materials
ArticleApplication of Digital Image Correlation (DIC)
Method for Road Material Testing
Jarosław Górszczyk * , Konrad Malicki and Teresa Zych
Faculty of Civil Engineering, Cracow University of Technology, 31-155 Cracow, Poland* Correspondence: jgorszcz@pk.edu.pl; Tel.: +48-12-6282368
Received: 25 April 2019; Accepted: 19 May 2019; Published: 24 July 2019


Abstract: The theoretical part of the paper presents the framework of the digital image correlation(DIC) method as well as its advantages and limitations. The DIC technique that can be usedin static and fatigue tests is a non-contact, non-interferometric optical method for measuring thesurface deformation of structural elements, and material samples. In the experimental part of thepaper, the implementation of the DIC method for the selected laboratory tests of building materials
is described. The results of the tests on the samples of the materials used in road construction,i.e., asphalt mixtures (HMA), stone, soil stabilized with a hydraulic binder, and geosynthetics
are discussed. The conducted research pointed out the possibilities of using the DIC method toevaluate the deformation of road materials in laboratory tests, taking into account their specificity.
The variety of samples of tested material allowed to indicate the areas in which the DIC method andthe algorithms used to evaluate the results give a significant advantage compared to tensometricmeasurement methods.
Keywords: digital image correlation; DIC technique; image analysis; tension test; displacement
measurement; geogrid; geosynthetics

1. Introduction
The measurement of displacement/deformation at any point of a structural element/material
subjected to external loads is important in many engineering applications. The obtained strain values
are used to visualize the strength problems in a structural member.
The conventional instruments that measure strains (i.e., strain gauges, extensometers) cannot
generate strain maps. A full-field strain measurement is possible using the Digital Image Correlation(DIC) technique. This method belongs to the group of optical, non-interferometric techniques,
which determine deformation by comparing the changes in the image of the surface of a tested object
before and after deformation [1]. The DIC technique enables new and more complex investigations.
The DIC technique was introduced in the 1980s [2–5]. Since then the quality of digital images
has improved enormously and multiple major modifications of algorithms have been implemented.
Nowadays the DIC method with high sensitivity and accuracy is utilized for testing specimens and,most recently, the whole structures. It can be used in laboratory or outdoor environments. The DIC
techniques are applied not only in the field of engineering, but also in medicine, mining, conservationof cultural heritage, etc., [6–11].
The examples of the application of the DIC method for testing road materials are presented in thepaper. At the beginning, the theoretical fundamentals, advantages, and limitations of the DIC method
are described. Then the implementation of this method for laboratory testing of selected materials usedin the road construction is presented. The results of the tests carried out on asphalt mixtures (HMA),
stone, soil stabilized with a hydraulic binder and geosynthetics are discussed. The conducted researchallowed evaluation of the possibility of implementing the DIC method to assess deformation of road
Materials 2019, 12, 2349; doi:10.3390/ma12152349

www.mdpi.com/journal/materials

deformation of road materials in laboratory tests. A wide spectrum of studied materials enabled to
identify the areas in which the DIC method and the algorithms used in it give a significant advantage
over traditional and tensometric measurement methods.
Materials
2019, 12, 2349of DIC
2. Fundamentals

2 of 192.1. Digital Image Correlation (DIC) System for Registration and Analysis of Displacement/Deformationmaterials in laboratory tests. A wide spectrum of studied materials enabled to identify the areas in
Fieldswhich the DIC method and the algorithms used in it give a significant advantage over traditional andDigital measurement
Image Correlation(DIC) is a precise, non-contact, and non-interferometric opticaltensometricmethods.
method used for measuring the displacement/deformation of a structural element/material subjected2.
of TheDICidea of the DIC method is based on the principles of continuum mechanics
to Fundamentals
external loading.
(rigid body mechanics) [12]. The system consists mainly of a digital camera/cameras and specialised
2.1. Digital Image Correlation (DIC) System for Registration and Analysis of Displacement/Deformation Fields
computer software (e.g., Istra 4D programme [8]). A camera/cameras is/are used to capture
Digital Image(DIC) isofa theprecise,non-contact,
non-interferometric
opticalmethodconsecutive
imagesCorrelation(photos/pictures)
surfaceof a testedand
object/material
before andduringtheusedfor measuring
theThedisplacement/deformation
a structuralsubjectedby
to theexternal
deformation
period.
obtained digital imageofdata(a serieselement/materialof photos) is analysed
DIC
loading.
idea of the
DIC method
is based on thecorrelation
principles ofcontinuum
mechanics(rigidbody
software The(computerprograms).
The mathematicalanalysis
is applied.
Finally,a setof
mechanics)
[12].
The
systemconsists
mainlyofadigitalcamera/cameras
andspecialised
computerdisplacement/deformation maps for the entire specimen surface is created [4,6,13]. Stress fields cansoftware(e.g.,from
Istrastrain
4D programme[8]). A camera/cameras is/are used to capture consecutive images
be evaluatedfields.
(photos/pictures)of the surface
of a tested object/material
beforeand during
deformation
period.
The measurementof displacement/deformation
using
the DIC
systemthe
requiresthe followingTheobtained
digital
image
data
(a
seriesofphotos)isanalysedby
theDIC
software(computerconsecutive stages [8,12]:
programs). The mathematical correlation analysis is applied. Finally, a set of displacement/deformation•
Preparation of the surface of the analysed specimen (patterning)
maps for the entire specimen surface is created [4,6,13]. Stress fields can be evaluated from strain fields.
•
Calibration of a device according to the required procedure (using the DIC system calibration)
The measurement of displacement/deformation using the DIC system requires the following•
Recording images of the examined object/material surface before and during its
consecutive stages [8,12]:
movement/deformation•
Image analysis(evaluation)a specialised
computerprogramme (software)•
Preparationof thesurface ofusing
the analysed
specimen(patterning)•
Visualization
the results.
•
Calibrationof ofa device
according to the required procedure (using the DIC system calibration)•
Recordingimages of
examined
object/material
beforeits movement/deformation
At the beginninga thetestedspecimenneeds to besurface
preparedbyandtheduring
applicationof a unique pattern•
Image
analysis(evaluation)usingaspecialised
computerprogramme(software)of random speckles/dots to its surface (Figures 1–3). The speckle pattern can be the natural texture of•Visualizationof theorresults.
the specimen
surfacein many cases artificially made by spraying black paint on the whitebackground of the specimen surface (Figures 1–3). Other techniques can be applied as well [1]. The
At the beginning a tested specimen needs to be prepared by the application of a unique pattern ofspeckle-free DIC method was also developed and applied [14]. A special preparation of the specimen
random speckles/dots to its surface (Figures 1–3). The speckle pattern can be the natural texture of thesurface with the pattern containing the characteristic speckles enables the observation of the changes
specimen surface or in many cases artificially made by spraying black paint on the white backgroundin the position of these dots during the deformation period [12,15–17].
of the specimen surface (Figures 1–3). Other techniques can be applied as well [1]. The speckle-freePrior to the loading of the examined object/material surface and before taking photos during its
DIC method was also developed and applied [14]. A special preparation of the specimen surface with
deformation, the calibration/positioning of the device (camera/cameras) according to the appropriate
the pattern containing the characteristic speckles enables the observation of the changes in the position
procedure is required, e.g., using a special calibration plate (target) (Figure 4) [8,16].
of these dots during the deformation period [12,15–17].

