As a rut resistant mixture, SMA is a type of gap-graded hot mix asphalt that
acquires its resistance from stone skeleton of coarse aggregate. Coarse aggregate
fraction of this type of mixture is glued together with a durable and moisture-resistant
mortar comprising asphalt cement (typically 5.5-7%), filler (typically 8-12%)
and stabilizing additive (Brown and Manglorkar, 1993).
According to the reports, stone matrix asphalt has indicated more durability
as a surface asphalt concrete (Al-Hadidy and Yi-qiu, 2009a).
Some of positive properties of SMA mixtures comprise its high rut resistance,
high skid resistance, high durability, improved resistance to reflective cracking,
better drainage condition and reduced noise pollution (Nejad
et al., 2010). The main reason of using a gap gradation in these
mixtures is to establish stone on stone condition to resist against permanent
deformation. Because of both gap graded nature of this mixture and high amount
of binder, drain down has been regarded as the main problem in using this type
of asphalt concrete. To prevent this problem, the use of some additives may
be essential. There are two mainly types of additives used in these mixtures:
Fibers and Polymers. Fibers such as polyester fiber, mineral fiber
and cellulose fiber are often used in ordinary SMA mixtures to significantly
decrease the amount of binder drain down (Tayfur et al.,
2007; Behbahani et al., 2009). Fibers could
relatively change the viscoelasticity of mixture; improves dynamic modulus,
tensile strength and moisture susceptibility, creep compliance, rutting resistance
and fatigue life (Abtahi et al., 2010) while
significantly lowers the amount of draindown of asphalt mixtures. In contrast,
a polymer is often used in asphalt mixtures to substantially improve the mechanical
properties of asphalt mixtures and relatively decrease the binder drain down.
Different types of polymers (such as Styrene-Butadiene-Styrene (SBS), Ethylene-Vinyl-Acetate
(EVA), polyethylene or polypropylene) may be used to gain desired mixture properties
(Al-Hadidy and Yi-qiu, 2009a). To combine binder with
polymer, it is often necessary to heat the binder up to 180°C. This could
bring about some problems in terms of aging of the binder which cause asphalt
concrete to be more brittle during its service life. One way to solve this problem
is by using antioxidants. The main aim of using antioxidant is to reduce the
amount of oxidative hardening of the binder. Oxidation is a kind of irreversible
chemical reaction of oxygen with binder. Oxidation of asphalt can occur throughout
the life of an asphalt pavement such as during mixing, field placement and during
its service life (Apeagyei, 2010).
With regard to use different additives in SMA mixtures, Sharma
and Goyal (2006) evaluated the effect of crumb rubber and natural fiber
on performance of SMA mixtures. They concluded that the crumb rubber could positively
affect the engineering properties of SMA mixtures such as resistance to moisture
damage, rutting, aging and permeability more than natural fiber could. Al-Hadidy
and Tan (2009) investigated the influence of Low Density Polyethylene (LDPE)
on life of SMA mixtures and performed performance tests such as Marshall, moisture
sensitivity and low-temperature cracking test and the benefit of modification
was evaluated by mechanistic-empirical method. As they reported, it was observed
that LDPE could efficiently reduce the binder drain down and improve the properties
of asphalt binder as well as SMA mixture; 6% of LDPE could be the best dosage
to construct the SMA mixtures. Moreover, the injection of LDPE to the SMA mixture
resulted in a life of pavement 1.359 times more than unmodified SMA mixture.
Kumar et al. (2007) added different types of
fibers (natural and patented) and crumb rubber to SMA mixtures. Based on the
results, natural fiber was comparable to the patented fiber in terms of Marshall,
rutting and flexural fatigue tests; however, natural fiber resulted in a better
condition in terms of aging of asphalt concrete. In addition, crumb rubber indicated
better efficiency in improving the properties of SMA mixtures compared to the
fibers. As presented by Al-Hadidy and Yi-qiu (2009c)
polypropylene could also enhance the performance of SMA mixtures in terms of
Marshall, moisture sensitivity and compressive strength. It did not need to
add any fiber to decrease the amount of binder drain down when using polypropylene.
This type of additive could also improve the service life of the SMA mixture
1.48 times more than unmodified one. A comparative study was performed by Tayfur
et al. (2007) which included the rutting performance of SMA mixtures
modified with different additives such as amorphous polyalphaolefin, cellulose
fiber, polyolefin, bituminous cellulose fiber and styrene butadiene styrene.
