Advances in Materials Science and Engineering

Advances in Materials Science and Engineering / 2019 / Article

Research Article | Open Access

Volume 2019 |Article ID 6384360 | 16 pages | https://doi.org/10.1155/2019/6384360

Experimental Study on the High-Temperature and Low-Temperature Performance of Polyphosphoric Acid/Styrene-Butadiene-Styrene Composite-Modified Asphalt

Academic Editor: Luigi Nicolais
Received14 Nov 2018
Accepted23 Dec 2018
Published20 Jan 2019

Abstract

The high-performance asphalt materials are used to replace the ordinary road asphalt that cannot meet the requirements of natural environment and traffic situation, which is the effective way to solve poor asphalt pavement durability. In this paper, polyphosphoric acid- (PPA-) modified asphalt and polyphosphoric acid (PPA)/styrene-butadiene-styrene (SBS) composite-modified asphalt with different PPA content were prepared by using two-type asphalt. The effect of PPA modifier on asphalt was analyzed by using the creep elastic recovery rate, accumulating strain and creep modulus tests. The results showed that asphalt types and the PPA could significantly improve the elastic recovery rate of asphalt, reduce the cumulative strain and creep stiffness of the viscosity part, improve the high-temperature performance, and reduce the permanent deformation of the asphalt under repeated load. The high-temperature performance and low-temperature performance of PPA-modified asphalt were studied by the chemical and physical modification techniques. The advantages of modified asphalt are well developed while reducing the price of it, which has important technical and economic significance.

1. Introduction

The main tasks of contemporary highway construction study are to optimize pavement structure, apply high-performance materials, improve construction technology, and decrease risk assessment [1]. The asphalt pavement in China has the main problems of weak operation performance and durability. The early damage of rutting, cracks, and pits is very serious. The actual service life of pavement is much lower than that of design life [2, 3]. The phenomenon of rapid increase in traffic, overloading of vehicles, serious channelized traffic, and extreme weather is becoming more frequent. However, the existing pavement design system and material performance cannot meet the actual requirements of the traffic conditions. Using high-performance asphalt materials to replace ordinary road asphalt is an effective way to solve the problem of poor durability of asphalt pavement and serious early damage and reduce the cost of pavement maintenance.

According to the modification principle of polymer on asphalt performance, modified asphalt can be divided into reactive-modified asphalt and swelling dispersing-modified asphalt [4, 5]. Polymer-modified asphalt mainly includes SBS and styrene-butadiene-rubber (SBR) powder. Fine-grained polymer particles absorb light components of asphalt and change the colloidal structure of asphalt. The synergistic effect of polymer and base asphalt improves the high- and low-temperature performance and mechanical properties of asphalt. Physically modified asphalt needs professional grinding and shearing equipment, and the production cycle is long and the production process is complex. SBS and SBR modifiers are expensive, and segregation easily occurs during storage and transportation of physically modified asphalt due to the difference between polymer particles and asphalt surfaces [1, 6, 7]. SBS-modified asphalt cannot achieve ideal performance in practice, and the price of SBS modifier is relatively high.

PPA has been used as a modifying agent to produce modified asphalt for decades [810]. In the early 1870s, TOSCO company claimed that PPA could be used as a modifying agent to improve the softening point, viscosity, and needle penetration of asphalt. After the asphalt PG grading index system was established, adding PPA to asphalt can improve the high temperature grade of asphalt [1113]. In 2004, Martin expounded the influence of PPA addition on the high temperature grade and critical cracking temperature of asphalt PG. He believed that the application of PPA can increase 2∼3 grades of asphalt PG and increase the critical pyrolysis temperature by 3°C [14]. Edward used viscoelasticity performance testing method to analyze the rheological performance of PPA-modified asphalt. He considered that PPA had a remarkable influence on medium- and high-temperature rheological behavior of asphalt. Repeated creep tests showed that PPA can significantly improve the asphalt resistance to permanent deformation [15, 16].

