ASTM D6938-23

Standard Test Methods for In-Place Density and Water Content of Soil and Soil-Aggregate by Nuclear Methods (Shallow Depth)

SIGNIFICANCE AND USE
4.1 The test method described is useful as a rapid, nondestructive technique for in-place measurements of wet density and water content of soil and soil-aggregate and the determination of dry density.  
4.2 The test method is used for quality control and acceptance testing of compacted soil and soil-aggregate mixtures as used in construction and also for research and development. The nondestructive nature allows repetitive measurements at a single test location and statistical analysis of the results.  
4.3 Density—The fundamental assumptions inherent in the methods are that Compton scattering is the dominant interaction and that the material is homogeneous.  
4.4 Water Content—The fundamental assumptions inherent in the test method are that the hydrogen ions present in the soil or soil-aggregate are in the form of water as defined by the water content derived from Test Methods D2216, and that the material is homogeneous. (See 5.2)
Note 1: The quality of the result produced by this standard test method is dependent on the competence of the personnel performing it, and the suitability of the equipment and facilities used. Agencies that meet the criteria of Practice D3740 are generally considered capable of competent and objective testing/sampling/inspection, and the like. Users of this standard are cautioned that compliance with Practice D3740 does not in itself ensure reliable results. Reliable results depend on many factors; Practice D3740 provides a means of evaluating some of those factors.
SCOPE
1.1 This test method describes the procedures for measuring in-place density and moisture of soil and soil-aggregate by use of nuclear equipment (hereafter referred to as “gauge”). The density of the material may be measured by direct transmission, backscatter, or backscatter/air-gap ratio methods. Measurements for water (moisture) content are taken at the surface in backscatter mode regardless of the mode being used for density.  
1.1.1 For limitations see Section 5 on Interferences.  
1.2 The total or wet density of soil and soil-aggregate is measured by the attenuation of gamma radiation where, in direct transmission, the source is placed at a known depth up to 300 mm (12 in.) and the detector(s) remains on the surface (some gauges may reverse this orientation); or in backscatter or backscatter/air-gap the source and detector(s) both remain on the surface.  
1.2.1 The density of the test sample in mass per unit volume is calculated by comparing the detected rate of gamma radiation with previously established calibration data.  
1.2.2 The dry density of the test sample is obtained by subtracting the water mass per unit volume from the test sample wet density (Section 11). Most gauges display this value directly.  
1.3 The gauge is calibrated to read the water mass per unit volume of soil or soil-aggregate. When divided by the density of water and then multiplied by 100, the water mass per unit volume is equivalent to the volumetric water content. The water mass per unit volume is determined by the thermalizing or slowing of fast neutrons by hydrogen, a component of water. The neutron source and the thermal neutron detector are both located at the surface of the material being tested. The water content most prevalent in engineering and construction activities is known as the gravimetric water content, w, and is the ratio of the mass of the water in pore spaces to the total mass of solids, expressed as a percentage.  
1.4 Two alternative procedures are provided.  
1.4.1 Procedure A describes the direct transmission method in which the probe extends through the base of the gauge into a pre-formed hole to a desired depth. The direct transmission is the preferred method.  
1.4.2 Procedure B involves the use of a dedicated backscatter gauge or the probe in the backscatter position. This places the gamma and neutron sources and the detectors in the same plane.  
1.4.3 Mark the test area ...

General Information

• Status: Published
• Publication Date: 30-Apr-2023
• ICS: 13.080.20 - Physical properties of soils
• Technical Committee: D18 - Soil and Rock
• Drafting Committee: D18.08 - Special and Construction Control Tests

Relations

• Refers: ASTM D4959-24 - Standard Test Method for Determination of Water Content of Soil By Direct Heating
• Effective Date: 01-Feb-2024
• Refers: ASTM D3740-23 - Standard Practice for Minimum Requirements for Agencies Engaged in Testing and/or Inspection of Soil and Rock as Used in Engineering Design and Construction
• Effective Date: 01-Nov-2023
• Refers: ASTM D3740-19 - Standard Practice for Minimum Requirements for Agencies Engaged in Testing and/or Inspection of Soil and Rock as Used in Engineering Design and Construction
• Effective Date: 01-Oct-2019
• Refers: ASTM D2216-19 - Standard Test Methods for Laboratory Determination of Water (Moisture) Content of Soil and Rock by Mass
• Effective Date: 01-Mar-2019
• Refers: ASTM D4944-18 - Standard Test Method for Field Determination of Water (Moisture) Content of Soil by the Calcium Carbide Gas Pressure Tester
• Effective Date: 15-Feb-2018
• Refers: ASTM D2487-17e1 - Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System)
• Effective Date: 15-Dec-2017
• Refers: ASTM D2487-17 - Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System)
• Effective Date: 15-Dec-2017
• Refers: ASTM D2488-17 - Standard Practice for Description and Identification of Soils (Visual-Manual Procedures)
• Effective Date: 15-Jul-2017
• Refers: ASTM D2937-17e2 - Standard Test Method for Density of Soil in Place by the Drive-Cylinder Method
• Effective Date: 01-Feb-2017
• Refers: ASTM D2937-17 - Standard Test Method for Density of Soil in Place by the Drive-Cylinder Method
• Effective Date: 01-Feb-2017
• Refers: ASTM D4643-17 - Standard Test Method for Determination of Water Content of Soil and Rock by Microwave Oven Heating
• Effective Date: 01-Feb-2017
• Refers: ASTM D2937-17e1 - Standard Test Method for Density of Soil in Place by the Drive-Cylinder Method
• Effective Date: 01-Feb-2017
• Refers: ASTM D4253-16 - Standard Test Methods for Maximum Index Density and Unit Weight of Soils Using a Vibratory Table
• Effective Date: 01-Mar-2016
• Refers: ASTM D4253-16e1 - Standard Test Methods for Maximum Index Density and Unit Weight of Soils Using a Vibratory Table
• Effective Date: 01-Mar-2016
• Refers: ASTM D4254-16 - Standard Test Methods for Minimum Index Density and Unit Weight of Soils and Calculation of Relative Density
• Effective Date: 01-Mar-2016

Overview

ASTM D6938-23 is a standard test method developed by ASTM International for determining the in-place density and water content of soil and soil-aggregate using nuclear methods at shallow depths. This test is widely recognized in the construction and geotechnical engineering industries for providing rapid, nondestructive measurements that are critical for quality control, acceptance testing, and research applications. The method helps evaluate both the wet and dry density of compacted soil, which is essential for ensuring the long-term performance of earthwork and pavement projects.

