Winter 1995
Vol. 59· No. 2

The Nuclear Asphalt Content Gauge

by Kevin N. Black

Introduction

About 93 percent of the 3.4 million kilometers of paved roads in the United States are asphalt, and accurate monitoring of the asphalt cement content of these roads is important to ensure that the mix design-tolerance is met. Improper mix designs and amounts of asphalt cement are major causes of premature failure in pavement structures. If cement content is too low, a pavement will crack; if the content is too high, rutting and shoving will result.

Asphalt concrete mixes are composite materials consisting of aggregate, asphalt, air, and other components that collectively develop structural characteristics capable of supporting highway traffic. One of the critical factors in the design of these mixes is the proportion of the asphalt cement component. Many tests are used to evaluate the properties of mix designs, such as voids (air content), asphalt cement content, gradation, voids in mineral aggregate, stiffness, and other properties to ensure the quality of the mix design.

There are three major methods for determining asphalt content:

• Solvent extraction.
• Nuclear asphalt content gauge.
• Metering methods (automatic recordation, tank stabs).

These methods can generally be divided into direct tests and indirect tests.

Direct tests measure the actual property of interest in a sample -- in this case, the asphalt cement content. They are destructive or degradative since they determine the amount of asphalt by decomposing the mix into its individual components. Although they are usually very accurate, they are time-consuming and use hazardous solvents. In fact, the environmental hazards and restrictions associated with these chemicals are prompting additional interest in other analysis methods.

Indirect tests usually measure some other property that can be correlated with the desired property. They typically reduce the time needed to get an answer and often are used when actual property measurements are difficult or costly to make.

In the case of asphalt cement content, there is an important difference between direct and indirect methods. The direct method, using an extraction test, allows gradation and asphalt cement content to be determined from the same sample. The indirect tests only provide the asphalt content; therefore, gradation tests must be performed separately. Although two separate indirect tests are required to replace the extraction method, they yield similar accuracy, can be performed more quickly, and have minimal effect on the environment.

This article discusses one specific indirect test, the nuclear test, in an effort to provide accurate information on its abilities and its limitations and to encourage its use. For more information about asphalt content determination, refer to the Federal Highway Administration publication entitled, Asphalt Content Determination Manual. (1)

Background

The direct tests for determining asphalt content are classified as chemical techniques and are commonly known as solvent extraction methods. In these tests, asphalt cement content is determined using solvents such as methylene chloride, trichloroethylene or 1,1,1-trichloroethane to separate the asphalt cement from the aggregate. These chemicals are volatile organic compounds that create health, environmental, and disposal problems. These methods are being reevaluated by various states because of environmental and health concerns and because greater restrictions are being placed on their use.

For the past 15 years, the highway community has been evaluating the use of indirect test methods since they do not present the environmental and health concerns attributable to the solvent extraction methods. Indirect tests for determining asphalt mix properties include metering methods and the nuclear method.

Automatic recordation -- the accurate measuring of the amount of asphalt cement used in the batching process -- can be measured in the following three ways:

• Meter readings (instantaneous value for part of a batch).
• Weigh-bucket readings (average of total batch).
• AC storage tank volume readings (average of many batches).

The assumption is made that the amount of asphalt cement measured using one of these techniques (essentially by weight or volume) was properly blended with the aggregate to produce the required mix design. These methods have been in use for more than 25 years. However, they are not fully supported by all users because it is not clear that all the asphalt coated the aggregate properly. The testing frequency may also be insufficient to catch variations in production.

The nuclear asphalt content gauge is an indirect test that determines the amount of asphalt by measuring the amount of hydrogen in the mix. This method has been gaining support as states become more confident with its operation.

Table 1 lists a comparison of the advantages and disadvantages of the current methods used in asphalt content determination.

Table 1 -- Advantages/Disadvantages of Methods to Determine Asphalt Cement Content

Theory

The traditional, direct test methods represent a macroscopic evaluation of the asphalt contribution to a mix.

These methods compute the asphalt cement content based on the ratio of the mass (weight) of asphalt to the mass of the sample.

The nuclear method represents a microscopic analysis with the measurements based on the actual number of hydrogen atoms contained within the mix. Its principle of operation is the same as that used for moisture-content determinations in nuclear moisture-density gauges employed in construction. Neutrons are transmitted into the asphalt mix, and their movement through the material is influenced by the hydrogen composing or surrounding the aggregate. Usually, the source of hydrogen is water -- for example, when making moisture determinations on soils. In asphalt-content evaluations, the source of the hydrogen is predominantly the asphalt cement, but it can also be water in the mix and hydrogen contained within minerals in the aggregate.

During the calibration process, samples of the material are tested to establish a relationship between counts and asphalt content. The nuclear gauge "counts" the neutrons influenced by the hydrogen as they pass through the sample, establishing the hydrogen versus neutron count relationship depicted in figure 1. Correlating these counts to asphalt content using mathematical regression techniques permits the evaluation of asphalt content for any sample having the same mix design. This is the basis by which the nuclear asphalt content gauge relates the hydrogen content, gauge counts, and the asphalt cement content.

