LECTURE NO. 18 (Handout) ASPHALT CONCRETE MIXTURES - PowerPoint PPT Presentation

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LECTURE NO. 18 (Handout) ASPHALT CONCRETE MIXTURES

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LECTURE NO. 18 (Handout) ASPHALT CONCRETE MIXTURES Objectives: To introduce asphalt concrete To explain the asphalt concrete mixture To explain the types of asphalt ... – PowerPoint PPT presentation

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Title: LECTURE NO. 18 (Handout) ASPHALT CONCRETE MIXTURES


1
LECTURE NO. 18 (Handout)ASPHALT CONCRETE MIXTURES
  • Objectives
  • To introduce asphalt concrete
  • To explain the asphalt concrete mixture
  • To explain the types of asphalt concrete mixtures
  • To explain the requirements of aggregates for
    asphalt concrete

2
ASPHALT CONCRETE Introduction
  • Asphalt concrete is basically a mixture of
    asphalt cement and aggregates, hot-mixed in an
    asphalt plant and then hot-laid to form the
    surface course of a flexible pavement
  • The properties of asphalt concrete depend on
  • the quality of its components (i.e., asphalt
    binder and aggregates)
  • the mix proportions
  • construction process
  • Asphalt concrete must provide a stable, safe, and
    durable road surface

3
ASPHALT CONCRETE Stability
  • Stability of the asphalt concrete depends on the
    strength and flexibility of the mixture and the
    degree of compaction during placing
  • The strength must be sufficient to carry the load
    without shear occurring between particles. The
    structure must remain intact.
  • The main contributor to strength is friction
    between grains.
  • A dense-graded mixture, composed of particles
    with rough faces, with a relatively thin asphalt
    film between them is best for high-friction
    strength.
  • Flexibility is also important as the pavement
    distributes the imposed load by deflecting
    slightly as the load is applied, without cracking
    or permanent deformation.
  • To meet this requirement, a more open-graded
    mixture, with a higher asphalt content, is best.
  • Strength and flexibility are evaluated by various
    tests, depending on the design method being used

4
ASPHALT CONCRETE Safety
  • Safety is very important for the surface course
  • Safety is achieved by making the surface course
    skid resistant and able to allow quick drainage
    of water from the surface.
  • Skid resistance is enhanced by using smaller
    sized, very hard aggregates for the surface
    course.
  • This provides more points of contact for the
    development of friction forces.
  • Open-graded surface courses are used in very
    heavy traffic areas to allow immediate drainage
    of rainwater before it can result in
    hydroplaning.

5
ASPHALT CONCRETE Durability
  • Durability of the asphalt concrete is critical to
    ensure that it maintains the stability and skid
    resistance properties for the design service life
  • Asphalt ages, and pavements become denser (i.e.,
    aged) with time and traffic
  • Pavements fail (i.e., durability of pavement is
    lost) due to
  • changes in the aggregates
  • permanent deformation or rutting
  • cracking, either due to fatigue, or low
    temperatures
  • bleeding of asphalt to the surface

6
ASPHALT CONCRETE MIXTURE
  • Asphalt concrete mixture basically consists of
    asphalt cement, aggregates, and air

7
ASPHALT CONCRETE MIXTURE---contd.
  • Out of the total asphalt added to the mixture,
    some of the asphalt is absorbed in the pores of
    the aggregate particles
  • The portion of asphalt absorbed by aggregate
    particles is called absorbed asphalt
  • The net amount of asphalt available to coat and
    bind aggregates together is called effective
    asphalt

8
ASPHALT CONCRETE MIXTURE---contd.
  • The mass/volume relationships of a compacted
    asphalt mixture are illustrated in the following
    figure

9
ASPHALT CONCRETE MIXTURE---contd.
10
ASPHALT CONCRETE MIXTURE---contd.
11
ASPHALT CONCRETE MIXTURE---contd.
12
ASPHALT CONCRETE MIXTURE---contd.
13
TYPES OF ASPHALT CONCRETE MIXTURES
  • Asphalt concrete mixtures can be classified into
    following two types based on whether hot-mixed,
    hot laid or cold-mixed, cold-laid
  • Hot-mixed, hot-laid asphalt (HMA) concrete
    mixture
  • Cold-mixed, cold-laid asphalt concrete mixture
  • Asphalt concrete mixtures can be classified into
    following two types based on whether
    in-situ-mixed or plant-mixed
  • Road-mixed or in place-mixed asphalt concrete
    mixture
  • Plant-mixed asphalt concrete mixture
  • HMA concrete mixtures can be classified into
    following three types based on type of aggregate
    grading used
  • Dense-graded HMA concrete mixture
  • Stone matrix asphalt (SMA) concrete mixture
  • Open-graded HMA concrete mixture
  • Asphalt concrete mixtures can be classified into
    following three types based on type additives
    used
  • Rubber-modified asphalt concrete mixture
  • Polymer-modified asphalt concrete mixture
  • Sulfur-modified asphalt concrete mixture

