Research Article | Open Access
Performance Evaluation of Warm- and Hot-Mix Asphalt Mixtures Based on Laboratory and Accelerated Pavement Tests
A number of warm-mix asphalt (WMA) technologies are used to reduce the temperature at which the asphalt mixtures are produced and compacted, apparently without compromising the performance of the pavement. The main objective of this study is to determine whether the use of an innovative wax-based LEADCAP WMA additive influences the performance of the asphalt mixture, which is produced and compacted at significantly low temperatures. The WMA pavement using LEADCAP additive (WMA-LEADCAP) along with a control HMA pavement was evaluated with respect to their performances of rutting resistance, crack resistance, and viscoelastic property based on the laboratory dynamic modulus test, indirect tensile strength test, and in-door accelerated pavement test (APT) results. With the limited data carried out, the LEADCAP additive is effective in producing and paving asphalt mixture at approximately 30°C lower temperature than a control HMA mixture, and the performances of WMA-LEADCAP pavement are comparable to a control HMA pavement.
Hot-mix asphalt (HMA) is produced at temperatures between 140°C and 160°C and even higher mixing temperatures can be required to produce some mixtures, namely, asphalt rubber and polymer-modified asphalt (PMA) mixtures. These temperatures ensure that the aggregate is dry, the asphalt coats the aggregate, and the mixture has a suitable workability. Several temperature reduction technologies, called warm-mix asphalt (WMA), have been recently developed in the asphalt paving industry. The initial concept of WMA technologies was introduced in European to reduce greenhouse gas emission due to warn of global warming. Each country within Europe Union was confronted with greenhouse gas reduction targets as a result of the 1997 Kyoto treaty on climate change . The use of WMA technology has become a mainstream of the asphalt pavement construction in the United States and the world due to the reduced fuel consumption, less carbon dioxide emission, reduced oxidation of asphalt, early opening to traffic, and a better working environment for workers. Most WMA technologies work by lowering the viscosity of the asphalt which allows to better coat aggregate surfaces and reduces the target temperatures to reach adequate workability of the mixture. The WMA technology has become popular in recent years because of the environmental and economic benefits in the world . A number of WMA processes have been developed in Europe and the United States since the late 1997. There were approximately 20 WMA technologies being marketed in the United States in a short period of time span of 5 years from 2005 to 2010 . However, WMA technology is still new concept and the performance of any WMA technology has not been verified yet. Therefore, many agencies and contractors have hesitated to apply such new technology and they want to know if WMA technology is comparable to the conventional HMA pavement in terms of construction efforts as well as long-term performance. Therefore, it is essential that the overall performance of WMA pavement is truly as good as HMA pavement. On a life-cycle basis, if WMA pavement does not perform so well, there will not be long-term environmental benefits or energy savings. Thus, several authors have been studying the performance of the WMA additives, binders, and mixtures in order to improve their behavior [4–6].
Korea Institute of Construction Technology (KICT) and Kumho Petrochemical Co. LTD. have jointly invented an innovative WMA additive, which is named low energy and low carbon-dioxide asphalt pavement (LEADCAP). It controls the crystallization so that it does not become brittle at low temperature. The new WMA additive is positively charge to enhance the bonding of asphalt binder to negatively charged aggregate surface.
The main objectives of this study are (1) to introduce the recently developed LEADCAP additive in Korea and (2) to evaluate the performance of WMA-LEADCAP mixture produced and compacted at approximately 30°C lower than a control HMA mixture based on the stiffness and strength characteristics analyzed from the laboratory dynamic modulus and indirect tensile strength tests and permanent deformation analyzed from in-door accelerated pavement testing (APT). For such objectives, the WMA-LEADCAP mixture has been compared against the conventional HMA mixture in this paper. However, interested readers are directed to Yang et al.  for various laboratory performance evaluation of LEADCAP technology applied to the different mixture type and comparison with other WMA additive.
2. Characteristics of LEADCAP Technology
A number of new processes and products are available to reduce the temperature at which the asphalt mixtures are produced and compacted without changes of their volumetric characteristics and performance. These technologies are classified as (1) organic additive including paraffin wax, (2) foaming additive including zeolite, (3) foaming process, and (4) chemical additive including surfactants and emulsion. Among those technologies, the principle of organic additives is that the viscosity of asphalt is reduced at the temperature above the melting point in order to produce asphalt mixtures at lower temperatures. In Korea, a new WMA additive, which is named low energy and low carbon-dioxide asphalt pavement (LEADCAP), was developed in 2008. As shown in Figures 1(a): LEADCAP-Base and 1(b): LEADCAP-Modified, the developed additive is classified as an organic WMA additive, which has a wax-based composition including a crystal controller and an adhesion promoter [8, 9]. The crystal controller adjusts the wax crystallization at the low temperature, preventing the binder to show a brittle behavior and the adhesion promoter acts as an effective bonding agent between aggregates and asphalt. As a result, this additive should help improving crack resistance at low temperature and enhancing the moisture susceptibility of WMA mixtures. LEADCAP additive works by reducing the viscosity of the asphalt. Its melting point is 100°C and crystallization point is 90°C. It is typically added at the rate of 1.0% to 3.0% by weight of asphalt binder. Kim et al.  and Lee et al.  evaluated moisture susceptibility and stripping resistance of WMA-LEADCAP mixtures and it was found that the WMA-LEADCAP mixture exhibited higher moisture resistance than a control HMA and other WMA mixtures.
