Reference Manual
Module 2-5. Field Sampling and Testing
MODULE 2-5. FIELD SAMPLING AND TESTING
1.
INSTRUCTIONAL OBJECTIVES
This module describes procedures for conducting an effective field sampling, field testing, and laboratory
testing program. This can be a key component in an overall project level evaluation process, especially
when there is uncertainty in the layer conditions and properties of the existing pavement.
At the conclusion of this module, the participant should be able to accomplish the following:
1. Explain the purpose of conducting field sampling and testing.
2. Describe typical field sampling and testing procedures.
3. Describe commonly used laboratory test methods and their applications.
4. Describe the use of field and laboratory test results as part of the rehabilitation design process.
2.
INTRODUCTION
When performing a pavement evaluation, it is often necessary to conduct a more detailed investigation of
the in-place materials of a candidate pavement section. Typically, an analysis of these material properties
can be accomplished by (1) conducting in situ field testing, or (2) retrieving field samples and performing
laboratory tests. The results of these tests are used to validate NDT results (such as falling FWD
deflection testing), to help identify the mechanisms of certain distresses, or simply to obtain
representative engineering properties of the existing pavement layers. The emergence of new mechanistic
pavement design procedures has also increased the need for the assessment of the material properties of
the existing pavement structure.
This module begins with a presentation of common definitions, as well as a more detailed discussion of
the purpose of field and laboratory sampling and testing as it is used in the pavement evaluation process.
Next, the more common field sampling, field (in situ) testing, and laboratory testing methods (and their
applications) are discussed. This module addresses the testing procedures used to evaluate the properties
of all pavement layers including the HMA surface layer, bound pavement layers (asphalt and portland
cement), and granular and cohesive subgrade soils. Special emphasis is placed on the relatively new
techniques used to obtain engineering properties of existing pavement layers for the purpose of
mechanistic pavement design procedures.
3.
DEFINITIONS
General Definitions
Field Sampling. The collection of material samples from an in-place pavement. Examples of field
sampling techniques include coring, subsurface boring, split-spoon sampling, and trenches and test pits.
Field Testing. The conduct of testing in the field on the in situ pavement layer materials. This process
does not involve the actual collection of pavement material samples, but rather focuses on gathering direct
data (typically related to material strength) that characterizes the properties of the in situ materials. An
example of a field testing method is the dynamic cone penetrometer (DCP), which is used to assess
material strength.
Laboratory Testing. The assessment of material sample properties via the use of various testing
procedures performed in a controlled laboratory setting. Typical laboratory testing of pavement materials
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focuses on assessing material properties such as strength, deformation behavior under different loading
conditions, durability, and susceptibility to moisture damage. Examples of common laboratory tests
include visual inspection, soil classification, stiffness testing (e.g., resilient modulus), and strength testing
(e.g., indirect tensile strength testing).
Field Sampling and Testing
Augering. The process of removing an unbound material sample using a rotating helical (screw-type) bit,
which is typically power driven. The samples are used to determine gradations and moisture contents.
Coring. The process of drilling (cutting) and removing cylindrical material samples (cores) from the
pavement. Coring is conducted with the use of a hollow cylindrical diamond-bladed core barrel that is
attached to an anchored heavy-duty drill. Typical core barrels have inside diameters of 50-, 100-, or 150mm (2-, 4-, or 6-in).
Dynamic Cone Penetrometer (DCP). Manual or automated device used to measure the in situ strength of
subgrade soils or unstabilized base and subbase layers. The test consists of measuring the penetration rate
(mm/blow or in/blow) of a cone that occurs from dropping an 8 kg (16.7 lb) hammer on the device from a
fixed height of 57.5 cm (22.6 in). DCP penetration rates are used to identify pavement layer boundaries
and subgrade strata, as well as estimates of the strength and stiffness of those layers.
Shelby (Push) Tubes. Shelby tubes are thin-walled tubes (typically 75 mm [3 in] in diameter) used to
obtain relatively undisturbed soil samples. Typically used in soft cohesive soils, these thin-walled tubes
are pressed (not driven) into the soil.
Split-Spoon Sampler. The split-spoon sampler (also referred to the split-barrel sampler) is an open-ended
cylindrical tube used to retrieve soil samples in the field. The samples retrieved using this sampler have
most likely experienced some large strain disturbances; therefore, they most likely are used for soil
identification and classification purposes.
Standard Penetration Test (SPT) N-Value. The N-value is the output of the Standard Penetration Test
(SPT). The SPT is a very common in situ test used to measure the in situ strength of subgrade soils. This
test is conducted by driving a split-spoon sampler into the ground with blows from a 63.5-kg (140-lb)
hammer. The number of blows required to drive the sampler over the distance from 150 to 450 mm (6 to
18 in) below the surface is reported as the N-Value. Although not as common as other field-test methods
used in pavement evaluation procedures, the SPT continues to be used by many State agencies.
Test Pit. A form of sampling associated more with research and forensic investigations, test pits provide
the best means of obtaining undisturbed samples. Ranging in size from one-quarter to a full lane width,
they also provide a means of assessing subsurface rutting in HMA pavements.
Laboratory Testing
California Bearing Ratio (CBR). A laboratory test method that measures the resistance of a soil or
aggregate sample to penetration by a piston (1,935 mm2 [3 in2]) that is being pressed into the soil at a
standard rate of 1.3 mm (0.05 in) per minute. The penetration record for the sample is compared to the
penetration record for a standard, well-graded stone. The ratio of the load in the sample to the load in the
standard material, multiplied by 100, is the CBR for the sample material.
Hveem Resistance Value (R-Value). The R-Value is the Hveem Resistance value determined from
conducting a laboratory test with the Hveem Stabilometer. The Hveem Stabilometer indicates the overall
resistance of an unbound material sample to a compressive load force, as the device measures the
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Module 2-5. Field Sampling and Testing
transmitted horizontal pressure associated with the application of a vertical load. The R-Value is
calculated using the measured pressures.
Indirect Tensile Test. A test commonly used to determine the tensile strength of a bound material
specimen whereby tensile stresses are measured across the horizontal diameter of the specimen while a
compressive force is applied in the vertical direction.
Resilient Modulus. An engineering property of the base, subbase, subgrade layers often used in
mechanistic pavement design, equivalent to the ratio of the amplitude of the repeated axial stress to the
amplitude of the resultant recoverable axial strain. For unbound materials, the test is performed in a
triaxial stress state, whereas for bound materials, it is performed either in uniaxial compression or in
indirect tension.
4.
PURPOSE OF FIELD SAMPLING AND TESTING
The importance of adequately characterizing the structural characteristics of the existing pavement has
long been recognized, but recent years have seen an increase in activities as agencies have placed a
stronger emphasis on rehabilitating their existing pavement networks. In addition, modern mechanistic
pavement design procedures require more detailed material property information. The following describe
the typical situations in which field sampling and testing should be considered when evaluating an HMA
pavement in preparation for some rehabilitation treatment.
Calibration/Verification of NDT Data
The emphasis on accurately characterizing the existing pavement structure has led to a significant
increase in the number of NDT techniques for project level pavement evaluation. The most common of
these NDT techniques is FWD deflection testing. However, ground penetrating radar (GPR) is also
commonly used, and many other techniques are presently in the development stage. These NDT
techniques, which are discussed in more detail in module 2-3, are powerful tools for identifying
weaknesses and defects in the existing pavement structure. By properly interpreting this NDT
information, more knowledgeable decisions can be made regarding the most feasible rehabilitation
alternatives to consider.
Despite the significant advancements in these NDT technologies, it is often necessary to verify and/or
calibrate the data generated with these devices. This is best accomplished by taking cores of the existing
pavement structure at predetermined locations to compare with the results generated with NDT. Ideally,
thickness cores would be taken at the location of each NDT deflection test; however, this is not practical
from a time-based or economic standpoint. Therefore, each agency has developed its own procedures for
determining the appropriate number and location of cores for NDT thickness determination.
As a general rule, the higher the variability of the NDT results, the more important coring is to verifying
those results. By properly interpreting the NDT data, the most appropriate locations for taking cores can
be determined. Coordinated in this fashion, these two approaches are very complementary and can lead
to extremely accurate characterization of the entire pavement section being considered for rehabilitation.
Absence of NDT Data
In the absence of any project-level NDT data, it is important to develop a thorough laboratory testing
program. However, in developing the testing program, the engineer must rely solely on condition survey
data and historical construction records (if available). If the distresses in the pavement are of a similar
nature throughout the section, the number of field samples collected can be evenly distributed throughout
the project. Alternatively, if it is apparent that different distresses are present throughout the project, the
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field data collection plans must be tailored to adequately address these differences. Again, the engineer
must use judgment in determining the number of required samples and their locations.
Identification of Causes of Pavement Deficiencies
A particularly important application of field sampling and testing is its role in helping to determine the
causes of observed distresses. Types of deficiencies that may warrant the use of additional materials
sampling and testing include structural inadequacies, poor drainage conditions, accelerated HMA
deterioration (due to oxidation and moisture susceptibility), foundation movements (due to swelling soil
or frost heave), and base failures.