Key Topics

• In-Place Density and Water Content Measurement
  The standard outlines procedures for measuring both wet (total) density and gravimetric water content of soil and soil-aggregate using nuclear equipment, commonly referred to as a nuclear gauge. These values are fundamental for assessing soil compaction and moisture during construction.

• Testing Procedures
  Two core procedures are provided:

  • Direct Transmission Method: The preferred approach, where the probe extends into a pre-formed hole beneath the gauge for more representative measurements.
  • Backscatter Method: In this method, all sources and detectors remain on the material surface, enabling surface-level readings.

• Calibration and Standardization
  The nuclear gauge must be calibrated using appropriate reference standards as per ASTM guidance. Routine standardization ensures the accuracy of readings and addresses potential aging of radioactive sources.

• Quality Control and Statistical Analysis
  The method supports repetitive measurements at the same location, enabling statistical evaluation and robust quality assurance. The reliability of results depends on proper equipment handling, operator competence, and appropriate site preparation.

• Correction for Interferences
  The standard identifies potential sources of error such as non-homogeneous soils, chemical composition, large voids, and oversize particles. Procedures for corrections and sample verification are recommended to enhance result accuracy.

Applications

ASTM D6938-23 is primarily used in the following areas:

• Construction Quality Control:
  Ensure compaction requirements are met for foundations, roadbeds, embankments, and earthworks by verifying in-place density and moisture levels.
• Acceptance Testing:
  Confirm that placed soils and soil-aggregate materials meet specified standards before further construction activity or project sign-off.
• Research and Development:
  Facilitate the study of soil behavior under compaction and moisture variation for improved geotechnical designs and earthwork methods.
• Repeatability for Statistical Assessment:
  The nondestructive, rapid nature of nuclear methods allows repetitive and consistent measurements at a single site, supporting statistical process control and trend analysis.

Related Standards

This standard often references and is used in conjunction with several other ASTM and industry standards, including:

• ASTM D2216 - Laboratory Determination of Water (Moisture) Content of Soil and Rock by Mass
• ASTM D1556 - Density and Unit Weight of Soil in Place by the Sand-Cone Method
• ASTM D2167 - Density and Unit Weight of Soil in Place by the Rubber Balloon Method
• ASTM D2487 / D2488 - Soil Classification and Identification
• ASTM D6013 - Guide for Calibration Facility Setup for Nuclear Surface Gauges
• ASTM D3740 - Minimum Requirements for Agencies Engaged in Testing and/or Inspection of Soil and Rock

Practical Value

• Nondestructive and Time-Efficient:
  The nuclear gauge method enables quick, onsite assessment of soil density and moisture content without disturbing the compacted layers.
• Enhanced Quality Assurance:
  Accurate compaction and moisture control are vital to prevent future settlement, structural failure, or reduced lifespan of pavements and earthworks.
• Complies with Regulatory Requirements:
  Using ASTM D6938-23 ensures adherence to internationally recognized best practices, which is critical for contract compliance and public project approvals.

Keywords

ASTM D6938-23, nuclear gauge, in-place density, water content, soil testing, soil-aggregate, field compaction, construction quality control, nondestructive testing, moisture measurement, standard test methods, geotechnical engineering

