1. Asphalt Pavements and Materials
Chapter 6
Asphalt Pavements and Materials

2. 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

3. 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

4. 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

5. 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. 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 % measured at 60 C .
Example AC 2.5 has a viscosity of AC has a viscosity of 4000 poises

7. 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%

8. 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 % asphalt and 3% emulsifying agent
Grades
Rapid setting rs or crs
Medium setting ms or cms
Slow setting ss or css

9. 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.

10. 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

11. 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 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

12. 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

13. 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

14. 6-3 Asphalt Concrete Properties
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

15. 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

16. 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

17. 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

18. 6-4 Aggregates Aggregates Aggregates should Coarse aggregates
Fine aggregates
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

19. 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

20. 6-5 Asphalt Concrete 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

21. 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

22. 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.

23. 6-6 Superpave Mix Design