Critical levels of structural distress (fatigue cracking and rutting) typically trigger the need for coring to
determine pavement layer thicknesses and basic properties of the HMA surface layer and any stabilized
subsurface layers. The potential for asphalt stripping, poor binder quality, or off-target binder quantity
are additional reasons for coring the HMA surface layer.
In cases where drainage deficiencies or foundation movement (settlements and heaves) exist, it is
recommended that boring samples of the base and subgrade soil be obtained. In these instances, the
gradation and the permeability of the base, as well as the Atterberg limits and soil classification of the
subgrade soil should be determined in the laboratory.
An additional investigation of swelling subgrade soils is also recommended if their presence is suspected
(Hall et al. 1989). The presence of frost heaving and swelling soils can typically be easily identified by
the development of irregular swells and depressions in the pavement profile. If possible, corrective
actions should be programmed to remedy or alleviate frost heaving and swelling soils problems.
Localized areas of repeated frost heave or swelling soils can be removed and replaced. Large areas can
potentially be addressed by soil stabilization or improved drainage.
5.
FIELD SAMPLING AND IN SITU TESTING
Field Sampling Methods
Coring
By far, the most common field sampling method is coring, which is the process of cutting cylindrical
material samples (cores) from an in-place pavement. This sampling method allows the engineer to obtain
a cross section of all desired pavement layers. Coring is accomplished with the use of a hollow,
cylindrical, diamond-tipped core barrel attached to a rotary core drill. The drill is anchored (either to the
pavement or a coring rig) and held perpendicular to the pavement surface while the rotating core barrel is
used to slowly cut into the pavement surface. Cores are drilled and retrieved from the pavement and
tested in accordance with ASTM D-979 (Practice for Sampling Bituminous Paving Mixtures) (ASTM
2000a) and ASTM D-5361 (Sampling Compacted Bituminous Mixtures for Laboratory Testing) (ASTM
2000b).
Coring is most often used to determine/verify layer thicknesses and provide samples (HMA surface and
stabilized layers) for resilient modulus or indirect tensile strength testing. A visual inspection of retrieved
cores can also provide valuable information when trying to assess the causes of visual distress or poor
pavement performance.
Cores are commonly cut with diameters of 50-, 100-, or 150-mm (2-, 4-, or 6-in), the selection of which
depends on the purpose. Dimensions of the retrieved cores are determined based on the types of tests to
which the samples will be subjected. If thickness verification is all that is needed, 50-mm (2-in) diameter
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Module 2-5. Field Sampling and Testing
cores are sufficient. The required diameter of samples taken for laboratory testing is dependent on the
thickness of the HMA layer and the type of laboratory test being performed.
Many times, two or more tests may be performed on a single core, provided that the first test does not
modify the properties of the HMA material to be evaluated by subsequent tests. One example of such a
testing progression is the determination of thickness and unit weight, followed by resilient modulus (in
indirect tension) and then by asphalt extraction, aggregate gradation, and asphalt content.
Other Materials Sampling Methods
In addition to coring, there are other methods used to obtain material samples from subsurface layers (i.e.,
subgrade soil, subbase, and base). The methods presented here are those primarily used to sample
unstabilized granular materials or cohesive soils.
Augers
The simplest method of obtaining disturbed (experiencing significant deformation during sampling) base,
subbase, or soil samples is with the use of helical or post-hole augers. Helical augers (see figure 2-5.1)
typically range in diameter from 75-mm to 300-mm (3- to 12-in) and can either be hand- or power-driven.
Auger samples are most commonly used to identify soil strata and for some classification tests even
though the physical state of the material is completely altered by the sampling process (Peck, Hanson, and
Thornburn 1974). Auger sampling is conducted in accordance with ASTM standard D-1452, Standard
Practice for Soil Investigation and Sampling by Auger Borings (ASTM 2000c).
(a) Helical auger
(b) Iwan or post-hole auger
Figure 2-5.1. Examples of augers (Peck, Hanson, and Thornburn 1974).
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Split-Spoon (Split-Barrel)
Soil samples may also be obtained by driving or pushing into the ground an open-ended cylindrical tube
called a split-spoon (or split-barrel) sampler. Since this method of sampling may also produce large shear
strain disturbance in the sample, the provided samples are most commonly used for the purposes of
identification, classification, and moisture and density testing. Unconfined compression testing can be
conducted on samples if they can be retrieved with little disturbance (note: the quality of the sample
obtained greatly depends on the care taken during the sampling procedure). The split-spoon sampler
shown in figure 2-5.2 has an outside diameter of 50 mm (2 in) and an inside diameter of 38 mm (1.5 in).
Split-spoon sampling is most suited for the sampling of fine-grained soils, and is accomplished in
accordance with ASTM D-1586, Standard Practice for Penetration Test and Split-Barrel Sampling of
Soils (ASTM 2000d).
A = 25 to 50 mm (1.0 to 2.0 in)
B = 0.457 to 0.762 m (18.0 to 30.0 in)
C = 34.93 ± 0.13 mm (1.375 ± 0.005 in)
D = 38.1 ± 1.3 – 0.0 mm (1.50 ± 0.05 – 0.00 in)
E = 2.54 ± 0.25 mm (0.10 ± 0.02 in)
F = 50.8 ± 1.3 – 0.0 mm (2.00 ± 0.05 – 0.00 in)
G = 16.0º to 23.0º
1 in = 25.4 mm
Figure 2-5.2. Example of a split-spoon sampler (ASTM 2000d).
Shelby (Push) Tubes
Shelby tubes are commonly used when it is necessary to obtain a relatively undisturbed soil sample
suitable for laboratory testing of structural properties or other tests that may be influenced by soil
disturbance (ASTM 2000e). A Shelby tube, an example of which is presented in figure 2-5.3, is a thinwalled metal tube that is pressed (not driven with a hammer) into the soil with relatively continuous
penetration force. Although this sampling method can be used on some coarse-grained materials, it is
typically used to obtain samples in soft cohesive soils. Shelby tube sampling is accomplished in
accordance with ASTM D-1587, Standard Practice for Thin-Walled Tube Sampling of Soils for
Geotechnical Purposes (ASTM 2000e).
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Module 2-5. Field Sampling and Testing
1 in = 25.4 mm
Figure 2-5.3. Example of a Shelby (Push) Tube (ASTM 2000e).
Test Pits (Trenches)
Test pits (or trenches) are used as a method of viewing the cross section of all pavement layers in its
undisturbed condition. Although not commonly used because of the cost and time required for their
conduct, these are an excellent method of identifying problem pavement layers and assessing drainage
adequacy. Another specific application of test pits includes determining the locations and amounts of
permanent deformation (rutting) throughout the depth of the pavement structure. Test pits may be dug in
more than one area of a project to compare areas of different performance.
In Situ Field Testing
There are a number of methods that are used to test the in situ strength of paving materials. Some of the
more commonly used methods are included in this section.
Dynamic Cone Penetrometer (DCP)
The DCP is a device for measuring the in situ strength of paving materials and subgrade soils. The
principle behind the DCP is that a direct correlation exists between the “strength” of a soil and its
resistance to penetration by solid objects (Newcomb and Birgisson 1999). Although the DCP was
introduced in the 1960s, highway agencies are just now becoming familiar with it or are starting to use it,
although it is extensively used by the United States Air Force. Currently, ASTM is working to develop
standardized procedures for this test.
The DCP consists of a cone attached to a rod that is driven into the soil by the means of a drop hammer
that slides along the penetrometer shaft (Newcomb and Birgisson 1999). Figure 2-5.4 shows a schematic
of the DCP apparatus (US ARMY 1989). The test is performed by driving the cone into the
pavement/subgrade by raising and dropping the 8 kg (16.7 lb) hammer from a fixed height of 57.5 cm
(22.6 in). Earlier versions of the DCP used a 30º cone angle with a diameter of 20 mm (0.8 in)
(Newcomb and Birgisson 1999). More recent versions of the DCP use a 60º cone angle and also have the
option of using a 4.6-kg (10 lb) hammer for weaker soils (Newcomb and Birgisson 1999).
During a DCP test, the cone penetration (typically measured in mm or inches) associated with each drop
is recorded. This procedure is completed until the desired depth is reached. A representative DCP
penetration rate (PR) (mm or inches of penetration per blow) is determined for each layer by taking the
average of the penetration rates measured at three defined points within a layer: the layer midpoint,
midpoint minus 50 mm (2 in), and midpoint plus 50 mm (2 in). DCP penetration rates can be used to
identify pavement layer boundaries and subgrade strata, and to estimate the CBR values of those layers.
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Handle
Hammer
(8 kg
[17.6 lb])
Cone angle 60o
57.5 cm
(22.6 in)
20 mm
(0.79 in)
1 m (39.4 in)
(variable)
1 in = 25.4mm
Steel rod
(16 mm
[0.64 in])
1 lb = 0.454 kg
Cone
Figure 2-5.4. Dynamic cone penetrometer (US ARMY 1989).
DCP results have been correlated with the CBR for a broad range of material types (including finegrained soils and gravel). The most commonly used empirical correlations express CBR as a function of
the DCP Penetration Index (DPI), defined as penetration in millimeters per blow (Newcomb and
Birgisson 1999). Other correlations relating soil strength parameters to DPI have been developed by
Kleyn (1975), Smith and Pratt (1983), and Livneh (1987; 1989). One of the most widely used
correlations between DPI and CBR is the following developed by Webster, Grau, and Williams (1992) for
the manual DCP:
CBR 
292
DPI 1.12
(2-5.1)
where:
CBR = California Bearing Ratio.
DPI = DCP Penetration Index (measured in mm per blow).
Recent research has also resulted in variations of this equation that are applicable for heavy and lean clays
(Webster, Brown, and Porter 1994). These new correlations are illustrated in figure 2-5.5.
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Figure 2-5.5. Correlations between DPI and CBR (Webster, Brown, and Porter 1994).
Another example of an empirical relationship between CBR and DPI is the following relationship used in
Norway (Newcomb and Birgisson 1999):
CBR  2.57  1.25  log DPI
(2-5.2)
Automated DCPs are now being developed in which the hammer is picked up and dropped automatically.
Research results have indicated that CBR values computed using automated DCP results (obtained using
the Israeli automated DCP) are about 15 percent greater than CBR values computed using DPI from the
manual DCP (Newcomb and Birgisson 1999).
Standard Penetration Test (SPT)
The SPT is one of the most common in situ geotechnical tests used all over the world. The popularity of
this test method is mainly due to the abundance of experience over the years and its relative simplicity
