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Chapter 6. Asphalt Pavements and Materials. Asphalt Pavements and Materials. Asphalt or bituminous materials Combined with aggregates make common pavement choices Multiple layers for highways Thin dust control layers – rural roads. 6-1 Asphalt Paving Materials.

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chapter 6

Chapter 6

Asphalt Pavements and Materials

asphalt pavements and materials
Asphalt Pavements and Materials
  • Asphalt or bituminous materials
    • Combined with aggregates make common pavement choices
      • Multiple layers for highways
      • Thin dust control layers – rural roads
6 1 asphalt paving materials
6-1 Asphalt Paving Materials
  • Bitiuminous materials – defined as hydrocarbons that are soluble in carbon disulphate
    • Usually hard at normal temp
    • Soften when heated
  • Bitumens used in paving
    • Native asphalts – obtained from asphalt lakes
    • Rock asphalts – rock deposits containing bituminous materials
    • Tars – distillation of coal
    • Petroleum asphalts – distillation of crude oil
6 1 3 viscosity
6-1.3 Viscosity
  • Grades and temperatures at which they are used depend on viscosity
  • Viscosity of asphalt varies with temperature
    • Ranging from solid to thin liquid
  • Viscosity decreases (material becomes more fluid) as temp increases
  • Absolute viscosity is measured in Pa . s (Si units ) and poises (tradition units ) 1 poise = 0.1 Pa.s
  • Kinematic viscosity measured in cm2 and stokes or centistokes 1 stoke = 100 centistokes = 1 cm2/s
  • Kinematic viscosity equals absolute viscosity divided by density
6 1 3 viscosity1
6-1.3 Viscosity
  • Figure 6-2 (temperature – viscosity relationships
    • Ac 10 is harder then AC5
      • reduce viscosity to 1.5cm2/s asphalt A must be heated to 167 C and sphalt B only to 163 c
  • Temperature limits corresponding to viscosities of 1.5 to 3.0 cm2/s are sometimes used
    • Asphalt A would require the plant to operated at temperatures between 153 c and 167 c
  • Minimum temperature for spraying
    • 2.0cm2/s
    • Asphalt B would be 156 C
6 1 4 paving products
6-1.4 Paving Products
  • Asphalt cements
    • Primary asphalt products produced by distillation of crude oil
    • Various viscosity grades
    • Most common being AC 2.5, AC 5, AC 10, AC 20, AC 40
      • Viscosity grades indicate the viscosity in hundreds of poises + - 20% measured at 60 C .
      • Example AC 2.5 has a viscosity of 250 + - 50 AC has a viscosity of 4000 poises + - 800
6 1 4 paving products1
6-1.4 Paving Products
  • Liquid Asphalts (cutback asphalts)
    • Mixed with solvent to reduce viscosity
    • Commonly heated and then sprayed on aggregates
    • Solvent evaporates
    • Types of grade based on solvent
      • Rc rapid curing gasoline
      • Mc medium curing kerosene
      • Sc slow curing diesel fuel
    • Solvent contents between 15% to 40%
6 1 4 paving products2
6-1.4 Paving Products
  • Asphalt emulsions
    • Mixtures of asphalt cement and water and emulsifying agent
    • Emulsifying agent breaks up the asphalt cement and disperses it - in the form of very fine droplets
    • Emulsion contains 55 -75% asphalt and 3% emulsifying agent
    • Grades
      • Rapid setting rs or crs
      • Medium setting ms or cms
      • Slow setting ss or css
6 1 5 quality control tests
6-1.5 Quality Control Tests
  • Viscosity
  • Penetration values - measuring depth of penetration of a standard needle into asphalt cement
  • Ductility – asphalt sample is cast in a mold consisting of two jaws then placed in a water bath. One jaw is moved away from the other at a standard rate – distance it moves before the thread between the two breaks is the ductility in centimeters
  • Thin film oven test – asphalt paving materials in use are found as extremely thin layers joining aggregate particles . A thin sample is heated in an oven for a period of time and the properties of the sample afterward are obtained as an indication of the rate of aging or hardening of the asphalt
  • Solubility – the purity of the asphalt can be checked
  • Flashpoint – determines the temperature to which asphalt materials may safely be heated.
6 2 superpave asphalt b inders
6-2 Superpave Asphalt Binders
  • SHRP – strategic highway research program
    • Developed superpave standards
      • Widely used in North America
    • Research in fail or crack due to the following
      • Permanent deformation or rutting occurring at high temperatures as the asphalt softens and the mix loses elasticity
      • Fatigue cracking due to high volume of load applications and aging of the asphalt
      • Low temperature cracking as asphalt becomes brittle and the pavement shrinks in cold weather
6 2 superpave asphalt binders
6-2 Superpave Asphalt Binders
  • Superpave bases specifications on high and low pavement temp expect at geographic locations
    • High temp grades vary from 46 c to 82 c
    • Low temp grades range from -10c to -46
    • Example grade pg 58-28 would be maximum temperatures of 53 c to 58 c and minimum temp down to -28 c
    • Specification also require adjustments to binder grade based on loads
      • Example pg 64 may be increase to pg 70 for slow moving or parked heavy transport vehicles or pavements for ESAL traffic loads over 30 million
6 2 superpave asphalt binders1
6-2 Superpave Asphalt Binders
  • Plot of high or low temp recorded at a site for number of years yield frequency distribution curve
  • A measure of the variability of the curve is the standard deviation
  • Average temp from the curve is used to specify the binder grade
    • Reliability is 50%
    • In on e half of the years the actual temperature will be higher then average
