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Partial-depth patching mixes must rapidly gain strength to allow the roadway to be reopened to traffic quickly. A patch should also bond well to the substrate to prevent the patch from separating from the existing material and be durable enough to withstand harsh winters. The objective of the research described in this report is to develop improved guidelines for evaluation of pre-bagged commercial patching mixtures and to recommend effective construction practices. To achieve these objectives, 13 different cementitious materials were selected and tested to determine key properties including strength gain, shrinkage, bond strength, and durability. The impact of the proposed research will be a better performing patch material as well as performance criteria that can be used to compare the materials tested in this program to new materials that will certainly be developed in the future. This research was conducted in four main phases, literature review and development of a testing plan and three phases of laboratory testing campaigns. The most commonly available acceptance specification for partial-depth patching materials is the ASTM C928. This specification was followed and the outcomes of each of the recommended tests were evaluated in context of the performance of the patching materials. Several additional tests were developed and conducted to evaluate the bonding properties of patching materials; correlations between lab measured properties were also evaluated. Through aforementioned testing and analysis, a laboratory testing based acceptance procedure was developed for partial-depth patching materials to be used by MnDOT.

This report summarizes efforts of using the disk-shaped compact tension (DCT) test to measure thermal fracture properties of asphalt mixtures on five asphalt paving projects in Minnesota during the 2013 construction season. Five construction projects throughout the state were chosen by a team of researchers at the Minnesota Department of Transportation (MnDOT) and University of Minnesota Duluth (UMD) representing differing climatic conditions, construction practices, and asphalt PG binder grades. Contractors from these varying projects provided UMD with mix design and production pills and MnDOT with loose production mix and raw materials for specimen fabrication. Testing was done to verify mixes met the required fracture energy value of 400 J/m2. If DCT results did not meet this requirement, mix adjustment recommendations were made by the research team. When recommendations were accepted, test sections with adjusted mix were paved. DCT testing was conducted on both adjusted and unadjusted production mix. Results of these efforts showed a drop in fracture energy between mix design and production for each project. The cause is not known at this time, but will be investigated in future research. Preliminary distress surveys indicated projects with mill and overlay experienced higher amounts of cracking compared to projects with reclaim or new construction. It should be noted distress surveys were conducted 9 months after initial paving, with the roadways subjected to only one season of freezing conditions. Condition of underlying pavement structure was not investigated before paving began in the cases of mill and overlays

The asphalt mixture design and acceptance procedures for Minnesota Department of Transportation are currently governed primarily by the mixture composition requirements put forth through use of various volumetric measures (such as, air content, asphalt film thickness, aggregate gradation etc.). The asphalt binder has been required to meet performance criteria through the Superpave asphalt binder specifications. This study looked at use of laboratory performance test for asphalt mixtures. The study was conducted in three phases, first phase focused on merging the asphalt mix design records with the pavement performance data to determine effects of mix design parameters on asphalt pavement cracking performance. Second and third phase used a series of field sections across Minnesota to conduct field performance evaluations as well as laboratory tests on field cored samples. The testing for second an third phase of the study focused on using disk-shaped compact tension (DCT) fracture energy test as a laboratory performance test. The findings form he first phase of study indicated that the asphalt binder type as defined by the Superpave performance grade (PG) plays an important role in affecting the field cracking performance, majority of mixture design parameters did not indicate a consistent effect on field cracking performance, this reinforces the need for use of laboratory performance test as a mixture design tool as well as acceptance parameter. The DCT testing results showed trends consistent with previous and other on-going research studies, whereby the asphalt mixtures with higher fracture energies corresponded with pavements with lower amount of transverse cracking.

The work detailed in this report represents a continuation of the research performed in phase one of this national pooled fund study. A number of significant contributions were made in phase two of this comprehensive research effort. Two fracture testing methods are proposed and specifications are developed for selecting mixtures based on fracture energy criteria. A draft SCB specification, that received approval by the ETG and has been taken to AASHTO committee of materials, is included in the report. In addition, alternative methods are proposed to obtain mixture creep compliance needed to calculate thermal stresses. Dilatometric measurements performed on asphalt mixtures are used to more accurately predict thermal stresses, and physical hardening effects are evaluated and an improved model is proposed to take these effects into account. In addition, two methods for obtaining asphalt binder fracture properties are summarized and discussed. A new thermal cracking model, called "ILLI-TC," is developed and validated. This model represents a significant step forward in accurately quantifying the cracking mechanism in pavements, compared to the existing TCMODEL. A comprehensive evaluation of the cyclic behavior of asphalt mixtures is presented, that may hold the key to developing cracking resistant mixtures under multiple cycles of temperature.

The long-term performance of pothole patches largely depends on the selection of the patching method. A number of pothole patching methods are in practice in Minnesota and other nearby states. However; pavement maintenance crews often encounter problems in selecting the most appropriate patching method because proper guidelines are not available. The objective of this project was to investigate the effectiveness of different pavement patching methods and to develop simple decision trees and a best practices manual. The performance of 20 different pothole patches; which were patched with four different types of patching methods and located at five different construction sites; were monitored for approximately two years. Based on the observed performance of the pothole patches considered in this study; two forms of decision trees and a best practices manual have been developed for selecting the most appropriate patching method for a given pothole condition. The developed decision trees can be used to select the patching method based on the location of the pothole (e.g.; along longitudinal joints; localized potholes; etc.); construction season; condition of the pothole; and pothole area and depth. The best practices manual provides guidelines on the selection of patching method; pothole preparation; placement of patching materials; and compaction.

