Chapter 6 Asphalt Pavements and Materials Asphalt Pavements

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 • 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 • 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 cm 2 and stokes or centistokes 1 stoke = 100 centistokes = 1 cm 2/s • Kinematic viscosity equals absolute viscosity divided by density

6 -1. 3 Viscosity • Figure 6 -2 (temperature – viscosity relationships – Ac 10 is harder then AC 5 • reduce viscosity to 1. 5 cm 2/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 cm 2/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. 0 cm 2/s – Asphalt B would be 156 C

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 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 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 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 • 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 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 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 • 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 -10 c 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 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 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 • 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 • 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 • 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 • • • 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 • 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 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 • 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 • 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 • 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.

6 -6 Superpave Mix Design