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Comparing the effects of nano-silica and hydrated lime on the properties of asphalt concrete
Construction and Building Materials 218 (2019) 308–315
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Comparing the effects of nano-silica and hydrated lime on the properties of asphalt concrete Hasan Taherkhani a,⇑, Milad Tajdini b,⇑ a b
Civil Engineering Department, University of Zanjan, 45371-38791 Zanjan, Iran Civil Engineering Department, University of Tabriz, Tabriz, Iran
h i g h l i g h t s Asphalt concrete has been modified using dosages of hydrated lime and nano-silica. Additives improved tensile strength, and resistance against fatigue and freeze-thaw. More improvement was achieved using nano-silica than hydrated lime. More resilient modulus was achieved using nano-silica than hydrated lime.
a r t i c l e
i n f o
Article history: Received 13 October 2018 Received in revised form 9 May 2019 Accepted 16 May 2019 Available online 24 May 2019 Keywords: Asphalt concrete Nano-silica Hydrated lime Freeze-thaw cycles Resilient modulus Fatigue cracking
a b s t r a c t In this study, the effects of nano-silica (NS) and hydrated lime (HL) were investigated on some engineering properties of asphalt concrete. The unconditioned indirect tensile strength (ITS) and that after subjecting to one, three and five freeze-thaw cycles; resilient modulus at 5, 25, and 45 °C; and stresscontrolled fatigue cracking behavior of the mixtures modified by different percentages of each additive were examined. Results showed that the both additives improved the ITS of the mixture. The ITS of the mixture modified by 6% of NS was approximately 10% higher than that of the mixture containing 2.5% of HL. However, NS was more effective than lime in improving the resistance against freeze-thaw cycles. Moreover, it was shown that 20 and 26.7% of ITS was lost after five freeze-thaw cycles in the mixtures containing 6% of NS and 2.5% of HL, respectively. The both additives improved the resilient modulus and resistance against the fatigue cracking of the mixture with a slightly higher improvement for NS. The fatigue life of the mixture modified by 6% of NS was approximately 5.8% higher than that of the mixture containing 2.5% of HL. The results also showed that, in general, the temperature sensitivity of the resilient modulus of the mixtures modified with lime and NS was lower than that of the control mixture, with higher sensitivity for the mixtures modified with NS. Moreover, the sensitivity decreased with increasing the modifier content and decreasing the temperature. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Nowadays, due to the rapid increase of traffic volume, heavier and larger vehicles, new configuration of tires, use of wide-base tires, and increased tire pressures, it is a common practice to modify asphaltic mixtures to improve their performance. In addition, new technologies and achievements in understanding the behavior of materials have enabled researchers to study the effects of different modifiers on asphaltic mixtures [34]. Different types of additive, including waste materials, materials found in nature or ⇑ Corresponding authors. E-mail addresses: [email protected] (H. Taherkhani), [email protected] (M. Tajdini). https://doi.org/10.1016/j.conbuildmat.2019.05.116 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
produced by industry, and carefully engineered products, have been used for the modification of asphaltic mixtures [34]. Some examples of the most-commonly used modifiers are fillers, waste tires, fibers, polymers, and extenders [25]. Choosing the most appropriate additive requires the consideration of the extent of the effects the additive on the desired properties, economical evaluation, and impacts on the environment. Stiffness, fatigue behavior, and durability are among the main engineering properties of asphaltic mixtures. The resilient modulus of asphaltic mixtures is related to their ability to distribute traffic load on the underlying layers, resistance against fatigue, thermal cracking and rutting. Fatigue behavior is important as it is related to the occurrence of alligator cracking in asphaltic pavements. The durability of asphaltic mixtures is the resistance against the detri-
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mental effects of environmental conditions such as moisture and cycles of freeze and thaw. The damage caused by moisture and freeze-thaw is a common distress in asphaltic pavements. Moisture may cause weakness of the bond between the binder and aggregate particles and separation of aggregate particles from the binder. This distress is generally known as stripping which, in its critical state, is manifested by potholes on the surface of the pavement and is hazardous for traffic. Freeze-thaw cycles accelerate the moisture damage. Studies have shown that, on average, moisture damage occurs after three to four years from construction, and in some cases, in the first year, in unmodified asphaltic mixtures [13]. This type of distress can be overcome by the use of antistripping agents, used in filler or liquid forms. For more than four decades, hydrated lime (HL) has been a wellknown material for improving the resistance against moisture and frost damage of asphaltic mixtures. The decrease of asphalt quality, as a result of the oil crisis in 1970s, made the mixtures prone to moisture and frost damage, and the modification of asphaltic mixtures by HL improved the performance of the mixtures. HL is composed of calcium hydroxide [Ca(OH)2] which is produced from quicklime by slaking it with water using a hydrator as a specific equipment. Different laboratory test methods and field surveys have shown that HL is equal to or better than additives such as chemical anti-stripping agents and cement. However, the difference between the effects of additives was not equally shown by different test methods. For example, when the retained Marshall stability is used, the difference between the additives is not remarkable. However, multiple freeze-thaw methods, e.g. repeated Lottman and Texas freeze-thaw pedestal, in addition to Hamburg wheel track tests are more differentiating test methods. The longtime use of HL in asphalt mixtures has revealed that the mechanical properties of asphaltic mixtures such as stiffness, resistance against cracking, rutting and ageing are improved [9,13,21,23,26]. Lesueur has extensively reviewed the studies conducted around the world concerning the lime effects on the properties of asphalt mixtures. State agencies in North America estimate that by using HL at the usual range of 1–2%, the durability of asphalt mixtures in highways increases by 25–50% [14]. The use of HL in the wearing course of motorways in France also extended the durability by 20– 25%. Due to the many benefits achieved by using HL in asphaltic mixtures, a large amount of this material is utilized for this application. In the United States alone, 40 million tons of asphaltic mixtures containing lime are produced every year. European countries started the application of HL later than the United States. However, European countries currently use a significant amount of asphalt mixtures containing HL [10] In recent years, new technologies, e.g. nanotechnology, have assisted pavement engineers in employing new additives for the modification of asphaltic mixtures. Nanotechnology involves manufacturing materials in the nano-scale with dimensions approximately within the range of 1–100 nm (nanometer) while they still retain characteristics comparable to the same materials in real size [7,35]. Nano-size materials have a more specific area, resulting in more atoms on the surface area of the particles, causing a remarkable change in the surface energies and surface morphologies of the whole material as well as an alteration in the physicochemical properties of the material [24,29]. Therefore, research engineers have been encouraged to use nano-materials into pavement materials. Some examples of materials used in pavement engineering in nano-scale are HL, polymerized or plastic powders, clay, silica, fibers and nanotubes [34]. Adding nano-materials into asphaltic mixtures results in the achievement of numerous benefits, such as increased storage stability by polymer modification of asphalt; improvement in resistance against moisture damage, permanent deformation, low-temperature and fatigue cracking;
and decreased ageing [31]. These benefits, in turn, lead to a decrease in maintenance cost. Nano-silica (NS) is a nano-material used for the modification of asphaltic materials. Low cost and high performance features are the main advantages of NS [19]. Yao et al. and Yang and Tighe found that the addition of NS into asphaltic mixtures improved their resistance against rutting and fatigue cracking [31,32]. Yousoff et al. [34] found that the NS modification of asphaltic mixtures increased the resistance against plastic deformation and fatigue cracking at moderate temperatures, elastic modulus and Marshall stability. The phase angle and dynamic shear modulus of an asphaltic binder decreases and increases, respectively, by NS modification [17]. Yousoff et al. found that the modification of asphaltic mixtures with NS increased the resistance against moisture damage, rutting, and fatigue cracking. They also found that oxidative ageing could be decreased by NS modification [34]. Enieb and Diab found that the NS modification of asphalt increased the stiffness, tensile strength and resistance against fatigue cracking, plastic deformation and moisture damage [8]. This study was aimed to compare the effects of a traditional asphalt mixture modifier, i.e. HL, with a new nano-technology by using a product modifier, i.e. NS, on some of the engineering properties of asphalt concrete. The former is cheaper and is used as a fraction of aggregate filler, and the latter is more expensive and is mixed with a binder. This study focused to compare these materials in terms of resistance against moisture damage after different freeze-thaw cycles, resilient modulus, temperature sensitivity, and resistance against fatigue cracking, which were not studied before. The results of this research also extended the knowledge on the properties of asphalt mixtures modified by each additive. 2. Research methodology 2.1. Materials Four different materials, including an asphaltic binder, an aggregate, NS, and HL, were used in this study. A 60/70 penetration-grade asphalt cement, produced by Pasargard Oil Company in Iran, was utilized for the fabrication of laboratory specimens. Some properties of the asphalt cement are presented in Table 1. The source of the aggregates was the Haftjooy quarry in Southwest Tehran, Iran, collected from a local asphalt plant in Alborz Province, Iran. In order to control the requirements of specification and obtain the required data to be used in the mix design, some preliminary tests were performed on coarse, fine, and filler fractions. The properties of the aggregates are presented in Table 2. X-ray fluorescence was utilized for determining the compositions of the coarse, fine, and filler fractions of the aggregates. The results are depicted in Table 3. As observed in Table 3, the silicate content
Table 1 Properties of the asphalt cement used in this study. Test
Standard
Results
Density in 15 °C (gr/cm3) Penetration in 25 °C (0.1 mm) Softening Point (°C) Penetration Index (PI) Ductility in 25 °C (cm) Solubility in Trichloroethylene (%) Flash Point (°C) Loss in weight after thin film oven test (%) Penetration after thin film oven test Ductility after thin film oven test (cm) Viscosity at 120 °C (centistokes) Viscosity at 135 °C (centistokes) Viscosity at 160 °C (centistokes)
ASTM-D70 ASTM-D5 ASTM-D36 – ASTM-D113 ASTM-D2042 ASTM-D92 ASTM-D1754 – – ASTM-D2170 ASTM-D2170 ASTM-D2170
1.018 64 50.5 0.48 ›100 99.8 299 0.03 44 ›100 576 326 127
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Table 2 Properties of aggregates used in this study. Properties
Standards
Materials
Sand Equivalent % Percentage of loss in Los Angeles test % Plasticity Index % Angularity in one and two sides % Moisture absorption % Density Flakiness % % Loss in Magnesium sulfate % %
ASTM D2419 ASTM C131 ASTM D4318 ASTM D5821 ASTM C127, 128, D854 ASTM C127, 128, D854 BS812 ASTM C88
Table 3 Compositions of aggregates according to XRF analysis. Minerals
Coarse aggregates
Fine aggregates
Filler
– 27.2 – 98.2/96.9 1.58 2.6 11 0.47
89 – N.P. – 2.25 2.6 – 0.69
– – N.P. – – 2.79 – –
study. The lime was sieved and the passing sieve #200 was utilized in the mixtures.
