lec 14 highway engineering - asphalt types tests and specifications

Lecture 14 157

Highway Eng. Bituminous Materials 14 15

Dr. Firas Asad

In this lecture;

A. Sources.

B. Types.

C. Properties.

D. Specifications and Tests.

Bituminous Materials

Information listed in this lecture is mainly taken from mainly taken from the Iraqi General Specifications for Roads and Bridge (SCRB, 2007), Standard Specifications for Engineering Materials and Methods of Sampling and Testing (AASHTO, 2013), Traffic and Highway Engineering (Garber, 2009), Handbook of Highway Engineering (Fwa, 2006), http://www.pavementinteractive.org (Accessed on 2015) and Highways (OFlaherty, 2007).

A- Sources of Asphalts

According to the Asphalt Institute Handbook (1989), the word asphalt is believed to

be derived form the ancient UAccadianU term Asphaltic. Asphalt is mans

oldest engineering material. In USumer U, about 6000 B.C, there existed a successful

shipbuilding industry that produced and used asphalt. In building the Ziggurats, such

as Tower of Babel, asphalt was used for mortar.

Bituminous materials are used widely all over the world in highway construction.

These hydrocarbons are found in natural deposits or are obtained as a

product of the distillation of crude petroleum (refining process).

The bituminous materials used in highway construction are either asphalts or tars.

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Bitumenous Matreials

Tar

Manufactured (from Coal)

Dest. Distill.

Asphalt

Natural Deposits

Rock Asphalt

Lake Asphalt

Manfuctured (Petrolum

Refininjg Process)

Fractional Distill.

Destructive Distillation

All bituminous materials consist primarily of bitumen and have strong adhesive

properties with colours ranging from dark brown to black. They vary in consistency

from liquid to solid; thus, they are divided into liquids, semisolids, and solids.

B- Types of Asphalts

UB-1 Tar

Road Tar is a viscous liquid with adhesive properties obtained from the destructive

distillation of organic materials such as bituminous coal. Their properties

are significantly different from petroleum asphalts. In general, they are more

susceptible to weather conditions than similar grades of asphalts, and they set more

quickly when exposed to the atmosphere. Tars nowadays are used rarely for

highway pavements due to environmental and health concerns as well as inherent

undesirable physical characteristics.

UB-2 Asphalt (Bitumen)

Regarding the source, Asphalt can be found in the form of Unatural deposits U in

different parts of the world (natural asphalt); or it can be obtained as a product of

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The largest natural deposit of lake asphalt

occurs on the Island of Trinidad in

South America. The main lake of asphalt

covers an area of roughly 35 ha, has a depth of

about 90 m, and is estimated to contain 10 to

15 million tonnes of material.

Rock asphalt is a natural deposit of sandstone

or limestone rocks filled with asphalt. Rock

asphalt can be used to surface roads after

suitable processing. This process includes

adding suitable mineral aggregates, asphalt

binder, and oil, which facilitates the flowing of

the material. Rock asphalt is not used widely

because of its high transportation costs.

the distillation of crude petroleum

(petroleum asphalt). Natural deposits of asphalt

occur as either native asphalt (lakes) or rock asphalt. The largest deposit of native

asphalt is known to have existed in Iraq several thousand years ago.

In contrast, petroleum asphalt is obtained from the distillation of petroleum during

the refining processes. The refining processes can be divided into two main groups:

fractional distillation and destructive distillation. The fractional distillation processes

involve the separation of the different materials in the crude petroleum without

significant changes in the chemical composition of each material.

Types of Petroleum Asphalt

Bituminous binders can be classified into three general groups: asphalt cement,

asphalt cutbacks, and emulsified asphalt.

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Blown asphalt and road tars are also other types of bituminous material that now

are not used commonly in highway construction.

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B-2-1 Asphalt Cement (Graded Asphalt)

Asphalt cements (AC) are obtained after separation of the lubricating oils. They are

semisolid hydrocarbons with certain physiochemical characteristics that make them

good cementing agents. They are also very viscous, and when used as a binder for

aggregates in pavement construction, it is necessary to heat both the aggregates

and the asphalt cement prior to mixing the two materials. The particular grade of

asphalt cement has been designated by its penetration and viscosity, both of which

give an indication of the consistency of the material at a given temperature.

