Presentation on theme: "Aggregates Usually refers to a soil that has in some way been processed or sorted. Soils are materials that are used as-is. An example would be a finished."— Presentation transcript:
0SUPERPAVE FHWA Condensed Superpave Asphalt Specifications Lecture Series
1AggregatesUsually refers to a soil that has in some way been processed or sorted.Soils are materials that are used as-is. An example would be a finished subgrade surface. Aggregates are materials that have been specifically sorted or processed to achieve given properties. This block will present general background information about how aggregates are obtained and processed.
2Aggregate Size Definitions 1009072654836221594100998972654836221594Nominal Maximum Aggregate Sizeone size larger than the first sieve to retain more than 10%Maximum Aggregate Sizeone size larger than nominal maximum sizeFor HMA pavements these are the definitions for gradations.
3Sieve Size (mm) Raised to 0.45 Power Percent Passing100max density linerestricted zonecontrol pointnommaxsizemaxsizeTo specify aggregate gradation, two additional features are added to the 0.45 chart: control points and a restricted zone. Control points function as master ranges through which gradations must pass. They are placed on the nominal maximum size, an intermediate size and the dust size.The restricted zone resides along the maximum density gradation between the intermediate size (either or 2.36 mm) and the 0.3 mm size. It forms a band through which gradations should not pass. Gradations that pass through the restricted zone have often been called “humped gradations” because of the characteristic hump in the grading curve that passes through the restricted zone. In most cases, a humped gradation indicates a mixture that possesses too much fine sand in relation to total sand. This gradation practically alwaysresults in tender mix behavior, which is manifested by a mixture that is difficult to compact during construction and offers reduced resistance to permanent deformation during its performance life. Gradations that violate the restricted zone possess weak aggregate skeletons that depend too much on asphalt binder stiffness to achieve mixture shear strength. These mixtures are also very sensitive to asphalt content and can easily become plastic.Sieve Size (mm) Raised to 0.45 Power
4Superpave Aggregate Gradation Percent Passing100Design Aggregate StructureThe term used to describe the cumulative frequency distribution of aggregate particle sizes is the design aggregate structure. A design aggregate structure that lies between the control points and avoids the restricted zone meets the requirements of Superpave with respect to gradation. Superpave defines five mixture types as defined by their nominal maximum aggregate size:Sieve Size (mm) Raised to 0.45 Power
5Superpave Mix Size Designations Superpave Nom Max Size Max SizeDesignation (mm) (mm)37.5 mm25 mm19 mm12.5 mm9.5 mmThese are the five gradations developed for Superpave.
6Gradations * Considerations: - Max. size < 1/2 AC lift thickness - Larger max size+ Increases strength+ Improves skid resistance+ Increases volume and surface area of agg which decreases required AC content+ Improves rut resistance+ Increases problem with segregation of particlesSeveral factors need to be considered in selecting a desirable aggregate gradation. The maximum size of the aggregate needs to be at a minimum less than one half of the planned lift thickness. Current construction practices with Superpave gradations indicate that this needs to be changed to less than one-third of the lift thickness.Larger maximum size aggregate gradations have several advantages such as improved aggregate interlock, improved skid resistance and improved rut resistance. Local availability of aggregates will usually dictate the largest size aggregate. Also, the larger maximum size gradations also tend to have more problems with gradation separation (segregation) during construction.- Smaller max size+ Reduces segregation+ Reduces road noise+ Decreases tire wear
7Percent Crushed Fragments in Gravels Quarried materials always 100% crushedMinimum values depended upon traffic level and layer (lift)Defined as % mass with one or more fractured facesThe appropriate percentages of each aggregate stockpile are combined and then split on the 4.75 mm screen. The material retained on the 4.75 mm screen are used to determine the percent crushed faces
8Percent Crushed Fragments in Gravels 0% Crushed % with 2 or MoreCrushed FacesThis is a measurement of coarse aggregate angularity. The amount of crushing (angularity) is important because it determines the level of internal shear resistance which can be developed in the aggregate structure. Round, uncrushed aggregates tend to “roll” out from under traffic loads and therefore have a low rutting resistance.
