We are in the heart of asphalt paving season, but some local agencies may still be gearing up because of the fiscal calendar and perhaps because of the COVID-19 pandemic. Regardless, local agency personnel are sometimes at a disadvantage with paving simply because the terminology can be so alien. Let’s talk about some of those terms so you can be a better consumer.
You may hear in your travels reference to Superpave and warm mix, so let’s start there briefly, if for no other reason than a historical context. The SUperior PERforming Asphalt PAVEments (Superpave) mix design method emerged in the 1990s from the Strategic Highway Research Program (SHRP) and was intended to replace the Hveem and Marshall methods. The goal of Superpave was to balance the high-temperature rutting and low-temperature cracking characteristics of asphalt concrete (which is what fancy people call asphalt). While Marshall mixes are still used in some parts of the country, we are pretty much Superpave people in the Mid-Atlantic. Durable, long-lasting pavement is the goal, and you can learn more from an early introduction by the Federal Highway Administration and the National Asphalt Pavement Association.
Asphaltic concrete and bituminous asphalt concrete just two of the terms used by academics and researchers for what in practice we more commonly call hot mix, asphalt, blacktop, hot sticky stuff, or just plain pavement. If you’re really old, you call it macadam or tarmac, and there’s nothing wrong with that, either.
Many of us became accustomed to the term hot mix, but in Delaware our mixes are now primarily Warm Mix Asphalt (WMA), a proven technology in the first round of FHWA’s Every Day Counts. Simply put, this is a technology that allows producers to lower the temperature at which the material is mixed and placed on the road and the benefits can include reduction of paving costs, extending the paving season, improved compaction, greater hauling distances, and improved working conditions by reducing exposure to fuel emissions, fumes, and odors. Producers use asphalt foaming technologies, organic additives, or chemical additives to lower temperatures 30 to 120 degrees Fahrenheit lower than traditional hot-mix asphalt. Explore the EDC site to learn more about WMA.
The mix design you will commonly work with in Delaware is PG 64-22. Relax, this is simple. PG is simply a reference to Performance Grade binder (i.e., the asphaltic material used to coat the aggregate). The numbers are in reference to the average 7-day maximum pavement design temperature, 64˚C (147 ˚F), and the average 7-day minimum pavement design temperature, -22˚C (-8 ˚F). You may also see PG 70-22 and PG 76-22, but these are typically used where there will be high volumes of fast moving (large) trucks or high volumes of slower moving trucks, respectively. For many lower volume, lower speed roads without a lot of heavy trucks or turning movements, PG 64-22 will be typical.
When you watched the FHWA/NAPA video referenced earlier, you saw the mysterious gyratory compactor. For most of us, there’s not much to see there, but the quality assurance folks at the production plants and at laboratories like DelDOT’s Materials and Research Lab use this instrument to fine tune mix designs and ensure compliance. In Delaware, generally 115 gyrations are used in areas where there is lower truck traffic and 160 in high truck areas.
You may see carbonate vs. non-carbonate stone choices. Non-carbonate stone is more resistant to polishing than carbonate stone and so, unless you have higher volume, higher speed roads, there’s no need to pay the extra money because you probably don’t have skidding issues.
Aggregate (stone) is also part of the mix specification. Typical aggregate sizes are 9.5 mm (⅜”) and 12.5 mm (½”). Smaller aggregate provides a smoother finish; larger aggregate provides greater bulk strength. For the country club entrance, you might see 4.75 mm, but you don’t need it for your subdivision street. Now you would also use 4.75 mm in a thin lift mix, but that is a topic for another day. In thicker base courses, you will often see 19 mm mix, but you don’t want to drive on that typically.
Recycled asphalt pavement (RAP) will often be included as part of your mix, perhaps as much as 20%. When they mill up your old pavement, that’s where this goes. It saves virgin natural resources and costs. You may also see recycled asphalt shingles (RAS). You should be able to ask your contractor for the mix design and it should specify whether and how much recycled product is included. This and other information should also be on the material delivery tickets you receive with each truck – don’t just accept them; review them to ensure you are getting the product you ordered.
Release agents are used on the paver and in the truck beds and these are typically emulsified oil, soap solution, or citrus compounds. Never permit the use of diesel fuel.
The screed is the last piece of the paver to touch the asphalt mat before the rollers come into finish compaction. The screed condition and its use is critical to achieve a high percentage of initial compaction and provide a smooth riding mat.
“Forty degrees and rising” is a common phrase to reference when you can pave. Typically, 40 ˚F is the minimum ambient air temperature for paving; 
hence it’s forty degrees and the temperature is going up. Thicker lifts and warm mix asphalt can sometimes allow you to go below forty degrees, but should not pave when air temperatures are below freezing. You never want to pave on frozen pavement or when it is raining or going to rain.
Tack is essential to good bonding and it is, unfortunately, one of the more commonly compromised components of paving. Sometimes, performance grade binder is used (for thin lift, for example) for even greater adhesive qualities, but tack is typically an emulsion (it turns out oil and water do mix, but only under the right circumstances) that goes down brown in an even coat of 0.05 to 0.15 gallons/square yard. Emulsions used in Delaware is typically CRS-2P – there is a whole system of secret codes (C for cationic, RS for rapid setting, 2 for high viscosity asphalt, P for polymer). After a period of time that can range from 10 minutes to much longer, depending upon atmospheric conditions, the emulsion “breaks” and turns black as it releases the water from the emulsion. Do not compromise on tack. Or surface preparation. Or compaction. Or segregation.
A lute is a special hand tool with a serrated edge used by the crew to adjust the mat prior to final compaction. It should not be confused with a garden rake, which should never be used. Lutes and only square point shovels on your job, please and thank you and they should be used sparingly. On most jobs, an experienced paver operator should be able to put down a mat that requires only minor adjustments. Of course, there may be some need around manholes or driveway entrances, also, but overuse or use of pointed shovels or rakes will increase segregation and reduce the pavement performance.
We’ve mentioned segregation twice, so let’s talk about why it’s evil. Segregation is the opposite of homogenous, at least for our purposes. We want a well-mixed asphalt product (binder, aggregate, additives) and from the time it is mixed at the plant to how it is loaded in the trucks, dumped in the paver hopper, moved through the paver, and overworked with hand tools, it can become segregated. When it does, potholes develop prematurely and the 
strength of the mat is uneven. Segregation will try to compromise your pavement at every step of the job and it is evil. A good contractor will exercise best practices to minimize the potential for segregation and a good project owner will make sure the contractor does so. If short pavement life is your goal, you will love segregation.
That’s not all you need to know about asphalt paving, but it is a good start on the terminology. If you are the project owner, you are writing the checks, and you are entitled to the project you specified. That begins with knowing what you’ve specified and what you are receiving. The Delaware T2/LTAP Center offers a half-day training workshop once or twice a year to help beginners get comfortable with asphalt construction and you should sit in on it the next chance you get (fingers crossed – this fall). You will be a lot more comfortable after this training course, so be on the lookout for it.
The Delaware T2/LTAP Center’s Municipal Engineering Circuit Rider is intended to provide technical assistance and training to local agencies and so if you have pavement questions or other transportation issues, contact Matt Carter at matheu@udel.edu or (302) 831-7236.
You must be logged in to post a comment.