Simplify the Search for Optimum Asphalt Content

Simplifying the search for Optimum Asphalt Content (OAC)

Author’s Note: This article presents a simple procedure that can be used to estimate optimum asphalt content. A more detailed version of this article, titled “Simple Analytical Procedure to Estimate Optimum Asphalt Content,” was originally published in ASTM International’s Journal of Testing and Evaluation, Vol 53, No. 2, 2025.

Asphalt producers allocate significant personnel and material resources to develop hundreds of hot-mix asphalt (HMA) mix designs annually. Traditional mix design methods such as the Marshall, Superpave and Hveem methods require multiple samples to establish the optimum asphalt content (OAC). For example, the Marshall method requires at least 17 samples to determine the optimum asphalt content. This includes two samples for measuring theoretical maximum specific gravity (Gmm) and three samples at five different asphalt content levels to understand how asphalt content affects air voids. The Superpave method requires at least 12 samples to estimate OAC.

Express Your RAP, RAS Content

Many state and federal highway agencies are strongly pursuing the implementation of balanced mix design (BMD), which requires fabrication and testing of substantially more samples to develop a compliant asphalt mix. Unlike traditional volumetric mix design methods, all of which have volumetric requirements such as limits on the voids in mineral aggregate (VMA), voids filled with asphalt (VFA), etc., at the design asphalt content for a preselected design air voids level, BMD introduces performance testing to evaluate cracking and rutting potential of mixes into the mix design process.

While some state agencies have opted to retain traditional volumetric requirements as a starting point for mix development, others have moved toward a more performance-focused approach that may reduce emphasis on volumetrics. Nonetheless, since developing a mix design using BMD or otherwise is a detailed, time-consuming process that demands significant effort, volumetrics most likely will continue to play an important role, either as a screening tool or a starting point for trial blends.

Current practice on mix design requires specific volumetric criteria to ensure performance and durability. Among these, one of the difficult challenges for mix designers is achieving the minimum required VMA. The Asphalt Institute’s MS-2 manual even points out that it’s not uncommon for designers to finish their laboratory mix design, only to find out that the VMA falls short of the requirement. To overcome this, designers tweak the aggregate proportions and asphalt content until the mix meets the VMA criteria.

But the challenge does not stop at the design stage. During production, VMA can collapse because of aggregate breakdown, usually by about 0.2% to 0.5%, according to the article “Fix Your Mix” published in Asphalt Institute’s Asphalt magazine in June 2016. To stay ahead of this problem, designers typically aim for a VMA slightly above the minimum, giving their mixes extra leeway for real-world conditions.

Current mix design methods are iterative, relying on trial and error to select the right aggregate structure and binder content to meet various volumetric criteria. It is like trying to navigate an unfamiliar city using a paper map by unfolding and refolding the map, squinting at tiny street names, taking wrong turns, and often backtracking to reach the destination. It takes patience and a lot of effort. Yet, just like switching from paper maps to a GPS makes navigation smoother and faster, a simple tool would be useful to take the hassle out of estimating OAC.

Balance Your Mix Design for Asphalt

Proposed Estimating Procedure

The proposed procedure is based on three well-established equations used in volumetric asphalt mix design and a newly developed equation based on these.

Using these three equations, an analytical expression shown in Equation 4 is derived to estimate the optimum asphalt content (P̂_b). A detailed explanation of this equation is provided in the referenced ASTM paper.

Pa—Air voids (%)

Pb—Percent Binder Content by total weight of mix (%)

Gmb—Bulk Specific Gravity of compacted specimen

Gmm—Theoretical Maximum Specific Gravity of loose mix specimen

Gsb—Bulk Specific Gravity of combined aggregate blend

Gse—Effective Specific Gravity of aggregate

Gb—Bulk Specific Gravity of asphalt binder

VMA—Voids in Mineral Aggregate (%)

It is useful to note that, for fixed air voids (Pa), the estimated bulk specific gravity of a compacted sample (Gmb) decreases as asphalt content (Pb) increases in Equation 1. This occurs because the theoretical maximum specific gravity of the mix (Gmm) decreases with increasing asphalt content. Similarly, for a fixed VMA, the estimated Gmb increases as Pb increases in Equation 2. The intersection of these two curves represents the point at which the mix simultaneously satisfies the target Pa and VMA criteria. The corresponding Gmb at this point is referred to as the critical bulk specific gravity (Gmb−crit) and the corresponding Pb is the estimated OAC of the mix.

Based on Equations 3 and 4, only six inputs are required to estimate OAC. These inputs are:

• Trial asphalt content, Pb (at which the loose mix for Gmm testing is prepared)
• Gmm at trial asphalt content
• Gb of the binder
• Gsb of the combined aggregate blend
• Target air voids, Pa
• Target VMA for the mix

Compaction’s Role

There was no mention of compaction effort throughout this discussion, which raises an important question: Can we accurately estimate optimum asphalt content without factoring the compaction level? The Asphalt magazine article “A Look at Superpave5” from May 2019 addresses the issue of the effect of compaction level on Superpave mix design methodology by stating “A common myth is that laboratory compactive effort (N_design) will change asphalt content, but data does not support that.”

