Materials and Structures
DOI 10.1617/s11527-015-0675-6
ORIGINAL ARTICLE
Laboratory and field evaluation of recycled unbound layers
with cement for use in asphalt pavement rehabilitation
Iuri S. Bessa . Ana L. Aranha . Kamilla L. Vasconcelos .
Amanda H. M. Silva . Liedi L. B. Bernucci
Received: 17 April 2015 / Accepted: 7 July 2015
Ó RILEM 2015
Abstract The use of recycling techniques in the
rehabilitation of old pavements has become an
important tool for pavement engineering practices.
The reduced costs and the scarce natural resources are
the main reason for which the development and
improvement of recycling processes have been studied
by many researchers around the world. The main
objective of this paper is to investigate the structural
responses of an asphalt pavement after using deep
recycling techniques of granular layers with an
addition of cement for the rehabilitation of asphalt
I. S. Bessa (&) K. L. Vasconcelos L. L. B. Bernucci
Department of Transportation Engineering, Universidade
de São Paulo, Av. Professor Almeida Prado, Travessa 2 –
No. 83, Cidade Universitária, São Paulo, SP 05508-070,
Brazil
e-mail: iuribessa@usp.br
K. L. Vasconcelos
e-mail: kamilla.vasconcelos@gmail.com
L. L. B. Bernucci
e-mail: liedi@usp.br
A. L. Aranha
Regulatory Transport Agency of São Paulo State –
ARTESP, Rua Iguatemi, 105, Itaim Bibi, São Paulo,
SP 01451-011, Brazil
e-mail: analuisa.aranha@gmail.com
A. H. M. Silva
Concessionary Arteris S.A., Av. Pres. Juscelino
Kubitschek, 1455, 98 andar, Itaim Bibi, São Paulo,
SP 04543-011, Brazil
e-mail: amanda.marcandali@arteris.com.br
pavements. Laboratory analysis using resilient modulus, indirect tensile strength (ITS), and unconfined
compressive strength (UCS) tests were performed for
mixes with 5 and 6 % of cement content, two
compaction energies and three curing periods. In the
field, two sections using 250 and 300 mm thicknesses
of a recycled base layer were constructed with 5.0 %
of cement addition. An evaluation of these experimental test tracks was conducted by performing
deflections and by mechanical characterization of
samples extracted from the test site at different ages.
The results indicated differences in UCS and ITS
values for the laboratory mixes. Field specimens from
the 300 mm-section presented higher stiffness than the
250 mm. Studies have shown that the resilient moduli
determined from nondestructive test based on backcalculation procedures are similar from those determined in the laboratory through modulus of elasticity
tests.
Keywords Design of recycled granular base layer Portland cement Mechanical behavior Field
evaluation Deflection measurements
1 Introduction
In general, the maintenance of highways is done by
processes of restoration that normally include the
application of new asphalt layers. This practice
increases the consumption of natural resources,
Materials and Structures
leading to major environmental impacts, and is not
always a good technical solution, because of reflective
cracking problems [4, 15, 26]. The use of new
aggregates for asphalt pavement rehabilitation starts
a new cycle of exploration of ore deposits, material
production, and transportation. It causes not only
environmental impacts, but also financial implications
in the overall construction costs. Besides being environmentally friendly, pavement recycling has become
an interesting option due to its cost, which can be up to
50 % less than conventional rehabilitation techniques
[30].
In order to reduce the impact caused by the
exploration and transportation of scarce natural
resources, several government agencies have been
promoting the use of recycled materials in highway
construction [31]. With the increasing use of recycling
techniques, a lot of research has been done in order to
propose the application of recycling materials in the
different asphalt pavement layers.
The most common process of pavement recycling is
the reuse of the milled asphalt layers as aggregate for
the production of new mixes. However, pavements
with more severe structural problems might need a
reinforced layer or a deeper intervention, up to the
granular base and subbase layers. Recycling base
layers can promote an enhancement to the structural
conditions of deteriorated asphalt pavements. The
addition of Portland cement promotes an enhancement
in the pavement structure, once it becomes semi-rigid
instead of flexible.