Masson used the methods of gel permeation chromatography, nuclear magnetic resonance, and thin-layer chromatography to study the evolution process of chemical reaction of PPA in asphalt. During the reaction of PPA with asphalt, esters and salts are produced in the active part of asphalt, which ionizes saturated aromatic aromatization, aromatic cyclization, and colloidal components in asphalt. Several possible reaction mechanism hypotheses are proposed, and new asphaltene micelles were prepared. In addition, PPA had the effect of activating a part of asphalt double bond to have cross-bonding, extending, and strengthening asphaltene network structure [17]. His research result of ionic bond in the reaction process showed that PPA was a short-chain active oligomer. PPA can have segregation in the asphalt of high dielectric constant and have chemical reaction. Baumgardner analyzed the influence of PPA on asphalt aging resistance by studying the relative amount of carbonyl and acylamino and considered that the addition of PPA could improve the aging resistance of asphalt [18]. John used fluorescence microscope to observe the microstructure of PPA/SBS compound-modified asphalt, considering that the addition of PPA could form dispersed SBS single-point particles into long-chain structure and further form an extensive flocculent network to improve the capacity of asphalt against deformation [1921]. Naresh studied the aged microstructure of PPA/SBS compound-modified asphalt. It proved that PPA promoted the aging resistance of SBS-modified asphalt from micro perspective [22].

Different base asphalts were used to study the influence of PPA addition on asphalt PG high temperature grade and four components. Experimental data indicated that the addition of 2% PPA could improve the high temperature grade of asphalt. The addition of PPA could increase the content of asphaltene by over 50% [2325]. The influence of PPA on the storage stability of SBS-modified asphalt was studied by calculating the rutting factor of the upper and lower asphalt specimens in the segregation test tube. When the PPA stoichiometry was small, the storage stability of SBS-modified asphalt was improved. When PPA exceeded its optimum content, serious segregation occurs in the composite-modified asphalt [2628].

The rheological performance and aging resistance of PPA/SBS-modified asphalt were studied. Compared with sulfur/SBS composite-modified asphalt, PPA/SBS composite-modified asphalt has better high temperature stability, but poor low temperature performance. This phenomenon can be improved by increasing the content of SBS-modifying agent [29].

The effects of PPA modification on asphalt binders had been studied [1]. The preparation process, the influence of reasonable modifying agent content, and stirring temperature of PPA/SBS-modified asphalt on its performance also had been studied [3033].

Therefore, different contents of PPA were added to SBS-modified asphalt in this paper. Combined with chemical modification and physical modification technology, the high-temperature performance and low-temperature performance of PPA-modified asphalt and PPA/SBS composite-modified asphalt were studied. In terms of reducing the price of modified asphalt and giving full play to the advantages of modified asphalt, this study has important technical and economic significance.

2. Experiments

Experimental method: different proportions of SBS or SBS + PPA material were added to SK90 and ZH90 bitumen to produce SBS-modified bitumen or SBS/PPA composite bitumen. Through high temperature test and low temperature test, the material property changes of SBS-modified asphalt or SBS/PPA composite asphalt could be obtained after the corresponding test. The experimental process is shown in Figure 1.

2.1. Materials
2.1.1. Asphalt Binders

The SK90 and Zhenhai 90 (ZH90) were selected as two asphalt binders in this paper. According to The Testing Regulations of Asphalt and Asphalt Mixture, basic performance tests and four component analysis of asphalt binders were conducted. The results are shown in Table 1.


Physical properties and chemical componentSK90ZH90

Penetration (25°C, 0.1 mm)86.581.0
Softening point (°C)45.546.0
Ductility (15°C, cm)>100>100
Penetration ratio (%)72.070.6
Ductility (cm)9.38.0
Mass loss (%)0.91.0
Performance grade (°C)58–2258–22
Stiffness modulus (MPa, −12°C)231243
Creep rate (MPa/S, −12°C)0.3510.342
Asphaltene (%)8.637.34
Resin (%)17.4029.28
Aromatics (%)44.7836.62
Saturates (%)24.2023.41

2.1.2. PPA-Modifying Agent

Industrial PPA with H2PO3 content of 105% was selected as a modifying agent in this research. Its main technical indexes are shown in Table 2.


Technical index105% H2PO3

P2O5 concentration74.2
Surface tension (N/cm2)74 × 10−5
Specific heat capacity (J/g·°C)1.498
Boiling point (°C)354

2.1.3. SBS-Modifying Agent

Yueyang Petrochemical 1301 (YH791) was selected as an SBS-modifying agent in this research. Its main performance indexes are shown in Table 3.


ItemYH791

StructureLine-type
PS/PB30/70
Percentage of liquid volume (%)0
300% stretching stress (MPa)1.90
Elongation (%)700
Permanent deformation (%)45
Hardness60
Total ash (%)0.20

2.2. Specimen Preparation
2.2.1. PPA-Modified Asphalt

When base asphalt was heated to 160°C, PPA-modifying agent which accounted for 1%, 1.5%, 2%, and 2.5% of the asphalt mass was added. FLUCK high-speed shearing emulsifying machine was used to shear it at the speed of 1000 r/min for 20 minutes, and then PPA-modified asphalt with different PPA contents was prepared, as shown in Figure 2.

