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 concrete mixtures
• To explain the requirements of aggregates for
  asphalt concrete

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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

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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

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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.

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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

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ASPHALT CONCRETE MIXTURE

• Asphalt concrete mixture basically consists of
  asphalt cement, aggregates, and air

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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

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ASPHALT CONCRETE MIXTURE---contd.

• The mass/volume relationships of a compacted
  asphalt mixture are illustrated in the following
  figure

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ASPHALT CONCRETE MIXTURE---contd.
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ASPHALT CONCRETE MIXTURE---contd.
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ASPHALT CONCRETE MIXTURE---contd.
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ASPHALT CONCRETE MIXTURE---contd.
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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

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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

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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

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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

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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

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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)
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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.

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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

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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

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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

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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.

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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

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AGGREGATES FOR ASPHALT CONCRETE
MIXTURESSpecifications for mineral filler
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AGGREGATES FOR ASPHALT CONCRETE
MIXTURESSpecifications for fine aggregate
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AGGREGATES FOR ASPHALT CONCRETE
MIXTURESSpecifications for coarse aggregate