Transportation Engineering - I
Highway Material
. Dr. Attaullah Shah
Types of Roadway material
• Currently, there are two primary types of pavement surfaces — Portland
cement concrete (PCC) and hot-mix asphalt concrete (HMAC).
•
Below this wearing course are material layers that provide structural
support for the pavement system. These may include either (a) the
aggregate base and sub base layers, or (b) treated base and sub base
layers, and the underlying natural or treated subgrade. The treated
layers may be cement-treated, asphalt-treated or lime-treated for
additional structural support.
• There are various methods by which pavement layers are designed. For
example, HMAC may be designed using the Marshall, Hveem, or
Superpave mix design systems. PCC may be designed using the
American Concrete Institute (ACI) or the Portland Cement Association
(PCA) method.
Hot-Mix Asphalt Concrete
• HMAC consists primarily of mineral aggregates, asphalt cement (or
binder), and air.
• It is important to have suitable proportions of asphalt cement and
aggregates in HMAC so as to develop mixtures that have desirable
properties associated with good performance.
• These performance measures include the resistance to the three primary
HMAC distresses: permanent deformation, fatigue cracking, and low
temperature cracking.
• Permanent deformation refers to the plastic deformation of HMAC under
repeated loads. This permanent deformation can be in the form of rutting
(lateral plastic flow in the wheel paths) or consolidation (further
compaction of the HMAC after construction).
• Aggregate interlock is the primary component that resists permanent
deformation with the asphalt cement playing only a minor role. Angular,
rough-textured aggregates will help reduce permanent deformation. To a
significantly lesser extent, a stiffer asphalt cement may also provide
some minor benefit.
• Cracking can be subdivided into two broad categories: load
associated cracking and non-load associated cracking. Load
associated cracking has traditionally been called fatigue cracking. In
this scenario, repeated stress applications below the maximum
tensile strength of the material eventually lead to cracking.
• Factors associated with the development of fatigue cracking include
the in-situ properties of the structural section, asphalt cement,
temperature, and traffic.
• Non-load associated cracking has traditionally been called lowtemperature cracking. During times of rapid cooling and low
temperatures, the stress experienced by the HMAC may exceed its
fracture strength. This leads to immediate cracking.
Aggregates Specification and test
• Traditional aggregate specifications for HMA include the American
Association of State Highway and Transportation Officials (AASHTO)
M29 (ASTM D1073) “Standard Method of Test for Fine Aggregate for
• Bituminous Paving Mixtures,
• ” ASTM D692 “Standard Specification for Coarse Aggregate for
Bituminous Paving Mixtures,” and
• ASTM D242 “Standard Specification for Mineral Filler for Bituminous
Paving Mixtures.”
• The quality of aggregates depend on the following:
– coarse aggregate angularity
– fine aggregate angularity
– flat, elongated particles, and
– clay content
Asphalt Cement Specification and Tests
• Penetration Grading System
• ASTM D946 “Standard Specification for Penetration-Graded Asphalt
Cement for Use in Pavement Construction”
• This specification includes five penetration grades ranging from a
hard asphalt graded at “40-50” to a soft asphalt cement graded at
“200-300.” The sections below discuss the tests used to classify
penetration grades
• Following tests conducted to classify the penetration grades:
•
Penetration Test: AASHTO T49 (ASTM D5) “Standard Method of Test for
Penetration of Bituminous Mixtures” In this procedure, a needle is typically
loaded with a 100-g weight and allowed to penetrate into an asphalt
cement sample for 5 sec. Prior to conducting the test, the asphalt cement
sample is brought to the testing temperature, typically 258C (778F).
•
Flash Point Test (ASTM D92) “Standard Method of Test for Flash and Fire
Points by Cleveland Open Cup” In this procedure, a brass cup partially filled
with asphalt cement is heated at a given rate. A flame is passed over the
surface of this cup periodically and the temperature at which this flame
causes an instantaneous flash is reported as the flash point.
• Ductility Test Ductility is the number of centimeters a standard briquette
of asphalt cement will stretch before breaking.
• This property is determined using AASHTO T51 (ASTM D113) “Standard
Method of Test for Ductility of Bituminous Mixtures” (AASHTO, 2003).
• Solubility Test Solubility is the percentage of an asphalt cement sample
that will dissolve in trichloroethylene. This property is determined using
AASHTO T44 (ASTM D2042) “Standard Method of Test for Solubility of
Bituminous Materials” (AASHTO, 2003).
• Thin-Film Oven Test The TFO test is used to approximate the effect of
short-term aging during the mixing process. This test is conducted using
AASHTO T179 (ASTM D1754) “Standard Method of Test for Effect of Heat
and Air on Asphalt Materials (Thin-Film Oven Test)” (AASHTO, 2003).
• Absolute and Kinematic Viscosity Tests: Viscosity can be defined as a
fluid’s resistance to flow. In the asphalt paving industry, two tests are used
to measure viscosity — absolute and kinematic viscosity tests. Absolute
viscosity is determined using AASHTO T202 (ASTM D2171) “Standard
Method of Test for Viscosity of Asphalt by Vacuum Capillary Viscometer”
(AASHTO, 2003). Kinematic viscosity is determined using AASHTO T201
(ASTM D2170) “Standard Method of Test for Kinematic Viscosity of Asphalts
(Bitumen)” (AASHTO, 2003).