1. DIC
Figure 1.
DIC method—Stages
method—Stages [17].
[17]. (The
(TheCreative
Creative Commons
Commons Attribution
Attribution (CC(CC BYBY 4.0) license(http://creativecommons.org/licenses/by/4.0/)).
(http://creativecommons.org/licenses/by/4.0/)).

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Figure 2. DIC method—Reference image: facets (also called subsets) marked with blue squares andFigure 2. DIC method—Reference image: facets (also called subsets) marked with blue squares
centrepointsof thefacets/subsets locatedat theintersectionof theyellow
virtualwith
gridblue[17,18].
Figure2. DIC
method—Reference
image:
facets(also called
subsets)
markedsquares andand centre points of the facets/subsets located at the intersection of the yellow virtual grid [17,18].
centre points of the facets/subsets located at the intersection of the yellow virtual grid [17,18].
(The Creative Commons Attribution (CC BY 4.0) license (http://creativecommons.org/licenses/by/4.0/)).
Figure 2. DIC method—Reference image: facets (also called subsets) marked with blue squares andcentre points of the facets/subsets located at the intersection of the yellow virtual grid [17,18].

Figure 3. Graphical illustration of the selected region deformation of a scanned surface (a square
Figure3. Graphical illustrationof the selected region
deformationa scanned(a Commons
squarefacet/subset/subimage)in a two-dimensional
coordinate
system of[17,18].
(The surface
Creative
facet/subset/subimage)ina
two-dimensional
coordinate
system[17,18].
Figure3.
Graphical
illustrationoftheselectedregiondeformationofascannedsurface(asquareAttribution (CC BY 4.0) license (http://creativecommons.org/licenses/by/4.0/)).
facet/subset/subimage) in a two-dimensional coordinate system [17,18].

Prior to the loading of the examined object/material surface and before taking photos during its
Figure 3. Graphical illustration of the selected region deformation of a scanned surface (a square
deformation, the calibration/positioning of the device (camera/cameras) according to the appropriate
facet/subset/subimage) in a two-dimensional coordinate system [17,18].
procedure is required, e.g., using a special calibration plate (target) (Figure 4) [8,16].

(a)
(a)

(b)
(b)

Figure 4. 3D-DIC system- Calibration: (a) The example of a calibration target; (b) Calibration live
image[8].4. 3D-DIC system- Calibration: (a) The example of a calibration target; (b) Calibration live
Figure(a)(b)
image [8].
Next,a photo/picture/image
before loading(“reference
image”)and thena seriesof photos
Figure4. 3D-DIC system- Calibration:(a) Theexample ofa calibration
target;(b) Calibration
Figure 4. 3D-DIC system- Calibration: (a) The example of a calibration target; (b) Calibration live
a photo/picture/image
before1 loading(“reference
and lighting
then a series
photoslive[8].
duringNext,
the imagedeformation
period (Figure
are taken,
applyingimage”)
the properto theoftested
image [8].

during thedeformation
1 arethecameras
properislighting
to the5 tested
specimen.
Typically,
the DICperiod
system(Figure
equipped
withtaken,
one orapplying
two digitalused (Figures
andNext,Typically,a photo/picture/image
beforeloadingwith(“reference
image”)
andthen a isseriesof(Figures
photos during
theDIC
systemequippedoneortwo
digitalcamerasused
5 and6)specimen.
[8,9,17].
Next, a photo/picture/image before loading (“reference image”) and then a series of photos
thedeformation
period(Figure1
aretaken,
applying
theproperlighting
tothetestedspecimen.
[8,9,17].
A 6)
single-camera
system is applied
measurein-planedeformationsof theplanarobjects.toAthe
cameraduringthe deformation
period to(Figure
1 aretaken,
applying theproperlighting
testedTypically, the DIC system equipped with one or two digital cameras is used (Figures 5 and 6) [8,9,17].
single-camera
systemisDIC
appliedtomeasurein-plane
ofcameras
the planar
objects.
A camera
specimen.
Typically,thesystemequippedwith surface
onedeformations
or twodigitalis used(Figures
5of
and
is A
placedwithits
opticalaxis
normaltothespecimen(Figure5).
Two-dimensional
results
A single-camera system is applied to measure in-plane deformations of the planar objects.
6)placed[8,9,17].with its optical axis normal to the specimen surface (Figure 5). Two-dimensional results ofisA camera is placed with its optical axis normal to the specimen surface (Figure 5). Two-dimensional
A single-camera system is applied to measure in-plane deformations of the planar objects. A camera
is placed with its optical axis normal to the specimen surface (Figure 5). Two-dimensional results ofMaterials 2019, 12, x FOR PEER REVIEW
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deformation are obtained. Therefore, the system is called two-dimensional digital image correlation
deformation are obtained. Therefore, the system is called two-dimensional digital image correlation(2D-DIC).
If the object surface
is non-planar,or theif 3D
deformation
after loading,the 2D-DIC
resultsof deformation
obtained.
Therefore,systemis calledoccurs
two-dimensional
digitalimage(2D-DIC).
If the object are
surfaceis non-planar,
or if 3D
deformation
occursafter loading,the 2D-DIC
correlation(2D-DIC).
Iftheobjectsurfaceisnon-planar,orif
3D
deformation
occursafter
loading,
method is no longer applicable [1,16,19].
method is no longer applicable [1,16,19].
the 2D-DIC method is no longer applicable [1,16,19].

Figure5. 2D-Digital
Image
Correlation[17,18,20].
Figure 5. 2D-Digital Image
Correlation
system[17,18,20].
(Thesystem
Creative
Commons Attribution (CC BY
Figure 5. 2D-Digital Image Correlation system [17,18,20].
4.0) license (http://creativecommons.org/licenses/by/4.0/)).

Figure 6. 3D-Digital Image Correlation system [17,18,20]. (The Creative Commons Attribution (CC BY
Figure 6. 3D-Digital Image Correlation system [17,18,20].
4.0) license (http://creativecommons.org/licenses/by/4.0/)).
Figure 6. 3D-Digital Image Correlation system [17,18,20].

