Scott Schram

Laboratory Characterisation of Recycled Asphalt Pavement as a Base Layer

Asphalt rehabilitation projects produce about 100 million tons of recycled asphalt pavement (RAP) per year from millings, presenting a major solid waste concern. Literature indicates that RAP has a structural value as a pavement layer. However, limited research exists to quantify its structural capacity with fundamental engineering properties, especially for high RAP contents. Properties such as the resilient modulus (M R) of unbound layers are needed in the mechanistic–empirical pavement design guide. This paper investigates the behaviour of base layer mixture containing RAP at different quantities using resilient modulus (M R) from laboratory testing. RAP from millings was blended in varied quantities with local aggregates. Densities also varied among test specimens. Resilient modulus tests showed that as RAP content increased, M R increased. Results also showed a strong positive correlation between M R and density. Conclusions indicate RAP has a potential to be used in high percentages in pavement base layer applications. Doing so may help alleviate a growing environmental problem while providing a strong pavement foundation.

Improving Prediction Accuracy in Mechanistic-Empirical Pavement Design Guide

Model calibration plays a fundamental role in the implementation of the mechanistic-empirical pavement design guide. The data used in the default calibration effort, which were afforded by the Long Term Pavement Performance (LTPP) database, have a network-level inference space. As implementation proceeds, state highway agencies may be inclined to calibrate at a local network level. However, with a focus on the calibration data set to local project-level conditions, model prediction error can be reduced further. Under this study, Nebraska Department of Roads Pavement Management System data were used to calibrate two design guide smoothness models at the local project level. The focused data set was categorized by annual daily truck traffic and surface layer thickness. Results showed that project-level calibrations reduced default model prediction error by nearly twice that of network-level calibration. This study offers a window into the accuracy that can be achieved with local focus calibrations of design...

Reporting Results from the Hamburg Wheel Tracking Device

As the list of states adopting the Hamburg wheel tracking device continues to grow, there is a need to evaluate how the results are used. AASHTO T-324 does not standardize the analysis and reporting of test results. Furthermore, the processing and the reporting of the results of manufacturers are not uniform. This inconsistency is partly the result of the variation of agency reporting requirements. Some requirements include only the midpoint rut depth; others include the average across the entire length of the wheel track. No guidance is given when the stripping infection point (SIP) is reported. To eliminate bias in reporting, statistical analysis was performed on more than 135 test runs on adjoining gyratory specimens. Measurement location was found to be a source of significant variation for rut depth in the Hamburg wheel tracking device. This variation was likely the result of the nonuniform wheel speed across the specimen, geometry of the specimen, and air void profile. Eliminating this source of bias when rutting results are reported is feasible although the feasibility depends on the average rut depth at the final pass. When the results of a test that has an average rut depth of less than 12 mm at the final pass is reported, the average of the measurements along a 6.4-in. path beginning at 0.5 in. from the specimen edge nearest the gear housing should be reported. When the average final-pass rut depth is greater than 12 mm, it is reasonable to report the average of the measurements just off the center of the rutting track along a 3.2-in. path beginning at 2.1 in. from the specimen edge nearest the gear housing. SIP values were also analyzed. It is reasonable to report the average of the SIP values across all locations while calculated values measured more than 2 in. from the specimens edge are discarded and for cases in which the ratio of the stripping slope to creep slope exceeds 2.0. Validation is needed for multiple machines.

Integration of Mechanistic-Empirical Pavement Design Guide Distresses with Local Performance Indices

The pavement management system (PMS) is the organizational entity within a state highway agency responsible for the condition of the pavement network. Visual distress surveys are typically combined into an index to provide an overall measure of performance. Decision makers use these familiar indices in a number of facets. A recent survey suggests every state highway agency has implemented or plans to implement the Mechanistic–Empirical Pavement Design Guide (MEPDG). As they do so, maintaining the role these local indices play is critical for upholding the systems continuity. However, using the MEPDG output directly in calculating performance indices becomes problematic because local distresses and MEPDG distresses are not always congruent. Therefore, there is a need to develop procedures for calculating local performance indices with locally calibrated MEPDG output. Doing so will allow interchangeable use of both while preserving the role of the local indices. The Nebraska Department of Roads (NDOR) PMS serves as a model case. NDOR employs three indices in network-level PMS analyses for flexible pavements. MEPDG flexible distress models were calibrated by using local agency data and input into the existing index functions. This paper explores how the current measures of network condition used in decision making can coexist with the new design methodology. This connection allows mechanistic–empirical analyses of fund allocation, needs estimations, performance modeling, planning, and remaining service life. In addition, local indices can provide much more meaningful failure criteria in the MEPDG to local designers. Practical methods for index calculations are introduced. Detailed guidance for local calibration is also presented.

