CE222 Materials: Asphaltic Materials Notes have been updated to reflect material covered in lectures |
Lecture 1
What is Asphalt?
Asphalt is a type of bituminous material
In other words, it contains BITUMEN which is defined as a hydrocarbon that is completely soluble in carbon disulfide (CS2)
Two types of bituminous materials
ASPHALT - almost pure bitumen, from petroleum refining or natural sources
TAR - 50% bitumen, from destructive distillation of organic materials
Tar is no longer used for highway construction - it is also considered to be a health hazard
Sources of Asphalt
Natural Sources or petroleum refining
Most commercial asphalt is from petroleum refining
Natural Sources
Lake asphalts (Bermudez and Trinidad)
Gilsonite - occur in faults, large deposit in Utah, very hard substance, used in varnish/paper/powered form on roads
Rock asphalts - sandstone or limestone impregnated with asphalt (0 to 20% asphalt), usually too expensive for use in paving, found in KY, OK, AR, AL, TX, UT, CA
Petroleum Asphalts
Asphalt is obtained from crude oil by a process called FRACTIONAL DISTILLATION
Crude oil is a mixture of different hydrocarbons which are mutually soluble - fractional distillation is used to separate the fractions based on boiling point
The asphalt properties depend on the crude source and the manufacturing process
How is asphalt used for paving?
Asphalt is used in a wide range of different types of applications for road construction
Types of Pavements: Flexible, Rigid, Composite
Flexible pavements generally include one or more layers of asphaltic materials
Rigid pavement is made using portland cement concrete
Composite pavement is a combination - usually pcc overlaid with asphaltic material
Flexible Pavement
Typical pavement structure
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Surface course: AC
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Binder course: AC
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Base
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Sub-base
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Sub-grade
Pavement Layers
Surface - asphalt cement, or surface treatment, or seal coat
Binder - asphalt cement
Base - asphalt cement, or stabilized gravel, or unstabilized gravel
Sub-base - stabilized gravel or unstabilized gravel
Sub-grade - existing soil
Surface and binder course maybe combined
Base and sub-base maybe combined
Full-depth asphalt pavement - AC surface and AC base
Types of asphalt paving materials
Types: asphalt concrete and surface treatments
Asphalt Concrete
Dense Graded
Open Graded
Stone Matrix Asphalt
Dense Graded - most common mix type; consists of the full range of size of aggregates so that there is very little voids in the matrix of the mix
Open Graded - designed to have high permeability; useful in areas of high rain
Stone Matrix Asphalt - larger aggregate than traditional used in the USA; used to reduce rutting
Mix may be made using Hot or Cold binder
Surface Treatments
Used as surface for low to moderate volume roads
Also used to seal and rejuvenate old asphalt concrete pavements
Consist of layer of aggregate over a layer of asphalt binder
The aggregate is rolled into the binder
Does little to increase the structural capacity of the roadway.
Hot or Cold binder may be used.
Lecture 2
Pavement Performance
The most important reason for studying asphaltic materials is to gain a better understanding of how we can design and make longer lasting pavements
Causes of Pavement Deterioration
The many different factors that contribute to pavement deterioration include
material properties (asphalt binder and aggregate)
mix proportions
mix properties
mix production and transportation
pavement construction
Types of Pavement Deterioration
Pavement deterioration takes place in different ways but the pavement deterioration mechanisms that are attributable to the asphaltic materials are
permanent deformation
thermal cracking
fatigue cracking
Asphaltic materials are designed to slow down the occurrence of these types of deterioration. In order to produce high performing pavements we must understand how the material properties and characteristics affect performance
Today's class - discussion of the asphalt binder properties which affect performance
Asphalt Binder Properties
Asphalt is a material with very complex viscoelastic properties. The viscoelastic means that its response under load is a combination of a
viscous response (time dependent and non-recoverable - like a dashpot)
elastic response (instantaneous and recoverable - like a spring)
Perhaps the most dramatic feature of asphalt is that all its properties change significantly with temperature. This feature is both good and bad. Why? How?
In selecting a suitable asphalt the engineer must consider the following
What is the consistency of the asphalt at different temperatures?
What is its tensile strength at low temperature?
How will the asphalt change due to aging?