Equipment

Currently, there are three gauges on the market for determining the asphalt content. Two are made by Troxler Corporation, Models 3241-C and 3242, and one by CPN Inc., Model AC-2. The other nuclear density gauge manufacturers, Seaman and Humboldt, do not currently offer asphalt content gauges.

These gauges consist of a control unit, sample chamber, and specimen pan. Within the control unit are the electronics, including a microprocessor programmed to compute the asphalt content based on the theory described above. The sample chamber contains the nuclear source and the detector tubes. It is also where the specimen pan containing the uncompacted sample is exposed to the neutron source. Figure 2 illustrates the typical gauge components.

One gauge manufacturer has developed a compact accessory tray that permits asphalt content to be determined on a compacted laboratory specimen, rather than on uncompacted material. Another optional feature provides a means for transferring the calibration from one gauge to another. These additional features may reduce the chance of certain errors, but they do not necessarily provide faster or more consistent results.

Sample Preparation

The procedure to accurately perform the test is described by American Association of State Highway and Transportation Officials (AASHTO) T-287 and American Society for Testing and Materials (ASTM) D 4125. In addition to these tests, the moisture content must also be evaluated, using AASHTO T-110, to ensure that moisture makes no contribution to the hydrogen count. Each state department of transportation must make certain that the tests are followed as prescribed to ensure repeatability of the results.

The material can either be heated to remove moisture by drying in accordance with AASHTO T-255 or extracted as per AASHTO T-110. Drying can be done using a microwave oven if the aggregate is not highly absorptive. The length of time for heating will vary, but it should not exceed 30 minutes. The sample temperature should not be allowed to go higher than 110 degrees Celsius according to AASHTO T-287. Determination of the moisture content by extraction will yield a percentage that can then be subtracted from the asphalt content reading to provide a corrected asphalt content.

The sample size may vary according to the gauge, but generally, the sample should fill the test pan as illustrated in figure 2. Typically, the amount of material required is about 7000 grams. Since the volume is constant, owing to the pan's size, the mass should be very close to the manufacturer's recommended mass to ensure that the proper compacted density is achieved for each sample. Consistent preparation of samples with respect to mass, density, and temperature is essential to ensure accuracy and repeatability.

Calibration of Gauge

Calibration is conducted using the aggregate and asphalt that will be used in a specific mix design. Each calibration is only valid for the one mix design on which it was made. Changes in mix design, sources of aggregate, or sources of asphalt cement will require recalibration of the instrument. The importance of proper calibration cannot be overstated. If the gauge is not calibrated using the procedures described in the operator's manual, the measured asphalt content will not be accurate.

The operator performs the calibration using a minimum of three tests, each test using a known but different asphalt content. Specifically, one test should have the asphalt content at 1 percent below the mix design target, one at the design asphalt content, and one 1 percent above the target. Each sample should be tested for a 16-minute count to obtain the maximum precision. From a plot of this count versus the asphalt content data, the gauge uses a regression analysis to determine the equation of the curve best describing that material. The relationship is then stored in memory and used to compute the asphalt content of other samples tested. More specific instructions can be found in the gauge operator's manuals or in AASHTO T-287 or ASTM D 4125, which outline the calibration procedure.

It should be noted that a "blank" sample (dry, hot aggregate, without asphalt) must be placed in the gauge and measured. This will establish a baseline for the aggregate, and it can also be helpful if questionable readings are obtained during the testing process.

Test Results

The nuclear asphalt content gauge can be used for evaluating the asphalt cement content in many types of bituminous mixtures, including:

• Hot mix (surface and binder).
• Recycled asphalt pavement (RAP).
• Surface treatments.
• Material containing asphalt additives and modifiers.

Several states have undertaken testing programs to validate the use of the nuclear gauge in asphalt content determinations. (1) Most tests have been very positive by indicating a good correlation with extraction tests. Based on these and other continuing test programs, states are beginning to accept the results from nuclear asphalt content gauges for determining asphalt content of binder and surface mixes. Table 2 contains a list of states and their method for measuring asphalt cement. (1)

Table 2 -- Asphalt Content Determination Methods Used by States

Tests of asphalt cements containing additives and certain treatments have been done; however, additional tests are still required before the use of the gauge on these products may gain complete acceptance. Material additives requiring further evaluation include mixes containing polymers and anti-strip agents.

Other problems and sources of error can all be eliminated or compensated for if proper instruction is provided to the technician. Improper operation of the gauge may occur when an operator is either ignorant of the factors that contribute to erroneous measurements or fails to follow the prescribed procedures for using the gauge. As an example, corrective measures may only require moving a gauge to a relatively isolated area a few meters from any object known to have a high hydrogen content.

Many of the factors that contribute to inaccuracies in the asphalt content gauge also cause errors in nuclear moisture-density gauges. Since most states use moisture-density gauges, operators should already be familiar with many of the causes of errors.

