Technical Note #32 - Use of Nuclear Gauges for - NATA

Technical Note 32 - July 2011
Issued: July 2011
Use of Nuclear Gauges for Testing Soils and Asphalt
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Technical Note 32 - Use of Nuclear Gauges for Testing Soils and Asphalt
Contents
1. Introduction.........................................................................................................................................................................................4
2. Test Methods......................................................................................................................................................................................4
3. Principles of Operation .......................................................................................................................................................................4
4. Measurement Modes..........................................................................................................................................................................5
4.1 Direct Transmission Mode .....................................................................................................................................................5
4.2 Backscatter Mode ..................................................................................................................................................................6
4.3 Moisture Measurement Mode ................................................................................................................................................7
5. Calibration of Gauges.........................................................................................................................................................................8
6. Nuclear Gauge Checks in the Laboratory ..........................................................................................................................................9
6.1 Standard Count Check (Form 1a Troxler and Humboldt, Form 1b CPN) ..............................................................................9
6.2 Gauge Function Checks ......................................................................................................................................................10
6.2.1 Drift Test.................................................................................................................................................................10
6.3 Consistency Check (Form 2) ...............................................................................................................................................11
6.3.1 Initial Check............................................................................................................................................................11
6.3.2 Monthly Check........................................................................................................................................................11
6.4 Leak and Radiation Level Tests ..........................................................................................................................................12
7. Use of Gauges..................................................................................................................................................................................12
7.1 Maximum Particle Size of Material to be Tested .................................................................................................................12
7.2 Density Range to be Tested ................................................................................................................................................12
7.3 Density/Moisture Offsets......................................................................................................................................................13
7.4 Checking of Services ...........................................................................................................................................................13
7.5 Location of Test Sites ..........................................................................................................................................................13
7.6 Surface Preparation.............................................................................................................................................................13
7.6.1 Initial Preparation....................................................................................................................................................13
7.6.2 Source Rod Hole ....................................................................................................................................................14
7.6.3 Use of Sand/Fines from Tested Material................................................................................................................14
7.6.4 Roller Marks in Asphalt ..........................................................................................................................................14
7.7 Depth of Testing...................................................................................................................................................................14
7.7.1 General...................................................................................................................................................................14
7.7.2 Client Specified Depth............................................................................................................................................15
7.7.3 Unspecified Layer Thicknesses..............................................................................................................................15
7.7.4 Asphalt Thickness ..................................................................................................................................................15
7.7.5 Testing Over Pipes.................................................................................................................................................15
7.7.6 Importance of Locking Handle into Place...............................................................................................................16
7.8 Placement of the Probe in the Hole .....................................................................................................................................16
7.9 Counts..................................................................................................................................................................................16
7.9.1 Standard Counts ....................................................................................................................................................16
7.9.2 Density and Moisture Counts .................................................................................................................................16
7.9.3 Testing in Trenches................................................................................................................................................17
8. Radiation Safety ...............................................................................................................................................................................17
8.1 National Codes of Practice ..................................................................................................................................................17
8.2 State and Territory Radiation Safety Acts and Regulations.................................................................................................17
9. References .......................................................................................................................................................................................18
10. Notes....................................................................................................................................................................................18
10.1 Density.................................................................................................................................................................................18
10.2 Moisture ...............................................................................................................................................................................18
10.3 Location of Secondary Blocks..............................................................................................................................................18
10.4 Counts..................................................................................................................................................................................18
10.5 Asphalt Layers .....................................................................................................................................................................19
Appendix A: Standards Australia Test Methods ........................................................................................................................................20
Appendix B: Calibration Relationships.......................................................................................................................................................21
Appendix C: Worksheets ...........................................................................................................................................................................24
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Technical Note 32 - Use of Nuclear Gauges for Testing Soils and Asphalt
1. Introduction
Nuclear surface-moisture density gauges (nuclear gauges) have been used for the testing
of compaction of soils and asphalt for over twenty five years in Australia. In more recent
times, these gauges have also been used to test the compaction of concrete.
Thin-layer density gauges were developed for the compaction control testing of thin
asphalt layers (see Note 5) and were introduced in Australia over ten years ago.
2. Test Methods
There is a number of test methods covering the use and calibration of nuclear gauges.
These are listed in Appendix A of this technical note. A number of state road authorities
have also issued test methods. This technical note covers the Standards Australia
methods only.
The common brands on the Australian market are Campbell-Pacific (CPN), Humboldt and
Troxler.
3. Principles of Operation
Nuclear gauges contain a source of gamma rays (Note 1) for density measurement and,
when a moisture measurement is required, a source of neutrons (Note 2).
Density measurement is based on the scattering and absorption of gamma radiation which
in turn can be related to the density of the material being tested.
Moisture measurement is based on the slowing down (thermalisation) of fast neutrons,
which is a function of the hydrogen content of the material being tested.
The measurements are made by detectors built into the gauges which are displayed as
counts. The counts are taken over at least one minute, but additional counting time will
improve the accuracy of the measurement (see Note 4).
Each nuclear gauge is provided with a standard reference block which needs to be kept
with the gauge , and is used for determining standard counts both in the laboratory and in
the field. Use of a standard reference block from another nuclear gauge is not technically
valid.
As background radiation may affect the readings, standard counts on a standard reference
block provided by the gauge manufacturer are made at the site where the gauge is used.
The actual counts measured are normalised against these standard counts to provide
count ratios. Calibration equations provided by the calibrating authority are used to
convert count ratios to density and water/moisture content values.
The activity of the radioactive source decreases with time due to radioactive decay. The
decrease is about 2% per year for the gamma source and 0.15% for the neutron source.
Hence, standard counts are made and are used to normalise the raw count to provide a
count ratio (raw count/standard count).
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Technical Note 32 - Use of Nuclear Gauges for Testing Soils and Asphalt
Some nuclear gauges contain electronic circuitry and firmware which convert the detected
counts into measures of density/moisture content respectively using the calibration
constants.