3. Applications of WMA-LEADCAP Technology in the Field
To investigate both performance and constructability of WMA-LEADCAP mixtures in the field, nine WMA-LEADCAP field trials were conducted. Table 1 summarizes information of nine WMA-LEADCAP field trials included with paving length and amount of production in Korea. A total of 2,358 tons of WMA-LEADCAP mixtures was paved in a total length of 4,090 m. The first WMA-LEADCAP field trial section was constructed with dense-graded asphalt mixture in 2008. Since then, WMA-LEADCAP field trials were successfully completed on the national highway and national expressway from 2008 to 2011 in Korea. It should be noted that performance graded (PG) of asphalt was 64-22 except only one section as summarized in Table 1.
3.1. Measurements of Field Air Voids
To compare compatibility between WMA-LEADCAP pavement and a control HMA pavement in the test section, two samples were cored to determine field air voids from both pavements. Table 2 shows individual field air voids of WMA-LEADCAP and the control HMA pavements collected from 2nd WMA-LEADCAP field trial. The air voids of WMA-LEADCAP pavement ranged between 5.2% and 5.4% and those of the control HMA pavement ranged between 4.7% and 5.5%. On the basis of the field measurements of air voids, it can be postulated that the air voids of the WMA-LEADCAP and the control HMA pavements were not significantly different. These results indicate that the LEADCAP additive would be effective in producing and compacting WMA mixture that is comparable to a control HMA mixture. It was also observed that there is no significant distress in both the LEADCAP-WMA and the control HMA field trial sections for 3 years in service life.
3.2. Measurements of Fuel Consumption and Emissions
During the production of the WMA-LEADCAP mixture at 130°C and a control HMA mixture at 160°C, the use of bunker C oil and carbon dioxide (CO2) was measured in the plant. Fuel consumption and emission data were collected from 2nd WMA-LEADCAP field trial. As summarized in Table 3, the decreased production temperatures led to 32% of energy savings, which resulted in 32% reduction of CO2, 18% reduction of CO, 24% reduction of SO2, and 33% reduction of NOX. These results indicate how effective the LEADCAP additive was in reducing the energy consumption and the emissions to the atmosphere, when producing WMA mixtures .
3.3. Feasibility of WMA-LEADCAP Technology
As summarized in Table 4, six WMA-LEADCAP field trials in foreign countries were completed to examine feasibility of LEADCAP-B and LEADCAP-M additives to different types of asphalt and mixtures at different weather conditions, plant types, and mixing and compaction temperatures. As summarized in Table 4, LEADCAP additives were used in dense-graded asphalt mixture in Portugal, Italy, and Thailand, PG 82-22 SBS-modified porous asphalt mixture in Japan, PG 76-22 SBS-modified SMA asphalt mixture in China, and dense-graded asphalt mixture with RAP materials in United States from fall season in 2010 to fall season in 2011. The LEADCAP additive was added to the asphalt mixture as a plant-mixed type in Portugal, Italy, Thailand, and China and to asphalt as premixed type in Japan and United States. It can be postulated that the LEADCAP additives can be used in all types of asphalt mixtures at any field conditions without modification of asphalt plant [12–15].
4. Laboratory Mix Design
The identical mix design parameters and testing conditions were adopted for WMA-LEADCAP mixture and a control HMA mixture. For this study, the unmodified PG 64-22 binder and the granite aggregate were used for both mixtures. The maximum nominal aggregate size (MNAS) of 19.0 mm is selected for surface layer. As shown in Figure 2, aggregates were sieved and blended to produce dense gradation of MNAS of 19.0 mm, which comprised 38% of 19 mm stockpile, 23% of 13 mm stockpile, 34% of sand, and 4% of filler. Table 5 summarizes superpave binder test results of base asphalt and base asphalt with 2.0% LEADCAP-B additive in terms of softening point, viscosity, , , stiffness, and m-value. Both asphalts are determined as PG 64-22, which indicates LEADCAP additive does not affect influence on the PG grade of base asphalt. LEADCAP additive is determined to add by 2.0% of asphalt binder in weight. Following ASTM D 6927, optimum asphalt content was specified as 5.2% for both the WMA-LEADCAP, and the control HMA mixtures by weight of total mix . The mixing and compacting temperatures of the control HMA mixture are 155°C and 145°C, respectively while the mixing and compacting temperatures of WMA-LEADCAP mixture are about 30°C lower than those of the control HMA mixture, for example, 130°C for mixing temperature and 115°C compacting temperature.
