How to Design Mixes with Shingles

You have to be careful with the low temperature properties of recycled shingles

Peterson Pacific Shingle Grind 11196515

The use of reclaimed asphalt pavement (RAP) and recycled asphalt shingles (RAS) in asphalt pavement is increasing because those materials provide economic and environmental benefits. Recycled materials reduce the amount of virgin aggregate and virgin asphalt binder required to produce asphalt, and that lowers the cost of the end product.

But the binders in RAP and RAS are different from each other, and the differences must be taken into account when designing an asphalt mixture. Shingle binders are made much differently than paving grade binders, and the two behave differently. RAS binders are designed to withstand hot summer sunshine. “But if you put paving grade binders up there on the roof, they would all be in the gutter,” says Ramon Bonaquist, chief operating officer of Advanced Asphalt Technologies, Sterling, VA. “They’re much softer. Shingle binders are much harder.

“From a low-temperature standpoint, paving grade binders have relaxation capabilities,” says Bonaquist. “At low temperatures, paving grade binders still creep. They will creep if you put a load on them over time. Shingle binders are much more elastic. They do not creep. And it’s that ability to relax that gives paving grade binders the ability to stop thermal cracks. As a shingle contracts in cold weather, there’s nothing to stop it from contracting.

“When an asphalt pavement contracts, it basically drags along the ground and you develop tensile stresses that have to be relaxed,” says Bonaquist. That’s part of what the binder specification does. It minimizes the amount of those stresses and it also makes sure the binder can relax. Shingle binders won’t relax; they are much more elastic. They do not creep at low temperatures.”

A mix designer has to compensate for the stiffness in the RAS binder, says Ronald Collins, president of Pavement Technology Inc. and former state Materials and Research Engineer for the Georgia DOT. “You’ve got to put in some kind of rejuvenator, or use a softer virgin binder,” says Collins. “You’ve got to have some method of softening that binder to bring it back to about the grade that the design person is looking for.”

Collins says if you want to save money on binder, the particle size of the RAS is important. “When you grind the shingles, the smaller the particle, the more asphalt that is released out of it,” says Collins. “And the more economical it is.” Some states specify 3/8-inch minus for their RAS. One Missouri producer is making the transition to one-tenth-inch minus, to realize the total value of the binder in the shingles. The larger the particle is, the more difficult it is to melt it down.

A producer needs to have enough dwell time in the hot mix plant for the recycled asphalt to melt. The Astec Double Barrel, Collins says, is well-designed for melting and mixing RAS particles and RAP. A Double Barrel heats the virgin aggregate in an inner drum, and mixes the RAS and RAP in an annular space between the inner and outer drums, away from the burner flame.

“Generally I think you will find that the Astec Double Barrel is superior for using shingles over some of the other plants that push the mix through the drum faster,” says Collins. “The particle needs to stay in there long enough to melt and release the asphalt. If you can’t get most of the asphalt released, then you’re affecting the economics of the mix.”    

Recycled shingles come in two forms, manufactured waste and tear-offs. Manufactured waste shingles are newly made, and are wasted by the shingle manufacturer because the color is not right, or some similar flaw. Tear-offs are just that, worn and oxidized shingles that have been torn from a roof. They have to be cleaned of wood and debris. “The tear-offs are much more oxidized than the manufactured waste, and it’s very difficult to get them heated to where they will release the asphalt,” says Collins.

Recycled shingles are rich with binder. “You can get as much asphalt, theoretically, out of say, 5 percent shingles as you can with 25 or 30 percent RAP,” says Collins. If shingles have 20 percent asphalt in them, and you use 5 percent shingles, that is one percent going to the total mix. But if a RAP has 4 percent asphalt in it, and you use 30 percent, that’s a little over 1 percent of binder in the total mix. So there’s a lot of economic benefit to using shingles.”

Vive la difference

Bonaquist and his firm recently performed a research study that evaluated the properties of recycled asphalt binders from 18 sources in Wisconsin. RAP from 12 sources was evaluated, as was RAS from 6 sources. The data were used to evaluate the binder replacement criteria contained in the 2011 Wisconsin DOT Standard Specifications for Highway and Structure Construction using a reliability analysis.

“RAP and RAS don’t change the grade of a binder in the same way,” says Bonaquist. “The RAS binders change the binder grade much more rapidly than RAP binders do. That’s very clear from our study. And with some agencies, recycle is recycle and they really don’t care where it’s coming from, but these two binders are much different. RAP binders are a lot softer than RAS binders.

“The second thing that comes from this study is that there is a way to engineer the binder in your mixture to meet a certain performance grade AND meet whatever target values you have for recycle content,” says Bonaquist. “And that is through the use of these blending charts. That’s a very positive thing from a producer’s standpoint.”

Bonaquist gives a hypothetical example of what happens to the low temperature grade of a binder with the addition of RAS. Suppose that a typical RAP binder in Wisconsin changes the low temperature grade by 0.1 degree Celsius per 1 percent of binder. So if you put 20 percent of RAP binder in the mixture, you’ll have 0.1 times 20 – and change the low temperature grade of the binder by two degrees. RAS binders, on the other hand, are going to change the low temperature grade by 0.2 degrees C per 1 percent added. That’s double the change of RAP binders. So that same amount, if you put in 20 percent, is going to change the low temperature grade by 0.2 times 20, or 4 degrees.

