Stroup-Gardiner, Mary

A 5-year study of newly constructed pavements showed that a reduction in in situ air voids occurred both within and between wheelpaths for highways with an average daily traffic (ADT) load of less than 10,000 vehicles. Regardless of the level of voids immediately after construction, mixtures in the upper 65 mm (2.5 in.) within the wheelpath indicated a reduction in voids by 3 to 5 percent (e.g., from 10 to 6 percent voids), and by between 2 to 4 percent between the wheelpaths. Because only limited densification occurred below this depth for lower–traffic-volume facilities, reducing the mix design level of air voids from 4 percent to 2 percent for the lower lifts was suggested so that lower initial voids could be obtained during construction. An evaluation of older pavements indicated that moisture damage to the lower pavement layers was typical; thus, a change in mix design procedures might also help improve durability by increasing the film thickness. Pavements with high traffic volumes (>50,000 ADT) consistently indicated an increase in voids over time in the upper lift [40 mm (1.5 in.)], little change in the middle 65 mm (2.5 in.), and a decrease in the bottom 65 mm (2.5 in.). The hypothesis suggested to explain these findings was that a loss of material in the upper lifts was occurring, most probably due to moisture damage as the upper, more permeable wear course, commonly used in Minnesota, allowed water trapping at the wear and binder course (i.e., less permeable) interface. A further investigation of in situ void changes on an interstate indicated that for a pavement constructed with the same fine gradation in all lifts, traffic compacted the mixtures in a manner similar to that in low-volume roads. When the initial in situ voids increased from around 7 percent to nearly 10 percent, the influence of traffic on the densification was substantially increased. Content Note: This is the author’s version of a work that was accepted for publication in the Transportation Research Record: Journal of the Transportation Research Board, Issue Number: 1575, Publisher: Transportation Research Board ISSN: 0361-1981. The final version can be found at https://doi.org/10.3141/1575-01.

At the Minnesota Road Research Project (Mn/ROAD), asphalt concrete mixtures were used to evaluate both warm and cold temperature material properties with selected text methods and a wide range of testing parameters. These parameters were selected to approximate different levels of environmental conditions, traffic speeds, traffic loads, and, in certain cases, confining pressures. The underlying theories used to calculate stress and strain from various loading configurations also were rigorously evaluated to determine the appropriateness of comparing results from one testing configuration to another. Mn/ROAD mixtures were evaluated as the first step in linking laboratory measurements and test method selection to live traffic pavement responses and performance. A comparison of axial and diametral testing using harmonic loading showed that experimental results did not agree with theory. That is, the complex deviatoric modulus determined for diametral testing should have been less than the Young's modulus determined from testing axially loaded samples. This was not the case. This suggested that a further examination of the sample instrumentation, testing variability, and the possibility of anisotropic mixture behavior due to particle orientation during compaction are needed to resolve these differences. Other findings indicated that the influence of load duration is minimized as the test temperature decreases, there was little influence in rest period times in repeated loading tests on modulus, and confining pressure only had a significant influence on modulus above about room temperature.

This report presents the material characterization for the Minnesota Road Research Project (Mn/ROAD) bituminous materials. This effort will provide the historical base line information on properties needed for the validation of future pavement evaluation and design models. The objectives of the work were to 1) Document construction of Mn/ROAD, 2) Establish a series of test methods for characterizing the materials and 3) Develop a data base of material properties to develop mechanistic pavement design procedures. Documentation on construction included mixture design, construction techniques and a summary of test results. The laboratory test methods represent a wide variety of tests developed by the Strategic Highway Research Program, the National Cooperative Highway Research Program and the Federal Highway Administration. The materials represent those tested during the mixture design, construction and post construction phases of Mn/ROAD.

The research performed for this report was intended to recommend alternative mix design procedures and parameters for evaluation of asphalt mixture sensitivity, with more of an emphasis on volumetric relationships. Three Mn/DOT projects were selected to represent the following durability issues: 1) debonding of asphalt from aggregate, 2) cohesion problems, and 3) mix design problems. Materials were obtained from these construction projects and evaluated in the laboratory. Gradations were varied from the project specifications so that mixtures with more and less asphalt were evaluated along with the project mixture. Testing included the temperature susceptibility and moisture sensitivity of the mixtures, in addition to the net adsorption test on the aggregates. The results suggested means for identifying moisture sensitivity mechanisms in mixtures during the mixture design phase, although these need to be confirmed through more extensive investigation. Aggregate mineralogy, gradation, and mixture proportioning can all play a role in improving the durability characteristics of asphalt mixtures. Recommendations are made for continued research and implementation of an improved approach to asphalt mixture design.