5. In-Door Accelerated Pavement Test
The accelerated pavement tester (APT) is widely used in the world to apply a prototype wheel loading with appropriate legal load limit to a structural pavement system to determine pavement response and performance under accumulated damage in a compressed time period. APT can evaluate pavement performance as a function of various variable factors such as materials and thickness. APT was adopted to evaluate the performance of various new asphalt materials such as stone mastic asphalt (SMA), porous asphalt, very thin layers of asphalt pavement, and warm-mix asphalt mixtures using several products . Heavy vehicle simulator (HVS), one of accelerated pavement testers, was conducted to compare the rutting performance of three different warm-mix asphalt products under wet conditions at pavement temperature of 50°C at 50 mm pavement depth using various loads on a standard dual-wheel configuration and a unidirectional trafficking mode [18, 19].
For this study, accelerated pavement test (APT) was employed to compare the rutting resistance of LEADCAP-WMA pavement against a control HMA pavement. As shown in Figure 3, APT with a standard dual-wheel configuration with 8.2 ton and a unidirectional trafficking mode was conducted at pavement temperature of 40°C at 25 mm pavement depth on the test bed at indoor facility.
5.1. Building Test Beds
To build 6.25-m LEADCAP-WMA test bed along with 6.26-m control HMA test bed, as shown in Figure 4, WMA mixture using 2.0% LEADCAP of asphalt weight was produced at 130 ± 5°C and compacted at 110°C ± 5°C while a control HMA mixture was produced at 160 ± 5°C and compacted at 130°C ± 5°C.
5.2. Measuring Rut Depth
Figure 5 shows the layout of the test bed for each of the rutting measurement. The numbers indicated fixed locations on the test bed were used for measurements of transverse profile of both the WMA-LEADCAP and the control HMA pavements after given trafficking loading. The failure criterion of rutting resistance was specified as 12.0 mm for the test. The pavement temperature at 25 mm was maintained at 40°C ± 4°C to examine rutting potential.
5.3. Rutting Resistance
Testing on the both test beds was conducted under the pavement temperatures remained constant throughout APT trafficking. The rutting behavior for two test beds is compared in Figure 6. The solid circles indicate the rut depth of the WMA-LEADCAP test bed, and the blank solid circles represent that of the control HMA test bed. Each point in Figure 6 represents the average rut depth of the three different locations as a function of accumulated load repetition. It should be noted that the error bars are shown to represent the highest and lowest rut depths measured after given trafficking loading. As shown in Figure 6, the WMA-LEADCAP and the control HMA test beds exhibit similar trends of rut depth over accumulated load repetitions. The final rut depth of the control HMA test bed is slightly higher than that of the WMA-LEADCAP test bed after the 56,000 APT trafficking loading. The final rut depth of the control HMA test bed reached the failure criterion (12.4 mm) while that of the WMA-LEADCAP test bed was slightly less than the criterion (11.9 mm).
5.4. Dynamic Modulus
To measure the dynamic modulus of each pavement section, core samples with 150 mm diameter were taken from untrafficking zone at APT test bed. The core samples were prepared for indirect tensile strength test by cutting both the top and bottom ends to obtain 38 mm thick specimens. After obtaining specimens of the appropriate dimensions, air void measurements were taken. The average air void contents for the WMA-LEADCAP and specimens ranged between 7.5% and 6.4%, respectively. The dynamic modulus test was performed in stress-controlled mode under six frequencies of 20, 10, 5, 1, 0.5, and 0.1 Hz at three different testing temperatures of 5, 20, and 35°C. The target for horizontal strains was 30 microstrains, which resulted in 60 to 100 microstrains in the vertical direction, depending upon the temperature and Poisson’s ratio. MTS 810 machine was used and the vertical and horizontal deformations were measured using MTS extensometers. These extensometers were mounted on each of the specimen faces using a 38 mm gauge length.