In situ recycling of granular base materials has
become an important rehabilitation strategy in asphalt
pavements. The benefits are reduced construction
time, which leads to less traffic disruption, an increase
in cost effectiveness, and sometimes an improvement
in construction quality compared with conventional
rehabilitation techniques.
Full-depth reclamation of asphalt pavements is a
cost-effective practice that reduces the use of virgin
base materials. The process includes the milling of the
existing unbound granular base, normally with the
addition of cement or lime, and the proper compaction
of the new layer [25]. According to ARRA [5], the
main distresses that can be treated with the use of this
technique are longitudinal, transversal, fatigue and
reflective cracking, permanent deformation, loss of
bond properties between the asphalt layers, and
insufficient structural capacity, among others.
The objective of this study was to investigate the
design of in-place recycled base layers with the
addition of Portland cement through laboratory analysis and field evaluation. The expected outcomes were
to compare the results obtained for field samples to the
results of laboratory specimens and try to select the
best test methods that should be done to provide good
information for the design road asphalt pavements.
Figure 1 presents pavement structures before and after
the rehabilitation strategy. The main focus of this
paper is on the recycled base layer with cement.
2 Recycling of unbound pavement layer
2.1 Recycling of base layer
Cement treated granular materials have been applied
in road construction for many decades [35]. Yuan et al.
[39] incorporated different percentages of reclaimed
asphalt pavement (RAP) to recycled base materials
with the addition of several cement contents. UCS
results were found to be directly proportional to the
cement content added to the mixes. The more cement
content was added, the higher the UCS values were.
The optimum cement content increased as the RAP
content also increased. Camargo et al. [13] compared
four different mixes to be used as base layers of asphalt
pavements. The mixes were: (i) base material with
crushed conventional aggregate, (ii) unstabilized fulldepth reclaimed pavement material (RPM), (iii)
laboratory-mixed RPM with fly ash, and (iv) fieldmixed RPM mixed with fly ash. The mixes were tested
with respect to their mechanical behavior in the
laboratory and to their field performances. In relation
to the resilient modulus, the crushed aggregate and the
unstabilized RPM mixes resulted in similar values, but
once the RPM was stabilized with fly ash in the
laboratory, the resilient modulus (Mr) value after
7 days of curing increased in more than 1000 %.
2.2 Soil-aggregate gradation
According to Yoder and Witczak [38], soil-aggregate
mixes with little or no fines (passing through
0.075 mm- or No. 200 sieve) have better stability
due to the grain-to-grain contact from the coarse
particles, but they are very difficult to handle during
construction; however, the use of heavy equipment
Materials and Structures
Fig. 1 Pavement structure, a before and b after the rehabilitation strategy
might solve this issue on field. On the other hand, soilaggregate mixes with great amount of fines have no
grain-to-grain contact and are easy to handle and can
be compacted quite readily. Peng and He [28] studied
two gradations of granular material by adding cement
to it. The coarser gradation resulted in lower compressive strength values for cement contents that were
4.5 % or higher. For a cement content value of 4.0 %,
both gradations presented similar resistance results.
Xiao et al. [37] investigated different types of
aggregate to be employed in base layers by using
several parameters related to aggregate gradations,
including the Bailey method and the coefficients of
curvature and uniformity. The authors concluded that
the gravel-to-sand ratio exhibited the best correlation
with the mechanical behavior of soil-aggregate base
course materials, including recycled materials. The
highest shear strength was reached for an optimal
intermediate gravel-to-sand ratio of 1.5, regardless of
the aggregate type tested. Baus and Li [11] investigated different gradations of aggregates used in base
layers and concluded that poorly-graded materials
(i.e., with curvature coefficient less than 1.0) tend to
have lower rutting resistance in comparison to wellgraded materials.
The RAP material has been evaluated as aggregate
(coarse and fine) for base layers in different studies [24,
17, 20]. Yuan et al. [39] investigated different gradations of RAP materials to be incorporated to recycled
base layers and concluded that by increasing the fines
content (particles passing No. 40 sieve) the UCS values
also increased. The RAP materials and cement were
added to two granular materials with different grain
size distribution. When mixing 50 % of RAP with
50 % of each one of those materials, the results were
very similar for both mixes. It is important to mention
that even though the specified gradation and density
may be the same, bases constructed with aggregates
from different sources may exhibit different performances. This might be explained by differences in the
aggregate particles resistance and shape properties.