Table 3 -- Likely Sources of Error in Measuring Asphalt Content With Nuclear Gauge

Licensing

State departments of transportation are required to maintain strict accounting methods for gauge ownership, training, use, maintenance, and disposal under Nuclear Regulatory Commisssion (NRC) guidelines. States are usually licensed by the NRC although some state governments, known as agreement states, have been given the authority by the NRC to regulate control of nuclear gauges and other sources of low-level radiation within their states. Licensing programs generally consist of the following:

• Radiation safety program.
• Personnel monitoring using a dosimeter, TLD (Thermoluminescent Dosimeter film badges).
• Security.
• Storage.
• Periodic leak testing.
• Training.

Some gauges use a Californium-252 isotope that emits low amounts of radiation, and the gauges have other design features that remove the requirement for a license. Only one gauge manufacturer currently makes this type of gauge.

Safety

Although exposure to radiation is always a safety concern, nuclear gauges have been used in the highway industry for decades with no documented evidence of either environmental or health problems. Safety courses are given to all users and radiation exposure histories should be continually monitored.

The millirem represents the amount of radiation absorbed by the body, and it is the unit used in reporting exposure levels found on the radiation badges or dosimeters worn by gauge operators. These units represent the measurement of radiation absorbed rather than the amount emitted. For reference purposes, one chest x-ray, typically given in medical examinations, is 40 millirems, and the typical exposure level for nuclear gauge operators is between 100 and 200 millirems per year.

Economics

Although the primary reason for promoting the use of the nuclear gauge is environmental, there are also economic incentives. Several factors must be considered when doing the economic analysis. These include:

• Size of the project (number of tests to be performed).
• Material costs (including equipment).
• Labor costs.

Some projects may be too small for the gauge to be cost-effective because of the time required for calibration. Calibration may require hours and must be done for each new mix (i.e., for base, binder, and surface). Most jobs require many tests, and as seen in Table 4, the cost of using the nuclear gauge becomes attractive as the number of tests required increases. (2) If a mix will be in production more than one day, the use of the nuclear asphalt content gauge can almost always be justified. For fewer than three tests per mix type, it probably would never be cost-effective to use nuclear gauges.

Table 4 -- Comparative Costs

While extraction equipment typically costs $1,500 or more, new nuclear gauges currently cost between $5,000 and $6,000. Although initial costs tend to favor the extraction method, the greatest costs associated with extraction procedures are those associated with the purchase and disposal of chemicals for each test. For a laboratory conducting a significant number of tests, the "per test" cost for nuclear gauge testing is considerably lower than for extraction tests.

Summary

This article discusses one of several new instruments and techniques being incorporated into highway construction, maintenance, and testing programs. It describes the equipment and its operation in addition to licensing, safety, and economic factors that support its use. It also addresses the gauge's more favorable environmental operation over traditional methods.

This method is both accurate and cost-effective. Since environmental concerns are making chemical methods increasingly costly, the use of the nuclear asphalt content gauge should be encouraged.

Some extractions must still be incorporated into the testing plan to provide a check on nuclear methods, monitor the asphalt content in recycled asphalt pavement, serve as an option for jobs too small to warrant nuclear asphalt content gauge use, and provide gradation control. Eventually, however, biodegradable solvents or other methods of performing these tests and checks are likely to replace the hazardous chemicals used today.

Over the next few years, three solvents that have traditionally been used in extraction tests are going to come under additional regulation. Trichloroethane is being phased out and will no longer be manufactured after 1996. Methylene chloride and trichloroethylene are also having additional restrictions placed on their use. Biodegradable solvents are an option; however, many of these still have shortcomings in safety (flammability) and longer testing times.

The changes that are occurring require state highway agencies to replace chemical tests with new methods that are environmentally clean and can be performed quickly.

References

1. Asphalt Content Determination Manual, Publication No. FHWA-IP-90 008, Federal Highway Administration, Washington, D.C., 1990.
2. James Warren, et al. Current Industry Practices and Procedures for Determining Asphalt Cement Content in Hot Mix Asphalt, National Asphalt Paving Association, Washington, D.C., 1991.

Acknowledgements

Support for this article was provided by Dr. Terry Mitchell of the Materials Division of the Office of Engineering and Highway Operations Research and Development, Federal Highway Administration (FHWA). This article was adapted from a paper titled "The Nuclear Asphalt Content Gauge for Measuring Asphalt Content in Mixes" by Kevin N. Black. The unabridged paper and bibliography is available from FHWA regional offices and from the Construction and Maintenance Division, Office of Engineering, FHWA, 400 Seventh St., S.W., Washington, DC 20590.

Kevin N. Black is a highway engineer in the Materials Branch, Construction and Maintenance Division, Federal Highway Administration (FHWA). He developed this article as a supplement to the Materials Notebook, which is distributed to FHWA field offices to provide guidance on issues related to construction materials.