Aggregates
SiO2 % Al2O3 % Fe2O3 % CaO % MgO % Na2O % K2O % CO2 % Loss of Ignition %
Coarse aggregates
Fine aggregates
Filler
48.36 12.5 6.4 15.5 2.2 2.16 0.96 10.41 12.23
48.11 8.67 58 18.48 1.86 1.71 0.94 12.75 14.47
47.07 11.05 5.2 16.92 2.5 2.1 1.03 12.11 14/13
2.2. Research plan The effects of NS and HL on the properties of the mixtures were investigated by adding different concentrations of NS, namely, 2, 4, and 6% (by the weight of the binder) and HL, namely, 1.5, 2 and 2.5% (by the weight of the total aggregate) into the mixtures. The dosages were chosen according to the results of previous studies [8,34]. In total, seven mixture types were studied, denoted by C (the control mixture without additives), 2% NS, 4% NS, 6% NS, 1.5% L, 2% L, and 2.5% L. The numbers show the additive content: NS shows that the additive is NS and L shows HL. NS was added using the wet method, in which the desired content of NS was added to the binder and the modified binder, was used for mixing with the aggregates. The desired lime content was replaced to the same weight of the natural filler fraction of the aggregate. The properties chosen for study in this research were the ITS of the unconditioned mixtures and those being conditioned using one, three, and five freeze-thaw cycles as per the AASHTO T283 standard method; the resilient modulus at 5, 25, and 45 °C; and the controlled stress fatigue cracking test at 25 °C. Three specimens were utilized in each test condition. In total, 168 specimens were made in this study. 2.3. Mix design and fabrication of specimens
Fig. 1. Aggregate gradation of the mixtures.
of the aggregates was high and thus they were classified as siliceous aggregates. The gradation of the aggregates in the mixtures was selected from the specified bands in the Iranian Asphalt Pavements Code (Iran Highway Asphalt Paving [16] with a maximum aggregate size of 19 mm. The gradation of the aggregates and the specification band are shown in Fig. 1. NS used in this study was provided by Novin Fadak Company (Iran). Some of the physical and chemical properties of NS are shown in Table 4. HL used in this project was provided by Petrokimia Akam Co. (Iran). Table 5 demonstrates the properties of the lime used in this
The mixtures were designed according to the Marshall method following the ASTM D6927 standard. The optimum asphalt content of the control mixture (without additives) was found to be 5.3%. The specimens containing different concentrations of NS and HL were made using the same binder content of the control mixture. The volumetric properties of the modified mixtures were controlled to satisfy the requirements of specification. The NS modification of asphalt cement was performed by adding NS to the asphalt heated to 160 °C, and mixing by a high-shear mixer rotating at 4000 rpm for 40 min. By using scanning electron microscopy (SEM), the effectiveness of dispersing the NS particles into the binder was evaluated. Fig. 2 shows the SEM images of the unmodified binder and that containing 2% of NS. From the figure, it can be concluded that the mixing method was successful in dispersing the NS
Table 4 Properties of nano-silica. Chemical composition Composition Value
SiO2 (%) >99
Ti (ppm) <120
Ca (ppm) <70
Na (ppm) <40
Fe (ppm) <220
Bulk density (gr/cm3) 0.1>
Color White
SSA (m2/g) 10–40
Particle size (nm) 20
Purity (%) +99
Physical properties True density (gr/cm3) 2.85
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H. Taherkhani, M. Tajdini / Construction and Building Materials 218 (2019) 308–315 Table 5 Physical and chemical properties of the hydrated lime used in this study. Properties
Values
Retained on sieve 30#
Retained on sieve 200#
CaO content %
Non-hydrated CaO Content %
Co2 Content %
Moisture %
Density (gr/cm3)
0.15
0.45
94.5
5.15
3.15
0.72
2.17
Fig. 2. SEM images of (a) unmodified asphalt binder, 500_ magnification; (b)modified binder, 20,000 _ magnification.
ITS (kPa)
ITS-Dry 900 800 700 600 500 400 300 200 100 0
ITS-Dry 1 cycle 3 cycles 5 cycles
C 560 359 297 224
2%NS 612 465.1 429.6 401.47
1 cycle
4%NS 707 593.68 563.83 519.64
3 cycles
6%NS 765 680.85 650.25 612
2.4. Experiments
5 cycles
1.5%L 635 486.25 438.15 382.58
2%L 683 564.15 530.34 471.27
2.5%L 714 617.61 585.54 523
Mixtures
Fig. 3. Unconditioned and conditioned ITS test results.
particles into the binder. Moreover, Fig. 3 shows that the particles are well distributed in the binder as groups with different sizes. Table 6 presents some of the properties of the nano-silica modified binders. It was observed that the penetration grade, which reflected the stiffness of the binder at intermediate temperatures, and ductility, which was an indication of the binder cohesion, dropped with increasing the NS content. Moreover, the penetration index and softening point grew with increasing the content of NS, indicating that the sensitivity to temperature decreased by the NS modification.
2.4.1. ITS tests Tensile strength is related to the thermal and fatigue cracking and permanent deformation resistance of asphaltic mixtures [15]. The higher the tensile strength, the more is the resistance against rutting and cracking. Change in tensile strength under environmental conditions such as moisture is used for evaluation of the durability of asphalt mixtures. The mixtures were subjected to certain conditions, and ITS was measured and compared with that of the mixture which did not undergo the conditioning. The present study aimed to investigate the change in the ITS of the mixtures after undergoing different freeze-thaw cycles. In the AASHTO T283 standard method, an asphaltic mixture is subjected to only one freeze-thaw cycle. For each mixture type, 12 specimens were made with the air void content ranging from 6.5 to 7.5% and divided into four groups. The ITS of the unconditioned mixtures in Group 1 was measured after placing the specimens in a water tank set at 25 °C for 2 h. The specimens in Groups 2–4 underwent one, three and five freeze-thaw cycles, respectively. The ITS of the samples in the four groups was measured using a Marshall test set-up. By measuring the maximum vertical load required for specimen failure, ITS in kPa was estimated using Eq. (1) [3].
ITS ¼
2000F pHd
ð1Þ
Table 6 Properties of the unmodified and nano-silica modified binder. Nano-silica content %
0 1 3 5
Properties Penetration grade (1/10 mm)
Softening point (°C)
Penetration Index
Ductility (cm)
68.9 67.8 62.7 54.6
48.9 49 50.4 54.8
0.69 0.7 0.56 0.14
100 94.5 87 70.67
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where F is the maximum vertical load required for breaking the specimen in N, d is the diameter in mm and His the thickness of the specimen in mm. In each group, the average of ITS for the three specimens was calculated and used as the unconditioned or conditioned ITS of the mixture. The tensile strength ratio (TSR) of each mixture was calculated using Eq. (2).
ITSC TSR ¼ 100 ITSU
ð2Þ
where ITSC denotes the ITS of the mixtures conditioned under one freeze-thaw cycle and ITSU represents the ITS of the unconditioned specimens. 2.4.2. Resilient modulus tests Stiffness is an important property of asphaltic mixtures in pavement surface layers, which indicates how well it can spread the traffic load onto the underlying layers. Resilient modulus is usually used for characterising the stiffness of asphaltic mixtures, which is defined as the ratio of the deviator stress to the recoverable strain of a specimen under cyclic loading. By using a UTM 10 test set-up, the resilient modulus of the mixtures in this study was measured at different temperatures of 5, 25, and 45 °C, following the ASTM D4123 standard method. Cylindrical specimens with a thickness of, approximately, 40 mm and 100 mm in diameter were utilized in this test. The samples were fabricated by cutting Marshallcompacted specimens. For temperature conditioning, they were placed in the temperature-controlled cabinet of the test set-up, 4 h before commencing the test. Five pulses of the raised load were applied along the diameter. The load pulses were applied at a constant amplitude of, approximately, 15% of the load required for their breaking in the ITS test and frequency of 1 Hz (0.1 s of loading time and 0.9 s of rest time). The software connected to the test set up calculated the resilient modulus using Eq. (3) [1]. The average of the modulus for the five cycles was employed as that of each mixture.
