Dangers of Reheating Asphalt Mix
BY Jayanth Kumar Rayapeddi Kumar, P.E.
Reheating asphalt mix influences moisture levels in aggregate over time
In an asphalt laboratory, ensuring accurate test results on reheated mix comes down to a lesson we can learn from a batch of leftover French fries: moisture migration over time.
Consider that the humble French fry is beautifully engineered: crispy on the outside, soft on the inside, nicely salted, and just hot enough to make anyone forget about the will-start-eating-healthy resolutions they made. Now, think about what happens when you take the leftover fries, put them in a container, let them go cold and soggy, and reheat them the next morning. The magic is gone. Technically, they are the same potatoes, but because of time, moisture has migrated, fundamentally changing the product quality.
Asphalt mixes also go through metamorphosis. Fresh asphalt mix that comes out of the plant is hot, the binder viscosity is low, aggregates are nicely coated, and the mix is highly workable. But if the sample is stored in a cardboard box and cooled to ambient temperature, the properties of the mix change.
Leaving the sample inside the oven for a few extra hours could allow the aggregates to absorb more binder.
This happens because aggregates have natural pores. While the mix is hot and the binder viscosity is low, the binder slowly travels through those pores. Technically, this is known as asphalt binder absorption (Pba).
Because this absorption is a one-way street, reheating the cold mix will never force the binder back to the surface. This means Pba directly steals from effective asphalt content (Pbe), which is the actual binder left on the outside coating aggregates. Mathematically, when Pba goes up, Pbe goes down, triggering a domino effect across several volumetric properties.
The severity of this metamorphosis depends on the percent of asphalt binder absorption, which depends on the aggregate type used in the mix. A low-absorption aggregate like granite isn’t as “thirsty” as a high-absorption aggregate like the volcanic basalt used in Hawaii.
Reheating a sample that uses low-absorptive aggregates might only cause a minor nudge in the test results. On the other hand, reheating a sample with high-absorptive aggregates can be the difference between the mix passing or failing. To make matters worse, the number of times a sample is allowed to cool to ambient temperature and reheated to test temperature only compounds the problem.
This means two different labs could test the exact same split-sample of a mix produced using highly absorptive aggregates and get wildly different results, simply because one lab let the sample cool down completely and reheated the sample prior to testing.
Because the reheated sample has absorbed some of the binder, lowering the effective asphalt content, the mix is harsher to compact, leaving extra pockets of empty space known as air voids. To make matters worse, the reheating cycle also affects the maximum specific gravity, also known as the Rice value.
This is exactly where reheating of mixes strikes a project where it hurts. Typically, cores drilled from the project are compared against the Rice value to compute the percent density, which feeds the pay factor calculation. When the Rice value is calculated using a reheated sample, the value is skewed compared to the Rice value calculated using a fresh sample. This can easily lead to erroneous density values, making a good project in the field look like it failed on paper.
So, the question is, how do we prevent our lab results from turning into yesterday’s soggy fries? Here are some rules to ensure what’s paved is actually what’s tested:
Test the mix fresh whenever possible: This is perhaps the best defense against binder absorption. Remove the cooling phase to obtain the volumetric test results. However, this is easier said than done. Acceptance testing labs have their own challenges to test samples before they cool down to ambient temperature.
Establish a correlation factor: If you know samples will be cooled and reheated for independent testing, establishing a correlation factor early in the project to account for the data shift is a good idea.
Watch the clock: When a sample goes into the oven for reheating, it is critical to ensure the sample doesn’t stay inside the oven longer than it needs to after reaching testing temperature. Leaving the sample inside the oven for a few extra hours could allow the aggregates to absorb more binder.
Avoid multiple reheatings: To avoid compounding the problem, it’s best to avoid putting a sample through the vicious cycle of reheating-cooling-reheating.
At the end of the day, it’s important to make sure the lab results reflect the true quality of the pavement on the road. After all, nobody wins when a perfectly good paving job gets penalized over how the sample was handled. Just like nobody wants to be served yesterday’s soggy fries.
Jayanth Kumar Rayapeddi Kumar is a quality control engineer at Jas. W. Glover Ltd. in Hawaii and a licensed professional engineer in the state of Hawaii. He has over 14 years of experience in the asphalt industry and is an active member of ASTM Committees D04 (Road and Paving Materials) and E36 (Accreditation and Certification).