A number of different tests have been developed in the SHRP for evaluating these factors
Consistency tests include
viscometer (used at 60 and 135oC)
dynamic shear rheometer (at intermediate temperatures)
bending beam rheometer (at low temperatures)
Lecture 3
Asphalt Binder Tests
High Temperatures
At high temperatures the asphalt is liquid-like. The viscometers are used to measure asphalt viscosity - this information helps the engineer to decide
What is the best temperature for mixing the asphalt with the binder
What is the appropriate temperature for compaction during construction
Intermediate Temperatures
The dynamic shear rheometer is used to measure visco-elastic properties such as complex modulus and the phase angle at pavement surface temperature. These tests tell the condition of the asphalt at intermediate service temperatures. This information is needed to design against fatigue cracking and permanent deformation.
Low Temperatures
At low temperatures the asphalt is glass-like. The bending beam rheometer is used to measure the stiffness modulus. And the direct tensile test is used to measure the tensile strength.
Both of these parameters are used to assess whether or not the asphalt pavement will be able to resistant to low temperature cracking
Binder Aging
Asphalt properties change over time on exposure to high temperature and the atmosphere. This process is referred to as aging.
The main mechanism for aging are oxidation and volatilization (this is same thing that happens to plastic which gets hard and starts to crack after some time).
Aging is important because the rate and amount of aging affect how long the pavement will be able to perform adequately
Aging is considered to occur in two stages
during production and construction (short-term aging)
during the life of the pavement (long-term aging)
The rolling thin film oven test is used to assess short-term aging
The pressure aging vessel is used to assess long-term aging
Binder Specifications
The specification sets the minimum standard for the important properties of the asphalt
Asphalts are selected for a given application based on their grade (which is determined by the specification)
The SHRP specification system is based on the extreme temperatures that the asphalt is expected to see during service in the pavement
For example, an asphalt rated PG58-28 means that that asphalt should be used where
the maximum average pavement temperature is 58C
the minimum pavement temperature is -28 C
PG64-22 is used in Kentucky
PG64-34 is used in Nebraska
PG52-28 is used in Michigan
PG52-34 is used in Wisconsin
Specification Testing
Tests used for determining the specifications include
Viscometer
Dynamic Shear Rheometer (DSR)
Bending Beam Rheometer
Direct Tension
Tests are conducted on
Original asphalt - get properties needed for mixing
R-TFOT residue - to try to get properties of the binder during and just after construction
PAV residue - to try to get properties of the binder in the mix late in the life of the pavement
Lecture 4
Aggregate Properties
Aggregate properties have a significant effect on the performance of asphalt pavements since most of the applied load is carried by the aggregate. The aggregate forms the backbone of the pavement structure.
The most important aggregate properties include
Gradation - the stability of the mix (ability to resist rutting) is largely determined by gradation
Particle shape and surface roughness - angular particles results in a much higher internal resistance (helps to improve stability). particle roughness also increases internal resistance.
Particle toughness - the particles much be able to resist crushing and freezing and thawing cycles
Cleanliness - clay and other foreign substance on aggregate may reduce the adhesion between asphalt and aggregate
Affinity for asphalt - water should not be able to easily strip asphalt from aggregate particles (the agregate should be hydrophobic).
Absorption - the absorption of asphalt into the aggregate is important - if too little the bond between asphalt and aggregate will not be too good, if too much the asphalt is essentially lost and the mix will be very expensive.
In the SHRP mix design, the aggregate is tested for these properties
Mixture Properties
In The SHRP procedure volumetric properties are used to ensure that the mix will perform properly
The characteristics that are measured include
air void content (Va)
voids in the mineral aggregate (VMA)
voids filled with asphalt (VFA)
Significance of Volumetric Properties
Air Void
Voids between the aggregate particles in the compacted mix that are filled with air
The mix should have an optimum amount of air voids
If air void content is too little there is no space for the asphalt as the pavement compact under traffic - leads to rutting and bleeding
If the air void content is too much asphalt will age rapidly
Voids in the Mineral Aggregate (VMA)
Voids between aggregate particles in the compacted mix that is filled with either air or asphalt
It the VMA is too low then there is not enough space for the effective asphalt and for air voids
Pavement Performance
Summary
In order to design a high performance mix the engineer must
select asphalt with right properties
select aggregate with right properties
determine the right combination of asphalt and aggregate
ensure that the mix has optimum properties
