In earlier blogs, I looked at some of the factors affecting asphalt pavement performance. In this column, I am going to look at lab mix designs and how they can influence asphalt pavement performance.
There are several recognized types of lab design procedures, including Superpave, Marshall, Hveem, and specialty methods (like designing OGFCs). Each of the procedures has their advantages and probably some disadvantages. Superpave is the current state-of-the-practice and is widely used.
There are several steps in the lab mix design process. The steps include selecting the materials, optimizing the aggregate gradation (a good practice for Marshall designs, as well as for Superpave), determining the appropriate binder content, and finally, determining the selected mix’s performance properties (moisture damage or stripping resistance, rutting resistance, etc.)
Design life is an important consideration in selecting the ingredient materials. High type facilities or projects designed for long service lives should include better quality and longer lasting materials. If you know a pavement will serve for only a limited time, then the cost of premium materials is probably not justified.
Utilizing an appropriate laboratory compaction effort is important, whether Superpave, Marshall, Hveem or some proprietary or specialized mix design procedure is applied. The compactive effort forces the aggregate particles together and reduces the space available for asphalt binder and air voids. A lab compaction that is too low will result in too much asphalt and possibly a mixture which is susceptible to rutting in that particular application. Similarly, too much lab compaction results in a mix with too little asphalt and long-term durability becomes a concern.
Performance considerations
Some of the performance considerations addressed in the mix design process include:
Rutting resistance
The mix should not distort or displace under traffic. For the aggregate, the crushed particle count and gradation influence the rutting susceptibility. The asphalt binder?s grade/stiffness and content also affect the pavement stability.
Cracking resistance
Fatigue and reflection cracking are common pavement problems. Stiffer mixes are more rutting resistant, but typically more prone to brittle behavior or cracking. Asphalt binder properties have more influence on fatigue and reflection cracking than does the aggregate properties.
Low temperature cracking
Areas with cool temperatures experience pavement stresses due to thermal shrinkage, leading to crack formation. Select an asphalt binder with appropriate low temperature characteristics for the pavement location. Durability. Sufficient binder is needed to achieve an adequate film thickness on the aggregate particles. Proper film thickness reduces hardening and aging of the binder. Sufficient binder must be present to provide workability/lubrication of the mass of mixture to facilitate compaction so that permeability and aging is minimized.
Stripping
Moisture will eventually find its way into a pavement. The mix must be evaluated for moisture damage susceptibility and adjustments to the ingredient materials must be made, as needed. This can be done by using an anti-stripping additive, or by changing the asphalt binder or aggregate source, or by some combination of adjustments to produce a less susceptible total slate of materials.
VMA
The voids in mineral aggregate or VMA is critical to the workability and durability of an asphalt mix. The VMA is the volume of space in the mix available for the asphalt to occupy. In many designs, the VMA spec is the most difficult criteria to meet. The design goal is to balance the need for enough asphalt to bind the aggregate particles into a mass with having enough space to avoid bleeding of the asphalt when it expands with higher temperatures.
Mix stability
Traditionally, the stability or “strength” of a mix has been measured by the Marshall stability or the Hveem stabilometer number. The Asphalt Mixture Performance Tester has been introduced as the companion test to the Superpave mix design process for evaluating mix stiffness. The Hamburg test device and Loaded Wheel Testing devices are also used by some agencies to evaluate mix strength and rutting resistance.
Additional resources
There are several good resources for learning more about designing asphalt mixes. The Asphalt Institute has training sessions and manuals available through their website, www.asphaltinstitute.org. A fairly recent addition to the mix design toolbox is the Bailey Method of analyzing asphalt mixtures. The Bailey method is a tool that can be used to develop and analyze hot asphalt mixes in the lab and field. It provides a good starting point for a mix design and an aid for making adjustments at the mixing plant to improve air voids, VMA and the overall workability of the mix, for both Marshall and Superpave designs. The Bailey Method focuses on the aggregate?s packing characteristics and he resulting influence on mix volumetrics and compactability. Bailey training is available through AI.
Selecting the final design
For most applications, the final mix design will be the most economical one that meets all the design criteria. The design asphalt content will likely be a compromise to balance all the properties. Generally, the mix should not be designed to maximize one particular property. An exception might be for a specific application. For example, in extreme loading conditions, a stiffer, stronger mix with higher air voids and a lower asphalt content might be selected to minimize the rutting potential.
Verification of actual mix properties
In addition to the lab mix properties, it is important to recognize that plant produced mixes may vary significantly from the lab design. Adjustments to the gradation and aggregate structure and binder content may be necessary. It is important to verify the volumetric and performance properties of the actual, produced mixture as quickly as possible in order to make timely adjustments.
Conclusion
Developing an appropriate lab mix design is a critical step in achieving asphalt pavement performance. It is well worth the time and expense to do the lab design well. Some challenges remain in lab designs—particularly, in the design of some warm mixes, but that is material for another time. With the procedures available today, the likelihood of designing a well-performing mix is excellent.
