Highway Material . Dr. Attaullah Shah

Transportation Engineering - I

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 low-temperature 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

• Average speed– Time mean speed

Arithmetic mean– Space mean speed

Harmonic mean

TLv

TLv avg

,

n

vv i

i

t

ii

i

is t

nL

ntLv

st vv

Individual Speed

600 700

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

Location (ft)

Vehi

cle

Time of Passing (sec)

Speed (ft/sec)

(1)

Spot Speed

mph) or (mi/hr

ft/sec

09.345280/3600*50

500.00.2600700

1

v

Time Mean Speed

Mile post

Observation Period

(ft/sec) 6.458

6045404035504550

tv

Space Mean Speed

Observation Period

(mi/hr)(ft/sec) 1302447.12.25.25.29.223.22

8*100 ..vs

Obs

erva

tion

Dis

tanc

e

Volume (q)• Number of vehicles passing a point

during a given time interval• Typically quantified by Rate of Flow

(vehicles per hour)

vph) or (veh/hr 11523600/258

q

Time-Space Plot

0

100

200

300

400

500

600

700

800

900

1000

0 5 10 15 20 25

Time (sec)

Dist

ance

(ft)

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)

(veh/mi)(mi/hr)(veh/hr) kvq *

(veh/mi)

(mi/hr)(veh/hr)

22.381.30/1152

*1.301152

k

k

Spacing (s)• Front bumper to front bumper

distance between successive vehicles

S1-2S2-3

T=3sec

Headway (h)• Time between successive vehicles

passing a fixed point

T=0 sech1-2=3sec

Spacing and Headway

spacing

headway

Spacing and Headway

A

What are the individual headways and the average headway measured at location A during the 25 sec period?

Spacing and Headway

A

What are the individual headways and the average headway measured at location A during the 25 sec period?

600 700

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

Location (ft)

Vehi

cle

Time of Passing (sec)

h1-2

h2-3

Lane Occupancy• Ratio of roadway occupied by

vehicles

L1L2L3

D

D

LLO i

i

Clearance (c) and Gap (g)• Front bumper to back bumper

distance and time

Spacing (ft) or headway (sec)

Clearance (ft) or Gap (sec)

avg

avgavgavg v

Lhg

avgavgavg vgc *