The form of the calibration equations is shown in the Standards Australia test methods AS
1289.5.8.4, AS 2891.14.3 and AS 2891.14.4 and is also shown on the calibration
authority’s certificate (refer Appendix B).
4. Measurement Modes
Nuclear gauges are designed to use the emission and detection of gamma radiation for
determining density in two measurement modes- direct transmission and backscatter.
The measurement of density by the nuclear gauge provides a wet density result. If
suitable calibration for moisture and material dependent moisture offsets have been
determined, dry density results can also be calculated using the nuclear gauge electronics.
Nuclear gauges can generally be operated in the following modes.
4.1 Direct Transmission Mode
The direct transmission method involves placing the detector on the surface and the
source within the material (see fig.1 and note 1). The gamma radiation emitted from the
source then passes through the material to be measured before it is detected. This
method is partially destructive in that it requires a hole to be formed in the material under
test in order to position the source. It provides a measure of the average density of the
material between the source and the detector. Measurement positions are normally
provided to a depth of 300 mm in increments of 25 mm.
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SOURCE
Technical Note 32 - Use of Nuclear Gauges for Testing Soils and Asphalt
Figure 1: Direct Transmission Mode
4.2 Backscatter Mode
DETECTOR
GAMMA
RADIATION
PATHS
The backscatter mode enables the test to be performed rapidly as a non-destructive
method. The gauge is placed above the material to be tested (i.e. on the surface see fig 2
and note 2). The gamma radiation emitted from the source is scattered back towards the
detector to be measured.
The backscatter method commonly utilises one measurement position (e.g. BS
(backscatter) in Troxler and Humbolt gauges) or two measurement positions (e.g. BS and
AC (asphaltic concrete) in CPN gauges).
Backscatter mode has a restricted measurement depth and the accuracy of its
measurements are biased toward the surface of the material. For the BS measurement
position, about 80 to 90 percent of the measurement is made in the top 50 mm of the
material. It therefore does not provide a measure of the average density of the material.
Also, as it is very sensitive to surface roughness, the backscatter method is less precise
than the direct transmission method and is not used for the measurement of soil density.
Backscatter mode has been retained for the density measurement of asphalt where
problems associated with surface roughness and measurement depth are reduced and
where its rapid and non-destructive nature make it more attractive despite slightly reduced
accuracy.
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Technical Note 32 - Use of Nuclear Gauges for Testing Soils and Asphalt
The nuclear thin-layer density gauge uses two backscatter geometries to provide
independent measures of material density at two depths. Mathematical computation of
responses from the two geometries reduce the influence of the underlying layer on density
measurement. The use of this type of gauge is restricted to asphalt having a nominal
maximum particle size not greater than 40 mm and a nominal layer thickness between 25
and 100 mm. This gauge does not have a moisture mode, hence moisture measurements
are not possible.
SOURCE
Figure 2: Backscatter Mode
4.3 Moisture Measurement Mode
DETECTOR
GAMMA RADIATION
PATH
The emission of neutron radiation and detection of slow neutron radiation for the
determination of moisture content is not appropriate for direct transmission measurement.
If moisture is to be determined it is conducted only in the "backscatter" mode with the
source and detector positioned close together to provide a linear relationship between
detected radiation and moisture content.
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Technical Note 32 - Use of Nuclear Gauges for Testing Soils and Asphalt
The effective measurement depth for moisture content varies according to the moisture
content of the material and decreases with increasing moisture content. For a moisture
content range of 0.1 to 0.3 t/m 3 , the measurement depth is about 250 to 200 mm
respectively. However, detection of slow neutrons relies on diffusion to the detector and,
as such, moisture content measurements are biased towards the surface of the material.
This bias will not adversely affect the accuracy of moisture content measurement, provided
that the water within the material is evenly distributed. As a consequence, reliable
measurements of moisture can only be made to a depth of about 150 mm (see fig 3).
MOISTURE
DETECTION
Figure 3: Moisture Measurement Mode
5. Calibration of Gauges
SOURCE
DETECTOR
The Standards Australia test methods require that nuclear gauges be calibrated to
establish the relationships between the count ratios and density and moisture values.
The procedure for calibration of nuclear gauges is detailed in the Australian Standards:
AS 1289.5.8.4 for gauges used for testing of soils, and
AS 2891.14.3 and AS 1289.14.4 detail the calibration procedures for gauges used
for testing asphalt.
Calibrations are performed against standard blocks of known density or moisture content,
as relevant. Nuclear gauges used in direct transmission mode need to be calibrated at
each nominal depth at which they are to be used.
Arising from the calibration is a set of coefficients, for each depth calibrated, for use in the
density equation, and a single set of coefficients for use in the moisture equation (see
Appendix B).
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Technical Note 32 - Use of Nuclear Gauges for Testing Soils and Asphalt
Modern gauges have the facility to have these coefficients programmed into the gauge so
that direct readings of wet density and moisture can be made.
The direct reading of wet density is verified by the calibrator to ensure that the correct
constants have been programmed into the gauge. As Troxler and Humboldt gauges take
into account the effect of water on the gamma mass attenuation coefficient of the material
under test when calculating the wet density, these gauges need to be calibrated in the
moisture mode when direct reading of wet density is required.
The field moisture content and field dry density can be determined directly using a nuclear
gauge when the gauge is calibrated in the moisture mode and the moisture intercept for
the materials being tested has been determined. The moisture readings and direct dry
density readings from the gauge shall not be used. The moisture offset is the difference
between the moisture content determined by oven drying (AS 1289.2.1.1) and the
moisture content determined by the nuclear gauge moisture content (see AS 1289.5.8.1
Appendix A).
6. Nuclear Gauge Checks in the Laboratory
Examples of worksheets are shown in Appendix C of this technical note.
6.1 Standard Count Check (Form 1a Troxler and Humboldt, Form 1b CPN)
The Standards Australia test methods require that a standard count check be made on
each day of use for both density and moisture modes. This is required to ensure correct
operation of the gauge and that there is no drift in the response of the gauge.
