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EVALUATION OF COLD RECYCLED ASPHALT MIXTURES MODIFIED WITH CONVENTIONAL AND POLYMER MODIFIED EMULSION
ABSTRACT
Cold recycling (CR) is a partial recycling process through which a portion of the existing recycled asphalt pavement (RAP) material is rejuvenated and placed back on the road as a new base course without introducing heat during the recycling process. Although CR technology is not a new concept, it has not been understood as well as Hot Mixture Asphalt (HMA) and Warm Mixture Asphalt (WMA). Currently, a nationwide standard for CR mixture design does not exist. Furthermore, sufficient data is not available to indicate if there is a difference between the performance of cold mixtures stabilized with conventional emulsions versus those stabilized with polymer modified emulsions.
In this thesis, the performance of lab produced CR specimens with conventional asphalt emulsion and polymer modified asphalt emulsion is investigated through conducting a series of tests. CR specimens were prepared in precise procedures to ensure reliable and consistent specimen production for testing. Through basic volumetric measurements and performance related tests including strength, durability, moisture damage resistance, and rutting resistance, a general CR mixture design procedure was established. Statistical analysis was conducted to determine the difference between performance of CR mixtures modified with conventional and polymer modified emulsion. Although no significant difference in performance was observed for most of the performance measures, polymer modified emulsion was shown to improve the rutting resistance of CR mixtures compared with conventional emulsion.
TABLE OF CONTENTS
LIST OF FIGURES………………………………………………………………………………………………. vii
LIST OF TABLES………………………………………………………………………………………………… xi
ACKNOWLEDGEMENTS…………………………………………………………………………………… xii
CHAPTER 1 INTRODUCTION……………………………………………………………………………… 1
1.1 Background………………………………………………………………………………………………….. 1
1.2 Asphalt Pavement Distresses………………………………………………………………………….. 1
1.2.1 Rutting (Permanent Deformation)……………………………………………………………… 3
1.2.2 Fatigue Cracking…………………………………………………………………………………….. 3
1.2.3 Thermal Cracking (Low Temperature Cracking/Transverse Cracking)………….. 4
1.3 Asphalt Recycling Methods……………………………………………………………………………. 5
1.4 Objectives…………………………………………………………………………………………………….. 8
1.5 Scope of Work………………………………………………………………………………………………. 8
1.6 Organization of the Thesis……………………………………………………………………………… 9
CHAPTER 2 LITERATURE REVIEW………………………………………………………………….. 10
2.1 Cold Recycling (CR)……………………………………………………………………………………. 10
2.1.1 Cold In-Place Recycling (CIR)……………………………………………………………….. 11
2.1.2 Cold Central Plant Recycling (CCPR)……………………………………………………… 13
2.2 Recycled Asphalt Pavement (RAP) Aggregate in CR Process…………………………… 14
2.3 Asphalt Emulsion………………………………………………………………………………………… 15
2.3.1 Asphalt Emulsion Classification……………………………………………………………… 16
2.3.2 Breaking Mechanism of Asphalt Emulsion………………………………………………. 17
2.4 CR Mixture Design……………………………………………………………………………………… 20
2.4.1 RAP Material Evaluation……………………………………………………………………….. 22
2.4.2 Selection of Recycling Agent………………………………………………………………….. 23
2.4.3 Curing in the Lab…………………………………………………………………………………… 26
2.4.4 Compaction in the Lab…………………………………………………………………………… 27
2.5 Important Engineering Properties of CR Mixture…………………………………………….. 28
2.5.1 Air Void……………………………………………………………………………………………….. 28
2.5.2 Tensile and Compressive Strength…………………………………………………………… 30
2.5.3 Durability…………………………………………………………………………………………….. 30
2.5.4 Moisture Damage Resistance………………………………………………………………….. 31
2.5.5 Rutting Resistance…………………………………………………………………………………. 32
2.6 Summary of Literature Review……………………………………………………………………… 33
CHAPTER 3 MATERIALS AND LABORATORY TESTS……………………………………… 35
3.1 Materials…………………………………………………………………………………………………….. 35
3.2 Mixture Design……………………………………………………………………………………………. 35
3.2.1 Determination of Asphalt Content in RAP Materials…………………………………. 36
3.2.2 Determination of RAP Material Gradation and Set Design Gradation………….. 36
3.2.3 Laboratory Production of CR Mixture……………………………………………………… 41
3.2.4 Use of Polymer Modified Asphalt Emulsion…………………………………………….. 45
3.2.5 Density Measurement…………………………………………………………………………….. 46
3.2.6 Determination of the Optimum Water and Emulsion Content……………………… 48
3.3 Durability of Lab Produced CR Specimens…………………………………………………….. 51
3.4 Moisture Damage Resistant of Lab Produced Specimens…………………………………. 53
3.5 Rutting Resistant of Lab Produced Specimens………………………………………………… 58
CHAPTER 4 TEST RESULTS AND ANALYSIS…………………………………………………… 65
4.1 Mixture Design……………………………………………………………………………………………. 65
4.1.1 Density of All Specimens……………………………………………………………………….. 65
4.1.2 Air Void Distribution…………………………………………………………………………….. 69
4.1.3 IDT Strength Results……………………………………………………………………………… 74
