Highway materials

Basic road construction materials includes soils, aggregates, bitumen and Portland cement

Bitumen is defined as an amorphous, black or dark-colored, (solid, semi-solid, or viscous) cementitious substance ,composed principally of high molecular weight hydrocarbons, and soluble in carbon disulfide.

For civil engineering applications, bituminous materials include primarily asphalts and tars

Bitumen

Asphalts may occur in nature (natural asphalts) or may be obtained from petroleum processing (petroleum asphalts). Tars do not occur in nature and are obtained as condensates in the processing of coal, petroleum, oil-shale, wood or other organic materials

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Asphalt binders are most commonly characterized by their physical properties.  An asphalt binder’s physical properties directly describe how it will perform as a constituent in HMA pavement.

Physical Properties

Durability is a measure of how asphalt binder physical properties change with age (sometimes called age hardening).  In general, as an asphalt binder ages, its viscosity increases and it becomes more stiff and brittle. 

Durability

Age hardening is a result of a number of factors, the principal ones being Oxidation.  The reaction of oxygen with the

asphalt binder. Volatilization.  The evaporation of the lighter

constituents of asphalt binder.  It is primarily a function of temperature and occurs principally during HMA production.

Polymerization.  The combining of like molecules to form larger molecules.  These larger molecules are thought to cause a progressive hardening.

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Separation.  The removal of the oily constituents, resins or asphaltenes from the asphalt binder by selective absorption of some porous aggregates.

There is no direct measure for asphalt binder aging.  Rather, aging effects are accounted for by subjecting asphalt binder samples to simulated aging then conducting other standard physical tests

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Asphalt binder aging is usually split up into two categories

Short-term aging.  This occurs when asphalt binder is mixed with hot aggregates in an HMA mixing facility.

Long-term aging.  This occurs after HMA pavement construction and is generally due to environmental exposure and loading.

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Typical aging simulation tests are: Thin-film oven (TFO) test Rolling thin-film oven (RTFO) test Pressure aging vessel (PAV)

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RTFO

PAV

Rheology is the study of deformation and flow of matter.  Deformation and flow of the asphalt binder in HMA is important in determining HMA pavement performance.  HMA pavements that deform and flow too much may be susceptible to rutting and bleeding, while those that are too stiff may be susceptible to fatigue or thermal cracking.  HMA pavement deformation is closely related to asphalt binder rheology. 

Rheology

Penetration test following basic procedure: Melt and cool the asphalt binder sample under controlled

conditions. Measure the penetration of a standard needle into the asphalt

binder sample under the following conditions: Load = 100 grams Temperature = 25° C (77° F) Time = 5 seconds The depth of penetration is measured in units of 0.1 mm and

reported in penetration units (e.g., if the needle penetrates 8 mm, the asphalt penetration number is 80).  Penetration grading is based on the penetration test.

The standard penetration test is:AASHTO T 49 and ASTM D 5: Penetration of Bituminous Materials

Penetration Test

The softening point is defined as the temperature at which a bitumen sample can no longer support the weight of a 3.5-g steel ball. Basically, two horizontal disks of bitumen, cast in shouldered brass rings , are heated at a controlled rate in a liquid bath while each supports a steel ball. The softening point is reported as the mean of the temperatures at which the two disks soften enough to allow each ball, enveloped in bitumen, to fall a distance of 25 mm (1.0 inch) (AASHTO, 2000).

The standard softening point test is:AASHTO T 53 and ASTM D 36: Softening Point of Bitumen (Ring-and-Ball Apparatus)

Softening Point

The ductility test measures asphalt binder ductility by stretching a standard-sized briquette of asphalt binder to its breaking point.  The stretched distance in centimeters at breaking is then reported as ductility. 

The standard ductility test is: AASHTO T 51 and ASTM D 113: Ductility of

Bituminous Materials

Ductility Test

The rotational viscometer (RV) is used in the Superpave system to test high temperature viscosities (the test is conducted at 135° C (275° F)).  The basic RV test measures the torque required to maintain a constant rotational speed (20 RPM) of a cylindrical spindle while submerged in an asphalt binder at a constant temperature.  This torque is then converted to a viscosity and displayed automatically by the RV.