The check is to be made at the same location in the laboratory each day to ensure that the
background radiation is generally the same.
The check location must be at least two metres away from any other building walls or large
structure and at least ten metres from any other radioactive source, including other
gauges.
The laboratory standard count check for both density and moisture is made on the
standard reference block supplied by the manufacturer. The check is performed in
accordance with the manufacturer’s instructions over a counting period of at least four
minutes.
Records of the standard counts obtained during the standard count check must be
maintained as well as the mean value of the previous four standard count checks. This
provides the lower and upper limits against which the current check is being compared, as
detailed in AS1289.5.8.1, Appendix B.
The pre-scale factor, which should be obtained from the manufacturer, must be taken into
account when checking the lower and upper limits. For the purpose of the equations
shown in AS 1289 5.8.1, this value is “64” for Humboldt and most Troxler gauges and “4”
for Campbell Pacific gauges for 4 minute counts. (See Table 1).
If a gauge fails the standard count check, the check needs to be repeated once. If it fails
again, the gauge shall be removed from service and repaired. In this case, the previous
working day’s field density and/or moisture test results need to be reviewed to ascertain if
they have been affected.
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Technical Note 32 - Use of Nuclear Gauges for Testing Soils and Asphalt
The standard count check is quite separate from and additional to the standard counts
measured at the work site during daily operations.
TABLE 1 PRE-SCALE FACTORS
(AS 1289 5.8.1 Appendix A)
Nuclear Gauge Type Pre-scale
Factor
CPN 4
Troxler- except Models
3450 and 4640B
64
Troxler Models 3450
and 4640B
32
Humboldt 64
6.2 Gauge Function Checks
Monthly gauge function checks are performed to ensure that the nuclear sources are
stable. There are two main function checks performed:
Stability Test, also known as the Statistical Stability or Stat Test;
Drift Test.
(CPN - Form 3a, Humboldt - Form 3b, Troxler - Form 3c)
A suitable location for the function checks needs to be selected, preferably at the same
location as that used for the standard count check. The same site is used each time these
checks are made. The site location should at least two metres away from any building
walls or large structure and at least ten metres from any other radioactive source (i.e. other
gauges).
The stability test procedure is detailed in the manufacturer’s handbook.
6.2.1 Drift Test
On completion of the stability test the gauge needs to remain on for three hours then five
sets of standard density and moisture counts need to be obtained immediately. The mean
of the five standard density counts and the mean of the five moisture standard counts is
then calculated.
For Troxler gauges, the density drift and moisture drift are calculated as detailed in the
manufacturer’s handbook. Density drift should not exceed 0.5% and moisture drift should
not exceed 1.0%.
A drift test is not specified for Humboldt gauges by the manufacturer, however, there are
no technical issues preventing a facility from performing a drift test and applying the same
acceptance criteria.
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Technical Note 32 - Use of Nuclear Gauges for Testing Soils and Asphalt
CPN gauges automatically turn the power off after 60 minutes if keyboard functions are not
used, therefore a drift test cannot be performed.
6.3 Consistency Check (Form 2)
A consistency check is performed to ensure that the mechanical parts of the gauge used
for locating and locking the source rod have not worn and that the electronics within the
nuclear gauge are operating consistently.
The density system consistency check is performed at least monthly to confirm the
calibration for each source rod position. Such checks are performed on a standard density
block as defined in AS 1289.5.8.4, or a dry secondary block of naturally occurring stone
(see Note 3). The block must be stored in the laboratory or covered and protected from
the rain and moisture ingress.
6.3.1 Initial Check
Following the return of a nuclear gauge from calibration, the laboratory must immediately
(i.e. prior to operational use and preferably within one month of calibration) check wet
density readings at each calibrated source rod position on the secondary block to confirm
that consistent density readings are being obtained (see Note 3).
The height of secondary block is required to be at least 50 mm greater than the greatest
source rod depth for which the gauge is calibrated.
The results of these checks should be compared to the last consistency density readings
taken prior to calibration. If these results differ by more than 0.04 t/m 3 , checks should be
made with the calibration authority. The laboratory should also review test results
obtained immediately after recalibration to ensure that no sudden change in density results
has occurred which may be attributed to the recalibration.
If a nuclear gauge is moved to another location where a different secondary block is used
then the following needs to be performed.
A further consistency check is made at each calibrated source rod position on the
secondary block which was used for the initial check after calibration. These checks
should meet the requirements in AS 1289.5.8.1, i.e within 0.02 t/m 3 of the initial reading
after calibration. The difference (ρs) between the density measured at the initial check
(ρi) and the density measured at this check is calculated.
Consistency checks shall be made on the secondary block used at the location to
where the gauge has been moved. This value must then be used as the initial value for
consistency checks at that particular location.
The difference between the values obtained in subsequent monthly consistency checks
(see 6.3.2 below) should not exceed (0.02 – ρs) t/m 3 .
6.3.2 Monthly Check
For each calibrated source rod position, further density measurements on the secondary
block using the nuclear gauge are made at intervals not exceeding one month, or sooner if
it is suspected that the gauge is malfunctioning.
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Technical Note 32 - Use of Nuclear Gauges for Testing Soils and Asphalt
These readings are compared to the relevant initial reading. If the difference between the
initial reading and the monthly reading is greater than 0.02 t/m 3 , the gauge needs to be put
out of service and repaired. Each monthly check is recorded.
6.4 Leak and Radiation Level Tests
State/Territory regulations require leak tests which check the integrity and the shielding of
the source to be performed annually or more often if required.
Annex 3, ISO 9978 Radiation protection – Sealed radioactive sources – Leakage test
methods describe how leak tests are performed. These are generally arranged through the
state authority which regulates the operation of radioactive devices.
Laboratories possessing equipment for checking gamma and neutron radiation levels must
ensure that this apparatus is recalibrated regularly in keeping with the requirements of the
appropriate State/Territory regulatory body (see Table 2).
