Hard Doesn't Mean Stale

By: Bill Wilson

"Fresh, no preservatives" sounds good when you’re about to eat something—like asphalt.

When engineers in Tazewell County, Ill., were getting ready to chew up the road for a cold in-place recycling (CIR) project, they looked at a unique application from Koch Pavement Solutions, Chicago.

ReFlex Cold In-Place Recycling uses an emulsion fit for a specific environment, and the key ingredient isn’t the kind aging on the shelf.

"We call it engineered emulsion because we’re not pulling out an emulsion that just happens to be sitting at our plant that day," Todd Thomas, civil engineer for Koch Pavement Solutions, told Roads & Bridges. "We’re formulating the emulsion for the material and the climate on the job. It contains no rejuvenating or fuel oils, and we’re doing that because it’s a lot more environmentally friendly product."

The five-mile stretch marked for CIR in Tazewell County—Washington Road—has had an off-and-on relationship with asphalt pavers over the years.

"We took core samples last year and mix samples this year," Norm Johansen, Tazewell County engineer, told Roads & Bridges. "We had 11 in. of blacktop on there, so what we were doing was restablizing what we had because it was just layer after layer after layer."

This $1.2 million project called for 3 in. of CIR capped with a 2 1/4-in. Superpave mix. After studying the pavement history, Koch moved in with its core samples, which were sent to the lab and crushed to expected CIR gradations. The top 3 in. of the cores were examined and two sets of designs were made, one for expected gradation and another "in case the gradation might be a little coarser to what we might see," said Thomas.

"That established two emulsion contents and when we were on the project we were measuring the gradations and determining if it falls closer to this medium gradation design we did or the coarse gradation design."

After measuring gradations almost every day Koch determined the gradations leaned toward the medium design, and the emulsion content was set at 31/2%.

The mix design actually required four different tests—Raveling; Dry Marshall Stability; Conditioned Marshall Stability; Thermal Cracking.

Duplicate samples of the reclaimed asphalt pavement are adjusted to the field moisture content, treated with the design quantity of emulsion, mixed for 60 seconds and compacted to 150-mm specimens using a Superpave gyratory compactor. The specimens are placed on a modified Hobart mixer (as specified in TB-100) fitted with a 600 ±15 g abrasion head and hose assembly. The samples are abraded for 15 minutes and the difference in mass before and after the test is reported as the percent raveling loss.

"What we’re looking for in that raveling test is no more than 2% mass loss during the test," said Thomas. "How this all relates to the field is you’ll often see during construction a raveling of CIR because it does not have enough asphalt emulsion."

Additives are used to help in the resistance to raveling, especially for cooler weather.

The Dry Marshall Stability test is run at three emulsion contents. Specimens are cured in an oven, and what engineers are looking for is a Marshall Stability of 1,500 lb or greater when tested at 40?C.

Half of the specimens are tested for Dry Marshall Stability, while the other half are placed in a vacuum container after oven curing. The vacuum actually sucks water into the voids of the specimens, and the aim is to achieve 60-75% saturation of the voids. Koch then soaks the specimens at 25?C for 23 hours. The last hour they’re soaked at 40?C.

"What we’re looking for there is a 70% or greater on the percent of retained stability," said Thomas.

The Indirect Tensile Test, AASHTO TP 9-96, is modified for testing the predicted thermal cracking temperature. Samples, 150-mm in diam., are prepared at the design emulsion content and ±1% of design air voids using the Superpave Gyratory Compactor. The specimens are cured at 60?C for 48 hours, then checked every two hours until there is no more than 0.05% change in mass. Samples are tested at 10ûC intervals bracketing the LTPP Bind specification temperature for the climate and depth in the pavement structure. The critical cracking temperature is the intersection of the calculated pavement thermal stress curve, derived from the tensile creep test, and the tensile strength line, derived from the results of the tensile strength test.

The paving train consisted of two milling machines, a mixing machine, a pickup machine and an asphalt paver. A shoulder milling machine led the way, grinding 6 ft of pavement, and a 12-ft milling machine followed to pick up material left by the first pass and to chew across to the center line. The mixing machine added the emulsion, a solvent-less material with 65% residue that had a penetration of 138 dmm, according to Thomas. The emulsion was to be heated at no more than 120?C, but there was an initial problem with the temperature—it was too high.

"The higher the heat, the quicker (the asphalt) sets," said Johansen. "We had to send back one load of emulsion because it was way too hot and it was setting up before we could get it on the ground."

The pickup machine then sent the new, recycled asphalt to the asphalt paver. Koch established a starting rolling pattern of two breakdown steel double-drum passes, two light pneumatic roller passes, five heavy pneumatic roller passes and two finish steel double-drum passes. After making a check, the number of breakdown passes was upped to five or six. A nuclear density gauge was used to set the rolling pattern, and the approximate density achieved was 132 lb per cu ft.

For smoothness, a 1/4-in. bump using a 10-ft straight edge was allowed.

"We had never done this before so we had some growing pains, especially getting started," Dennis Tresenriter, assistant county engineer for Tazewell County, told Roads & Bridges. "With the ReFlex system we were able to pick the road up and lay it back down. We increased the structural quality of it and essentially ended up with 51/4 in. of construction, whereas our normal method is to just mill a couple of inches and put back a couple of inches, so you don’t gain anything there.

"We were able to overlay within about 10 to 11 days. With normal CIR you may have to wait 30 or 40 days."

Tazewell County completed a similar CIR job on Springfield Road, and after the third summer there are no signs of cracking.

"Normally, we have cracking by the second winter," said Tresenriter.

Officials expect to get 13-15 years service life out of Washington Blacktop.

Koch has used ReFlex Emulsion Cold In-Place Recycling on a total of 13 jobs, including 10 this year.