Mr ¼ Fð0:27 þ tÞ=ðH DtÞ
ð3Þ
in which, F is the magnitude of the vertical load(N), t is the Poisson ratio, H is the sample thickness (mm), Dtis the horizontal recoverable deformation along the thickness (mm) and Mr is the resilient modulus (MPa). The Poisson’s ratio at different temperatures was utilized using Eq. (4) [30].
v ¼ 0:15 þ
0:35 1 þ eð3:18490:042T Þ
ð4Þ
2.4.3. Fatigue tests Fatigue tests were conducted on the mixture at 25 °C to compare the effects of additives on resistance against fatigue cracking. Three specimens of each mixture were tested and the average of the fatigue life of the specimens was used for analysis. An indirect tensile fatigue test was conducted on cylindrical specimens compacted by the Marshall method. Vertical dynamic load, which induces a uniform indirect tensile stress of 200 kPa, was applied along the diameter of the specimens using UTM-10. Sinusoidal
loading was utilized on the specimens at the frequency of 10 Hz without rest time. Fatigue life was recorded when the specimens failed and could no longer withstand the stress. The test conditions were chosen to compare different mixtures under the dynamic tensile stress at intermediate temperatures. The stress level of 200 kPa, frequency of 10 Hz and temperature of 25 °C are common to be used in fatigue tests. 3. Results and discussion 3.1. Results of indirect tensile strength Fig. 3 shows the ITS test results on the dry and conditioned mixtures. The results of the unconditioned mixtures showed that the ITS of the mixtures modified by HL and NS was higher than that of the control mixture, indicating that the resistance against cracking could be improved by the additives. However, in general, NS was more effective than HL. It is clear that the ITS of the mixture containing 6% of NS in the dry condition was89% higher than that of the control mixture, and that the mixture containing 2.5% of HL had an ITS 72% higher than that of the control mixture. The results also indicated that ITS increased with growing the additive content. The increase in ITS can be related to the stiffening effects of the additives on the binder. Both NS and HL resulted in the increased stiffness of the binder, making the mixture sustain more tensile stress before failure. The increase in stiffness due to NS modification is resulted from the interaction of a large number of nano-size particles and the binder. The particles absorbed the solvents of the binder, leading to the stiffening of the binder [27]. The stiffening effect of HL is due to the inclusion of a high volume of particles in the binder. HL had a high porosity which increased its volume in the binder, resulting in the increased stiffness of the binder [22]. Moreover, Fig. 3 shows the ITS of the mixtures modified with NS and HL after conditioning under one, three, and five freeze-thaw cycles. It is obvious that ITS decreased with increasing the number of freeze-thaw cycles. It can also be observed that the effect of the additives was higher on conditioned ITS than on dry ITS. For example, the dry ITS of the 6% NS mixture was approximately 90% higher than that of the control mixture. However, the ITS of the 6% NS after five freeze-thaw cycles was 173% higher than that of the control mixture after the same conditioning. Moreover, the dry ITS of the 2.5% L was 72% higher than that of the control mixture while ITS after five freeze-thaw cycles was 133% higher than that of the control mixture. These results indicated that the both additives were highly effective on the improvement of the mixture performance in detrimental moisture and freeze-thaw conditions. Nevertheless, NS was more effective than HL for improving resistance against moisture damage and freeze-thaw cycles. In order to investigate the decrease of ITS under different freeze-thaw cycles, the percentage of reduction in dry ITS after undergoing different freeze-thaw cycles was calculated for the mixtures, as shown in Table 7. As can be observed, the major reduction in ITS occurred after undergoing the first freeze-thaw cycle, after which the rate of reduction decreased. Moreover, the reduction of ITS was much higher for the control mixture than
Table 7 Percentage of loss in ITS after different freeze-thaw cycles. Number of Freeze-thaw cycles
1 3 5
Name of Mixture C
2%NS
4%NS
6%NS
1.5%L
2%L
2.5%L
36 46 60
24 30 34
16 20 26.5
11 15 20
23.4 31 39
17.4 22.3 31
13.5 18 26.7
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more prone to disintegration under the following freeze-thaw cycles than those modified with NS. 3.2. Resilient modulus tests results Fig. 5 depicts the resilient modulus of the mixtures modified by different percentages of NS and HL at the temperatures of 5, 25, and 45 °C. In order to obtain a higher reproducibility of results, all the values presented were the average of at least three specimens tested. It is clear that the resilient modulus increased with increasing the NS and HL content. The stiffening effect of HL is attributed to its higher inside porosity, compared with other mineral fillers [22]. Due to the high inside porosity of HL, the air voids in dry compacted state were about 65%, while they were 30–34% for the mineral filler [22]. Based on the theories in suspension rheology, the viscosity of a liquid increases with increasing the volume of the solid particles inside the liquid [5,20]. The volume of the solid particles of HL in the asphalt increases with increasing the HL content, as the air voids inside HL particles sum up to the porosity between particles. The stiffening effect of NS is related to its higher specific surface area. The high specific surface area results in more interaction with the asphalt binder and more absorption of the oils in the asphalt cement, leading to a higher stiffness [27,28]. It can also be observed that the mixtures modified with NS had higher resilient moduli than those modified with HL. Moreover, the difference between the stiffness of the mixtures containing NS and HL was higher at lower temperatures than at higher temperatures. The increased resilient modulus results in a decrease in asphalt layer thickness, reducing the pavement cost and environmental pollution. Moreover, the difference between the resilient modulus of the mixtures modified with NS and HL was smaller than the difference between their costs. For example, the resilient modulus of the mixture containing 6% of NS at moderate temperature (which is usually used for pavement design purposes) was only 0.5% higher than that of the mixture containing 2.5% of lime. However, the difference between the costs of these mixtures was much higher. Using the price of the additives, the percentages of increase in cost compared with the control mixture are presented in Table 8. As can be observed, the cost increased by 237% by using 6% of NS while increased by 2.51% by using 2.5% of lime. Therefore, using HL appears to be more economical than using NS. The improvement in durability may justify the use of NS in asphalt mixtures. Fig. 6 shows the variation of resilient modulus with temperature for the mixtures containing different concentrations of NS and HL. This figure was produced to compare the temperature sensitivity of the mixtures. It is clear that the resilient modulus of the control and modified mixtures decreased with increasing temperature. On the contrary, the rate of decrease, which is an indication of the temperature sensitivity of resilient modulus, varies with
Resilient Modulus (MPa)
for the modified mixtures. In addition, the decrease of ITS was lower in the mixtures modified with NS than in those modified with HL, indicating that NS is superior to the traditional additive of HL in conditions where mixtures experience exposure to moisture after repeated freeze-thaw cycles. The results also revealed that lower reduction in ITS was obtained when the additives content increased. Indirect TSR using one freeze-thaw cycle is used in many specifications for the moisture susceptibility of asphaltic mixtures. It is defined as the ratio of the ITS of the mixture conditioned using one freeze-thaw cycle, as described in Section 5.1, to the ITS of the unconditioned mixture. The Iranian Asphalt Pavement Code [16] requires a minimum TSR of 75% for asphalt concrete mixtures. Fig. 4 depicts the TSR of the mixtures. It is clear that the control mixture did not satisfy the requirement of specification because of using the hydrophilic siliceous type of aggregates in this mixture. Compared with limestone aggregates, siliceous aggregates have worst adhesive properties toward asphaltic binders [2,6,13,11]. In case of using limestone aggregates in asphaltic mixtures, both anionic and cationic surfactants which are naturally present in asphaltic binders strongly bond with calcium ions on the surface of aggregate particles. However, in siliceous aggregates, only cationic surfactants strongly bond with silica atoms and anionic surfactants are easily displaced by water [6], making siliceous aggregates hydrophilic. Nevertheless, all the modified mixtures were shown to have a TSR higher than that required by specification, indicating that the problem of moisture susceptibility of the control mixture can be resolved by any additive contents utilized in this research. Additionally, the results showed that TSR increased with increasing the additives content. Moreover, NS increased the stiffness of the binder through the interaction of nano-size particles with the binder, which can be the reason for the increase of resistance against moisture damage in mixtures containing NS. The adhesion of the binder to the aggregate surface is hardly lost at higher stiffness values of the binder [12]. The HL increases resistance against moisture damage in a different way. Two mechanisms of aggregate surface modification and binder modification through chemical interaction have been found to be effective in HL modification [22]. One of the effects of HL is that it allows the precipitation of calcium ions onto the aggregate surface. These ions bond with the acids from the asphaltic binder, which form water-insoluble salts and improve the adhesion of the binder and aggregate [22]. Binder modification by HL is induced by the neutralization of the acidic moieties of the binder by the basic using the solid particles of HL. The results also revealed that NS was slightly more effective than HL for improving resistance against moisture damage. However, as mentioned earlier, the mixtures modified with HL were
5°C
3000
25°C
45°C
2500 2000
1500 1000 500 0 5°C 25°C 45°C
C 1495 1119 708.8
2%NS 1817 1473 925.5
4%NS 2218 1845 1240.5
6%NS 2725 2293.7 1675.2
1.5%L 1842 1579 940.7
2%L 2167 1924 1313.3
2.5%L 2431 2199 1654.5
Mixtures Fig. 4. TSR of the mixtures.