2.2.2. SBS-Modified Asphalt

The base asphalt was heated up to 180°C; solubilizer and SBS-modifying agent which accounted for 4.5% of base asphalt mass were added. FLUCK high-speed shearing emulsifying machine was used to operate at the speed of 4000 r/min for 40 min, and then stabilizer was added to shear for 5 min. Finally, it was put into a constant temperature oven at 160°C to grow for 2 hours, as shown in Figure 3.

2.2.3. PPA/SBS Compound-Modified Asphalt

The base asphalt was heated up to 180°C; solubilizer and SBS-modifying agent which accounted for 3.5% of base asphalt mass were added. FLUCK high-speed shearing emulsifying machine was used to operate at the speed of 4000 r/min for 40 min, and then stabilizer was added to shear for 5 min. It was put into a constant temperature oven at 160°C to grow for 1.5 hours. Then, PPA which accounted for 0.5%, 1.0%, 1.5%, and 2.0% of asphalt mass was added to it to be sheared at the speed of 1000 r/min for 10 min, as shown in Figure 4.

2.3. High-Temperature Test
2.3.1. High-Temperature Repeated Creep Test

Dynamic shearing rheometer was applied to PPA-modified asphalt with different modifying agent contents, SBS-modified asphalt, and PPA/SBS compound-modified asphalt with different PPA contents to carry out high-temperature repeated creep test. The test temperature of modified asphalt was 60°C, load intensity was 100 kPa, load was 1 s, unload was 9 s, and repetitive cycle quantity 100 times.

SK90 base asphalt and ZH90 base asphalt were selected. PPA that was 0.5%, 1.0%, 1.5%, 2.0%, and 2.5% of asphalt mass was added to record the accumulated strain values of load and unload on 10 times, 20 times, 30 times, 40 times, 50 times, 60 times, 70 times, 80 times, 90 times, and 100 times, as shown in Figure 5.

2.3.2. Temperature Sweep Test

Strain-controlled mode was used in this paper. The test frequency was 10 rad/s and test temperature was from 20°C to 60°C. Temperature sweep test was conducted to PPA-modified asphalt with different modifying agent contents, SBS-modified asphalt, and PPA/SBS compound-modified asphalt with different PPA contents to record the complex modulus and phase angle under every test temperature.

2.4. Low-Temperature Test

To study the effect of raw materials on the low-temperature performance of PPA-modified asphalt and PPA/SBS compound-modified asphalt, the bending beam rheological (BBR) test was used to test the stiffness modulus S and creep rate m at low temperature.

PPA-modifying agent that was 0.5%, 1.0%, 1.5%, 2.0%, and 2.5% of asphalt mass was added to prepare PPA-modified asphalt and compare with base asphalt in terms of the low-temperature performance. First, it was placed in the rolling thin film oven for aging. The aging temperature was 163°C and aging time was 85 min. PAV pressure aging was conducted to the asphalt that had short-term aging to simulate the long-term aging condition of pavement using process. The aging temperature was 100°C, pressure was 2.1 MPa, and aging time was 20 h. Temperature was increased until the flow condition for the specimen with pressure aging and casting was conducted to the asphalt beam test specimen to test the stress-strain characteristics at −6°C, −12°C, −18°C, and −24°C, as shown in Figure 6.

To study the low-temperature performance of PPA/SBS compound-modified asphalt, SBS-modified asphalt with 3.5% and 4.5% SBS-modifying agent was prepared. In the 3.5% SBS-modified asphalt, PPA modifier that was 0.5%, 1.0%, 1.5%, and 2.0% of asphalt mass was added to prepare PPA/SBS compound-modified asphalt. RTFOT short-term aging test was conducted to SBS-modified asphalt and PPA/SBS compound-modified asphalt with swelling development first, and further PAV pressure aging test was performed on it. Then, asphalt beam specimen was formed. Stiffness modulus S and creep rate m of asphalt were tested at low temperature to analyze the rheological characteristics of compound-modified asphalt at low temperature.