measuringthe
thedisplacements/deformations
displacements/deformations
in 3D
space
at least
two cameras
aretousedto
For measuring
in 3D
space
at least
two cameras
are usedrecordFor measuring the displacements/deformations in 3D space at least two cameras are used torecordtheofimageof aobject
testedobjectfrom directions
different directions[16]. Three-dimensional
digital
image
the
image
a testedfromdifferent[16]. Three-dimensional
digital image
correlation
record the image of a tested object from different directions [16]. Three-dimensional digital image
correlation(3D-DIC)
system isinpresented(3D-DIC)
systemis presented
Figure 6. in Figure 6.
correlation (3D-DIC) system is presented in Figure 6.
Currently, modern and developed DIC systems, consisting
consisting ofof more than two cameras
cameras (three,(three,Currently, modern and developed DIC systems, consisting of more than two cameras (three,four, six and even eight cameras) are designed to observe spatial objects from several directions
four, six and even eight cameras) are designed to observe spatial objects from several directionsareintended
to analyse
the material
behaviour
duringvibration,drop
simultaneously. These
Thesesystems
systemsareintended
to analyse
the material
behaviour
duringvibration,
simultaneously. These systems are intended to analyse the material behaviour during vibration, drop
tests, crashTheyhigh-speed
systems systems
with highwith
sampling
frequencies(several thousanddrop
tests, tests,
crashetc.
tests,
etc.areThey
are high-speed
high sampling
frequencies(severaltests, crash tests, etc. They are high-speed systems with high sampling frequencies (several thousandfps) contrary
low-speedsystems (typicalmeasuringstatic deformations)with the sampling
thousandfps)tocontrary
to low-speed
systemsfor(typicalfor measuring
static deformations)with thefps) contrary to low-speed systems (typical for measuring static deformations) with the sampling
frequencies
of maximallyseveral fpsseveral[4,16].fps [4,16].
sampling
frequencies
of maximallyfrequencies of maximally several fps [4,16].
Nowadays, aamore
morecomprehensive
comprehensiveanalysis
analysisis alsopossibleusing
the DIC
systems
equipped
Nowadays,is alsopossibleusing
the DIC
systems
equipped
withNowadays, a more comprehensive analysis is also possible using the DIC systems equippedwitha microscope
adapter,whichallowsfor microscopic
observationsa tested
specimenduringamicroscope
adapter,whichallowsfor microscopic
observationsof a oftestedspecimenduringthewith a microscope adapter, which allows for microscopic observations of a tested specimen during
the deformation
perioddeformation
period[19].[19].
the deformation period [19].
In thethe field
fieldofofexperimental
experimentalmechanics
mechanics
roadmaterials,
2D-DICtechniqueis commonly
of of
road
materials,
the the2D-DICtechniqueis commonlyused
In the field of experimental mechanics of road materials, the 2D-DIC technique is commonlyused
as a practicaland effective
tool
for measuring
asa practicaland effective
tool formeasuringstrainsstrains[1]. [1].
used as a practical and effective tool for measuring strains [1].
After recording the images obtained from one or more CCD cameras, the subsequent stage of
After recording the images obtained from one or more CCD cameras, the subsequent stage ofthe study is image analysis (evaluation)(evaluation) in a specialised
specialised computer
computer programme
programme (e.g.,(e.g., Istra
Istra 4D)
4D) [4,8].
[4,8].
the study is image analysis (evaluation) in a specialised computer programme (e.g., Istra 4D) [4,8].
DICsystem
systemis basedoncomparison
the comparison(correlation)
digital Aimages.
A mathematical
The DIC
is basedon the(correlation)of digitalofimages.
mathematicalcorrelation
The DIC system is based on the comparison (correlation) of digital images. A mathematicalcorrelation
analysis
is applied
to compare
digitalimagestaken before
deformation
analysis
is applied
to compare
digitalimages taken
beforedeformation(“reference
image”) (“reference
and duringcorrelation analysis is applied to compare digital images taken before deformation (“reference
image”)and during
the (“deformed
deformationimage”)
process(Figures(“deformed
image”) (Figures
1–3)[16–18,21].
At gridfirst, athedeformation
process1–3) [16–18,21].
At first,
a virtual,
regularimage”)and during
the deformation
process (“deformed
image”) (Figures
1–3)[16–18,21].
At first,ofa

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points is placed onto the “reference image”, also called the “specific image” or the “region of interest” (afragment of the object seen/scanned by the camera before deformation). Each point at the intersectionof the virtual grid is the centre of a facet/subset/subimage (“correlation region”/”calculation area”)—asmall square region/element of the image (Figures 1 and 2). The minimal facet size is determined by
the size of the created pattern in such a way that every facet has to contain white and black colours(e.g., random black speckles/points on the white background) in order to ensure proper correlation.
Due to a unique pattern, each facet can be distinguished from other facets and thus can be identified in
the “deformed image” (Figure 3). Based on the recorded changes in the location of facets/points in
two consecutive images (taken before and during the deformation process), specialised DIC software
performs the calculation of the displacement/deformation (for the entire analysed specimen surface)
using a correlation algorithm. Depending on the DIC system, NCC (Normalised Cross-Correlation),
LSM (Least-Squares Matching), etc., are applied as correlation criteria [1,19].
The last stage of the study is the visualization of the results. The output data is a set ofdisplacement/deformation maps [4,6,8,13,16,18,21].
2.2. Advantages and Limitations of Using DIC
The following main advantages of the DIC system can be distinguished [3,8,15,16]:•••

The ability to measure the displacements/deformations of samples of various sizes, from microsamples examined using a microscope to large-size objects such as fragments of bridges
The possibility of the determination of Poisson’s ratio and Young’s modulus of materials, detectionof cracks and damage, the measurement of vibration, modal analysis and the validation of FEM
numerical models
The system is easy to use in laboratory and field conditions.

A comparison of the DIC technique with traditional gauges and sensors techniques (extensometers)
is presented in Table 1.
Table 1. Comparison of the DIC method with measurements by using extensometers [5,8,16].
DIC System

Extensometers

Non-contact measurement

Contact measurement

Unlimited number of deformation measurements

An extensometer can be used only once(a glued extensometer cannot be peeled off without
damaging it)

The possibility of testing samples of any shape andmaterial; the tested surface of the sample does nothave to be flat

A surface on which the extensometer is glued has tobe flat

The ability to measure deformation in all directions(along X-axis, Y-axis, Z-axis), on a plane or inthree-dimensional space

The ability to measure deformation only in the chosen
direction

Full-field deformation analysis

Results of the deformation at selected points of thesample, i.e., at the points where the sensors areattached

A measurement of the real maximum displacements
and deformations

A measurement limited by the maximum value of thedeformation of an extensometer

A quick preparation of a random pattern of black
dots on the white background on the sample surface
by spraying paintA time-consuming process of placing theextensometer on the surface of a sample (gluing, etc.)

The need to clean the surface of a sample before testing

The DIC system also has some limitations, such as:

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The dependence of the system on natural lighting conditions; the need to apply artificial lightwhen registering images with high frequency
The need to use calibration tables appropriate to the size of the tested sample area and capacious
storage media required to archive recorded images and to obtain research results.

3. Application of DIC System for Testing Deformation of Road Materials—Results of Own
Investigations
In the field of road engineering, the DIC system enables the study of the mechanical parameters ofanisotropic and heterogeneous road materials, such as asphalt mixtures (commonly used as a material
for pavements), road stones, soils stabilized with a binder, geosynthetics, etc., [5,8,9,22]. The DIC
technique is also applied for testing the deformation of structures, e.g., road pavement structures.
According to Grygierek et al. [23], this method enables the assessment of the influence of a vehicle
wheel load on the resulting deformation of wearing course.
The DIC system is used for various purposes, such as [22,24,25]:••••

The evaluation of the correctness of the applied test procedures
The assessment of local changes in the parameters of materials due to their heterogeneity
The description of the cracking process
The validation of theoretical and numerical models.

