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This project focuses on the second construction phase of the Minnesota Road Research facility (Mn/ROAD) and evaluates three typical, locally available, surfacing aggregates along with a rollover section from the initial phase for performance. The project results indicate that the adsorption test did not predict the performance of the sections in this experiment. All of the aggregates were characterized as marginal in terms of moisture and frost susceptibility. The sections with the greatest percentage of fines typically performed better than sections with a low percentage of fines. The Minnesota Department of Transportation issued a technical memorandum to change the specification from 0-15% to 8-15% passing the No. 200 sieve for Class 1 surfacing aggregate. The project also compared freezing and thawing rates on the aggregate sections to nearby hot mix asphalt (HMA) sections. Soil at any particular depth froze four to five days before HMA sections. The aggregate sections also thawed at exponential slower rates as depth increased from 11 to 35 days, which means that an aggregate surfaced road will freeze sooner and thaw slower than an HMA surfaced road. This information impacts the management of spring load restrictions and winter load limits.

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.

Low-volume roads constructed in regions susceptible to freezing and thawing periods are often at risk of load-related damage during the spring-thaw period. The reduced support capacity during the thawing period is a result of excess melt water that becomes trapped above the underlying frozen layers. Many agencies place spring load restrictions (SLR) during the thaw period to reduce unnecessary damage to the roadways. The period of SLR set forth by the Minnesota Department of Transportation is effective for all flexible pavements; however, experience suggests that many aggregate-surfaced roads require additional time relative to flexible pavements to recover strength sufficient to carry unrestricted loads. An investigation was performed to improve local agencies' ability to evaluate the duration of SLR on aggregate-surfaced roadways. This was accomplished through seasonal measurements of in situ shear strengths, measured using the dynamic cone penetrometer (DCP), on various Minnesota county routes. In situ strength tests were conducted on selected county gravel roads over the course of three years. Strength levels recorded during the spring-thaw weakened period were compared to fully recovered periods that typically occur in late spring/summer. The results indicate that aggregate-surfaced roads generally require 1 to 3 additional weeks, over that of flexible pavements, to reach recovered bearing capacity. Additionally, a strong correlation was found between duration required to attain given strength recovery values and climatic and grading inputs.

This study evaluated the performance and cost of commonly used dust palliatives using a mobile air sampling technique. Treatments of calcium chloride, magnesium chloride, and organic polymer-plus-binder were evaluated at standard application rates during the first year and at variable rates during the second year. The treatments were applied to a variety of subject roads that were located throughout Minnesota. Average daily traffic levels varied from 25 to 700 vehicles per day. The overall data trend showed that treatments reduced dust levels and measurements showed that aggregate surface moisture content was the best predictor of dust control efficiency. Positive relationships were measured between dust control efficiency and other variables in the study, generally reinforcing the concept that higher application rates may be more successful on gravels containing greater amounts of material passing the #200 sieve. A negative relationship was measured between dust control efficiency and sand equivalency, showing that treatments on gravels containing more sand material were less effective. In addition to dust control, study participants observed a secondary benefit of surface stabilization, which lasted for a period of time. Treated sections that developed surface stabilization were able to reduce maintenance activities to intersection areas only.