The Mechanical Properties of the Dry Process CRM Asphalt Mixtures
Following Short-term and Long-term Ageing
M. M. Rahman
Jacobs Babtie Pavement Management & Engineering, Jacobs Babtie, UK
G. D. Airey & A. C. Collop
Nottingham Centre for Pavement Engineering, School of Civil Engineering, The University of Nottingham, UK
ABSTRACT: Rubber produced from the scrap tyres, known as crumb/ground rubber, can be
used in asphalt mixtures either as a binder modifier (wet process) or as a fine and/or coarse
aggregate replacement (dry process). In both wet and dry processes, rubber particles react
with bitumen at high temperatures during the manufacturing stage. Compared to the wet
process, the reaction time in the dry process is considerably less (maximum six hours) and
slower due to the larger particle sizes. Consequently, it is generally assumed that the effect of
rubber-bitumen reaction in the dry processed mixture is less and, therefore, has a limited
effect on the mixture performance. To evaluate the effect of rubber-bitumen interaction on the
mixture’s mechanical properties, a laboratory investigation has been conducted on a range of
dense graded dry process crumb rubber modified (CRM) asphalt mixtures containing 0 %
(control), 3 % and 5 % crumb rubber by the total aggregate mass. The mixtures were
subjected to two stages of ageing, short-term (maximum six hours at 160 °C at the loose
condition to simulate the production to laying period) and long-term (five days at 85 °C of
compacted specimen to simulate approximately 10 years in service condition). The stiffness
modulus, fatigue and permanent deformation resistance have been determined using the
Nottingham Asphalt Tester (NAT) and compared with mixtures tested in their unaged state.
The results demonstrated that the influence of short-term ageing on mechanical properties is
far greater compared to long-term ageing. In addition, the results also indicated that,
irrespective of rubber contents in the mixtures, the load spreading ability (stiffness modulus)
increased in all mixtures following both short-term and long-term ageing. The twoperformance indicators, fatigue and resistance to permanent deformation, marginally
improved following short-term ageing of the loss mixtures, but generally deteriorated after
long-term ageing of the compacted CRM specimen.
KEY WORDS: Crumb rubber, short-term ageing, long-term ageing, stiffness, fatigue and
permanent deformation
1
INTRODUCTION
As new European legislation will prohibit the use of shredded scrap tyres for land filling by
2006, governments in the European countries are increasingly under pressure to find
alternatives to landfill disposal (Hird et al. 2002). The lack of proper controlled use of scrap
tyres would lead to added disposal costs, increase additional illegal dumping or inadequate
storage and could increase the risk of fire and environmental damage. In addition, road traffic
is predicted to increase by 17 % in the UK alone within next five years and consequently the
number of post consumer tyres arising in UK is likely to increase (Department of
Environment, 2000).
Within the expanding recycling market, using crumb rubber in asphalt mixtures either as a
binder modifier (known as the wet process) or as an aggregate (known as the dry process) has
shown the potential to use a significant number of scrap tyres in environmentally friendly
manner. The fundamental difference between the wet process and dry process is that in the
wet process, the fine rubber particles (.075 mm to 1.2 mm) are charged into the bitumen at
high temperature prior to mixing with the aggregate to allow reaction between them to
produce modified bitumen (Bahia and Davis, 1994, Kim et al. 2001). In the dry process,
coarser rubber particles (0.4 to 10 mm) are added directly to the mixtures with an assumption
that the rubber crumb is solely part of the aggregate and that the reaction between bitumen
and crumb rubber is negligible (Heitzman, 1992). This is usually achieved by limiting the
time at which the bitumen and crumb rubber are maintained at high mixing (reaction)
temperatures and specifying a coarse granulated crumb rubber with a low surface area and
smooth (less reactive) sheared surfaces. Recent research on rubber-bitumen interaction has
demonstrated that during the mixing period as well as during transportation and laying, rubber
crumb does swell and the rate and amount of bitumen absorption by rubber is significantly
higher which causes the residual bitumen to be stiffer and elastic, changes the shape and
rigidity of the rubber and consequently the performance of the asphalt mixture (Singleton et
al. 2000, Airey et al. 2003). Similar research conducted on the dry process CRM mixtures
demonstrated that compared to the conventional mixtures, there is significant increase in
mixture stiffness on rubber modified mixtures subjected to short-term age conditioning Airey
et al. 2004).