9Coarse Aggregate Angularity Criteria Traffic Depth from SurfaceMillions of ESALs < 100 mm > 100 mm< 0.3< 1< 3< 10< 30< 100 10055/--65/--75/--85/8095/90100/100--/--50/--60/--80/75Superpave requirements are based on both the traffic level and the depth of the layer below the surface. The crushing requirements for low traffic volumes are low or none, regardless of depth. As traffic levels increase, so do the required percentages of crushed faces.There is a higher level of crushing required in the upper 100 mm of the pavement because this is the region which is subjected to the highest shear due to traffic loads. Higher shear forces require a higher level of resistance to shear.First number denotes % with one or more fractured facesSecond number denotes % with two or more fractured faces
10Background History of Specifications Asphalt CementsBackgroundHistory of SpecificationsThis block of instruction will cover traditional penetration and viscosity grading systems used in the US. While the Superpave binder specification is quickly replacing these specifications in practice, the vast majority of the literature will refer to these standards. People in this industry also commonly ask “So, what AC grade is a PG 64-22?”. These traditional specifications will remain the baseline against which field experience is translated into an understanding of the new Superpave binder specification.At the conclusion of this block the student will understand:* How to use traditional binder specifications* The limitations of these specifications which led to the development of the Superpave binder specification.This material is covered in detail in Chapter 2 “Asphalt Refining, Uses, and Properties” in the recommended text book.
11Background Tar Asphalt Resistant to petroleum products Soluble in petroleum productsGenerally a by-product of petroleum distillation processCan be naturally occurringTarResistant to petroleum productsGenerally by-product of coke (from coal) productionStudents commonly use the words “asphalt” and “tar” interchangeably. However, asphalt is actually a waste product from the refining of petroleum crude oil while tar is a coal by-product. These chemical distinctions are important in a selecting the appropriate product for a given application.For example, if protection from oil and fuel spills is desired, then a coating which will not be dissolved by these products is needed. Since motor oils, fuel, and asphalt are all derived from petroleum, any spills on this pavement can damage the asphalt-based coating as well as the asphalt concrete. The old chemistry rule-of-thumb applies: Like dissolves like. Because tar is a coal rather than asphalt-based product, sealers from this material will not be damaged by petroleum product spills.
12Penetration Testing Sewing machine needle Specified load, time, temperature100 gInitialPenetration in 0.1 mmAfter 5 secondsThe penetration test started out using a No. 2 sewing machine needle mounted on a shaft for a total mass of 100 g. This needle was allowed to sink into (penetrate) a container of asphalt cement at room temperature (25 oC) for 5 seconds. The consistency (stiffness) of a given asphalt was reported as the depth in tenths of a millimeter (dmm) that the needle penetrated the asphalt.
13Penetration Specification Five GradesThis test was used to standardize the penetration grading system approach for specifying asphalt cements. This specification uses the penetration of the original asphalt cement in the grade names. That is, a penetration grade asphalt will have a penetration value for the original asphalt of between 120 and 150 tenths of a millimeter.
14DuctilityThis test evaluates the ability of an asphalt sample to stretch at a rate of 5 cm/min at 25oC. The distance the samples can be pulled is measured directly from the centimeter scale mounted to the top of the tank.The significance of the ductility test to indicate performance-related properties has been debated for a number of years due to its empirical nature and poor reproducibility of test results.In general, asphalts with lower ductility have a greater tendency to produce pavements which have excessive cracking
15Typical Penetration Specifications Flash Point, CDuctility, cmSolubility, %There are five penetration grades of asphalt cements with discrete ranges. That is, there is no overlap in penetration values between grades. It is possible to have an asphalt which will not meet any penetration grading requirement. This table presents two of these grades.Note the flash point decreases with increasing penetration. This is because softer asphalts usually have a higher percentage of lighter ends which will “flash” at lower temperatures. The specification allows for this difference.Since these fractions of the asphalt are also easily removed by heating, this means that there will be a higher percentage mass loss on aging. This greater loss of the softer asphalt components is reflected in the differences in requirements on the percent of the original penetration retained after aging. That is, the more light ends lost during aging, the greater the stiffening affect due to aging. This loss of the softer asphalt components is also reflected the ductility requirements.Retained Pen., %Ductility, cm NA
16Viscosity Graded Specifications These disadvantages led to the development of the viscosity grading system in the 1970's which is detailed in ASTM D This one ASTM standard actually contains three separate specifications designated as: Table 1, Table 2, and Table 3. The first two specifications are based on the original properties of the asphalt while the last table is based on the properties of the asphalt after rolling thin film oven aging. Each of these tables and differences between them will be discussed in the following slides.