2.3 Cement in recycled base course
Cement stabilization is not always used in conjunction
with base recycling, but should be considered when
the strength of the existing material is poor. Guthrie
et al. [18] produced mixtures of recycled aggregate
base materials with the addition of different percentages of RAP and cement. The mixtures’ UCS values
were measured, and the results indicated that increasing the amount of cement also increased the values of
UCS for any RAP percentage. McConnell [21], Abdo
[2], and Peng and He [28] tested cement-stabilized
aggregate base layer samples. Samples with higher
amount of cement resulted in higher UCS values.
Also, for the first two studies, results indicated a linear
correlation between the UCS values and the cement
content used in the mixtures.
Haichert et al. [19] evaluated mixes of granular
base materials and their results indicated that the
addition of 1 or 2 % of cement to those mixtures would
produce mixtures with similar or better performance
than recycled Portland cement concrete materials. On
the other hand, adding more cement could produce
stiffness that would be so high that it could result in
brittle failures. Soares et al. [32] used a modeling
technique to study the addition of cement to base and
subgrade layers of asphalt pavements. They concluded
that adding only 2 % of cement in the base layer could
reduce the pavement deflections and the horizontal
strain at the bottom of the asphalt mixture layer.
Materials and Structures
Chai et al. [14] and Miller et al. [23] performed field
analysis using the falling weight deflectometer (FWD)
in an asphalt pavement sections with and without
cement stabilization, while and Trichês and Santos
[36] used the Benkelman beam test. The results
indicated that the deflections found on the pavement
segments with cement addition decreased and their
rigidity increased after only a few days of curing.
Gaspard [16] tested samples from field pavement
sections with variation on the cement content. Mixtures with 5 % of cement resulted in UCS values that
reached their criterion value, based on the minimum
required compression strength outlined by the Louisiana Department of Transportation and Development
(LaDOTD). Syed and Scullion [34] reported the
efficiency of recycling techniques used by the Texas
Department of Transportation (TxDOT). Normal
practices from this department include the breaking
up of the existing pavement to a depth of 250 mm and
then mixing it with a stabilizer material and adequate
moisture.
Camargo et al. [13] studied recycled mixtures with
fly ash for base layers in the laboratory and in the field.
The main conclusion was that the CBR and UCS
values for the field samples were lower than the
laboratory specimens, which probably indicated that
the test methods used to characterize this type of
material were not the most suitable ones.
3 Laboratory study
The first approach considered in this paper was a
laboratory study regarding the behavior of recycled
granular layer with variation on: (i) the compaction
energy, and (ii) the cement content. The curing period
was also evaluated in this first approach.
Samples were constituted by soil and aggregate
materials with the addition of water and Portland
cement. The soil-aggregate blend was milled from the
original base layer of a high-traffic highway in Brazil
that, according to the rehabilitation project, needed to
be replaced. Basic characterization tests, such as sieve
analysis and equilibrium field moisture content, were
evaluated. It was known that the base material could
vary along the highway, once there was not a rigid
control of what was used in the previous constructions.
The authors assume that the material collected and
used for the laboratory tests are representative of the
highway base material.
Samples were prepared according to the modified
Proctor compaction test to determine the optimal
moisture content of the granular mixes [8]. All
mixtures were characterized with respect to their
mechanical behavior. Tests for Mr, UCS, and indirect
tensile strength (ITS) were performed with variation
on the cement content and laboratory compaction
energy. The results will be shown and detailed in the
following sections.
3.1 Variation of cement content
Mixtures with two different cement contents were
compacted by using the modified Proctor energy
(eight layers and 26 blows per layer). Figure 2
presents the results obtained for the different mixtures
analyzed in terms of ITS tests [7]. Each data point is an
average value calculated from results obtained for
three different specimens with same characteristics.
The error bars regarding the standard deviation are
also plotted.