However, materials produced using both the wet and the dry processes have the potential
to demonstrate improved properties compared to conventional asphalt materials. These
improved benefits include increased fatigue life or fatigue resistance, reduced reflective
cracking and low temperature cracking, improved tensile strength, ductility, resilience, skid
resistance, resistance to rutting (Epps, 1994, McQuillen et al. 1988, Way, 2000, Oliver, 2000,
Fager, 2001, Amirkhanian, 2001). It is important to note that although mixture production in
the dry process is logistically easier where larger quantities of recycled crumb rubber are
possible to consume, it has been a far less popular method than the wet process due to
inconsistent field performance. To evaluate the concern regarding the performance of the dry
process, it is important to evaluate whether greater rubber-bitumen interaction during
production stage could generate long-term durability concerns and if it does, how significant
it would be compared to the conventional mixtures and/or same mixtures without short term
aged. The primary objectives of this paper are, therefore, to study the short-term and longterm ageing affect on the mechanical properties such as stiffness, resistance to permanent
deformation and fatigue properties on a range of dry process CRM asphalt mixtures.
2
AGEING STUDY OF ASPHALT MIXTURES
Ageing is primarily associated with the loss of volatile components and oxidation of the
bitumen during asphalt mixture construction (short-term ageing) and progressive oxidation
during service life in the field (long-term ageing). Bitumen slowly oxidises when in contact
with air (oxygen) increasing the viscosity and making the bitumen harder and less flexible.
The degree of viscosity is highly dependent on the temperature, time and the bitumen film
thickness. Excessive age hardening can result in brittle bitumen with significantly reduced
flow capabilities, reducing the ability of the bituminous mixture to support the traffic and
thermally induced stresses and strains, which contribute to various forms of cracking in the
asphalt mixture (Brown et al. 1995).
To study the ageing effect on the laboratory-fabricated specimen to simulate field
condition, it is important to account how the asphalt mixture ages in asphalt plant and also in
the service life. Briefly, the mixture ages as it goes through the plant in production stage, and
during storage and transportation, until it cools down to normal temperature. Ageing also
continues at a slower rate throughout the service life of the pavement where reaction proceeds
at a higher rate in hot climates or during the summer months when temperatures are higher.
One way to account for these changes is to condition the laboratory prepared mixtures in such
a way as to simulate the ageing that happens during construction and service stage. To
simulate the ageing that occurs during construction (up to the point of compaction) stage, the
short-term conditioning is primarily conducted on the loose mixtures. On the other hand, to
simulate the ageing that occurs over the many years that pavement is in service, long-term
ageing is used on the compacted specimen in a force draft oven.
Although there are various procedures that are effective in producing aged mixtures in an
accelerated manner, none have been conclusively validated with the short-term and/or longterm field performance of bituminous pavements (British Standard Institution, 1994, Scholz,
1995). However, for this investigation, two methods, Link Bitutest testing protocol (Scholz,
1995, Scholz and Brown, 1996) for short-term ageing and the SHRP methodology, as set out
in project A-003A American Association of State Highways and Transportation Officials,
1994) for Long-Term Oven Ageing (LTOA) were adopted. In this method, a 2-day ovenageing regime appears representative of up to 5 years in service and an ageing period of 4 or 5
days is used to simulate the ageing process for 10-year-old projects. It is important to note
that although five days ageing represents approximately 10 years of service life where ageing
generally increases the stiffness, it is not known whether the magnitude of the stiffness
increase resulting from accelerated ageing procedures accurately represents that which occurs
in the actual pavement material (Scholz, 1995, Scholz and Brown, 1996, Brown et al. 1995).