17Types of Viscosity Tubes Two viscosity measurements are used in this specification: Absolute viscosity (60 oC) and kinematic viscosity (135 oC). Both use the principle of the rate of flow through a known area to measure viscosity. Because asphalt is still very thick (stiff) at 60 oC, a vacuum is needed to move the asphalt through the tube in a reasonable time. At 135 oC, gravity and a falling head pressure is sufficient to get the asphalt to flow.Zietfuchs Cross-Arm TubeAsphalt Institute Tube
18Table 1 Example AC 2.5 AC 40 Visc, 60C 250 + 50 4,000 + 800 PenetrationThis table compares two of the viscosity graded Table 1 specifications. The viscosity grading system provides immediate information as to the mean anticipated viscosity at 60 oC. For example, an AC 25 will have a mean viscosity of 250 Poise (2.5 times 100). Because of the limited allowable range (coefficient of variance of 20 percent), there is no overlap between the AC grades.Visc, 60C <1, <20,000Ductility
19Penetration Grades Viscosity, 60C (140F) AC 40 40 50 100 AC 20 60 50 7085100120150200300Penetration GradesAC 40AC 20AC 10AC 5AC 2.510050Viscosity, 60C (140F)This figure provides a general comparison of the various traditional specifications. While there is no direct relationship between the specifications, there is a general relationship between stiffness and viscosity. Higher penetration numbers correspond with lower viscosities.105
20Asphalt Cements New Superpave Performance Graded Specification This block of instruction will cover the tests, concepts and use of the new Superpave binder specifications. At the end of this block the student will be familiar with the :* Concepts behind the PG binder grading system.* Tests used for determining performance-related binder properties.* Selection of an appropriate PG binder grade.
21PG SpecificationsFundamental properties related to pavement performanceEnvironmental factorsIn-service & construction temperaturesShort and long term agingThe PG grading system was developed to address the short comings seen in the traditional asphalt cement specifications.This specification is referred to as a binder rather than an asphalt cement specification. The difference is that a binder can be either a neat (unmodified) or modified asphalt cement. The term “asphalt cement” usually refers to an unmodified asphalt cement.
22High Temperature Behavior High in-service temperatureDesert climatesSummer temperaturesSustained loadsSlow moving trucksIntersectionsViscous LiquidThe viscous component of the binder response dominates its warm temperature behavior and is seen as permanent deformation. The magnitude of this deformation is increased with the time that the load is applied.
23Pavement Behavior (Warm Temperatures) Permanent deformation (rutting)Mixture is plasticDepends on asphalt source, additives, and aggregate propertiesPermanent deformation or rutting of the pavement is the result of non-recoverable or plastic deformation due to traffic loads. At the warmer temperatures, the aggregate structure carries a major portion of the loads. Stiffer binders help to keep the aggregate structure intact as well as help resist deformation in the binder matrix.
24Permanent Deformation Ruts can be very visible in extreme cases such as the one shown in this photo. Other places where rutting can be observed are at stop lights. In many cases, the crosswalk lines can highlight this type of distress.Courtesy of FHWAFunction of warm weather and traffic
25Low Temperature Behavior Cold climatesWinterRapid LoadsFast moving trucksElastic SolidAt cold temperatures, or under very quick loads, the binder response is predominately elastic.Hooke’s Laws = t E
26Pavement Behavior (Low Temperatures) Thermal cracksStress generated by contraction due to drop in temperatureCrack forms when thermal stresses exceed ability of material to relieve stress through deformationMaterial is brittleDepends on source of asphalt and aggregate propertiesA length of pavement can be considered to be a semi-infinite constrained beam. As the temperature drops the asphalt concrete wants to contract but is restrained. This results in internal stresses building up as the temperature drops. Thermal cracks occur when the contraction-induced stresses exceed the tensile strength of the mixture.A number of researchers have shown that the low temperature behavior of the asphalt concrete pavement is highly dependent upon the properties of the binder.