and cost effectiveness (Newcomb and Birgisson 1999). This SPT is conducted in accordance with ASTM
D-1586 (ASTM 2000d), and a complete analysis of the statics and dynamics of the SPT is given in
studies by Schmertmann (1979) and Schmertmann and Palacios (1979). Although the SPT can be
performed in a wide variety of subgrade soils, the most consistent results are found in sandy soils where
large gravel particles are absent (Newcomb and Birgisson 1999).
The SPT consists of driving a standard split-spoon sampler into the ground with blows from a 63.5-kg
(140-lb) hammer. The raising and dropping of the hammer shall be performed in accordance with the
guidelines provided in ASTM D-1586 (ASTM 2000d). The number of blows associated with each 150
mm (6 in) is recorded. Penetration through the first 150 mm (6 in) of soil is considered to be a seating
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drive. The sum of the number of blows required for the second and third 150 mm (6 in) of penetration
(i.e., 150 to 450 mm below the starting elevation) is termed the “standard penetration resistance,” or the
“N-Value” (ASTM 2000d). The basis for using the N-value is that this measure of resistance to soil
penetration is correlated with the relative density, the unit weight, the angle of internal friction, the
undrained shear strength, and the elastic modulus of a soil (Kulhawy and Mayne 1990).
Although there are a number of recognized potential sources of error associated with the SPT, there are
many published correlations between SPT and important mechanical soil properties such as undrained
shear strength, unconfined compressive strength, angle of internal friction, and relative density (Kulhawy
and Mayne 1990). An example of a general relationship for fine-grained soils that relates undrained shear
strength and N-value (measured in blows per meter) is shown in table 2-5.1 (Kulhawy and Mayne 1990).
Table 2-5.1. Approximate relationship between undrained shear strength and N-value
as determined from the Standard Penetration Test method.
N-Value
(blows/m)
0–6
6 – 12
12 – 24
24 – 45
45 – 90
> 90
Consistency
Very soft
Soft
Medium
Stiff
Very stiff
Hard
Approximate
su/pa
< 1/8
1/8 – ¼
1/4 – ½
1/2 – 1
1–2
>2
This information was further generalized into the following relationship (Kulhawy and Mayne 1990):
s u / p a  0.06 N
(2-5.3)
where:
su = Undrained shear strength.
pa = Atmospheric pressure (included to make the resulting number dimensionless and thus
independent of the units of measure.)
N = Number of blows per meter.
A recent survey shows that a number of State highway agencies continue to use the SPT to determine
subgrade soil stiffness (Newcomb and Birgisson 1999). The primary advantages of the SPT include its
availability, past experience with the method (large experience database), and that fact that it is relatively
quick and simple to perform; primary disadvantages of the SPT include its many potential sources of error
(such as the method of winding the hammer rope around the cathead on the drill rig) and its inaccuracy in
soils containing coarse boulders, cobbles, or coarse gravel (because the sampler can become obstructed
and give artificially high blow counts) (Newcomb and Birgisson 1999). Furthermore, in soft and
sensitive clays, the SPT results should also not be relied upon as they have not been found to be
consistent with actual field conditions (Newcomb and Birgisson 1999).
Defining a Sampling Plan
Determining the number of required material samples and their locations is not always an easy task. In
today’s practice, the development of a material sampling plan for pavement evaluation is primarily
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dependent upon four factors: (1) observed pavement distress, (2) variability along the site (particularly as
characterized by NDT), (3) the traffic on the given roadway, and (4) economics.
First, the type, severity, extent, and variation of visible distress on a pavement greatly affect the locations
and amount of field sampling and testing. Targeted coring is often required to investigate the specific
distress mechanisms of areas where high distress concentrations are observed. Conversely, a random
(objective) coring plan may be more appropriate when observed distress appears to be somewhat uniform
over the entire project. Engineering judgment plays a key role in determining what types of samples to
take, where to take them, and the quantity needed to adequately assess the material characteristics of the
in-service pavement.
Overall project variability is the second factor in identifying sampling locations because of its effect on
rehabilitation design and future pavement performance. The best indicators of project variability are
deficiencies in pavement condition and NDT results. Accordingly, their variability should be considered
in determining an appropriate number of samples as well as sampling locations. In general, more
variability means more sampling.
Worker and driver safety concerns associated with lane closures on high-trafficked roadways is the third
factor that plays a key role in determining the locations for sampling and the maximum number of
samples allowed. For example, in urban areas with high traffic volume, it may be nearly impossible (or
prohibited) to close a lane. Even if lane closures are allowed, the agency may have to greatly reduce the
number of traffic lane samples (due to lane closure time constraints) or require that testing be completed
at night. Additional subgrade testing in the core holes (e.g., split-spoon sampling, Shelby tube sampling,
DCP, and so on) may also be limited or avoided due to the same concerns of lane closure time and safety.
Because of these considerations, once an engineer determines the maximum number of in situ samples
allowed, it is important that their locations be carefully selected. Such lane closure restrictions and safety
related issues are typically not an issue on roadways with lower traffic volumes.
The fourth primary factor affecting the development of the sampling and testing plan is economics.
Although the cost of sampling and testing is small in comparison to the total rehabilitation cost, most
agencies have a limited budget that determines the types and amount of sampling and testing that can be
conducted for a given project. Therefore, it is paramount that engineering judgment be applied to achieve
a balance between the amount of testing required to adequately assess a pavement’s condition and the cost
of the sampling program.
The evaluation of in-service HMA pavements typically includes a plan for coring (i.e., number, size, and
locations of core samples) so that as much information about the HMA surface and stabilized layers can
be obtained from the inspection and testing of core samples. However, since subsurface layer sampling or
in situ testing is also generally conducted at core locations, it is important that sampling locations be
chosen appropriately. The engineer should always develop an adequate sampling and testing plan while
making an effort to minimize cost and minimize the amount of traffic disruption on the in-service
pavement. The following section presents recommendations for developing a plan for obtaining and
handling samples from existing HMA pavements (both pavement layer and subgrade soil samples).
Core Samples
As mentioned previously, coring is the primary method of obtaining samples of the HMA surface layer
and the underlying base and subbase pavement layers. A number of different tests can be conducted on
core samples to determine layer material strength, composition, and durability (these methods are
discussed in detail in section 7 of this module, Laboratory Testing Methods). The recommended practices
for sampling bituminous mixtures are presented in ASTM D-979, Practice for Sampling Bituminous
Paving Mixtures (ASTM 2000a) and ASTM D-5361, Sampling Compacted Bituminous Mixtures for
Laboratory Testing (ASTM 2000b).
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Depending on the types, extent, severity, and variability of visible distress, the engineer must determine
the most appropriate approach for retrieving core samples from a pavement. This decision typically
involves determining if a random objective coring approach is warranted, or if it is more appropriate to
focus on targeting cores in areas where a high concentration of distress exists. As indicated above, these
decisions should take into consideration certain economic- and safety-related constraints.
Random Coring
If the purpose of sampling is to assess the overall quality of the in situ materials or the overall variability
within a section, it should be done randomly and objectively so that the samples taken are not weighted
towards unusually poor or unusually sound materials (Van Dam et al. 2000). Past field condition survey
results typically give a good indication of how uniform the material quality is along a project.
Targeted Coring
Coring is often targeted in identified problem areas. Such problem areas are typically areas in which
unusual distress types or unusually high distress concentrations are observed. Sampling used to
investigate the sources of such observed deficiencies is not random as the number and locations of
samples are chosen subjectively using engineering judgment. When coring is conducted in problem
areas, additional coring should also be considered in areas with no visible distress for comparative
purposes.
Handling and Packaging of Core Samples
Core samples for laboratory testing must be protected from contamination, damage, and other processes
that might change the character of the material being examined or tested (Van Dam et al. 2000). In an
attempt to reduce material contamination (typically the result of materials from other layers at the site, or
material carried by coring rigs and other equipment from other sites) the surface of all HMA surface layer
cores should be rinsed with fresh water prior to wrapping or packing for transport and/or shipping.
Finally, accurate sample-related information including sample location, manner of retrieval, and field
condition of samples should be recorded for future reference.
If bulk samples of unbound granular base/subbase materials are taken from core holes, they are typically
stored in bags or placed in jars until testing can be performed. It is important that these granular material
samples be adequately stored (i.e., in a controlled moisture and temperature environment) so as to obtain
testing values that are representative of the in situ material properties.
Subgrade Soil Investigation
The investigation of soil properties along a pavement section is an additional consideration when
developing a comprehensive sampling and testing plan. This process generally consists of reviewing all
available subgrade soil information and taking a relatively small number of soil samples or in situ tests to
verify expected soil conditions. More comprehensive subgrade soil investigations are not generally
conducted unless specific soil-related problems are identified, or if extensive data is required to validate
NDT results.
Review of Available Information
Many sources of information are available to assist the pavement engineer in evaluating the different
subgrade soil properties that may need to be considered in the design of the rehabilitation strategy
selected without an intensive field-testing program. These include:
2-5.12
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
Previous engineering “soil reports” for the project.