In addition, Brown et al (Brown et al. 1995) reported that this test method appeared to be
sensitive to the volumetric proportion of bitumen and air void contents and could only be used
for comparative purpose.
3
MIXTURE DESIGN
As specified in BS 4987-1:2001A, a continuously graded, 20 mm maximum aggregate size
dense bitumen macadam (DBM) asphalt mixture was used to manufacture a range of control
and dry process crumb rubber modified (CRM) asphalt mixtures. The design of CRM
mixtures consisted of replacing a portion of the aggregate fraction with granulated crumb
rubber between 2 and 8 mm in size. For simplicity, the crumb rubber were grouped into two
single size fractions; passing 6.3 mm and retained on 3.35 mm, and passing 3.35 mm and
retained on 0.3 mm, and added to the DBM mixtures gradation by substituting similar sizes of
aggregate fraction. As the majority of the granulated crumb rubber was less than 3.35 mm, the
two fractions were not replaced in equal amounts but consisting of 20 % < 6.3 mm & > 3.35
mm and 80 % < 3.35mm & > 0.3 mm.
Figure 1 shows the grading envelopes of the DBM mixture and the design grading that was
used in the study. All the mixtures were produced with a Middle East 100/150 penetration
grade bitumen, as specified in BS EN 12591, with a binder content of 5.25 % by mass of total
mixture.
100
20mm DBM_0%CRM
% Cumalative passing
80
20mm DBM_3%CRM
20mm DBM_5%CRM
60
BS4987-1:2001 max
40
BS4987-1:2001 min
20
0
0.01
0.1
1
10
100
Sieve size (mm)
Figure 1: 20 mm DBM aggregate gradation
The CRM asphalt mixtures were batched gravimetrically by replacing either 3 % or 5 % of
the aggregate fraction with granulated crumb rubber. The gravimetric gradings were then
converted to volumetric gradings to check that they were still within the grading envelopes of
the 20 mm DBM asphalt mixture. The mixtures were then compacted using gyratory
compactor with the following compaction parameters:
Axial pressure and Gyratory angle:
0.6 MPa & 1.25°
Speed of gyration:
30 gyrations per minute,
Compaction temperature:
155°C,
Specimen dimension:
D: 100 mm & H: 90mm
Trimmed (final) specimen height:
60 mm.
All the specimens (control and CRM) were compacted to a design density (target air voids
content) or 600 (maximum number) gyrations whichever occurred first. A total of six control
and CRM asphalt mixtures were therefore produced in the laboratory with the following
variables:
Crumb rubber content by mass of total aggregate:
0, 3 and 5 %,
Target air void content:
4%
Short-term ageing
0 and 6 hours
For simplicity, the different materials were coded as follows; crumb rubber content as R0, R3
and R5, short-term conditioning as C0 and C6, and finally code “A” for long-term aged and
“U” for unconditioned mixtures. For example, UR3-C6 refers to the unconditioned CRM
asphalt mixture with 3 % rubber content by mass, short-term conditioned for 6 hours.
4
LABORATORY AGEING TEST
In the Link Bitutest protocol, to simulate hardening during plant mixing, requires loose
mixtures, prior to compaction, to be aged in a forced draft oven at a temperature either 130 °C
or related to the desired compaction temperature, whichever is higher. In this investigation,
the compaction temperature of 155 °C was chosen in the short-term ageing where
uncompacted mixtures were placed on a shallow tray and aged in a force-draft oven for 6
hours.
In the SHRP LTOA protocol, the procedure consisted of performing LTOA test on
compacted mixtures that have already undergone the short-term oven ageing procedure and
then the compacted specimens are placed in a forced draft oven at 85 °C for 120 hours
(5days). At the end of the ageing period, the oven is switched off and left to cool to room
temperature before removing the specimens. The specimens are not tested until at least 24
hours after removal from the oven. Initial trials on CRM mixtures showed that the specimens
required extra protection as significant vertical expansion and rubber plucking (similar to
ravelling) was observed during high temperature (85 °C) curing and consequently it was
impossible to test the damaged specimen in the NAT. To overcome this problem, a simple
protective measure was undertaken by covering the specimen with aluminium foil, which was
clamped using a thin perforated sheet metal.