Mechanistic―Empirical Modeling in Network-Level Pavement Management

Researchers and practitioners have long recognized the advantages of mechanistic modeling. As agencies implement the Guide for Mechanistic–Empirical Design of New and Rehabilitated Pavement Structures [referred to as the Mechanistic–Empirical Pavement Design Guide (MEPDG)] in the local project-level pavement management system (PMS), its potential as a planning tool in network-level analyses remains unrecognized. A unique, large-scale application of MEPDG is presented. The Nebraska Department of Roads uses a decision tree that systematically screens and identifies candidates for entry into a multiyear optimization program. The process uses linear deterioration rates that have been shown to provide an R2 value as low as 14%. In a retroactive 5-year analysis, 86 sections were prescreened by using the existing models. Predictions indicated that 85 of the 86 sections would have been candidates for maintenance in the first 5 years, whereas field data suggested that only 23 should have been targeted. Calibrated MEPDG distress models calculated with local performance indices replaced the existing linear models. When the analysis was repeated with the mechanistic–empirical models, 35 of the 86 sections qualified for entry into the 5-year analysis, a 70% improvement in the accuracy of the forecasted cost. This research fulfills a timely need for the transitioning of network-level PMS toward mechanistic practices. Consideration of fundamental material properties, climatic conditions, and the structural response to traffic loading provides improved accuracy in planning. Furthermore, production variability and prediction uncertainties can be quantified and used as an additional probabilistic decision-making parameter. Detailed results of the implementation of MEPDG in the local network-level PMS are presented.

Long-Term Evaluation of Cold-in-Place Recycling and Factors Influencing Performance

AbstractCold in-place recycling (CIR) is a type of rehabilitation strategy that has shown to significantly improve the condition of flexible pavements with adequate subgrade support. The process in...

Effects of High Reclaimed Asphalt-Pavement Content on the Binder Grade, Fatigue Performance, Fractionation Process, and Mix Design

AbstractThe primary objective of this research is to examine the effects of reclaimed asphalt pavement (RAP) amounts and fractionation methods on the mix design of high-RAP content surface mixes. T...

Specifications for Aggregate Frictional Qualities in Flexible Pavements: Case Study

Asphalt mixtures can be engineered to meet a pavement facilitys surface friction demands. Researchers recommend laboratory testing on mixtures and aggregates to ensure desirable friction in the field. Some state highway agencies use this testing, and other agencies specify properties related to the aggregates. Aggregate structure, source, grain size, and chemistry have been shown to affect surface friction, yet these effects have been little validated in the field. The Iowa Department of Transportation (DOT) classifies frictional qualities of aggregates on the basis of grain size, hardness, and abrasion. Mixtures are assigned one of three designations that specify the frictional class and amount needed to satisfy the facilitys demands. A case study analyzed the effectiveness of the classification system and validated material specification limits for flexible Interstate pavements. A comprehensive review of mixture designs and friction measurements from the past 30 years showed that friction was highly sensitive to aggregate classification and could be improved with modifications to specification limits. A correlation between average ratio of wet-to-dry crashes and corresponding friction numbers was developed from more than 14,000 crash records. The developed model provided a desirable minimum friction level and was validated with a multimethod approach. The analysis showed the average ratio of wet-to-dry crashes remained constant when friction levels exceeded 38. Historically, this level was achieved when 30% or more of the coarse aggregate and 25% or more of the fine aggregate was a crushed quartzite, granite, or steel slag. Iowa DOT specifications for asphalt mixtures have been updated to reflect these findings.

Laboratory and Field Evaluation of HMA with High Contents of Recycled Asphalt Pavement

In an attempt to conserve natural resources such as materials and energy there is a trend to increase the amount of recycled asphalt pavement (RAP) in asphalt pavement construction. Currently in Iowa, the amount of RAP materials allowed for the surface layer is limited to 15% by weight. The objective of this paper is to develop quality standards for inclusion of RAP content higher than 15% in asphalt mixtures. To determine if the higher percentage of RAP materials greater than 15% can be used in Iowa’s state highways, three test sections with target amounts of RAP materials of 30%, 35% and 40% were constructed on Highway 6 in Iowa City. To meet Superpave mix design requirements for mixtures with high RAP contents, it was necessary to fractionate the RAP materials. Based on an extensive sieve-by-sieve analysis of RAP materials, the optimum sieve size to fractionate RAP materials was then identified. Three test sections with actual RAP contents of 30.0%, 35.5% and 39.2% were constructed and the average field densities measured from the cores were 95.3%, 94.0%, and 94.3%, respectively. Field mixtures were compacted in the laboratory to evaluate moisture sensitivity using a Hamburg Wheel Tracking Device and rut depths after 20,000 passes were less than 3 mm for all three test sections. The binder was extracted from the field mixtures from each test section and tested to identify the effects of RAP materials on the performance grade of the virgin binder. Based on Dynamic Shear Rheometer and Bending Beam Rheometer tests, the virgin binders (PG 64-28) from test sections with 30.0%, 35.5% and 39.2% RAP materials were stiffened to PG 76-22, PG 76-16, and PG 82-22, respectively. Finally, a condition survey of the test sections was conducted to evaluate their short-term pavement performance.