As-built plans.

Materials testing information.

Pedologic soil data.
Module 2-5. Field Sampling and Testing

United States Department of Agriculture County soil reports.

State soil manuals based on “pedologic soil type.”

Soil association maps.

Unpublished surficial soils data available from Soil Conservation Service Office and land
grant universities.

Climatic information.

Geological data (Federal and State geological surveys are excellent contacts).

Climatic information.

National Oceanic and Atmospheric Administration (NOAA).

Agricultural experiment stations.

State water surveys.

Federal geological survey.

Weather atlases.

Moisture-accelerated distress zones (see Carpenter, Darter, and Dempsey 1979).
Since the subgrade soil is already in place, a soil study may have been completed before the pavement
was originally constructed. That information should be obtained and used as the starting point for
subgrade soil evaluations. Soil parameters, material types, and thickness profiles should be plotted along
the project strip map for comparison with the strip chart of the distress. This side-by-side comparison
will provide insight as to whether the problems are related to the subgrade soil.
The most concise and complete collection of subgrade soil data at the project level will be contained in
the as-built plans and material testing records from the original construction, if they are available. If these
sources of information are not available, then an alternative source of subgrade soil information is the soil
surveys produced by the Soil Conservation Service of the United States Department of Agriculture. They
include complete information on engineering properties of the soils, such as:

Soil classification (AASHTO or Unified).

Grain size distribution.

Atterberg limits.

Shrink-swell potential.

Water table location.

Frost action potential.

Average daily maximum and minimum temperature by month, and average monthly precipitation
for each major soil type in the survey area.

Permeability, drainage characteristics, and infiltration capacities.
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Determining Sampling Needs (Data Analysis)
The project location is clearly shown on available soil maps and the different soil types present can be
determined with enough precision to satisfy a preliminary survey and to establish limits on the site. The
soil types can be compared with the distress survey and previous soil studies to determine potential
subgrade soil problem areas. The deflections obtained from FWD testing should also be used to help in
determining potential problem areas. The different problem areas provide an indication of where
sampling should be concentrated to develop the most data for the least expenditure of money and time.
The information developed in the preliminary survey should be analyzed to show where problems may
exist that require detailed material property descriptions to be developed. In general, sampling of the
subgrade soil when considering rehabilitation options is not as extensive as is needed for new pavement
design. Detailed sampling of the base and surface may be needed if there is a large amount of variability
in distress types along the project or if different materials are known to have been used. Specific
recommendations concerning sampling frequency cannot be made without having the distress strip map
of the existing project, the original soils data, and the original construction records for the pavement
materials.
Handling and Packaging of Soil Samples
When subgrade soil samples are taken, it is important that they be handled with care to ensure that the
obtained test values are representative of the in situ conditions. Due to the volume of work and the
likelihood of delays in testing some of the delivered samples, proper storage conditions must be
maintained for all samples. As soil mechanical properties are sensitive to moisture changes, adequate
storage (i.e., controlled moisture and temperature conditions) of soil samples is critical to obtaining
representative testing results (SHRP 1988). Typically, samples are in the form of bagged bulk samples,
jars, and thin-walled tubes. Jar samples and Shelby tube samples are assigned the higher priority for
testing as they have the highest potential for sample moisture loss (SHRP 1988).
6. LABORATORY TESTING METHODS
This section presents some of the common testing methods used in the evaluation of pavement layer
materials. Many of the tests that are used in the initial pavement design and as part of QC/QA activities
during construction can also be used to evaluate materials that have been in service. However, there are
many new tests that may be of more use to the designer in evaluating the existing pavement and in
determining the most appropriate rehabilitation strategy. The types of tests discussed here are divided
into general categories of visual inspection, material characteristic determination, strength testing, and
HMA specialty tests. Each test type has its own area of applicability and it is important that each test be
conducted on representative material samples. The following sections briefly summarize these test
procedures.
Visual Inspection
While being the simplest of testing methods, a visual inspection of a sample is a very efficient way of
obtaining a quick assessment of material composition, as well as aiding in the identification of any
obvious layer material deficiencies.
A visual inspection of soil samples will typically give an approximate classification based on observed
texture, color, odor, particle size, plasticity, structure, moisture, and density. The field classification and
description are based on a combination of experience of field personnel, SPT blow count, and certain
other simple field tests (e.g., grittiness, cohesiveness, finger pressure, and other senses) (IDOT 1999).
2-5.14
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Module 2-5. Field Sampling and Testing
Any field classification and description are usually verified or corrected by further laboratory testing of
the soil samples (including tests for particle size, Atterberg limits, moisture, and density).
As mentioned previously, a visual inspection of cores allows the engineer to check layer thicknesses,
investigate the material integrity of all layers, and check the bond conditions between stabilized layers.
This type of information typically gives an initial indication into the causes of any visual distress and
allows the engineer to choose more targeted laboratory and field tests (if necessary) to complete the
required pavement evaluation. Visual examination of cores and material samples provides valuable
information about environmentally induced deterioration in pavement layers, including the HMA surface
and other stabilized layers. No evaluation should be considered complete until an experienced engineer
has completed a visual examination of the materials.
Determination of Material Characteristics (for Subsurface Layer Materials)
After the initial field classifications (from the visual inspection) are completed, the collected material
samples (soil samples, granular base samples, and cores) are typically subjected to more detailed testing
methods to determine specific characteristics. These material constituency tests measure intrinsic
properties of the subsurface layer materials that may affect performance directly or indirectly. Included
among these material characteristics are soil classification, gradation, moisture content, and density.
These tests are primarily run to show whether the properties of the materials have changed since
construction. Over time, materials undergo changes in their properties. Intrusion of subgrade soil fines
into a granular base course can lower its shear strength, CBR value, or other strength-related properties.
They also can have an adverse effect on the layer’s permeability. Loss of density in the subgrade soil or
base due to climatic influences greatly alters the support potential. Changes in the moisture condition are
perhaps the most detrimental that can occur as moisture has a tremendous impact on the structural
behavior of almost all unbound materials. Construction records containing original test results may be
compared with the present condition of each material. Any significant changes in the properties of the
subgrade soil, for example, may be suggestive of a problem in the material.
Material characteristics (such as soil classification, gradation, moisture content, and density) are easily
determined and provide the only direct indication of how the material has changed from the time it was
constructed. These tests are described in the next sections and should be used in conjunction with other
material tests (e.g., strength-related testing) in order to fully characterize the properties of a material.
Soil Classification
The AASHTO method of soil classification uses the grain size distribution and the plasticity
characteristics of the soil to differentiate among soils based on their potential to perform as a subgrade
soil under a pavement structure. Other soil classifications in use include the Unified Soil Classification
system, the Federal Aviation Administration (FAA) classification system, pedologic classifications, and
the United States Department of Agriculture classification system.
Some correlations have been developed relating soil classifications to traditional strength parameters, as
shown in figure 2-5.6 (PCA 1984). It is important to realize that the relations between soil types and soil
strengths are not precise and a wide range of strengths are possible for a given soil type. For instance, a
soil with a CL designation in the Unified System could have a CBR of 5 to 15. One reason for this
variation is that fine-grained soils are highly dependent on the sample’s level of saturation.
HMA Pavement Evaluation and Rehabilitation
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Module 2-5. Field Sampling and Testing
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Figure 2-5.6. Soil classification related to strength parameters (PCA 1984).
2-5.16
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Module 2-5. Field Sampling and Testing
Gradation
Unbound base and subbase courses have traditionally been graded to achieve maximum density at nearoptimum moisture content with minimal compactive effort during construction. One potential problem
that can occur when maximum density is the only criteria for selecting a gradation is that some materials
tend to have low permeability. If drainage is a concern during the rehabilitation design process, then the
permeability of a base, subbase, or subgrade soil may be estimated using the results of a grain-size
(gradation) analysis of the material.
Moisture Content and Density
The moisture content and density relationship for a soil is a critical factor affecting the strength and
deformation properties of any prepared soil. Each soil has an individual moisture-density relationship
determined in the laboratory that, when properly constructed, will produce the maximum sustainable
support. Figure 2-5.7 illustrates some typical compaction curves for a variety of soils (Oglesby and Hicks
1982). During construction, the subgrade soil and pavement materials were compacted to a certain
density at a specified moisture content. This combination provided the strength values assumed in the
original pavement design. However, both the moisture content and density can change after construction.
Well-graded loamy sand
Dry Unit Weight (pci)
135
Well-graded sandy loam
Medium-graded sandy loam
125
Lean sandy silty clay
115
Lean silty clay
105
95
5
10
15
20
Loessial silt
Heavy clay
Poorly-graded sand
25
Moisture Content (%)
1 pci = 27,683 kg/m3
Figure 2-5.7. Typical compaction curves for different soil types (Oglesby and Hicks 1982).
Some agencies have developed moisture-density curves for all of their known soils. This allows them to
identify the moisture-density relationship for a soil from a catalog after only determining one moisturedensity point. Such one-point moisture-density curves should be used very carefully and only when
developed specifically for soils in the local area. The development of such curves and the use of this
method for determining maximum density and optimum moisture content of a soil are conducted in
accordance with AASHTO designation T 272, Standard Method of Test for Family of Curves—One-Point
Method (AASHTO 2000).
The moisture-density relationship for a given subgrade soil has a significant effect on how the soil
responds to loading. Figure 2-5.8 illustrates the effect of an increase in moisture content on the
permanent strain in a fine-grained soil (AASHTO soil classification: A-7-6[23]) (Thompson and
Dempsey 1977). Figure 2-5.9 shows the dramatic effect of an increase in density on the permanent
deformation of a well-graded granular material.
HMA Pavement Evaluation and Rehabilitation
2-5.17
Repeated Deviator Stress, D (psi)
Module 2-5. Field Sampling and Testing
40
Reference Manual
N=5000
Optimum moisture
30
Optimum +4% moisture
20
1 psi = 6.89 kPa
10
AASHTO Classification: A-7-6(23)
0
1
2
3
4
Permanent Strain, P (%)
5
6
Figure 2-5.8. Influence of moisture content on permanent strain response of a loess-derived soil
(Thompson and Dempsey 1977).

