Reclaiming the street

Jan. 24, 2008

Recently, the Regional Transportation Commission (RTC) in Reno, Nev., completed the rehabilitation of three downtown streets using full-depth reclamation (FDR) with portland cement.

Recently, the Regional Transportation Commission (RTC) in Reno, Nev., completed the rehabilitation of three downtown streets using full-depth reclamation (FDR) with portland cement.

Using a pavement condition index system, the city discovered that portions of Mira Loma Drive, Sierra Street and Hunter Lake Drive were experiencing base failures and in need of immediate repair. Lumos and Associates, who served as engineer and construction manager, needed to consider several project-specific limitations such as maintaining curb lip elevations, the presence of cobbles in the subgrade, insufficient or contaminated base and shallow utilities. The street’s urban setting and its high traffic volumes added to the challenge, as did a limited budget.

A number of rehabilitation options were evaluated, including full removal and replacement, partial removal and replacement incorporating geotextiles, thick hot-mix asphalt (HMA) layers and FDR using cement. FDR proved to be the winner, meeting the roadways’ structural criteria while also saving time and money and reducing construction traffic.

Sierra Nevada Construction started with the pulverization of the existing roadway to a depth at or slightly below the final bottom of the treatment elevation. Five to 6 in. of pulverized material was removed to ensure that the final surfacing course would match existing curb and gutter elevations. A cement content of 3.5% was spread directly onto the pulverized materials. The materials were blended and, with the addition of water, brought to optimum moisture content. This material was then graded and compacted to the required density.

The RTC mandated that the roadways be repaved within seven days. Because early strength was a requirement, these roadways were designed to achieve the highest balance between strength and durability. There were concerns, however, that shrinkage cracking could reflect back up through the newly placed HMA surfacing.

To minimize this potential, the roadways used a relatively new technique known as “microcracking” or “precracking” the FDR prior to placement of the surface treatment. For this application, the completed cement-stabilized base is exposed to several passes of a large vibrating roller after a short curing period to create a fine network of cracks. This process minimizes the development of wider, more severe cracks.

Each roadway section was pulverized, blended with cement, shaped and compacted in one day and then moisture-cured for two days. The microcracking process was performed at the two-day mark, and all sections were paved within 12 hours after microcracking. The roadways were reopened in half the time required by the RTC.

The strong, durable cement-treated FDR base layer provided such a stable deck that the expensive HMA thicknesses were reduced by 1 to 3 in., decreasing asphalt quantities by as much as 30%. FDR cut construction time from eight weeks to six weeks, allowing for quicker back-tonormal traffic loads and less traffic congestion.

Environmentally, FDR with cement also cut down on the amount of virgin natural resources needed and the amount of hauling and fuel used on the projects.

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