Standards Content (Sample)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: D6938 − 23
Standard Test Methods for
In-Place Density and Water Content of Soil and Soil-
Aggregate by Nuclear Methods (Shallow Depth)
This standard is issued under the fixed designation D6938; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope* ratio of the mass of the water in pore spaces to the total mass
of solids, expressed as a percentage.
1.1 This test method describes the procedures for measuring
in-place density and moisture of soil and soil-aggregate by use
1.4 Two alternative procedures are provided.
of nuclear equipment (hereafter referred to as “gauge”). The
1.4.1 Procedure A describes the direct transmission method
density of the material may be measured by direct
in which the probe extends through the base of the gauge into
transmission, backscatter, or backscatter/air-gap ratio methods.
a pre-formed hole to a desired depth. The direct transmission is
Measurements for water (moisture) content are taken at the
the preferred method.
surface in backscatter mode regardless of the mode being used
1.4.2 Procedure B involves the use of a dedicated backscat-
for density.
ter gauge or the probe in the backscatter position. This places
1.1.1 For limitations see Section 5 on Interferences.
the gamma and neutron sources and the detectors in the same
plane.
1.2 The total or wet density of soil and soil-aggregate is
measured by the attenuation of gamma radiation where, in 1.4.3 Mark the test area to allow the placement of the gauge
over the test site and to align the probe to the hole.
direct transmission, the source is placed at a known depth up to
300 mm (12 in.) and the detector(s) remains on the surface
1.5 Units—The values stated in SI units are to be regarded
(some gauges may reverse this orientation); or in backscatter or
as standard. The values given in parentheses are provided for
backscatter/air-gap the source and detector(s) both remain on
information only and are not considered standard. Reporting
the surface.
the test results in units other than SI shall not be regarded as
1.2.1 The density of the test sample in mass per unit volume
nonconformance with this standard.
is calculated by comparing the detected rate of gamma radia-
1.6 All observed and calculated values shall conform to the
tion with previously established calibration data.
guide for significant digits and rounding established in Practice
1.2.2 The dry density of the test sample is obtained by
D6026.
subtracting the water mass per unit volume from the test
1.6.1 The procedures used to specify how data are collected,
sample wet density (Section 11). Most gauges display this
recorded, and calculated in this standard are regarded as the
value directly.
industry standard. In addition, they are representative of the
1.3 The gauge is calibrated to read the water mass per unit
significant digits that should generally be retained. The proce-
volume of soil or soil-aggregate. When divided by the density
dures used do not consider material variation, purpose for
of water and then multiplied by 100, the water mass per unit
obtaining the data, special purpose studies, or any consider-
volume is equivalent to the volumetric water content. The
ations for the user’s objectives; and it is common practice to
water mass per unit volume is determined by the thermalizing
increase or reduce significant digits of reported data to be
or slowing of fast neutrons by hydrogen, a component of water.
commensurate with these considerations. It is beyond the scope
The neutron source and the thermal neutron detector are both
of this standard to consider significant digits used in analysis
located at the surface of the material being tested. The water
methods for engineering design.
content most prevalent in engineering and construction activi-
1.7 Limitations—This test method is not applicable to clean
ties is known as the gravimetric water content, w, and is the
gravel or clean crushed rock due to excessive surface voids
which have the potential to affect gauge measurements.
1.8 This standard does not purport to address all of the
This test method is under the jurisdiction of ASTM Committee D18 on Soil and
Rock and is the direct responsibility of Subcommittee D18.08 on Special and
safety concerns, if any, associated with its use. It is the
Construction Control Tests.
responsibility of the user of this standard to establish appro-
Current edition approved May 1, 2023. Published May 2023. Originally
ɛ1
priate safety, health, and environmental practices and deter-
approved in 2006. Last previous edition approved in 2017 as D6938–17a . DOI:
10.1520/D6938-23. mine the applicability of regulatory limitations prior to use.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D6938 − 23
1.9 This international standard was developed in accor- 3. Terminology
dance with internationally recognized principles on standard-
3.1 Definitions—See Terminology D653 for general defini-
ization established in the Decision on Principles for the
tions.
Development of International Standards, Guides and Recom-
3.2 Definitions of Terms Specific to This Standard:
mendations issued by the World Trade Organization Technical
3.2.1 nuclear gauge—a device containing one or more
Barriers to Trade (TBT) Committee.
radioactive sources used to measure certain properties of soil
and soil-aggregates.
2. Referenced Documents
3.2.2 thermalization—the process of “slowing down” fast
2.1 ASTM Standards:
neutrons by collisions with light-weight atoms, such as hydro-
D653 Terminology Relating to Soil, Rock, and Contained
gen.
Fluids
3.2.3 test count, n—the measured output of a gamma ray or
D698 Test Methods for Laboratory Compaction Character-
neutron detector for a specific type of radiation for a given test.
istics of Soil Using Standard Effort (12,400 ft-lbf/ft (600
3.2.4 prepared blocks—blocks prepared of soil, solid rock,
kN-m/m ))
concrete, and engineered materials, that have characteristics of
D1556 Test Method for Density and Unit Weight of Soil in
various degrees of reproducible uniformity.
Place by Sand-Cone Method
D1557 Test Methods for Laboratory Compaction Character-
4. Significance and Use
istics of Soil Using Modified Effort (56,000 ft-lbf/ft
4.1 The test method described is useful as a rapid, nonde-
(2,700 kN-m/m ))
structive technique for in-place measurements of wet density
D2167 Test Method for Density and Unit Weight of Soil in
and water content of soil and soil-aggregate and the determi-
Place by the Rubber Balloon Method
nation of dry density.
D2487 Practice for Classification of Soils for Engineering
4.2 The test method is used for quality control and accep-
Purposes (Unified Soil Classification System)
tance testing of compacted soil and soil-aggregate mixtures as
D2488 Practice for Description and Identification of Soils
used in construction and also for research and development.
(Visual-Manual Procedures)
The nondestructive nature allows repetitive measurements at a
D2216 Test Methods for Laboratory Determination of Water
single test location and statistical analysis of the results.
(Moisture) Content of Soil and Rock by Mass
4.3 Density—The fundamental assumptions inherent in the
D2937 Test Method for Density of Soil in Place by the
methods are that Compton scattering is the dominant interac-
Drive-Cylinder Method
tion and that the material is homogeneous.
D3740 Practice for Minimum Requirements for Agencies
Engaged in Testing and/or Inspection of Soil and Rock as 4.4 Water Content—The fundamental assumptions inherent
Used in Engineering Design and Construction
in the test method are that the hydrogen ions present in the soil
D4253 Test Methods for Maximum Index Density and Unit or soil-aggregate are in the form of water as defined by the
Weight of Soils Using a Vibratory Table water content derived from Test Methods D2216, and that the
