Ben C Cox

Merits of reclaimed asphalt pavement-dominated warm mixed flexible pavement base layers

Use of reclaimed asphalt pavement (RAP) has increased considerably over the past few years; approximately 85% of the RAP available is used within either hot mix asphalt (HMA) or warm mix asphalt (WMA). Within this time frame a number of research efforts have been performed, but most efforts have focused on RAP contents of 50% or less. This paper describes a laboratory effort that studied WMA with RAP contents of 50–100% in the areas of rutting, moisture damage, durability, cracking, and mixing uniformity. Lack of a RAP surplus coupled with performance data presented in this paper indicates that mixtures with more than 50% RAP do not, for most applications, add value to the highway system in present day. WMA with 50% RAP for use as an underlying (or base) pavement layer performed adequately in all performance areas investigated, durability and cracking included.

In-Place Recycling Moisture-Density Relationships for High-Traffic Applications

This paper evaluates cold in-place recycling (CIR) moisture-density relationships. Phase 1 describes the Mississippi Department of Transportation (MDOT) US-49 CIR project and performs complementary Proctor compaction testing, a key component of MDOT’s US-49 design/construction protocols. Phase 2 evaluates the Superpave Gyratory Compactor (SGC) in place of Proctor compaction as MDOT is interested in its future use. Over 300 SGC specimens were compacted using different materials, gradations, binders, and moisture contents. The key finding is that moisture content had no meaningful effect on dry density in the range where Proctor compaction would detect an optimum moisture content (OMC). Because SGC-compacted dry density appears unrelated to moisture content (within reason), Proctor OMC does not seem as informative for CIR as for soil or crushed stone. Therefore, SGC compaction is recommended, likely at a fixed moisture content, for CIR mixture design.

Vacuum Sealing-Based Volumetric Density Measurement Approach for Cold In-Place Recycling

Recent economic limitations have fostered increased use of pavement rehabilitation methods such as cold in-place recycling (CIR). CIR could be more appealing to departments of transportation provided variability were lower and performance were more predictable. Because of the nature of the materials and construction processes, CIR is likely to be more variable than plant-mixed asphalt, and it is believed that the performance variability of CIR can be reduced through enhanced density control. The many density control methods described in the literature for CIR are often vague. This study used ASTM D6857 vacuum sealing to obtain the maximum theoretical specific gravity (Gmm) of reclaimed asphalt pavement (RAP) and a gravity-proportioning equation to estimate CIR Gmm (specific gravity after cement, emulsion, or a combination is mixed with RAP). Vacuum sealing was also used to obtain the bulk specific gravity (Gmb) of compacted CIR (AASHTO T 331). In all, 240 Gmm and 156 Gmb tests were conducted. D6857 and AASHTO T 209 detect differences between RAP and hot-mix asphalt Gmm, but the differences led to manageable errors for CIR density control that should still allow for some improvements over many current practices. The equation developed in the study estimated CIR Gmm with reasonable error, and T 331 suitably characterized CIR Gmb at the often high air void levels that were observed. The findings show that CIR Gmm and Gmb, as determined by vacuum sealing and the gravity-proportioning equation, constitute a reliable, convenient, and implementable approach to controlling density and likely reducing performance variability.

Cold In-Place Recycling Moisture-Related Design and Construction Considerations for Single or Multiple Component Binder Systems

