Contractor’s Guide for Handling PMA
BY Ron Corun and Bob Kluttz
A primer for the handling and use of polymer-modified asphalts and highly modified asphalts
Authors’ Note: This article is based on the Association of Modified Asphalt Producers training workshop. The original presentation was developed by Ron Corun and edited by Bob Kluttz. No AI was used in writing this article.
Currently up to 20% of all paving asphalt is polymer modified. The most common polymer-modified asphalt (PMA) binders are in the range of PG64-28 to PG76-28. Key for all modified asphalts is to be safe and follow the manufacturers’ recommendations.
Handling and Storage
PMAs are not all created the same way. Mixing modified asphalts from different suppliers can cause the mixture to fail to meet the performance grade (PG) requirements.
Some guidelines help reduce contamination at the terminal. Make sure that the tanker trunk is empty before loading, then make sure to load from the correct loading arm. It is important to minimize the heel in both trucks and tanks. As little as 6 inches of residue in a tank can cause up to 10% contamination by a different grade.
At the asphalt mixing plant, use dedicated tanks, if at all possible. If a dedicated tank is not available, empty the tank as much as possible from the previous material and then add two or three loads to fill the tank before testing. Diluting PMA may cause the PG grade to fail.
Vertical tanks are optimum as they provide more efficient agitation. Most PMAs today do not require agitation to prevent separation, the exception being ground tire rubber (GTR). Horizontal tanks work fine for most PMAs but it’s important to circulate to achieve uniform temperatures above and below the heating coils.
For proper circulation it is important that the product should be pumped out of one end of the tank with the return to the other end of the tank. If the circulation is all at one end of the tank the other half of the tank will be a dead zone. It is also important to ensure that the return line goes into the tank below the liquid level. Otherwise, you are spraying liquid asphalt through the remaining air in the tank and essentially running RTFO on your virgin binder.
As mentioned above, beware of mixing modified asphalts from different suppliers as not all PMAs are created with the same components by the same process and may not be fully compatible. Direct fire heating is not desired. Direct fire heat tubes will have hot spots, which cause two problems. Hotspots will damage the polymer network and lead to loss of PG grade and/or undesired viscosity rise. Hot spots can also lead to coking, which effectively insulates the line requiring ever increasing heat to maintain temperature.
Highly modified asphalt (HP or HiMod) is increasing in use around the United States. Handling and storing HiMod is not much different than handling and storing conventional PMA. The same general guidelines apply, but shelf life of a HiMod may be substantially shorter than a standard PMA. Check with your supplier to get his recommendations.
An ideal tank storage temperature is 320-330°F. A key point for HiMod as well as all PMA is do not overheat it. Overheating can cause the HiMod to further shorten the shelf life. The hotter the temperature, the more rapidly aging can take place in the binder, so it is important to keep the temperature as low as comfortably possible for easy pumping.
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Long-term storage of PMA is feasible, but cooling is required. For 60 days or longer, the heat must be turned down or completely off. When bringing back up to temperature, do it slowly. The tank contents must heat up uniformly through conduction and convection until the material is pumpable. Use increments of 20°F over three to four days to get the material back up to circulation temperature.
Mix Production
Pay attention to optimum mixing and compaction temperatures for PMA. The classic rule of rotational viscosities at 135 and 165°C does not apply to PMA. The low shear rotational viscosity of conventional asphalt is similar to the viscosity at high shear—mixing and compaction. PMAs, however, are shear thinning. The apparent viscosity in a drum is much lower than that from a rotational viscometer. Consequently, rotational viscosity will yield a much higher temperature than is actually recommended. Again, your provider should recommend ideal temperatures for their product.
Ensure that all plant equipment is in good condition and suitable for PMA. PMAs tend to be higher in viscosity so they will draw higher amperage at the pump. The pump should be calibrated and the strainer likely will require larger than standard holes—you want a minimum of 1/8-inch holes.