3. Results and Discussions

3.1. High-Temperature Repeated Creep Test
3.1.1. Repeated Creep Test of PPA-Modified Asphalt

Figure 7 gives the accumulated strain of PPA-modified asphalt with the change of load times. It can be seen from Figure 7 that the accumulated strain of repeated creep of modified asphalt increased gradually with the rise of the times of loading effect under the same temperature and load effect. Whether for SK90 base asphalt or ZH90 base asphalt, its accumulated strain of repeated creep decreased with the mixing amount of PPA modifier increasing. When the mixing amount of PPA was 0.5%, the accumulated strains of repeated creep of SK90 asphalt or ZH90 asphalt for 100 times were 199.8% and 206.3%. When the mixing amount of PPA was 1.0%, the final accumulated strains of two asphalts were 105.2% and 109.3%. When the mixing amount of PPA was 1.5%, the final accumulated strains of two asphalts were 64.7% and 68.5%. When the mixing amount of PPA was 2.0%, the final accumulated strains of two asphalts were 49.8% and 52.4%. When the mixing amount of PPA was 2.5%, the final accumulated strains of two asphalts were 17.9% and 20.9%. Obviously, when the mixing amount of PPA was over 1.5%, the accumulated strain slowed down, corresponding to the decreased rate of modifier mixing amount. For the two asphalts selected in this task, the accumulated strain of SK90 asphalt was always less than ZH90 asphalt under the same modifier mixing amount. With the modifier mixing amount increasing, the difference of accumulated strain of two modified asphalts was getting smaller under the same mixing amount.

Figure 8 presents the influence of PPA mixing amount on the elastic recovery rate of modified asphalt. It can be seen by analyzing Figure 8 that the elastic recovery rate of asphalt increased obviously with the mixing amount of PPA increasing. With regard to PPA-modified SK90 asphalt, when PPA mixing amount was 0.5%, the elastic recovery rate was only 3.6, but when PPA mixing amount was 2.5%, its elastic recovery rate was up to 115.8. With regard to ZH90 base asphalt, when PPA mixing amount was 0.5%, the elastic recovery rate was 4.1, but when PPA mixing amount was 2.5%, its elastic recovery rate was up to 107.6. Obviously, the addition of PPA mixing amount strengthened the deformation recovery capability of asphalt. Meanwhile, it was found by observing curve shape that the elastic recovery rate fluctuation of asphalt was large before repeated creep-recovery test was conducted for 50 times, indicating that the delayed elastic recovery of asphalt was not stable. Basically, elastic recovery capacity tended to increase with the load time increasing, but after creep-recovery effect was conducted for over 50 times, the deformation recovery rate tended to be consistent. Except the influence of equipment system error, it could be considered that the delayed elastic deformation recovery of asphalt tended to be stable gradually after 50 times. Delayed elastic deformation recovery became a constant. Compared with SK90 base asphalt and ZH90 base asphalt, it was found that the addition of PPA had consistent improvement rule on the elastic recovery rate of both. From the test results, the modification result of SK90 asphalt was slightly superior to that of ZH90 asphalt.

The constitutive equation of Burgers model was used to fit stress and strain relationship in the 50th and 51st times of repeated creep-recovery tests to obtain generalized Voigt (Gv) from average values, which was used as the viscous part of creep stiffness modulus and evaluating high-temperature permanent deformation characteristics of asphalt. The test results of two asphalts with different PPA mixing amounts are as shown in Figure 9.

It can be found by analyzing Figure 9 that the Gv value of modified asphalt was increasing with PPA mixing amount increasing, but when PPA mixing amount was over 1.5%, the slope of PPA mixing amount-Gv curve slowed down significantly. Obviously, when the mixing amount is beyond certain value, the improvement effect of PPA on the high-temperature performance of modified asphalt will be weakened.

3.1.2. Repeated Creep Test of PPA/SBS Compound-Modified Asphalt

Figure 10 shows that influence of PPA on accumulated strain of PPA-modified asphalt with the load effect times. It can be seen from Figure 10 that the addition of PPA could obviously improve its shearing strength when SBS modifier mixing amount was 3.0%. The accumulated strain of compound modified-asphalt with the addition of PPA was far less than modified asphalt with the addition of 3.0% SBS. The accumulated strain of repeated creep of compound-modified asphalt for 100 times with the addition of 0.5% PPA was only 1/6 of modified asphalt with the addition of 3.0% SBS. For PPA with the addition of 0.5% compound-modified asphalt, the final accumulated strain and modifier mixing amount were close to the accumulated strain value of 4.5% SBS-modified asphalt. With PPA mixing amount increasing, the accumulated strain of compound-modified asphalt was decreasing gradually, but when PPA mixing amount was more than 1.0%, the increase of mixing amount reduced the decreasing effect on accumulated strain. Compound-modified asphalt with the PPA mixing amounts of 1.5% and 2.0% had a considerable improvement effect on shearing strength.