5
MIXTURE VOLUMETRIC
The volumetric proportions for the control and CRM asphalt mixtures are presented in Table
1. The mean volumetric values together with their minimum, maximum and standard
deviation have been quoted for each mixture. It is important to note that whilst it has been
possible to produce CRM mixtures at target air void contents (4 %) in terms of their average
results, difficulties associated with compacting the CRM mixtures, particularly the higher
crumb rubber content (5 %) mixtures, has meant that there has been a considerable amount of
scatter in the mixture void content.
Table 1: Volumetric results of control and CRM mixtures
Mixture
R0_C0
R0_C6
R3_C0
R3_C6
R5_C0
R5_C6
6
avg
2.6
2.6
4.7
4.0
4.2
4.5
Air void content (%)
max
min
2.9
2.3
3.4
2.1
5.8
2.7
6.0
2.7
5.5
1.5
5.8
3.3
std
0.2
0.4
0.9
1.1
1.3
0.7
EXPERIMENTATION
The mechanical properties of the primary and secondary aggregate asphalt mixtures were
measured using the Nottingham Asphalt Tester (NAT) (Cooper and Brown, 1989). The three
main parameters that were measured were the stiffness modulus (load spreading capacity) of
the asphalt mixtures and the two main pavement distress mechanisms of permanent
deformation and fatigue cracking.
6.1 Indirect Tensile Stiffness Modulus (ITSM) test
The stiffness moduli of the primary and secondary asphalt mixtures were measured using the
Indirect Tensile Stiffness Modulus (ITSM) test using the following parameters as described in
British Standard DD213 (British Standard Institution, 1993):
Specimen dimension:
H: 100mm, D: 60mm
Test temperature:
20 °C,
Loading rise-time:
124 milliseconds, and
Peak transient horizontal deformation:
5 µm.
Poisson’s ratio:
0.35
6.2 Confined Repeated Load Axial Test (CRLAT)
The permanent deformation resistance of the different asphalt mixtures was determined by
means of the Confined Repeated Load Axial Test (CRLAT) using a direct uniaxial
compression configuration on specimens with a diameter of 100 mm and a height of 60 mm.
The CRLA tests were performed in accordance with the British Standards DD185 using the
following test parameters (British Standard Institution, 1994):
Test temperature:
40 °C,
Test duration:
7200 seconds (3600 cycles) with a load pattern 1
second loading on (load application period) followed
by one second off (rest period),
Axial stress:
100 kPa
Confining pressure:
50 kPa,
Conditioning stress:
10 kPa for 600 seconds.
The results are interpreted as a measure of total strain (%) after 3600 cycles and average
strain rate (microstrain/cycle) between 1500 and 3600 cycles.
6.3 Indirect Tensile Fatigue Test (ITFT)
The fatigue resistance of the asphalt mixtures was determined by means of the Indirect
Tensile Fatigue Test (ITFT) with an experimental arrangement similar to that used for the
ITSM but under repeated loading and with slight modifications to the testing modulus
crosshead. The ITFT tests were performed using the following test parameters:
Test temperature:
20 °C,
Loading condition: Controlled-stress,
Loading rise-time:
120 milliseconds, and
Failure indication:
9 mm vertical deformation.
7
RESULT ANALYSIS
7.1 Stiffness Modulus
The average stiffness modulus values for the control and CRM mixtures are presented in
Table 2. It can be seen that in terms average stiffness modulus, irrespective of short-term
conditioning, all the control and CRM mixtures have undergone an increase in stiffness. In the
following sections, the results are discussed to analyse the effect of rubber content and the
relative influence of short-term and long-term ageing on mixture stiffness.