The results show that there was no major difference
among the mixtures in the first 3 days of curing. After
that, it is possible to observe that the materials with
bigger amount of cement presented higher indirect
tensile resistance. Samples with 6 % of cement had an
overall value of ITS 30 % higher than the samples
with 5 % of cement. Normally, a lower water/cement
ratio provides better resistance to the materials [22]
and this could be noticed for the mixtures tested.
Water/cement ratios of 1.30 and 1.08 were used for
both tested mixtures (5 and 6 % of cement content,
respectively). UCS tests were performed according to
1.4
5% of cement
6% of cement
1.2
ITS (MPa)
2.4 Field applications of recycled base layer
1.0
0.8
0.6
0.4
0.2
0.0
3
7
Curing period (days)
Fig. 2 ITS results for variation of cement content
28
Materials and Structures
12.0
5% of cement
6% of cement
UCS (MPa)
10.0
8.0
6.0
4.0
2.0
0.0
7
28
Curing period (days)
Fig. 3 UCS results for variation of cement content
the AASHTO standard procedure (ASTM D2166)
after 7 and 28 days of curing, as presented in Fig. 3.
For all the mixes evaluated, as curing time
increased, UCS values also increased, approximately
30 % from 7 to 28 days. It is interesting to notice that
the ratio between UCS and ITS is approximately nine
times for all curing time and cement content. Resilient
modulus was also determined by using the triaxial test
method [1]. The results for each cement content and
28-day curing time are presented in Fig. 4.
For the two cement contents tested, Mr values after
28 days of curing for 0,1 MPa of confining stress were
approximately 8000 and 9000 MPa for 5 and 6 % of
cement content, respectively. These values are comparable to mixtures of soils with 10 % of cement, cited
in the literature [10]. In respect to the influence of the
confining pressure, it can be noticed that the increase
in this parameter causes an increase in Mr, showing a
non-linear elasticity of this material.
3.2 Variation of compaction energy
The Brazilian standards for compaction of soil-aggregates-cement and soil–cement mixtures normally
10,000
Mr (MPa)
8,000
6,000
4,000
5% of cement (28 days)
6% of cement (28 days)
2,000
0
0
0.1
0.2
Confining pressure (MPa)
Fig. 4 Resilient modulus results for variation of cement
content
specify the use of the intermediate compaction energy
(five layers and 19 blows per layer), although this
energy is not considered at international specifications. European and North American procedures
normally indicate the use of the modified Proctor
compaction. The use of higher compaction energy
leads to lower optimal moisture content. In this study,
intermediate and modified energy was investigated for
the 5 %-cement mixture, with optimum moisture
content of 7.5 and 6.5 %, respectively.
The compressive strength was evaluated. No
specific test methods used to mechanically characterize recycled soil-aggregate mixtures with cement
addition were found in the literature. Since the
material studied in this research (mix of soil, aggregate
and cement) generally presents behavior that is neither
a concrete material, nor a granular material, the
authors decided to analyze the influence of different
test methods in the mechanical behavior of this type of
material, some concrete and granular materials (or
asphalt concrete) specifications were evaluated. The
first method was the ASTM C39 [6], which is used for
concrete specimens. In this method, the loading ratio is
0.25 MPa per s. The second method used for granular
materials was the ASTM D2166 [9], which defines a
displacement rate of 1 mm per min. Again 18 samples
were produced with the same dimensions and composition and also using the same compaction effort. The
curing period and the test method were the only
variables.
The results are presented in Fig. 5. It can be seen
that the compaction energy had a major influence on
the results, once the modified compaction produced
mixes with higher values of UCS. Once again, each
data point is an average value calculated from results
obtained for three different specimens, while the error
bars mean the standard deviation.
The effect of compaction energy was also investigated in previous studies for other stabilized materials.
Bejarano and Harvey [12] studied recycled base
materials with the addition of pulverized asphalt
concrete, untreated and treated with 3 % of lime.
When the compaction energy was increased, the
results were generally 50 % higher. Suleiman et al.
[33] analyzed the effect of the compaction energy on
the mechanical behavior of previous concrete samples.
Lower compaction energy produced samples with
higher void ratios and consequently lower UCS values
after 7 days of curing (at least 30 % lower).