14
TYPES OF ASPHALT CONCRETE MIXTURES Hot-mixed
asphalt (HMA) concrete
  • A hot-mixed asphalt (HMA) concrete is a mixture
    of asphalt, fine aggregate or both fine and
    coarse aggregates, and mineral filler (optional)
  • The grade of asphalt to be used depends on
  • the type of construction
  • climatic conditions
  • amount and nature of traffic
  • HMA concrete is produced and laid in the
    following steps
  • both aggregate and asphalt are heated prior to
    mixing to drive off moisture from the particles
    and make the asphalt sufficiently fluid (maximum
    temperatures for heating asphalt cement and
    emulsified asphalt are 176.6 F and 82.2 F,
    respectively)
  • after heating, all the raw materials are mixed in
    the plant, and the hot mixture is transported to
    the paving site and spread on a loosely compacted
    layer to a uniform, even surface with the help of
    a paving machine
  • while the mixture is hot it is compacted by
    heavy, motor-driven rollers to produce a smooth,
    well-compacted paving course
  • Since the aggregates are thoroughly dried prior
    to mixing, stripping of asphalt (i.e., removal
    from the pavement) will not take place in
    hot-mixed, hot-laid asphalt pavements

15
TYPES OF ASPHALT CONCRETE MIXTURES Cold-mixed
asphalt concrete
  • Like a hot-mixed asphalt concrete, cold-mixed
    asphalt concrete is also a mixture of asphalt,
    fine aggregate or both fine and coarse
    aggregates, and mineral filler (optional)
  • Cold-mixed asphalt concrete is produced and laid
    at normal temperature, however, some heating of
    both the aggregates and asphalt may be required
    during winter season
  • Drying of aggregates is not necessary except when
    the particles have surface moisture
  • To improve bonding, commercial additives are
    needed in this type of asphalt concrete

16
TYPES OF ASPHALT CONCRETE MIXTURES Road-mixed
and plant-mixed asphalt concretes
  • A bituminous surface or base course produced by
    mixing aggregates and asphalt at the jobsite is
    called road-mixed or mixed-in place asphalt
    concrete
  • A mixture of aggregates and emulsified or cutback
    asphalt prepared at a central mixing plant and
    spread and compacted at the jobsite at near
    ambient temperature is called plant-mixed,
    cold-laid asphalt concrete

17
TYPES OF ASPHALT CONCRETE MIXTURES Dense-graded
HMA concrete
  • A dense-graded HMA concrete is produced using
    well-graded aggregates, and intended for general
    use
  • When properly designed and constructed, a
    dense-graded HMA concrete is relatively
    impermeable
  • Dense-graded HMA concrete mixes are generally
    referred to by their nominal maximum aggregate
    size
  • They can further be classified as either
    fine-graded or coarse-graded. 
  • Fine-graded mixes have more fine and sand sized
    particles than coarse-graded mixes

18
TYPES OF ASPHALT CONCRETE MIXTURES Dense-graded
HMA concrete---contd.
  • Fine- and course-graded definitions for
    dense-graded HMA concrete are presented in the
    following Table

Mixture Nominal Maximum Aggregate Size Coarse-Graded Mix Fine-Graded Mix
37.5 mm (1.5 inches)     lt 35 passing the 4.75 mm (No. 4 Sieve) gt 35 passing the 4.75 mm (No. 4 Sieve)
25.0 mm (1.0 inch)       lt 40 passing the 4.75 mm (No. 4 Sieve) gt 40 passing the 4.75 mm (No. 4 Sieve)
19.0 mm (0.75 inches) lt 35 passing the 2.36 mm (No. 8 Sieve) gt 35 passing the 2.36 mm (No. 8 Sieve)
12.5 mm (0.5 inches)     lt 40 passing the 2.36 mm (No. 8 Sieve) gt 40 passing the 2.36 mm (No. 8 Sieve)
  9.5 mm (0.375 inches) lt 45 passing the 2.36 mm (No. 8 Sieve) gt 45 passing the 2.36 mm (No. 8 Sieve)
19
TYPES OF ASPHALT CONCRETE MIXTURES Stone matrix
asphalt (SMA) concrete
  • Stone matrix asphalt (SMA) is a gap-graded HMA
    that is designed to maximize deformation
    (rutting) resistance and durability by using a
    structural basis of stone-on-stone contact  
  • Because the aggregates are all in contact, rut
    resistance relies on aggregate properties rather
    than asphalt binder properties.
  • Since aggregates do not deform as much as asphalt
    binder under load, this stone-on-stone contact
    greatly reduces rutting.
  • SMA is generally more expensive than a typical
    dense-graded HMA (about 20 - 25 percent) because
    it requires more durable aggregates, higher
    asphalt content and, typically, a modified
    asphalt binder and fibers. 
  • In the right situations it should be
    cost-effective because of its increased rut
    resistance and improved durability.
  • SMA, originally developed in Europe to resist
    rutting and studded tire wear, has been used in
    the U.S. since about 1990.