Fig. 5. Resilient modulus of the nano-silica modified mixtures.
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Table 8 Percentage of increase in cost compared with the control mixture. Name of mixture
2%NS
4%NS
6%NS
1.5%L
2%L
2.5%L
Percent of increase in cost
79%
158%
237%
1.6%
2%
2.51%
Resilient Modulus (MPa)
2800 2300 1800 1300 800 300 0
10
20
30
40
50
Temperature (°C) Control
2%NS
4%NS
1.5%L
2%L
2.5%L
6%NS
Fig. 7. Fatigue life of the mixtures.
Fig. 6. Variation of the resilient modulus with temperature for the nano-silica modified mixtures.
modifier type, modifier content, and temperature range. While the resilient modulus of the control mixture decreased at a constant rate with increasing temperature from 5 to 45 °C, for the mixtures containing HL and NS, the rate of decrease depended on the range of temperature. When temperature increased from 5 to 25 °C, the rate was lower than when temperature increased from 25 to 45 °C. Furthermore, the rate decreased with increasing the HL and NS content. Table 9 shows the percentages of decrease in the resilient modulus of the mixtures after 20 °C of increase in temperatures (from 5 to 25 and from 25 to 45 °C). It can be observed that the sensitivity to temperature increases with increasing temperature and decreases with increasing HL and NS content. Moreover, the results showed that the NS modified mixtures were less sensitive to temperature than the HL modified mixtures. The increase of sensitivity to temperature with increasing temperature can be described by the mechanical contrast between asphalt-swollen HL and NS particles with the asphaltic matrix, as proposed by Lesueur et al. [22]. At lower temperatures, the contrast is not significant compared with that at higher temperatures, which explains the smaller difference between the resilient modulus at 5–25 °C and 25–40 °C.
also shows that fatigue life increased with increasing the NS and HL content. Previous studies [8,33] have also shown that NS inclusion improves resistance against fatigue cracking. Consistent with the findings of Enieb and Diab [8], the improvement at higher NS content was not significant. Moreover, most of the limited studies in the literature on the fatigue behavior of HL-modified asphaltic mixtures have shown that HL improves resistance against fatigue cracking [4,18,23].
3.4. Application in engineering From the results of this research, it is concluded that the ratio of the NS to HL cost is much higher than the ratio of performance indices of these mixtures. Therefore, authors of this paper recommend using HL rather than NS. Comparing the cost and improvement in the performance of the mixtures modified with different dosages of HL indicated that the use of 2.5% of HL was more economical than the use of 1.5 and 2% of HL. However, more research is needed to be conducted on the other performance properties of the mixtures for making an economic decision on selecting the appropriate additive and their content.
3.3. Fatigue tests results 4. Conclusions One of the main failure modes used for the structural design of pavements is fatigue cracking in asphaltic layers. Asphaltic layers must be sufficiently thick to ensure that fatigue cracks do not appear until the end of pavement design life. Fig. 7 illustrates the fatigue life of the mixtures under the constant stress of 200 kPa at 25 °C. As can be observed, the both additives increased resistance against fatigue cracking. However, NS was slightly superior to HL for enhancing resistance against fatigue cracking. The figure
In this study, an asphaltic concrete made of siliceous aggregate was modified with different percentages of NS and HL. Moreover, the ITS of the mixtures in dry condition and conditioned after subjecting to different freeze-thaw cycles as well as the resilient modulus at different temperatures was measured. The following are brief conclusions, which are valid over the range of additives and test conditions utilized in this study.
Table 9 Percentages of reduction in resilient modulus by increase of temperature. Rise in temperature (°C)
5–25 25–40
Mixture C
1.5%L
2%L
2.5%L
2%NS
4%NS
6%NS
25 36.6
14.2 40.4
11.2 31
9.5 24
19 37.1
16.8 32.2
15.8 27
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The both additives improved the ITS of the mixtures, with more improvement resulted by NS. Compared with the control mixture, ITS was improved by 89 and 72% by adding 6% of NS and 2.5% of HL, respectively. The highest loss in the ITS of the mixtures occurred after conditioning under the first freeze-thaw cycle, with the highest in the unmodified mixture, followed by the one modified with HL. The loss of ITS after conditioning decreased with increasing the percentage of the additives, with a lower loss for the NS modified mixtures. Compared with dry condition, 20% and 26.75% loss of ITS was obtained after five freeze-thaw cycles for mixtures containing 6% of NS and 2.5% of HL, respectively. Resistance against the moisture damage of the NS modified mixtures was higher than against that of the mixtures containing HL. The TSR values of 89 and 86.4% were obtained by adding 6% of NS and 2.5% of lime, respectively. The resilient modulus of asphalt concrete grew with increasing the additive content, with more resilient modulus for the nanomodified mixtures. The temperature sensitivity of the resilient modulus of the mixtures decreased with increasing the HL and NS content and decreasing temperature. The temperature sensitivity of the resilient modulus of the NS and HL-modified mixtures was lower than that of the control mixture, with a higher sensitivity for the mixture modified with NS. NS and HL improved the fatigue resistance of asphaltic mixtures, with a higher improvement for NS modification. The fatigue life of the mixtures containing 6% of NS and 2.5% of HLwas3.6 and 3.4 times, respectively, of the fatigue life of the control mixture. Analysis of cost showed that using HL was more economical than using NS for modification.
Declaration of Competing Interest None. References [1] ASTM D7369-11, (2011) ‘‘Standard Test Method for Determining the Resilient Modulus of Bituminous Mixtures by Indirect Tension Test,” ASTM International, West Conshohocken, PA. [2] U. Bagampadde, U. Isacsson, B.M. Kiggundu, Classical and contemporary aspects of stripping in bituminous mixes, Road Mater. Pavement Des. 5 (2004) 7–43. [3] F.L. Carneiro, A. Barcellos, Tensile strength of concrete, RILEM Bull. No 3, Int. Assoc. Test. Res. Lab. Mater. Struct. (1953) 97–123. [4] S.R. Chari, K. Jacob, Influence of lime and stone dust fillers on fatigue properties of bituminous concrete mixes, Highway Res. Bull. (New Delhi) (1984). [5] P. Coussot, Rheometry of Pastes, Suspensions and Granular Materials, Applications in Industry and Environment, John Wiley & Sons, 2005. [6] C.W. Curtis, K. Ensley, J. Epps, Fundamental Properties of Asphalt-Aggregate Interactions Including Adhesion and Absorption, National Research Council Washington, DC, USA, 1993. [7] A. Diab, Z. You, Z. Hossain, M. Zaman, Moisture susceptibility evaluation of nanosize hydrated lime-modified asphalt-aggregate systems based on surface free fnergy concept, Transp. Res. Rec.: J. Transp. Res. Board (2014) 52–59.
