Tests on Bitumen

Experience in using bitumen in engineering projects has led to the adoption of certain test procedures that are indicative of the characteristics that identify adequate performance levels. Some of the tests have evolved with the development of the industry and are empirical methods. Consequently it is essential that they are carried out in strict compliance with the recommended procedures if they are to be accurate measurements of the bitumen's properties.

1. Penetration Test

2. Flash Point Test

3. Solubility Test

4. Ductility Test

5. Viscosity Test

6. TFOT test

Test 1. Penetration Test on Bitumen

The penetration test is one of the oldest and most commonly used tests on asphalt cements or residues from distillation of asphalt cutbacks or emulsions. The standardized procedure for this test can be found in ASTM D5 [ASTM, 2001]. It is an empirical test that measures the consistency (hardness) of an asphalt at a specified test condition.

Procedure of Penetration Test on Bitumen:

In the standard test condition, a standard needle of a total load of 100 g is applied to the surface of an asphalt or Liquid bitumen sample at a temperature of 25 °C for 5 seconds. The amount of penetration of the needle at the end of 5 seconds is measured in units of 0.1 mm (or penetration unit). A softer asphalt will have a higher penetration, while a harder asphalt will have a lower penetration. Other test conditions that have been used include

1. 0 °C, 200 g, 60 sec., and

2.46 °C, 50 g, 5 sec.

The penetration test can be used to designate grades of asphalt cement, and to measure changes in hardness due to age hardening or changes in temperature.

Read a detailed procedure of Bitumen Penetration Test with Sample Data

Test 2. Flash Point Test on asphalt:

The flash point test determines the temperature to which an asphalt can be safely heated in the presence of an open flame. The test is performed by heating an asphalt sample in an open cup at a

specified rate and determining the temperature at which a small flame passing over the surface of the cup will cause the vapors from the asphalt sample temporarily to ignite or flash. The commonly used flash point test methods include

1.The Cleveland Open Cup (ASTM D92)

2.Tag Open Cup (ASTM D1310).

The Cleveland Open-Cup method is used on asphalt cements or asphalts with relatively higher flash points, while the Tag Open-Cup method is used on cutback asphalts or asphalts with flash points of less than 79 °C. Minimum flash point requirements are included in the specifications for asphalt cements for safety reasons. Flash point tests can also be used to detect contaminating materials such as gasoline or kerosine in an asphalt cement. Contamination of an asphalt cement by such materials can be indicated by a substantial drop in flash point.

When the flash point test is used to detect contaminating materials, the Pensky-Martens Closed Tester method (ASTM D93), which tends to give more indicative results, is normally used. In recent years, the flash point test results have been related to the hardening potential of asphalt. An asphalt with a high flash point is more likely to have a lower hardening potential in the field.

Read a detailed procedure of Bitumen Penetration Test with Sample Data

Test 3. Solubility Test on asphalt bitumen

Asphalt consists primarily of bitumens, which are high-molecular-weight hydrocarbons soluble in carbon disulfide. The bitumen content of a bituminous material is measured by means of its solubility in carbon disulfide.

Procedure for Solubility test on Bitumen

In the standard test for bitumen content (ASTM D4), a small sample of about 2 g of the asphalt is dissolved in 100 ml of carbon disulfide and the solution is filtered through a filtering mat in a filtering crucible. The material retained on the filter is then dried and weighed, and used to calculate the bitumen content as a percentage of the weight of the original asphalt. Due to the extreme flammability of carbon disulfide, solubility in trichloroethylene, rather than solubility in carbon disulfide, is usually used in asphalt cement specifications. The standard solubility test using trichloroethylene is designated as ASTM D 2042.

The solubility test is used to detect contamination in asphalt cement. Specifications for asphalt cements normally require a minimum solubility in trichloroethylene of 99.0 percent.

Unfortunately, trichloroethylene has been identified as a carcinogen and contributing to the depletion of the earth’s ozone layer. The use of trichloroethylene will most likely be banned in the near future. There is a need to use a less hazardous and non-chlorinated solvent for this purpose. Results of several

investigations have indicated that the solvent n-Propyl Bromide appears to be a feasible alternative to trichloroethylene for use in this application.

Test 4. Ductility Test on Asphalt

The ductility test (ASTM D113) measures the distance a standard asphalt sample will stretch without breaking under a standard testing condition (5 cm/min at 25 °C). It is generally considered that an asphalt with a very low ductility will have poor adhesive properties and thus poor performance in service. Specifications for asphalt cements normally contain requirements for minimum ductility.