In the study of road materials conducted at Cracow University of Technology, the 2D digital image
correlation system (which provides strain measurements for planar surface specimens) was applied.
The following road materials were tested: Hot-mixed asphalt (HMA), soil stabilized with a hydraulicbinder, stone (porphyry), and geosynthetics. The results of our own tests of mechanical properties ofroad materials are presented in this Chapter.
3.1. Road Materials Tested
Hot-Mixed Asphalt (HMA) (asphalt concrete) AC11S was used in the study.
The tested soil was silty loam with optimum compaction moisture of 13.8%. As a binder,
a two-component road binder was used for stabilizing the soil. The used road binder causes theagglomeration of fines and improves soil characteristics through the waterproofing effect [26]. The test
was carried out on 30 cylindrical specimens of soil with the dimensions of 80 mm × 80 mm.
Stone samples were prepared from porphyry rock.
Geosynthetics that perform various functions in civil engineering, e.g., strengthening, filtering,and separating were also investigated [27–30]. Geosynthetics research was carried out on the three
types of reinforcing geogrids: biaxial glass fibre geogrid, biaxial polyester fibre geogrid and triaxial
hexagonal polypropylene geogrid. Glass fibre geogrids are designed for the reinforcement of asphalt
layers [31–33]. Polyester geosynthetics are used in transportation engineering to strengthen a weaksoil substrate, reinforce embankments and to stabilise landslides [34–36]. Polypropylene geogrids witha hexagonal arrangement of ribs are used for the stabilisation of unbound granular layers by way ofinterlock with the aggregate [37–39].
3.2. Preparation of Samples of Road Materials
The use of the DIC system requires a proper preparation of road material samples for laboratory
tests. It involves painting the selected sample surfaces with white paint and then applying as many
black spots as possible, which the DIC system converts into a grid of characteristic elements [38].
The preparation of samples (painting HMA samples with white paint and spotting a geosynthetic
sample with black colour) is shown in Figure 7 [24,25]. The prepared samples of soil and stone, covered
with a layer of the pattern in the form of a white background with randomly distributed black points,
are also presented in Figure 7.

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(a)

(b)

(c)

(d)

Figure 7.
samplesurfacefor fortesting
withwiththe DIC
method:(a) Painting
HMA HMA(Hot7. Preparation
Preparationofofthe
thesamplesurfacetesting
the DIC
method:(a) Painting
Mixed
Asphalt)
sampleswith whitesoil (b)sampleto test; (c)toStone
prepared(Hot-Mixed
Asphalt)
sampleswith paint;
white(b)paint;
soil preparedsample prepared
test; sample(c) Stone
sampletotest—atowhitebackgroundsprinkledwith pepper(so called“pepper
method”);(d)(d)
spottingaprepared
test—awhite backgroundsprinkledwith pepper(so called“pepper
method”);
spotting
geosynthetic
samplewithblackcolour.
a geosynthetic sample with black colour.

3.3. DICDIC Set-Up
Set-Up
3.3.
The laboratorylaboratory stand
stand based
based onon thethe MTS
servo-hydraulic testing
testing systemsystem andand thethe DICDICThe
MTS Landmark
Landmark servo-hydraulicmeasurement
systemwasusedfortestingroadmaterials.
measurement system was used for testing road materials.
The most
most important
important elements
elements ofTheof thethe laboratorylaboratory stand
stand areare asas follows
follows [24]:
[24]:

••
••
••
••
••

A servo-hydraulic testing machine station with
with aa thermal
thermal chamberchamber
A digital camera
Linear lighting
A controller
controller andand aa computer
computer forfor controlling
controlling thethe DICDIC systemsystemA
A place/standplace/stand toto analyse
analyse theresults ofof thethe DICDIC system,
system, equipped,
equipped, forA
the resultsfor example,
example, with
with thethe ISTRA
ISTRA
4D
programme.
4D programme.

A servo-hydraulictesting
machinecancanbe usedto toperform
staticandA servo-hydraulictesting
machinebe usedperform
staticandfatigue
fatiguetests
testsofofroadroad materialmaterial
samples. The accuracy
accuracyclassclassofofforce
forcetransducers
transducers
testing
machineequals0.5a range
in a range
the
of of
thethetesting
machineequals0.5 inof theofforce◦◦
valuevalue
from 1%
to1%
100%.
The climaticchamberallowstesting
at temperaturesfrom from−40 −40
C to°C
+60
C.
force
from
to 100%.
The climaticchamberallowstesting
at temperatures
to +60The
laboratory
standpreparedfortheindirecttensile
test
is
shown
in
Figure
8.
°C.
The laboratory stand prepared for the indirect tensile test is shown in Figure 8.

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(a)
(a)

(b)
(b)

Figure
stand
using
the
DIC
methodfortheindirecttensiletest:
HMA
sample;(b)
Figure 8.
fortheindirecttensiletest:(a) (a)HMA
sample;(b) soil
8. Laboratory
Laboratory stand
standusing
usingthetheDIC
DICmethod
methodfortheindirecttensile
test:
(a)
HMA
sample;(b)soil[40,41].
sample[40,41].
soil sample
sample[40,41].

Geosynthetics
research waswascarriedSpecialGeosynthetics
Geosynthetics researchresearch
was carried
carried out
out onon aa laboratorylaboratory stand
stand adapted
adapted forfor this
this purpose.
purpose. Special
Special
jaws
used
to
attachgeosynthetics
samplesinadirecttensiletest.
Thelaboratory
standduring
the
jaws
were
used
to
attachgeosynthetics
samplesinadirecttensiletest.
Thelaboratory
standjaws were used to attach geosynthetics samples in a direct tensile test. The laboratory stand during
duringgeosynthetics
test
is
shown
in
Figure9.
thethe geosynthetics
geosynthetics test
test isis shown
shown inin Figure
Figure 9.
9.

(a)
(a)

(b)
(b)

FigureLaboratory stand
stand forfor geosynthetics(a) Sample
Figure 9.
9. Laboratory
geosynthetics testing
testing using
using thethe DICDIC method:
method: (a)Sample ofof hexagonal
hexagonal
geogrid;(b)
sampleoforthogonal
geogrid[24,38].
[24,38].
geogrid; (b) sample of orthogonal geogrid [24,38].

The
The DICDIC set-up
set-up used
used inin theconsisted ofof onecamera andadequate multiple
multiple LED
LED lightlight
the study
study consisted
one camera
and anan adequate
sourcePhotographs
were
taken
duringloadingofasampleatadeterminedload
increase.
source [4,8].
[4,8].
Photographs
were
taken
duringloadingofasampleatadeterminedload
increase.
[4,8]. Photographs were taken during loading of a sample at a determined load increase. The
The
distance
of
from
the
observed
samplewaschosensoThedistanceof camerathecamerafromobserved
samplewaschosenthatthethecorrect
correctsystemsystemcalibration
calibration
distanceof thethecamerafromthetheobserved
samplewaschosensosothatthat
thecorrect
systemcalibration
could
could bebe obtained.
obtained. The
The post-processing
post-processing ofof thethe measurement
measurement results
results was
was performed
performed using
using thethe ISTRA
ISTRA
4D
The
determinationoftheanalysed
field
and
the
division
of
this
area
into
the
grid
of
4D programme.
programme.
The
determinationoftheanalysed
field
and
the
division
of
this
area
into
the
grid
programme. The determination of the analysed field and the division of this area into the gridofelements
werecarriedoutbysoftware
in
first
stage
of
analysis.
As
result,theofelements
werecarriedbysoftwarein thestageof theanalysis.
As analysis
the analysis
result,elements
werecarriedoutoutby thethethesoftwarein thethefirstfirststage
of thetheanalysis.
As thetheanalysis
result,
the
maps
of
and
deformationsofofsamplewere
obtained.
themapsof displacements
deformationsof surface
thesurfaceof testedthetestedsamplewere
obtained.
maps
of displacements
displacements
andanddeformationsof thethesurfaceof thethetestedsamplewere
obtained.
3.4.
Test Results
3.4.
3.4. Test
Test
Results ofof Displacements
Displacements andand Deformations
Deformations ofof RoadRoad Materials
Materials Using
Using thethe DICDIC Method
MethodGeosynthetics
For otherother
Geosynthetics
Geosynthetics wide-width
wide-width tensile
tensile tests
tests were
were carried
carried out
out according
according toto thethe standard
standard [42].
[42]. For
For
other
road
materials
themodulusofelasticity,
Poisson’s
ratio,
compressive
strength,andindirecttensileroad
road materials
materials thethe modulus
modulus ofof elasticity, Poisson’s ratio, compressive
compressive strength,
strength, andand indirect tensile
tensilestrength
strength
procedures[40,41,43–48].
strength were
were determined
determined according
according toto thethe standards
standards andand procedures
procedures[40,41,43–48].
[40,41,43–48].