Table 2: Stiffness results for control and CRM asphalt mixtures
Mixture
Stiffness (MPa)
5 days oven aged
std
avg
std
Unaged
avg
R0-C0
R0-C6
R3-C0
R3-C6
R5-C0
R5-C6
2907
4702
2518
3794
1907
2903
248
455
306
608
239
260
3528
5584
2843
4415
2801
3133
% Increase in
stiffness due to longterm oven ageing
233
310
359
746
276
293
21 %
19 %
13 %
16 %
47 %
8%
7.2 Rubber Content
The average stiffness values for the control and CRM mixtures together with the percentage
stiffness increase with long-term oven ageing are presented graphically in Figure 2 which
shows plot of mixtures subjected to various degrees of short-term age conditioning prior to
compaction. It is apparent from the results that irrespective of rubber content in the mixtures,
stiffness values in general increased due to age hardening of bitumen. This relatively uniform
increase in stiffness for all (with or without short-term conditioning) control mixtures
indicates that bitumen hardening is the main contributory element to improve the load
spreading capacity. In terms of rubber content, compared to R3 mixtures, considerable
reduction of percentage stiffness increase between R5-C0 and R5-C6 mixtures is probably the
influence of rubber softening due to the rubber-bitumen interaction which compensates the
effect of bitumen hardening. Therefore, in terms of rubber content, the results indicate that in
addition to bitumen hardening, softening of rubber due to rubber-bitumen interaction also
contributes to the overall mixture stiffness.
100
6000
Initial stiffness
5000
90
stiffness after long term ageing
80
Stiffness ( MPa)
% stiffness increase
4000
70
0 hour conditioning
60
50
3000
40
2000
30
20
1000
10
0
0
R0-C0
R3-C0
R5-C0
R0-C6
R3-C6
Figure 2: Stiffness modulus of control and CRM asphalt mixtures
R5-C6
% increase in stiffness due to oven ageing
6 hours conditioning
7.3 Short-term and Long-term Ageing
A relative influence of both short-term and long-term ageing on the control and CRM
mixtures is presented in Table 3 including percentage change in stiffness for different ageing
conditions. The results were calculated to observe the change in stiffness between 0-6, 0-120
and 6-120 hours of oven ageing for different mixtures. The results show that as expected, the
relative effect of short-term conditioning is significantly higher compared to long-term ageing
which points towards the importance of the mixing, transportation and laying period on
mixture’s ageing. It is interesting to note that irrespective of rubber content, the stiffness
increase in long-term aged C6 CRM mixtures is relatively lower than C0 mixtures. Three out
of four C6 mixtures have shown lower increases in stiffness suggesting that the softening
effect of rubber particles following rubber-bitumen interaction may have compensated the
hardening of bitumen due to combined influence of rubber-bitumen interaction and oxidation.
Table 3: Comparative stiffness results for short and long-term aged control and CRM
mixtures
Mixture
R0
R3
R5
8
Average stiffness modulus (MPa)
Short-term
Long-term
C0
2907
2518
1907
C6
4702
3794
2903
C05days C65days
3528
5584
2843
4415
2801
3133
Percentage change (%)
Short-term
Long-term
C0-C6
62
51
52
C0-C120 C6-C120
21
19
13
16
47
8
RESISTANCE TO PERMANENT DEFORMATION
The permanent deformation results for control and CRM mixtures are plotted in Figures 3
(a)&(b) and are presented as measures of total strain (%) after 3600 cycles and average strain
rate (microstrain /cycle) between 1800 and 3600 cycles in Table 4.