Read a detailed procedure of Bitumen Penetration Test with Sample Data

Test 5. Viscosity Tests on Bitumen Asphalt

The viscosity test measures the viscosity of an asphalt. Both the viscosity test and the penetration test measure the consistency of an asphalt at some specified temperatures and are used to designate grades of asphalts. The advantage of using the viscosity test as compared with the penetration test is that the viscosity test measures a fundamental physical property rather than an empirical value. Viscosity is defined as the ratio between the applied shear stress and induced shear rate of a fluid.

When shear rate is expressed in units of 1/sec. and shear stress in units of Pascal, viscosity will be in units of Pascal-seconds. One Pascal-second is equal to 10 Poises. The lower the viscosity of an asphalt, the faster the asphalt will flow under the same stress. For a Newtonian fluid, the relationship between shear stress and shear rate is linear, and thus the viscosity is constant at different shear rates or shear stress. However, for a non-Newtonian fluid, the relationship between shear stress and shear rate is not linear, and thus the apparent viscosity will change as the shear rate or shear stress changes.

Asphalts tend to behave as slightly non-Newtonian fluids, especially at lower temperatures. When different methods are used to measure the viscosity of an asphalt, the test results might be significantly different, since the different methods might be measuring the viscosity at different shear rates. It is thus very important to indicate the test method used when viscosity results are presented.

The most commonly used viscosity test on asphalt cements is the Absolute Viscosity Test by Vacuum Capillary Viscometer (ASTM D2171).

The standard test temperature is 60 °C. The absolute viscosity test measures the viscosity in units of Poise. The viscosity at 60 °C represents the viscosity of the asphalt at the maximum temperature a pavement is likely to experience in most parts of the U.S. When the viscosity of an asphalt at a higher temperature (such as 135 °C) is to be determined, the most commonly-used test is the Kinematic Viscosity Test (ASTM D2170), which measures the kinematic viscosity in units of Stokes or centi-Stokes.

Kinematic viscosity is defined as: When viscosity is in units of Poise and density in units of g/cm, 3 the kinematic viscosity will be in units of Stokes. To convert from kinematic viscosity (in units of Stokes) to

absolute viscosity (in units of Poises), one simply multiplies the number of Stokes by the density in units of g/cm3.

Test-6: Rolling Thin-Film Oven

Overview

The Rolling Thin-Film Oven (RTFO) procedure (Figure 1) provides simulated short term aged

asphalt binder for physical property testing. Asphalt binder is exposed to elevated temperatures to

simulate manufacturing and placement aging. The RTFO also provides a quantitative measure of the

volatiles lost during the aging process.

Figure 1. Rolling thin-film oven test.

The basic RTFO procedure takes unaged asphalt binder samples in cylindrical glass bottles and

places these bottles in a rotating carriage within an oven. The carriage rotates within the oven while

the 325°F (163°C) temperature ages the samples for 85 minutes. Samples are then stored for use in

physical properties tests or the PAV.

The standard Rolling Thin-Film Oven test is:

AASHTO T 240 and ASTM D 2872. Effect of Heat and Air on a Moving Film of Asphalt

(Rolling Thin-Film Oven Test)

Background

One of the basic tenets of the Superpave PG binder specification is that tests should be as closely

tied with field performance as possible. Seeing as the constituent asphalt binder in HMA undergoes

significant aging during the manufacturing and placement processes (Figure 2), a method to

simulate the aging is important in investigating and predicting early age HMA pavement behavior

and distresses. Specifically, the Superpave PG binder specification calls for short term aged asphalt

binder to be tested at high temperatures to determine fatigue and rut resistance.

Figure 2: HMA placement.

Asphalt Binder aging by Loss of Volatiles

Although many different factors contribute to asphalt binder aging, the key component of concern for

the RTFO is the loss of volatiles. The loss of smaller molecules from the asphalt binder, often termed

“volatiles” increases an asphalt’s viscosity.

Loss of Volatiles Occurrence

Asphalt binders typically lose volatiles during the manufacturing and placemen processes. The

elevated temperature of these processes ages the asphalt binder by driving off a substantial amount

of volatiles. Field tests have shown that in-place asphalt binder does not lose a significant amount of

volatiles over its life (Corbett and Merz, 1975[1]).

Aging Simulation

The RTFO aging procedure is used to simulate aging during mixing and placement, while the PAV

aging procedure is used to simulate aging during in-service life. Therefore, asphalt binder tests

concerned with mix and placement properties (such as the DSR) are conducted on RTFO aged

samples, while asphalt binder tests concerned with in-service performance (such as the DSR, BBR

and DTT) are performed on samples first aged in the RTFO and then in the PAV.

