When the National Asphalt Pavement Association (NAPA) released its 7th Annual Asphalt Pavement Industry Survey IS 138 September 2017, it included information showing a continuing increase in the use of warm-mix asphalt (WMA) product in the United States for the 2016 construction season. The increase in use was somewhat slower than increases of past years, but the authors of the report reminded us that “as demand for asphalt pavement mixtures increases in the public sector, WMA use is expected to similarly increase.”

With that in mind, this annual look at WMA best practices focuses on another fact from the NAPA report: “Production plant foaming, representing nearly 77 percent of the market, is the most commonly used warm-mix technology; chemical additive technologies accounted for a little more than 21 percent of the market.”

That doesn’t mean asphalt mix producers should hold chemical additive use for WMA production in any less esteem than mechanical means of WMA production. In fact, a number of excellent WMA additive producers can be found in the pages of this publication and should be called on for more information, including best practices regarding injection and how their products can assist in much more than mere temperature modification. For the confines of this article, let’s dive into mechanical techniques.

Foam All the Time

Water and Evotherm (or Evoflex) get added to the liquid AC to create a foamed product at this facility. Photo courtesy Stansteel.

Once a producer has installed a WMA system, one key to proper operation is consistent use. Lennie Loesch, CEO of Stansteel, Louisville, Kentucky, shared why daily use of the system is important.

“The best practices for warm-mix systems depend upon the nature of the contractor’s business and what arrangement they have with their state or provincial department of transportation,” Loesch said. “The No. 1 best practice is to use the foaming system every day for all mixes.

After a reasonably short period, the plant personnel will find that there are different adjustments and different percentages of either water or liquid chemicals that they would use for each configuration.

“Again, as a best practice, the advanced foaming systems are not restricted to just water, but can handle a wide range of liquid chemicals, cutbacks, rejuvenators, anti-strip materials and more. The producers and contractors that report the absolute best results are those that operate the systems all day, every day, and get the tremendous other benefits, such as extended haul distances, better release of the material of all mixes from the silo or from the truck beds, better in-place compaction, lower mix temperatures, and less energy use. The list goes on and on.”

Personnel can get samples of the foamed (or blended) product via the Safe-T-Station™ immediately after the Accu-Shear Advanced for continuous drum plant liquid blending unit. Photo courtesy Stansteel.

Using the system on a regular basis not only gets the personnel accustomed to it, but also gives the plant itself a chance to catch up to the technology. Consider how many mix designs must be stored for customers, DOTs, counties, etc.

“The control of the warm-mix foaming system should be integrated with the plant control to track the use of liquids and also use different combinations of liquids, depending on the result desired,” Loesch recommended. “Some of the most advanced blending systems have the capability of blending two, four, five or seven different liquids with the base AC on an on-demand basis. This is done based on material specifications required and the additives to be incorporated, but also has the tremendous benefit of modifying the performance grade of the AC.

“As an example, starting with a base material of a PG64-22 and adjusting it to a more enhanced PG70-22 or PG76-22 right at the plant and on demand. Also, as an example, certain chemicals have proven properties at very low mix temperatures, and those can be blended in a very similar manner. Terminal additive blends are expensive and often are run into mix designs that do not call for the expensive additive due to lack of liquid storage space. Blending on demand from a base liquid has a high return on investment. Rubber or ground tire rubber (GTR) mixes have a short life once produced. Foaming GTR mixes provides all the benefits, longer storage, longer trucking times and reduced roller patterns to achieve density. Operating a system that is cycle dependable and repeatable over a wide range of production is mandatory.”

Keep it in Good Repair

The Tri-Mix WMA system from Tarmac brings the materials into a vortex mixing zone. Photo courtesy Tarmac.

Using a system on a daily basis is one way to keep an eye on it, but plant managers will want to remember the daily walk-around for good preventive maintenance. Ron Heap, CEO of Tarmac International Inc., Lee’s Summit, Missouri, recommended a best practice for the end of the shift: get the asphalt out of the system. “Use compressed air to purge the asphalt injection nozzles at the end of a run to maintain clean nozzles,” Heap said.

On the front end of operations, Loesch had some maintenance ideas for operators.

