Derek Tompkins

Reappraisal of Recycled Concrete Aggregate as Coarse Aggregate in Concretes for Rigid Pavements

State departments of transportation began using recycled concrete aggregate (RCA) as aggregate in portland cement concrete pavement in the United States in the late 1970s. Although RCA is rarely used in current U.S. rigid pavement slabs, the impetus for its continued use remains the same: a lack of landfill space, a shortage of nearby quality natural aggregates, or both. However, as American pavement engineers and researchers place a greater emphasis on sustainable, reusable roadways, the status quo for RCA in American roadways should be reconsidered along with these new priorities. This study proposes to revisit the use of recycled concrete as aggregate in rigid pavement slabs by using overlooked research to address the concerns that prevented the wide-scale adoption of recycled concrete as an aggregate in pavement slabs by state departments of transportation. Experiences encountered in countries (mostly restricted to Europe) where the use of RCA in rigid pavement is more common are also described. New opportunities for the use of RCA as a structural component in pavement concretes are detailed.

Design and Construction of Sustainable Pavements: Austrian and German Two-Layer Concrete Pavements

Several recent efforts in U.S. pavement research deal with creating sustainable technologies and designs that use composite pavements, that is, pavements with heterogeneous asphalt, concrete layers, or both, resulting from new construction. Although there are several asphalt-over-concrete composites in the United States, in the case of two-layer portland cement concrete (PCC) pavements there has been little research or expertise in the United States despite the wide acceptance of two-layer PCC as a solution for pavements in parts of the European Union (EU). The EU experience is described in this paper in terms of German and Austrian case studies. After the case studies, the major benefits of two-layer PCC and potential challenges in importing, adapting, and implementing German and Austrian techniques are discussed.

Benefits of the Minnesota Road Research Project

The Minnesota Department of Transportation began construction on the Minnesota Road Research Project (MnROAD) in 1991 and opened the full-scale pavement research facility to live traffic in 1994. Since the time of its construction, MnROAD, the first major test track since the AASHO Road Test of the 1950s and 1960s, has provided many lessons in pavement testing and pavement engineering on behalf of the greater pavement community. Researchers at the University of Minnesota reviewed these lessons from the first phase of MnROAD (the facilitys first 10 years of operation) for a project titled MnROAD Lessons Learned. The Lessons Learned project involved more than 50 interviews; 300 published and unpublished reports, papers, and briefs; and an online survey of pavement professionals. This paper, based on the Lessons Learned project, presents a sample of the lasting benefits of MnROAD at the local, state, and national levels. Furthermore, the paper provides extensive references for these benefits in the hope of increasing awareness of this pavement test facilitys underpublicized contributions to pavement engineering.

Composite Pavement Systems Volume 1: HMA/PCC Composite Pavements

Composite pavements have proved in Europe and the United States to have long service life with excellent surface characteristics, structural capacity, and rapid renewal when needed. This project developed the guidance needed to design and construct new composite pavement systems. Volume 1 presents the state of the practice and guidelines for designing and constructing new hot-mix asphalt (HMA) concrete over a portland cement concrete (PCC) composite pavement that takes full advantage of using differing materials. Volume 2 provides guidance on the design and construction of two-layer, wet-on-wet PCC pavements where the upper layer is a thin high-quality layer (hard nonpolishing aggregate, higher cement content, higher quality binder) and excellent surface characteristics with the lower layer containing a higher percentage of local aggregates and recycled materials. Both volumes detail performance data on existing composite pavement systems and provide step-by-step guidance on the design of composite pavements using mechanistic-empirical design methods for both types of new composite pavements.

Low-Volume-Road Lessons Learned: Minnesota Road Research Project

The Minnesota Department of Transportation built the Minnesota Road Research Project (MnROAD) and its low-volume road (LVR) between 1990 and 1993. The 2.5-mi LVR consists of a two-lane roadway that originally contained gravel, hot-mix asphalt, and concrete test sections designed for low-volume road research. Each of these test sections is trafficked by a controlled five-axle tractor-semitrailer to simulate conditions of rural roads in two load configurations, resulting in the same equivalent axle loads. Over the years, a number of activities and studies have taken place that have used information from MnROADs LVR. The first 10 years of findings related to the LVR in the areas of facility, hot-mix asphalt, portland cement concrete, aggregate surfacing, seasonal load limits, and non-pavement-related lessons learned are summarized.

Evaluation of Characterization and Performance Modeling of Cementitiously Stabilized Layers in the Mechanistic-Empirical Pavement Design Guide

Cementitious stabilization of aggregates and soils is an effective technique to increase the stiffness of base and subbase layers. Furthermore, cementitious bases can improve the fatigue behavior of asphalt surface layers and subgrade rutting over the short and long term. However, it can lead to additional distresses such as shrinkage and fatigue in the stabilized layers. Extensive research has tested these materials experimentally and characterized them; however, very little of this research attempts to correlate the mechanical properties of the stabilized layers with their performance. The Mechanistic–Empirical Pavement Design Guide (MEPDG) provides a promising theoretical framework for the modeling of pavements containing cementitiously stabilized materials (CSMs). However, significant improvements are needed to bring the modeling of semirigid pavements in MEPDG to the same level as that of flexible and rigid pavements. Furthermore, the MEPDG does not model CSMs in a manner similar to those for hot-mix asphalt or portland cement concrete materials. As a result, performance gains from stabilized layers are difficult to assess using the MEPDG. The current characterization of CSMs was evaluated and issues with CSM modeling and characterization in the MEPDG were discussed. Addressing these issues will help designers quantify the benefits of stabilization for pavement service life.

