Chapter 6

1. 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 + - 20% measured at 60 C . • Example AC 2.5 has a viscosity of 250 + - 50 AC has a viscosity of 4000 poises + - 800

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 55 -75% 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 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

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 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

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 • 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

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 • 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