Axial Strain (%)
10
1
UR0C0
AR0C0
UR3C0
AR3C0
UR5C0
AR5C0
0.1
1
10
100
1000
10000
1000
10000
Number of load cycles (N)
a
Axial Strain (%)
10
UR0C6
AR0C6
UR3C6
AR3C6
UR5C6
AR5C6
1
0.1
1
10
100
Number of load cycles (N)
b
Figure 3: CRLAT results for control and CRM mixtures conditioned a) 0hr b) 6hrs prior to
compaction
Table 4: Permanent deformation parameters for control and CRM asphalt mixtures
Mixture
Total Strain (%)
Unaged
Long-term aged
avg
std
avg
std
Strain Rate (microstrain/cycle)
Unaged
Long-term aged
avg
std
avg
std
R0-C0
R0-C6
R3-C0
R3-C6
R5-C0
R5-C6
0.76
0.40
4.65
3.75
4.60
4.74
0.61
0.23
2.09
3.86
1.27
1.75
0.34
0.25
0.68
0.58
1.05
1.37
1.12
1.10
6.85
4.30
9.00
4.30
0.2
0.2
0.5
1.0
1.5
1.3
0.33
0.14
0.78
1.77
0.24
1.25
0.8
0.4
4.1
2.3
3.9
4.9
0.2
0.2
0.7
0.7
1.6
1.8
In terms of total strain, relative to the control mixtures, the replacement of part of the
aggregate fraction with crumb rubber results in an increase in total permanent strain in both
unconditioned and long-term aged state. This is probably due to the combined effect of highly
elastic nature and reduced mixture cohesion following high temperature long-term ageing.
However, it is important to note that the differences in material ranking by means of total
strain can be attributed to the sensitivity of any initial slack in the test apparatus due to rough
top surface of CRM specimens as well as the fundamental differences in what this parameter
is measuring as any delayed elastic response that cannot be recovered during the one second
recovery period will be added to the final strain measurement. For the highly elastic
(rubberised), lower stiffness modulus CRM mixtures, the proportion and actual strain
magnitude of the delayed elastic component will inevitably be relatively high resulting in an
increase in total strain. On the other hand, the strain rate parameter is a more direct
measurement of the viscous response (permanent strain) of the material. For this reason the
strain rate parameter can be considered to be a more reliable and accurate means of assessing
the permanent deformation performance of the dry process CRM asphalt mixtures and
therefore exclusively used to analyse the effect of rubber content and short-term and long-tern
ageing.
8.1 Rubber Content
Table 5 presents a comparative rutting performance in terms of the control and CRM mixtures
in their unconditioned state and following long-term oven ageing. The results are also
compared with control mixtures by calculating the percentage changes for CRM mixtures in
their unconditioned and long-term aged conditioned states. The results show that, in general,
compared to control mixture, the resistance to permanent deformation for continuously graded
CRM mixtures decreases significantly following long-term ageing. It is also interesting to
note that in unconditioned state, although the total strains for the 5 % CRM mixtures were
greater than those of the 3 % CRM mixtures, the strain rates tend to be generally lower for
mixtures with higher rubber content. In addition, compared to unconditioned state test results,
the percentage changes relative to control in long-term aged mixtures are also generally
inferior and increase is generally higher for 5 %CRM mixtures indicating that the long-term
deformation resistance decreased with increasing rubber content in the mixtures.
Table 5: Permanent deformation performance of control and CRM mixtures subjected to
LTOA
strain rate (µε/cycle)
Mixture
C0
C6
0 % rubber
U
A
3 % rubber 5 % rubber
U
A
U
A
0.61
0.23
2.09
3.86
0.8
0.40
4.1 1.27
2.3 1.75
3.9
4.9
Percentage change (%) relative to
control
3 % rubber
5 % rubber
U
A
U
A
243
1578
413
475
108
661
388
1125
8.2 Short-Term and Long-Term Ageing
The effects of short-term and long-term ageing on the performance in terms of rutting
resistance of the control and CRM mixtures are shown in Table 6. The results are evaluated
by calculating the percentage change compared to unconditioned state testing. In terms of
unconditioned results, as expected, hardening the binder of the control mixtures leads to a
decrease in strain rate (improved rutting resistance). In terms of long term ageing, it can be
observed that compared to the unconditioned state results, the strain rate for C6 control
mixtures is slightly inferior to that of the C0 mixtures.