4.1.4 IDT Strength with Various Polymer Modified Asphalt Emulsions………………. 78
4.2 Durability Study………………………………………………………………………………………….. 79
4.3 Moisture Damage Resistance Study……………………………………………………………….. 80
4.3.1 Air Void of Specimens in Moisture Damage Resistance Study…………………… 80
4.3.2 Degree of Saturation of Specimens in Wet Subset…………………………………….. 84
4.4 Rutting Resistance Study……………………………………………………………………………… 94
4.4.1 Air Void of Specimens in Rutting Resistance Study………………………………….. 94
4.4.2 Permanent Deformation Results……………………………………………………………… 95
CHAPTER 5 CONCLUSIONS AND RECOMMENDATIONS………………………………. 103
REFERENCES…………………………………………………………………………………………………… 106
APPENDIX A MINITAB OUTPUT OF STATISTICAL ANAYLSIS RESULTS……… 110
CHAPTER 1 INTRODUCTION
1.1 Background
Traffic levels and vehicle loads are increasing each year. Excluding commercial vehicles, the general public requires safer and more efficient roadways with high service levels, which leads to a dramatic increase in demand to rehabilitate existing pavements. Although billions of dollars of government funding are spent on maintaining and rehabilitating existing pavements every year, due to the complexity and scale of the road system in the United States, the budget is still constrained. Due to this reason, low cost, efficient maintenance and rehabilitation methods are gaining popularity. Paving with cold mixtures using recycled asphalt materials is one of the popular techniques, and the focus of this thesis research.
Asphalt recycling is not a new concept. Cold recycling/rehabilitation of roadways with asphalt binder dates back to the early 1900’s (ARRA 2001). Moderate advancements in asphalt recycling technology and equipment occurred until the mid 1970’s due to two reasons: the worldwide petroleum crisis of the early 1970’s and the development and introduction of large scale cold planing equipment in 1975 (ARRA 2001). When properly implemented, the cold recycling techniques is a highly beneficial road rehabilitation approach from technical, economical, and environmental perspectives (Kennedy, Tam and Solaimanian 1998).
The cold recycling (CR) makes use of 100 percent of recycled asphalt pavement (RAP) materials in the recycling process, produces a new and improved pavement layer. It provides economical and environmental benefits when properly implemented.
1.2 Asphalt Pavement Distresses
An asphalt pavement is a multilayered structure. The pavement structure is designed and constructed in a way to carry loads transferred from passing vehicles and resist adverse climatic effects. Each layer is made up of a combination of materials, generally aggregate, binder, and sometime special additives. The optimum combination of these materials at each layer is achieved through an engineered mixture design. A properly designed mixture of materials along with optimum thickness provides the required structural capacity for each layer. This process is called the structural design (Mallick and El-Korchi 2013). The pressure or stress caused by vehicles is the highest at the top layer, and the lowest at the bottom layer of a pavement structure (Figure 1). In a typical pavement structure, higher quality materials are used at layers closer to the surface.
Figure 1 Typical asphalt pavement cross section and traffic load transfer pattern (Washington State DOT 2005)
Even the best designed and constructed pavements develop distresses of varying severity during their service life. However, a highly sound pavement structure lasts for decades before manifesting such distresses. There are three primary distresses observed in asphalt pavements: rutting (permanent deformation), fatigue cracking, and thermal cracking (low temperature cracking or transverse cracking).
1.2.1 Rutting (Permanent Deformation)
A rut is defined as a longitudinal depression in the wheelpath, with or without transverse displacement (Figure 2). It is a physical distortion of the surface material and a rut depth of 0.5 in. (0.013 m) is considered as a rutting failure. Rutting is the result of the accumulation of repeated loading from the passing vehicles, which leads to the increase of the permanent deformation. Rutting can be classified into three severities (Mallick and El-Korchi 2013). Low to moderate severity rutting is a one-dimensional densification or vertical compression near the center of the wheelpath, caused by densification of mixtures with excessive air voids. Moderate to high severity rutting is a depression in the wheelpath along with humps on either side of the depression, caused by lateral flow due to plastic deformation, resulting from shear failure of the mixture under traffic and generally associated with very low air voids in the mixture. Rutting accompanied by cracks on the surface of the pavement, is caused by rutting in underlying layers, such as subgrade or subbase.
