Title: LECTURE NO. 18 (Handout) ASPHALT CONCRETE MIXTURES
1LECTURE 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 -
2ASPHALT 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
3ASPHALT 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
4ASPHALT 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.
5ASPHALT 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
6ASPHALT CONCRETE MIXTURE
- Asphalt concrete mixture basically consists of
asphalt cement, aggregates, and air
7ASPHALT 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 -
8ASPHALT CONCRETE MIXTURE---contd.
- The mass/volume relationships of a compacted
asphalt mixture are illustrated in the following
figure
9ASPHALT CONCRETE MIXTURE---contd.
10ASPHALT CONCRETE MIXTURE---contd.
11ASPHALT CONCRETE MIXTURE---contd.
12ASPHALT CONCRETE MIXTURE---contd.
13TYPES 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
-
14TYPES 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
15TYPES 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
16TYPES 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
17TYPES 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
18TYPES 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)
19TYPES 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.
20TYPES 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
21TYPES 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
22AGGREGATES 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
23AGGREGATES 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.
24AGGREGATES 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
25AGGREGATES FOR ASPHALT CONCRETE
MIXTURESSpecifications for mineral filler
26AGGREGATES FOR ASPHALT CONCRETE
MIXTURESSpecifications for fine aggregate
27AGGREGATES FOR ASPHALT CONCRETE
MIXTURESSpecifications for coarse aggregate