Figure 7 presents the dynamic modulus test results of the WMA-LEADCAP and the control HMA mixtures. A reference temperature of 5°C was used to create sigmoidal dynamic modulus mastercurves of each mixture. The dynamic modulus mastercurves in semilog and log-log scales in Figures 7(a) and 7(b) can be used to evaluate the linear viscoelastic characteristics of mixtures in low temperature range and high temperature range, respectively. It is observed that the WMA-LEADCAP mixture exhibits higher stiffness than the control HMA mixture at high reduced frequencies (i.e., lower temperature and/or faster frequency) whereas both mixtures show similar stiffness at low reduced frequencies (i.e., higher temperature and/or slower frequency). Generally, the stiffness at low reduced frequencies is related to rutting performance of the mixture and thus, both mixtures are expected to have similar rutting resistance. With respect to the horizontal phase angle mastercurves presented in Figure 7(c), the control HMA mixture shows less elastic behavior (high phase angle values) than WMA-LEADCAP mixture at high temperatures but similar elastic properties at high reduced frequencies. In terms of the vertical phase angle, both mixtures exhibit similar elastic behavior for all frequency range. Based on the dynamic modulus test results, it is expected that the rutting resistance of the WMA-LEADCAP mixture would be better than or at least similar to that of the control HMA mixture, which corresponds to the APT results.
5.5. Indirect Tensile Strengths
The indirect tensile strength (ITS) test was developed to characterize the crack resistance of hot-mix asphalt (HMA) pavement. To evaluate a creak resistance between the WMA-LEADCAP and the control HMA mixtures in terms of before and after trafficking loading with APT, ITS test was conducted with cored specimens from APT test beds. The ITS values according to mix types and trafficking loading are plotted in Figure 8. Each data point in Figure 8 represents the average ITS values of the three replicates. A large solid circle in Figure 8 indicates the average for each mixture type as function of mixture types. As seen in Figure 8, ITS results of the control HMA mixture before trafficking loading shows a larger variation than that of the WMA-LEADCAP mixture. However, the average ITS value was similar between WMA-LEADCAP mixture and the control HMA mixture under the same trafficking loading condition. It implies that the WMA-LEADCAP mixture produced under lower mixing temperature has similar crack resistance with a control HMA mixture. It can be observed from Figure 8 that the ITS values decreased as trafficking condition changed. The ITS values of two mix types after trafficking loading were decreased with 80% of strength before given trafficking loading. It indicates that the ability of crack resistance is getting weaker as trafficking loading increased. The statistical test chosen for this comparison was the equal variance two-tailed paired t-test. The null hypothesis is that the ITS test results are the same. A t-test with significance levels of 0.05 is performed to evaluate where there are differences in the variances of ITS results between two mix types under the same trafficking loading conditions. value as seen in Figure 8 is greater than 0.05 indicating the ITS results between the two groups are not different. These results indicate that both the WMA-LEADCAP and the control HMA mixtures have similar crack resistance at ambient temperature (25°C).
6. Summary and Conclusions
The use of warm-mix asphalt (WMA) for the construction of roads around the world is growing rapidly. A number of WMA technologies have been developed and successfully implemented, apparently without compromising the performance of the pavement.
A new organic WMA additive (LEADCAP) was developed to reduce the temperature at which the asphalt mixtures are produced and compacted without changes of volumetric and performance characteristics. Several WMA field trial sections using LEADCAP additive (WMA-LEADCAP) were successfully constructed in the world.
On the basis of the limited WMA-LEADCAP field trials, it indicates that the LEADCAP additive is effective in producing and compacting asphalt mixtures. As a result, it is concluded that the WMA-LEADCAP pavement achieved a comparable field air void as the control HMA pavement at a significantly lower temperature. The energy savings and the air quality improvements obtained with the WMA-LEADCAP mixture were observed in an asphalt plant, but long-term performance and durability of WMA-LEADCAP pavement should be further investigated.
To compare the performance of the WMA-LEADCAP against the control HMA mixtures, the WMA-LEADCAP test bed along with the HMA test bed was built and evaluated with respect to their performances of crack and rutting resistances based on the laboratory dynamic modulus test, indirect tensile strength test, and in-door accelerated pavement test (APT) results.
The WMA-LEADCAP and the control HMA test beds exhibit similar trends of rut depth over accumulated load repetitions at the accelerated pavement test (APT). The WMA-LEADCAP mixture exhibits higher stiffness than the control HMA mixture at high reduced frequencies whereas both mixtures show similar stiffness at low reduced frequencies. It is expected that the rutting resistance of the WMA-LEADCAP mixture would be better than or at least similar to that of the control HMA mixture, which corresponds to the APT results. The average indirect tensile strength values were similar values between WMA-LEADCAP and the control HMA mixtures under same trafficking loading condition. The indirect tensile strength values decreased as trafficking condition changed. It indicates that the ability of crack resistance is getting weaker as trafficking loading increased. With the limited test results presented in this paper, it can be concluded that the performance of the WMA-LEADCAP mixture produced under lower mixing temperature would be comparable to that of a control HMA mixture. To make more general conclusion, however, LEADCAP additive should be applied and evaluated for various mixtures, for example, different aggregate sources and binder grade, and so forth.
This research was supported by a grant from a Strategic Research Project (Development of Low-Carbon and Low-Cost Asphalt Pavements) funded by the Korea Institute of Construction Technology and Korea Institute of Construction & Transportation Technology Evaluation and Planning.
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