3.4.1.
3.4.1. Hot-Mixed
Hot-Mixed Asphalt
Asphalt (HMA)(HMA)

Materials 2019, 12, 2349

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Materials
Materials 2019,2019, 12,
12, x
x FOR
FOR PEER
PEER REVIEWREVIEW

99 of
of 20
20

3.4.1. Hot-Mixed Asphalt (HMA)The
view ofof thethe analysed
analysed Hot-Mixed
Hot-Mixed Asphalt
Asphalt (asphalt
The view(asphalt concrete
concrete AC11S)
AC11S) sample
sample andand thethe grid
grid ofofelements on
on itsits surface
surface areare presented
presented inin Figure
Figure 10.
10.
elements
Figure
10.

(a)
(a)

(b)
(b)

Figure 10.
HMA sample:
sample: (a)(a)
Viewof thethe sample
sample before
before testing;testing; (b)(b) virtual
virtual grid
grid onon thethe surfaceof the(a) View
View ofFigure10. HMAsurface ofthetested sample
sample [25].
tested[25].

The
The indirect
indirect tensile
tensile test
test waswas carried
carried out
out forfor Hot-Mixed
Hot-Mixed Asphalt
Asphalt (HMA).
(HMA). By
By registering
registering changes
changesthepositionthegridpointsonthesurfaceofthesampleduring
the
test,
its
in
of
in the position of the grid points on the surface of the sample during the test, its displacements
displacements andanddeformations were
Figure 11
11 shows
shows thethe horizontal
horizontal displacements
displacements ofof thethe analyzed
analyzed surface
surfacedetermined. Figure
deformations
were determined.
determined.
ofof aa cylindricalcylindrical HMA
HMA sample
sample inin selected
selected phases.
phases. Vertical
Vertical displacement
displacement maps
maps generated
generated byby thethe DIC
DIC
software
in
the
same
test
are
presented
inFigure12.
presented inin Figure
Figure 12.
12.
software in the same test are presented

(a)
(a)

(b)
(b)

Figure11.
Horizontal
displacements
of
the
in
indirecttensile(a)Figure 11.
11.Horizontal
Horizontaldisplacements
displacementsof HMA
the HMA
HMA
samplein thethe tensile
indirecttensiletest:(a) TheTheof thesamplesample
in the indirect
test:(a) Thetest:
distribution
distribution
ofdisplacements,
horizontal displacements,
displacements,
the
load
of
0.4 kN;kN;(b)offinal
final
phase ofof thethe test—visible
test—visible
vertical
distribution
horizontal
the
load
0.4
(b)
phase
verticalof horizontalof
the load of 0.4
kN;
(b)of
final
phase
the test—visible
vertical cracksof thecracksthesample,sample,ofof 3.2thekNload[25].ofcracksofthe
theloadsample,theloadof 3.2
3.2 kNkN [25].
[25].

Using the DIC method, displacements resulting only from sample deformations, excluding
displacements associated with the motion of a rigid body, can also be determined for mineralasphalt mixtures.
Moreover, other important mechanical parameters can be specified, such as Poisson’s ratio
for various load levels as well as the deformations accompanying the destruction of the sample.
These parameters are used in the mechanistic design of road pavement structures.

Materials 2019, 12, 2349

10 of 19

In addition, the DIC method can be applied to examine the effect of temperature (its daily andannual changes) on the mechanical parameters of asphalt mixtures during thermo-mechanical tests inaMaterials
climatic
chamber
2019,12, x FOR[49–51].
PEER REVIEW
10 of 20(a)(b)

Figure 12. Vertical displacement of the HMA sample in the indirect tensile test: (a) The distributionof horizontal displacements, the load of 0.8 kN; (b) advanced phase of the test—the formation of local
cracks (discontinuities), the load of 1.7 kN [25].

Using the DIC method, displacements resulting only from sample deformations, excluding
displacements associated with the motion of a rigid body, can also be determined for mineral-asphalt
mixtures.
Moreover, other important mechanical parameters can be specified, such as Poisson’s ratio forvarious load levels as well as the deformations accompanying the destruction of the sample. These(a)(b)
parameters are used in the mechanistic
design of road pavement structures.
Inaddition,the DIC
method canbeHMAappliedto examine
the effectoftemperature(its daily
Verticaldisplacement
ofthe
the
HMA
sampletheindirect
indirecttensiletest:
The
distribution
Figure12. Verticaldisplacementofsampleininthetensiletest:(a)(a)
The
distributionof andannualchanges)on the mechanicalparameters
ofadvanced
asphalt
duringthermo-mechanicalof horizontal
displacements,
theload
0.8kN;
kN;(b)(b)advancedmixtures
phaseofofthe
thetest—theformation ofof local tests
horizontal
displacements,
the
load
ofof
0.8
phase
test—theformation
in a climaticchamber [49–51].
cracks (discontinuities),
the load of 1.7 kN
kN [25].
[25].
3.4.2.Using
Soil Stabilized
Stabilized
withBinder
3.4.2.
Soil
aa Binder
the DIC withmethod,displacements resulting only from sample deformations, excluding
displacements
associatedwiththe motionof a rigidgrid
body, canalso be determined
for mineral-asphalt
The viewview ofsoil
sampleareareshowninTheof thethe samples
samples ofofsoilsoilandandthethevirtual
virtual gridon
onthe
thesurface
surfaceofofthe
thesoilsampleshown
mixtures.
Figure13.
in Figure 13.
Moreover, other important mechanical parameters can be specified, such as Poisson’s ratio forvarious load levels as well as the deformations accompanying the destruction of the sample. These
parameters are used in the mechanistic design of road pavement structures.
In addition, the DIC method can be applied to examine the effect of temperature (its daily andannual changes) on the mechanical parameters of asphalt mixtures during thermo-mechanical tests
in a climatic chamber [49–51].

3.4.2. Soil Stabilized with a Binder
The view of the samples of soil and the virtual grid on the surface of the soil sample are shownin Figure 13.