“Most binders are designed to be pretty close to the mid-point of the grade,” says Bonaquist. “So a minus 28 would have a true continuous grade of about minus 31. So that four degrees (of change from RAS) would drop you by a grade. When you add 20 percent shingle binder, you’re going to worsen a typical binder by one performance grade.”

So if somebody says they run 5 percent shingles, they mean 5 percent of the total mix. The shingles will have a binder content of almost 20 percent. So you’re going to have a one percent contribution from the shingles, and if you have a 5 percent mixture, that means you’ll have one divided by five, or 20 percent recycled binder in there.

“That is the reason that in Wisconsin their way of specifying it is by the amount of virgin binder that you replace with recycled binder,” says Bonaquist. “That way your specification goes directly toward the performance of the binder and I think nationally this concept is beginning to catch on. We should be specifying the recycle contents based on the amount of binder that we’re putting into the mixture.

Naturally, Bonaquist says low temperature performance grades and thermal cracking are not a problem in Brownsville, Texas, where it’s warm the year around. But it certainly matters in Milwaukee, Wisconsin, or any cold-weather climate. “We have a lot of available tools we can use,” says Bonaquist. “There is hesitancy on the part of both producers and even agencies to use these blending charts. You have to extract the binder, recover it, and test it. People don’t like the fact that we use solvents to extract and recover the binders and test them, but that’s what you have to do right now in order to use these recycled materials effectively.”

Superpave methods

Designing mixes with shingles is standard practice for David Madden, vice president of construction for Madden Contracting, Minden, LA. The company owns and operates nine asphalt plants – five in Texas and four in Louisiana. Three of the Louisiana plants are Astec Double Barrel units. One Astec Double Barrel is located in Texas, and three of the plants in Texas are Astec parallel flow plants.

If a mixture requires 5 percent asphalt binder, Madden says he may make three mixes in the laboratory – one at 4.8, one at 5.0 percent and one at 5.2 percent. Using Superpave design methods, the mix designer evaluates the three mixtures and determines which one best meets N-max, N-design and N-initial gyratory compaction parameters. Those are numbers of rotations in the gyratory compactor. N-max gives you 100 percent density, a point at which rutting theoretically occurs, so N-design is lower and leaves some air voids in the mix. Every DOT has a different idea of those three numbers, Madden says.  

Madden also does an ignition oven test to determine the asphalt content in the shingles. “If 5 percent is the magic number that your shingles are giving you theoretically in the mix, about 1 percent binder is what the ignition oven will tell you, but you’re probably only really getting 0.6 percent of effective asphalt from the shingles that truly will regenerate and be live,” says Madden. “And there’s a little Kentucky windage in that. So you say, OK, I’m going to put in 4.4 percent virgin binder, because I’m going to get 0.6 percent from my shingles.”

We asked Madden if he usually runs RAS and RAP together. “I’ve run it both ways,” came the answer. “It just depends on what I have the most flavor of. I have no problem running shingles by themselves, or virgin mix with RAS and RAP, it doesn’t make any difference to me. You take care of all of those issues when you do your original mix design.

“I love the Astec Double Barrel,” says Madden. “It’s probably the most efficient drum on the market that I have ever run. I really, really like the performance, and production. You do have to maintain the pugmill tips, but they’re easy to get to. The outer barrel has pugmill tips for mixing.”

Because a Double Barrel performs aggregate drying in the inner drum and mixing in the outer drum, a producer does not get blue smoke that is often associated with running high recycled contents, Madden says. “If you take a parallel flow drum, and run high RAP and RAS, you’re going to have blue smoke, which means you’re burning the liquid asphalt, because everything is in one barrel. So there’s a drawback to the parallel flow plant as opposed to the counterflow. The Double Barrel is a counterflow plant.”  

Bonaquist’s solution

Bonaquist says AASHTO’s M323 binder blending analysis works well with material from RAP. “But you cannot attain this continuous grade that’s necessary for the M323 procedure with RAS binders,” he says. “You just can’t do it, even though another AASHTO standard, PP53, tells you to do that. There’s a significant problem when you apply this blending chart analysis as it’s described in M323 for RAS binders.

What’s the solution? “We came up with a recommendation of how to apply a blending chart analysis when you have recycled asphalt shingles,” says Bonaquist. “And in essence what we do is to recover the shingle binder, and blend it with some virgin binder and essentially use the blending chart twice. That seems to work reasonably well with recycled shingles.”

He explains further. “We make a blend of 70 percent virgin binder and 30 percent of the recovered shingle binder. We grade that blend, and then extrapolate from that blend to obtain the properties at 100 percent shingle binder. And then we use those 100 percent properties in the blending chart analysis.”

Bonaquist says that is a reasonable way to get estimates of what the blended binder grade will be in mixtures that have both RAP and shingles in them.