Rotational (or Brookfield) Viscometer (RV)

Asphalt cement like most other materials, volatilizes (gives off vapor) when heated.  At extremely high temperatures (well above those experienced in the manufacture and construction of HMA) asphalt cement can release enough vapor to increase the volatile concentration immediately above the asphalt cement to a point where it will ignite (flash) when exposed to a spark or open flame.  This is called the flash point. For safety reasons, the flash point of asphalt cement is tested and controlled.

The fire point, which occurs after the flash point, is the temperature at which the material (not just the vapors) will sustain combustion.

Safety Tests

A typical flash point test involves heating a small sample of asphalt binder in a test cup.  The temperature of the sample is increased and at specified intervals a test flame is passed across the cup.  The flash point is the lowest liquid temperature at which application of the test flame causes the vapors of the sample to ignite. The test can be continued up to the fire point – the point at which the test flame causes the sample to ignite and remain burning for at least 5 seconds.

Standard flash point tests are: AASHTO T 48 and ASTM D 92: Flash and Fire Points by

Cleveland Open Cup (more common for asphalt cement used in HMA)

AASHTO T 73 and ASTM D 93: Flash-Point by Pensky-Martens Closed Cup Tester

Fire and flash point

Penetration Grading Viscosity Grading Superpave Performance Grade (PG)

Grading Systems

The penetration grading system was developed in the early 1900s to characterize the consistency of semi-solid asphalts.  Penetration grading quantifies the following asphalt concrete characteristics:

Penetration depth of a 100 g needle 25° C (77° F) Flash point temperature Ductility at 25° C (77° F) Solubility in trichloroethylene Thin-film oven test(accounts for the effects of short-term aging)

Penetration grading’s basic assumption is that the less viscous the asphalt, the deeper the needle will penetrate. This penetration depth is empirically correlated with asphalt binder performance. 

Penetration Grading

Therefore, asphalt binders with high penetration numbers (called "soft") are used for cold climates while asphalt binders with low penetration numbers (called "hard") are used for warm climates.

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In the early 1960s an improved asphalt grading system was developed that incorporated a rational scientific viscosity test.  This scientific test replaced the empirical penetration test as the key asphalt binder characterization.  Viscosity grading quantifies the following asphalt binder characteristics:

Viscosity at 60° C (140° F) Viscosity at 135° C (275° F) Penetration depth of a 100 g needle applied for 5 seconds at

25° C (77° F) Flash point temperature Ductility at 25° C (77° F) Solubility in trichloroethylene Thin film oven test(accounts for the effects of short-term

aging)

Viscosity Grading

Viscosity is measured in poise (cm-g-s = dyne-second/cm2, named after Jean Louis Marie Poiseuille). The lower the number of poises, the lower the viscosity and thus the more easily a substance flows.  Thus, AC-5 (viscosity is 500 ± 100 poise at 60° C (140° F)) is less viscous than AC-40 (viscosity is 4000 ± 800 poise at 60° C (140° F)).

Viscosity:

The previous grading systems are somewhat limited in their ability to fully characterize asphalt binder for use in HMA pavement.  Therefore, as part of the Superpave research effort new binder tests and specifications were developed to more accurately and fully characterize asphalt binders for use in HMA pavements.  These tests and specifications are specifically designed to address HMA pavement performance parameters such as rutting, fatigue cracking and thermal cracking.

Superpave Performance Grade (PG)

Superpave performance grading (PG) is based on the idea that an HMA asphalt binder’s properties should be related to the conditions under which it is used.  For asphalt binders, this involves expected climatic conditions as well as aging considerations.  Therefore, the PG system uses a common battery of tests (as the older penetration and viscosity grading systems do) but specifies that a particular asphalt binder must pass these tests at specific temperatures that are dependant upon the specific climatic conditions in the area of use.

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Superpave performance grading uses the following asphalt binder tests: Rolling thin film oven (RTFO) Pressure aging vessel (PAV) Rotational viscometer (RV) Dynamic shear rheometer (DSR) Bending beam rheometer (BBR) Direct tension tester (DTT)

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Superpave performance grading is reported using two numbers – the first being the average seven-day maximum pavement temperature (°C) and the second being the minimum pavement design temperature likely to be experienced (°C).

Thus, a PG 58-22 is intended for use where the average seven-day maximum pavement temperature is 58°C and the expected minimum pavement temperature is -22°C.  Notice that these numbers are pavement temperatures and not air temperatures

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