As the main results of this research, SBS showed the highest resistance to permanent
deformation and fibers indicated the least improving effect among different
used additives. Behbahani et al. (2009) used
different fibers to construct SMA mixtures. Based on this research, German cellulose
fiber had a better effect on properties of SMA mixtures rather than Iranian
cellulose and mineral fibers. Al-Hadidy and Yi-qiu (2009b)
also investigated the effect of SBS and Starch on life of SMA mixtures. Different
performance tests such as Marshall, indirect tensile strength and resilient
modulus were conducted to evaluate the SMA mixtures modified with 5% of aforementioned
additives. Based on their report, Starch could be a good replacement of SBS;
however, in the most performed tests SBS showed an insignificant better influence.
The efficiency of starch was more than SBS in terms of reducing the moisture
susceptibility of SMA mixture.
The main objective of this study was to perform a laboratory investigation on SMA mixtures containing regular additives (SBS and mineral fiber) and a compound additive (Rheofalt® wkr-2) which is a polymer-based modifier includes antioxidant to reduce the age-hardening of asphalt concrete.
MATERIALS AND METHODS
Materials used: Required aggregate was obtained from an asphalt plant in Roudehen around Tehran. The properties of aggregate were presented in Table 1. Moreover, Rock dust, Passed through sieve No.200 was used as mineral filler. The bulk specific gravity of used filler was 2.702 kg cm-3.
Asphalt cement for this study was AC-60/70, provided from Tehran Refinery and Pasargad Oil Company, Tehran, Iran. Physiochemical properties of this cement were provided in Table 2.
|| Properties of coarse and fine aggregates
As drain down inhibitors, mineral fiber and SBS were added to the SMA mixtures
at the rate of 0.4% (by weight of the mixture) and 5% (by weight of the binder),
respectively. Mineral fiber was uniformly mixed with aggregate before adding
asphalt cement. The used SBS was ITERPRENE SBS/G-L®. In order
to mix the binder with polymer, 5% of SBS by weight of asphalt cement was added
to the heated binder (170°C) according to the previous studies (Yildirim,
2007). At this temperature, the rotation speed was regulated at 1500 rpm.
After about 30 min the speed increased up to 4000 rpm for 1 h to make a homogeneous
binder. The specification of the used SBS in this study is as shown in Table
In addition, one type of compound additive was used in this study. The physical properties of Rheofalt® wkr-2 which consists of EVA, waxes and antioxidants is presented in Table 4. This type of additive does not need a high shear stirrer to blend with bitumen. A typical rotating stirrer is adequate for this purpose.
Mixture design: The mixing and compaction temperatures for constructing
SMA specimens were selected in a way that the viscosities are 170±20
and 280±30 cSt, respectively (Brown and Manglorkar,
1993). This criterion was considered to determine the mixing and compaction
temperature for base and modified binders. The selected dosages of Rheofalt®
wkr-2 in this study were 5, 10 and 15% by weight of the binder. However, construction
temperatures for SBS modified binder were 175°C and 165°C recommended
by manufacturer. The mixture design method used for SMA was as proposed by NCHRP
Report No. 425 (Brown and Cooley, 1999). To evaluate volumetric
properties of mixtures with different gradation limits, a number of 12 samples
(four identical samples for each) were fabricated according to the Marshall
mix design (ASTM D-1559) procedure. Laboratory specimens were prepared using
fifty blows of the Marshall hammer per side.
|| Physical properties of asphalt cement
Seventy-five compaction blows were not used since they would not result in
a significant increase in density over that provided by 50 blows (Al-Hadidy
and Yi-qiu, 2009b) and would also cause more breakdown in coarse aggregate
(Asi, 2006). The optimum gradation should be selected
in such a way that the VCA ratio was less than 1 in order to establish stone
on stone condition in all SMA samples. After optimizing the gradation for SMA
mixtures, it is important to select Optimum Binder Content (OBC) to meet the
requirements presented in Table 5. With this regard, two series
of SMA mixtures should be prepared for each set of the mixture to calculate
the OBC for control/SBS/wkr-2 and mineral fiber modified mixtures. Different
percentages of binder should be added to each subset to both evaluate volumetric
properties of mixtures and determine the amount of OBC. It should be noted that
the OBC for each subset in this section was considered in such a way that all
specimens met 4% air void.