At startup, circulate unmodified asphalt first. Once everything is hot, switch to PMA and circulate again before starting production. Switch back to unmodified asphalt after the shutdown of the shift to leave unmodified asphalt in the pump and the strainer.
The slat conveyor must be properly sized and in good condition. PMA will increase the amperage draw on the conveyor. It’s good practice to start out at reduced tonnage with unmodified mix to get everything up to temperature.
Again, not much is different with HiMod. The ideal mixing temperature is dependent on the mix as always, but 325 to 340°F is generally good. Check with your supplier on recommended warm-mix additives and again start up a little hotter and slower to heat up the whole system, slat conveyor, silos and trucks, to make sure every point in your process is allowed to get up to good operating temperature. On overnight modified asphalt storage there is varying guidance. If you are new at this, or if you have older storage systems, it is probably unwise to store overnight. If you’re well experienced with modified asphalts, if your silos in particular are in excellent condition, well insulated and well maintained, it may be quite feasible to store overnight.
Construction and More Thoughts on HiMod
When transporting to the paver, make sure as always that the truck bed is smooth and clean. Use of release agents is recommended, but do not use hydrocarbons. Tarping is a very good thing and probably required in your state. These same practices apply for general PMA and for HiMod. Placing hot-mix asphalt (HMA) generally requires no modifications to equipment. Handwork likely will be more difficult, depending on the mix. The ambient temperature guideline recommended by the Association of Modified Asphalt Producers is 50°F and rising.
Compaction equipment, again, is normal. Keep the mix temperature high enough for good compaction, but no higher. Every 10°F increase doubles the amount of fumes and the higher temperature can damage the PMA polymer structure. Keep the roller up right behind the screed. If available, direct temperature and density measurement is great. Compacting PMAs may actually be easier than unmodified asphalts as it’s been noted that PMA may help reduce or eliminate a tender zone.
There is not much different when placing HiMod. Aim for the temperature behind the screed to be about 300°F. Hotter is not better as it can lead to shoving. Keep the breakdown roller close to the paver. The only significant caveat, and this is an important one, is cooling. HiMod mixes are not any stiffer at operating temperatures than conventional PMA mixes. The cooling rate is about the same. However, the HiMod mix will stiffen up faster than a conventional PMA so your working time on the mix is shorter. This applies to cleanup as well.
Best practices from some states that regularly use HiMod have shown laydown of HiMod binders does not pose a significant problem as reported so far but keep in mind general good construction practices. Prevent end of load segregation. HiMod binders may magnify poor construction practices. Balance your production rate, plant to truck to paver to rollers. Keep the mix moving. Delays will add to cooling of the material, which will give you an increase in viscosity and difficult handling.
Be ready when the trucks arrive on the project as temperature is critical for the mix. Keep compactors tight with the paver. Adjust the rolling pattern as needed to maintain target density. Plant storage may be somewhat different as previously noted. HiMod binders do have a limited shelf life, so scheduling and frequent communication with the binder supplier and the agency is important. Do not store indefinitely. Follow supplier specific handling guidelines, storage temperature, storage time and circulation. Recommended best practices may vary with suppliers, so as with all modified asphalt follow your supplier’s advice.
Some final comments:
- Do not overheat. High temperature accelerates viscosity rise. Monitor viscosity daily.
- Warm-mix additives are good.
- Unused high polymer binder can readily be diluted to a PG76-22 or -28.
- Repeat—do not overheat.
A good quality control plan is as important or more so with PMA than it is with conventional unmodified asphalt. Especially if the plant uses both unmodified and PMA, all of your materials, all your tanks, pumps, valves, sampling points, all need to be well labeled. Establish standard procedures and hardware settings for asphalt flow into the storage and into the HMA plant. In summary, PMA improves the performance of asphalt pavements. Understand the product that you’re using and treat it with respect. Follow supplier’s recommendations and use best practices.