  • Temp of twice the standard deviation above the average would be 98% reliable
    • In only 2 out of 100 years would this be exceeded
6 2 2 superpave r evised t ests
6-2.2 Superpave Revised Tests
  • Rolling thin film oven test – similar to the thin film oven test – 8 small samples are placed in bottles and rotated for 75 min while being subjected to air streams – two samples are tested to find loss
  • Pressure aging vessel test – samples from RFTO are placed in a pressure aging vessel and are aged for 20 hours under a constant air pressure – measures the aging that occurs over long period of time
  • Rotational Viscometer test – measures the viscosity of the original binder at high temp to ensure that it is fluid enough for the mixing operation
  • Dynamic shear rheometer test – samples form the original binder are placed between a base plate and an oscillating plate – test measure the elastic and viscous properties of the binder at various ages
  • Bending beam rheometer test – are formed into a beam at low temp and loaded until cracking occurs
  • Direct tension test – indicates the binders resistance to pavement cracking in the winter by measuring the load required to cause failure in tension
6 3 asphalt concrete properties
6-3 Asphalt Concrete Properties
  • Asphalt concrete
    • Asphalt cement
    • Aggregates
    • Air
  • Amount of asphalt absorption is less than water absorption for the same aggregates usually about 50%
  • Mass/volume relationships of a compacted asphalt paving mix are illustrated in 6-9
  • Formulas on page 221
  • Sample problems Page 222 -225
6 3 2 asphalt concrete properties
6-3.2 Asphalt Concrete Properties
  • Stability of asphalt concrete
    • Depends on strength and flexibility of mixture
    • Strength must be sufficient to carry the load without shear occurring
    • Structure must remain intact
    • Main contributor to strength is friction between partials
    • Flexibility important to distribute imposed loads by deflecting slightly without cracking or permanent deformation
6 3 3 safety
6-3.3 Safety
  • Skid resistance and drainage of water from surface – key safety issues
  • Skid resistance improves with smaller sized – hard aggregates for surface course
    • Provides more contract for development of friction force
6 3 4 durability
6-3.4 Durability
  • Key to maintaining the stability and skid resistance over service life
  • As it ages pavements become more dense
  • Pavements fail due
    • Changes in the aggregates
    • Permanent deformation or rutting
    • Cracking either due to fatigue or low temperatures
    • Bleeding of asphalt to the surface
  • To prevent failure
    • Use strong sound durable aggregates
    • Use asphalt binders that resistance fatigue cracking rutting and low temperature cracking
    • Maximum temperature during mixing to prevent premature hardening
    • Maximum percentage of air voids to reduce permeability and movement of air and water
    • Minimum percentage of air voids helps to ensure that asphalt does not bleed to the surface
    • Minimum percentage of vma to ensure that sufficient space is left for asphalt cement
    • Minimum and maximum percentages for vfa – asphalt content must not be too high for stability – air voids content must not be too high for light traffic areas or too low for heavy traffic areas
6 4 aggregates
6-4 Aggregates
  • Aggregates
    • Coarse aggregates
    • Fine aggregates
    • Mineral filler
  • Mineral filler
    • Defined as fines
    • Important to produce dense graded strong material
    • Limestone dust most common used
  • Aggregates should
    • Well graded
    • Hard
    • Sound
    • Rough surfaced
    • Cubical
    • Hydrophobic (hater hating)
    • Free form deleterious substances
6 4 2 superpave aggregates r equirements
6-4.2 Superpave aggregates Requirements
  • Gradation requirement include control points for minimum and maximum percent passing certain sieve sizes
  • Restricted zone limits the amount of medium to fine sand with their rounded grains
  • Table 6-5 page 229
6 5 asphalt concrete mix design
6-5 Asphalt Concrete Mix Design
  • Mix Design
    • Selection of aggregate proportions to meet specification
    • Conducting trial mixes at a range of asphalt contents and measuring the resulting physical properties of the samples
    • Analyzing the results to obtain the optimum asphalt content and to determine if the specifications can be met
    • Repeating with additional trial mixes using different aggregate blends until a suitable design is found
  • Two methods for making and evaluating trial mixes
    • Marshall method
    • Hveem method
6 5 2 marshall method
6-5.2 Marshall Method
  • Aggregates are blended in proportions that meet specification
  • Mixing and compacting temperatures for the asphalt cement being used are obtained from temperature – viscosity graph
  • A number of briquettes are mixed using 1200 g of aggregates and asphalt cement content at various percents both above and below the expected optimum
  • Density of the briquettes is measured to allow calculation of the void properties
  • Briquettes are heated to 60 c . Stability and flow values are obtained in a compression test in the marshal apparatus pictured on page 234
  • Example 6-6 and Example 6-7
6 5 4 hveem method
6-5.4 Hveem Method
  • Obtain the estimated optimum asphalt content by the centrifuge kerosene equivalent method
  • Prepare test briquettes at a range of asphalt contents above and below the estimated optimum
  • Conduct stabilometer tests to obtain stabilometer values in the Hveem apparatus
  • Conduct swell tests on two samples containing the estimated optimum asphalt content.