Permanent Strain, p (%)
3.5
95%
AASHTO T-99
d = 45 psi
2.5
c = 15 psi
100%
AASHTO T-99
1.5
0.5
0
10
100
1000
10,000
Number of Load Applications, N
Figure 2-5.9. Permanent deformation as a function of load application for two
compaction efforts on a granular material.
Cyclic freeze-thaw (without moisture change and the resultant heaving) causes drastic changes in the
strength and repeated load behavior of fine-grained soils. Figure 2-5.10 shows that the resilient modulus
value decreases with exposure to freeze-thaw cycles (Thompson and Dempsey 1977). The most dramatic
decrease occurs after only one freeze-thaw cycle, whereas subsequent freeze-thaw cycles create much
smaller decreases in the resilient modulus. During the first freeze-thaw cycle, moisture in the material
freezes, causing expansion. Then, upon thawing, the soil particles are no longer tightly compacted and a
reduction in resilient modulus occurs. Subsequent freeze-thaw cycles have a substantially smaller effect.
Unfortunately, the soil can never regain strength lost due to freezing and thawing.
2-5.18
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Module 2-5. Field Sampling and Testing
15
TAMA B
Resilient Modulus, ER (ksi)
14
Number of
Freeze-Thaw
Cycles
12
10
0
8
6
1
5
10
4
2
0
0
AASHTO Classification: A-7-6(27)
4
8
12
16
20
Repeated Deviator Stress, D (psi)
Figure 2-5.10. Influence of cyclic freeze-thaw on the resilient behavior of a fine-grained soil
(Thompson and Dempsey 1977).
Laboratory Strength-Related Testing Methods
The ability of a pavement structure to adequately carry repeated traffic loadings is very much dependent
on the strength, stiffness, and deformation-resistance properties of each layer. Strength tests, or tests that
are indicative of material strength, have long been a popular method of assessing the quality of a
pavement layer. However, measures of elastic or resilient modulus have a greater significance because of
their effect on the way pavements respond to load. The types of tests used depend on the type of material
making up a given layer (stabilized or unstabilized) and the function of the layer (surface, base, subbase,
or subgrade soil material).
There are various laboratory test methods that are used to measure material strength, stiffness, or its
ability to resist deformation or bending. Some of the more common tests used in the assessment of HMA
pavement paving materials include the following:

CBR.

R-value.

Triaxial testing.

Resilient modulus.

Unconfined compressive strength (bound or stabilized materials and cohesive soils).

Indirect tensile strength (bound or stabilized materials).

Resilient modulus.

Dynamic modulus.
A general description of each of these test types, along with details of their appropriate applications, is
described in the following sections.
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California Bearing Ratio (CBR)
The CBR test measures the resistance of an unbound soil or aggregate sample to penetration by a piston
with an end area of 1,935 mm2 (3 in2) being pressed into the soil at a standard rate of 1.3 mm (0.05 in) per
min. A schematic of the test and typical data is shown in figure 2-5.11. The load resulting from this
penetration is measured at given intervals and the resulting loads at sequential penetrations are compared
to the penetration recorded for a standard, well-graded crushed stone. The ratio of the load in the soil to
the load in the standard material (at 2.5 mm [0.1 in] penetration), multiplied by 100, is the CBR of the
soil.
Step 1
Compact
Step 2
Soak
Step3
Load
Bearing Value (psi)
2,500
Good crushed base
100%
Spec. requirement
2,000
80%
Good granular base
1,500
50%
Good subbase
30%
Very good subgrade
20 %
Fair to good subgrade
1,000
500
Poor subgrade
10 %
5%
Very poor subgrade
0
0.1
0.2
0.3
Penetration (in)
0.4
0.5
1 psi = 6.89 kPa
Figure 2-5.11. CBR testing procedures and load penetration curves for typical soils
(Oglesby and Hicks 1982).
2-5.20
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Module 2-5. Field Sampling and Testing
The CBR test method provides an indicator of the strength of both subgrade soils and untreated base and
subbase materials. Fine-grained cohesive soils are generally compacted at the optimum moisture content
for testing. Granular materials are generally compacted at several moisture contents above and below
optimum. The samples are soaked for 96 hours prior to testing to simulate saturated conditions that may
develop under the pavement. Weights may be added to the surface of the sample to simulate overburden
pressures of a pavement structure. CBR values will typically range from 2 to 8 for clays and 70 to 90 for
crushed stones (PCA 1992).
The CBR test is an empirical test that has been used extensively in pavement design. It is important that
the test be conducted in strict accordance with AASHTO T 193, Standard Method of Test for The
California Bearing Ratio, if results for different soils are to be comparable (AASHTO 2000). The major
advantages of this test are the simple equipment requirements and the large amount of data available for
correlating results with field performance. A major drawback is that this test procedure cannot be
conducted on materials prepared to conditions approximating field conditions to develop any comparison
with existing data. The CBR test can also be performed in the field in accordance with ASTM D-4429,
Standard Test Method for CBR (California Bearing Ratio) of Soils in Place (ASTM 2000f).
Resistance Value
A stabilometer may be used to assess the stability of a soil (or unbound material) sample as it measures
the transmitted horizontal pressure associated with the application of a vertical load (PCA 1992). In
accordance with ASTM D-2844, Standard Test Method for Resistance R-Value and Expansion Pressure
of Compacted Soils, the test consists of enclosing a cylindrical sample (100 mm [4 in] in diameter and 6
mm [0.25 in] tall) in a membrane and loading it vertically over the full face of the sample to a given
pressure (ASTM 2000g). The resulting horizontal pressure is measured and used to calculate the
Resistance Value (R-value), which gives an indication of the stiffness of the material. The R-value
method has been used most frequently in several Western States. The test result is empirical and does not
represent a fundamental soil property. Also, the results cannot be used with any analytical evaluation of
the structural adequacy of the material in the pavement.
Triaxial Strength Testing
The triaxial test is a compressive strength test in which a soil (or unbound material) sample is placed in a
triaxial cell and a confining pressure is applied to the sample in the chamber prior to the test. The
confining pressures are applied to simulate the confining conditions of the materials in place. A vertical,
axial load is then applied to the sample until it fails. Several samples are tested under several confining
pressure levels to develop a relationship between the vertical load at failure and the associated confining
pressure. Texas and Kansas are two States that have design procedures based on this test. Several types
of equipment and variations in test procedures have been developed (PCA 1992). The test procedure is
described in ASTM D-2850 (ASTM 2000h).
Unconfined Compressive Strength
A very popular test on PCC and other cement- and lime-treated materials is the unconfined compressive
strength test. The popularity of this test method is primarily because it is an easy test to perform, and
many of the desirable characteristics of concrete are qualitatively related to its strength (Neville 1996).
The unconfined compression test can be performed on all stabilized materials used in pavement
construction.
For in service pavement evaluation purposes, the test is performed on cylindrical PCC (or stabilized base)
core samples. The test procedure consists of placing a prepared cylinder of material (with a height-todiameter ratio of 2) in the testing apparatus, and applying a load at a constant rate of deformation,
HMA Pavement Evaluation and Rehabilitation
2-5.21
Module 2-5. Field Sampling and Testing
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typically 1.3 mm (0.05 in) per min. The load on the sample and the deformation are recorded. The data
are normally plotted as stress (load per cross section area) versus strain (deformation per original length).
It is important to note that the process of drilling the core potentially causes damage to the sample that
may cause the observed strength test results to be lower than the actual in situ material strength. This test
result is not used directly in design, but can be correlated with the flexural strength and the elastic
modulus.
Indirect Tensile Strength
The indirect tension test, also called the splitting tensile test, can be used to determine the tensile strength
of HMA cores or any stabilized pavement layer. The procedure is described in ASTM D-4123, Standard
Test Method for Indirect Tension for Resilient Modulus of Bituminous Mixtures (ASTM 2000i) and is a
simplified application of the test equipment used to determine resilient modulus. The test involves
applying a vertical load at a constant rate of deformation on the diameter of a cylindrical sample (as
shown in figure 2-5.12). The sample will fail in tension along the vertical diameter of the sample and the
indirect tensile strength is calculated from the following equation:
t 
2Pult
LD
(2-5.4)
where:

t
Pult
L
D
=
=
=
=
Indirect tensile strength, Pa (lbf/in2).
Vertical compressive force at failure, N (lbf).
Length of sample, m (in).
Diameter of sample, m (in).
This test is particularly valuable for rehabilitation testing as it is performed on cores taken from the
pavement as well as on samples prepared in the laboratory.
Resilient Modulus of Unbound Materials
Since the release of the 1986 version of the AASHTO Guide for Design of Pavement Structures
(AASHTO 1986), there has been an increased emphasis in the use of the resilient modulus to characterize
subgrade soils and unbound base/subbase materials. The resilient modulus test provides a material
parameter that more closely simulates the behavior of the material under a moving wheel. For example, a
moving wheel imparts a dynamic load pulse to the pavement structural layers and the subgrade soil as
illustrated in figure 2-5.13. The applied dynamic loading causes the development of stresses
(compressive [z], tangential [t], and radial [r]) and deflections in the pavement structure (see figure 25.13). The magnitude of the stresses and strains are a function of tire load, contact pressure, point of
interest within the pavement structure, the number of pavement layers, and the thickness, elastic modulus,
and Poisson’s ratio of those layers. An adequate rehabilitation design is one that will sufficiently handle
these developed stresses, strains, and deflections, at the critical locations within the pavement structure,
over the chosen design period.
The moving loads produce deformation, as shown in figure 2-5.14, that can be used to calculate the
following deformation properties of the soil:

Total Strain. Total deformation under the load, t.

Resilient Strain. Deformation recovered when the load is removed, r.

Permanent (Plastic). Strain deformation not recovered when the load is removed, p.
2-5.22
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Module 2-5. Field Sampling and Testing
Load, P
v
Diameter, D
r
Length, L
P
t =


2P
LD


Tension
Compression
Figure 2-5.12. Indirect tension test (Mindess and Young 1981).
Wheel Load, P
h1, E1, 1
h2, E2, 2
h3, E3, 3
z
r
r
t
Figure 2-5.13. Three dimensional stress states in a typical pavement structure.
HMA Pavement Evaluation and Rehabilitation
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Deviator stress, D
Module 2-5. Field Sampling and Testing
D
Stress or load pulse
Time
Deviator stress, D
Total Strain, t
Plastic
p
Resilient
r
MR =
D
r
Deformation of soil sample (strain)
Figure 2-5.14. Typical repeated load response.
Fine-Grained Materials
In the laboratory, the resilient modulus test is conducted by placing a compacted soil specimen in the
triaxial cell, as shown in figure 2-5.15. The specimen is subjected to an all-around confining pressure,
orc, and a repeated axial stress (deviator stress), D, is applied to the sample. The number of times
the axial load is applied to the sample varies, but typically ranges from 50 cycles to 200 cycles. During
the test, the recoverable axial strain, r is determined by measuring the recoverable deformations across
the known gauge length. The test is run at various combinations of deviator stress and confining pressure.
For the LTPP P-46 protocol (AASHTO T 307-99), the deviator stress varies from 13.8 to 68.9 kPa (2 to
10 psi) and the confining stress varies from 13.8 to 41.4 kPa (2 to 6 psi) for fine-grained soils (AASHTO
2000).
2-5.24
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Module 2-5. Field Sampling and Testing
Figure 2-5.15. Resilient modulus test apparatus.
The resilient modulus is determined using equation 2-5.5 for each combination of deviator stress and
confining pressure:
MR 
D
r
(2-5.5)
where:
MR
D
z
r
=
=
=
=
Resilient modulus.
Deviator stress (z – c).
Total vertical stress.
Recoverable strain.
The resilient modulus set up can also be used to develop data about the deformation potential of the
materials by recording the permanent deformation produced by the repeated loadings. Typical permanent
strain-to-number of stress repetitions responses are shown in figure 2-5.16. Permanent strain accumulates
very rapidly when the repeated stress is large in relation to the strength of the soil.
HMA Pavement Evaluation and Rehabilitation
2-5.25
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0
25% Q U
0.5
50% Q U
1.0
75% Q U
1.5
2.0
AASHTO A-4
100% Q U
Q U = 17 psi
Q U = Unconfined compressive strength
2.5
3.0
1
10
100
1000
Number of Stress Repetitions
1 psi = 6.89 kPa
Figure 2-5.16. Effect of load magnitude and repetitions on permanent strain (Thompson et al. 1977).
Coarse Grained Materials
Resilient modulus testing can also be conducted on granular materials; however, the resulting
relationships are much different from fine-grained, cohesive soils. While granular materials are also
stress-sensitive, they exhibit “stress hardening” characteristics, in which the resilient modulus increases
with increasing stress states. This is due to increased interlock between the individual aggregate particles.
The resilient modulus as a function of the applied stress state in granular materials is given by the
following equation:
M R  k n
(2-5.6)
where:
MR
k, n

1
2, 3
=
=
=
=
=
Resilient modulus.
Experimentally-derived factors.
Bulk stress =1+2+3.
Major principal stress.
Intermediate and minor principal stresses.
For coarse-grained materials, the testing stress states in the LTPP Protocol P46 range from 21 to 138 kPa
(3 to 20 psi) for confining pressure and from 21 to 276 kPa (3 to 40 psi) for the deviator stress (AASHTO
2000).
Figure 2-5.17 illustrates the resilient modulus for a sandy gravel as a function of the bulk stress. For
coarse-grained soils the major factors that influence resilient modulus are bulk stress, density, percent
crushed particles, and moisture content.
2-5.26
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Module 2-5. Field Sampling and Testing
1 psi = 6.89 Pa
Figure 2-5.17. Resilient modulus vs. bulk stress for a sandy gravel (Fischer et al. 1984).
Test Procedures
Resilient modulus testing of unbound materials is performed in accordance with one of two current
procedures: AASHTO T307-99, Determining the Resilient Modulus of Soils and Aggregate Materials, or
AASHTO T 292-96, Resilient Modulus of Subgrade Soils and Untreated Base/Subbase Materials
(AASHTO 2000). AASHTO T 307-99 is based on LTPP Protocol P-46.
Selection of Design Modulus
The many factors that affect the resilient modulus value make it difficult to select a single resilient
modulus value for use in rehabilitation design. It is generally recommended that resilient modulus values
be selected for the state of stress that exists beneath the pavement. Von Quintus and Killingsworth (1997)
recommend that subgrade resilient modulus be determined at a depth of 460 mm (18 in). Typically, this
may mean confining pressures of 21 kPa (3 psi) and saturated conditions (wet of optimum) (Elliott et al.
1988). However, some advocate the use of unconfined testing (i.e., 0 confining pressure) because the
effect of confining pressure is low on saturated samples, because it provides a conservative estimate of
the resilient modulus, and because it is a quicker test to perform (Thompson and Robnett 1976).
Resilient Modulus of Bituminous Mixtures
Like the resilient modulus test for unbound materials, the resilient modulus test for bituminous mixtures
provides an estimate of the material’s modulus of elasticity. The most popular test method for resilient
modulus of bituminous mixtures is ASTM D-4123, Standard Test Method for Indirect Tension Test for
Resilient Modulus of Bituminous Mixtures (ASTM 2000i). As suggested by the title, the test setup is
similar to that used for determining the indirect tensile strength. In fact, the tests can be run in sequence
to determine both the resilient modulus and tensile strength of the material. The primary differences are
1) the test equipment must be able to apply repeated haversine loads, and 2) strain gauges must be
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mounted on the sample to measure its lateral deformation during loading. The test can be run on either
100- or 150-mm (4- or 6-in) diameter samples.
The equations for computing the tensile strength and strain to failure for the indirect tensile test are used
for computing stress and strain in the resilient modulus test. The applied stress is calculated the same way
as that for tensile strength; however, the specimen is not loaded to failure when measuring resilient
modulus. For the resilient modulus test, the specimen is normally loaded to a stress level between 5 and
20 percent of the estimated indirect tensile strength. The load is typically applied for 0.1 seconds along
with a no-load (rest) period of 0.9 seconds. Hence, the sample receives one load cycle per second
(Roberts et al. 1991).
The equation used to calculate the resilient modulus is independent of the specimen diameter and is as
follows:
MR 
P
(0.27   )
Ht
(2-5.7)
where:
MR
P
H
t
μ
=
=
=
=
=
Resilient modulus, Pa (lbf/in2).
Applied load, N (lbf).
Horizontal deformation, m (in).
Sample thickness, m (in).
Poisson’s ratio (typically 0.35 for HMA).
The test is typically run at more than one temperature in order to determine the sensitivity of the resilient
modulus to changes in temperature.
Dynamic Modulus of Asphalt Mixtures
This is a test procedure that is designed to provide an estimate of the elastic modulus of an asphalt
mixture under a repeated uniaxial compressive load. The measured property is sometimes referred to as
the complex modulus. Although the test method has been around for many years, it is now receiving
more attention because of its selection for use in characterizing HMA materials in the 2002 AASHTO
Guide for Design of New and Rehabilitated Pavement Structures. The procedure is described in ASTM
D-3497, Standard Test Method for Dynamic Modulus of Asphalt Mixtures (ASTM 2000j).
Cyclic sinusoidal loading is applied to the sample at a fraction of the mixture’s strength. The dynamic
modulus is simply a function of the maximum applied stress and the resulting vertical strain in the sample
(as measured by linear variable displacement transducers), that is:
E* 