In recent years, cold in-place recycling (CIR) has gained momentum because of its economic, performance, and sustainability characteristics; as a result CIR markets are likely to expand into, for example, higher traffic routes. To further understand how to continue improving CIR for existing applications as well as future applications, better techniques are needed with regard to interfacing design and construction. Moisture is one key area in which design and construction are often disconnected. This study’s objective, therefore, was to evaluate the moisture (and associated early-age strength and stability) aspects of CIR, particularly within a framework that could consider hydraulic cement, bituminous emulsion, or combinations of both binders. A universal design framework that accommodates any binder or combination thereof while representing early-age field conditions has advantages for agencies, not only in its reasonable characterization of construction processes, but also in its facilitation of competition and creativity in the process of selecting materials and proportions. This study was organized in three phases. Phase 1 documented moisture instrumentation of a cement CIR project. Data were successfully obtained throughout compaction and curing and were used in Phases 2 and 3 alongside supplemental field and laboratory testing. Phase 2 evaluated moisture’s role in compaction; Phase 3 evaluated moisture–strength and moisture–stability relationships for various curing protocols. Phase 2 concluded that high (>6%) moisture content, typical of Proctor compaction, is generally unnecessary. Thus, Proctor compaction is discouraged in favor of a fixed design moisture content. Phase 3 concluded that humid (35% to 50% humidity) and dry 40°C oven curing protocols are candidates for universal CIR design.

Characterising emulsion effects on aged asphalt concrete surfaces using Bending Beam Rheometer mixture beams

A function of asphalt emulsions is often to rejuvenate the surface of an aged asphalt concrete roadway. However, it is unclear as to the effectiveness of different types of asphalt emulsions, as most current testing is empirical and does not give an indication of field performance. Using asphalt concrete beams from the surface of a roadway in the Bending Beam Rheometer is a new approach that may give an indication of an asphalt emulsions ability to decrease the stiffness. Seven emulsions at three application rates on two roadways were examined to identify the effect of emulsion on the stiffness and m-value of asphalt concrete. The addition of asphalt emulsion generally decreased the stiffness of the aged pavement samples; however, results were fairly erratic and inconsistent compared to change in m-value. The addition of emulsion consistently increased the m-value. Requiring an aged asphalt m-value increase of 0.05–0.06 was suggested as an initial value for consideration within specifications for projects where rejuvenation is a first-order concern.

Merits of Asphalt Concrete Durability and Performance Tests When Applied to Cold In-Place Recycling

Within the pavement industry, there exists a continually growing sustainability emphasis. Cold in-place recycling (CIR) of flexible pavements has some potential sustainability implications but has yet to be soundly established on a large scale or fully characterized in terms of performance. Currently-established design and testing procedures for traditional asphalt provide a logical starting point but should be evaluated and possibly modified to accommodate differences within CIR relative to traditional asphalt (e.g. use of mixing water). This papers objective is to evaluate CIR using several available performance tests originally developed for asphalt concrete and assess their ability to characterize CIR, specifically for a diverse array of binding agents. Tests evaluated are the Cantabro durability test, the bending beam rheometer (BBR) mixture beam test, wheel tracking in the Asphalt Pavement Analyzer (APA) and Hamburg Loaded Wheel Tester (HLWT), a loaded wheel fatigue test, and an indirect tensile (IDT) crack characterization test. Based on results of this paper, Cantabro and BBR tests are not recommended, HLWT and loaded wheel fatigue do not appear promising, APA appears promising, and IDT appears promising and should be studied further. INTRODUCTION AND BACKGROUND Within the pavement industry, there exists a continually growing sustainability emphasis. Cold in-place recycling (CIR) of flexible pavements is one pavement rehabilitation technique with potential sustainability benefits. However, CIR processes have yet to be soundly established on a large scale. Further, CIR is not fully distinguished in terms of performance relative to other recycling options such as traditional asphalt mixtures incorporating high percentages of reclaimed asphalt pavement (RAP). Traditional plant recycling aligns more closely with that of traditional asphalt and, consequently, is reasonably established. Conversely, CIR introduces factors not present in plant recycling or traditional asphalt such as binders with vastly differing properties (e.g. portland cement and asphalt emulsion), cold mixing temperatures, use of mixing water, and similar. Therefore, while currently established design and testing procedures for traditional asphalt mixtures provide a

State of Knowledge for Cantabro Testing of Dense Graded Asphalt

AbstractThis paper uses information collected over several years to determine where the Cantabro test usefully characterizes dense graded asphalt (DGA). The Cantabro mass loss (CML) procedure is si...