material is homogeneous. (See 5.2)
D4254 Test Methods for Minimum Index Density and Unit
Weight of Soils and Calculation of Relative Density
NOTE 1—The quality of the result produced by this standard test method
D4643 Test Method for Determination of Water Content of
is dependent on the competence of the personnel performing it, and the
suitability of the equipment and facilities used. Agencies that meet the
Soil and Rock by Microwave Oven Heating
criteria of Practice D3740 are generally considered capable of competent
D4718 Practice for Correction of Unit Weight and Water
and objective testing/sampling/inspection, and the like. Users of this
Content for Soils Containing Oversize Particles
standard are cautioned that compliance with Practice D3740 does not in
D4944 Test Method for Field Determination of Water (Mois-
itself ensure reliable results. Reliable results depend on many factors;
ture) Content of Soil by the Calcium Carbide Gas Pressure Practice D3740 provides a means of evaluating some of those factors.
Tester
5. Interferences
D4959 Test Method for Determination of Water Content of
5.1 In-Place Density Interferences
Soil By Direct Heating
5.1.1 Measurements may be affected by the chemical com-
D6026 Practice for Using Significant Digits and Data Re-
position of the material being tested.
cords in Geotechnical Data
5.1.2 Measurements may be affected by non-homogeneous
D7013 Guide for Calibration Facility Setup for Nuclear
soils and surface texture (see 10.2). Excessive voids in the
Surface Gauges
prepared test surface beneath the gauge can cause density
D7759 Guide for Nuclear Surface Moisture and Density
measurements that are lower than the actual soil density.
Gauge Calibration
Excessive use of fill material to compensate for these voids
may likewise cause biased density measurements, or biased
water content measurements, or both.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
5.1.3 Measurements in the Backscatter Mode are influenced
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
more by the density and water content of the material in
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. proximity to the surface.
D6938 − 23
5.1.4 Measurements in the Direct Transmission mode are an 6.1.4 Slow Neutron Detector—Any type of slow neutron
average of the density from the bottom of the probe in the soil detector such as boron trifluoride or helium-3 proportional
or soil aggregate back up to the surface of the gauge. counter.
5.1.5 Gravel particles or large voids in the source-detector
6.2 Reference Standard—A block of material used for
path may cause higher or lower density measurements. Where
checking instrument operation, correction of source decay, and
lack of uniformity in the soil due to layering, aggregate or
to establish conditions for a reproducible reference count rate.
voids is suspected, the test site shall be excavated and visually
6.3 Site Preparation Device—A plate, straightedge, or other
examined to determine whether the test material is representa-
suitable leveling tool that may be used for planing the test site
tive of the in situ material in general and whether an oversize
to the required smoothness, and in the Direct Transmission
correction is required in accordance with Practice D4718.
Method, guiding the drive pin to prepare a perpendicular hole.
5.1.6 Oversize particles or large voids in the source-detector
path may cause higher or lower density measurements. Where 6.4 Drive Pin—A pin of slightly larger diameter than the
lack of uniformity in the soil due to layering, aggregate or
probe in the Direct Transmission Instrument used to prepare a
voids is suspected, the test site shall be excavated and visually hole in the test site for inserting the probe.
examined to determine if the test material is representative of 6.4.1 Drive Pin Guide—A fixture, generally part of the site
the in situ material in general and if an oversize correction is preparation device, that keeps the drive pin perpendicular to
required in accordance with Practice D4718. the test site.
5.1.7 The measured volume is approximately 0.0028 m
6.5 Hammer—Heavy enough to drive the pin to the required
3 3 3
(0.10 ft ) for the Backscatter Mode and 0.0057 m (0.20 ft ) for
depth without undue distortion of the hole.
the Direct Transmission Mode when the test depth is 150 mm
6.6 Drive Pin Extractor—A tool that may be used to remove
(6 in.). The actual measured volume is indeterminate and varies
the drive pin in a vertical direction so that the pin will not
with the apparatus and the density of the material.
distort the hole in the extraction process.
5.1.8 Other radioactive sources must not be within 9 m (30
ft) of equipment in operation. 6.7 Slide Hammer, with a drive pin attached, may also be
used both to prepare a hole in the material to be tested and to
5.2 In-Place Water (Moisture) Content Interferences
extract the pin without distortion to the hole.
5.2.1 The chemical and elemental composition of the mate-
rial being tested can affect the measurement and adjustments
6.8 Probe, a slender, elongated device, part of the gauge,
may be necessary (see 10.6). Materials containing high that is inserted into the soil under measurement by the gauge.
amounts of carbon or molecularly bound hydrogen will cause
This device may contain either a radioactive source, a radiation
measurements in excess of the true value. Some chemical detection device, or both. Probes containing only a radioactive
elements such as boron, chlorine, and cadmium will cause
source are commonly referred to as “source rods.”
measurements lower than the true value.
7. Hazards
5.2.2 The water content measured by this test method is not
necessarily the average water content within the volume of the
7.1 These gauges utilize radioactive materials that may be
sample involved in the measurement. Since this measurement
hazardous to the health of the users unless proper precautions
is by backscatter in all cases, the value is biased by the water
are taken. Users of these gauges must become familiar with
content of the material closest to the surface. The volume of
applicable safety procedures and government regulations.
soil and soil-aggregate represented in the measurement is
7.2 Effective user instructions, together with routine safety
indeterminate and will vary with the water content of the
procedures and knowledge of and compliance with Regulatory
material. In general, the greater the water content of the
Requirements, are a mandatory part of the operation and
material, the smaller the volume involved in the measurement.
storage of these gauges.
Approximately 50 % of the typical measurement results from
the water content of the upper 50 to 75 mm (2 to 3 in.).
8. Calibration
5.2.3 Other neutron sources must not be within 9 m (30 ft)
8.1 Gauge calibration shall be performed in accordance with
of equipment in operation.
Guides D7013 and D7759.
6. Apparatus
9. Standardization
6.1 Nuclear Density / Moisture Gauge—While exact details
9.1 Nuclear moisture density gauges are subject to long-
of construction of the apparatus may vary, the system shall
term aging of the radioactive sources, which may change the
consist of:
relationship between count rates and the material density and
6.1.1 Gamma Source—A sealed source of high-energy
water content. To correct for this aging effect, gauges are
gamma radiation such as cesium or radium.
calibrated as a ratio of the measurement count rate to a count
6.1.2 Gamma Detector—Any type of gamma detector such
rate made on a reference standard or to an air-gap count (for the
as a Geiger-Mueller tube(s).
backscatter/air-gap ratio method).
6.1.3 Fast Neutron Source—A sealed mixture of a radioac-
tive material such as americium, radium and a target material 9.2 Standardization of the gauge shall be performed at the