Design of Hot-Mix Asphalt Concrete
• Mix design method named after Marshall. AASHTO adopted this mix
design procedure as AASHTO R-12 “Standard Recommended Practice
for Bituminous Mixture Design Using the Marshall and Hveem
Procedures”
– Step 1. Aggregate Evaluation
– Step 2. Asphalt Cement Evaluation
– Step 3. Preparation of Marshall Specimens
– Prepare the Marshall specimens in accordance to the
requirements set in AASHTO R-12. Compact three replicate
specimens at five asphalt contents.
– Step 4. Marshall Stability and Flow
– Step 5. Density and Void Analysis
– Step 6. Tabulating and Plotting Test Results
– Step 7. Optimum Asphalt Content Determination
Emulsified and Cutback Asphalts
• Asphalt cement can be emulsified with an emulsifying agent and water to
form asphalt emulsions or dissolved in suitable petroleum solvents to
form cutback asphalts.
• Cutback asphalts consist primarily of asphalt cement and a solvent.
The speed at which they cure is related to the volatility of the solvent
(diluent) used.
• Cutbacks made with highly volatile solvents will cure faster as the solvent
will evaporate more quickly. Conversely, cutbacks made with less volatile
solvents will cure slower as the solvent will evaporate slower.
• The standard practice for selecting cutback asphalts is covered in ASTM
D2399 “Standard Practice for Selection of Cutback Asphalts” (ASTM,
2003)
• Asphalt Emulsions Asphalt emulsions consist primarily of asphalt
cement, water, and an emulsifying agent. They should be stable enough
for pumping, mixing, and prolonged storage.
Pavement Distresses and Performance
• These distresses could be developed due to traffic load repetitions,
temperature, moisture, aging, construction practice, or combinations.
• Fatigue Cracking: are a series of longitudinal and interconnected cracks
caused by the repeated applications of wheel loads. This type of cracking
generally starts as short longitudinal cracks in the wheel path and
progress to an alligator cracking pattern (interconnected cracks) as
shown in Figure. This type of distress will
eventually lead to a loss of the structural integrity
of pavement system.
• Rutting: Rutting is defined as permanent deformation
in the wheel path as shown in Figure.Rutting can occur due to: (a)
unstable HMA, (b) densification of HMA, (c) deep settlement in the
subgrade.
Traffic Flow
• Complex: between vehicles and drivers,
among vehicles
• Stochastic: variability in outcome, cannot
predict with certainty
• Theories and models
– Macroscopic: aggregate, steady state
– Microscopic: disaggregate, dynamics
– Human factor: driver behavior
Speed (v)
• Rate of motion
• Individual speed
L
L
v
, vavg 
T
T
• Average speed
– Time mean speed
Arithmetic mean
– Space mean speed
Harmonic mean
vt 
v
i
i
n
L
nL
vs 

ti  ti
i n i
vt  vs
Individual Speed
(1)
Time of
Location (ft) Speed
Passing (sec) 600
700 (ft/sec)
700 600
v1 
 50 ft/sec
2.0  0.0
 50* 3600/ 5280 34.09 (mi/hr or mph)
Vehicle
Spot
Speed
1
0.0
2.0
50.0
2
4.4
6.7
45.0
3
6.0
8.0
50.0
4
11.4
14.3
35.0
5
15.0
17.5
40.0
6
17.5
20.0
40.0
7
21.1
23.3
45.0
8
23.3
25.0
60.0
Time Mean Speed
Mile
post
Observation Period
vt 
50  45  50  35  40  40  45  60
 45.6 (ft/sec)
8
Observation Distance
Space Mean Speed
Observation Period
100* 8
vs 
 44.2(ft/sec)  30.1(mi/hr)
2  2.3  2  2.9  2.5  2.5  2.2  1.7
Volume (q)
• Number of vehicles passing a point during
a given time interval
• Typically quantified by Rate of Flow
(vehicles per hour)
Time-Space Plot
1000
900
800
Distance (ft)
700
q
600
500
400
300
200
100
0
0
5
10
15
Time (sec)
20
25
8
 1152 (veh/hr or vph)
25 / 3600
Volume (q)
Density (k)
• Number of vehicles occupying a given
length of roadway
• Typically measured as vehicles per mile
(vpm),
or vehicles per
mile per lane
(vpmpl)
Density (k)
Density (k)
q(veh/hr)  v(mi/hr) * k(veh/mi)
1152 (veh/hr)  30 .1(mi/hr) * k
k  1152 / 30 .1  38 .22 (veh/mi)
Spacing (s)
• Front bumper to front bumper distance
between successive vehicles
S2-3
S1-2
Headway (h)
• Time between successive vehicles passing
a fixed point
T=0 sec T=3sec
h1-2=3sec
Spacing and Headway
spacing
headway
Spacing and Headway
What are the individual headways and the average headway measured at
location A during the 25 sec period?
A
Spacing and Headway
What are the individual headways and the average headway measured at
location A during the 25 sec period?
Time of
Location (ft)
Passing (sec) 600
700
Vehicle
A
1
0.0
2.0
2
4.4
6.7
3
6.0
8.0
4
11.4
14.3
5
15.0
17.5
6
17.5
20.0
7
21.1
23.3
8
23.3
25.0
h1-2
h2-3
Lane Occupancy
• Ratio of roadway occupied by vehicles
L2
L3
LO 
L
i
i
D
L1
D
Clearance (c) and Gap (g)
• Front bumper to back bumper distance
and time
Clearance (ft) or Gap (sec)
g avg  havg 
Lavg
vavg
cavg  gavg * vavg
Spacing (ft) or headway (sec)