20
TYPES OF ASPHALT CONCRETE MIXTURES Open-graded
HMA concrete
  • An open-graded HMA mixture is designed to be
    water permeable (dense-graded and SMA mixes
    usually are not permeable)
  • Open-graded mixes use only crushed stone (or
    gravel) and a small percentage of manufactured
    sands. 
  • There are three types of open-graded mixes
    typically used in the U.S.
  • Open-graded friction course (OGFC).  Typically 15
    percent air voids, no minimum air voids
    specified, lower aggregate standards than Porous
    European mixes (PEM).
  • Porous European mixes (PEM).  Typically 18 - 22
    percent air voids, specified minimum air voids,
    higher aggregate standards than OGFC and requires
    the use of asphalt binder modifiers.  See Figure
    2.13.
  • Asphalt treated permeable bases (ATPB).  Less
    stringent specifications than OGFC or PEM since
    it is used only under dense-graded HMA, SMA or
    PCC for drainage

21
TYPES OF ASPHALT CONCRETE MIXTURES Rubber-,
polymer-, sulfur-modified asphalt concretes
  • Asphalt rubber also called crumb rubber, which
    is a recycled product from old tires, is added
    in quantity ranging from 1 to 5 (by wt. of
    asphalt) as additive in the production of HMA
    concrete for improving binding property of
    aggregate.
  • Rubber addition increase the viscosity and the
    softening point of the asphalt
  • Polymers (such as ethyl vinyl acetate, latex,
    silicone, and epoxies) are added to asphalt as
    additive to produce polymer-modified asphalt
    concrete
  • Polymer addition increases dispersion, ductility,
    and adhesiveness of asphalt
  • Sulfur is added to asphalt concrete to provide
    higher stiffness at elevated temperatures

22
AGGREGATES FOR ASPHALT CONCRETE MIXTURES
Introduction
  • In asphalt concrete mixture, aggregates may
    constitute about 70-75 by volume or 90-95 by
    weight
  • The roles of aggregates in a asphalt concrete
    pavement are to
  • distribute wheel loads through point-to-point
    contact (aggregate interlock) ensuring stability
    of pavement (i.e., resistance to pavement
    deformation under load)
  • provide resistance to abrasion and skid
  • A number of factors, such as quality, gradation,
    shape, stiffness, and quantity of aggregates,
    determine effectiveness of aggregates in load
    transfer and the stability of pavement
  • Aggregates for asphalt concrete are usually
    classified as coarse aggregates, fine aggregates,
    and mineral filler
  • Mineral filler is often used in asphalt concrete
    mixtures to supply the fines (smaller than 75 ?m
    or No. 200 sizes)
  • Fines are very important in producing a
    dense-graded, strong material.
  • Many natural sands do not contain the amount or
    type of fines required. Limestone dust is the
    most common material used for mineral filler

23
AGGREGATES FOR ASPHALT CONCRETE MIXTURESGeneral
requirements
  • Aggregates should be
  • well-gradeddense, including mineral filler (if
    required) for strength
  • hardfor resistance to wear and to polishing due
    to traffic.
  • soundfor resistance to breakdown due to freezing
    and thawing.
  • rough surfacedcrushed rough surfaces give higher
    friction strength and a better surface for
    adhesion of the asphalt cement.
  • free from cubicalthin, elongated aggregate
    particles because they are broken easily.
  • hydrophobic (or water hating) some siliceous
    aggregates such as quartz are hydrophobic (water
    liking), meaning that they have a greater
    affinity for water than for asphalt, due to their
    surface charges. This may lead to stripping, as
    asphalt coating comes away from the particle in
    the presence of water.
  • free from deleterious substancesclay particles,
    dust, dirt, and lightweight pieces may lead to a
    lower quality asphalt film on the aggregate
    particles or may result in breaking of some of
    the particles.

24
AGGREGATES FOR ASPHALT CONCRETE MIXTURESTypes of
aggregate gradation
  • Following two types of aggregate gradation are
    commonly used in pavement construction (i) open
    gradation and (ii) dense gradation

25
AGGREGATES FOR ASPHALT CONCRETE
MIXTURESSpecifications for mineral filler
26
AGGREGATES FOR ASPHALT CONCRETE
MIXTURESSpecifications for fine aggregate
27
AGGREGATES FOR ASPHALT CONCRETE
MIXTURESSpecifications for coarse aggregate
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