Pushing limits

Oct. 16, 2009

Aging makes even asphalt stiffen up. Humans with stiff joints sometimes take supplements to try to restore suppleness. Asphalt engineers who want to reuse asphalt from previous pavement in their new pavement usually add a softer binder to their hot-mix asphalt (HMA) to compensate for the harder binder that comes with the aged (in other words, oxidized) pavement.

Aging makes even asphalt stiffen up. Humans with stiff joints sometimes take supplements to try to restore suppleness. Asphalt engineers who want to reuse asphalt from previous pavement in their new pavement usually add a softer binder to their hot-mix asphalt (HMA) to compensate for the harder binder that comes with the aged (in other words, oxidized) pavement.

An alternative method is to add the usual binder and mix the asphalt at a lower temperature. That is exactly what the Missouri Department of Transportation (MoDOT) is trying on the north outer road of I-44 in Franklin County.

A 2.8-mile-long pilot project on the I-44 outer road tests whether using the warm-mix asphalt (WMA) process will allow them to pave with a higher percentage of reclaimed asphalt pavement (RAP) and their standard grade of asphalt binder.

The three-lane roadway, which goes past the Six Flags Over Mid-America amusement park and many businesses, supports about 12,000 vehicles a day, with about 6.3% trucks. The center lane is a turn lane, so it sees less traffic than the two outer lanes.

The road needed to be repaired anyway. It was an old concrete pavement that had been overlaid with asphalt. The asphalt was in poor condition, with rutting and other problems.

“This was a project that was under contract,” Dale Williams, field materials engineer for MoDOT, told Roads & Bridges. “We were approached by MeadWestvaco to look at the possibility of using higher RAP percentages with warm-mix technology. They had worked with this contractor in the past on some projects, so it was kind of a three-way partnering.”

MoDOT has no rules against using as much RAP as the contrac-tor wants.

“Currently, in theory we allow 100% RAP in a mix,” said Williams, “but when you get above 20% RAP you have to do some binder testing and you have to use blending charts to determine if you have to use a softer grade of binder.”

Most contractors on most jobs would rather avoid the expense of the extra testing, so they stay below 20% RAP use.

“The theory is that with warm mix you’re not aging the binder when you go through the asphalt plant,” explained Williams. “The binder ages quite a bit as it goes through the asphalt plant. It oxidizes and gets hard and brittle. The biggest part of the oxidation takes place in the asphalt plant, because of the high temperatures in it. By using warm mix, we’re reducing the amount of oxidation that takes place in the asphalt plant, and we’re able to mix higher percentages of RAP, which has the harder, oxidized asphalt on it.”

The contractor on the high-RAP WMA project, Pace Construction of St. Louis, Mo., started by milling off 3.75 in. of the old asphalt pavement and repairing some of the concrete joints. Then they laid a 2-in. binder course of their standard mix of SP190C with PG 70-22 liquid asphalt (19-mm nominal maximum aggregate size).

The middle lane of the three-lane road received a 1.75-in. course of a standard hot mix of SP125C with 20% RAP and the same 70-22 liquid asphalt (19-mm nominal maximum aggregate size).

On the two outside lanes, Pace laid three test sections of the 1.75-in. course: a warm mix with 20% RAP and the Evotherm DAT additive, made by MeadWestvaco; a warm mix with 28% RAP and the additive; and a warm mix with 35% RAP and the additive.

Pace was already familiar with laying RAP-containing HMA.

“In the last two years, we have changed the majority of our mixes over to RAP,” Andy Ernst, vice president of construction operations for Pace Construction, told Roads & Bridges.

Ernst also is familiar with the characteristics of the RAP the company uses, because it is old MoDOT pavement.

“Here in Missouri, the only requirement that you have to put RAP in your mix is that the RAP that you use needs to come off of a previous MoDOT project. So most of our RAP comes from our own projects that we build, and we have pretty good control over that. Most of our work is with MoDOT.”

Pace produced the WMA to be about 250°F behind the screed and the HMA about 340°F. Since the plant was only a seven-mile haul from the project site, the material was not too much warmer than that coming out of the plant.

“We had some really good results on our volumetrics and our densities on the project,” said Ernst. “The mixes were real workable in the field. I think they’ll prove to be pretty durable.

“We actually utilized the same roller pattern for the warm mixes with the high RAP as we did for the hot mix.”

The roller pattern consisted of five passes with a 12-ton vibratory breakdown roller, then another five passes with an intermediate vibratory roller and then five passes with a finish roller in static mode.

As far as density, MoDOT targets 92-96% of theoretical maximum density, and Pace hit that mark.

The most challenging aspect of the project might have been doing the paving at night to avoid huge traffic backups. Even paving in the dark did not pose much of a problem for Pace.

“Providing enough lighting for the equipment is a big concern and making sure that the traffic is channelized into the proper lanes and you have plenty of signing available,” said Ernst. “On our equipment we use high-intensity light to give us high visibility. Once you have those in place, so everything is very visible, generally most of your problems are behind you.”

Pace used trimline channelizers with reflective banding to guide traffic around the work zone. To reduce the danger of having vehicles speeding by so close to the paving crew, Pace was able to employ local law enforcement to control the speed of the passing traffic.

“That is effective,” said Ernst. “That seems to be the only effective method of keeping an eye on [speeders].”

Pace has had excellent results on its previous RAP projects, said Ernst, and they hope the new pavement will last at least the 15-year life MoDOT expects of its usual asphalt pavement.

Now the only thing to do is to see what happens.

“We’ll monitor it periodically,” said MoDOT’s Williams. “Most of it will be visual. We’ll see if there’s any type of cracking in it. We’ll measure rutting on it. We’ll probably check it at least quarterly. I don’t expect to see any problems.”

Depending on the results of the test project, RAP use may change in Missouri.

“Where I’d like to go with this—and this is all hinging on what the rest of the test results show—would be that if you use a warm-mix technology, you would be able to go up to 35 or maybe even 45% RAP without having to adjust the grade of binder. That’s where I hope to go with the specification with this.”

Sponsored Recommendations

The Science Behind Sustainable Concrete Sealing Solutions

Extend the lifespan and durability of any concrete. PoreShield is a USDA BioPreferred product and is approved for residential, commercial, and industrial use. It works great above...

Proven Concrete Protection That’s Safe & Sustainable

Real-life DOT field tests and university researchers have found that PoreShieldTM lasts for 10+ years and extends the life of concrete.

Revolutionizing Concrete Protection - A Sustainable Solution for Lasting Durability

The concrete at the Indiana State Fairgrounds & Event Center is subject to several potential sources of damage including livestock biowaste, food/beverage waste, and freeze/thaw...

The Future of Concrete Preservation

PoreShield is a cost-effective, nontoxic alternative to traditional concrete sealers. It works differently, absorbing deep into the concrete pores to block damage from salt ions...