(2-5.8)
where:
E* = Dynamic (or complex) modulus, kPa (lbf/in2).
σ = Applied maximum stress, kPa (lbf/in2).
ε = Measured vertical strain, mm/mm (in/in).
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Module 2-5. Field Sampling and Testing
The dynamic modulus test is distinctly different from the resilient modulus test in terms of both the state
of stress (compression versus tension) but also in the loading pattern (sinusoidal versus haversine). Both
of these contribute to some significant differences in the estimates of the elastic modulus.
One weakness associated with the dynamic modulus test is the requirement of a 2:1 ratio between the
sample length and diameter. This requirement is essential because of the effect of friction between the
sample and the loading plates at each end. The 2:1 ratio usually means that cores from existing HMA
pavements cannot be tested.
Test Result Correlations
Since not all agencies are familiar with the different material tests, and since most agencies are gradually
adopting the use of the resilient modulus to characterize their subgrade soils and unbound materials, it is
useful to consider correlations between some of the various material strength indicators. This section
provides some typical correlations between resilient modulus and CBR and R-value. Correlations are
also available between the resilient modulus and soil properties such as the AASHTO classification,
plasticity index, and moisture content (especially for cohesive soils). In all cases, any relationships
should be applied with extreme caution.
Resilient Modulus vs. CBR
Figure 2-5.18 presents some approximate correlations between other materials strength indicators and the
resilient modulus value (Van Til et al. 1972). The design engineer should be aware that these correlations
are approximate and based on limited data; therefore, they should be applied with caution.
An approximate correlation for the resilient modulus of subgrade soil materials based on the CBR is:
M R  B * CBR
(2-5.9)
where:
MR = Resilient Modulus, lbf/in2.
CBR = California Bearing Ratio.
B = Coefficient = 750 – 3000 (1500 for CBR < 10).
Resilient Modulus vs. R-value
An approximate relationship for the resilient modulus of a subgrade soil (or other unbound material)
based on the resistance value (R-value) obtained using a stabilometer is (Oglesby and Hicks 1982):
M R  A  B(R)
(2-5.10)
where:
MR
R
A
B
=
=
=
=
Resilient modulus, lbf/in2.
Resistance value obtained using the stabilometer.
Constant = 772 – 1155 (1000 for R < 20).
Constant = 369 – 555 (555 for R < 20).
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Figure 2-5.18. Correlations with resilient modulus (Van Til et al. 1972).
7. RECOMMENDED SAMPLING AND TESTING FOR EVALUATION
The paving materials used in the surface, base, subbase, and subgrade soil layers must each be considered
as part of the overall pavement evaluation. Subsequently, the tests selected to assess material quality
differ between layer types. This section provides recommendations into the types of sampling and testing
that should typically be used to assess the quality of each layer.
Subgrade Soil
Desirable properties of a subgrade soil include adequate shear strength, adequate permeability, ease and
permanency of compaction, volume stability, and durability. Unlike some engineering materials (e.g.,
steel), it is difficult to assign one strength or stiffness value to the subgrade soil. Along a project, roadbed
soil properties can be highly variable. In addition, soil properties change with changes in moisture,
density, and confining pressure. Due to this variability, it is desirable in the analysis and design stages to
conduct an extensive survey to determine the subgrade soil properties as they change throughout the year
(although this step is rarely taken). It is then up to the designer and individual agencies to address the
variability that is encountered in the design process.
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Module 2-5. Field Sampling and Testing
In pavement rehabilitation projects, subgrade soil evaluation starts with an estimate of in situ properties,
goes through an analysis of the design assumptions, and proceeds to the selection of subgrade soil design
values. This process determines if the subgrade soil has provided the support that was assumed during
design. If the subgrade soil has performed as expected, the design assumptions were correct and can
probably be used again. If, however, the subgrade soil has not performed as assumed, then the design
assumptions may have been incorrect. The subgrade soil may have been the cause of the deterioration
and new subgrade soil design parameters may need to be determined.
The identification of subgrade soil properties for pavement rehabilitation design should account for the
variability of the subgrade soil along the length of the project, its variability by depth, and its projected
variability over the design period.
Base/Subbase Layers
The materials used in the base and subbase layers are typically different from those of the subgrade soil in
that they are either granular or stabilized. It may not be possible to obtain information about these
materials beyond that contained in the construction records, unlike the subgrade soil that can be studied in
soil maps. The base materials protect the subgrade soil, and must satisfy several independent
considerations in order for the pavement to perform satisfactorily.
In addition to resisting deformation under loadings, base layers must also resist the deterioration effect of
moisture in the base while maintaining minimum strength requirements. The minimum strength
requirements apply particularly when stabilized materials are used. Typical tests to characterize the
suitability of untreated base course materials include CBR, moisture content, gradation, density and
triaxial classification. Stabilized materials typically require additional testing to determine their tensile
strength, a major response parameter for stabilized pavement sections.
Surface (HMA) Layer
For the purposes of rehabilitation design, it is common for agencies to perform laboratory testing on the
HMA surface layer, particularly if the pavement is more than 10 or 15 years old and is showing some
significant signs of deterioration. In those situations, the HMA surface has lost some or most of its loadcarrying capacity and may be a candidate for removal and replacement. If otherwise overlaid, the layer
would likely be patched and then treated more as a base course in determining the overlay thickness
requirements.
There are three situations in which significant laboratory testing of the HMA layer may be justified. One
is the case where the existing HMA is to be re-used as part of the hot-mix recycling process (see module
3-8). The second situation is when the existing HMA pavement has failed prematurely. In this instance,
testing of the HMA surface layer may reveal a problem that can be addressed as part of the rehabilitation
design (and yield information useful in avoiding other premature failures). The third situation is when the
existing HMA pavement has some significant remaining life. Under this scenario, it is worthwhile to recharacterize the important properties of the HMA surface layer since, despite its apparent good condition,
it is unlikely that its properties are the same as when originally constructed. Thus, laboratory testing
would provide a better estimate of the aged, in situ properties that could be factored into the rehabilitation
design.
Although there would be some similarities, the tests required for these three situations are likely to be
different. In the first case, the testing would focus on those properties necessary to develop a recycled
mix design. In the second case, the full gamut of laboratory tests may be required if the observed
deterioration can be attributed to the properties of the HMA layer. In the final case, the emphasis of any
testing should be on the properties that would affect the subsequent rehabilitation design.
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8. SUMMARY
It is often necessary to conduct a more detailed investigation of the in-place materials of a pavement
section being evaluated for a rehabilitation project. The primary purposes of this field and laboratory
investigation are to calibrate/verify NDT data, provide material information where NDT data is not
available, and help determine the causes of any observed pavement deficiencies. Additional material
properties are also required by some of the modern mechanistic pavement design procedures. Many
different in situ field tests and laboratory test methods are available to determine the subgrade soil and
pavement layer material properties, especially those that are linked to pavement performance.
The types and amount of material sampling and testing is primarily dependent upon the following four
factors:

Observed pavement distress. The type, severity, extent, and variation of visible distress on a
pavement greatly affect the locations and amount of field sampling and testing. If the distress is
uniformly spread over the project, sampling is most likely conducted in a random (objective)
manner. Otherwise, sampling can be targeted in areas of high distress concentrations.

Variability. The variability along the project site will affect the amount of material and sampling
required. Projects with greater variability in material properties will require a greater amount of
testing in order that this variability can be properly characterized and accounted for in
rehabilitation design.

Traffic volume. The locations and number of allowable samples may be limited on higher
trafficked roadways due to worker and driver safety concerns. Such lane closure restrictions and
safety related issues are typically not an issue on roadways with lower traffic volumes.