Table 6: Permanent deformation performance as a function of short-term ageing
Mixture
R0
R3
R5
strain rate (µε/cycle)
0 hr conditioning
6 hrs conditioning
Change compared to unconditioned state (%)
0 hr
6 hrs
U (a)
A (b)
U( c)
A (d)
(a-b)/a
(c-d)/c
0.61
2.09
1.27
0.8
4.1
3.9
0.23
3.86
1.75
0.40
2.3
4.9
-31
-96
-207
-74
40
-180
The results also indicate that although short-term age hardening of the bitumen can
improve the rutting resistance, the excessive amount of bitumen oxidation can lead to a brittle
mixture. On the other hand, with the exception of R3-C6, all the other C0 and C6 CRM
mixtures have shown poorer rutting performance compared to unconditioned state results.
These observations indicate that although short-term ageing of the CRM mixtures marginally
improves the rutting resistance, its long-term influence is relatively minor compared to the
effect of increasing rubber content in the mixtures.
9
FATIGUE
The ITFT fatigue lines for control and CRM mixtures are shown in the Figures 4 (a) & (b)
and the regression line for strain and fatigue life with co-efficient of correlation for each
mixture tested in unconditioned and long-term aged conditioned state are presented in Table
7. . Using equations presented in Table 7, predicted fatigue life for 100µε and predicted strain
for million load cycles were calculated to study the effect of rubber content and the effect of
ageing.
Tesnile strain(µε)
10,000
1,000
UR0-C0
AR0-C0
UR3-CO
AR3-C0
UR5-C0
AR5-C0
100
10
10
100
1,000
10,000
100,000
1,000,000
a
Cycles to failure (Nf)
Tesnile strain(µε)
10,000
1,000
UR0-C6
AR0-C6
UR3-C6
AR3-C6
UR5-C6
AR5-C6
100
3% CRM
10
10
100
1,000
10,000
100,000
1,000,000
b
Cycles to failure (Nf)
Figures 4: ITFT fatigue line for control and CRM mixtures conditioned a) 0hr) 6hrs prior to
compaction
Table 7: Fatigue relationship for control and CRM asphalt mixtures
Mixture
UR0-C0
UR0-C6
UR3-C0
UR3-C6
UR5-C0
UR5-C6
AR0-C0
AR0-C6
AR3-C0
AR3-C6
AR5-C0
AR5-C6
Fatigue equation
Nf = 1.6 x1011ε -3.45
Nf = 1.0 x 1012ε -3.54
Nf = 4.8 x 1016ε -5.42
Nf = 8.9 x 1019ε -7.24
Nf = 1.0 x 1015ε -4.69
Nf = 9.0 x 1014ε -4.78
Nf =6.0x1012ε -4.01
Nf =6.0x1012ε -4.02
Nf =1.0x1013ε -4.10
Nf =3.0x1015ε -5.43
Nf =1.0x1014ε -4.22
Nf =9.0x1011ε -3.95
Fit (R2)
0.94
0.90
0.81
0.99
0.63
0.98
0.93
0.94
0.97
0.95
0.93
0.97
Strain equation
ε = 1534 x Nf -0.27
ε = 1879 x Nf -0.26
ε = 887 x Nf -0.15
ε = 563 x Nf -0.14
ε = 858 x Nf -0.13
ε = 1284 x Nf -0.20
ε =1371xNf-0.23
ε=1306xNf-0.24
ε =1509xNf-0.24
ε=671xNf-0.18
ε=1944xNf-0.22
ε=999xNf-0.25
9.1 Rubber Content
The results presented in Table 8 show that age hardening of control mixture leads to better
long-term fatigue performance. For CRM mixtures, there are significant reductions in both
predicted strains and fatigue lives of all aged CRM mixtures compared to their corresponding
unconditioned state. Although all the mixtures were produced using the same percentage of
bitumen using the same manufacturing technique, the rubber particles are believed to be the
main cause of absorption of the lighter fractions of bitumen during long-term ageing which
consequently leads to a stiffer material and increased brittleness.