27Thermal Cracking Courtesy of FHWA Thermal cracks are transverse cracks, usually at relatively evenly spaced intervals. The spacing gets closer together with increasing binder stiffness the colder the temperatures.Courtesy of FHWA
28Superpave Asphalt Binder Specification The grading system is based on ClimatePGMin pavement temperaturePerformance GradeThe binder designation is based on expected extremes of hot and cold pavement temperatures.Average 7-day max pavement temperature
29Pavement Temperatures are Calculated Calculated by Superpave softwareHigh temperature20 mm below the surface of mixtureLow temperatureat surface of mixturePave temp = f (air temp, depth, latitude)
30Concentric Cylinder Rheometers t Rq =Mi2 p Ri2 Lg =W RRo - RiThe rotational viscosmeter (also referred to at a Brookfield viscometer) is a concentric cylinder rheometer. This means that one cylinder rotates inside of another. Viscosity is determined from the amount of torque needed to rotate a cylinder (called a spindle) with a known geometry. Viscosity, as defined earlier, is the ratio of the shear stress to the strain rate. This viscometer uses information about the torque, speed, and geometry to obtain these measurements.
31Dynamic Shear Rheometer (DSR) Shear flow varies with gap height and radiusNon-homogeneous flowParallel PlatetR =2 Mp R3This type of rheometer has a parallel plate configuration. The stress and strain measurements are based on the assumption of a cylindrical geometry. This is why a great deal of effort is expended in preparing and trimming the specimen prior to starting the test.gR =R Qh
32Short Term Binder Aging Rolling Thin Film OvenSimulates aging from hot mixing and constructionShort term aging is accomplished using the same RTFO oven as has been traditionally used in the AR viscosity graded specification.
33Pressure Aging Vessel (Long Term Aging) Simulates aging of an asphalt binder for 7 to 10 years50 gram sample is aged for 20 hoursPressure of 2,070 kPa (300 psi)At 90, 100 or 110 CA pressure aging vessel (PAV) treatment of the RTFO binder is used to further age the binder. This simulates long term aging changes.
34Bending Beam Rheometer ComputerDeflection TransducerAir BearingLoad CellFluid BathThis test applies a static load to a simply supported beam of asphalt cement. Temperature is held constant using a liquid bath. A computer provides both equipment control and data acquisition.
35Direct Tension Test Load Stress = s = P / A D L sf D Le ef Strain Regardless of the type of equipment used, a sample of binder is molded into a “dog bone” shape with a uniform center cross section. The sample is pulled until the it breaks in the middle. The stress and strain at failure are recorded. This test requires a minimum strain before the sample fails.efStrain
36Summary Low Temp Fatigue Cracking Cracking Construction Rutting [DTT] [RV][DSR][BBR]This figure summarizes the testing required for the PG binder specification.RTFOShort Term AgingNo agingPAVLong Term Aging
37Superpave Binder Purchase Specification One of the primary purposes of the Superpave binder testing is to use that data for the development of a purchase specification for asphalt binders.