RTFO Aging of Modified Asphalt Binders

The RTFO has problems with highly viscous binders (e.g., some polymer modified asphalt binders

and PG 70-XX and higher) because they do not flow properly in the bottles as they are rotated.

Test Origins

The RTFO was developed as an improvement to the Thin-Film Oven Test (TFOT) for short term

asphalt binder aging. The TFOT placed asphalt binder samples in shallow pans (of the same

dimensions as those used for the PAV) and then heated them in an oven for an extended period of

time to accomplish simulated aging. The RTFO is an improvement over the TFOT because:

Fresh asphalt binder is continuously exposed to heat and air flow due to the rolling action of

the carousel (Video 1).

Video 1: RTFO rotating. Notice the small air nozzle on the bottom blowing on the bottles as they

rotate by.

Asphalt binder modifiers, if used, usually remain dispersed in the asphalt binder due to the

rolling action of the carousel (Figure 3).

Figure 3: RTFO bottles. The bottle at left is after the RTFO test, the bottle in the middle is

before the test.

No surface skin, which inhibits aging, forms on the asphalt binder because of the rolling

action of the carousel.

The test is reasonably short at 85 minutes.

Test Description

The following description is a brief summary of the test. It is not a complete procedure and should

not be used to perform the test. The complete procedure can be found in:

AASHTO T 240 and ASTM D 2872: Effect of Heat and Air on a Moving Film of Asphalt

(Rolling Thin-Film Oven Test)

Summary

Unaged asphalt binder is placed in a cylindrical jar, which is then placed in a carousel inside a

specially designed oven. The oven is heated to 325°F (163°C) and the carrousel is rotated at 15

RPM for 85 minutes. The carousel rotation continuously exposes new asphalt binder to the heat and

air flow and slowly mixes each sample. Figure 4 shows major RTFO equipment.

Figure 4: RTFO equipment.

Approximate Test Time

3 hours from sample preparation to scraping of final bottle.

Basic Procedure

1. Heat a sample of asphalt binder until it is fluid to pour. Stir sample to ensure homogeneity

and remove air bubbles.

2. If a determination of mass change is desired, label two RTFO bottles and weigh them

empty. These are designated as the “mass change” bottles. Record the weights.

3. Pour 1.23 oz (35 g) of asphalt binder into each bottle (Figure 5). Immediately after pouring

each bottle, turn the bottles on their side without rotating or twisting and place them on a

cooling rack.

Figure 5. Pouring hot asphalt binder into the RTFO bottle

4. Allow all bottles to cool 60 to 180 minutes.

5. After cooling, weigh the two mass change bottles again. Record the weights.

6. Place the bottles in the RTFO oven carousel (Video 2), close the door, and rotate

carousel at 15 RPM for 85 minutes . During this time, maintain the oven temperature at

325°F (163°C) and the airflow into the bottles at 244 in3/min (4000 ml/min).

Video 2: Inserting a bottle in the RTFO.

1. Remove the bottles one at a time from the carousel, setting the mass change bottles

aside. Residue from the remaining bottles should be transferred to a single container.

Remove residue from each bottle by first pouring as much material as possible, then

scraping the sides of the bottle to remove any remaining residue (Video 3). There is no

standard scraping utensil but at least 90 percent of the asphalt binder should be removed

from the bottle. RTFO residue should be tested within 72 hours of aging.

Video 3: Scraping the asphalt residue out of a RTFO bottle.

WARNING

Any scraping tool may be used but an average of 90% of the residue must be removed from the

sample bottles. It has been found that circumferential scraping is usually more effective than

lengthwise scraping.

1. After cooling the two mass change bottles for 60 – 180 minutes, weigh them and discard

their residue. Record the weights.

Results

Parameters Measured

Mass change of a sample as a percent of initial mass.

The RTFO is primarily used to simulate short term asphalt binder aging for use in other tests.

Specification

Table 1: Performance Graded Asphalt Binder RTFO Specification

Material Value Specification Property of Concern

Unaged binder Mass loss1 ≤ 1.0% None

Note 1

Although some samples can gain weight due to the oxidative products formed during the test

(Roberts et al., 1996[2]), there is currently no limit on mass gain.

Typical Values

Typical mass loss is in the range of 0.05 to 0.5 percent.