Figure 2 Rutting in asphalt pavement (Mallick and El-Korchi 2013)
1.2.2 Fatigue Cracking
Fatigue cracking is generally found in the wheelpath (Figure 3). Repeating tensile stress at the bottom of the asphalt mixture layer caused by traffic loads leads to fatigue cracking. It starts from the bottom and moves upward, beginning as a series of interconnected cracks at the surface and develops into a chicken wire/alligator pattern (Mallick and El-Korchi 2013). A low-severity fatigue cracking area is one in which there are no or few connecting cracks, the cracks are not spalled or sealed, and there is no evidence of pumping. A moderate-severity fatigue cracking area includes cracks that are interconnected, forming a complete pattern, and cracks may be slightly spalled or sealed with no evidence of pumping. In a high-severity fatigue cracking area, the cracks are interconnected with moderate or severe spalls, forming a complete pattern; pieces may move under traffic; cracks may be sealed; and pumping may be evident.
Figure 3 Fatigue cracking in asphalt pavement (Mallick and El-Korchi 2013)
1.2.3 Thermal Cracking (Low Temperature Cracking/Transverse Cracking)
Thermal cracking occurs in the form of transverse cracking, which is perpendicular to the pavement centerline (Figure 4). The thermal cracking can be caused by the fracture of asphalt mixture due to a severe drop in temperature or by thermal fatigue caused by repeated low and high temperature cycle (Mallick and El-Korchi 2013). A low severity thermal crack is one with a mean width of 6 mm, or a sealed crack with sealant material in good condition and a width that cannot be determined. A medium severity thermal crack is one with a mean width 6 and 19 mm, or any crack with a mean width 19 mm and adjacent low severity random cracking. A high severity thermal crack has a mean width >19 mm, or 19 mm, but with adjacent moderate to high severity random cracking.
Figure 4 Thermal cracking in asphalt pavement (Mallick and El-Korchi 2013)
When accurately designed and constructed, properly used, and appropriately maintained, an asphalt pavement should give a service life of fifteen to twenty years. Since design and construction of a new roadway is rather expensive, extending the service life of an existing road is a more economical choice. A major rehabilitation for the pavement is necessary to remove the distresses and keep it in service for an extended period beyond its original design life.
1.3 Asphalt Recycling Methods
When pavement condition deteriorates, there is a point when rehabilitation is required. Lots of benefits can be accomplished through the application of asphalt recycling processes. Those processes provide economical and sustainable solutions that largely reduce demands on raw materials, energy consumption, and production of greenhouse gases, while still guaranteeing the functionality and performance of the pavement.
The Asphalt Recycling and Reclaiming Association (ARRA) defined five categories of asphalt recycling methods (ARRA 2001). These categories are:
- Cold Planing (CP)
- Hot Recycling (HR)
- Hot In-Place Recycling (HIR)
- Cold Recycling (CR)
- Full Depth Reclamation (FDR)
Within these five broad categories of asphalt recycling methods, there are a number of sub-categories that further define asphalt recycling methods (ARRA 2001, StroupGardiner 2011), which are:
-
Hot In-Place Recycling (HIR)
- Resurfacing (Surface Recycling)
- Repaving
- Remixing
-
Cold Recycling (CR)
- Cold In-Place Recycling (CIR)
- Cold Central Plant Recycling (CCPR)
-
Full-Depth Reclamation (FDR) – Pulverization
- Mechanical stabilization – Bituminous stabilization
- Chemical stabilization
Cold planing (CP) is the controlled removal of an existing pavement to a desired depth, longitudinal profile, and cross-slope, using specially designed equipment. The resulting textured surface can be immediately used as a driving surface, or further treated with one of the other asphalt recycling methods, or overlaid with HMA.
Hot recycling (HR) is the process of combining Recycled Asphalt Pavement (RAP) with virgin aggregate, new asphalt binder, and/or recycling agents (as required) in a central plant to produce a recycled mixture. This method uses the heat-transfer method to soften RAP materials to permit mixing with virgin aggregates and asphalt binder and/or recycling agent. It is currently the most widely used asphalt recycling method in the world.
Hot in-place recycling (HIR) uses 100 percent recycling asphalt pavement on site. The process consists of heating and softening existing asphalt pavement, permitting it to be scarified or hot rotary milled to the specified depth. The scarified or loosened asphalt pavement is then thoroughly mixed and subsequently placed and compacted with conventional HMA paving equipment. When necessary, virgin aggregate, new asphalt binder, recycling agents and/or new HMA can be added. Typical treatment depths range from ¾ to 2 inches (20 to 50 mm); some equipment can treat up to 3 inches (75 mm).