38Min pavement temperature Average 7-day max pavement temperature Superpave Asphalt Binder SpecificationThe grading system is based on ClimatePGMin pavement temperaturePerformance GradeThe binder designation is based on expected extremes of hot and cold pavement temperatures.Average 7-day max pavement temperature
39Performance Grades ORIGINAL > 230 oC > 1.00 kPa > 2.20 kPa CECAvg 7-day Max, oCPG PG PG PG PG PG PG 821-day Min, oCORIGINAL> 230 oC(Flash Point) FP< oC(Rotational Viscosity) RV(Dynamic Shear Rheometer) DSR G*/sin > 1.00 kPa(ROLLING THIN FILM OVEN) RTFO Mass Loss < %(Dynamic Shear Rheometer) DSR G*/sin > 2.20 kPa(PRESSURE AGING VESSEL) PAVThis is the binder specification - it is defined by AASHTO MP -1.20 Hours, 2.07 MPa(110) (110) (110)(Dynamic Shear Rheometer) DSR G* sin < 5000 kPa28S < 300 MPam > 0.300( Bending Beam Rheometer) BBR “S” Stiffness & “m”- valueReport Value(Bending Beam Rheometer) BBR Physical Hardening> 1.00 %(Direct Tension) DT
40How the PG Spec Works Spec Requirement Remains Constant CECSpec RequirementRemains ConstantAvg 7-day Max, oCPG PG PG PG PG PG PG 821-day Min, oC5864ORIGINAL> 230 oC(Flash Point) FP< oC(Rotational Viscosity) RV(Dynamic Shear Rheometer) DSR G*/sin > 1.00 kPa(ROLLING THIN FILM OVEN) RTFO Mass Loss < %(Dynamic Shear Rheometer) DSR G*/sin > 2.20 kPa(PRESSURE AGING VESSEL) PAVThe approach to the PG system represents a change in philosophy. The specification requirement does not change; the temperature which the value has to meet changes with grade.20 Hours, 2.07 MPaTest TemperatureChanges(110) (110) (110)(Dynamic Shear Rheometer) DSR G* sin < 5000 kPa28S < 300 MPam > 0.300( Bending Beam Rheometer) BBR “S” Stiffness & “m”- valueReport Value(Bending Beam Rheometer) BBR Physical Hardening> 1.00 %(Direct Tension) DT
41PG Binder Selection PG 52-28 PG 58-22 PG 58-16 PG 64-10 > Many agencies haveestablished zonesPG 52-28Many states have divided their territory into different regions. How many regions depends on the variation of climates.PG 58-22PG 58-16PG 64-10
42Summary of How to Use PG Specification Determine7-day max pavement temperatures1-day minimum pavement temperatureUse specification tables to select test temperaturesDetermine asphalt cement properties and compare to specification limitsThis provides a brief summary of the steps needed to determine if an asphalt meets a particular PG specification.
43Asphalt Concrete Mix Design HistoryThis block will present background information on the traditional Marshall and Hveem mix design methods.At the conclusion of this block the student will have a general understanding of:The principal procedures and concepts used in Marshall and Hveem mix design techniquesThis material is covered in detail in Chapter 4 “Hot Mix Asphalt Mixture Design Methodology” of the recommended textbook.
44Hot Mix Asphalt Concrete (HMA) Mix Designs Objective:Develop an economical blend of aggregates and asphalt that meet design requirementsHistorical mix design methodsMarshallHveemNewSuperpave gyratoryThe objective of HMA mix design is to develop an economical blend of aggregates and asphalt. In the developing of this blend the designer needs to consider both the first cost and the life cycle cost of the project. Considering only the first cost may result in a higher life cycle cost.Historically asphalt mix design has been accomplished using either the Marshall or the Hveem design method. The most common method was the Marshall. It had been used in about 75% of the DOTs throughout the US and by the FAA for the design of airfields. In 1995 the Superpave mix design procedure was introduced into use. It builds on the knowledge from Marshall and Hveem procedures. The primary differences between the three procedures is the machine used to compact the specimens and strength tests used to evaluate the mixes. The current plan is to implement the Superpave procedures throughout the US for the design and quality control of HMA highway projects early in the next century. It appears that the Marshall method will continue to be used for airfield design for many years and that the Hveem procedure will continue to be used in California.
45Requirements in Common Sufficient asphalt to ensure a durable pavementSufficient stability under traffic loadsSufficient air voidsUpper limit to prevent excessive environmental damageLower limit to allow room for initial densification due to trafficSufficient workabilityNo matter which design procedure is going to be used the HMA mixture that is placed on the roadway must meet certain requirements.The mix must have sufficient asphalt to ensure a durable, compacted pavement by thoroughly coating, bonding and waterproofing the aggregate.Enough stability to satisfy the demands of traffic without displacement or distortion (rutting).Sufficient voids to allow a slight amount of added compaction under traffic loading without bleeding and loss of stability. However, the volume of voids should be low enough to keep out harmful air and moisture. To accomplish this the mixes are usually designed by 4% VTM in the lab and compacted to less than 7% VTM in the field.Enough workability to permit placement and proper compaction without segregation.