Figure 11 presents the effect of PPA mixing amount on the elastic recovery performance of PPA/SBS compound-modified asphalt. It can be seen by analyzing Figure 11 that the addition of 0.5% PPA could obviously improve the elastic recovery rate of SBS-modified asphalt. The elastic recovery rate of SBS-modified asphalt with the addition of only 3.5% modifier was only 10.4 (SK90) and 8.2 (ZH90). However, after 0.5% PPA was added, its elastic recovery rate was up to 44.7 and 48.3 that elastic recovery was increased by 4-5 times, but the performance of 1% PPA + 3.5% SBS compound-modified asphalt was significant due to SBS-modified asphalt with 4.5% SBS modifier mixing amount. When PPA mixing amount was over 1%, the improvement effect was weakened on the elastic recovery rate of compound-modified asphalt with a higher PPA mixing amount.

It can be seen from Table 4 that the addition of 0.5% PPA increases the Gv value of the modified asphalt by about 2 times. Gv of the viscous part of creep stiffness of 1.0% PPA + 3.5% SBS compound-modified asphalt was slightly greater than 4.5% SBS-modified asphalt. When PPA mixing amount was over 1.5%, the Gv value of compound-modified asphalt was small. The modification effect of PPA and SBS modifier on SK90 base asphalt was slightly superior to ZH90 asphalt. So the SK90 asphalt was chosen as the object of following temperature sweep test research.


PPA mixing amountGv (Pa)
SK90ZH90

3.5% SBS49985370
0.5% PPA + 3.5% SBS95319682
1.0% PPA + 3.5% SBS1025710092
1.5% PPA + 3.5% SBS1381413420
2.0% PPA + 3.5% SBS1443213996
4.5% SBS100419985

3.2. Temperature Sweep Test

Figure 12 showed the influence of PPA on storage modulus (G′) and loss modulus (G″) of PPA-modified asphalt. It can be seen from Figure 12 that the G′ and G″ of modified asphalt tended to reduce exponentially with the test temperature increasing when the mixing amounts of PPA were the same. However, the phase angle (δ) of PPA-modified asphalt tended to rise perpendicularly with the rise of temperature. When temperature was the same, the G′ and G″ of modified asphalt tended to increase with the mixing amount of PPA increasing. However, when the temperature was different, the increasing proportion of modulus would also be different. When temperature was at 20°C, the modified asphalt storage modulus of 2.5% PPA mixing amount was 2.8 times of 0.5% mixing amount, 2.1 times of 1.0% mixing amount, 1.5 times of 1.5% mixing amount, and 1.2 times of 2.0% mixing amount. When temperature was at 40°C, the modified asphalt storage modulus of 2.5% PPA mixing amount was 26.0 times of 0.5% mixing amount, 4.0 times of 1.0% mixing amount, 2.4 times of 1.5% mixing amount, and 1.4 times of 2.0% mixing amount. When temperature was at 50°C, the modified asphalt storage modulus of 2.5% PPA mixing amount was 30.0 times of 0.5% mixing amount, 6.0 times of 1.0% mixing amount, 2.5 times of 1.5% mixing amount, and 1.6 times of 2.0% mixing amount. Obviously, when temperature was low, the improvement effect of PPA mixing amount on modified asphalt storage modulus is not significant. With the temperature increasing, modified asphalt G′ of larger PPA mixing amount was far greater than modified asphalt with lower mixing amount. When PPA mixing amount was over 2.0%, the improvement effect on G′ was limited.

Under the same temperature, the δ of modified asphalt would be smaller with PPA mixing amount increasing. When PPA mixing amount was small, the rule of δ increasing with the rise of temperature was significant. However, when PPA mixing amount was increasing, the tendency of δ increasing with the rise of temperature slowed down. The difference δ of modified asphalt was very small for 1.5%, 2.0%, and 2.5% PPA mixing amounts. In addition, when the temperature was higher, the difference of three phase angles would be smaller, which referred that the content of viscous component and elastic component of asphalt was relatively fixed. With regard to PPA-modified asphalt, PPA first had chemical reaction with asphaltene. The relative content of other components of asphalt was stable. Obviously, when PPA mixing amount was over 1.5%, the functional groups of asphalt that could have chemical reaction with PPA were decreasing, and the improvement effect on asphalt was going down gradually.