such as beryllium, or a neutron emitter such as californium- start of each day’s use, and a record of these data shall be
252. retained for the amount of time required to ensure compliance
D6938 − 23
with either subsection 9.2.3 or Annex A3, whichever is 10.3 Turn on and allow the gauge to stabilize (warm up)
applicable. Perform the standardization with the gauge located according to the manufacturer’s recommendations (see Section
at least 9 m (30 ft) away from other nuclear moisture density 9.2.1).
gauges and clear of large masses of water or other items which
10.4 Procedure A—The Direct Transmission Procedure:
can affect the reference count rates.
10.4.1 Select a test location where the gauge in test position
9.2.1 Turn on the gauge and allow for stabilization accord-
will be at least 150 mm (6 in.) away from any vertical
ing to the manufacturer’s recommendations.
projection.
9.2.2 Using the reference standard, take a reading that is at
10.4.2 Make a hole perpendicular to the prepared surface
least four times the duration of a normal measurement period
using either (a) the drive pin guide, the guide pin extractor, a
(where a normal measurement period is typically one minute)
hammer, and drive pin, (b) a slide hammer, or (c) a drill. The
to constitute one standardization check.
hole shall be a minimum of 50 mm (2 in.) deeper than the
9.2.3 When available, the procedure recommended by the
desired measurement depth and of an alignment that insertion
gauge manufacturer shall be used to establish the compliance
of the probe will not cause the gauge to tilt from the plane of
of the standard measurement to the accepted range. Without
the prepared area.
specific recommendations from the gauge manufacturer, use
10.4.3 Mark the test area to allow the placement of the
the procedure described in Annex A3.
gauge over the test site and to align the probe to the hole.
9.2.4 If for any reason the measured density or moisture Follow the manufacturer’s recommendations if applicable.
becomes suspect during the day’s use, perform another stan- 10.4.4 Remove the hole-forming device carefully to prevent
dardization check. the distortion of the hole, damage to the surface, or loose
material to fall into the hole.
10.4.4.1 When preparing an access hole in cohesionless
10. Procedure
soils, care shall be taken in the preparation of the access hole;
10.1 When possible, select a test location where the gauge
measurements have the potential to be affected by changes to
will be placed at least 600 mm (24.0 in) away from any object
the density of surrounding material during the hole formation.
sitting on or projecting above the surface of the test location,
10.4.5 Place the gauge on the material to be tested, ensuring
when the presence of this object has the potential to modify
maximum surface contact as described previously in 10.2.4.
gauge response. Any time a measurement must be made at a
10.4.6 Lower the probe into the hole to the desired test
specific location and the aforementioned clearance cannot be
depth. Pull the gauge gently toward the back, or detector end,
achieved, such as in a trench, follow the gauge manufacturer’s
so that the back side of the probe is in intimate contact with the
correction procedure(s).
side of the hole in the gamma measurement path.
10.2 Prepare the test site in the following manner:
NOTE 2—As a safety measure, it is recommended that a probe
10.2.1 Remove all loose and disturbed material and addi-
containing radioactive sources not be extended out of its shielded position
tional material as necessary to expose the undisturbed surface
prior to placing it into the test site. When possible, align the gauge so as
to allow placing the probe directly into the test hole from the shielded
of the material to be tested.
position.
10.2.2 Prepare an area sufficient in size to accommodate the
10.4.7 Keep all other radioactive sources at least 9 m (30 ft)
gauge by grading or scraping the area to a smooth condition so
away from the gauge to avoid any effect on the measurement.
as to obtain maximum contact between the gauge and material
10.4.8 If the gauge is so equipped, set the depth selector to
being tested.
the same depth as the probe.
10.2.3 The depth of the maximum void beneath the gauge
10.4.9 Secure and record one or more one-minute density
shall not exceed 3 mm ( ⁄8 in.). Use either native material that
and water content readings. Read the in-place wet density
does not contain gravel or fine sand to fill the voids, and then
directly or determine one by use of the calibration curve or
smooth the surface with the site preparation device or other
table previously established.
suitable tool. The depth of the filler shall not exceed approxi-
10.4.10 Read the water content directly or determine the
mately 3 mm ( ⁄8 in.).
water content by use of the calibration curve or table previ-
10.2.3.1 If the grading or scraping of the test area dislodges
ously established.
rocks that leave a void greater than 3 mm ( ⁄8 in.), the dislodged
rock can be replaced, or a smaller rock in combination with 10.5 Procedure B—The Backscatter or Backscatter/Air-Gap
fine sand or native material that does not contain gravel can be
Ratio Procedure:
used to fill the void. The fine sand or native material that does 10.5.1 Seat the gauge firmly (see 10.4.5).
not contain gravel used to fill the remainder of the void not
10.5.2 Keep all other radioactive sources at least 9 m (30 ft)
filled by a smaller rock shall not exceed approximately 3 mm
away from the gauge to avoid affecting the measurement.
( ⁄8 in.).
10.5.3 Set the gauge into the Backscatter (BS) position.
10.2.4 The placement of the gauge on the surface of the 10.5.4 Secure and record one or more set(s) of one-minute
material to be tested is critical to accurate density measure- density and water content readings. When using the
ments. The optimum condition is total contact between the backscatter/air-gap ratio mode, follow the manufacturer’s in-
bottom surface of the gauge and the surface of the material structions regarding gauge setup. Take the same number of
being tested. The total area filled shall not exceed approxi- readings for the normal measurement period in the air-gap
mately 10 percent of the bottom area of the gauge. position as in the standard backscatter position. Calculate the
D6938 − 23
air-gap ratio by dividing the counts per minute obtained in the M × 100
m
w 5 (1)
air-gap position by the counts per minute obtained in the ρ
d
standard position. Many gauges have built-in provisions for
or,
automatically calculating the air-gap ratio and wet density.
M × 100
m
10.5.5 Read the in-place wet density or determine one by
w 5 (2)
ρ 2 M
m
use of the calibration curve or table previously established.
10.5.6 Read the water content or determine one by use of
where:
the calibration curve or previously established table (see
w = water content
3 3
Section 10.6).
ρ = dry density in kg/m or (lbm/ft ),
d
3 3
ρ = wet density in kg/m or (lbm/ft ), and
NOTE 3—Gauge measurements acquired using either Procedure A or
3 3
M = water mass per unit volume in kg/m or (lbm/ft )
m
Procedure B yield both density and water content values for the material
under test. It is good practice to record gauge density and water counts
11.2.3 If the water content reading was in “counts,” deter-
corresponding to the density and water values at the time of measurement
mine the water mass per unit volume by use of this reading and
in the event that data recording errors or improper probe depth errors are
previously established calibration curve or table. Then convert
of concern.
to gravimetric water content in accordance with 11.2.2.
10.6 Water Content Correction and Oversize Particle Cor-
11.2.4 Record water content to the nearest 0.1 %.
rection
11.3 Determine the Dry Density of the soil by one of the
10.6.1 For proper use of th
...