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[8] M. Enieb, A. Diab, Characteristics of asphalt binder and mixture containing nanosilica, Int. J. Pavement Res. Technol. 10 (2017) 148–157. [9] J. Epps, D. Little, The benefits of Hydrated Lime in Hot Mix Asphalt, National Lime Association, 2001. [10] EuLA (European Lime Association) (2010) Hydrated Lime: a Proven Additive for Durable Asphalt Pavements: Critical literature review.‘‘ Asphalt Task Force Report, September, 96. [11] J. Gronniger, M.P. Wistuba, P. Renken, Adhesion in bitumen-aggregatesystems: new technique for automated interpretation of rolling bottle tests, Road Mater. Pavement Des. 11 (2010) 881–898. [12] G.H. Hamedi, F. Moghadas Nejad, K. Oveisi, Investigating the effects of using nanomaterials on moisture damage of HMA, Road Mater. Pavement Des. 16 (2015) 536–552. [13] R.G. Hicks, Moisture Damage in Asphalt Concrete, Transportation Research Board, 1991. [14] R.G. Hicks, T.V. Scholz, Life Cycle Costs for Lime in Hot Mix Asphalt, Report & Software for National Lime Association, 2001. [15] B. Huang, Z. Zhang, W. Kingery, G. Zuo, Fatigue Crack Characteristics of HMA Mixtures Containing RAP, in: Proceeding 5th Int. Conf. on Cracking in Pavements, RILEM, 2004, pp. 631–638. [16] IHAP (IRAN HIGHWAY ASPHALT PAVING) CODE (2012). ‘‘Publication No 234”. management and planning Organization. [17] A. Jamshidi, M.R. Hasan, H. Yao, Z. You, M.O. Hamzah, Characterization of the rate of change of rheological properties of nano-modified asphalt, Constr. Build. Mater. 98 (2015) 437–446. [18] O.K. Kim, C.A. Bell, R. Hicks, The Effect of Moisture on the Performance of Asphalt Mixtures, Evaluation and Prevention of Water Damage to Asphalt Pavement Materials. ASTM International, 1985. [19] G. Lazzara, S. Miliioto, Dispersions of nanosilica in biocompatible copolymers, Polym. Degrad. Stab. 95 (2010) 610–617. [20] D. Lesueur, The colloidal structure of bitumen: consequences on the rheology and on the mechanisms of bitumen modification, Adv. Colloid Interface Sci. 145 (2009) 42–82. [21] D. Lesueur, Hydrated lime: A Proven Additive for Durable Asphalt Pavements– Critical Literature Review, European Lime Association (EuLA) Ed, Brussels, 2010. [22] D. Lesueur, J. Petit, H.J. Ritter, The mechanisms of hydrated lime modification of asphalt mixtures: a state-of-the-art review, Road Mater. Pavement Des. 14 (2013) 1–16. [23] D.N. Little, J.C. Petersen, Unique effects of hydrated lime filler on the performance-related properties of asphalt cements: physical and chemical interactions revisited, J. Mater. Civ. Eng. 17 (2005) 207–218. [24] C.P. Poole JR, F.J. Owens, Introduction to Nanotechnology, John Wiley & Sons, 2003. [25] J. Read, D. Whiteoak, The Shell Bitumen Handbook, Thomas Telford, 2003. [26] P. Sebaaly, D. Little, J. Epps, ‘‘The Benefits of Hydrated Lime in Hot Mix Asphalt, National Lime Association, Arlington (Virginia, USA), 2006. [27] H. Taherkhani, S. Afroozi, S. Javanmard, Comparative study of the effects of nano-silica and zycosoil nanomaterials on the properties of asphalt concrete, J. Mater. Civ. Eng. 29 (8) (2017) 04017054. [28] H. Taherkhani, S. Afroozi, The properties of nanosilica-modified asphalt cement, Pet. Sci. Technol. 34 (2016) 1381–1386. [29] R.W. Whatmore, J. Corbett, Nanotechnology in the marketplace, Comput. Control Eng. J. 6 (1995) 106. [30] M. Witczak, K. Kaloush, T. Pellinen, M. Von El-Basyouny, H. Quintus, Simple performance test for superpave mix design, National Cooperative Highway Res. Program. Rep. (2002) 465. [31] J. Yang, S. Tighe, A review of advances of nanotechnology in asphalt mixtures, Procedia-Soc. Behav. Sci. 96 (2013) 1269–1276. [32] H. Yao, Z. You, L. Li, S.W. Goh, C.H. Lee, Y.K. Yap, X. Shi, Rheological properties and chemical analysis of nanoclay and carbon microfiber modified asphalt with fourier transform infrared spectroscopy, Constr. Build. Mater. 38 (2013) 327–337. [33] H. Yao, Z. You, L. Li, C.H. Lee, D. Wingard, Y.K. Yap, X. Shi, S.W. Goh, Rheological properties and chemical bonding of asphalt modified with nanosilica, J. Mater. Civ. Eng. 25 (2012) 1619–1630. [34] N.I.M. Yusoff, A.A.S. Breem, H.N. Alattug, A. Hamim, J. Ahmad, The effects of moisture susceptibility and ageing conditions on nano-silica/polymermodified asphalt mixtures, Constr. Build. Mater. 72 (2014) 139–147. [35] W. Zhu, P.J. Bartos, A. Porro, Application of nanotechnology in construction, Mater. Struct. 37 (2004) 649–658.
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Comparing the effects of nano-silica and hydrated lime on the properties of asphalt concrete Hasan Taherkhani a,⇑, Milad Tajdini b,⇑ a b
Civil Engineering Department, University of Zanjan, 45371-38791 Zanjan, Iran Civil Engineering Department, University of Tabriz, Tabriz, Iran
h i g h l i g h t s Asphalt concrete has been modified using dosages of hydrated lime and nano-silica. Additives improved tensile strength, and resistance against fatigue and freeze-thaw. More improvement was achieved using nano-silica than hydrated lime. More resilient modulus was achieved using nano-silica than hydrated lime.
a r t i c l e
i n f o
Article history: Received 13 October 2018 Received in revised form 9 May 2019 Accepted 16 May 2019 Available online 24 May 2019 Keywords: Asphalt concrete Nano-silica Hydrated lime Freeze-thaw cycles Resilient modulus Fatigue cracking
a b s t r a c t In this study, the effects of nano-silica (NS) and hydrated lime (HL) were investigated on some engineering properties of asphalt concrete. The unconditioned indirect tensile strength (ITS) and that after subjecting to one, three and five freeze-thaw cycles; resilient modulus at 5, 25, and 45 °C; and stresscontrolled fatigue cracking behavior of the mixtures modified by different percentages of each additive were examined. Results showed that the both additives improved the ITS of the mixture. The ITS of the mixture modified by 6% of NS was approximately 10% higher than that of the mixture containing 2.5% of HL. However, NS was more effective than lime in improving the resistance against freeze-thaw cycles. Moreover, it was shown that 20 and 26.7% of ITS was lost after five freeze-thaw cycles in the mixtures containing 6% of NS and 2.5% of HL, respectively. The both additives improved the resilient modulus and resistance against the fatigue cracking of the mixture with a slightly higher improvement for NS. The fatigue life of the mixture modified by 6% of NS was approximately 5.8% higher than that of the mixture containing 2.5% of HL. The results also showed that, in general, the temperature sensitivity of the resilient modulus of the mixtures modified with lime and NS was lower than that of the control mixture, with higher sensitivity for the mixtures modified with NS. Moreover, the sensitivity decreased with increasing the modifier content and decreasing the temperature. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Nowadays, due to the rapid increase of traffic volume, heavier and larger vehicles, new configuration of tires, use of wide-base tires, and increased tire pressures, it is a common practice to modify asphaltic mixtures to improve their performance. In addition, new technologies and achievements in understanding the behavior of materials have enabled researchers to study the effects of different modifiers on asphaltic mixtures [34]. Different types of additive, including waste materials, materials found in nature or ⇑ Corresponding authors. E-mail addresses: [email protected] (H. Taherkhani), [email protected] (M. Tajdini). https://doi.org/10.1016/j.conbuildmat.2019.05.116 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
produced by industry, and carefully engineered products, have been used for the modification of asphaltic mixtures [34]. Some examples of the most-commonly used modifiers are fillers, waste tires, fibers, polymers, and extenders [25]. Choosing the most appropriate additive requires the consideration of the extent of the effects the additive on the desired properties, economical evaluation, and impacts on the environment. Stiffness, fatigue behavior, and durability are among the main engineering properties of asphaltic mixtures. The resilient modulus of asphaltic mixtures is related to their ability to distribute traffic load on the underlying layers, resistance against fatigue, thermal cracking and rutting. Fatigue behavior is important as it is related to the occurrence of alligator cracking in asphaltic pavements. The durability of asphaltic mixtures is the resistance against the detri-
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mental effects of environmental conditions such as moisture and cycles of freeze and thaw. The damage caused by moisture and freeze-thaw is a common distress in asphaltic pavements. Moisture may cause weakness of the bond between the binder and aggregate particles and separation of aggregate particles from the binder. This distress is generally known as stripping which, in its critical state, is manifested by potholes on the surface of the pavement and is hazardous for traffic. Freeze-thaw cycles accelerate the moisture damage. Studies have shown that, on average, moisture damage occurs after three to four years from construction, and in some cases, in the first year, in unmodified asphaltic mixtures [13]. This type of distress can be overcome by the use of antistripping agents, used in filler or liquid forms. For more than four decades, hydrated lime (HL) has been a wellknown material for improving the resistance against moisture and frost damage of asphaltic mixtures. The decrease of asphalt quality, as a result of the oil crisis in 1970s, made the mixtures prone to moisture and frost damage, and the modification of asphaltic mixtures by HL improved the performance of the mixtures. HL is composed of calcium hydroxide [Ca(OH)2] which is produced from quicklime by slaking it with water using a hydrator as a specific equipment. Different laboratory test methods and field surveys have shown that HL is equal to or better than additives such as chemical anti-stripping agents and cement. However, the difference between the effects of additives was not equally shown by different test methods. For example, when the retained Marshall stability is used, the difference between the additives is not remarkable. However, multiple freeze-thaw methods, e.g. repeated Lottman and Texas freeze-thaw pedestal, in