7. Use of Gauges
The Standards Australia and State Road Authority test methods describe how nuclear
gauge testing is to be performed. The test methods describe the minimum requirements
for the tests. This section of the technical note sets down some practical aspects of testing
and provides more detail than described in some test methods.
Laboratories should look at the risk management issues related to each test instance and
in areas of medium to high technical risk, consideration should be given to exceeding the
minimum requirements, e.g. count time, multiple readings or rotation of the gauge (see
Note 4). If the minimum requirements are exceeded this is to be recorded.
All checks and testing involving a nuclear gauge must be performed by, or in the presence
of a licensed operator. All operators are required to wear personal radiation monitoring
badges (refer Section 8 on Radiation Safety).
7.1 Maximum Particle Size of Material to be Tested
The Standards Australia test method for testing soils using a nuclear gauge limits the
maximum particle size of material to be tested to not more than 20% by mass of particles
retained on a 37.5 mm sieve.
In earthworks, the size of the particles in the material “as placed” often exceeds these
requirements. However, the field compaction of the material may break down the particles
to meet the standard. Tests should be made to ensure that oversize does not exceed the
requirement of 20% by mass of particles retained on a 37.5 mm sieve. In cases where the
amount of oversize is greater than 20%, the field density result using the nuclear gauge is
not a valid test result and should not be reported.
7.2 Density Range to be Tested
The Standards Australia Standard test methods for Calibration of Nuclear Gauges, AS
1289.5.8.4, AS 2891.14.3 and AS 2891.14.4, require that the calibration certificate
includes the wet density range for which the density calibration is valid.
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Technical Note 32 - Use of Nuclear Gauges for Testing Soils and Asphalt
If a nuclear gauge measures wet density values outside the calibrated range, the result
must not be reported under the scope of accreditation of NATA accreditation.
7.3 Density/Moisture Offsets
It is essential that moisture offsets be determined in accordance with AS 1289.5.8.1,
Appendix A (or State Road Authority method), for each type of material whenever soil
moisture content is to be determined using the nuclear gauge.
Similarly, it has been found that, with asphalt testing in the backscatter mode or with the
thin layer asphalt gauge, density offsets may need to be determined. Density offsets may
be positive or negative, or sometimes zero.
The method for determining these offsets is detailed in the appropriate test methods.
AS 1289.5.8.1 neither requires nor provides for wet density offsets. It has been found that
with soils that contain certain minerals, e.g. blast furnace slag and ironstone, density
offsets may be needed. In such cases the test results cannot be reported as complying
with AS 1289.5.8.1.
7.4 Checking of Services
When operating nuclear gauges in direct transmission mode, the location of services such
as water, and gas pipes under the surface being tested should be known, prior to
drilling/driving the hole for the source rod.
7.5 Location of Test Sites
The test methods do not detail the criteria for selection of test sites. These are usually
specified in the overall contract documents or by the client. In the event of no specification
requirements, the selection of sites should be made using the random stratified method
described in AS 1289.1.4.2.
If a test site has been selected using a random number method, the location should not be
changed as this will bias the selection procedure. If problems are encountered on
earthworks sites caused by site preparation or the use of earthmoving equipment, the test
site may be moved within a 500 mm radius of the site selected and the location of the new
site recorded.
The presence of a harsh or coarse looking asphalt surface is not sufficient reason for
moving a gauge from the randomly selected site.
7.6 Surface Preparation
7.6.1 Initial Preparation
Generally roller finished surfaces of road pavements, carparks, etc. and asphalts do not
require surface preparation other than sweeping the surface free from loose particles.
However, the scraper plate may be used to scrape earthen surfaces to ensure that the
gauge sits flat on the surface and is free of any rocking movement.
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Technical Note 32 - Use of Nuclear Gauges for Testing Soils and Asphalt
Earthworks require a flat surface to be prepared by a grader or other similar item of
equipment prior to any additional preparation by the tester. The surface then needs to be
scraped flat using the scraper plate which must allow the gauge to sit on the surface
without rocking movement.
7.6.2 Source Rod Hole
The hole for the source rod is to be drilled to the required depth (see Section 7.7 below)
plus at least 25 mm. It has been found that drilling using an appropriate power drill is
effective in most materials and essential for asphalt, hardened stabilized materials and
hardened concrete. The spiking or drilling equipment used must be relocated at least two
metres from the test site prior to testing with the nuclear gauge.
When a source rod hole is driven by the hammering of a spiking tool, cracking in the
surface of the soil may occur but this generally does not affect the test result provided the
crack does not extend from the source rod to the detectors and the soil has broken up.
Occasionally, a small hump (less than 5 mm) occurs when the driven rod is removed. This
may be flattened by placing the scraper plate over the hump and gently tapping it with a
mallet or hammer. If a larger hump or cracks wider than 1 mm appear, another site should
be selected and a new hole formed.
7.6.3 Use of Sand/Fines from Tested Material
While not all test methods specify that sand or fines from the parent material must be
used, fines will minimise the possibility of an air gap between the gauge and the surface of
the material being tested. Air gaps will significantly reduce the measured density.
The fines are required to have 100% passing a 0.425 mm sieve and should not form a
separate layer (i.e. less than 0.5 mm thick). The amount of fines should be just sufficient to
enable the nuclear gauge to sit on the surface without rocking.
Sand or fines must be used when testing dense graded asphalt. Fines from the asphalt
need to be obtained from the asphalt plant raw materials or from the hot bins.
7.6.4 Roller Marks in Asphalt
Occasionally roller patterns or marks occur in asphalt. Operators should ensure that this
does not cause the rocking of the gauge or air gaps to occur under the gauge. The use of
sand flattened with a straight edge will indicate the amount and extent of any patterns. It
may be possible to move the gauge within a 500 mm radius to avoid the roller marks. The
reason and the new location are to be recorded on the test worksheet.