47Marshall Mix DesignDeveloped by Bruce Marshall for the Mississippi Highway Department in the late 30’sWES began to study it in 1943 for WWIIEvaluated compaction effortNo. of blows, foot design, etc.Decided on 10 lb.. Hammer, 50 blows/side4% voids after trafficInitial criteria were established and upgraded for increased tire pressures and loadsPoint out that the criteria has been modified since initial development; but, the basic process is the same as it was when it was initially developed.
48Marshall Mix Design Select and test aggregate Select and test asphalt cementEstablish mixing and compaction temperaturesDevelop trial blendsHeat and mix asphalt cement and aggregatesCompact specimen (100 mm diameter)This slide outlines the major steps in the development of a Marshall mix design.
49Marshall Design Criteria Light Traffic Medium Traffic Heavy TrafficESAL < < ESAL< ESAL > 106CompactionStability N (lb.) (750) 5338 (1200) (1800)Flow, 0.25 mm (0.1 in) to to to 14Air Voids, % to to to 5Voids in Mineral Agg.(VMA) Varies with aggregate sizeThe criteria on this slide is that recommended by The Asphalt Institute. Most DOTs have their own requirements and they may vary some from that noted here.
50Asphalt Concrete Mix Design SuperpaveThis block of instruction will cover the Superpave procedures.At the conclusion of this block the student will have a general understanding of:The principal procedures involved in the Superpave mix design.The relationship between these procedures and paving specifications.This material is covered in detail in Chapter 4 “Hot Mix Asphalt Mixture Design Methodology” of the recommended textbook.
51Superpave Volumetric Mix Design GoalsCompaction method which simulates fieldAccommodates large size aggregatesMeasure of compactibilityAble to use in field labsAddress durability issuesFilm thicknessEnvironmentalThe goals of the first new mix design procedure for HMA pavements in over 50 years were to have a procedure that would simulate the real world. In the past we may have “allowed the mold to design road”. The Marshall and Hveem procedures used 4 inch (100 mm) molds with Superpave using 150 mm molds to use larger aggregates.By monitoring compaction throughout the process, may provide a measure of how the mix will compact during construction.It was also desired to have equipment that could be used to for field quality control purposes.
52Compaction Key Components of Gyratory Compactor height measurementcontrol and dataacquisition panelreactionframeloadingrammoldtilt barThese are the components of the machine.rotatingbase
53Compaction Gyratory compactor Axial and shearing action 150 mm diameter moldsAggregate size up to 37.5 mmHeight measurement during compactionAllows densification during compaction to be evaluatedRam pressure600 kPaThe compactor puts 600 kPa of pressure on the specimen and operates at 30 rpm.1.25o
54Three Points on SGC Curve % GmmNmaxNdesNiniThere are three critical points on the SGC compactor curve that are evaluated in Superpave. Ninital is of importance because it is desirable not to have mixes that compact too easily. Nmaximum is of importance to prevent having mixes that continue to compact under traffic loading.Log Gyrations
55SGC Critical Point Comparison %Gmm= Gmb / GmmGmb = Bulk Mix Specific Gravity from compaction at N cyclesGmm = Max. Theoretical Specific GravityCompare to allowable values at:NINI : %Gmm < 89%NDES: %Gmm < 96%NMAX: %Gmm < 98%
56Design Compaction Nmax Ndes based on Ndes Log Nmax = 1.10 Log Ndes % GmmNmaxNdes based onaverage design high air temptraffic levelLog Nmax = 1.10 Log NdesLog Nini = 0.45 Log NdesNdesNiniThe level of Ndesign is based on the climate and traffic levels.Log Gyrations
57Superpave Testing Specimen heights Mixture volumetrics Dust proportion Air voidsVoids in mineral aggregate (VMA)Voids filled with asphalt (VFA)Mixture density characteristicsDust proportionMoisture sensitivityThis data is available or must be calculated to complete the development of the Superpave mix design.
58Superpave Mix DesignDetermine mix properties at NDesign and compare to criteriaAir voids % (or 96% Gmm)VMA See tableVFA See table%Gmm at Nini < 89%%Gmmat Nmax < 98%Dust proportion to 1.2These properties are determined and compared to the specification criteria.
59Gyratory Compaction Criteria Superpave Mix DesignGyratory Compaction CriteriaThese properties are determined and compared to the specification criteria.