Asphalt cements are used mainly in the manufacture of hot-mix, hot-laid asphalt

concrete

B-2-2 Asphalt Cutbacks (Liquid Asphalt)

. Asphalt concrete can be used in a variety of ways, including the

construction of highways and airport pavement surfaces and bases, parking areas,

and industrial floors. The specific use of a given sample depends on its grade.

Asphalt cement obtained from the refining process can be blended with some of the

more volatile solvents such as kerosene to form a solution that has a viscosity far

below that of penetration grade asphalt and will act as a fluid at much lower

temperatures. However, when the solution is exposed to the atmosphere, the

volatile solvents evaporate leaving solely the asphalt (bitumen) in place. Such

solutions are termed cutbacks and the process of evaporation of the volatile

solvents is called curing. The speed at which it occurs will depend on the nature of

the solvent. The classification of cutbacks is based on the following two

characteristics:

- The viscosity of the cutback itself - The penetration of the non-volatile residue.

The asphalt cutbacks are used mainly in cold-laid plant mixes, road mixes (mixed-in-

place), and as surface treatments.

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1- Rapid curing cut back

RC is asphalt cement of penetration (85-100) dissolved on gasoline or naphthalene type solvent. Used in tack coat for cohesion purposes.

Grades: RC-70, RC- 250, RC- 800, RC-3000

RC-70 is rapid curing with kinematic viscosity at 60oC is 70 centistoke.

a- Thin film of rapid curing cutback asphalt between two paved layers .

Tack coat characteristics:

b- Applied immediately before paving

c- Spreading rate is 0.25 0.50 litre/m2 (by means of a pressure hand spray).

d- To prevent effect of dust and sliding effects.

e- To provide bond between layers.

f- One part Gasoline to two parts asphalt by volume.

2- Medium curing cut back:

MC is asphalt cement of penetration (120-150) dissolved on kerosene type solvent.

It is used in prime coat for cohesion purposes.

Grades: MC-30, MC- 70, MC- 250, MC-800, MC-3000

a- Thin film of MC cutback asphalt between two paved and unpaved layers . Prime coat characteristics:

b- Applied before 24 hours of paving

c- Spreading rate is 0.5 1.50 litre/m2.

d- Spreading temperature ranges 60 85 oC.

e- To prevent water seepage from subbase layer

f- To provide bond between subbase and paved layers.

g- One part kerosene to one and half parts asphalt cement by volume.

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3- Slow- curing cut back:

SC cut back composition is AC (200-300) + diesel oil

Grades: SC- 70, SC- 250, SC-800, SC-3000

SC used in: Soil and subbase stabilization.

B-2-3 Asphalt Emulsions

Emulsified asphalts are produced by breaking asphalt cement, usually AC(100-250),

into small particles and dispersing them in water with an emulsifier. These small

particles have like-electrical charges and therefore do not join together. They

remain in suspension in the liquid phase as long as the water does not evaporate or

the emulsifier does not break.

Asphalt emulsions consist of asphalt (55-70)% by weight, water, and an emulsifying

agent, which also may contain a stabilizer. Emulsified asphalts are used in cold-laid

plant mixes and road mixes (mixed in-place) for several purposes, including the

construction of highway pavement surfaces and bases and in surface treatments.

Emulsified asphalts is divided into three subgroups based on how rapidly the asphalt

emulsion returns to the state of the original asphalt cement. These subgroups are

rapid-setting (RS), medium-setting (MS), and slow-setting (SS).

Blown Asphalts (Oxidised)

Blown asphalt is obtained by blowing air of high temperatures through the semisolid

residue obtained during the latter stages of the distillation process. Blown asphalts

are relatively stiff and can maintain a higher softening point and firm consistency

compared to other types of asphalts; but less ductility.