7.7 Depth of Testing
7.7.1 General
The Standards Australia test methods define the test depth as the maximum depth that
allows the probe to be located in the testing position, and the probe to remain in the layer
being tested, i.e. the full depth of the layer. It is therefore important to obtain the correct
layer thickness.
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Technical Note 32 - Use of Nuclear Gauges for Testing Soils and Asphalt
A number of gauges have 50 mm depth increments for the source rod probe. This often
restricts the depth which can be tested, e.g. for a layer depth of 240 mm, the bottom 40
mm of the material cannot be tested. This is often the area where the lower compaction
occurs. Where gauge rod allows 25 mm increments, only 15 mm of the layer would not be
tested.
7.7.2 Client Specified Depth
In some cases the client will specify a test depth which does not place the source at the
depth required by the test method. Test reports must show the depth tested, the actual
layer depth and that the test was performed at the depth shown as requested by the client
7.7.3 Unspecified Layer Thicknesses
In some cases the actual thickness of the layer is difficult to ascertain from the information
supplied by the client.
In such cases the layer thickness needs to be determined prior to field density testing at a
site close to the test site. This requires a hole to be excavated at a distance, e.g. 1 metre,
from the test site, which does not affect the gauge reading.
As a guide, if the probe is placed 15 to 35 mm less than the actual layer thickness,
adequate depth of material is tested.
After the nuclear gauge readings have been taken and during the excavation of the soil, it
is often necessary to obtain a moisture and/or reference density sample. If, when
obtaining the reference sample, it becomes apparent that the source rod has been placed
in a lower layer, the result from the test site is to be discarded and another site selected in
accordance with Section 7.5 and tested.
7.7.4 Asphalt Thickness
Generally the depth of asphalt is well known and most testing is carried out using
backscatter mode (see Note 5).
When a thin-layer gauge is used (AS 2891.14.2), the layer thickness is very important
particularly when setting the design layer thickness value into the gauge and in the use of
density offsets.
If layer thickness is variable, as in, say, a regulation layer, an alternative method may be
required to measure density, e.g. coring.
7.7.5 Testing Over Pipes
When fill over pipes or the water table is being tested, the test probe should be between
35 and 55 mm above the top surface of the pipe or water table. Extreme care must be
taken to ensure that preparation of the hole does not cause damage to the pipe.
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Technical Note 32 - Use of Nuclear Gauges for Testing Soils and Asphalt
7.7.6 Importance of Locking Handle into Place
Once the probe depth has been selected, the probe is lowered into the hole to that depth.
It is extremely important to ensure that the test probe is set exactly to the depth and that
the handle is locked into place at that depth. Operators should check the handle location
each time a count is taken. A height error of only 3 mm will cause significant errors in
density readings particularly at the shallower depths.
7.8 Placement of the Probe in the Hole
Once the source rod is placed at the required depth, the gauge should be moved towards
the detector end so that the probe is against the inner edge of the hole, thus avoiding air
gaps. The placement of the probe needs to be checked each time a test is undertaken so
that consistent readings are obtained.
Prior to lifting the source rod from the hole, the nuclear gauge should be moved away from
the edge of the hole to avoid wear of the source rod.
7.9 Counts
In order to obtain density and moisture test results, the count ratios at each test site need
to be recorded.
Density Count Ratio = Density Count/ Site Density Standard Count
Moisture Count Ratio = Moisture Count/Site Moisture Standard Count
7.9.1 Standard Counts
The Australian Standard test methods require that standard counts be made at the work
site and not at every test site.
Standard counts need to be taken at least every four hours and at sites more than 2 km
from where the initial standard count for that work site was taken. Standard counts take
into account variations in the natural radiation at the site. This radiation may vary over
time and over long distances at the work site. In order to take into account these site
variations, it is recommended that standard counts be taken for each lot or area being
tested.
The standard counts are obtained with the gauge placed on the reference block supplied
with the nuclear gauge for a minimum of four minutes. It is important that the site where
the counts are taken is clear from vertical projections and at least two metres from
buildings or large structures and at least ten metres from any other nuclear gauges.
7.9.2 Density and Moisture Counts
The Standards Australia test methods specify a minimum count time for the test of one
minute for direct transmission and backscatter and four minutes for thin layer asphalt
gauges. (See Note 4.)
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Technical Note 32 - Use of Nuclear Gauges for Testing Soils and Asphalt
7.9.3 Testing in Trenches
When testing in trenches where there is a vertical projection of the trench walls, or when
vertical projections are within 1 metre of the test site, a standard count should be made at
each test site and used in the calculation of the count ratio.
See also 7.7.6 Testing Over Pipes.
8. Radiation Safety
8.1 National Codes of Practice
Code of Practice and Safety Guide for Portable Density/Moisture Gauges Containing
Radioactive Sources (2004) Published by Australian Radiation Protection and Nuclear
Safety Agency.
Code of Practice - Safe Transport of Radioactive Materials – Radiation Protection Series
No. 2. Published by Australian Radiation Protection and Nuclear Safety Agency.
8.2 State and Territory Radiation Safety Acts and Regulations
Each state and territory has legislation and regulations which cover the ownership,
operation, storage and transport of nuclear gauges (see Table 2).
Due to the potential risk of radiation to the public, it is essential that laboratories met all the
requirements of the relevant state or territory.
TABLE 2 STATE ACTS AND REGULATIONS
State Act Regulations
ACT Radiation Protection Act 2006
Radiation Protection Regulation 2007
NSW Radiation Control Act (1990) No 13
NT Radiation Protection Act (as in force at
5 October 2009)
Qld Radiation Safety Act 1999
SA Radiation Protection and Control Act,
1982
Tas Radiation Protection Act 2005
Radiation Control Regulation 2003
Radiation Protection Regulations (as in force at 5
October 2009)
Radiation Safety Regulation 2010
Radiation Protection and Control (Transport of
Radioactive Substances) Regulations 2003
Ionizing Radiation Regulations 2000
Radiation Protection Regulations 2006
Vic Radiation Act 2005 Radiation Regulations 2007
WA Radiation Safety Act 1975 Radiation Safety (General) Regulations 1983,
(Qualifications) Regulations 1980, (Transport of
Radioactive Substances) Regulations 2002
The contact addresses for the government organisation responsible for the administration
of the regulations for each state is available at http://arpansa.gov.au/stateoff.htm.