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
´1
Designation: D6938 − 17a D6938 − 23
Standard Test Methods for
In-Place Density and Water Content of Soil and Soil-
Aggregate by Nuclear Methods (Shallow Depth)
This standard is issued under the fixed designation D6938; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
ε NOTE—Editorially corrected units of measurement statement in June 2021.
1. Scope*
1.1 This test method describes the procedures for measuring in-place density and moisture of soil and soil-aggregate by use of
nuclear equipment (hereafter referred to as “gauge”). The density of the material may be measured by direct transmission,
backscatter, or backscatter/air-gap ratio methods. Measurements for water (moisture) content are taken at the surface in backscatter
mode regardless of the mode being used for density.
1.1.1 For limitations see Section 5 on Interferences.
1.2 The total or wet density of soil and soil-aggregate is measured by the attenuation of gamma radiation where, in direct
transmission, the source is placed at a known depth up to 300 mm (12 in.) and the detector(s) remains on the surface (some gauges
may reverse this orientation); or in backscatter or backscatter/air-gap the source and detector(s) both remain on the surface.
1.2.1 The density of the test sample in mass per unit volume is calculated by comparing the detected rate of gamma radiation with
previously established calibration data.
1.2.2 The dry density of the test sample is obtained by subtracting the water mass per unit volume from the test sample wet density
(Section 11). Most gauges display this value directly.
1.3 The gauge is calibrated to read the water mass per unit volume of soil or soil-aggregate. When divided by the density of water
and then multiplied by 100, the water mass per unit volume is equivalent to the volumetric water content. The water mass per unit
volume is determined by the thermalizing or slowing of fast neutrons by hydrogen, a component of water. The neutron source and
the thermal neutron detector are both located at the surface of the material being tested. The water content most prevalent in
engineering and construction activities is known as the gravimetric water content, w, and is the ratio of the mass of the water in
pore spaces to the total mass of solids, expressed as a percentage.
1.4 Two alternative procedures are provided.
1.4.1 Procedure A describes the direct transmission method in which the probe extends through the base of the gauge into a
pre-formed hole to a desired depth. The direct transmission is the preferred method.
This test method is under the jurisdiction of ASTM Committee D18 on Soil and Rock and is the direct responsibility of Subcommittee D18.08 on Special and Construction
Control Tests.
Current edition approved Nov. 1, 2017May 1, 2023. Published December 2017May 2023. Originally approved in 2006. Last previous edition approved in 2017 as
ɛ1
D6938–17.–17a . DOI: 10.1520/D6938-17AE01.10.1520/D6938-23.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D6938 − 23
1.4.2 Procedure B involves the use of a dedicated backscatter gauge or the probe in the backscatter position. This places the
gamma and neutron sources and the detectors in the same plane.
1.4.3 Mark the test area to allow the placement of the gauge over the test site and to align the probe to the hole.
1.5 Units—The values stated in SI units are to be regarded as standard. The values given in parentheses are provided for
information only and are not considered standard. Reporting the test results in units other than SI shall not be regarded as
nonconformance with this standard.
1.6 All observed and calculated values shall conform to the guide for significant digits and rounding established in Practice D6026.
1.6.1 The procedures used to specify how data are collected, recorded, and calculated in this standard are regarded as the industry
standard. In addition, they are representative of the significant digits that should generally be retained. The procedures used do not
consider material variation, purpose for obtaining the data, special purpose studies, or any considerations for the user’s objectives;
and it is common practice to increase or reduce significant digits of reported data to be commensurate with these considerations.
It is beyond the scope of this standard to consider significant digits used in analysis methods for engineering design.
1.7 Limitations—This test method is not applicable to clean gravel or clean crushed rock due to excessive surface voids which
have the potential to affect gauge measurements.
1.8 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of
regulatory limitations prior to use.
1.9 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
D653 Terminology Relating to Soil, Rock, and Contained Fluids
3 3
D698 Test Methods for Laboratory Compaction Characteristics of Soil Using Standard Effort (12,400 ft-lbf/ft (600 kN-m/m ))
D1556 Test Method for Density and Unit Weight of Soil in Place by Sand-Cone Method
D1557 Test Methods for Laboratory Compaction Characteristics of Soil Using Modified Effort (56,000 ft-lbf/ft (2,700
kN-m/m ))
D2167 Test Method for Density and Unit Weight of Soil in Place by the Rubber Balloon Method
D2487 Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System)
D2488 Practice for Description and Identification of Soils (Visual-Manual Procedures)
D2216 Test Methods for Laboratory Determination of Water (Moisture) Content of Soil and Rock by Mass
D2937 Test Method for Density of Soil in Place by the Drive-Cylinder Method
D3740 Practice for Minimum Requirements for Agencies Engaged in Testing and/or Inspection of Soil and Rock as Used in
Engineering Design and Construction
D4253 Test Methods for Maximum Index Density and Unit Weight of Soils Using a Vibratory Table
D4254 Test Methods for Minimum Index Density and Unit Weight of Soils and Calculation of Relative Density
D4643 Test Method for Determination of Water Content of Soil and Rock by Microwave Oven Heating
D4718 Practice for Correction of Unit Weight and Water Content for Soils Containing Oversize Particles
D4944 Test Method for Field Determination of Water (Moisture) Content of Soil by the Calcium Carbide Gas Pressure Tester
D4959 Test Method for Determination of Water Content of Soil By Direct Heating
D6026 Practice for Using Significant Digits and Data Records in Geotechnical Data
D7013 Guide for Calibration Facility Setup for Nuclear Surface Gauges
D7759 Guide for Nuclear Surface Moisture and Density Gauge Calibration
3. Terminology
3.1 Definitions—See Terminology D653 for general definitions.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
D6938 − 23
3.2 Definitions of Terms Specific to This Standard:
3.2.1 nuclear gauge—a device containing one or more radioactive sources used to measure certain properties of soil and
soil-aggregates.
3.2.2 gamma (radiation) source—a sealed source of radioactive material that emits gamma radiation as it decays.
3.2.3 neutron (radiation) source—a sealed source of radioactive material that emits neutron radiation as it decays.
3.2.4 Compton scattering—the interaction between a gamma ray (photon) and an orbital electron where the gamma ray loses
energy and rebounds in a different direction.
3.2.5 detector—a device to detect and measure radiation.
3.2.6 gravimetric water content—same as water content (as defined in Terminology D653), a nomenclature used in some scientific
fields to differentiate it from volumetric water content.
3.2.2 thermalization—the process of “slowing down” fast neutrons by collisions with light-weight atoms, such as hydrogen.
3.2.8 volumetric water content—the volume of water as a percent of the total volume of soil or rock material.
3.2.3 test count, n—the measured output of a gamma ray or neutron detector for a specific type of radiation for a given test.
3.2.4 prepared blocks—blocks prepared of soil, solid rock, concrete, and engineered materials, that have characteristics of various
degrees of reproducible uniformity.
4. Significance and Use
4.1 The test method described is useful as a rapid, nondestructive technique for in-place measurements of wet density and water
content of soil and soil-aggregate and the determination of dry density.
4.2 The test method is used for quality control and acceptance testing of compacted soil and soil-aggregate mixtures as used in