Blown asphalt generally is not used as a paving material. However, it is very useful

as a roofing material, for automobile undercoating, and as a joint filler for concrete

pavements.

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C- Important Properties of Asphalt Cement

C-1 Consistency

The consistency of asphalt materials is important in pavement construction because

the consistency at a specified temperature will indicate the grade of the material.

Asphalt materials can exist in either liquid, semisolid, or solid states; hence more

than one method is used for determining the consistency of asphalt materials.

The property used to describe the consistency of asphalt materials in the liquid state

is the viscosity; in the semisolid and solid states is the penetration test and the float

test. The ring-and-ball softening point test also may be used for blown asphalt.

C-2 Aging and Temperature Sustainability

When asphaltic materials are exposed to environmental elements, natural

deterioration gradually takes place, and the materials eventually lose their plasticity

and become brittle. This change is caused primarily by chemical and physical

reactions that take place in the material (weathering). For paving asphalt to act

successfully as a binder, the weathering must be minimized as much as possible. The

ability of an asphalt material to resist weathering is described as the Udurability U of the

material. Some of the factors that influence weathering are oxidation, volatilization

, temperature, and exposed surface area.

C-3 Rate of curing

Curing is defined as the process through which an asphalt material increases its

consistency as it loses solvent by evaporation. For example, the rate of curing

of any cutback asphalt material depends on the distillate used in the cutting-back

process. This is an important characteristic of cutback materials, since the rate of

curing indicates the time that should elapse before a cutback will attain a

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consistency that is thick enough for the binder to perform satisfactorily. The rate of

curing (increase) is affected by both inherent and external factors.

The important inherent factors are

Volatility of the solvent (increase)

Quantity of solvent in the cutback (decrease)

Consistency of the base material (decrease)

The important external factors are

Temperature (increase)

Ratio of surface area to volume (increase)

Wind velocity across exposed surface (increase)

C-4 Resistance to Water Action

In pavement construction, the asphalt should continue to adhere to the aggregates

even with the presence of water. If this bond is lost, the asphalt will strip from the

aggregates, resulting in the deterioration of the pavement. In hot-mix, hot-laid

asphalt concrete, where the aggregates are thoroughly dried before mixing,

stripping does not normally occur.

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D- Specifications and Tests of Asphalt Cement

Several tests are conducted on asphalt to determine its consistency and adequacy to

be used in highway construction according to the prescribed specifications.

Penetration Test

Penetration is the number of units of 0.1 mm penetration depth achieved during the

penetration test. This property is determined using AASHTO T49 (ASTM D5)

Standard Method of Test for Penetration of Bituminous Mixtures. This

specification includes five penetration grades ranging from a hard asphalt graded at

40-50 to a soft asphalt cement graded at 200-300. It is an empirical measure of

the asphalt cements hardness (consistency).

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 250C (770F).

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Ring-and-Ball Softening Point Test

The ring-and-ball softening point test is used to measure the susceptibility of blown

asphalt to temperature changes by determining the temperature at which the

material will be adequately softened to allow a standard ball to sink through it. The

figure below shows an example of the apparatus commonly used for this test.

The test is conducted by first placing a sample of the material to be tested in the

brass ring which is cooled and immersed in the water or glycerine bath that is

maintained at a temperature of 50C. The ring is immersed to a depth such that its

bottom is exactly 1 in. above the bottom of the bath. The temperature of the bath

then is increased gradually, causing the asphalt to soften and permitting the ball to

eventually sink to the bottom of the bath. The temperature at which the asphalt

material touches the bottom of the bath is recorded as the softening point.

Details of this test are given

in the ASTM Designation

D36-95 and AASHTO T-53. It

consists principally of a

small brass ring of 5/8 in.

inside diameter and 1/4 in.

high, a steel ball 3/8 in. in

diameter, and a water or

glycerine bath.

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Ductility Test

Ductility is the number of centimetres a standard sample 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.

The test is used mainly for semisolid or solid materials which first are heated gently

to facilitate flow and then poured into a standard mold to form a briquette of at

least 1 cm2 in cross section. The material then is allowed to cool to 25oC in a water

bath. The prepared sample is placed in the ductility machine shown in Figure and

stretched at a rate of 5 cm/min until it breaks. The distance (in centimetres) moved

by the machine is the ductility of the material.

