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This report describes a research project that provides Minnesota counties, and townships with information and procedures to make informed decisions on when it may be advantageous to upgrade and pave gravel roads. It also provides resources to assist county and township governments in explaining to the public why certain maintenance or construction techniques and policy decisions are made. The research involved three major efforts. The first is a historical cost analysis based on the spending history for low volume roads found in the annual reports of selected Minnesota counties. The effects of traffic volume and type of road surface on cost was included in the analysis. The second was the development of a method for estimating the cost of maintaining gravel roads, which is useful when requirements for labor, equipment and materials can be predicted. The third is the development of an economic analysis example that can serve as a starting point for analyses to aid in making specific decisions. Additional information was gleaned from numerous interviews with local road officials. Maintenance and upgrading activities considered included: maintenance grading, re-graveling, dust control/stabilization, reconstruction/re-grading, paving, and others. As part of this report, an analysis is developed that compares the cost of maintaining a gravel road with the cost of upgrading to a paved surface. This analysis can be modified to address local conditions. Such an analysis may be used as a tool to assist in making decisions about upgrading a gravel road to a paved surface.

Aggregate surfaced roads become coarser and coarser after a few years of service due to an inherent problem - dust emission. Fines in the surfacing material are kicked up by traffic and blown away by the wind as fugitive dust. One of the alternative rejuvenation methods is to replenish the missing fines to restore the gradation and plasticity of the in situ material. Savings in the material and cost could in return benefit the environment and financial condition of the jurisdiction. Control and experimental test sections were established in three counties of Minnesota and performance of the sections were assessed using methods including cross-section profile change surveys, gravel loss and loose aggregate measurements, gravel road condition ratings, International Roughness Index estimation, and field observations. Experimental sections in Jackson County did not perform satisfactorily. However, one of the test sections in Beltrami County performed favorably well. A five-year-cycle benefit-cost analysis revealed that a 20 percent cost savings was also achievable in that particular section. Another trial in Olmsted County tested whether modified Class 5 limestone aggregate is appropriate for gravel road surfacing.

Minnesota has a large network of aggregate roads. The majority of the system is maintained by counties and townships. Some of the aggregate roads need to be upgraded with a sealed surface for dust control or to provide a smoother driving experience, as well as for local economic development. Local road officials are often faced with the responsibility of upgrading the roads with a limited budget. Light surface treatments (LSTs) are considered an economical alternative to the conventional upgrade approaches using hot-mixasphalt (HMA) or concrete pavements. The currently used methods in Minnesota for the structural design of LSTs for aggregate roads were originally developed for structural design of flexible pavements. This research evaluated the design methods that can be used to design LSTs for aggregate roads. The methods evaluated include the MnDOT granular equivalent (GE) method and the MnDOT mechanistic-empirical method (MnPave design software), which are used in Minnesota, and the American Association of State Highway and Transportation Officials (AASHTO) flexible pavement design method and South Dakota aggregate road design method, which are practiced in other places in the United States. The results include a discussion of the applicable situations for each method. The research team also conducted a survey that was distributed to the county engineers in Minnesota to document their experiences with LSTs. Recommendations for improving the current design methods when applied to LSTs on aggregate roads are suggested based on the survey results.

Minnesota's local highway agencies are tasked with maintaining their low-volume road networks with available financial resources, prompting increased interest in lower-cost pavement rehabilitation alternatives. In-place cold recycling technologies, such as cold in-place recycling (CIR) and full-depth reclamation (FDR), provide lower-cost opportunities to renew deteriorated roads than traditional rehabilitation methods, particularly if surfaced with a thin surface treatment such as a chip seal (seal coat) or microsurfacing rather than hot-mix asphalt. However, the resulting road surface may not meet some road users’ expectations. This study investigated the performance and economics of four pavement rehabilitation alternatives involving recycling technologies. The alternatives included CIR and FDR treatments with either an asphalt overlay or thin surface treatment. Fifteen case study sections in Minnesota and neighboring states were selected for performance evaluation and lifecycle cost analysis (LCCA). Pavement condition surveys were performed to evaluate the study sections’ pavement distresses and roughness. The results indicated satisfactory performance for fourteen sections, while a CIR section with a chip seal surface using quartzite as cover aggregate had extensive transverse cracking. The cause of the distress is not clear. The LCCA results indicate a 14% to 42% lower lifecycle cost for CIR and FDR treatments with chip seal surfaces. Costs savings may be achieved if asphalt overlay thicknesses are reduced, though chip seal surfaces may be rougher and nosier and require more maintenance than asphalt overlays. A decision tree was developed to aid local agencies during the treatment selection process.