construction and also for research and development. The nondestructive nature allows repetitive measurements at a single test
location and statistical analysis of the results.
4.3 Density—The fundamental assumptions inherent in the methods are that Compton scattering is the dominant interaction and
that the material is homogeneous.
4.4 Water Content—The fundamental assumptions inherent in the test method are that the hydrogen ions present in the soil or
soil-aggregate are in the form of water as defined by the water content derived from Test Methods D2216, and that the material
is homogeneous. (See 5.2)
NOTE 1—The quality of the result produced by this standard test method is dependent on the competence of the personnel performing it, and the suitability
of the equipment and facilities used. Agencies that meet the criteria of Practice D3740 are generally considered capable of competent and objective
testing/sampling/inspection, and the like. Users of this standard are cautioned that compliance with Practice D3740 does not in itself ensure reliable
results. Reliable results depend on many factors; Practice D3740 provides a means of evaluating some of those factors.
5. Interferences
5.1 In-Place Density Interferences
5.1.1 Measurements may be affected by the chemical composition of the material being tested.
5.1.2 Measurements may be affected by non-homogeneous soils and surface texture (see 10.2). Excessive voids in the prepared
D6938 − 23
test surface beneath the gauge can cause density measurements that are lower than the actual soil density. Excessive use of fill
material to compensate for these voids may likewise cause biased density measurements, or biased water content measurements,
or both.
5.1.3 Measurements in the Backscatter Mode are influenced more by the density and water content of the material in proximity
to the surface.
5.1.4 Measurements in the Direct Transmission mode are an average of the density from the bottom of the probe in the soil or
soil aggregate back up to the surface of the gauge.
5.1.5 Gravel particles or large voids in the source-detector path may cause higher or lower density measurments.measurements.
Where lack of uniformity in the soil due to layering, aggregate or voids is suspected, the test site shouldshall be excavated and
visually examined to determine whether the test material is representative of the in situ material in general and whether an oversize
correction is required in accordance with Practice D4718.
5.1.6 Oversize particles or large voids in the source-detector path may cause higher or lower density measurements. Where lack
of uniformity in the soil due to layering, aggregate or voids is suspected, the test site shouldshall be excavated and visually
examined to determine if the test material is representative of the in situ material in general and if an oversize correction is required
in accordance with Practice D4718.
3 3 3 3
5.1.7 The measured volume is approximately 0.0028 m (0.10 ft ) for the Backscatter Mode and 0.0057 m (0.20 ft ) for the Direct
Transmission Mode when the test depth is 150 mm (6 in.). The actual measured volume is indeterminate and varies with the
apparatus and the density of the material.
5.1.8 Other radioactive sources must not be within 9 m (30 ft) of equipment in operation.
5.2 In-Place Water (Moisture) Content Interferences
5.2.1 The chemical and elemental composition of the material being tested can affect the measurement and adjustments may be
necessary (see Section 10.6). Hydrogen in forms other than water and carbon Materials containing high amounts of carbon or
molecularly bound hydrogen will cause measurements in excess of the true value. Some chemical elements such as boron, chlorine,
and cadmium will cause measurements lower than the true value.
5.2.2 The water content measured by this test method is not necessarily the average water content within the volume of the sample
involved in the measurement. Since this measurement is by backscatter in all cases, the value is biased by the water content of the
material closest to the surface. The volume of soil and soil-aggregate represented in the measurement is indeterminate and will vary
with the water content of the material. In general, the greater the water content of the material, the smaller the volume involved
in the measurement. Approximately 50 % of the typical measurement results from the water content of the upper 50 to 75 mm (2
to 3 in.).
5.2.3 Other neutron sources must not be within 9 m (30 ft) of equipment in operation.
6. Apparatus
6.1 Nuclear Density / Moisture Gauge—While exact details of construction of the apparatus may vary, the system shall consist
of:
6.1.1 Gamma Source—A sealed source of high-energy gamma radiation such as cesium or radium.
6.1.2 Gamma Detector—Any type of gamma detector such as a Geiger-Mueller tube(s).
6.1.3 Fast Neutron Source—A sealed mixture of a radioactive material such as americium, radium and a target material such as
beryllium, or a neutron emitter such as californium-252.
6.1.4 Slow Neutron Detector—Any type of slow neutron detector such as boron trifluoride or helium-3 proportional counter.
6.2 Reference Standard—A block of material used for checking instrument operation, correction of source decay, and to establish
conditions for a reproducible reference count rate.
D6938 − 23
6.3 Site Preparation Device—A plate, straightedge, or other suitable leveling tool that may be used for planing the test site to the
required smoothness, and in the Direct Transmission Method, guiding the drive pin to prepare a perpendicular hole.
6.4 Drive Pin—A pin of slightly larger diameter than the probe in the Direct Transmission Instrument used to prepare a hole in
the test site for inserting the probe.
6.4.1 Drive Pin Guide—A fixture fixture, generally part of the site preparation device, that keeps the drive pin perpendicular to
the test site. Generally part of the site preparation device.
6.5 Hammer—Heavy enough to drive the pin to the required depth without undue distortion of the hole.
6.6 Drive Pin Extractor—A tool that may be used to remove the drive pin in a vertical direction so that the pin will not distort
the hole in the extraction process.
6.7 Slide Hammer, with a drive pin attached, may also be used both to prepare a hole in the material to be tested and to extract
the pin without distortion to the hole.
6.8 Probe, a slender, elongated device, part of the gauge, that is inserted into the soil under measurement by the gauge. This device
may contain either a radioactive source, a radiation detection device, or both. Probes containing only a radioactive source are
commonly referred to as “source rods.”
7. Hazards
7.1 These gauges utilize radioactive materials that may be hazardous to the health of the users unless proper precautions are taken.
Users of these gauges must become familiar with applicable safety procedures and government regulations.
7.2 Effective user instructions, together with routine safety procedures and knowledge of and compliance with Regulatory
Requirements, are a mandatory part of the operation and storage of these gauges.
8. Calibration
8.1 Gauge calibration shall be performed in accordance with Guides D7013 and D7759.
9. Standardization
9.1 Nuclear moisture density gauges are subject to long-term aging of the radioactive sources, which may change the relationship
between count rates and the material density and water content. To correct for this aging effect, gauges are calibrated as a ratio
of the measurement count rate to a count rate made on a reference standard or to an air-gap count (for the backscatter/air-gap ratio
method).
9.2 Standardization of the gauge shall be performed at the start of each day’s use, and a record of these data shouldshall be retained
for the amount of time required to ensure compliance with either subsection 9.2.3 or Annex A3, whichever is applicable. Perform
the standardization with the gauge located at least 9 m (30 ft) away from other nuclear moisture density gauges and clear of large
masses of water or other items which can affect the reference count rates.
9.2.1 Turn on the gauge and allow for stabilization according to the manufacturer’s recommendations.