July 2011 Construction Materials Testing Page 17 of 30

Technical Note 32 - Use of Nuclear Gauges for Testing Soils and Asphalt
9. References
1. VicRoads Test Method RC 900.04 (2002)
2. Code of Practice and Safety Guide for Portable Density/Moisture Gauges
Containing Radioactive Sources (2004) Published by Australian Radiation
Protection and Nuclear Safety Agency.
3. Code of Practice - Safe Transport of Radioactive Materials – Radiation
Protection Series No. 2. (2008) Published by Australian Radiation Protection
and Nuclear Safety Agency.
10. Notes
10.1 Density
An isotope of Cesium (Cs137) is used as the source of gamma radiation used in nuclear
gauges for the measurement of density. The quantity of radio-active material used in the
gamma source is usually either 0.37 or 0.296 GBq.
10.2 Moisture
An isotope of Americium (Am241) in combination with Beryllium is used as the source of
neutrons for nuclear surface moisture-density gauges for the measurement of moisture
content. The quantity of radioactive material used in the neutron source is usually either
1.48 or 1.85 GBq.
10.3 Location of Secondary Blocks
The location of the secondary block when performing consistency checks is important. In
particular, the block must be clear from vertical projections , be at least two metres from
buildings and at least ten metres from any other nuclear gauges. The following are
examples of interferences during consistency checks:
Example 1- it was observed that the metal handles of a trolley used to move the block
projected not only above the surface of the block but also above the gauge detectors. As
the block could move on the trolley, inconsistent readings were being obtained due to the
influence of the handles.
Example 2- The block was relatively narrow, and the readings were altered by testers
walking too close to the block during the counts. It is important to keep other staff away
from the block during the counts as is normally required for safety. The presence of
samples too close to the block during testing will also influence the consistency readings.
10.4 Counts
Manufacturer’s literature indicate that greater accuracy can be obtained using higher count
times. Operators should consider this in relation to the risks associated with the job.
July 2011 Construction Materials Testing Page 18 of 30

Technical Note 32 - Use of Nuclear Gauges for Testing Soils and Asphalt
If an anomalous result is obtained, the nuclear gauge may be rotated through 90 or 180
degrees after the initial count at the site and a second count be made. If the resultant
density results from the two counts are not within 0.075 t/m 3 both results should be
discarded. Such variations may be due to an air gap or a large stone or voids in the path
between the source and the detectors. The reason for the difference(s) should be
checked by carefully excavating a hole in the area covered by the nuclear gauge
measurement.
10.5 Asphalt Layers
When asphalt is laid on a surface which is highly variable (e.g. the lower layers of a multilayer
pavement) or where there is a crown in the surface, the asphalt layer depth may be
different at each site. In such cases, the layer thickness may need to be determined using
cores or probes.
July 2011 Construction Materials Testing Page 19 of 30

Technical Note 32 - Use of Nuclear Gauges for Testing Soils and Asphalt
Appendix A: Standards Australia Test Methods
The following Standards Australia test methods are required for the use and calibration of
nuclear gauges in construction materials testing:
AS 1289.5.8.1 (2007) Methods of testing soils for engineering purposes - Soil
compaction and density tests - Determination of field density and
field moisture content of a soil using a nuclear surface moisturedensity
gauge - Direct transmission mode
AS 1289.5.8.4 (2009) Methods of testing soils for engineering purposes - Soil
compaction and density tests - Nuclear surface moisture-density
gauges - Calibration using standard blocks
AS 2891.14.1.1 (1996) Methods of sampling and testing asphalt - Field density tests -
Determination of field density of compacted asphalt using a
nuclear surface moisture-density gauge - Direct transmission
mode
AS 2891.14.1.2 (1999) Methods of sampling and testing asphalt - Field density tests -
Determination of field density of compacted asphalt using a
nuclear surface moisture-density gauge - Backscatter mode
AS 2891.14.2 (1999) Methods of sampling and testing asphalt - Field density tests -
Determination of field density of compacted asphalt using a
nuclear thin-layer density gauge
AS 2891.14.3 (1999) Methods of sampling and testing asphalt - Field density tests -
Calibration of nuclear thin-layer density gauge using standard
blocks
AS 2891.14.4 (1999) Methods of sampling and testing asphalt - Field density tests -
Calibration of nuclear surface moisture-density gauge -
Backscatter mode
July 2011 Construction Materials Testing Page 20 of 30

Technical Note 32 - Use of Nuclear Gauges for Testing Soils and Asphalt
Appendix B: Calibration Relationships
1. Density Measurement
A relationship exists between the detected gamma radiation and the density of the material
which depends on the type of calibration blocks used.
When a nuclear gauge is calibrated in accordance with AS 1289.5.8.4, AS 2891.14.3 and
AS 2891.14.4, the calibration report should provide the form of equation and the constants
determined by calibration for field use.