addition to Hamburg wheel track tests are more differentiating test methods. The longtime use of HL in asphalt mixtures has revealed that the mechanical properties of asphaltic mixtures such as stiffness, resistance against cracking, rutting and ageing are improved [9,13,21,23,26]. Lesueur has extensively reviewed the studies conducted around the world concerning the lime effects on the properties of asphalt mixtures. State agencies in North America estimate that by using HL at the usual range of 1–2%, the durability of asphalt mixtures in highways increases by 25–50% [14]. The use of HL in the wearing course of motorways in France also extended the durability by 20– 25%. Due to the many benefits achieved by using HL in asphaltic mixtures, a large amount of this material is utilized for this application. In the United States alone, 40 million tons of asphaltic mixtures containing lime are produced every year. European countries started the application of HL later than the United States. However, European countries currently use a significant amount of asphalt mixtures containing HL [10] In recent years, new technologies, e.g. nanotechnology, have assisted pavement engineers in employing new additives for the modification of asphaltic mixtures. Nanotechnology involves manufacturing materials in the nano-scale with dimensions approximately within the range of 1–100 nm (nanometer) while they still retain characteristics comparable to the same materials in real size [7,35]. Nano-size materials have a more specific area, resulting in more atoms on the surface area of the particles, causing a remarkable change in the surface energies and surface morphologies of the whole material as well as an alteration in the physicochemical properties of the material [24,29]. Therefore, research engineers have been encouraged to use nano-materials into pavement materials. Some examples of materials used in pavement engineering in nano-scale are HL, polymerized or plastic powders, clay, silica, fibers and nanotubes [34]. Adding nano-materials into asphaltic mixtures results in the achievement of numerous benefits, such as increased storage stability by polymer modification of asphalt; improvement in resistance against moisture damage, permanent deformation, low-temperature and fatigue cracking;
and decreased ageing [31]. These benefits, in turn, lead to a decrease in maintenance cost. Nano-silica (NS) is a nano-material used for the modification of asphaltic materials. Low cost and high performance features are the main advantages of NS [19]. Yao et al. and Yang and Tighe found that the addition of NS into asphaltic mixtures improved their resistance against rutting and fatigue cracking [31,32]. Yousoff et al. [34] found that the NS modification of asphaltic mixtures increased the resistance against plastic deformation and fatigue cracking at moderate temperatures, elastic modulus and Marshall stability. The phase angle and dynamic shear modulus of an asphaltic binder decreases and increases, respectively, by NS modification [17]. Yousoff et al. found that the modification of asphaltic mixtures with NS increased the resistance against moisture damage, rutting, and fatigue cracking. They also found that oxidative ageing could be decreased by NS modification [34]. Enieb and Diab found that the NS modification of asphalt increased the stiffness, tensile strength and resistance against fatigue cracking, plastic deformation and moisture damage [8]. This study was aimed to compare the effects of a traditional asphalt mixture modifier, i.e. HL, with a new nano-technology by using a product modifier, i.e. NS, on some of the engineering properties of asphalt concrete. The former is cheaper and is used as a fraction of aggregate filler, and the latter is more expensive and is mixed with a binder. This study focused to compare these materials in terms of resistance against moisture damage after different freeze-thaw cycles, resilient modulus, temperature sensitivity, and resistance against fatigue cracking, which were not studied before. The results of this research also extended the knowledge on the properties of asphalt mixtures modified by each additive. 2. Research methodology 2.1. Materials Four different materials, including an asphaltic binder, an aggregate, NS, and HL, were used in this study. A 60/70 penetration-grade asphalt cement, produced by Pasargard Oil Company in Iran, was utilized for the fabrication of laboratory specimens. Some properties of the asphalt cement are presented in Table 1. The source of the aggregates was the Haftjooy quarry in Southwest Tehran, Iran, collected from a local asphalt plant in Alborz Province, Iran. In order to control the requirements of specification and obtain the required data to be used in the mix design, some preliminary tests were performed on coarse, fine, and filler fractions. The properties of the aggregates are presented in Table 2. X-ray fluorescence was utilized for determining the compositions of the coarse, fine, and filler fractions of the aggregates. The results are depicted in Table 3. As observed in Table 3, the silicate content
Table 1 Properties of the asphalt cement used in this study. Test
Standard
Results
Density in 15 °C (gr/cm3) Penetration in 25 °C (0.1 mm) Softening Point (°C) Penetration Index (PI) Ductility in 25 °C (cm) Solubility in Trichloroethylene (%) Flash Point (°C) Loss in weight after thin film oven test (%) Penetration after thin film oven test Ductility after thin film oven test (cm) Viscosity at 120 °C (centistokes) Viscosity at 135 °C (centistokes) Viscosity at 160 °C (centistokes)
ASTM-D70 ASTM-D5 ASTM-D36 – ASTM-D113 ASTM-D2042 ASTM-D92 ASTM-D1754 – – ASTM-D2170 ASTM-D2170 ASTM-D2170
1.018 64 50.5 0.48 ›100 99.8 299 0.03 44 ›100 576 326 127
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Table 2 Properties of aggregates used in this study. Properties
Standards
Materials
Sand Equivalent % Percentage of loss in Los Angeles test % Plasticity Index % Angularity in one and two sides % Moisture absorption % Density Flakiness % % Loss in Magnesium sulfate % %
ASTM D2419 ASTM C131 ASTM D4318 ASTM D5821 ASTM C127, 128, D854 ASTM C127, 128, D854 BS812 ASTM C88
Table 3 Compositions of aggregates according to XRF analysis. Minerals
Coarse aggregates
Fine aggregates
Filler
– 27.2 – 98.2/96.9 1.58 2.6 11 0.47
89 – N.P. – 2.25 2.6 – 0.69
– – N.P. – – 2.79 – –
study. The lime was sieved and the passing sieve #200 was utilized in the mixtures.
Aggregates
SiO2 % Al2O3 % Fe2O3 % CaO % MgO % Na2O % K2O % CO2 % Loss of Ignition %
Coarse aggregates
Fine aggregates
Filler
48.36 12.5 6.4 15.5 2.2 2.16 0.96 10.41 12.23
48.11 8.67 58 18.48 1.86 1.71 0.94 12.75 14.47
47.07 11.05 5.2 16.92 2.5 2.1 1.03 12.11 14/13
2.2. Research plan The effects of NS and HL on the properties of the mixtures were investigated by adding different concentrations of NS, namely, 2, 4, and 6% (by the weight of the binder) and HL, namely, 1.5, 2 and 2.5% (by the weight of the total aggregate) into the mixtures. The dosages were chosen according to the results of previous studies [8,34]. In total, seven mixture types were studied, denoted by C (the control mixture without additives), 2% NS, 4% NS, 6% NS, 1.5% L, 2% L, and 2.5% L. The numbers show the additive content: NS shows that the additive is NS and L shows HL. NS was added using the wet method, in which the desired content of NS was added to the binder and the modified binder, was used for mixing with the aggregates. The desired lime content was replaced to the same weight of the natural filler fraction of the aggregate. The properties chosen for study in this research were the ITS of the unconditioned mixtures and those being conditioned using one, three, and five freeze-thaw cycles as per the AASHTO T283 standard method; the resilient modulus at 5, 25, and 45 °C; and the controlled stress fatigue cracking test at 25 °C. Three specimens were utilized in each test condition. In total, 168 specimens were made in this study. 2.3. Mix design and fabrication of specimens
Fig. 1. Aggregate gradation of the mixtures.
of the aggregates was high and thus they were classified as siliceous aggregates. The gradation of the aggregates in the mixtures was selected from the specified bands in the Iranian Asphalt Pavements Code (Iran Highway Asphalt Paving [16] with a maximum aggregate size of 19 mm. The gradation of the aggregates and the specification band are shown in Fig. 1. NS used in this study was provided by Novin Fadak Company (Iran). Some of the physical and chemical properties of NS are shown in Table 4. HL used in this project was provided by Petrokimia Akam Co. (Iran). Table 5 demonstrates the properties of the lime used in this
The mixtures were designed according to the Marshall method following the ASTM D6927 standard. The optimum asphalt content of the control mixture (without additives) was found to be 5.3%. The specimens containing different concentrations of NS and HL were made using the same binder content of the control mixture. The volumetric properties of the modified mixtures were controlled to satisfy the requirements of specification. The NS modification of asphalt cement was performed by adding NS to the asphalt heated to 160 °C, and mixing by a high-shear mixer rotating at 4000 rpm for 40 min. By using scanning electron microscopy (SEM), the effectiveness of dispersing the NS particles into the binder was evaluated. Fig. 2 shows the SEM images of the unmodified binder and that containing 2% of NS. From the figure, it can be concluded that the mixing method was successful in dispersing the NS
Table 4 Properties of nano-silica. Chemical composition Composition Value
SiO2 (%) >99
Ti (ppm) <120
Ca (ppm) <70
Na (ppm) <40
Fe (ppm) <220
Bulk density (gr/cm3) 0.1>
Color White
SSA (m2/g) 10–40
Particle size (nm) 20
Purity (%) +99
Physical properties True density (gr/cm3) 2.85
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H. Taherkhani, M. Tajdini / Construction and Building Materials 218 (2019) 308–315 Table 5 Physical and chemical properties of the hydrated lime used in this study. Properties
Values
Retained on sieve 30#
Retained on sieve 200#
CaO content %
Non-hydrated CaO Content %
Co2 Content %
Moisture %
Density (gr/cm3)
0.15
0.45
94.5
5.15
3.15
0.72
2.17
Fig. 2. SEM images of (a) unmodified asphalt binder, 500_ magnification; (b)modified binder, 20,000 _ magnification.