Asphalt mixtures are commonly specified using volumetric controls in combination with aggregate gradation limits; like most transportation agencies; MnDOT also uses this approach. Since 2010 onward; several asphalt paving projects for MnDOT have been constructed using coarser asphalt mixtures that are manufactured with lower total asphalt binder contents. Due to the severe cold climate conditions in Minnesota; there are concerns of premature cracking and inferior durability in asphalt mixtures with lower asphalt binder contents. This research project evaluated 13 low asphalt binder content mixes from 10 actual field projects to determine whether there is potential for poor cracking performance and high permeability. Assessment of field performance indicated an average of 7.75 years of life until 100% transverse cracking level is reached. The pavement structure played a significant factor in controlling the cracking rates. Thin overlays showed almost ten times inferior transverse cracking performance as compared to asphalt wearing courses on full-depth reclamation. Asphalt mixture volumetric factors did not show a statistically significant effect on cracking rates; however; the asphalt binder grade did show a strong effect. Eight out of the 13 coarse asphalt mixtures evaluated in this study have higher permeability than the typical dense graded asphalt mixtures. Performance evaluations using lab measured properties predicted poor thermal cracking performances. No discernable trends were observed between measured or predicted cracking performance and mix volumetric measures. Use of performance tests based on specifications for design and acceptance purposes is reinforced through this study.

At present, like many other agencies, the Minnesota Department of Transportation asphalt material specifications rely primarily on volumetric properties to ensure good field performance. There have been considerable amounts of research efforts to develop so called "asphalt performance tests" that can link laboratory-measured parameters to pavement performance. Research efforts are also undertaken to refine the asphalt mix-design method so that laboratory tests and procedures can be incorporated into material specification. This research project explored availability of such tests, their suitability, and their use by other agencies.

This project aimed to validate loose mix aging procedures for cracking resistance evaluation of asphalt mixtures in balanced mix design (BMD) with a broad range of field projects covering various mixture components, pavement ages, and climatic conditions. To that end, a two-phase research approach was followed, with Phase I focusing on a literature review, research gap analysis, and development of Phase II work plan. The literature review topics included development and preliminary field validation of existing loose mix aging procedures; the impact of loose mix aging on asphalt binder and mixture properties; and effects of silo storage, mix hauling, mix reheating, specimen storage, and asphalt weathering on asphalt binder and mixture properties. The literature was then critically reviewed to identify research gaps that might hinder the implementation of loose mix aging for cracking resistance evaluation in BMD, including lab-to-field aging correlation, applicability to asphalt mixtures containing additives, selection of laboratory tests and parameters to assess loose mix aging, and implementation of loose mix aging into BMD. Finally, a Phase II work plan was developed to address the knowledge gaps identified through the literature review and research gap analysis, which include two major tasks: 1) further validation of 95°C loose mix aging maps, and 2) conversion of different loose mix aging procedures based on a kinetics aging model.

Good fracture properties are an essential requirement for asphalt pavements built in the northern part of the US and in Canada for which the predominant failure mode is cracking due to high thermal stresses that develop at low temperatures. Currently, there is no agreement with respect to what experimental methods and analyses approaches to use to investigate the fracture resistance of asphalt materials and the fracture performance of asphalt pavements. This report presents a comprehensive research effort in which both traditional and new experimental protocols and analyses were applied to a statistically designed set of laboratory prepared specimens and to field samples from pavements with well documented performance to determine the best combination of experimental work and analyses to improve the low temperature fracture resistance of asphalt pavements. The two sets of materials were evaluated using current testing protocols, such as creep and strength for asphalt binders and mixtures as well as newly developed testing protocols, such as the disk compact tension test, single edge notched beam test, and semi circular bend test. Dilatometric measurements were performed on both asphalt binders and mixtures to determine the coefficient of thermal contraction. Discrete fracture and damage tools were utilized to model crack initiation and propagation in pavement systems using the finite element method and TCMODEL was used with the experimental data from the field samples to predict performance and compare it to the field performance data.

Asphalt overlays are commonly used to rehabilitate deteriorated Portland cement concrete (PCC) pavements. However, mechanically or thermally driven movements at joints and cracks in the underlying pavement usually lead to development of reflective cracks in the overlay. The formation and propagation of reflection cracking is controlled by the mechanical properties of the asphalt and the condition of the overlaid pavement. Current state of practice for asphalt overlay design is policy oriented and lacking an engineered design approach. There is need for establishing state of practice in design of overlays as well as for assessment of PCC pavement condition and recommending improvements to existing pavement prior to overlay construction. The objective of this study is to develop a simple decision tree-based tool for selecting suitable asphalt mixtures and overlay designs to prolong overlay lives by lowering reflective cracking and improving in-situ density. This research will leverage the current National Road Research Alliance (NRRA) effort of constructing, instrumenting, and monitoring 12 MnROAD test sections, laboratory performance tests on asphalt mixtures from the test sections, and past field performance data. The proposed tool incorporates field performance data, performance modelling, and life-cycle cost analysis to develop best practices for rehabilitation of PCC with asphalt overlays.