(a)

(b)

Figure 13. Soil
Soil samples:
samples: (a)(a)View
View ofof thethe samples
samples preparedprepared forfor thethe compressive
compressive test; (b) virtual grid on
the surface of the sample [40].

measuredsurfacewaswascurved.
Using
the 2D-DIC
method,the results
In thethe compressive
compressivetests,
tests,thethe
measuredsurfacecurved.
Using
the 2D-DIC
method,theshouldonly
beconsideredonthesurfaceperpendiculartothecamera
(in
the
axis
of
symmetryoftheresults should only be considered on the surface perpendicular to the camera (in the axis of symmetry
sample), where the calibration process were performed. To assess the displacements on the entire
curved surface of the sample, it is recommended to use the 3D-DIC system.
The modulus of elasticity under uniaxial compression, modulus of elasticity under indirect tension
and Poisson’s ratio were tested for soil stabilized with a binder. The results of the mechanical properties(a) summarized in Table 2.
(b)of chemically stabilized soil are
Figure 13. Soil samples: (a) View of the samples prepared for the compressive test; (b) virtual grid on
the surface of the sample [40].

In the compressive tests, the measured surface was curved. Using the 2D-DIC method, theresults should only be considered on the surface perpendicular to the camera (in the axis of symmetry

Materials 2019, 12, x FOR PEER REVIEW

11 of 20of the sample), where the calibration process were performed. To assess the displacements on theentire curved surface of the sample, it is recommended to use the 3D-DIC system.
The modulus of elasticity under uniaxial compression, modulus of elasticity under indirect
Materials 2019, 12, 2349
11 of 19
tension and Poisson’s ratio were tested for soil stabilized with a binder. The results of the mechanicalproperties of chemically stabilized soil are summarized in Table 2.
Table 2. Mechanical properties of the tested soil [40].
Table 2. Mechanical properties of the tested soil [40].

Material
Properties
Mean
Material
Properties
MeanValue
Value
Modulusof ofelasticity
Moduluselasticity
670
670
under
uniaxial
compression
Ec E(MPa)
under
uniaxial
compressionc (MPa)Modulusofelasticity
Modulus of elasticity
583
583
under
indirecttensionEitE(MPa)
underindirecttensionit (MPa)
Poisson’s ratio (-)
0.22
Poisson’s ratio (-)
0.22
Compressive strength Rc (MPa)2.64
Compressive strength Rc (MPa)2.64
Indirect tensile strength Rit (MPa)0.27Indirect tensile strength Rit (MPa)0.27

CoefficientofofVariation
Variation
Coefficient
22%
22%
34%
34%
11%
11%
5%
5%
24%
24%

The mean value of Young’s modulus of the tested soil was equal to 670 MPa in a compressive
The mean value of Young’s modulus of the tested soil was equal to 670 MPa in a compressive
test and583 MPa in an indirect tensile test. The value of Poisson’s ratio was equal to 0.22 with thetest and 583 MPa in an indirect tensile test. The value of Poisson’s ratio was equal to 0.22 with thecoefficientof variation
11%.
Accordingtheindirect
indirecttensilestrengthis equal
to about
coefficientof variation
11%.
According toto thethe results,
results, thetensilestrengthis equal
to about
10% ofthevalueofcompressive
strength.
This
resultisconsistentwiththeresultspresented
10% of the value of compressive strength. This result is consistent with the results presented in thein theliterature[40,41].
literature[40,41].
maps
of displacementsand
and principal
principal strains
surfaceobtainedin theThe Themapsof displacements
strainsofofthe
thespecimen
specimensurfaceobtainedin thecompressivein theindirecttensile
tensiletest
test areare shownshown inand
15,15,
respectively.
compressive
test test
andandin theindirect
inFigures
Figures1414
andrespectively.

(a)

(b)

Figure14. Soilsampleunder
uniaxialcompression:
compression: (a)of ofthetheDICDIC
method,(b) verticalFigure14. Soilsampleunder
uniaxial(a)Virtual
Virtualgrid
gridmethod,(b) verticaldisplacementsof PEERthesamplethecompressive
compressive test
test [40].
displacements
thesample
in in
the
[40].
Materials
2019, 12, ofx FOR
REVIEW
12 of 20(a)(b)

Figure15. Mapsof maximal
trueprincipal
principalstrains
strains ofsurfacein theindirecttensiletest: test:
Figure15. Mapsof maximal
trueof thethesoilsoilsample
samplesurfacein theindirecttensile(a) Phase
immediately
beforecracking;(b)(b)sample
sample with
with cracks(a) Phase
immediately
beforecracking;
cracks[41].
[41].

3.4.3. Stone (porphyry)
Stone (porphyry) beams taken from the stone slab pavement (see Figure 16) were examined in athree-point bending test using the DIC technique. The test results in the form of displacement maps
are shown in Figure 17.

(a)
(a)

(b)
(b)

Materials
2019,15.
12,Maps
2349 of
12 of 19
FigureFigure15.
Mapsof maximal
maximal true
true principal
principal strains
strains ofof thethe soil
soil sample
sample surface
surface inin thethe indirect
indirect tensile
tensile test:
test:

(a)
(a) Phase
Phase immediately
immediately before
before cracking;cracking; (b)(b) sample
sample with
with cracks
cracks [41].
[41].

3.4.3.
Stone (porphyry)(porphyry)
3.4.3.
3.4.3. Stone
Stone(porphyry)Stone(porphyry)
beams
taken
from
the
stone
slab
pavement(see
Figure16)
wereexaminedin
Stone
Stone (porphyry)(porphyry) beams
beams taken
taken fromfrom thethe stonestone slab
slab pavement
pavement (see(see Figure
Figure 16)16) were
were examinedexamined inin aaathree-point
bending
test
using
the
DIC
technique.
The
test
results
in
the
form
of
displacement
maps
three-point
three-point bending
bending test
test using
using thethe DICDIC technique.
technique. The
The test
test results
results inin thethe form
form ofof displacement
displacement maps
maps
are
shown
in
Figure17.
areare shownshown inin Figure
Figure 17.
17.

(a)
(a)

(b)
(b)

FigureFigure 16.
16. Stone
Stone (porphyry)(porphyry) sample:
sample: (a)
(a)
Stone
slab
in
the
road
pavement
cracking due
due toto bending;bending; (b)(b)(a)Stone
Stoneslab
slabininthe
theroadroadpavement
pavementcracking
stonestone beam
beam preparedprepared forfor aa three-point
three-point bending
bending test
test using
using DIC
DIC [25].
[25].

(a)
(a)

(b)
(b)

Figuredisplacementsofrocksamplethree-point
bending
test:
Initialphase
Figure 17.
17. Horizontal
Horizontal displacements
displacementsof
ofaaarockrocksample
sampleinin
ina aathree-point
three-point
bending
test:(a)Initialphase
bending
test:(a)(a)
Initial
phase
of
of
the
test;
(b)
total
displacement;
final
phase
of
the
test
a
clearlyvisiblevertical
crack
in
the
sample.
of
the
test;
(b)
total
displacement;
final
phase
of
the
test
a
clearlyvisiblevertical
crack
in
the
sample.
the test; (b) total displacement; final phase of the test - a clearly visible vertical crack in the sample.