ITS (kPa)
ITS-Dry 900 800 700 600 500 400 300 200 100 0
ITS-Dry 1 cycle 3 cycles 5 cycles
C 560 359 297 224
2%NS 612 465.1 429.6 401.47
1 cycle
4%NS 707 593.68 563.83 519.64
3 cycles
6%NS 765 680.85 650.25 612
2.4. Experiments
5 cycles
1.5%L 635 486.25 438.15 382.58
2%L 683 564.15 530.34 471.27
2.5%L 714 617.61 585.54 523
Mixtures
Fig. 3. Unconditioned and conditioned ITS test results.
particles into the binder. Moreover, Fig. 3 shows that the particles are well distributed in the binder as groups with different sizes. Table 6 presents some of the properties of the nano-silica modified binders. It was observed that the penetration grade, which reflected the stiffness of the binder at intermediate temperatures, and ductility, which was an indication of the binder cohesion, dropped with increasing the NS content. Moreover, the penetration index and softening point grew with increasing the content of NS, indicating that the sensitivity to temperature decreased by the NS modification.
2.4.1. ITS tests Tensile strength is related to the thermal and fatigue cracking and permanent deformation resistance of asphaltic mixtures [15]. The higher the tensile strength, the more is the resistance against rutting and cracking. Change in tensile strength under environmental conditions such as moisture is used for evaluation of the durability of asphalt mixtures. The mixtures were subjected to certain conditions, and ITS was measured and compared with that of the mixture which did not undergo the conditioning. The present study aimed to investigate the change in the ITS of the mixtures after undergoing different freeze-thaw cycles. In the AASHTO T283 standard method, an asphaltic mixture is subjected to only one freeze-thaw cycle. For each mixture type, 12 specimens were made with the air void content ranging from 6.5 to 7.5% and divided into four groups. The ITS of the unconditioned mixtures in Group 1 was measured after placing the specimens in a water tank set at 25 °C for 2 h. The specimens in Groups 2–4 underwent one, three and five freeze-thaw cycles, respectively. The ITS of the samples in the four groups was measured using a Marshall test set-up. By measuring the maximum vertical load required for specimen failure, ITS in kPa was estimated using Eq. (1) [3].
ITS ¼
2000F pHd
ð1Þ
Table 6 Properties of the unmodified and nano-silica modified binder. Nano-silica content %
0 1 3 5
Properties Penetration grade (1/10 mm)
Softening point (°C)
Penetration Index
Ductility (cm)
68.9 67.8 62.7 54.6
48.9 49 50.4 54.8
0.69 0.7 0.56 0.14
100 94.5 87 70.67
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where F is the maximum vertical load required for breaking the specimen in N, d is the diameter in mm and His the thickness of the specimen in mm. In each group, the average of ITS for the three specimens was calculated and used as the unconditioned or conditioned ITS of the mixture. The tensile strength ratio (TSR) of each mixture was calculated using Eq. (2).
ITSC TSR ¼ 100 ITSU
ð2Þ
where ITSC denotes the ITS of the mixtures conditioned under one freeze-thaw cycle and ITSU represents the ITS of the unconditioned specimens. 2.4.2. Resilient modulus tests Stiffness is an important property of asphaltic mixtures in pavement surface layers, which indicates how well it can spread the traffic load onto the underlying layers. Resilient modulus is usually used for characterising the stiffness of asphaltic mixtures, which is defined as the ratio of the deviator stress to the recoverable strain of a specimen under cyclic loading. By using a UTM 10 test set-up, the resilient modulus of the mixtures in this study was measured at different temperatures of 5, 25, and 45 °C, following the ASTM D4123 standard method. Cylindrical specimens with a thickness of, approximately, 40 mm and 100 mm in diameter were utilized in this test. The samples were fabricated by cutting Marshallcompacted specimens. For temperature conditioning, they were placed in the temperature-controlled cabinet of the test set-up, 4 h before commencing the test. Five pulses of the raised load were applied along the diameter. The load pulses were applied at a constant amplitude of, approximately, 15% of the load required for their breaking in the ITS test and frequency of 1 Hz (0.1 s of loading time and 0.9 s of rest time). The software connected to the test set up calculated the resilient modulus using Eq. (3) [1]. The average of the modulus for the five cycles was employed as that of each mixture.
Mr ¼ Fð0:27 þ tÞ=ðH DtÞ
ð3Þ
in which, F is the magnitude of the vertical load(N), t is the Poisson ratio, H is the sample thickness (mm), Dtis the horizontal recoverable deformation along the thickness (mm) and Mr is the resilient modulus (MPa). The Poisson’s ratio at different temperatures was utilized using Eq. (4) [30].
v ¼ 0:15 þ
0:35 1 þ eð3:18490:042T Þ
ð4Þ
2.4.3. Fatigue tests Fatigue tests were conducted on the mixture at 25 °C to compare the effects of additives on resistance against fatigue cracking. Three specimens of each mixture were tested and the average of the fatigue life of the specimens was used for analysis. An indirect tensile fatigue test was conducted on cylindrical specimens compacted by the Marshall method. Vertical dynamic load, which induces a uniform indirect tensile stress of 200 kPa, was applied along the diameter of the specimens using UTM-10. Sinusoidal
loading was utilized on the specimens at the frequency of 10 Hz without rest time. Fatigue life was recorded when the specimens failed and could no longer withstand the stress. The test conditions were chosen to compare different mixtures under the dynamic tensile stress at intermediate temperatures. The stress level of 200 kPa, frequency of 10 Hz and temperature of 25 °C are common to be used in fatigue tests. 3. Results and discussion 3.1. Results of indirect tensile strength Fig. 3 shows the ITS test results on the dry and conditioned mixtures. The results of the unconditioned mixtures showed that the ITS of the mixtures modified by HL and NS was higher than that of the control mixture, indicating that the resistance against cracking could be improved by the additives. However, in general, NS was more effective than HL. It is clear that the ITS of the mixture containing 6% of NS in the dry condition was89% higher than that of the control mixture, and that the mixture containing 2.5% of HL had an ITS 72% higher than that of the control mixture. The results also indicated that ITS increased with growing the additive content. The increase in ITS can be related to the stiffening effects of the additives on the binder. Both NS and HL resulted in the increased stiffness of the binder, making the mixture sustain more tensile stress before failure. The increase in stiffness due to NS modification is resulted from the interaction of a large number of nano-size particles and the binder. The particles absorbed the solvents of the binder, leading to the stiffening of the binder [27]. The stiffening effect of HL is due to the inclusion of a high volume of particles in the binder. HL had a high porosity which increased its volume in the binder, resulting in the increased stiffness of the binder [22]. Moreover, Fig. 3 shows the ITS of the mixtures modified with NS and HL after conditioning under one, three, and five freeze-thaw cycles. It is obvious that ITS decreased with increasing the number of freeze-thaw cycles. It can also be observed that the effect of the additives was higher on conditioned ITS than on dry ITS. For example, the dry ITS of the 6% NS mixture was approximately 90% higher than that of the control mixture. However, the ITS of the 6% NS after five freeze-thaw cycles was 173% higher than that of the control mixture after the same conditioning. Moreover, the dry ITS of the 2.5% L was 72% higher than that of the control mixture while ITS after five freeze-thaw cycles was 133% higher than that of the control mixture. These results indicated that the both additives were highly effective on the improvement of the mixture performance in detrimental moisture and freeze-thaw conditions. Nevertheless, NS was more effective than HL for improving resistance against moisture damage and freeze-thaw cycles. In order to investigate the decrease of ITS under different freeze-thaw cycles, the percentage of reduction in dry ITS after undergoing different freeze-thaw cycles was calculated for the mixtures, as shown in Table 7. As can be observed, the major reduction in ITS occurred after undergoing the first freeze-thaw cycle, after which the rate of reduction decreased. Moreover, the reduction of ITS was much higher for the control mixture than
Table 7 Percentage of loss in ITS after different freeze-thaw cycles. Number of Freeze-thaw cycles
1 3 5
Name of Mixture C
2%NS
4%NS
6%NS
1.5%L
2%L
2.5%L
36 46 60
24 30 34
16 20 26.5
11 15 20
23.4 31 39
17.4 22.3 31
13.5 18 26.7
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more prone to disintegration under the following freeze-thaw cycles than those modified with NS. 3.2. Resilient modulus tests results Fig. 5 depicts the resilient modulus of the mixtures modified by different percentages of NS and HL at the temperatures of 5, 25, and 45 °C. In order to obtain a higher reproducibility of results, all the values presented were the average of at least three specimens tested. It is clear that the resilient modulus increased with increasing the NS and HL content. The stiffening effect of HL is attributed to its higher inside porosity, compared with other mineral fillers [22]. Due to the high inside porosity of HL, the air voids in dry compacted state were about 65%, while they were 30–34% for the mineral filler [22]. Based on the theories in suspension rheology, the viscosity of a liquid increases with increasing the volume of the solid particles inside the liquid [5,20]. The volume of the solid particles of HL in the asphalt increases with increasing the HL content, as the air voids inside HL particles sum up to the porosity between particles. The stiffening effect of NS is related to its higher specific surface area. The high specific surface area results in more interaction with the asphalt binder and more absorption of the oils in the asphalt cement, leading to a higher stiffness [27,28]. It can also be observed that the mixtures modified with NS had higher resilient moduli than those modified with HL. Moreover, the difference between the stiffness of the mixtures containing NS and HL was higher at lower temperatures than at higher temperatures. The increased resilient modulus results in a decrease in asphalt layer thickness, reducing the pavement cost and environmental pollution. Moreover, the difference between the resilient modulus of the mixtures modified with NS and HL was smaller than the difference between their costs. For example, the resilient modulus of the mixture containing 6% of NS at moderate temperature (which is usually used for pavement design purposes) was only 0.5% higher than that of the mixture containing 2.5% of lime. However, the difference between the costs of these mixtures was much higher. Using the price of the additives, the percentages of increase in cost compared with the control mixture are presented in Table 8. As can be observed, the cost increased by 237% by using 6% of NS while increased by 2.51% by using 2.5% of lime. Therefore, using HL appears to be more economical than using NS. The improvement in durability may justify the use of NS in asphalt mixtures. Fig. 6 shows the variation of resilient modulus with temperature for the mixtures containing different concentrations of NS and HL. This figure was produced to compare the temperature sensitivity of the mixtures. It is clear that the resilient modulus of the control and modified mixtures decreased with increasing temperature. On the contrary, the rate of decrease, which is an indication of the temperature sensitivity of resilient modulus, varies with