Figure 13 showed the influence of PPA on G′ and G″ of PPA/SBS compound-modified asphalt. It can be seen from Figure 9 that whether SBS-modified asphalt or SBS/PPA compound-modified asphalt, G′ and G″ tended to reduce exponentially with the rise of temperature. When temperature was low, the difference of G′ and G″ of modified asphalts was large. However, with the rise of temperature, the difference value of modulus among asphalts was going down gradually. When temperature was the same, the G′ and G″ of 3.5% SBS-modified asphalt were the lowest. The G′ and G″ of PPA/SBS compound-modified asphalt increased with the rise of PPA addition, but when temperature was below 60°C, the storage modulus of 4.5% SBS-modified asphalt was between 1.0% PPA + 3.5% SBS compound-modified asphalt and 1.5% PPA + 3.5% SBS compound-modified asphalt. When temperature increased to 60°C, the storage modulus of 4.5% SBS-modified asphalt was greater than PPA/SBS compound-modified asphalt. This is because that SBS modifier is not dissolvable in base asphalt, and its styrene glass state temperature is very low. In the range of test temperature, styrene chain segment had an excellent elastic behavior. When the temperature was higher, its advantages were outstandingly relative to other asphalts. However, it is a chemical reaction for PPA to adjust the relative components of asphalt to have asphalt modification. In the reaction process, the produced asphaltene is solid. Although it had higher modulus, its elastic behavior was poor, and it was dispersed in asphalt in the form of micelle that it was not able to fully give play to its performance as elastomer.

Figure 14 is the Influence of PPA on the phase angle (δ) of PPA-modified asphalt. It can be seen from Figure 14(a) that when PPA mixing amount was higher at the same temperature, the δ of modified asphalt would be smaller. When PPA mixing amount was small, the rule of phase angle increasing with the rise of temperature was significant. However, when PPA mixing amount was increasing, the tendency of phase angle increasing with the rise of temperature was slowing down. The difference of modified asphalt phase angle was very small for 1.5%, 2.0%, and 2.5% PPA mixing amounts. In addition, when the temperature was higher, the difference of three phase angles was smaller, which referred that the content of viscous component and elastic component of asphalt was relatively fixed. For PPA-modified asphalt, it first had chemical reaction with asphaltene. In this process, the relative content of other components of asphalt was stable. Obviously, when PPA mixing amount was over 1.5%, the functional groups of asphalt that could have chemical reaction with PPA were decreasing, and the improvement effect of PPA on asphalt was going down gradually.

It can be seen from Figure 14(b) that the phase angles of SBS-modified asphalt and PPA/SBS compound-modified asphalt increased with the rise of test temperature, but the increasing ranges of phase angles of different modified asphalts were different. According to the slope of phase angle-temperature curve, the slope of 3.5% SBS-modified asphalt and 0.5% PPA + 3.5% SBS compound-modified asphalt curve was the biggest, and the slope of 4.5% SBS-modified asphalt was the slowest. Thus, it can be considered that when modifier mixing amount was small, the temperature sensibility of asphalt was poor, but with the increase of modifier mixing amount, the temperature sensibility of asphalt is improved continuously. However, the temperature sensibility of several compound-modified asphalts applied to the test was poor compared with 4.5% SBS-modified asphalt.

3.3. Low-Temperature Performance Test
3.3.1. Stiffness Modulus

It can be seen by analyzing Figure 15(a) that the low-temperature creep stiffness modulus S of base asphalt and PPA-modified asphalt increased with temperature decreasing. Under the same temperature, the S of modified asphalt added with PPA was greater than that of base asphalt, and the stiffness modulus increased with PPA mixing amount increasing. With regard to SK90-modified asphalt, the S of modified asphalt with PPA mixing amounts of 0.5%, 1.0%, 1.5%, 2.0%, and 2.5% at −6°C was 1.17 times, 1.32 times, 1.49 times, 1.99 times, and 2.18 times of base asphalt. Thus, it can be considered that when PPA mixing amount is less than 1.0%, the difference of S of modified asphalt is not large from base asphalt, but when the mixing amount is over 1.5%, the S of modified asphalt increases rapidly. The S of modified asphalt with PPA mixing amounts of 0.5%, 1.0%, 1.5%, 2.0%, and 2.5% at −18°C was 1.11 times, 1.16 times, 1.34 times, 1.49 times, and 1.74 times of base asphalt, but the stiffness modulus of modified asphalt with PPA mixing amounts of 0.5%, 1.0%, 1.5%, 2.0%, and 2.5% at −24°C was 1.09 times, 1.21 times, 1.30 times, 1.40 times, and 1.65 times of base asphalt. Obviously, with the temperature further decreasing, after PPA mixing amount was increased, the increasing effect on S of asphalt was decreasing gradually.