Materials and Structures
(a) 10.0
UCS (MPa)
8.0
Displacement rate controlled method (ASTM D2166)
Loading rate controlled method (ASTM C39)
6.0
4.0
2.0
0.0
7
28
Curing period (days)
(b)10.0
Modified energy
UCS (MPa)
8.0
Intermediate energy
6.0
4.0
2.0
0.0
7
28
base material adding cement, and the cold recycling
with emulsion addition done on site to constitute the
new binder layer. Two sections were constructed,
having variation only at the recycled base layer
thickness (250 and 300 mm). After milling and
material removing of the surface asphalt layer, the
existing base layer constituted by a soil-aggregate
material was exposed and 5 % of cement by mass were
added spreading on the top of the base layer. The
addition of water was done by the recycling equipment, which was also responsible for keeping the
process of mixing and homogenization of the existing
material—soil and aggregates—combined with the
addition of cement and water. After recycled base
compaction by vibratory roller, the tack coat was
applied on the finished base layer and the binder was
laid after 3 days.
Curing period (days)
Fig. 5 Variation of: a test method and b compaction energy, on
UCS results
In relation to the different test methods, the method
with a constant displacement rate (for granular materials) resulted in lower values if compared to the
method used for concrete samples. This can be related
to the time of loading in which both tests were
performed. The displacement rate controlled test was
performed in 150 s, while the loading rate controlled
test was finished after 20 s. The student’s t test was
done and the results indicated that the average values
were statistically different when comparing both UCS
methods. Paul and Gnanedran [27] performed stiffness
modulus tests in stabilized granular materials. Their
results indicated that tests with higher values of
loading rate led to higher values of stiffness modulus.
4 Field study
The construction of the experimental test site was done
at highway BR-381, located between São Paulo and
Minas Gerais states, in Brazil. A semi-rigid pavement
was constructed with a surface layer of hot mix asphalt
(HMA), binder course of cold recycled mix with
asphalt emulsion, and the base course of granular
recycled material with addition of cement, as illustrated in Fig. 1. This was part of a rehabilitation
project of the highway, with the base recycling
processes done in-place by reusing the preexisting
4.1 Pavement structural condition
The experimental test site was monitored through nondestructive and destructive tests. The non-destructive
tests were performed by using the FWD, which applies
a circular loading pulse to the pavement surface. By
the application of load, deflection sensors on pavement
surface capture the displacement at different points.
The output is the deflection bowls with results from
different points of the pavement section evaluated. For
the experimental test site, the FWD test was performed
four times: (i) the first one was done before pavement
rehabilitation, (ii) the second one right after the
construction using recycling (in 2011); (iii) the third
one, 1 year after the construction (in 2012); and (iv)
the last one, one and a half years after the construction
(in 2013). Also, a preliminary FWD test was done in
the original pavement section, before the recycling
procedures (in 2010). The deflection bowls from the
two analyzed pavement sections in this study are
presented in Fig. 6.
The same tendency was observed for the deflection
bowls of the two sections. The evaluation before the
rehabilitation confirms the pavement distress with a
high deflection due to fatigue cracks of asphalt layers.
Analyzing the maximum deflections and deflection
bowl parameters it can be noticed that the asphalt layer
of the first track was more deteriorated than the second
one, but after the rehabilitation process the deflections
values became almost the same for both sections. The
evaluation conducted right after construction (2011)
Materials and Structures
Distance from point of load application (cm)
Deflections (10-2 mm)
(a)
0
20
40
60
80
100
120
0
10
20
after rehabilitation
30
40
before rehabilitation
50
4.2 Characterization of field samples
60
70
80
2010
2011
2012
2013
Distance from point of load application (cm)
(b) 0
Deflections (10-2 mm)
layer and compared the results to the original pavement before the rehabilitation process. They observed
that there was a decrease of at least 42 % in the
deflections after the construction of the new recycled
base layer.