Drain down: In order to evaluate the potential of drain down for SMA mixtures, a test procedure described by NCHRP Report No. 425 was conducted for all mixtures. Thus, for each set of mixture, a standard wire basket containing uncompacted SMA sample along with a plate were placed in an oven at the mixing temperature. After about 1 h, the amount of binder drain down was calculated by Eq. 1:
||Initial plate mass
|| Weight of plate plus drained materials
||Loose sample mass
Marshall test: Although, Marshall test is usually carried out for HMA
mixtures in order to perform the process of mixture design, Marshall stability
and flow values for SMA mixtures are generally measured for information but
not for acceptance. The Marshall stability for SMA mixtures is significantly
lower than that for dense graded mixtures (Brown and Manglorkar,
|| Properties of ITERPRENE SBS/G-L®
|| Physical properties of rheofalt® wkr-2
This is not an indication that dense graded mixtures are more stable than SMA
mixtures but it declares that Marshall stability may not be appropriate for
SMA samples. The volumetric properties might be more applicable in designing
SMA mixtures than Marshall stability.
The Marshall quotient is calculated as the ratio of the Marshall stability
to the flow. This ratio clearly represents the performance properties of asphalt
mixture such as stiffness, resistance to the shear stress and permanent deformation
(Mirzahosseini et al., 2011). The higher Marshall
quotient value a mixture has, the more resistant to cracking it should be.
To prepare desired specimens, three identical samples for each type of SMA
mixtures were compacted at 4% of air void. Designing the mixtures at 3% air
voids definitely results in a higher probability of fat spots and permanent
deformation (Brown et al., 1997a). All specimens
were submerged in 60°C water for 30-40 min before the test.
Mechanism and concept: Moisture can affect asphalt mixtures in three different
ways: loss of cohesion, loss of adhesion and aggregate degradation. The loss
of cohesion and adhesion are more prominent to the process of stripping. A reduction
in cohesion results in a reduction in strength and stiffness. The loss of adhesion
is the physical separation of the asphalt cement and aggregate, primarily caused
by the action of moisture (Khosla and Harikrishnan, 2007).
There is a close relationship between the permeability of the mixture and possibility
of moisture damage to occur. If asphalt pavements are impenetrable, moisture
damage would seldom happen; however, it may result in a excessive permanent
deformation of pavement under heavy traffic and occurrence of bleeding phenomenon
at high temperatures (Terrel and Al-Swailmi, 1994).
In the case of moisture damage measurement, tensile strength is one of the
most critical parameters to be always taken into consideration for performance
evaluation. This test is used to determine the tensile properties of the asphalt
concrete which can be further related to the cracking properties of the pavement.
The tensile strength is primarily a function of the binder properties. The amount
of asphalt binder in a mixture and its stiffness affect the tensile strength.
Tensile strength also depends on the absorption capacity of the used aggregates
(Khosla and Harikrishnan, 2007).
The moisture sensitivity of a mixture could be evaluated by performing the
AASHTO T-283 test. As defined in this procedure, the indirect tensile strength
of mixtures should be measured by applying compressive loads along a diametrical
plane through two opposite loading strips for both dry and preconditioned specimens.
This test does not seem to be a very accurate indicator of stripping but it
could help to minimize the problem (Brown et al.,
Specimen preparation and test procedure: Each SMA mixture in this phase should be compacted at 6±1% of air void by adjusting the number of blows as described by NCHRP Report No. 425. Two subsets (three identical samples for each) were produced in the laboratory for each set of mixtures. One subset was tested dry at 25°C and the other one was preconditioned before testing in warm-water bath for 24 h at 60°C. Maximum applied force was measured for each set; then, Indirect Tensile Strength (ITS) and Tensile Strength Ratio (TSR) were calculated by Eq. 2 and 3:
||Tensile strength (MPa)
|| Maximum load (N)
|| Specimen thickness (mm)
|| Specimen diameter (mm)
As mentioned by NCHRP Report No. 425, SMA mixtures should meet applicable specification (70% minimum) as their TSR values. Therefore, mixtures with TSR less than 70% are moisture susceptible and it might be necessary to use antistripping additives or adjust mixtures to meet the requirement.
Dynamic creep test
Basic concept: As a general description, rutting is the longitudinal settlement
of the pavement surface along the wheel load path due to the progressive movement
of paving materials under traffic loading. The amount of the depression could
significantly depend on the quality of asphalt concrete as well as other layers
(Tapkin, 2008). Based on previous researches, rutting
was considered as the most important distress in asphalt pavement. Although
aggregate, asphalt cement and air void are the three important parameters of
asphalt concrete which influence the rutting performance of pavements, aggregate
shape and texture have a significant role in determining the level of aggregate
interlocking and also the amount of pavement rutting (Xiao
et al., 2010). Therefore, the main reason why SMA mixtures have been
widely used is that the texture of these types of mixture causes them to be
superior to conventional hot mix asphalt in terms of permanent deformation.