Resilient Modulus (MPa)
for the modified mixtures. In addition, the decrease of ITS was lower in the mixtures modified with NS than in those modified with HL, indicating that NS is superior to the traditional additive of HL in conditions where mixtures experience exposure to moisture after repeated freeze-thaw cycles. The results also revealed that lower reduction in ITS was obtained when the additives content increased. Indirect TSR using one freeze-thaw cycle is used in many specifications for the moisture susceptibility of asphaltic mixtures. It is defined as the ratio of the ITS of the mixture conditioned using one freeze-thaw cycle, as described in Section 5.1, to the ITS of the unconditioned mixture. The Iranian Asphalt Pavement Code [16] requires a minimum TSR of 75% for asphalt concrete mixtures. Fig. 4 depicts the TSR of the mixtures. It is clear that the control mixture did not satisfy the requirement of specification because of using the hydrophilic siliceous type of aggregates in this mixture. Compared with limestone aggregates, siliceous aggregates have worst adhesive properties toward asphaltic binders [2,6,13,11]. In case of using limestone aggregates in asphaltic mixtures, both anionic and cationic surfactants which are naturally present in asphaltic binders strongly bond with calcium ions on the surface of aggregate particles. However, in siliceous aggregates, only cationic surfactants strongly bond with silica atoms and anionic surfactants are easily displaced by water [6], making siliceous aggregates hydrophilic. Nevertheless, all the modified mixtures were shown to have a TSR higher than that required by specification, indicating that the problem of moisture susceptibility of the control mixture can be resolved by any additive contents utilized in this research. Additionally, the results showed that TSR increased with increasing the additives content. Moreover, NS increased the stiffness of the binder through the interaction of nano-size particles with the binder, which can be the reason for the increase of resistance against moisture damage in mixtures containing NS. The adhesion of the binder to the aggregate surface is hardly lost at higher stiffness values of the binder [12]. The HL increases resistance against moisture damage in a different way. Two mechanisms of aggregate surface modification and binder modification through chemical interaction have been found to be effective in HL modification [22]. One of the effects of HL is that it allows the precipitation of calcium ions onto the aggregate surface. These ions bond with the acids from the asphaltic binder, which form water-insoluble salts and improve the adhesion of the binder and aggregate [22]. Binder modification by HL is induced by the neutralization of the acidic moieties of the binder by the basic using the solid particles of HL. The results also revealed that NS was slightly more effective than HL for improving resistance against moisture damage. However, as mentioned earlier, the mixtures modified with HL were
5°C
3000
25°C
45°C
2500 2000
1500 1000 500 0 5°C 25°C 45°C
C 1495 1119 708.8
2%NS 1817 1473 925.5
4%NS 2218 1845 1240.5
6%NS 2725 2293.7 1675.2
1.5%L 1842 1579 940.7
2%L 2167 1924 1313.3
2.5%L 2431 2199 1654.5
Mixtures Fig. 4. TSR of the mixtures.
Fig. 5. Resilient modulus of the nano-silica modified mixtures.
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Table 8 Percentage of increase in cost compared with the control mixture. Name of mixture
2%NS
4%NS
6%NS
1.5%L
2%L
2.5%L
Percent of increase in cost
79%
158%
237%
1.6%
2%
2.51%
Resilient Modulus (MPa)
2800 2300 1800 1300 800 300 0
10
20
30
40
50
Temperature (°C) Control
2%NS
4%NS
1.5%L
2%L
2.5%L
6%NS
Fig. 7. Fatigue life of the mixtures.
Fig. 6. Variation of the resilient modulus with temperature for the nano-silica modified mixtures.
modifier type, modifier content, and temperature range. While the resilient modulus of the control mixture decreased at a constant rate with increasing temperature from 5 to 45 °C, for the mixtures containing HL and NS, the rate of decrease depended on the range of temperature. When temperature increased from 5 to 25 °C, the rate was lower than when temperature increased from 25 to 45 °C. Furthermore, the rate decreased with increasing the HL and NS content. Table 9 shows the percentages of decrease in the resilient modulus of the mixtures after 20 °C of increase in temperatures (from 5 to 25 and from 25 to 45 °C). It can be observed that the sensitivity to temperature increases with increasing temperature and decreases with increasing HL and NS content. Moreover, the results showed that the NS modified mixtures were less sensitive to temperature than the HL modified mixtures. The increase of sensitivity to temperature with increasing temperature can be described by the mechanical contrast between asphalt-swollen HL and NS particles with the asphaltic matrix, as proposed by Lesueur et al. [22]. At lower temperatures, the contrast is not significant compared with that at higher temperatures, which explains the smaller difference between the resilient modulus at 5–25 °C and 25–40 °C.
also shows that fatigue life increased with increasing the NS and HL content. Previous studies [8,33] have also shown that NS inclusion improves resistance against fatigue cracking. Consistent with the findings of Enieb and Diab [8], the improvement at higher NS content was not significant. Moreover, most of the limited studies in the literature on the fatigue behavior of HL-modified asphaltic mixtures have shown that HL improves resistance against fatigue cracking [4,18,23].
3.4. Application in engineering From the results of this research, it is concluded that the ratio of the NS to HL cost is much higher than the ratio of performance indices of these mixtures. Therefore, authors of this paper recommend using HL rather than NS. Comparing the cost and improvement in the performance of the mixtures modified with different dosages of HL indicated that the use of 2.5% of HL was more economical than the use of 1.5 and 2% of HL. However, more research is needed to be conducted on the other performance properties of the mixtures for making an economic decision on selecting the appropriate additive and their content.
3.3. Fatigue tests results 4. Conclusions One of the main failure modes used for the structural design of pavements is fatigue cracking in asphaltic layers. Asphaltic layers must be sufficiently thick to ensure that fatigue cracks do not appear until the end of pavement design life. Fig. 7 illustrates the fatigue life of the mixtures under the constant stress of 200 kPa at 25 °C. As can be observed, the both additives increased resistance against fatigue cracking. However, NS was slightly superior to HL for enhancing resistance against fatigue cracking. The figure
In this study, an asphaltic concrete made of siliceous aggregate was modified with different percentages of NS and HL. Moreover, the ITS of the mixtures in dry condition and conditioned after subjecting to different freeze-thaw cycles as well as the resilient modulus at different temperatures was measured. The following are brief conclusions, which are valid over the range of additives and test conditions utilized in this study.
Table 9 Percentages of reduction in resilient modulus by increase of temperature. Rise in temperature (°C)
5–25 25–40
Mixture C
1.5%L
2%L
2.5%L
2%NS
4%NS
6%NS
25 36.6
14.2 40.4
11.2 31
9.5 24
19 37.1
16.8 32.2
15.8 27
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The both additives improved the ITS of the mixtures, with more improvement resulted by NS. Compared with the control mixture, ITS was improved by 89 and 72% by adding 6% of NS and 2.5% of HL, respectively. The highest loss in the ITS of the mixtures occurred after conditioning under the first freeze-thaw cycle, with the highest in the unmodified mixture, followed by the one modified with HL. The loss of ITS after conditioning decreased with increasing the percentage of the additives, with a lower loss for the NS modified mixtures. Compared with dry condition, 20% and 26.75% loss of ITS was obtained after five freeze-thaw cycles for mixtures containing 6% of NS and 2.5% of HL, respectively. Resistance against the moisture damage of the NS modified mixtures was higher than against that of the mixtures containing HL. The TSR values of 89 and 86.4% were obtained by adding 6% of NS and 2.5% of lime, respectively. The resilient modulus of asphalt concrete grew with increasing the additive content, with more resilient modulus for the nanomodified mixtures. The temperature sensitivity of the resilient modulus of the mixtures decreased with increasing the HL and NS content and decreasing temperature. The temperature sensitivity of the resilient modulus of the NS and HL-modified mixtures was lower than that of the control mixture, with a higher sensitivity for the mixture modified with NS. NS and HL improved the fatigue resistance of asphaltic mixtures, with a higher improvement for NS modification. The fatigue life of the mixtures containing 6% of NS and 2.5% of HLwas3.6 and 3.4 times, respectively, of the fatigue life of the control mixture. Analysis of cost showed that using HL was more economical than using NS for modification.
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