The Federal Highway Administration reinforces this in its December 2010 Tech Brief titled “Superpave Mix Design and Gyratory Compaction Levels” stating “there is a misconception that gyratory compaction level controls the voids and asphalt binder content of the mix. In truth, it is the gradation and aggregate properties and their resistance to compaction that controls the air voids and resulting binder content.” The Tech Brief also notes that, because of the need to stay competitive, asphalt producers modify their design aggregate structure within the design limits to make it possible to obtain a new aggregate skeleton such that a minimal amount of asphalt binder is used to achieve target air voids. It further points out that the air voids of a mix are controlled by VMA and not by compaction level. Other researchers have arrived at similar conclusions. While the above is true for the mix design process, it is also true that for given aggregate properties and a fixed gradation, as compaction level increases, the binder content must decrease and vice-versa to achieve the target air voids.

How AMIBIND Helps Crews Lower Temps Without Changing Plants

The premise of the procedure presented in this article is that the aggregate structure and compaction level have indeed a substantial effect on the resulting mix volumetrics. However, Equation 4 provides an estimated OAC achieved with some unspecified compaction level only for a trial aggregate blend. If volumetrics cannot be satisfied with that OAC at the selected “design” compaction level, then the designer will need to modify the aggregate structure. This is no different than with any other design method, but it is done with minimal testing. The procedure is particularly useful when working with new aggregates and blends. It offers a simple, analytical, and efficient starting point by estimating the likely optimal Pb based on the target Pa and VMA.

Based on this information, the designer can fabricate one set of specimens to check whether the mix meets all the target requirements. If it does, the mix is compliant. If not, the designer will know early on that adjustments to the aggregate structure are needed, without trial-and-error using multiple test batches. The single point verification ensures less waste of time, effort and materials.

From Equations to Application

With the six input values, a mix designer can quickly use Equations 3 and 4 to generate the estimated OAC for the blend. Once OAC is known, the designer can prepare two samples to determine Gmm, and compact two gyratory or three Marshall specimens to determine Gmb and calculate Pa and VMA. If the calculated volumetrics meet the target criteria, the job mix formula can be considered established. The same blend can then be tested at other asphalt contents to complete the mix design.

Evaluation of the Regressed Air Voids Approach for Mix Design

Confidence in the Procedure

The proposed procedure was validated using a combination of laboratory-prepared mixes, previously formulated designs from multiple asphalt producers, and long-term pavement performance (LTPP) data. More than 65 mix designs in total were evaluated.

Estimated OACs from the procedure were compared to the values from traditional mix design methods. The results demonstrated excellent agreement, with a correlation coefficient of 0.99. Full details of the validation process are provided in the ASTM paper.

Benefits of the Proposed Procedure

Optimum Asphalt Content: Solved Analytically

For any given aggregate blend, the OAC can be estimated analytically using Equation 4, requiring just six inputs, all of which are already part of traditional mix design methods. This equation is built on well-established volumetric equations that have long been the foundation of Marshall and Superpave mix design approaches.

A Quicker Route to Job Mix Formula

Unlike traditional mix design methods, which can require up to 17 and 10 samples using the Marshall and Superpave methods, respectively to establish the OAC, the proposed procedure significantly reduces laboratory effort. Assuming no alternative blends are being evaluated, OAC can be determined with four to five samples, which saves time and materials to establish the job mix formula.

Build the Test Section for Validation of Plant Produced Mixture

An Easier Way to Manage VMA

It’s not unusual for mix designers to reach the end of the mix design process and discover that the VMA criterion hasn’t been met. Also, it’s industry knowledge and experience that VMA can collapse during production. The proposed procedure offers a practical solution to both issues. It gives the mix designer the flexibility to input a target VMA that accounts for expected collapse during production. For instance, if the design VMA requirement for a mix is 15.0, but experience shows VMA typically drops by about 0.5% during production, the designer might enter 15.5 to estimate OAC. This proactive adjustment helps ensure the final mix meets targets not just during design but also in production.

Streamlining Testing for BMD

In the context of BMD, any approach that can simplify volumetric verification can be a valuable tool in a designer’s toolbox. In this regard, the proposed procedure may offer a way to ease some of the added workload that comes with BMD. By reducing the number of specimens needed to validate a selected aggregate blend, this method provides an opportunity to streamline the early stages of mix development.

Four Tips for Lab Sampling

Final Thoughts

Designing asphalt mixes to meet volumetric criteria can be simplified. The procedure presented in this article and detailed in the ASTM paper referenced earlier provides a more efficient, analytical approach to estimating OAC using fewer samples and familiar equations. We believe it offers a practical option that may help save time and resources, while simplifying the search for optimum asphalt content.

Request access to the longer, more technical paper titled “Simple Analytical Procedure to Estimate Optimum Asphalt Content” in Vol 53, Issue 2 of ASTM International’s Journal of Testing and Evaluation here: https://store.astm.org/jte20240242.html

Adrian Ricardo Archilla is a professor in the Department of Civil, Environmental, and Construction Engineering at the University of Hawaiʻi at Manoa, with more than 25 years of experience in asphalt materials characterization and pavement engineering research. Reach him at archilla@hawaii.edu.