The mechanical parameters obtained in the tests are related to the durability of a road pavement
made of stone slabs. In many cases, the insufficient tensile strength in bending of stone slabs causes apremature failure of a road pavement (see Figure 16). Therefore, the obtained parameters can be usedin both theoretical and numerical analyses as well as during the design of pavement structures [40,41].
3.4.4. Biaxial Glass Fibre Geogrid and Polyester Geogrid
The view of the analyzed samples of biaxial orthogonal geogrids and the grid of elements on thepolyester geogrid surface are shown in Figure 18.
In the tests of the samples of geogrids, the values and distributions of linear strains were obtained.
The tests were carried out according to the standard [42]. Tables 3 and 4 present a comparison of thevalues of vertical linear strains determined using two measurement techniques for various load levels.
The obtained results show no significant differences, indicating the effectiveness of the DIC system inthis study.

used in both theoretical and numerical analyses as well as during the design of pavement structures[40,41].
3.4.4. Biaxial Glass Fibre Geogrid and Polyester Geogrid
The view of the analyzed samples of biaxial orthogonal geogrids and the grid of elements on the
Materials
2019, 12,geogrid
2349
polyestersurface are shown in Figure 18.

(a)

13 of 19(c)(b)

Figure18. Geogrids
samples
prepared(a) Glass
Glassfibre
fibregeogrid;
geogrid;polyestergeogrid;
Figure18. Geogrids
samples
preparedfor
fortesting:testing: (a)(b)(b)
polyestergeogrid;(c) (c)virtualgrid gridfor thegeogrid(geosynthetics)
virtualfor polyester
the polyester
geogrid(geosynthetics) [52].
[52].
Table 3. Linear vertical strains (cross machine direction)—glass fibre geogrid; average values of strains
In the tests of the samples of geogrids, the values and distributions of linear strains werefrom 5 samples [52].
obtained. The tests were carried out according to the standard [42]. Tables 3 and 4 present a
comparisonofTensometricthe values ofTechnique
vertical linear DIC
strains
determined
twoBase)measurement
techniques
Load(Averaging
forusing
a 60 mm
Ratio
of Resultsfor
various load levels. The obtained results show no significant differences, indicating the effectivenesskN system in 200
1.05
10−4
209 × 10−4
of18.49
the DIC
this×study.
−4
22.10 kN
1.09
250 × 10273 × 10−4
−4
−4
16.89
kN
1.08 of
300
×
10
324
×
10
Table 3. Linear vertical strains (cross machine direction)—glass fibre geogrid; average values
strains from 5 samples [52].

Table 4. Linear vertical strains (machine direction)—polyester geogrid; average values of strains fromDIC (Averaging
Materials 2019, 12, x FOR PEER REVIEWTensometric
14 of 20
Ratio of Results
5 samples [52]. Load
Techniquefor a 60 mm Base)−4 in the form
−4
The selected
resultslaboratory
tests
maps of strains are presented in
18.49
kN of Technique
200 × 10
209of× colored10
Load
Tensometric
DIC (Averaging
for a 60 mm Base) 1.05 Ratio of Results
Figure 19.
22.10 kN
250 × 10−4
273 × 10−4
1.09
−4
−4
2.64 kN
1.12
300
×
10
336
×
10
−4
−4
16.89 kN
300 × 10324 × 10 −4
1.08
−4
6.27
kN
1.09 of the
600
×
10
654
×
10
The DIC method enables to determine the field of deformations for the entire surface
−4
−4
10.83
kN
1.07 This
900
×
10
965
×
10
geogrid sample along with the solution of the issue of principal strains and principal directions.
Table 4. Linear vertical strains (machine direction)—polyester geogrid; average values of strains fromis a great
advantage
5 samples[52]. of this method because it is unattainable using traditional measuring methods—
extensometers(strain ofgauges).
In addition,
DIC method
allowsforofthevisualizationof
The selected results
laboratory
tests in thetheDICform
of coloredmaps
strains
are presentedin(Averaging
deformations, which
are specificTensometricfor geogrids as heterogenic
and anisotropic
structures.
Load
Ratio
of
Results
Figure 19.
Techniquefor a 60 mm Base)
2.64 kN
300 × 10−4
336 × 10−4
1.12
6.27 kN
600 × 10−4
654 × 10−4
1.09
10.83 kN
900 × 10−4
965 × 10−4
1.07

(a)

(b)

Figure19. 19.
Verticalstrains
ofofbiaxial(a)Maps
Mapsofofthe
thedistributionof verticalstrains
FigureVerticalstrainsbiaxialgeogrids:
geogrids: (a)distributionof verticalstrains
on theon thesurface
of
the
glass
fibre
geogridsample;
(b)
maps
of
the
distributionofverticalstrainsonthe surface
surface of the glass fibre geogrid sample; (b) maps of the distribution of vertical strains on the surfaceof thepolyestergeogridsample[52].
of the polyester geogrid sample [52].

3.4.5. Triaxial Polypropylene Geogrid
The view of the triaxial hexagonal polypropylene geogrid sample and the grid of elements on
its surface are shown in Figure 20.

Materials 2019, 12, 2349

14 of 19

The DIC method enables to determine the field of deformations for the entire surface of the(a)(b)
geogrid sample along with the solution of the issue of principal strains and principal directions.
This
is a great
advantageof this(a)method
it is ofunattainable
Figure19. Verticalstrainsof biaxial geogrids:
Maps of because
the distribution
vertical strainsusing
on the traditional measuringmethods—extensometers(straingauges).
In addition,
the DIC
methodsurfaceof the glass fibre geogrid sample;(b) mapsof the distributionof verticalstrains
on theallows
surface for the visualization ofof thedeformations,polyester geogridwhichsampleare[52].specific for geogrids as heterogenic and anisotropic structures.
3.4.5. Triaxial
Polypropylene
Geogrid
3.4.5.
Triaxial Polypropylene
Geogrid
The view Theof theview
triaxial
hexagonal
geogrid sample
and thesample
grid of andelements
on of elements on itsof thetriaxialpolypropylene
hexagonal polypropylene
geogridthe grid
its surface are shown in Figure 20.

surface are shown in Figure 20.

Materials 2019, 12, x FOR PEER REVIEW

15 of 20

Materials 2019, 12, x FOR PEER REVIEW

(c)

(c)
(a)
(b)
Figure 20. Triaxial hexagonal polypropylene geogrid: (a) Geogrid sample prepared for testing (after
Figure 20.
Triaxial
geogrid:
Geogridsamplepreparedforgeogrid:
testing(afterpaintingwithwhite hexagonal
paint and polypropylene
spotting with black
colour);(b) virtual
gridof a hexagonal
geogrid
Figure20.(a)
Triaxial
hexagonal
polypropylene(a)Geogrid sample prepared for t
painting
with
white
paint
and
spottingwithblack
colour);(b)
virtualgridofahexagonal
geogridnode;(b) virtual grid of a hexago
painting with white paint and spotting with black colour);node; (c) geogrid node before painting [38].
(c) geogrid node before painting [38].
node; (c) geogrid node before painting [38].

Laboratory tests allowed to determine load-strain curves as well as selected mechanicalLaboratory tests allowed to determine load-strain
curvesas wellas selectedmechanicalparameters
Laboratory
tests
allowedto determineload-strain
curves as well as selec
parameters of the tested samples. During the tensile test, when the material was loaded, it wasparametersofthetestedsamples.
Duringthetensiletest,of the tested samples. During the tensile test, when the material was loaded, it was observed thatwhen
all the material wasobserved that all the samples were destroyed by breaking the nodes. The central part of the sample
thatTheall thesamplesweredestroyed
breaking
the nodes.
The central pa
the samples were destroyed by breaking observed
the nodes.
centralpartof thesamplebyafter
the test
isafter the test is shown in Figure 21.
after
the
test
is
shown
in
Figure21.
shown in Figure 21.

