Lab Corner: When you hear hoofbeats

By Mike Anderson, P.E.

One of the best things about the internet is the ability to find pretty much any information you want. It may also be one of the worst things. WebMD is not a friend to those of us who periodically can’t sleep and participate in a late-night search of symptoms – real or imagined.

I inevitably find myself fixated on the cause of the symptoms being something very rare and ultimately fatal, although I know better. I try and remind myself of the advice they give doctors trained here in the U.S. – if you hear hoofbeats, think horses, not zebras. In other words, look for the more common, likely answers first before jumping to the rare. The same advice can be offered to asphalt technologists when we are conducting forensic analyses of asphalt pavements, trying to understand why there was a failure.

So, what do we look for when performing a forensic analysis on a failed, or failing, asphalt pavement? The basics – asphalt binder content (volume), aggregate gradation and percentage of air voids in the compacted mix (density) – are a great place to start to evaluate the likely contribution of the impact of materials and construction to the distress. Failures that are related to materials and construction often come back to the information we get from a few simple tests.

How do we proceed in our lab?

• First, if we receive cores or slabs, we cut them to the appropriate size, separating the mix layer(s) of interest. After cutting, we need to make sure the sample is completely dry before testing. You can dry the cut specimen in front of a fan until the mass doesn’t change appreciably. A faster alternative if you’re impatient and want to return to your WebMD search is to use a device like a CoreDry. Simple equipment; very handy for quick drying.

• Next, we determine the bulk specific gravity of the compacted mix specimen (Gmb) using the saturated surface dry (SSD) method in AASHTO T 166 or ASTM D2726. Note that this method is only valid for specimens without open or interconnected voids or those specimens that absorb less than 2.0 percent water during the procedure. For cores, the latter caveat is most important.

In those cases, you should dry the core again and determine the specific gravity using AASHTO T 275 (paraffin-coated specimens) or AASHTO T 331 (vacuum-sealed specimens). You can guess which one is preferred by most technicians.

• Third, we heat the core to drive out any residual moisture and break it down into a loose mix sample. Typically, 110°C will get the water out but may not make it easy to break up the core. AASHTO R 67 Appendix X2 is a good reference. If you do need to heat the sample hotter than 110°C only do it for a short period of time. Just like binder testing, we recommend heating at the lowest temperature for the shortest time to get the job done.

• Fourth, we separate the sample to an appropriate size and determine the maximum theoretical specific gravity (Gmm) of the mixture following AASHTO T 209 or ASTM D2041. You may have to combine two or more cores to get enough material for a valid test.

• Last, we perform a solvent extraction procedure (AASHTO T 164 or ASTM D2172) to determine asphalt binder content and get the extracted aggregate gradation. You can use the ignition oven procedure (AASHTO T 308), of course, but if you want to recover the asphalt binder you have to perform a solvent extraction followed by a recovery procedure (we use ASTM D7906). Being the Asphalt Institute, we always want the asphalt binder; we leave no binder behind.

What information do we get from this testing?

• Asphalt binder content – critical to performance. Too much and you could have rutting or flushing; too little and you could have cracking.

• Aggregate gradation – provides information on nominal maximum aggregate size, relative fineness of the aggregate blend, and the amount of dust, or percent passing the #200 (0.075 mm) sieve.

• Gmm – used with asphalt binder content to calculate the effective specific gravity, Gse , which can give you an idea if the aggregate has changed from the design.

• Gmb – used with Gmm to determine the percentage of air voids in the compacted sample, telling you how open or closed the mix is, which can be related to rutting, flushing and cracking.

If you performed a solvent extraction with recovery, you could even test the physical properties of the asphalt binder as it exists in the mix and determine some aggregate properties as well.

The tests and properties discussed above are your workhorses. They should give you enough initial information to judge if the failure you are investigating could be related to materials and construction or something else. There are all kinds of other testing that you can do for further investigation, but the conclusions from that testing might not be so black and white.

For more information on the testing and training services provided by the Asphalt Institute Laboratory, located deep in the heart of the Bluegrass, please contact Mike Anderson, Gary Irvine, or Wes Cooper. We’re all thoroughbreds here.

Mike Anderson is the Director of Research and Laboratory Services at the Asphalt Institute.