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in accordance with ASTM C 42 while others were tested to determine their seismic modulus using the free-free resonant column method developed at the University of Texas at El Paso. The primary reason for performing the seismic modulus was to obtain the resilient modulus for the reclaimed base, which will be required input for the new American Association of State Highway and Transportation Officials Mechanistic-Empirical Pavement Design Guide. Based on the seismic modulus testing results, the lowest UCS value of 260 psi (1.8 MPa) would roughly correspond to a stiffness of 200,000 psi (1380 MPa), which is considered excellent in terms of the reclaimed base's ability to support traffic loads and minimize the stress that is transferred to the subgrade. Table 1 - Summary of pavement condition survey Durability The durability of a roadway base subjected to wetting-drying and/or freezing-thawing cycles is a critical parameter for any roadway's satisfactory performance. Durability issues are especially challenging in wet, northern climates where deeply penetrating freeze-thaw patterns can cause an unstabilized pavement base to lose strength and stiffness. Of the 79 projects that were part of the study, more than 50 were in areas with moderate to severe winter weather conditions. Volume change and loss of strength caused by traffic loads, environmental conditions and water movement within pavement layers cause heaving roadways, posing a serious safety risk to drivers. County engineers say road heaving due to winter freeze and rutting due to spring thaw are among their biggest challenges. The FDR process has proven very successful in combating freeze-thaw challenges. The heaving has been eliminated and the engineers are pleased to report that their roads are operable in cold-weather conditions. Overall, the FDR process has been a very positive experience for agencies in northern areas that have severe weather. The agencies have successfully provided public roads that do not heave in the winters or lose shear strength during spring thaws, allow businesses to efficiently move goods, and have enhanced road safety. The FDR process has enabled counties to build “all weather” roads. Construction Most agencies use equipment called a “reclaimer” to pulverize their old, distressed flexible pavements so that the maximum size of the crushed pavement is no more than 2.5 to 3.0 inches (63 to 75 mm). If thicker sections are required, some agencies add aggregate or soil base material and blend them with the pulverized pavement. Water and cement are then added in either a dry or slurry form to the pulverized material to form a stabilized mixture, which is compacted and becomes the base or subbase of the new pavement structure. While most agencies use the standard Proctor (ASTM D558) procedure to deter- mine compaction requirements, some now actually require the use of modified Proctor (ASTM D1558) energy or similar in the laboratory evaluation. Whether standard or modified, the required in-place field density for all agencies is between 95 and 98 percent of the laboratory-measured density. Depending on the agency, the curing of the completed FDR base and its opening to traffic varies between one-half and seven days. Some prefer to use moist curing over a period of three to seven days while others prefer the use of a bituminous coating or a curing compound that can allow the road to be opened to traffic within one-half to one day. Innovative Techniques Some agencies use a process called “microcracking” to reduce reflection cracking. This procedure uses a compaction roller on the surface of the cement-stabilized base one to two days after construction. The effect of the roller is to initiate numerous tiny microcracks in the base to absorb the shrinkage, rather than single shrinkage cracks that are wider. The tiny cracks are too small to reflect up through the asphalt surface. Design Agency officials realize the importance of design, and do their best in spite of shoestring budgets to perform a proper engineering investigation prior to design and construction of the FDR process. Most agencies tend to follow PCA recommendations while others rely on past experience when deciding on the thickness of the reclaimed base and the amount of cement to be added to the mix. FDR base thicknesses typically range from 6 to 12 inches (150 to 300 mm) depending on the materials and traffic volumes. In most cases, samples are compacted with varying cement contents using the standard Proctor test (ASTM D 558). Following an agency's experience, the minimum cement content is based on achieving a 7-day target UCS of between 150 and 600 psi (1.0 and 4.1 MPa) with most ranging from 300 to 400 psi (2.1 to 2.8 MPa). Some agencies in cold climates check the proposed mixture for frost susceptibility by performing freeze-thaw tests as recommended in ASTM D 560. Agencies realize that strength and performance are not the same thing and that durability is the key issue in the design of the FDR mixtures. This study showed that the mini- mum cement content should be based on the mixture passing the durability test (ASTM D 559 and D 560 or the Tube Suction Test (TST) as described below). Some agencies do not allow the blending of subgrade soils into the reclaimed layer because the silt and clay content of these soils can sometimes influence the shrinkage and durability characteristics of the reclaimed mix. However, because of budget constraints, other agencies are forced to cut into subgrade soils and blend them with the reclaimed pavement and base material. In these instances, agencies are obtaining enough reclaimed material to widen their roads up to 4 feet (1.2 m). Many agencies address durability issues during the design phase by subjecting duplicate samples to UCS measurements as per ASTM D 1633. One sample will employ standard curing techniques, and the other will employ a 4hour soak or 24- to 72-hour freeze. The soaked or frozen sample is required to retain between 75% and 85% of the strength obtained from the standard cured sample. TST is another technique to address moisture sensitivity issues. Standard test specimens are placed in a 1/4-inch (6 mm) deionized water bath whose surface is monitored for ten days by measuring the dielectric constant with a probe. Conclusions The FDR with cement process is a popular technique used by state, county, and city highway agencies that seek a speedy and cost-effective method to improve their roads. Agencies that use the process save between 25% and 50% over conventional reconstruction methods. There was no evidence of structural failure in the FDR sections. The distress identified on the pavement surface was restricted to the hot-mix asphalt (HMA) overlay and was not a result of failures in the stabilized base layer. This investigation has also provided evidence that the FDR pavement sections with sealed shrinkage cracks are performing satisfactorily. Another major benefit of the FDR with cement process is its environmental soundness. The sources of good-quality aggregate and HMA are limited and FDR conserves virgin construction materials and makes smart economic and strategic sense. Stabilizing the old HMA surface, granular base, and underlying subgrade soil with portland cement creates a solid foundation for pavements. 14 Special advertising supplement to the McGraw-Hill Construction Regional Publications

Full-Depth Reclamation

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