Full depth reclamation (FDR) is the rehabilitation technique in which the full thickness of the asphalt pavement and a predetermined portion of underlying materials (base, subbase and/or subgrade) are uniformly pulverized and blended to provide an upgraded, homogenous base material. It consists of pulverization/reclamation of the existing materials, adding more materials when necessary, mixing, initial shaping of the resulting mix, compaction, final shaping or “tight blading,” and application of an asphalt surface or wearing course. The whole process is performed on the roadway without the addition of heat. Treatment depth varies depending on the thickness of existing pavement, but generally ranges between 4 to 12 inches (100 to 300 mm).
Cold recycling (CR) consists of recycling asphalt pavement without the application of heat during the recycling process to produce a rehabilitated pavement. The cold in-place recycling (CIR) process is undertaken on site and generally uses 100 percent of RAP materials generated during the process. With typical treatment depth within 2 to 4 inches (50 to 100 mm), the recycling agent used in the CIR process is limited to asphalt emulsion or an emulsified asphalt agent. If chemical additives like Portland cement, lime, and fly ash are added, treatment depth can increase up to 5 to 6 inches (125 to 150 mm). Cold Central Plant Recycling (CCPR) is the process in which the asphalt recycling takes place in a central location using a stationary cold mix plant. The CCPR mixtures can be used immediately or can be stockpiled for late use in such applications as maintenance blade patching or pothole repair. Similar to the CIR process, asphalt emulsion and emulsified asphalt agents are typically used as the recycling additive.
The focus of this thesis will be on cold recycling (CR) technology. Although the CIR and the CCPR technology are taken place in difference locations, the material investigation process is similar.
1.4 Objectives
This thesis is based on the work of a project initiated by the Pennsylvania Department of Transportation (PennDOT), in order to develop a methodology for use of cold recycled mixture using polymer modified emulsion and conventional emulsion. Tasks of this project include:
- Develop a general mixture design process for cold recycled asphalt mixture.
- Evaluate performance of polymer modified cold recycled asphalt mixtures for strength, durability, rutting resistance, and moisture damage resistance through laboratory testing.
- Compare performance of polymer modified cold recycled asphalt mixtures with cold recycled asphalt mixtures with conventional emulsion through laboratory testing.
- Investigate the effect of cement on cold recycled asphalt mixtures through laboratory testing and data analysis.
1.5 Scope of Work
The work discussed in this thesis covers laboratory testing with one single source of RAP materials produced from PennDOT District 1-0 to establish a mixture design procedure and engineering property investigation for lab produced mixture using recycled asphalt pavement and emulsion. Test protocols include characterization of RAP material; measurement of density and air voids of lab-produced CR specimens; and evaluation of strength, durability, moisture damage resistance, and rutting resistance of lab-produced CR mixtures. Two types of asphalt emulsions from two different manufacturers were used in the whole testing process. Type II portland cement was also employed as a recycling agent additive for comparison.
Characterization of RAP materials included gradation analysis of raw RAP materials and determination of asphalt content of raw RAP materials. Design gradation, optimum water and emulsion contents, curing and conditioning procedure, and gyration level were determined in the mixture design process. The density of every lab-produced specimen was measured before testing and the air void content of each specimen was obtained through measured density data. The compressive strength of each specimen was evaluated using the Indirect Tensile Strength Test (IDT). The Raveling test was used to determine the durability of CR mixtures. The rutting resistance of CR mixtures was evaluated by measuring permanent deformation of lab-produced mixtures using the Superpave Shear Tester (SST). The moisture damage resistance properties were investigated using Indirect Tensile Strength Test (IDT) based on modified AASHTO T283 procedure. Type Ⅱ portland cement was introduced in each stage of these tests, except the mixture design phase, for performance comparison.
1.6 Organization of the Thesis
This thesis consists of five chapters. The first chapter describes the introduction of this research, objectives, and scope of work, including basic knowledge about pavement distress and asphalt recycling methods; the second chapter gives the literature review summary on CR technology, recycled asphalt pavement used in CR process, asphalt emulsion, existing mixture design protocols of CR process, and engineering properties of CR mixture; chapter three introduces the research methodology, materials, test procedures and details of this research; test results and data analysis will be presented in chapter four; the last chapter will give conclusions and provide recommendations.
EVALUATION OF COLD RECYCLED ASPHALT MIXTURES MODIFIED WITH CONVENTIONAL AND POLYMER MODIFIED EMULSION