Asphalt Concrete Field Compactibility Models Focusing on Aggregate Properties and Moisture

Some aggregates used in asphalt production retain fine dust coatings after crushing and washing operations, and these coatings, especially when coupled with moisture, can be severe enough to cause asphalt mixture tender-zone characteristics. This papers objective was to investigate asphalt mixture field compactibility by focusing on aggregate properties (e.g., moisture content and dust coating). In cooperation with the Mississippi Department of Transportation (DOT), 12 asphalt mixtures and 44 test locations were selected and studied. Considered for selection were mixtures with which the Mississippi DOT had experienced tenderness issues that were thought to be related to aggregate-adhered fines or aggregate moisture. Field compactibility was investigated with multiple regression procedures where air void (Va) level was the selected response variable. For each paving project, 31 material, production, and construction properties were collected for consideration within regression models. Through screening and processing procedures, the set of properties was refined to develop three regression models with R2 values ranging from .74 to .86. These models incorporated terms for fine aggregate angularity (FAA), surface area of coarse aggregate (SA8), cold-feed aggregate methylene blue value (MBVCF), cold-feed aggregate moisture (wCF), and other mixture properties (e.g., asphalt content and lift thickness). Key findings were the following: aggregate-adhered fines (ASTM D5711) and FAA had no effect on Va; increasing wCF slightly negatively affected Va; and increasing MBVCF positively affected Va. In general, aggregate property effects on Va were less meaningful than other properties included (e.g., lift thickness). Overall, regression models could be valuable to agencies and contractors for general guidance regarding factors affecting Va.

Recommendations for Seal Treatment Rejuvenation Specifications Based on Bending Beam Rheometer Testing of Mixture Beams

Recently, use of pavement preservation technologies, such as bituminous seal treatments, has increased. Seal treatments are often used to rejuvenate aged asphalt pavements and can decrease permeability and retard oxidation, cracking, and raveling. Many factors affect rejuvenation, and current specifications governing how rejuvenation is characterized could be enhanced. Viscosity testing of asphalt binder extracted and recovered from a pavements near surface is the predominant means of characterizing rejuvenation (e.g., a product must reduce viscosity by 40% to be classified as a rejuvenator). This paper presents data that suggest other rejuvenation approaches are worth considering because (a) extraction and recovery can adversely affect viscosity results, (b) viscosity testing cannot be conducted without forced and unrealistic blending of aged binder and rejuvenator, (c) high test temperatures may not be the most informative for distresses of interest (e.g., cracking), and (d) viscosity could not detect rejuvenation behaviors of some proprietary products. Alternatively, this paper suggests rejuvenation specifications be developed with bending beam rheometer (BBR) testing of mixture beams sawn from laboratory-compacted asphalt surfaces (e.g., a product must increase the m-value by 0.040 to be classified as a rejuvenator). This approach has shown promise for viscosity testing, and many concerns about viscosity testing are alleviated with BBR testing. A specification approach that uses BBR testing of laboratory-compacted asphalt is described and recommended for rejuvenation characterization.

Multiyear Laboratory and Field Performance Assessment of High-Traffic US-49 Full-Depth Reclamation

This paper presents a case study of full-depth reclamation (FDR) on US-49 in Madison County, Mississippi. Laboratory and field test data are presented for FDR activities from construction through 53 months of service. US-49 was chosen as a case study because (a) the reclamation process encountered highly variable and large quantities of particles finer than 75 µm; (b) reclamation was performed to a depth of 16 in., which is relatively deep; (c) the presence of numerous fine particles in a relatively deep reclaimed layer used for a high-traffic application was not a typical FDR situation; and (d) FDR was initiated after construction began to alleviate issues that occurred during construction. Laboratory and field testing performed over a 53-month period suggest the pavement is performing well.