This report compiles a vast majority of research on polymer modified asphalt cements and mixtures. It covers a general discussion of polymer chemistry and terms typically used by polymer suppliers; asphalt cement chemistry; typical test methods historically used to evaluate modified asphalts; reported results; comparisons of polymer modified asphalt cements and mixtures; proposed binder specifications; and a summary of field trials reported in the literature. Based on this information, the report suggests several experimental designs for use in the completion of the laboratory and field trial phases of this project.

The durability of selected asphalt concrete components and mixtures from six projects were evaluated with the newly recommended net adsorption test and the more common ASTM D4867 (a modified Lottman), respectively. The net adsorption test was used to assess the durability of the adhesion of the asphalt to the aggregate surface in the presence of water. The ASTM D4867 method evaluated the retained strengths of compacted mixture resistance after freeze/thaw conditioning. The net adsorption results indicated at least two of the six projects could be susceptible to moisture related adhesion problems. The test method was adjusted to use the full aggregate gradation rather than only the fine fraction. A comparison of these results to those originally reported by Strategic Highway Research Program (SHRP) researchers showed good agreement in both trends of results and within laboratory test method variability (about 0.14 mg/g). The mixture testing indicated that three of the six projects had tensile strength ratios of less than 70 percent and could be expected to show some evidence of moisture related damage. An evaluation of these results suggested that any moisture sensitivity could be due to mix design parameters such as lower film thicknesses, and lower initial strengths rather than a chemical loss of adhesion at the asphalt-aggregate interface. The influence of conventional mix design parameters on the moisture sensitivity of asphalt concrete mixtures should be more thoroughly investigated prior to any consideration of the use of additives.

This report presents the results of a one-year study on large-stone asphalt mixtures (LSAM). A thorough review of the existing technology regarding materials, mix design, and performance is included. This study expanded upon the body of knowledge by exploring an easier means of mixture design and explaining the fundamental properties of large-stone mixtures with respect to the aggregate gradation. It was found that a dense LSAM gradation possesses better strength and durability properties than a more open LSAM gradation. Furthermore, a mix design methodology is presented wherein the optimum asphalt content for the mixture may be determined on the basis of aggregate and compacted sample properties. This eliminates the need for cumbersome Marshall stability and flow measurements. The frequency dependency of large-stone mixtures is more pronounced at low temperatures than that of a conventional mixture. The tendency for thermal cracking should be lower for a LSAM than for a conventional mixture. Finally, recommendations are made to develop a permissive specification for LSAM, and to adopt the volumetric mixture design procedure outlined in the report.

This research program was based on laboratory and field studies. All work was limited to the use of a 10-mesh crumb rubber in the dry process. An evaluation of the asphalt-rubber interactions indicated that there should be a reasonable level of interaction between the crumb rubber and the asphalt cement selected for the project. A suggested criterion for defining an acceptable level of interaction would be to establish a minimum viscosity of 15 Poise (Brookfield viscosity) for a neat asphalt cement modified with 20 percent crumb rubber. When designing a crumb rubber modified mixture, the aggregate gradation should be substantially gapped. The target gradation used in the construction of the Babbit, Minnesota test sections should be considered as a guideline for an acceptable gapped gradation. Stockpile gradations should be adjusted for crumb rubber gradations volumetrically; generally, 1 gram of crumb rubber occupies the same volume as 3 grams of aggregate for a given sieve size. The optimum asphalt content for CRM mixtures should be based on air voids from 1.5 to 3 percent. During construction, the crumb rubber (supplied in 50 Ibs. bags) should be added through the recycled asphalt (RAP) hopper (drum plant) or directly into the pug mill (batch plant). Laydown and compaction procedures should proceed as usual. Use of vibration on the rollers should be used at the discretion of the field engineer.

It is estimated that the production of new roofing shingles generates approximately 1,000,000 tons of waste annually in the US., and about 36,000 tons of this waste is in the Twin Cities Metro Area of Minnesota. With another 8.5 million tons of waste materials which are similar to those used in asphalt concrete, it seems viable that their use in hot-mix would be an attractive alternative to disposing of them in landfills. This report presents the results of an effort to evaluate the use of roofing waste generated by manufacturers and from reconstruction projects. It was shown that up to 5%, by weight of mixture, manufacturing waste roofing shingles could be used in asphalt concrete with a minimum impact on the properties of the mixture. At a level of 7.5%, a noticeable softening of the mixture occurs, and this might be detrimental to pavement performance. The use of shingles from roof reconstruction projects resulted in the embrittlement of the mixture which may be undesirable for low temperature cracking of pavements. The manufactured shingle waste seems to work well in stone mastic asphalt mixtures.