Designing mixes for top performance

Posted 08/18/2014


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.