It can be seen from the modification effect of PPA on ZH90 asphalt in Figure 15(b) that the S of modified asphalt with PPA mixing amounts of 0.5%, 1.0%, 1.5%, 2.0%, and 2.5% at −6°C was 1.32 times, 1.57 times, 1.68 times, 1.99 times, and 2.50 times of base asphalt. The increasing effect of PPA on S of ZH90 asphalt was significant. For ZH90 asphalt with PPA mixing amounts of 0.5%, 1.0%, 1.5%, 2.0%, and 2.5% at −24°C, the stiffness modulus of modified asphalt was 1.08 times, 1.19 times, 1.24 times, 1.30 times, and 1.61 times of base asphalt. It can be seen by comparing Figure 9 that the increasing effect of PPA on S of SK90 asphalt was significant. Obviously, the influence of PPA on the low-temperature performance of different base asphalts was different, and the S was small and low-temperature performance was good at a certain temperature. At another temperature, on the contrary, stiffness modulus could be large, indicating that this asphalt was sensitive to this temperature under such a low temperature.

Figure 16 shows the influence of PPA mixing amount on S of PPA/SBS-modified asphalt. It can be seen by analyzing Figure 16(a) that the stiffness modulus of SBS-modified asphalt, PPA/SBS compound-modified asphalt was increasing with the test temperature decreasing. In addition, according to the slope of curve, when test temperature decreased to −18°C, the curve slope of temperature-stiffness modulus increased, and the stiffness modulus of asphalt increased faster. Under the same test temperature, compared with SBS-modified asphalt that the modifier mixing amount was 3.5%, the stiffness modulus of 4.5% SBS-modified asphalt was larger. When the PPA mixing amount was higher, the stiffness modulus of compound-modified asphalt would be larger. With regard to SK90 asphalt, when test temperature was at −6°C, the stiffness modulus of compound-modified asphalt with PPA mixing amounts of 0.5%, 1.0%, 1.5%, and 2.0% was 1.35 times, 1.59 times, 2.06 times, and 2.52 times of 3.5% SBS-modified asphalt. When temperature was at −12°C, the stiffness modulus of compound-modified asphalt with PPA mixing amounts of 0.5%, 1.0%, 1.5%, and 2.0% was 1.07 times, 1.17 times, 1.34 times, and 1.52 times of 3.5% SBS-modified asphalt. When temperature was at −18°C, the stiffness modulus of compound-modified asphalt with PPA mixing amounts of 0.5%, 1.0%, 1.5%, and 2.0% was 1.06 times, 1.13 times, 1.28 times, and 1.43 times of 3.5% SBS-modified asphalt.

As shown in Figure 16(b), the increasing rule of S indicated that even the stiffness modulus of compound-modified asphalt increased with the rise of PPA mixing amount, the increasing range was different under different temperatures. When the test temperature was high, the increasing of stiffness modulus was sensitive to the change of PPA mixing amount, but when temperature was lower, the influence of PPA mixing amount was smaller on the stiffness modulus of compound-modified asphalt. The influence rule of mixing amount on the increase of stiffness modulus also indicated that when PPA mixing amount was less than 1.0%, the stiffness modulus of PPA/SBS compound-modified asphalt had no significant difference compared with 3.5% modifier mixing amount. When the S of SBS-modified asphalt with 4.5% modifier mixing amount was between 0.5% PPA/3.5% SBS compound-modified asphalt and 1.0% PPA/3.5% SBS compound-modified asphalt.

It can be found by comparing the influence of PPA and SBS modifier on the low-temperature performance of SK90 and ZH90 asphalts under different temperatures that the influence of two selected base asphalts on the test results of stiffness modulus of PPA/SBS compound-modified asphalt was not significant.

3.3.2. Creep Rate

It can be seen by analyzing Figure 17(a) that the creep rate of PPA-modified asphalt was going down with the test temperature decreasing, indicating that the stress relaxation capacity of asphalt would be worse when the temperature was lower. Under the same test temperature, the creep rate of PPA-modified asphalt was less than base asphalt. Moreover, when the PPA mixing amount was higher, the creep rate of modified asphalt was smaller, indicating that the addition of PPA made the stress relaxation capacity of asphalt worse at a low temperature. When test temperature was −24°C, the m value of PPA-modified asphalt with large modifier mixing amount had no linear relation with mixing amount. The reason that caused this phenomenon was because that the strain measured by instrument was small at a low temperature, and measuring error increased.

