great to see that there are many young people that have interest in our industry; we need, as members of UAPA, to strive to help educate and give these folks opportunities to join our ranks.

Later this month we will present our first Southern Utah Asphalt Pavement Seminar.  The response so far from members has been great.  I would challenge all of you to reach out and invite our city, county folks, engineering firms, testing labs and all others associated with our industry to attend with us.  Waylund Ludlow and Reed have done a great job planning this event.  I look forward to seeing many of your there.

Thanks, for all you do as members to support our group.  We have many people attending meetings with UDOT, APWA, Questar Gas, and others every week helping tell the story of our industry and helping build and maintain a better infrastructure for everyone to enjoy.  For that, I thank you!

-Craig Fabrizio

Staker Parson Companies

UAPA President 2014-2015

Executive Director Message - UAPA Newsletter November 2014

The frost in the air and a few flakes here and there must mean that the construction season in Utah is quickly coming to an end in the next few weeks.  By most accounts, it has been a good season full of challenges and opportunities.  And while things may be slowing down on a lot of fronts, there are still a number of things happening with UAPA that you should be aware of – here a just a few of the highlights:

• The first-ever Southern Utah Asphalt Pavement Seminar will take place on the 18^th of November at the Dixie Center in St. George.  This one-day conference promises to deliver excellent educational sessions while also bringing in the Mayor of Washington City and the Director of Region Four for UDOT.  We’re excited to help this event become a success year in and year out and with over one hundred (100!) people already registered, I’d say we are off to a great start!

• Following on the heels of the Southern Seminar, the 2015 Utah Asphalt Conference seems to be lurking right around the corner and both the Steering Committee and the Planning Committee are in full swing developing another great agenda and event.  We’ve already sold a number of booths to returning vendors who rate our conference as one of the best in the entire nation!  I’m excited to see what the conference this year will bring, and I am happy to take your input on how we can make it even better as we move forward.

• As an association, we continue our engagement with various other entities, owners, and associations.  We’re proud to be a member of the Utah Transportation Coalition and we look forward to what promises to be a productive legislative session for the state of Utah in 2015.  Similarly, we continue to engage Questar Gas in ongoing task group meetings striving to find common ground in developing better rates for asphalt suppliers and producers.  As always, we’re proud to partner with the APWA and are pleased that UAPA has been offered a seat at the Specifications Committee for the APWA.  We’ve also worked closely with UDOT to make a change to the Binder Certification Program this year that should help with mix design submittals in the off season, and we continue to work with the DOT on a number of other issues including the DOT’s move to an IRI specification; the microsurfacing specification; longitudinal joints; and SMA.

As always, I am very thankful for everything this association does for our communities and our industry.  I am thankful for the commitment of many individuals who have put forth and sacrificed many hours on behalf of UAPA.  It is amazing to see that as a collective, we are making a difference.  There’s always plenty to do and things to improve, and I am thankful to be working alongside so many of you to make a difference each and every day!  In a season of Thanksgiving, I offer my sincerest gratitude for all of you! Thank you!

-Reed

Designing Mixes for Top Performance

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.