Microstructural and rheological investigation of asphalt mixtures containing recycled asphalt materials

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Abstract

Using increased proportions of Reclaimed Asphalt Pavement (RAP) in asphalt pavements construction has become a top priority due to significant economic and environmental benefits. A similar trend has also emerged in Recycled Asphalt Shingles (RAS), which represent the main roofing material in US. In this paper, the effect of adding RAP and RAS on low temperature properties of asphalt mixtures is investigated using microstructural analysis and modeling of rheological data obtained on eight asphalt mixtures. Detailed information on internal microstructure of asphalt mixtures is obtained from digital images of asphalt mixtures beams and numerical estimations of spatial correlation functions. It is found that that microstructure spatial distribution is not affected by RAP and RAS addition. Mechanical analog (Sharpe, 2008 [18]) and semi empirical models are used to back-calculate binder creep stiffness from mixture experimental data. Differences between back-calculated results and experimental data suggest blending between new and old binder may be only partial.

Highlights

► We studied the use of recycled materials in asphalt mixtures at low temperature. ► We used Reclaimed Asphalt Pavement (RAP), and Recycled Asphalt Shingles (RAS). ► We used microstructural and analogical back-calculation modeling. ► Microstructure spatial distribution is not affected by RAP and RAS addition. ► The analogical model is a good tool to study blending of new and recycled binders.

Introduction

Over the past 20–30 years, the search for recycling alternatives has led federal, state, and local agencies to consider a variety of potentially recyclable materials for pavement and other construction applications. The list includes Reclaimed Asphalt Pavement (RAP), reclaimed Portland cement concrete, iron blast-furnace slag, fly ash, waste tire rubber, waste glass, and roofing shingles. Incorporating RAP in new asphalt pavements significantly reduces the usage of new materials, conserves natural resources and solves disposal problems [1]. To address this important issue, the National Cooperative Highway Research Program (NCHRP) has funded a number of projects, such as NCHRP 9-12 [2] and NCHRP 9-46 [3].

Roofing shingles represent another material that is available in large quantity for recycling. According to one estimate, about 10 million tons of waste bituminous roofing materials are generated in United States each year [4]. The use of Recycled Asphalt Shingles (RAS) in hot mix asphalt has seen increased acceptance from government agencies and construction contractors only in recent years. The main obstacle is the potentially detrimental effect of recycled asphalt materials on asphalt pavement durability due to the aged binders present in old pavements and roofing shingles.

Section snippets

Previous research efforts

Several past studies were performed to investigate the effect of RAP on recycled asphalt mixtures [5], [6], [7], [8] and concluded that RAP content had a significant influence on mixture properties. More recently, research efforts focused on design procedures, forensic evaluation and modeling [1], [9]; specifications were also developed with recommendations for selecting RAP content based on traffic level. For example, Minnesota Department of Transportation (MnDOT) Specification 2350/2360

Research approach

In this research, the addition of RAP, MWSS and TOSS to asphalt mixtures used in pavement applications is investigated based on changes in mixtures microstructure and on changes in mixtures and binders low temperature properties, respectively. The changes in material microstructure are evaluated using Digital Image Processing (DIP) and estimating 2 and 3 point correlation functions of the aggregate phase. Huet [17] analogical model [18], [19], [20] coupled with ENTPE transformation [21], [22],

Materials and testing

Eight different asphalt mixtures (Table 1) prepared with a PG58-28 binder were used in this study. The virgin aggregate materials used in the mixtures consisted of pit-run-sand, quarried ¾ in. (19 mm) dolostone, and quarried dolostone manufactured sand. The recycled material consisted of different amounts of RAP, TOSS and MWSS. More details on these materials can be found elsewhere [15].

Thin beams of asphalt mixtures (three replicates per mixture) were tested using the Bending Beam Rheometer (

Microstructural analysis

In most research efforts, asphalt mixture is considered either as a two-phase material [26], [27], or a three phase material [28], [29], and different micromechanical models are applied to describe and predict mixture behavior. Using low order microstructural information, such as volume fraction, leads to poor prediction of mixture properties [30]. More detailed information can be obtained from the estimation of spatial distribution of the micro-components of a material; this type of

Back-calculation of asphalt binder creep stiffness

Since no changes were found in the mixtures based on the microstructural investigation, the change in rheological properties due to the addition of aged binders is investigated next.

Summary and conclusions

In this paper, the effect on low temperature properties of asphalt mixtures due to the addition of various percentages of RAP, MWSS and TOSS was investigated. Microstructural analysis of the mixtures was first performed, followed by the analysis of changes in rheological properties of asphalt mixtures and corresponding asphalt binders. Asphalt mixture specimens (two-dimensional projections of BBR beams) were analyzed based on digital image processing, and microstructure properties were

Acknowledgment

The authors gratefully acknowledge Minnesota Department of Transportation (MnDOT) for providing the asphalt mixtures used in this investigation and the asphalt binder creep stiffness data and Dr. Raul Velasquez for his suggestions.

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