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Asphalt concrete pavement surrounding utility structure covers is prone to settlement, cracking, breaking up over time, and potholing, and these distresses are particularly common in wet-freeze climates (e.g., Minnesota). Several factors contribute to their formation, including design requirements, collar material type and cut shape, construction practices, frost heave, and backfill settlement. If not properlymaintained, the distressed pavement can lead to ride quality issues and hazards for vehicles and snowplows. Differences in design details and construction practices can result in different performance; however, differences in pavement performance around utility structure covers are not well documented. The main goal of this Minnesota Local Road Research Board (LRRB) project is to fill this knowledge gap bydocumenting regional agency best practices for adjusting utility covers and patching the surrounding pavement. Information is gathered through a review of existing information, a review of standard details from agencies in and around Minnesota, an agency survey, and follow-up discussions with agencies that are generally satisfied with their practices. No single best practice is identified for design details; however, common themes among agencies include the importance of both inspecting and testing during construction and achieving adequate compaction of all pavement patch layers. This document is developed to assist local transportation agency personnel and engineering consultants in improving design and maintenance of asphalt concrete pavement around utility covers. It highlights successful and unsuccessful regional practices and trends, factors contributing to pavement damage around utility covers, timing of inspections and maintenance, and a framework for evaluating and modifying practice.

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.

A performance monitoring and forensic study was conducted on several test sections with varying base and pavement characteristics. This report includes the performance data from the current phase, which was extended until pavement reconstruction, with the previously reported data, collected since original construction of the test sections. Each of the three original test sections were prepared with a different combination of base type and asphalt binder type. Three different crushed limestone base types were used in the construction: (Section 1) standard Class 5, (Section 2) permeable aggregate base (PAB), and (Section 3) Class 5 modified. Two different types of asphalt binders, PG 58-28 and PG 58-34, were incorporated to evaluate the effects of cold temperature cracking. All asphalt mixtures were designed according to MnDOT Specification 2350. In addition to the comparisons among sections, each road section was constructed, both with and without transverse sawn-and-sealed regularly spaced joints, to evaluate the relative performance for ride and cracking (both spacing and crack severity) of sawing and sealing (S&S) versus allowing thermal cracking to relieve tensile stress without sawing and sealing (non-S&S). Prior to reconstruction, the severity of cracking in Section 1 was significantly higher than in Sections 2 and 3, with no severe cracking observed in Section 3. This study confirmed previous research efforts on this project, indicating that the combination of PG58-34 binder with Class 5 modified base material (Section 3) and PAB base (Section 2) performed well as compared to the control section. Seasonal trends of reduced stiffness during spring thaw followed by rapid recovery were also observed in the FWD analysis.

This Research Summary is part of Report 2024-01, "Performance Monitoring of Olmsted CR 117 and 104 and Aggregate Base Material Update," published March 2024.

This report presents freeze-thaw durability results of an investigation regarding the application of high performance concrete (HPC) to prestressed bridge girders. This study included a total of 30 concrete mixes and more than 130 specimens, with the following variables: aggregate type, round river gravel, partially-crushed gravel, granite, high-absorption limestone, and low-absorption limestone; cementitious material composition, Type III portland cement only, 20 percent fly ash, 7.5 percent silica fume, and combination of 20 percent fly ash with 7.5 percent silica fume replacement by weight of cement; and curing condition heat-cured or seven-day moist-cured. No air-entraining agents were used in the study's initial phase to simulate the production of precast/prestressed bridge girders. Results indicate that it is possible to produce portland cement concrete with high strength and freeze thaw durability without the use of air-entraining agents. Overall, the moist-cured concrete specimens exhibited better freeze-thaw durability than the heat-cured concrete specimens. The reference concrete mixes--containing only portland cement performed better than the concrete containing pozzolan material of fly ash or silica fume. The low-absorption limestone aggregate concrete mixes exhibited the best freeze-thaw durability performance--in some cases, enduring more than 1,500 freeze-thaw cycles without failing. The study found that the moisture content of the coarse aggregate at the time of mixing had a significant impact on the concrete's freeze-thaw durability.

An increase in freeze-thaw events will result in detrimental impacts on pavement systems. However, the impacts of recent climate changes on freeze-thaw cycles have not been well studied, although they are of interest to a broad number of transportation agencies. In this study, the number of freeze-thaw events at typical air temperature sensor level (e.g., 6 feet above the earth’s surface) as well as at different pavement layers and critical sub-pavement locations such as saturated subgrade within the active zone were quantified. In response to global warming, current work resulted in rigorously quantified freeze-thaw events rooted in climate data from 1941 to 2020. Results indicated that in the recent 40 years (i.e., 1981-2020), Minnesota winters have become warmer by 1-2 °F daytime and 2-5 °F nighttime temperatures. With a decrease in freezing temperatures, the yearly number of freeze-thaw cycles tended to decrease at shallow pavement depths (< 6 inches), whereas remained sporadic at deeper pavement layers. The decreases in freeze-thaw events at shallower depths were significant during the early and late winter months. However, the annual freeze-thaw events at the air temperature sensor level were randomly distributed throughout the analysis period.

This Technical Summary pertains to report 2022-04, Effect of Warmer Minnesota Winters on Freeze-Thaw Cycles.

With the construction of four new test cells in 2008, the Minnesota Department of Transportation (Mn/DOT) now has six unique pervious pavement test sections at the MnROAD test facility. Recorded temperatures in the pervious pavements and subgrades were compared to impervious Portland Cement Concrete (PCC) test sections over the same time interval. It was found that the subgrade in pervious PCC and Hot Mix Asphalt (HMA) was up to 4 °C warmer in the winter than impervious PCC pavements. The frost depth in an impervious PCC pavement was found to be 45.7 cm deeper than in a pervious PCC pavement of similar thickness. One pervious pavement test cell experienced 60% less freezing cycles over a three year interval than impervious PCC pavements of similar thickness. The air trapped in the pavement voids was suspected to be the main reason for the reduced number of freeze-thaw cycles by creating an insulating effect. In another pervious pavement, entrapped air within the base material may also insulate the pavement from the subgrade.

Minnesota’s temperature changes over a wide range on a daily and a seasonal basis. Our roads are expected to perform without major distress over this range of temperature extremes. The effects of temperature cause thermal stress, warping and curling and joint design for Portland Cement Concrete (PCC), and freezing/thawing of moisture sensitive materials. Temperature is one of the most basic pieces of information needed to design a pavement. Temperature is used to specify Performance Grades (PG) for asphalt binders, timing of spring load limits, base thickness for frost free design and modeling parameters for pavement performance and design. This report provides a summary of the various temperature phenomena seen in select the pavement systems. All data is from the Minnesota Road Research Project (Mn/ROAD), located in south central Minnesota. All available data was not analyzed for this report, but rather a few selected cells were analyzed. Therefore, the temperature extremes may not be the exact maximum or minimum for all the cells that were monitored, but should be within a few degrees.

During the winter, soil under a roadway that has been saturated with water freezes and expands, causing the road to rise. When this occurs unevenly, the result is rougher roads during the winter and spring, and in some cases, cracked pavement surface layers. This problem of “frost heave” is very common in Minnesota, especially on lower volume roads, and can cause serious damage to concrete, asphalt and gravel pavements. It presents one of the greatest challenges to engineers to keep Minnesota’s 130,000 miles of roads well maintained throughout the year. Cracked and heaved pavements lead to a bumpy or otherwise poor driving experience for motorists.