Table 8: Fatigue life comparison as a function of rubber content
Strain @ 106 cycles (µε)
Cycles @ 100 µε ( in thousands)
Mixture 0 % Rubber 3 % Rubber 5 % Rubber 0 % Rubber 3 % Rubber 5 % Rubber
U
A
U
A
U
A
U
A
U
A
U
A
C0
C6
36
55
56
51
U: Unconditioned state
111
86
57
59
134
76
93
34
20
84
57
54
706
301
64
41
416
250
366
11
A: Long-term oven aged state
9.2 Short-Term and Long-Term Ageing
Table 9 shows that as with the effect of rubber content, the overall long-term fatigue
performance of the short-term aged CRM mixtures appear to be reduced. In general, the
predicted strains for the aged CRM mixtures are reduced compared to their unconditioned
state which indicate the consequence of the increased stiffness after long-term ageing.
Table 9: Fatigue life comparison as a function of short-term conditioning
Mixture
R0
R3
R5
Strain @ 106 cycles (µε)
0 hrs conditioning
6 hrs conditioning
U
36
111
134
A
56
57
93
U
55
86
76
A
51
59
34
Cycles @ 100 µε(in thousand)
0 hrs conditioning 6 hrs conditioning
U
20
706
416
A
57
64
366
U
84
301
250
A
54
41
11
In terms of predicted fatigue life, ageing appears to reduce the overall long-term fatigue
performance of both C0 and C6 CRM mixtures although for the control mixtures it appears
that fatigue life increases with ageing. In addition, generally the reduction of fatigue life is
greater for six hours ageing (C6) than without ageing (C0) for the CRM mixtures, suggesting
that in addition to normal oxidation, more absorption of lighter fractions reduces the cohesive
and adhesive strength of the bitumen and consequently increases the brittleness of the
material.
10 CONCLUSION
The long-term ageing performance of the dry process CRM continuously graded asphalt
mixtures were studied and compared to the performance of the conventional primary
aggregate mixtures produced under similar conditioning and compaction regimes. The
following conclusion can be drawn from these laboratory investigations:
•
The ageing of the CRM mixtures leads to an increase in stiffness through excessive loss
of the lighter fraction of bitumen because of the combined effects of normal oxidation
and rubber-bitumen interaction following short-term and long-term ageing. Compared
to the long-term ageing, short-term ageing for both control and CRM mixtures was
found to be more significant on the mixture’s stiffness indicating the importance of the
production stage.
•
The increase in stiffness generally appears to be lower for mixtures with higher rubber
contents indicating that the softening effect of flexible rubber particles may have
contributed to compensate the increased effect of bitumen hardening. In addition, the
short-term oven ageing was found to be less significant in both CRM mixtures, as
changes in stiffness are mostly dominated by the rubber content of the mixture.
•
The permanent deformation analysis in terms of total strain was found not to be suitable
for CRM mixtures because the rough top surface, highly elastic nature of the rubber
particles and low stiffness of CRM mixtures. However, strain rate was used as it
provides a more accurate way to assess the viscous response (permanent deformation)
of the bituminous mixtures in the steady state portion of the deformation curve.
Significant reductions in permanent deformation resistance were observed for all CRM
mixtures following long-term ageing. The effect of rubber content was found to be the
dominant factor in the permanent deformation resistance, where an increase in rubber
content in the mixtures reduced the overall rutting resistance.
•
The brittleness of the mixtures increases due to the loss of adhesive and cohesive
strength of the material resulting in a reduction in long-term fatigue life. However, the
predicted strain and fatigue lives calculated from fatigue and strain equations were still
better than similarly aged corresponding conventional mixtures. In addition, the
compaction effort did not appear to have any more significance on the long-term ageing
properties on the CRM mixtures then it had on the control mixtures.
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