Compared with Figure 17(b), it can be seen from the influence rule of PPA mixing amount on the creep rate of SK90 asphalt that m value of creep rate of modified was 0.94 time, 0.90 time, 0.80 time, 0.74 time, and 0.67 time of base asphalt when PPA mixing amounts were 0.5%, 1.0%, 1.5%, 2.0%, and 2.5% at −6°C. However, when it was at −12°C and −18°C, m values were 0.91 time, 0.82 time, 0.76 time, 0.67 time, 0.52 time and 0.87 time, 0.73 time, 0.68 time, 0.52 time, and 0.35 time of base asphalt. The data above indicated that m value characterizing asphalt relaxation performance would be more sensitive to the increase of PPA mixing amount when temperature was lower. When modifier mixing amount was over 1.0%, the creep rate of PPA-modified asphalt was only 70% to 80% of base asphalt.

Figure 18 gives the influence of PPA mixing amount on the creep rate of PPA/SBS-modified asphalt. It can be seen from Figure 18(a) that the m value of creep rate of SBS-modified asphalt and PPA modified-asphalt reduced with test temperature decreasing. When temperature was lower, the stress relaxation capacity of asphalt material was worse. Under the same temperature, the creep rate of SBS-modified asphalt with 4.5% modifier mixing amount the maximum, and the secondary was SBS-modified asphalt with mixing amount of 3.5%. The creep rate of PPA/SBS compound-modified asphalt reduced with PPA mixing amount increasing. Obviously, the addition of SBS modifier improved the stress relaxation capacity of asphalt under low temperature, but PPA made the relaxation capacity of compound-modified asphalt worse. It can be seen from Figure 18 that the creep rate of ZH90-modified asphalt was always larger compared with SK90 asphalt under the same temperature and modifier mixing amount, indicating that the stress relaxation capacity and pavement thermal thinking resistance of ZH90-modified asphalt could be better under low temperature.

4. Conclusion

(1)With the increase of PPA content, the recovery rate of asphalt increased obviously. When repeated creep-recovery test effect was less than 50 times, the fluctuation of elastic recovery value of asphalt was significant, but when creep-recovery effect was over 50 times, the deformation recovery rate tended to be stable. With the increase of PPA content, the Gv value of modified asphalt increased continuously. But when PPA content was over 1.5%, the slope of PPA content-Gv curve slowed down obviously.(2)The high temperature performance of SK90 asphalt was always better than that of modified ZH90 asphalt. The creep performance of 3.5% SBS + 1% PPA composite-modified asphalt was almost the same as that of 4.5% SBS-modified asphalt. When the PPA content exceeded 1%, the improvement effect of it on repeated creep recovery performance was not significant.(3)Temperature sweeping results showed that the storage modulus and loss modulus of PPA-modified asphalt increased with the increase of PPA content. When the test temperature was low, the performance difference of modified asphalt with different modifying agent content was more obvious. The phase angle of PPA-modified asphalt decreased with the increase of modifying agent content and increased with the increase of test temperature. From the analysis of the rate at which the phase angle increases with the increase of temperature, the addition of PPA improves the high temperature stability of asphalt.(4)The low-temperature stiffness modulus of PPA-modified asphalt increased obviously with temperature decreasing. The stiffness modulus of modified asphalt increased with the increase of PPA content, but with the further decrease of test temperature, the influence of PPA content on the stiffness of modified asphalt decreased. With the decrease of test temperature, the creep rate m value of PPA-modified asphalt decreased gradually. The more the content of PPA, the worse the stress relaxation ability of modified asphalt. Compared with SBS-modified asphalt with 3.5% SBS content, the stiffness modulus of PPA/SBS composite-modified asphalt increased and the creep rate decreased. Compared with SBS-modified asphalt, the low temperature cracking resistance of composite-modified asphalt was worse.

Data Availability

The data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest

The authors declare that there are no conflicts of interest regarding the publication of this article.

Acknowledgments

This work was supported by the Special Fund for Basic Scientific Research of Central College of Chang’an University (nos. 300102218413, 310821153502, and 300102218405) and the Department of Science &Technology of Shaanxi Province (nos. 2016 ZDJC-24 and 2017KCT-13).

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Copyright © 2019 Xueqian Li et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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