Figure 21. The central part of the polypropylene geogrid sample after the tensile test w
destroyed geogrid node [38].

Figure21.
21.TheThecentral
centralpartpartofofthe
thepolypropylene
polypropylenegeogrid
geogridsample
sampleafter
afterthethetensile
tensiletest
testwith
witha avisible
visibleFiguredestroyed
geogrid
node[38].
destroyed geogrid node [38].

Load-strain curves obtained in the tension test for geogrid samples are shown in Figure 22. Secant
stiffness of a polypropylene geogrid was also determined (according to the standard [42]). A graphMaterials 2019, 12, 2349

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Figure 21. The central part of the polypropylene geogrid sample after the tensile test with a visible
destroyed geogrid node [38].

showing secant stiffness lines for the assumed levels of strains 0.5%, 2%, 5%, 10% is presented in
Figure 22. The results of the tensile strength and secant stiffness are summarized in Table 5.

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Figure 22. Polypropylene geogrid samples: (a) Load-strain curves; (b) secant stiffness lines [38].

Load-strain curves obtained in the tension test for geogrid samples are shown in Figure 22.
Secant stiffness of a polypropylene geogrid was also determined (according to the standard [42]). A(a)graph showing secant stiffnesslines for the assumed levels of strains 0.5%,(b)2%, 5%, 10% is presented
in FigureFigure
22. Theof the tensile
strengthsecant stiffnessare(b)summarized
in linesTable
5.
22.results
Polypropylene
geogridsamples:and(a) Load-strain
curves;secant stiffness[38].
Table5.5.Mechanical
Mechanicalpropertiesofofthe
thetested
testedpolypropylene
polypropylenegeogrid
geogrid[38].
[38].
Table
properties

Material
Properties
Mean
Material
Properties
MeanValue
Value
Tensilestrength(kN/m)
16
Tensile strength (kN/m)
16
Tensilestrainat maximum
tensile13%
Tensilestrainat maximum
tensileloadload(-)(-)
13%
Secantstiffness
0.5%
strain(kN/m)(kN/m)
167
Secantstiffness
at at
0.5%
strain167
Secantstiffnessat2%
strain(kN/m)
142
Secant stiffness at 2% strain (kN/m)
142
Secant stiffness at 5% strain (kN/m)
150
Secant stiffness at 5% strain (kN/m)
150
Secant stiffness at 10% strain (kN/m)
134
Secant stiffness at 10% strain (kN/m)
134

Standard
StandardDeviation
Deviation
1.2
1.2
1.5%
1.5%
64.7
64.7
48.5
48.5
28.4
28.4
14.1
14.1

Coefficient
Coefficient of Variation
Variation
8%
8%
12%
12%
39%
39%
34%
34%
19%
19%
11%
11%

Dueto
to thethe observedof thespecialattention
was paid
to theDueobserveddamage
damagecharactercharacterof geogrids,
the geogrids,
specialattention
was
paidbehaviour
to the
of
the
nodes
in
the
tensiletest.
Thedisplacement
measurements
andtheanalysisofthedeformation
behaviour of the nodes in the tensile test. The displacement measurements and the analysisof thedistributiondistribution
were carriedoutcarriedusing themethod.
map showing
the distributionof principal
deformation
were
out DICusing
the DICA
method.
A map showing
the distributionofstrainsonthesurfaceofapolypropylene
sampleispresented
inFigure23.
principal strains on the surface of a polypropylene sample is presented in Figure 23.

(a)

(b)

Figuredistributionofofthe
theprincipal
principal
strains
1 theon surface
the surface
the polypropylene
Figure23.
23.MapMapofof the distribution
strains
1 onof theofpolypropylene
sample:sample:(a) Geogridview;(b) close-upof the[38].
node [38].
(a) Geogrid
view; (b)close-upof the node

The obtained strain maps point out that the largest of the principal strains appear in zones on
the edges of the nodes of the tested polypropylene geogrids. Therefore, maximum equivalent strains
and stresses can occur in these areas, which explains the observed nature of the grid damage in theform of cracks on the edges of the nodes. Cracks are perpendicular to the direction of the maximum
principal strains. The destruction of the sample occurred with the maximum principal strain 20%
greater than the tensile/vertical strain measured in the base of the extensometer [38].

Materials 2019, 12, 2349

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The obtained strain maps point out that the largest of the principal strains appear in zones on theedges of the nodes of the tested polypropylene geogrids. Therefore, maximum equivalent strains andstresses can occur in these areas, which explains the observed nature of the grid damage in the form of
cracks on the edges of the nodes. Cracks are perpendicular to the direction of the maximum principal
strains. The destruction of the sample occurred with the maximum principal strain 20% greater than
the tensile/vertical strain measured in the base of the extensometer [38].
4. Discussion
In recent years, researchers have been working on extending the range of application of the DIC(Digital Image Correlation) method. Most of the efforts are focused on the development of multi-camera
DIC systems, in which data obtained from individual systems are stitched together in a common
coordinate system [7].
With a history of more than 30 years, DIC has been developed into an effective and flexible
optical technique to measure surface deformations at macro-, micro-, and even nanoscale. Numerous
applications in broad fields have already proven that it is a practical and indispensable tool formeasuring deformations [1].
Optoelectronic, non-contact systems are increasingly being used to monitor materials andstructures in civil engineering. This also applies to materials used in road construction. The study ofmechanical parameters of road materials is an important issue from the point of view of the conditionand durability of road pavements. The implementation of the DIC method in the laboratory tests ofroad materials confirms the possibilities of wide applications of this method.
5. Conclusions
The results of the conducted research allow the formulation of the following conclusions:
•
•

•
•

•
•

The Digital Image Correlation (DIC) method implemented on a properly prepared laboratory setup
enables the effective measurements of deformation of road materials under various load conditions.
The great advantage of the DIC technique is the ability to determine displacements anddeformations in any direction and at any point (for the entire surface of the sample) along withthe solution of the issue of principal strains values and principal directions. This is unattainable
using traditional measurement methods (strain gauges).
The DIC method can visualize deformations of road materials, which are specific/complicated inshape (e.g., heterogenic and anisotropic geogrids).
The numerical algorithm used in the DIC system eliminates the displacements associated with the
motion (movement) of a rigid body (sample) during the test. This reduces the problem of slippingout a geosynthetic sample from the jaws of a testing machine.
The DIC method gives the possibility to validate the results obtained on the basis of numericalmodels [24].
The constantly improved DIC method opens new research fields for non-contact determination of
deformations of modern, more complex road materials under various load conditions.

Author Contributions: Conceptualization, J.G. and K.M.; methodology, J.G. and K.M.; formal analysis, J.G. andK.M.; investigation, J.G. and K.M.; resources, J.G. and K.M.; data curation, J.G. and K.M.; writing—original draft
preparation, J.G., K.M. and T.Z.; writing—review and editing, J.G., K.M. and T.Z.; visualization, J.G., K.M. andT.Z.; supervision, J.G. and K.M.; project administration, J.G. and K.M.; funding acquisition, J.G., and K.M.
Funding: This research received no external funding.
Conflicts of Interest: The authors declare no conflict of interest.

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