Reconstruction workshop highlights reclamation techniques

A workshop on full-depth reconstruction of city streets, presented Sept. 12 by the City of San Antonio in conjunction with the 2007 International Public Works Congress and Exposition highlighted various reclamation techniques available to road agencies today.

At the workshop, hundreds of public works personnel from around the United States viewed all phases of a full-depth street reconstruction project, including lime-slurry base soil stabilization, asphalt emulsion-stabilization of the pavement base, stabilized base compaction, and paving with warm mix asphalt.

Previously, the deteriorated pavement and layers below had been removed using a cold milling machine and stockpiled separately. This material later was brought back to site for use for the stabilized base layer.

San Antonio partnered with the Lime Association of Texas in producing the workshop. Chemical Lime Co. provided the lime slurry and delivered it to the site. Ergon Asphalt provided the asphalt emulsion for the reclaimed base as well as the process for the Evotherm warm-mix technology. Vulcan Materials Co. produced the warm-mix asphalt while Valero Asphalt provided the asphalt cement, with The Asphalt Institute providing technical support for both the base and surface layers. Engineering and construction testing services was provided by Fugro Engineering Consultants Inc.

Additional assistance and Wirtgen Group equipment to supplement city-owned Wirtgen Group equipment was provided by Cooper Equipment Co. Inc., San Antonio.

The field demo was preceded by a classroom session which brought together materials experts Larry Peirce, executive director, Lime Association of Texas, Gary Fitts, P.E., senior district engineer, The Asphalt Institute, and Harry Bush, manager, technical services, Vulcan Materials Company, San Antonio.

Demo presented in stages
Field work was staged so that different elements of reconstruction could be exhibited at the same time.

The street on which emulsion-base stabilization was taking place had been soil-stabilized before the workshop, and the street on which paving was taking place had been both soil-stabilized and base-stabilized previously. The street on which soil stabilization was taking place was milled down to the soil most recently but would be base-stabilized and paved after the workshop had ended.

"We were able to simultaneously construct the different treatments within the pavement structure on three different streets so the attendees could be 'moved' from one process to the next," says Texas Lime's Peirce. "Thus they would be able to see each of the different pavement layers being reconstructed at the same time."

The streets selected exhibited poor subgrade soils that were lime-stabilized. For the overlying base layers, a reclaimed asphalt emulsion stabilized base was constructed using existing surface and base materials milled up from the original roadway. And the pavement surface was the new "warm-mix" design incorporating Evotherm technology.

The streets reconstructed were residential drives just east of downtown San Antonio. The existing asphalt surface and black base sat directly on top of a highly expansive clay subgrade, which has caused many problems with performance, ultimately leading to premature failure.

"No matter what you design or build, your street is only as good as the foundation it sits on," Peirce says. "These three different treatments were implemented to completely reconstruct these pavements while using the existing materials to save money and expedite construction."

The existing, problematic, expansive clay subgrade was lime-stabilized using an amount of lime engineered to permanently change it into a strong and stable foundation layer.

After the lime-stabilized layer, the former hot-mix and asphalt base material that was milled up and stockpiled was brought back to the site, laid down and mixed with an asphalt emulsion to create a strong yet flexible base layer. And the final surface was completed with the warm-mix asphalt overlay.