However, the type of stabilizer can seriously affect the SMA rut resistance
capabilities (Brown et al., 1997a).
Various experimental tests such as static creep, dynamic creep, wheel tracking
and indirect tensile tests are normally used to assess the potential of permanent
deformation of asphalt mixtures. Among the mentioned methods, dynamic creep
test is found to be one of the best methods to evaluate the rutting performance
of asphalt mixtures (Kaloush et al., 2002). In
addition, the findings indicated that there is a very good correlation between
the measured rut depth obtained from the wheel tracking test and the deformation
strain obtained from the uniaxial creep test.
The most commonly used device in dynamic creep test is the Universal Testing
Machine (UTM-5) which it can be considered as the first generation of UTM. This
device is usually employed to determine the important mechanical properties
of asphalt mixtures under similar field conditions (i.e., similar loading and
temperature). As indicated in Fig. 1, The creep curve could
be divided into three zones: (1) primary zone, (2) secondary zone and (3) tertiary
zone. During the primary zone, the mixture volume decreases due to densification
and accumulated strain increases dramatically. In the secondary zone which can
be identified as a transition zone between the primary and the tertiary zones,
the relationship between accumulated strain and cycles is linear. The tertiary
zone can be named as appearance of the second mechanism of rutting in which
the shear deformation starts and rutting increases again. As a comparison criterion,
Kaloush et al. (2002) used Flow Number (FN) as
a point at which the tertiary zone in creep curve begins (Kaloush
et al., 2002).
In order to evaluate the creep behavior of SMA mixtures the UTM-5P in the laboratory
of Iran University of Science and Technology was used. The load on the specimens
was uniaxial and dynamic, representing the repeated application of axle loads
on the pavement structure.
All specimens for each alternative were prepared by Marshall compactor in
order to reach 4% of air void. Other conditions were also the same for different
type of mixtures; a loading level of 400 KPa (58.4 psi) was applied to all specimens;
the shape of loading was rectangular with a load and rest period of 1s; all
specimens were placed at 45°C for 4 h before testing; the test was continued
for each sample until 10000 cycles or the loaded specimen reach its tertiary
Statistical analysis: To investigate how different properties of SMA mixtures could relate to each other, a statistical analysis should be performed. In this research, the relationship between performance tests results such as, Marshall stability, Marshall Quotient, Indirect Tensile Strength (ITS) and dynamic creep test was inspected using linear regression by conducting the Statistical Package for the Social Sciences (SPSS V19) program. In addition, for each existed association, the relevant equation with the corresponding information was provided.
RESULTS AND DISCUSSION
Rheological tests: The Rheological tests including Penetration and Softening Point tests were performed for unmodified and modified binders and the value of Penetration Index (PI) was measured for each type of binder. Penetration Index is a term which describes the sensitivity of the binder to the temperature. Temperature susceptibility is the rate at which the consistency of binder changes. To evaluate the potential of temperature sensitivity of asphalt binder, penetration index could be calculated by using Eq. 4:
|| Testing temperature
|TR and B
||Ring and Ball softening point
|| Volumetric properties of different limits of gradation
|| Results of binder properties tests
Figure 2 presents the results of binder tests. From Fig. 2, it could be inferred that all percents of wkr-2 have the capability of improving the binder properties. For example, 5, 10 and 15% of this additive could reduce the binder penetration by 17, 29 and 40% and increase the softening point of the binder by 10, 24 and 43%, respectively. However, the improving effect of SBS on penetration and softening point of the binder were 30 and 24% which could express that the influence of 15% wkr-2 on asphalt cement properties was more significant than that of SBS. Moreover, as an indicator of temperature sensitivity of binder, PI values were provided in Fig. 3. It can be realized from Fig. 3 that both SBS and wkr-2 increased the P.I. value which could indicate that the temperature sensitivity of binder decreased after adding SBS and different percents of wkr-2.
Mixture design: According to Fig. 4, the mixing and compaction temperatures of base binder for preparing control and mineral included SMA specimens were 158°C and 146°C, respectively. From Fig. 4, it could be observed that the construction temperatures for wkr-2 modified binders were more than that for base binder; for instance, 15% of wkr-2 increased the mixing temperature up to 177°C.