The result of this test indicates the extent to which the material can be deformed

without breaking. This is an important characteristic for asphalt materials

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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. Minimum flash point requirements are typically

incorporated into asphalt cement specifications for

safety reasons. However, a change in flash point may

indicate the presence of contaminants.

Flash-Point Test

The flash point of an asphalt material is the temperature at which its vapors will

ignite instantaneously in the presence of an open flame. Note that the flash point

normally is lower than the temperature at which the material will burn.

This property is determined using AASHTO T48 (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.

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.

In this procedure, an asphalt cement sample is dissolved in trichloroethylene and

then filtered through a glass-fiber pad where the weight of the insoluble

material is measured. The solubility is calculated by dividing the weight of the

dissolved portion by the total weight of the asphalt cement sample. This test is used

to check for contamination in asphalt cement. Most specifications require a

minimum of 99% solubility in trichloroethylene. Technicians should handling

trichloroethylene carefully as it is a carcinogen .

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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). In this procedure, a 50-g asphalt cement sample is placed on a cylindrical flat-

bottom pan to a depth of about 3.2 mm (0.125 in.). The pan is then placed on a shelf

that rotates at 5 to 6 rev/min per minute in a ventilated oven maintained at 1630C

for 5 h. The sample, considered short-term aged after it is removed from the oven,

is then tested in accordance with the specification requirements.

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Viscosity (AC) and Aged Residue (AR) Viscosity Grading Systems

In the 1960s, the FHWA, ASTM, and AASHTO wanted asphalts to be graded by

viscosity at 600C (1400F). The main reason was to replace the empirical penetration

test with a more fundamental material property (viscosity) and to measure this

property at a temperature which approximates the average pavement surface

temperature on a hot summer day.

Table 7.3 shows the viscosity grading requirements from ASTM D3381 Standard

Specification for Viscosity-Graded Asphalt Cement for Use in Pavement

Construction. This specification includes five viscosity grades ranging from a hard

asphalt graded at AC-40 to a soft asphalt cement graded at AC-2.5.

At approximately the same time, the California Department of Highway (now

Caltrans) was developing an aged residue viscosity grading specification. This

specification includes five AR viscosity grades ranging from a hard asphalt graded at

AR-160 to a soft asphalt cement graded at AR-10.

Viscosity can be defined as a fluids resistance to flow. In the asphalt paving

industry, two tests are used to measure viscosity absolute and kinematic viscosity

tests. The relationship between absolute and kinematic viscosity is shown below:

Absolute and Kinematic Viscosity Tests

Kinematic Viscosity = Absolute Viscosity / Density

Absolute viscosity is determined using AASHTO T202 (ASTM D2171) Standard

Method of Test for Viscosity of Asphalt by Vacuum Capillary Viscometer. In this

procedure, a partial vacuum pulls an asphalt cement sample through the viscometer

maintained at a temperature of 600C (1400F). Two marks on the viscometer indicate

the starting and ending time for the test. The time taken to pull the asphalt cement

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from the beginning point to the end point is recorded and compared against the

time for a fluid of known viscosity. The ratio in these two times is used to calculate

the viscosity of the asphalt.

Kinematic viscosity is determined using AASHTO T201 (ASTM D2170) Standard

Method of Test for Kinematic Viscosity of Asphalts (Bitumens). In this procedure,

an asphalt cement sample is allowed to flow between two timing marks within a

Zeitfuchs Cross-Arm viscometer under the force of gravity while the viscometer is

maintained at a temperature of 1350C (2750F). Two marks on the viscometer

indicate the starting and ending time for the test. The time taken to pull the asphalt

cement from the beginning point to the end point is recorded. The kinematic

viscosity in units of centistokes is calculated by multiplying this time with the

calibration factor supplied by the viscometer tubes manufacturer (see Figure).

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lec 14 highway engineering - asphalt types tests and specifications

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