0
20
40
60
80
100
120
10
20
after rehabilitation
30
before rehabilitation
40
50
60
70
80
2010
2011
2012
2013
Fig. 6 Deflection bowls: a section with 250 mm thickness of
recycled base, and b section with 300 mm thickness of recycled
base
presented the highest deflection at the point of load
application for all segments, while the third evaluation
(2012) presented the lowest deflection (attributed to
the curing process of the recycled layers). The first
evaluation (2011) was done every 40 m of each
section, which resulted in six results for each distance
from the load application. For the other two FWD
evaluations (2012 and 2013), the results were collected every 10 m of the road section, which resulted
in at least 20 results for each point. The variability of
the tests resulted in CV values of less than 25 % for the
300 mm-section and approximately 10 % for the
250 mm-section. Student’s t tests indicate that for
the 300 mm-section the deflection values from 2012 to
2013 might be considered as statistically equals (in a
5 % significance level), which can mean that the
structural condition of the pavement did not change
during this 1 year period; the same could not be
observed for the 250 mm-section. Also, the comparison between the deflection bowls from both sections
indicate that the thickness of the recycled base courser
layer did not influence the structural condition of the
pavement sections, once the results were very similar.
Bejarano and Harvey [12] evaluated the deflection
bowls of asphalt pavements that had a recycled base
The field study in the experimental test site was also
done through destructive tests by extracting specimens
of the base layer. These specimens were characterized
in the laboratory in terms of rigidity and resistance.
The extracted samples of the recycled base layer
were sawed into 200 mm-height specimens to correspond the sample size requirements of triaxial test.
The bottom 200 mm from each sample was selected.
Tests for triaxial resilient modulus and elastic modulus
were performed and compared, once the material
presents characteristics between a cement concrete
and a granular material. An ultrasonic wave propagation test was also carried out to verify its correlation
with the elasticity tests. Resistance tests, such as ITS
and UCS, were also performed.
The triaxial resilient modulus test is normally used
for soil and granular materials [1]. The test basically
consists of applying different sets of axial loads to the
specimens while they are submitted to different
confining pressures. The presence of cement as a
constituent of the recycled granular base material
normally reduces, or eliminates, the influence of
confining pressure, different from what is expected
in soil and granular material.
The modulus of elasticity test is usually run for
concrete samples. It consists of the application of slow
linear increments of stress while measuring the
resultant strains. This test brings the specimens up to
rupture, with no confining pressure used according to
the Brazilian standard method NBR 8522 [3], through
a hydraulic testing machine.
The results of Mr (for confining pressure of
0.1 MPa) and modulus of elasticity are presented in
Table 1. The average values (avg.) calculated from
several specimens, the standard deviation (SD) and the
coefficient of variation (CV) are shown. Figure 7
presents the triaxial results with the inclusion of the
effect of the confining pressure on each sample tested.
The comparison between the results indicates that the
recycled base layer samples, for both 250 and
Materials and Structures
Layer thickness (mm)
Test method
250
300
300 mm-thick layers, presented similar Mr and modulus elasticity values.
Both sections (250 and 300 mm) were influenced
by the confining pressure in their Mr results. This
might indicate that the mixtures do not have a clear
behavior of cement concrete. The high values of Mr
also demonstrate that the material had a behavior of
cement-treated aggregates although the dependence of
confining stress. It is also worth highlighting that the
Mr results obtained with extracted specimens of the
base layer and compacted laboratory samples with
5 % of cement content were similar.
A correlation between the resilient modulus (for
confining pressure of 0.1 MPa) and the modulus of
elasticity results was done in order to verify if both
tests provide similar values, or at least the same
tendency. Figure 8 presents the correlation.
The results show that the values for modulus of
elasticity are consistently higher than the ones for
resilient modulus, but there was a moderate linear
correlation between both tests, once the coefficient of
determination (R2) was 0.76. Despite that, the increase
in modulus of elasticity values does not necessarily
provide a proportional increase in resilient modulus
values. Putri et al. [29] studied materials to be used as
highway subgrades and obtained the modulus of
elasticity and the triaxial resilient modulus. At the
same amplitude of axial stress, their samples resulted
in different values for modulus of elasticity and
resilient modulus. Considering axial stress amplitude
10,000
Mr (MPa)
8,000
6,000
4,000
25cm
30cm
2,000
0
0
0.1
Confining pressure (MPa)
Fig. 7 Resilient modulus results for field samples
0.2
Avg. (MPa)
SD (MPa)
CV (%)
43
Mr
6239
2706
ME
15,788
7232
46
Mr
6817
1362
20
ME
15,885
7113
45
Elasticity modulus (MPa)
Table 1 Triaxial resilient
modulus results
25,000
20,000
R² = 0.76
15,000
10,000
5,000
0
0
5,000
10,000
15,000
20,000
25,000
Resilient modulus (MPa)
Fig. 8 Correlation between modulus of elasticity and resilient
modulus of cement-treated soil-aggregate base
of 200 kPa, the value of modulus of elasticity was
50 % lower than the value of resilient modulus, the
opposite of that was found for the present study.