Economics. Most agencies have a limited budget that determines the types and amount of
sampling and testing that can be conducted for a given project. Engineering judgment must be
used to determine a sampling and testing plan that minimizes the amount of testing required to
adequately assess a pavement’s condition, while staying within the provided budget constraints.
The selected sampling and testing methods are dependent upon the particular pavement layer being
investigated (e.g., different test methods are used to assess subgrade soil and the HMA surface) and the
types of rehabilitation activities being considered (e.g., recycling requires a higher level of sampling).
Subgrade soils are primarily assessed based on their shear strength, their ability to resist deformation
under loadings, and their resistance to potential moisture damage. For base and subbase layers, the in situ
strength is of primary concern as the principal purpose of these collective layers is to protect the subgrade
soil from being overstressed. Sampling and testing of the surface layer focuses on assessing the strength,
stiffness, and elasticity of the HMA surface layer. The typical field sampling techniques, in situ testing
methods, and standard laboratory testing procedures used in a detailed material investigation are
discussed in this module.
When conducting an evaluation for pavement rehabilitation, the focus of all sampling and testing is to
facilitate the selection of the most appropriate rehabilitation technique. By determining material details,
and estimating the variability of material differences along a pavement, such sampling and testing
methods greatly help the engineer in making final rehabilitation technique recommendations.
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9.
Module 2-5. Field Sampling and Testing
REFERENCES
American Association of State Highway and Transportation Officials (AASHTO). 1986. Guide for
Design of Pavement Structures. American Association of State Highway and Transportation Officials,
Washington, DC.
American Association of State Highway and Transportation Officials (AASHTO). 2000. Standard
Specifications for Transportation Materials and Methods of Sampling and Testing, 20th Edition.
American Association of State Highway and Transportation Officials, Washington, DC.
American Society for Testing and Materials (ASTM). 2000a. “Practice for Sampling Bituminous Paving
Mixtures.” ASTM Standard Practice D-979. Annual Book of ASTM Standards, Volume 04.03. West
Conshohocken, PA.
American Society for Testing and Materials (ASTM). 2000b. “Sampling Compacted Bituminous
Mixtures for Laboratory Testing.” ASTM Standard Practice D-5361. Annual Book of ASTM Standards,
Volume 04.03. West Conshohocken, PA.
American Society for Testing and Materials (ASTM). 2000c. “Standard Practice for Soil Investigation
and Sampling by Auger Borings.” ASTM Standard Practice D-1452. Annual Book of ASTM Standards,
Volume 04.08. West Conshohocken, PA.
American Society for Testing and Materials (ASTM). 2000d. “Standard Practice for Penetration Test
and Split-Barrel Sampling of Soils.” ASTM Standard Practice D-1586. Annual Book of ASTM
Standards, Volume 04.08. West Conshohocken, PA.
American Society for Testing and Materials (ASTM). 2000e. “Standard Practice for Thin-Walled Tube
Sampling of Soils for Geotechnical Purposes.” ASTM Standard Practice D-1587. Annual Book of ASTM
Standards, Volume 04.08. West Conshohocken, PA.
American Society for Testing and Materials (ASTM). 2000f. “Standard Test Method for CBR
(California Bearing Ratio) of Soils in Place.” ASTM Standard Practice D-4429. Annual Book of ASTM
Standards, Volume 04.08. West Conshohocken, PA.
American Society for Testing and Materials (ASTM). 2000g. “Standard Test Method for Resistance RValue and Expansion Pressure of Compacted Soils.” ASTM Standard Practice D-2844. Annual Book of
ASTM Standards, Volume 04.08. West Conshohocken, PA.
American Society for Testing and Materials (ASTM). 2000h. “Standard Test Method for
Unconsolidated-Undrained Triaxial Compression Test on Cohesive Soils.” ASTM Standard Practice D2850. Annual Book of ASTM Standards, Volume 04.08. West Conshohocken, PA.
American Society for Testing and Materials (ASTM). 2000i. “Standard Test Method for Indirect
Tension Test for Resilient Modulus of Bituminous Mixtures.” ASTM Standard Practice D-4123. Annual
Book of ASTM Standards, Volume 04.03. West Conshohocken, PA.
American Society for Testing and Materials (ASTM). 2000j. “Standard Test Method for Dynamic
Modulus of Asphalt Mixtures.” ASTM Standard Practice D-3497. Annual Book of ASTM Standards,
Volume 04.03. West Conshohocken, PA.
Carpenter, S. H., M. I. Darter, and B. J. Dempsey. 1979. “Evaluation of Pavement Systems for MoistureAccelerated Distress.” Transportation Research Record 705. Transportation Research Board,
Washington, DC.
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2-5.33
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Elliott, R. P., S. I. Thornton, K. Y. Foo, K. W. Siew, and R. Woodbridge. 1988. Resilient Properties of
Arkansas Subgrades. Report No. FHWA/AR-89/004. Arkansas State Highway and Transportation
Department, Fayetteville, AK.
Fischer, J. A., M. R. Thompson, A. M. Ioannides, and E. J. Barenberg. 1984. “The Resilient Modulus of
Subgrade Reaction.” Transportation Research Record 954. Transportation Research Board, Washington,
DC.
Hall, K. T., J. M. Connor, M. I. Darter, and S. H. Carpenter. 1989. Rehabilitation of Concrete
Pavements, Volume III—Concrete Pavement Evaluation and Rehabilitation System. FHWA-RD-088073. Federal Highway Administration, McLean, VA.
Illinois Department of Transportation (IDOT). 1999. Geotechnical Manual. Bureau of Materials and
Physical Research, Illinois Department of Transportation, Springfield, IL.
Kleyn, E. G. 1975. The Use of the Dynamic Cone Penetrometer. Transvall Roads Department, Pretoria,
South Africa.
Kulhawy, F. H. and P. W. Mayne. 1990. Manual on Estimating Soil Properties for Foundation Design.
Report EL-6800. Electric Power Research Institute, Palo Alto, CA.
Livneh, M. 1987. “The Use of Dynamic Cone Penetrometer in Determining the Strength of Existing
Pavements and Subgrades.” Proceedings, 9th Southeast Asian Geotechnical Conference. Bangkok,
Thailand.
Livneh, M. 1989. “Validation of Correlations Between a Number of Penetration Tests and In Situ
California Bearing Ratio Tests.” Transportation Research Record 1219. Transportation Research Board,
Washington, DC.
Mindess, S. and J. F. Young. 1981. Concrete. Prentice-Hall, Inc., Englewood Cliffs, NJ.
Neville, A. M. 1996. Properties of Concrete. Fourth and Final Edition. John Wiley and Sons, Inc. New
York, NY.
Newcomb, D. E. and B. Birgisson. 1999. Measuring In Situ Mechanical Properties of Pavement
Subgrade Soils. Synthesis of Highway Practice 278. National Cooperative Highway Research Program,
Transportation Research Board, Washington, DC.
Oglesby, C. H. and G. L. Hicks. 1982. Highway Engineering. John Wiley & Sons, New York, NY.
Peck, R. B., W. E. Hanson, and T. H. Thornburn. 1974. Foundation Engineering. John Wiley & Sons,
Inc., New York, NY.
Portland Cement Association (PCA). 1984. Thickness Design for Concrete Highway and Street
Pavements. EB-109.01B. Portland Cement Association, Skokie, IL.
Portland Cement Association (PCA). 1992. PCA Soil Primer. EB-007.05S. Portland Cement
Association, Skokie, IL.
Roberts, F. L., P. S. Kandhal, E. R. Brown, D. Lee, and T. W. Kennedy. 1991. Hot-Mix Asphalt
Materials, Mix Design, and Construction. First Edition. National Asphalt Pavement Association,
Lanham, MD.
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Module 2-5. Field Sampling and Testing
Schmertmann, J. H. 1979. “Statics of SPT.” Journal of the Geotechnical Engineering Division, Vol.
105, No. GT5. American Society of Civil Engineers, New York, NY.
Schmertmann, J. H. and A. Palacios. 1979. “Energy Dynamics of SPT.” Journal of the Geotechnical
Engineering Division, Vol. 105, No. GT8. American Society of Civil Engineers, New York, NY.
Smith, R. B. and D. N. Pratt. 1983. “A Field Study of In Situ California Bearing Ratio and Dynamic
Cone Penetrometer Testing for Road Subgrade Investigations.” Australian Road Research Board 13(4).
Perth, Australia.
Strategic Highway Research Program (SHRP). 1988. Technical Provisions and Fee Schedules for
Contracts P-012, P-013, P-014, P-015, Laboratory Testing of Soils and Bituminous Materials.
Amendment No. 1. Strategic Highway Research Program, Washington, DC.
Thompson, M. R. and B. J. Dempsey. 1977. “Subgrade Soils: An Important Factor in Concrete
Pavement Design.” Proceedings, International Conference on Concrete Pavement Design. Purdue
University, West Lafayette, IN.
Thompson, M. R., T. C. Kinney, M. L. Traylor, J. R. Bullard, and J. L. Figueroa. 1977. Final Report—
Subgrade Stability. FHWA-IL-UI-169. Illinois Department of Transportation, Springfield, IL.
Thompson, M. R. and Q. L. Robnett. 1976. Final Report—Resilient Properties of Subgrade Soils.
UIUC-ENG-76-2009. Illinois Department of Transportation, Springfield, IL.
United States Army Engineer Waterways Experimental Station (US ARMY). 1989. Instruction Manual,
Dynamic Cone Penetrometer. U.S. Army Engineer Waterways Experimental Station, Vicksburg, MS.
Van Dam, T. J., K. D. Smith, M. J. Wade, L. L. Sutter, and K. R. Peterson. 2000. Guideline I—Field
Distress Survey, Sampling, and Sampling Handling Procedures for Distressed Concrete Pavements.
Final Draft, FHWA Project No. DTFH61-96-C-00073. Federal Highway Administration, McLean, VA.
Van Til, C. J., B. F. McCullough, B. A. Vallerga, and R. G. Hicks. 1972. Evaluation of AASHTO Interim
Guides for Design of Pavement Structures. NCHRP Report 128. National Cooperative Highway
Research Program, Highway Research Board, Washington, DC.
Von Quintus, H. and B. Killingsworth. 1997. Design Pamphlet for the Determination of Design
Subgrade in Support of the AASHTO Guide for the Design of Pavement Structures. Federal Highway
Administration, Washington, DC.
Webster, S. L., R. W. Brown, and J. R. Porter. 1994. Force Projection Site Evaluation Using the
Electric Cone Penetrometer (ECP) and the Dynamic Cone Penetrometer (DCP). Technical Report GL94-17. U.S. Air Force, Tyndall AFB, Florida.
Webster, S. L., R. H. Grau, and T. P. Williams. 1992. Description and Application of Dual Mass
Dynamic Cone Penetrometer. Instruction Report GL-92-3. U.S. Army Engineer Waterways Experiment
Station, Vicksburg, MS.
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NOTES
2-5.36
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