Footnotes (↵ returns to text)

1. Corbett, L.W. and Merz, R.E. (1975). Asphalt Binder Hardening in the Michigan Test Road After 18 Years of

Service. Transportation Research Record 544. Transportation Research Board, National Research Council,

Washington, D.C. pp. 27-34.↵2. Roberts, F.L.; Kandhal, P.S.; Brown, E.R.; Lee, D.Y. and Kennedy, T.W. (1996). Hot Mix Asphalt Materials,

Mixture Design, and Construction. National Asphalt Pavement Association Education Foundation. Lanham,

MD.↵

3. Asphalt Air Blowing Process Pilot Test Plant

The physical properties of asphalts may further be modified by 'air blowing'. This is an oxidation process which involves the blowing of air through the asphalts, either on a batch or a continuous basis, with the short residue at a temperature of 240°C to 320°C.

Description of the continuous blowing process

After preheating, the short residue is introduced into the blowing column

just below the normal liquid level. Air is blown through the bitumen by

means of an air distributor located at the bottom of the column. The air is

not only the reactant but also serves to agitate and mix the bitumen,

thereby increasing the surface area and rate of reaction. Oxygen is

consumed by the bitumen as the air ascends through the material. Steam

and water are sprayed into the vapour space above the bitumen

level, the former to suppress foaming and dilate the oxygen content of

waste gases and the latter cools the vapours to prevent after-burning. The "blown" product flows from the bottom

of the blowing column into a surge drum via an external draw-off line. In

this way the minimum level of product in the blowing column is controlled.

From the surge drum the blown product is passed through heat

exchangers to achieve the desired "rundown" temperatures and to

provide an economical means of preheating the short residue, before

pumping the product to storage. The penetration and softening point of the

blown bitumen are affected by:

model registered ©

* the viscosity of the feedstock* the temperature in the blowing column* the residence time in the blowing column* the origin of the crude oil used to manufacture the feedstock * the air-to-feed ratio

The blowing process dehydrogenates the short residue, resulting in oxidation and polycondensation,

increasing the overall molecular size of the asphaltenes already present in the feed and forming additional asphaltenes from the maltene phase. The reaction is exothermic. Therefore close temperature control of this process is required, which is achieved by regulating the air-to-short residue ratio in the blowing column.

Air-rectified or semi-blown bitumens

To produce penetration grade bitumens suitable for road construction, bitumens manufactured from some crudes require a limited amount of air blowing. This process is termed semi-blowing or air rectification. Used judiciously, semi-blowing can be applied to reduce the temperature susceptibility of the bitumen, ie increase its penetration index.

Fully blown bitumens

Fully blown or oxidised bitumens are produced by vigorous air-blowing of short residue or short residue blended with a heavy distillate. The position of the blowing curve is primarily dependent on the viscosity of the feed, ie the softer the feed the higher the curve. The severity of blowing depends on the temperature in the column and to a lesser extent on the residence time. Thus by controlling the viscosity of the feed and the conditions in the column all the blown grades of bitumen can be manufactured.

Oxidized bitumens

Oxidized bitumens are used almost entirely for industrial applications, eg roofing, flooring mastics, pipe coatings, paints, etc, and are specified and designated by both softening point and penetration tests, eg 85/40 is an oxidized grade bitumen with a softening point of 85+-5 ºC and a penetration of 40 +- 5.

The chemistry of the blowing process

The aim of the blowing process is theformation of asphaltenes. Three phenomena can be identified:

* Reactions during which the size of the molecules increases; formation of esters is particularly important; they not only account for about 60% of the oxygen in blown bitumen but also link up two different molecules and thus contribute to the formation of material of higher molecular weight; this mechanism results in an increase in the asphaltene content and a change in the colloid-chemical constitution and rheological properties of the bitumen

* Reactions during which the size of the molecule is unchanged; formation of cyclic hydrocarbons by means of dehydrogenation with H2O as a side product

* Reactions during which the size of the molecule decreases; separation of side branches from the molecules with blown distillate produced as a side product.

Spot Test

Publish date: August 16, 2007 | Author: Pavement Interactive

The spot test is used to determine whether or not an asphalt cement has been damaged during

processing due to overheating. This damage, called “cracking”, occurs because the actual molecules

are thermally broken apart. Cracked asphalt cements tend to be less ductile and more susceptible to

aging effects. Since modern refining practices rarely cause cracking, the spot test is not often

specified.

Basically, the spot test is a form of paper chromatography (a method for analyzing complex mixtures

by separating them into the chemicals from which they are made). A small drop of prepared asphalt

cement is dropped onto a filter paper. If the spot formed is uniformly brown then the test is negative.

If the spot formed is brown with a black center then the test is positive. Today, the spot test is rarely

used.