The equations take the form:
AS 1289.5.8.4 and AS 2891.14.4
Type A and B Blocks:
ln A ln ( DCR C)
B B
where ρ = wet density, in tonnes per cubic metre
DCR = density count ratio
A,B,C = calibration constants for the particular gauge
Type C Blocks:
P Q
ln ( DCR)
where ρ = wet density, in tonnes per cubic metre
DCR = density count ratio
P,Q = calibration constants for the particular gauge
AS 2891.14.3 System 1
All blocks:
1 G H ( DCR1)
where ρ1 = density, in tonnes per cubic metre
DCR1 = density count ratio for System 1
G = intercept constant for System 1
H = slope constant for System 1
July 2011 Construction Materials Testing Page 21 of 30

Technical Note 32 - Use of Nuclear Gauges for Testing Soils and Asphalt
AS 2891.14.3 System 2
Type A and B Blocks:
2
A ln ( DCR C)
B B
ln 2
where ρ2 = density, in tonnes per cubic metre
DCR2 = density count ratio
A,B,C =calibration constants for the particular gauge
Type C Blocks:
2 P Q ln ( DCR2)
where ρ2 = density, in tonnes per cubic metre
DCR2 = density count ratio
P,Q =calibration constants for the particular gauge
AS 1289.14.3 Depth factors
Depth factor equations are expressed in the following form:
K
K
1
2
A
A
11
21
e
e
A12t
A22t
A
A
13
23
where K1 = depth factor for System 1
K2 = depth factor for System 2
t = thickness, in metres
A12 = depth factor calibration constant for System 1
A22 = depth factor calibration constant for System 2
A11, A21 = 1
A13, A 23 = 0
Field density is calculated using the depth factors from the following equation:
K 2
1 K 1
2
K 2 K1
where ρ = density, in tonnes per cubic metre
ρ1 = density measures by System 1, in tonnes per cubic metre
ρ2 = density measured by System 2, in tonnes per cubic metre
K1 = depth factor for System 1
K2 = depth factor for System 2
July 2011 Construction Materials Testing Page 22 of 30

Technical Note 32 - Use of Nuclear Gauges for Testing Soils and Asphalt
2. Moisture Measurement
A relationship exists between the detected slow neutron radiation and the water content of
the material. This relationship is commonly expressed in the form:
W d(
MCR)
c
where MCR = moisture count ratio
W = water content of the material
d = moisture slope calibration constant
c = moisture intercept for the particular material
Nuclear gauges with more advanced electronics use this equation to display moisture
content directly for a given value of count ratio.
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Technical Note 32 - Use of Nuclear Gauges for Testing Soils and Asphalt
Appendix C: Worksheets
1. Standard Count Checks
(a) Troxler and Humboldt (Form 1a)
(b) Campbell Pacific (Form 1b)
1. Consistency Checks (Form 2)
2. Stability and Drift Checks
(a) Campbell Pacific (Form 3a)
(b) Humboldt (Form 3b)
(c) Troxler (Form 3c)
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Technical Note 32 - Use of Nuclear Gauges for Testing Soils and Asphalt
Form 581b1t&h June 2010 COMPANY AND LAB NAME
Limits for
Density : R D
Troxler &
(U and
NUCLEAR GAUGE STANDARD COUNT CHECK - TROXLER & HUMBOLDT (EXCEPT 3450 or 4640B models)
TEST METHOD AS 1289.5.8.1 Appendix B1
L) = DS
4
DS
= DS
PS
Humboldt are then : MeanDS 0.5
4
DS
64
MeanDS
Moisture : RM (U and L) = MS
Limits for Troxler &
8
MS
= MS
PS
8
Humboldt are then : MeanMS
MS
64
MeanMS
NUCLEAR GAUGE
Serial No :
Make & Model :
For this check, both DS (= Density Standard Count) and MS (= Moisture Standard Count), are taken at the same location (at the laboratory). PS = Prescale factor = 16 for Troxler & Humboldt, from the manufacturer’s handbook, under stability test
1
Date
2
CurrentDS
3
Mean of previous
4 DS values
(MeanDS)
0.5
4
MeanDS
5
CurrentDS
— MeanDS
|modulus|
6
Column 5 smaller
than Column 4 ?
yes/no
7
CurrentMS
8
Mean of previous
4 MS values
(MeanMS)
July 2011 Construction Materials Testing Page 25 of 30
9
MeanDS
10
CurrentMS
— MeanMS
|modulus|
11
Column 10 smaller
than Column 9 ?
yes/no
12
Operator
Initials
& action

Technical Note 32 - Use of Nuclear Gauges for Testing Soils and Asphalt
Form 581b1cpn June 2010 COMPANY AND LAB NAME
Density : RD
(U and
L) = DS
DS
= DS
PS
Limits for CPN gauges are then : MeanDS 2
4
NUCLEAR GAUGE STANDARD COUNT CHECK - CAMPBELL PACIFIC
TEST METHOD AS 1289.5.8.1 Appendix B1
4
MeanDS
July 2011 Construction Materials Testing Page 26 of 30
DS
4
Moisture: RM
Limits for
CPN
(U and
L) = MS
8
MS
= MS
PS
gauges are then : MeanMS
4
8
MS
4
MeanMS
NUCLEAR GAUGE
Serial No :
Make & Model :
For this check, both DS (= Density Standard Count) and MS (= Moisture Standard Count), taken at same location (at the laboratory) PS = Prescale factor = 1 for Campbell Pacific gauges
1
Date
2
CurrentDS
3
Mean of previous
4 DS values
(MeanDS)
2
4
MeanDS
5
CurrentDS
— MeanDS
|modulus|
6
Column5 smaller
than Column 4 ??
yes/no
7
CurrentMS
8
Mean of previous
4 MS values
(MeanMS)
4
9
MeanDS
10
CurrentMS
— MeanMS
|modulus|
11
Column 10 smaller
than Column 9 ??