The major difference found for the two tests is the
type of loading application. Figure 9 illustrates results
of the load and displacement for the same specimen
during the triaxial resilient modulus and modulus of
elasticity, respectively.
The graphs indicate that the load applied during the
modulus of elasticity test is ten times higher than the
one applied during the resilient modulus test. The time
of loading is also different. It takes 0.1 s for a loading
cycle to be finished at the resilient modulus test, while
it takes 60 s at the modulus of elasticity test. The
modulus of elasticity test applies an initial load of
approximately 3 kN, and this is capable of providing
an initial strain, or accommodation, into the specimens. In the resilient modulus test, there is no initial
loading and consequently no initial strain observed in
the specimens.
A structural analysis based on a linear elastic
approach was performed in order to verify the
influence of using either the resilient modulus or the
modulus of elasticity values for purposes of designing
an asphalt pavement with a recycled base layer.
Pavement performance was evaluated by using fatigue
model proposed by the Federal Highway Administration (FHWA), which uses horizontal tensile strain at
Materials and Structures
Stress
Strain
15,000
0.06
Table 2 Ultrasonic test results
0.05
Thickness (mm)
Avg. (km/s)
SD (km/s)
CV (%)
250
3.2
0.4
12.2
300
3.1
0.1
2.6
0.04
10,000
0.03
5,000
0
0.0
0.02
1.0
2.0
3.0
4.0
E ¼ q v2 Time (s)
Stress (N)
20,000
0.05
15,000
0.04
10,000
Stress
Strain
5,000
0.03
0
Strain (mm)
0.06
(b) 25,000
0.02
0
100
200
300
400
500
600
700
800
Time (s)
Fig. 9 Example of data obtained from: a triaxial resilient
modulus test, and b modulus of elasticity test
the bottom of the asphalt layer to calculate the number
of allowable equivalent single axle loads (ESALs).
There was an increase of 15 % in the maximum
allowable traffic when considering the modulus of
elasticity instead of the Mr.
The ultrasonic wave propagation test was done with
the objective of predicting the rigidity of the materials
by measuring the speed of propagation through the
specimens. The equipment used for this test is
normally used for concrete and mortar samples and a
good correlation with the modulus of elasticity of this
type of material can be obtained. For the recycled
granular material with cement, no correlations were
found in the literature, but one can conclude that the
more rigid the material, the faster is the wave
propagation. Table 2 presents the results for the
ultrasonic propagation test.
The results obtained were similar for both sections.
Correlations between the values of wave propagation
speed and values of elasticity and resilient moduli
were done. Equation (1) is used to determine the value
of modulus of elasticity of mortar samples based on
the ultrasonic test results (NCHRP Report 465).
Despite the difference in the type of material, the
equation was also used to calculate the values for the
materials of this research. Figure 10 presents the
correlation between these values and the values of
resilient modulus previously presented.
ð1 þ lÞ ð1 2lÞ
ð1 lÞ
ð1Þ
where, E is the modulus of elasticity, q is the specimen
specific gravity, and l is the Poisson’s ratio.
There was a good linear correlation between the
results from the two tests, i.e., the higher the Mr were,
the higher the values from the ultrasonic tests were
too. Despite that, the values provided by each test have
different orders of magnitude, indicating that the
ultrasonic test might only be a good tool for ranking
different mixes, but not for obtaining the resilient
modulus results.
Tests for ITS and UCS were performed on the
samples extracted from the experimental test site.
Table 3 presents the results. The average values
obtained for the field specimens indicate that the
250 mm-segment had, in general, higher results of ITS
and UCS.