This report documents the best practices of Australia and New Zealand in maintaining and upgrading aggregate roads. Compared to the United States, Australia and New Zealand have fewer resources to invest in road construction and maintenance. As a result, both countries have developed systems for economically constructing and maintaining roads. Although differences exist in climate, traffic, and road user expectations, studying the best practices of Australia and New Zealand offers opportunities to apply relevant practices.

The objectives of this study were to document the performance of roads using full-depth reclamation (FDR) and stabilized FDR (SFDR) in Minnesota; help develop SFDR design parameters appropriate for Minnesota; provide information on FDR/SFDR design procedures and specifications from beyond Minnesota; share current Minnesota FDR practices; and catalog the characteristics of some FDR/SFDR roads. A comprehensive literature review of FDR/SFDR projects and case studies was conducted; and an online survey was distributed to Minnesota local road agencies to determine the stabilizing agents used for SFDR projects. Eighteen FDR/SFDR test sections from eight counties were then selected for a case study; and performance data and core samples were collected for the sections. Minnesota gravel equivalency (GE) analysis was performed to back-calculate the granular equivalent factor for FDR/SFDR layers based on the design equivalent single axle loads (ESALs) and R-values for subgrade soils. The back-calculated GE values indicate that designers have likely been using GE values for FDR/SFDR layers that are consistent with current recommendations. It is recommended that the current GE values be generally retained for FDR/SFDR design. However; when slower-moving vehicles are the critical design consideration; relatively robust performance of FDR/SFDR layers may be expected. Visual distress surveys indicated that the FDR/SFDR bases studied are performing well in terms of destroying crack patterns that are often reflected through traditional hot mix asphalt (HMA) overlays. Therefore; decision makers may want to consider the use of FDR/SFDR as a base for reasons other than structural capacity.

The objective of this project was to provide a better understanding of how various virgin aggregate and recycled asphalt pavement (RAP) mixtures for surface layers affect the performance of gravel or crushed rock roads and; with further analysis; to determine the optimal RAP content for Minnesota gravel roads. This project included a literature review; preliminary laboratory testing; economic and feasibility analysis; and two field studies. Several studies regarding the use of RAP materials for road surfaces were reviewed. Then; laboratory tests on various RAP materials; one virgin aggregate; and mixtures of RAP materials and virgin aggregate were conducted to observe the effect of RAP on the index properties of the materials and the engineering properties of the mixtures. Initially; six test sections were constructed with various surface aggregates in two locations. Virgin RAP-aggregate blends having 15% to 60% RAP contents were placed as surface aggregate. Then; three more test sections were constructed using RAP-aggregate blends having 50%; 70%; and 80% RAP contents. Several field tests; including lightweight deflectometer; dynamic cone penetrometer; scrape; and dustometer tests; were performed to evaluate the test sections. This report provides insights regarding the effect of using RAP material on surface layers to reduce the use of virgin aggregates. It was concluded that the optimal RAP content for unpaved road surfaces changes according to the properties of the materials used; testing methods; and site conditions.

Geogrids have been widely used in roadway construction as reinforcement in pavement foundations. Geogrids have been effective in practice for reducing rutting damage, distributing traffic loads within the pavement foundation layers, increasing the resilient modulus of the base course, and stabilizing the subgrade layer. For this project, an integrated mobile accelerated test system (IMAS), an automated plate load test (APLT) device, and finite element simulation approaches were used to evaluate the effects of geogrid reinforcement. Test configurations were constructed by varying geogrid types (i.e., light-duty biaxial, heavy-duty biaxial, light-duty triaxial, and heavy-duty triaxial), geogrid locations in the base course (i.e., at the interface between the base and the subgrade or in the base course), and base aggregate thicknesses (6, 10, and 16 in) in the laboratory and in experimental field tests. The finite element method (FEM) models were calibrated based on the results from the experimental test sections. Then, the calibrated FEM models were used to determine granular equivalent (GE) values for the remaining sections. Testing results included resilient modulus, deflection, and permanent deformation of the pavement foundation to evaluate the structural benefits of geogrids as a function of the GE. The results of this research revealed that improvement in pavement performance using geosynthetic reinforcement depended on various factors and variables. A new formulation was proposed to predict the GE factor of geogrid reinforcement of flexible pavements. The products produced by this research include this report, which improves geogrid understanding, and a well-developed method to apply GE factors during pavement design. It is expected that one or more of the following benefits will be achieved during implementation: increased service life, reduced gravel and/or asphalt thickness, and reduced maintenance costs.