To optimize the gradation, three limits (upper, middle and lower) were first
considered as shown in Fig. 5. In addition, 6.2% of binder
was initially added to the mixture according to the report. The volumetric properties
of SMA mixtures needed to calculate the parameters such as VCA, VMA and Va are
presented in Table 6.
|| PI Values for modified binders
As shown in Table 6, the VCA ratio for middle limit is 0.927
at 6.95 percent of air void. This amount of air void in these samples seems
to be relatively high to be filled with binder and it seems not logical to select
the middle limit as optimum gradation. A more wisely way is to select the optimum
gradation between upper and middle limits in a way that the VCA ratio is nearer
to 1. The aim of this is to produce mixtures more economically in terms of the
amount of used binder. Therefore, the optimum gradation is as shown in Fig.
5. The properties of specimens prepared with this optimum gradation are
presented in Table 6. It is obvious that all VMA values were
more than 17% specified in Table 5.
The results of binder optimization showed that the amount of 7.1% for control/SBS/wkr-2 and 6.7% for mineral modified mixtures were considered as OBC (Table 7). The main reason for selecting the same OBC for control, SBS and wkr-2 modified mixtures is to make binder consistency be constant for all these mixtures.
Drain down: The results of the drain down test were presented in Fig.
6 which indicates that the drain down value for conventional mixture exceeded
the required specification. However, different additives were successful in
reducing the amount of binder drain down. For instance, different percents of
wkr-2 could decrease the amount of drain down in such a way that 15% wkr-2 was
more efficient than 5% of SBS. As could be expected, the capability of mineral
fiber was more than other additives which is in support of ideas developed by
Brown et al. (1997b). Therefore, based on the
results, there would not be any problems in using these additives in terms of
binder drain down.
|| Temperature-viscosity relationship for modified binders
|| Proposed limits and selected gradation of SMA-9.5
|| Results of selecting optimum binder content
Marshall test: As indicated in Fig. 7, all modified
mixtures had higher Marshall stability and less flow value than control mixtures
which clearly describes that different additives had positive effect on Marshall
properties of SMA mixtures. Among various SMA mixtures, those modified with
SBS indicated the best performance with the highest values of stability and
quotient (6.56 and 2.02, respectively) and the least value of flow (3.24). Based
on the results, 10% of wkr-2 showed the best performance in Marshall test considering
the results of different percents of wkr-2. The obtained results clearly endorse
the previous researches on positive effect of SBS in increasing the Marshall
properties of SMA mixtures investigated by Al-Hadidy and
Yi-qiu (2009c). In addition, mineral fiber could improve the performance
of SMA mixtures but this improvement was insignificant.
Moisture sensitivity: As could be seen in Table 8,
mineral fiber could relatively increase TSR value of control mixture by 4%.
|| Results of drain down test
Meanwhile, different percents of wkr-2 were also efficient in decreasing the
moisture susceptibility of SMA mixtures so that the effect of 10 and 15% of
this additive was more than 5% of SBS. The important point in this section is
that although the ITSdry value for 10% wkr-2 modified mixture was
more than that of 15% modified one, the TSR value was more for 15% wkr-2 modified
mixture compared to the 10% modified mixture.
|| Results of marshall test
||Relationship between Percents of wkr-2 and ITS/TSR values
of SMA Mixture
|| Moisture susceptibility results of SMA mixtures
This could be because of the highly decreased aging of the wkr-2 modified binder
compared with the SBS modified binder. All in all, the values of TSR for all
constructed specimens were above the desired requirement. This fact could explicitly
indicate that there would not be any problem in using these mixtures in terms
of moisture sensitivity; however, some of these mixtures were more resistant
to the detrimental effect of the moisture.
To evaluate the variation of ITS and TSR values with the percent of wkr-2, a binomial trend line was considered with the corresponding equation and accuracy using the SPSS program. It could be inferred from Fig. 8 that there was a very good relationship between the ITS or TSR parameters and percents of wkr-2. The R2 values for each predicted equation were above 0.98 which could clearly show that these equations are significantly accurate.
Dynamic creep test: Figure 9 presents the variation
of accumulated strain versus the number of cycles for all constructed SMA specimens.
|| Dynamic creep curves for SMA specimens
||Flow Numbers corresponding to different SMA mixtures
As could be expected, unmodified SMA mixture has the least resistance to rutting.