yes/no
12
Operator
Initials
& action

Technical Note 32 - Use of Nuclear Gauges for Testing Soils and Asphalt
Form 581b3 June 2010 COMPANY AND LAB NAME
NUCLEAR GAUGE DENSITY SYSTEM CONSISTENCY CHECK
Serial Number
Make & Model
Date of Calibration
Calibrat'n Report No
Block Material & No
TEST METHOD AS 1289.5.8.1, Appendix B3
Checked at
completion:
Date:
DIRECT
TRANSMISSION
DEPTH mm
Soon after calibration, record FOUR - MINUTE DENSITY READINGS (t/m3)
(minimum of 8 minutes of readings, maximum of 20 minutes of readings)
Four-min reading 1
STANDARD COUNT =
INITIAL DENSITY ρi t/m3
(Average of readings above)
At the required
monthly interval
DATE & Operator ---->
Standard Count
Current DENSITY ρc
At the required
monthly interval
DATE & Operator ---->
Standard Count
Current DENSITY ρc
At the required
monthly interval
DATE & Operator ---->
Standard Count
Current DENSITY ρc
At the required
monthly interval
DATE & Operator ---->
Standard Count
Current DENSITY ρc
Four-min reading 2
Four-min reading 3
Report No:
Four-min reading 4
Initiating Date:
ρi + 0.02 t/m3
ρi - 0.02 t/m3
Initiating Operator:
Record a single FOUR - MINUTE DENSITY READING (t/m 3 )
Record a single FOUR MINUTE DENSITY READING (t/m 3 )
Record a single FOUR MINUTE DENSITY READING (t/m 3 )
Record a single FOUR MINUTE DENSITY READING (t/m 3 )
Four-min reading 5
Criteria: Current Density ρc should always be within +- 0.02 t/m3 of Initial Density ρi. If required, plot on graph paper, Initial Density ρi,
and values of Current Density ρc, against date upon which the readings were recorded.
July 2011 Construction Materials Testing Page 27 of 30

Technical Note 32 - Use of Nuclear Gauges for Testing Soils and Asphalt
COMPANY AND LAB NAME
NUCLEAR GAUGE FUNCTION CHECK – CAMPBELL PACIFIC
TEST METHOD AS 1289.5.8.1 Appendix B2
Nuclear Gauge Checked Report No.
Serial Number Date
Make & Model Date Operator
STABILITY CHECK MC-1, 2
STABILIY CHECK MC–3
Perform initial & final checks, 3 hour intervals
Use standard count & Xi
15 sec
INITIAL
FINAL 4 min DENSITY MOISTURE
Counts
Density Moisture
Density Moisture
Std count
1 Time
2 Std 1
3 Xi 1
4 Result Pass/Fail Pass/Fail
5 Time
6 Std 2
7 Xi 2
8 Result Pass/Fail Pass/Fail
9 Time
10 Std 3
11 Xi 3
12 Result Pass/Fail Pass/Fail
13 Time
14 Std 4
15 Xi 4
16 Result Pass/Fail Pass/Fail
17 Time
18 Std 5
19 Xi 5
20 Result Pass/Fail Pass/Fail
Mean Start Stability Initial
SD Start Stability Final
/√M
Stab
Ratio
Pass
Yes/No
Pass
Yes/No
Pass
Yes/No
Pass
Yes/No
FUNCTION CHECK Complies/
Does Not
Comply
STABILITY RATIO Query: 0.65-0.75 & 1.25-1.35
STABLE: 0.75 – 1.25 Unstable: < 0.65 & > 1.35
July 2011 Construction Materials Testing Page 28 of 30

Technical Note 32 - Use of Nuclear Gauges for Testing Soils and Asphalt
COMPANY AND LAB NAME
NUCLEAR GAUGE FUNCTION CHECK - HUMBOLDT
TEST METHOD AS 1289.5.8.1 Appendix B2
Nuclear Gauge Checked
Report No.
Serial Number
Date
Make & Model Date Operator
STABILITY CHECK
DRIFT CHECK
Use shaded cells for autotest
Use shaded cells for autotest
One min DENSITY MOISTURE Four min DENSITY COUNTS
COUNTS COUNTS
1 1
2 2
3 3
4 4
5 5
6 D Mean
7 a C – D
8 b (C + D)/2
9 Drift 100*a/b
10 Limit Pass/Fail
11 No MOISTURE COUNTS
12 1
13 2
14 3
15 4
16 5
Q Mean
r P – Q
s (P + Q)/2
Drift 100 * r/s
Mean C: P: Limit Pass/Fail < ± 1.0%
SD C P Time 0:00 Start Stability
4/√M 4C/√C 4P/√P Time 3:00 Start Drift
Stab Ratio PASS/FAIL PASS/FAIL FUNCTION CHECK COMPLIES-Yes/No
STABILITY RATIO LIMITS Query: 0.50-0.60 & 1.4 – 1.5
0.60 – 1.40 Unstable: 1.50
July 2011 Construction Materials Testing Page 29 of 30

Technical Note 32 - Use of Nuclear Gauges for Testing Soils and Asphalt
COMPANY AND LAB NAME
NUCLEAR GAUGE FUNCTION CHECK - TROXLER
TEST METHOD AS 1289.5.8.1 Appendix B2
Nuclear Gauge Checked
Report No.
Serial Number
Date
Make & Model Date Operator
3411 Electronic TEST SLOW Density & Moist NORMAL Density & FAST Density & Moist
value 14646 +/- 2
4 min:
Moist
15 sec:
(3401 - 8192)
1 min:
STABILITY CHECK
DRIFT CHECK
3401/3411B Use shaded cells for 3440
3401/3411B Use shaded cells for 3440
One min DENSITY MOISTURE Four min DENSITY COUNTS
COUNTS COUNTS
1 1
2 2
3 3
4 4
5 5
6 D Mean
7 a C – D
8 b (C + D)/2
9 Drift 100*a/b
10 Limit Pass/Fail
11 No MOISTURE COUNTS
12 1
13 2
14 3
15 4
16 5
17 Q Mean
18 r P – Q
19 s (P + Q)/2
20 Drift 100 * r/s
Mean C: P: Limit Pass/Fail
SD C P Time 0:00 Start Stability < ± 1.0%
4/√M 4C/√C 4P/√P Time 3:00 Start Drift
Stab Ratio PASS/FAIL PASS/FAIL FUNCTION CHECK COMPLIES – Yes/No
STABILITY RATIO LIMITS Query: 0.12-0.18 & 0.35 – 0.40
0.18 – 0.35 Unstable: 0.40
July 2011 Construction Materials Testing Page 30 of 30