The results obtained for the field samples can be
compared to the ones obtained for the lab specimens,
especially if the 300 mm-thick layer is considered.
The ITS and UCS values from lab samples were on
average 1.0 and 9.0 MPa, respectively (Figs. 2, 3).
Observing the results from Table 3, it is possible to
conclude that the compaction on the field was done
properly, since the strength values were even higher
than the ones from samples compacted in the laboratory. Any minor difference between these results
Resilient modulus (MPa)
Stress (N)
20,000
Strain (mm)
(a) 25,000
12,000
y = 0.495x - 2703.3
R² = 0.84
10,000
8,000
6,000
4,000
2,000
0
0
5,000
10,000
15,000
20,000
25,000
30,000
E (MPa)
Fig. 10 Correlation between E obtained by ultrasonic test and
resilient modulus test
Materials and Structures
Table 3 Results of ITS and UCS tests
Layer thickness (mm)
ITS
UCS
Average value (MPa)
SD (MPa)
CV (%)
Average value (MPa)
SD (MPa)
CV (%)
250
1.48
0.33
21.97
14.43
5.12
35.49
300
1.32
0.17
12.95
10.85
3.21
29.57
Table 4 Results of
backcalculated moduli
Layer thickness (mm)
Moduli (MPa)
Backcalculated modulus
Mr
ME
250
15,000
6239
15,788
300
12,300
6817
15,885
might be explained by, e.g., possible changes in the
materials gradation in the field, which is less controlled than the gradation in the laboratory.
Table 4 presents rigidity results for the recycled
base layer provided by a backcalculation procedure
that included the use of the deflection bowls obtained
with the FWD test combined with the use of the
software BAKFAA. It is important to state that the
values are related to the FWD test performed in 2013.
The values of the backcalculated moduli are compared
to the ones obtained in laboratory tests using samples
extracted from the experimental test site. It is important to state that for the Mr values obtained through the
triaxial test, the value reported here is related to a
confining pressure of 0.1 MPa. The comparison indicates that the values obtained in ME tests are close to
the ones obtained through backcalculation, which
might indicate that in relation to rigidity, the ME might
be the right parameter when characterizing cement
stabilized material.
5 Summary and conclusions
This research project had the main objective of
studying the characteristics of a recycled granular
material with the addition of cement to be used as a
pavement base layer. A laboratory study was done to
investigate the addition of different cement contents
and its influence on the mechanical properties of the
recycled mixes. A field study was also conducted by
constructing an experimental test site with two different sections of pavement constituted by a recycled
base layer with 5 % of cement at different thicknesses.
The comparison between modulus of elasticity and
resilient modulus for characterizing rigidity indicated
that this type of material provided higher values of
modulus of elasticity compared to resilient modulus
values. The frequency of loading and the applied
stresses of these tests were different resulting in
different strain responses. The ultrasonic wave propagation test can be correlated to the resilient modulus
test if there is a need for ranking different mixtures.
Some of the main conclusions of this study are that
the use of deep recycling, i.e., the construction of
300 mm-thick recycled base layer, is viable. The
parameters to be considered in the design and in the
construction of this layer, specifically for the materials
and gradation distribution used in this research, should
be the modified energy effort and the cement content
of 5 %, which provided good laboratory results and
acceptable field performance. Structural analysis
indicated that the use of the recycled base material
resilient modulus instead of its modulus of elasticity in
the project of an asphalt pavement leads to lower
values of maximum traffic load. However regarding
the values of both tests, the backcalculated data are
very similar to the results from the modulus of
elasticity. The authors believe that the modulus of
elasticity test is the most indicated to characterize the
type of mixture studied in this research. This test
seems to provide results that are similar to results from
the field data.
The main practical implications of the present
research are the possibility of providing information
related to laboratory tests and studies that might be
useful for purposes of pavement design. The use and
evaluation of different test methods are important to
Materials and Structures
select the one that will most likely result on values that
can be related to real pavement structures. The field
studies have the objective of verifying the performance of the pavement base course layers, considering
that there is a lack of performance models for the type
of material studied in this work.
13.
14.
Acknowledgments The authors would like to acknowledge
the financial support of Capes, CNPq and ANTT, and also
Arteris for providing the experimental site.
15.
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