Modification of Mechanistic-Empirical Pavement Design Guide Procedure for Two-Lift Composite Concrete Pavements

Modifications made to the Mechanistic–Empirical Pavement Design Guide (MEPDG) to accommodate the design of a newly constructed composite pavement system featuring a thin portland cement concrete (PCC) layer placed over another PCC layer (PCC–PCC pavement) are described. In previous versions of the MEPDG, a newly constructed PCC–PCC pavement project was considered a bonded PCC overlay of an existing PCC pavement. As a result of this simplification, the MEPDG was not consistent in its predictions for a newly constructed PCC–PCC pavement and its structurally equivalent single-layer analogue. To remedy this inconsistency, researchers suggested modifications to the Enhanced Integrated Climatic Model (EICM) used by the MEPDG in terms of (a) number of locations through the slab used by EICM to determine the thermal gradient and (b) EICM calculation of the subgrade spring stiffness (or k-value). Before-and-after MEPDG sensitivity analyses are included to justify the modifications developed and implemented into the MEPDG with the assistance of the MEPDG development team. This research was conducted under SHRP 2 R21, Composite Pavement Systems.

Minnesota Road Research Data for Evaluation and Local Calibration of the Mechanistic–Empirical Pavement Design Guide's Enhanced Integrated Climatic Model:

This paper describes research to evaluate modeling of the thermal behavior of concrete and composite pavements by the Enhanced Integrated Climatic Model (EICM), the climate-modeling package used in the Mechanistic–Empirical Pavement Design Guide (MEPDG). First, the study uses temperature data collected at the Minnesota Road Research Project (MnROAD) facility from portland cement concrete (PCC) and asphalt concrete (AC)–PCC pavements to investigate benefits of AC overlays on the thermal characteristics of PCC slabs. Furthermore, the study validates EICM predictions of thermal gradients through the slabs and investigates the effect of MEPDG-user inputs for thermal conductivity of PCC. Overall, the paper examines measured data from MnROAD for AC-PCC pavements and their single-layer PCC counterparts and attempts to explain how similar pavement systems and their thermal characteristics are taken into account in the MEPDG. The paper concludes that evaluation of the material thermal inputs should be part of a process of local calibration and adaptation of the MEPDG.

Accelerated Loading Testing of Stainless Steel Hollow Tube Dowels

The use of dowels in rigid pavements to provide adequate load transfer is a common practice in current pavement design. Since the use of deicing salts in cold climates results in significant deterioration of dowels, the need to optimize performance and reduce pavement life-cycle cost prompts the need for alternatives to common round steel dowel bars. These alternative dowels must be able to withstand corrosion while providing adequate long-term load transfer performance across jointed slabs. Results are presented of the accelerated loading of two jointed test pavement specimens that incorporate one of two types of dowels: a stainless steel hollow tube dowel or an epoxy-coated solid steel dowel. The accelerated loading of these test specimens was accomplished courtesy of the second generation of the Minnesota Accelerated Loading Facility (MinneALF-2), a 10-year-old joint project between the Minnesota Department of Transportation and the University of Minnesota that allows for the realistic simulation of re...

Establishing the Interlayer Structural Response for Unbonded Concrete Overlays of Existing Concrete Pavements

An improved mechanistic empirical design procedure for unbonded concrete overlays of existing concrete pavements (UBOLs) should account for the effect of the interlayer on the structural response of the pavement. One approach is to use the Totsky model to characterize the interlayer. The Totsky model treats the interlayer as a bed of springs between two plates and is currently incorporated into the rigid pavement finite element software ISLAB. A difficulty encountered in implementing this model is that there are currently no guidelines as to what the interlayer k-value should be for different types of interlayers. The interlayer can be constructed of new or aged asphalt (open or dense graded) or a nonwoven geotextile fabric. To establish the k-values that accurately characterize each of these materials, an ISLAB model of a laboratory test was created so the k-values could be established by matching the measured and calculated difference between the deflections in the overlay and existing pavement. To supplement the use of the laboratory data in establishing the Totsky interlayer k-value, an analysis was carried out using falling weight deflectometer (FWD) data from UBOLs at the Minnesota Road Research Facility (MnROAD). Analyses were then performed to determine if the difference between k-values for different interlayer materials are statistically significant, and if the results from the laboratory analysis match those obtained from the MnROAD field data. The Totsky k-value recommended for use when modeling the response of an UBOL with an asphalt interlayer is 3500 psi/in and 425 psi/in for a fabric interlayer.