“One of the other significant maintenance practices is to make sure that the components on the system, including pump skids, are working correctly from the standpoint of pumps, meters and valves,” Loesch recommended. “Calibration with and checking of the interlock with the plant controls to raise and lower the use of liquids with the production rate and an overall check and verification of the components should be performed on at least a weekly basis. The operations or laboratory personnel should use a device similar to a Safe-T-Station™ to verify that foaming is taking place at a 5:1 or greater ratio. Plant QC testing is critical as well as seeking field performance feedback.” (See note 1)

Patrick Ahern of Ahern Industries Inc., San Antonio, spoke of the importance of calibration as it relates to the pump specifically. “When running polymers or crumb rubber, you want to use a special pump,” Ahern said. “Of course you have to calibrate anytime you change the asphalt or add something. If you’re having issues with the pump, calibrate until it does pump properly.” Besides calibrating the equipment to get proper results, Ahern reminded plant operators that the original equipment manufacturer (OEM) is a source for help. “Nineteen out of 20 plant operators forget to contact the OEM, but you need to contact the pump manufacturer for the rate. Make sure the pump OEM knows what you’re running through there. They have to know what the content is, what the viscosity is. They could have 10 or 15 varieties of a 3-inch pipe.”

Viking jacketed internal gear pump installed at an asphalt blending terminal. Photo courtesy Viking Pump.

Chad Wunderlich, the training director for Viking Pump, Cedar Falls, Iowa, shared that positive displacement pumps can handle the thick, viscous material. He recommended a gear pump with three mechanical seals to guard against leaks, and reminded plant managers, “You have to control the speed of the pump to control the flow of the material.”

Wunderlich reminded plant operators that gear pumps are simple, easy to maintain and highly reliable for handling asphalt, saying, “gear pumps can be fit to handle everything from clean, pavement-grade asphalts to filled asphalt coatings and everything in between. It’s important to clarify the specific asphalt handled to ensure pump construction and operating conditions are appropriate for the application.

Viking has been manufacturing pumps for asphalt handling for 85 years now. Traditionally these pumps have been cast iron, jacketed, with braided shaft packing.”

“The issue is good mixing,” Heap said. He explained for plant managers that the main liquid asphalt pump at the asphalt injection system is sending the liquid AC through the center of a machined stainless steel collar with the water being injected into the center of the collar from the circumference of the collar at multiple points at very high pressure. Within the collar, water (or a chemical additive) and asphalt meet, mix, expand and go forward.

They keep moving.

“We didn’t create much resistance at all in the mixing zone,” Heap said. “If the liquid AC is moving into the collar at 25 tons per hour going in, it’s still leaving the collar at 25 tons per hour on the way out with the added water or chemical.”

“Just as liquid asphalt should be frequently calibrated and belt scales should frequently be calibrated, so too should any of your pumps and metering systems for water, liquid chemicals, anti-strips or latex polymers for liquid AC modification,” Loesch said.

No plant operator will merely send an electronic signal to a pump and hope for the best. “As DOTs become more sophisticated, they will require frequent calibration, interlocking the control system with the pumps, blending modes and devices,” Loesch said. “Perhaps one of the best ways to ensure that proper foaming is done is to take actual samples of the foamed material after it has gone through the foaming device and before it’s injected with the aggregate, RAP and other materials. Taking samples will illustrate the expansion and foaming and, of course, the material can be tested to make sure it conforms to DOT guidelines.”

In the end, producing a foamed liquid gives the opportunity to coat all materials in the mixing process. Loesch explained: “With a properly designed system combining water with the AC, the total volume of the liquid AC is expanded up to 600 percent of what its original volume would be without foaming. Fundamentally, foaming provides a greater film thickness, and, with this extra volume of liquid, all of the solid ingredients, such as various size aggregates, sand, RAP, RAS and virtually anything else in the formula, are extremely well coated and mixed, resulting in a superior mixed asphalt. With this improved mix, there are many positive reports from the field, whether it be the asphalt producer’s paving crews or private customers or others; once the foaming system is on the plant, they say, ‘Wow, what did you do differently on that mix?’ Even though they often drop the temperature of the mixed material or make other modifications, the fact is a superior product to lay down in place and achieving desired air void compaction with fewer roller passes and just overall better workability and excellent mat densities.”

There’s the goal for any producer. Follow best practices to ensure your warm-mix system functions at peak performance to give your crews and customers an excellent product to place. As WMA tonnages continue to increase across the nation, watch quality increase as well when each producer follows best practices for top quality.

1. Stansteel makes the Safe-T-Station sampling device. Maxam Equipment, Kansas City, Missouri, makes the Safe-Sampler liquid asphalt sampling device. Both are designed to allow ground personnel—or lab techs—to safely collect a sample of liquid AC inline. See the Here’s How it Works features of these systems on the AsphaltPro website under the Departments tab.