9.2.2 Using the reference standard, take a reading that is at least four times the duration of a normal measurement period (where
a normal measurement period is typically one minute) to constitute one standardization check.
9.2.3 When available, use the procedure recommended by the gauge manufacturer shall be used to establish the compliance of the
standard measurement to the accepted range. Without specific recommendations from the gauge manufacturer, use the procedure
described in Annex A3.
D6938 − 23
9.2.4 If for any reason the measured density or moisture becomes suspect during the day’s use, perform another standardization
check.
10. Procedure
10.1 When possible, select a test location where the gauge will be placed at least 600 mm (24.0 in) away from any object sitting
on or projecting above the surface of the test location, when the presence of this object has the potential to modify gauge response.
Any time a measurement must be made at a specific location and the aforementioned clearance cannot be achieved, such as in a
trench, follow the gauge manufacturer’s correction procedure(s).
10.2 Prepare the test site in the following manner:
10.2.1 Remove all loose and disturbed material and additional material as necessary to expose the trueundisturbed surface of the
material to be tested.
10.2.2 Prepare an area sufficient in size to accommodate the gauge by grading or scraping the area to a smooth condition so as
to obtain maximum contact between the gauge and material being tested.
10.2.3 The depth of the maximum void beneath the gauge shall not exceed 3 mm ( ⁄8 in.). Use either native material that does not
contain gravel or fine sand to fill the voids, and then smooth the surface with the site preparation device or other suitable tool. The
depth of the filler shouldshall not exceed approximately 3 mm ( ⁄8 in.).
10.2.3.1 If the grading or scraping of the test area dislodges rocks that leave a void greater than 3 mm ( ⁄8 in.), the dislodged rock
can be replaced, or a smaller rock in combination with fine sand or native material that does not contain gravel can be used to fill
the void. The fine sand or native material that does not contain gravel used to fill the remainder of the void not filled by a smaller
rock shall not exceed approximately 3 mm ( ⁄8 in.).
10.2.4 The placement of the gauge on the surface of the material to be tested is critical to accurate density measurements. The
optimum condition is total contact between the bottom surface of the gauge and the surface of the material being tested. The total
area filled shouldshall not exceed approximately 10 percent of the bottom area of the gauge.
10.3 Turn on and allow the gauge to stabilize (warm up) according to the manufacturer’s recommendations (see Section 9.2.1).
10.4 Procedure A - The —The Direct Transmission Procedure:
10.4.1 Select a test location where the gauge in test position will be at least 150 mm (6 in.) away from any vertical projection.
10.4.2 Make a hole perpendicular to the prepared surface using either (a) the drive pin guide, the guide pin extractor, a hammer,
and drive pin, or (b) a slide hammer. hammer, or (c) a drill. The hole shouldshall be a minimum of 50 mm (2 in.) deeper than the
desired measurement depth and of an alignment that insertion of the probe will not cause the gauge to tilt from the plane of the
prepared area.
10.4.3 Mark the test area to allow the placement of the gauge over the test site and to align the probe to the hole. Follow the
manufacturer’s recommendations if applicable.
10.4.4 Remove the hole-forming device carefully to prevent the distortion of the hole, damage to the surface, or loose material
to fall into the hole.
NOTE 2—Care must be taken in the preparation of the access hole in uniform cohesionless granular soils. Measurements can be affected by damage to
the density of surrounding materials when forming the hole.
10.4.4.1 When preparing an access hole in cohesionless soils, care shall be taken in the preparation of the access hole;
measurements have the potential to be affected by changes to the density of surrounding material during the hole formation.
10.4.5 Place the gauge on the material to be tested, ensuring maximum surface contact as described previously in 10.2.4.
D6938 − 23
10.4.6 Lower the probe into the hole to the desired test depth. Pull the gauge gently toward the back, or detector end, so that the
back side of the probe is in intimate contact with the side of the hole in the gamma measurement path.
NOTE 2—As a safety measure, it is recommended that a probe containing radioactive sources not be extended out of its shielded position prior to placing
it into the test site. When possible, align the gauge so as to allow placing the probe directly into the test hole from the shielded position.
10.4.7 Keep all other radioactive sources at least 9 m (30 feet)ft) away from the gauge to avoid any effect on the measurement.
10.4.8 If the gauge is so equipped, set the depth selector to the same depth as the probe.
10.4.9 Secure and record one or more one-minute density and water content readings. Read the in-place wet density directly or
determine one by use of the calibration curve or table previously established.
10.4.10 Read the water content directly or determine the water content by use of the calibration curve or table previously
established.
10.5 Procedure B -The —The Backscatter or Backscatter/Air-Gap Ratio Procedure:
10.5.1 Seat the gauge firmly (see Note 210.4.5).
10.5.2 Keep all other radioactive sources at least 9 m (30 ft) away from the gauge to avoid affecting the measurement.
10.5.3 Set the gauge into the Backscatter (BS) position.
10.5.4 Secure and record one or more set(s) of one-minute density and water content readings. When using the backscatter/air-gap
ratio mode, follow the manufacturer’s instructions regarding gauge setup. Take the same number of readings for the normal
measurement period in the air-gap position as in the standard backscatter position. Calculate the air-gap ratio by dividing the counts
per minute obtained in the air-gap position by the counts per minute obtained in the standard position. Many gauges have built-in
provisions for automatically calculating the air-gap ratio and wet density.
10.5.5 Read the in-place wet density or determine one by use of the calibration curve or table previously established.
10.5.6 Read the water content or determine one by use of the calibration curve or previously established table (see Section 10.6).
NOTE 3—Gauge measurements acquired using either Procedure A or Procedure B yield both density and water content values for the material under test.
It is good practice to record gauge density and water counts corresponding to the density and water values at the time of measurement in the event that
data recording errors or improper probe depth errors are of concern.
10.6 Water Content Correction and Oversize Particle Correction
10.6.1 For proper use of the gauge and accurate values of both water content and dry density, both of these corrections need to
be made when applicable.
Prior to using the gauge-derived water content on any new material, the value should be verified by comparison to another
ASTM method such as Test Methods D2216, D4643, D4944, or D4959. As part of a user developed procedure, occasional samples
should be taken from beneath the gauge and comparison testing done to confirm gauge-derived water content values. All gauge
manufacturers have a procedure for correcting the gauge-derived water content values.
10.6.1.1 Prior to using the gauge-derived water content on any new material, the value shall be verified by comparison to another
ASTM method such as Test Methods D2216, D4643, D4944, or D4959. As part of a user developed procedure, occasional samples
shall be taken from beneath the gauge and comparison testing done to confirm gauge-derived water content values. All gauge
manufacturers have a procedure for correcting the gauge-derived water content values.
10.6.2 When oversize particles are present, the gauge can be rotated about the axis of the probe to obtain additional readings as
a check. When there is any uncertainty as to the presence of these particles it is advisable to sample the material beneath the gauge
to verify the presence and the relative proportion of the oversize particles. A rock correction can then
...