Mineral fiber had a positive effect in increasing the rutting performance of
SMA mixtures; however, this effect does not seem to be significant as expected
based on the study previously performed by Brown and Manglorkar
(1993). In addition, it can be inferred from Fig. 9 that
among wkr-2 modified mixtures, those modified with 10% had more stability to
resist permanent deformation. Although, the effect of wkr-2 was considerable
in terms of permanent deformation, SBS was more effective in decreasing the
accumulated strain of asphalt mixture. The better capability of SBS compared
to the mineral fiber to reduce the rutting potential of SMA mixtures has been
antecedently proven by Brown et al. (1997b) and
Tayfur et al. (2007).
The point at which each creep curve enters to its tertiary zone was considered
as Flow Point and the corresponding number of cycles was regarded as Flow Number
(FN). Flow number values for different types of SMA mixtures were obtained from
Fig. 9 and are presented in Fig. 10. As
presented in Fig. 10, the SBS modified mixture has the service
life about 6 times more than unmodified mixture in terms of rutting resistance
which could clearly represent the efficiency of SBS in this way.
|| Results of statistical analysis
Moreover, 10% of wkr-2 can increase the rutting life of asphalt mixture more
than 5 times compared to the unmodified mixture.
Statistical analysis of performance tests results: In order to correlate the results of performance tests, linear regression was used. The accuracy of presented equations was verified by the R2 coefficient and significant (p, 2-tailed). For each test, three identical samples were used. Each point in Fig. 11 represents the average of those three samples. According to Fig. 11, it was observed that there are very good correlations between different mixture tests that could illustrate that it is possible to predict the results of each test from the others. The R2 values for all established correlations were higher than 0.97 which indicate that the predicted equations are accurate enough. Moreover, the calculated p values for these correlations were less than 0.05 that could represent that the relationships were significant enough.
Based on this research, the following conclusions could be drawn:
||According to the results of Binder properties tests, it could
be observed that Rheofalt® wkr-2 had positive effect on increasing
the softening point and decreasing the penetration of asphalt binder so
that the effect of 15% Rheofalt® wkr-2 was more significant
than that of 5% SBS. In addition, both SBS and Rheofalt®
wkr-2 were capable of reducing the temperature sensitivity of asphalt cement;
however, the effect of 15% Rheofalt® wkr-2 was more significant
||Rheofalt® wkr-2 needed more temperature to
reach the required viscosity range compared to the unmodified binder which
clearly indicate that the initial cost of asphalt concrete production may
increase when using Rheofalt® wkr-2 to modify SMA mixture
||Lower and middle limit of gradation recommended by NCHRP Report
No. 425 are not suitable for constructing SMA mixtures with these used materials
(aggregate and asphalt binder), because they resulted SMA mixtures at high
air void percent even with 6.2% of binder. Also, upper limit did not meet
the VCA requirement. Therefore, the final gradation was selected between
upper and middle limits to reach the desired criteria
||All additives were efficient in reducing the drain down to
an allowable level. As expected, mineral fiber was more effective in this
way. Different percents of Rheofalt® wkr-2 were also capable
of enhancing the mixture in terms of drain down
||Although, the results of Marshall test for SMA mixtures have
been usually considered as unreliable data, it was observed that there were
good correlations between different performance properties of SMA mixture
and Marshall test results. This fact could explicitly describe that Marshall
test should be regarded as a valid test to compare different types of SMA
mixtures but the achieved data would not represent the real performance
of these mixtures
||By using Rheofalt® wkr-2 in SMA mixtures, moisture
susceptibility of these mixtures improved greatly so that this type of additive
could considered as a better modifier compared to SBS in terms of moisture
sensitivity; however, the values of ITS (in both dry and wet conditions)
for SBS modified mixtures were higher than those for Rheofalt®
wkr-2 modified mixtures. As a result, Rheofalt® wkr-2 could
be an appropriate replacement for typical additives in regions with high
amount of rainfall
||Based on the results of dynamic creep test, mineral fiber
had insignificant effect in decreasing the permanent deformation of SMA
mixture. In contrast, SBS was superior in improving the rutting performance
of SMA mixtures. Among various percents of Rheofalt® wkr-2,
10% of it could be the best dosage in terms of rutting. It seems that higher
percents of Rheofalt® wkr-2 decrease the integrity of asphalt
||Significant correlations were found between different properties
of SMA mixtures. It could be concluded that it would be possible to estimate
different performance properties of SMA mixtures from each other