Rutting Resistance of Asphalt Mixtures & Bitumen Layers

Introduction

Rutting (permanent deformation) is one of the most common and severe modes of distress that occurs on asphalt pavements. The rutting phenomenon in the bitumen layers appears as “longitudinal surface depression in the wheel path” due to the combined effect of densification and shear flow (i.e., shear failure) under the traffic loads. Rutting significantly impacts riding quality and road safety, leading to hydroplaning and a high potential for traffic accidents. Notably, rutting in asphalt mixes accelerates, particularly when the pavement structure is subjected to high service temperatures, heavy loading (overloading), and slow-moving traffic conditions. The rutting mechanism in the asphalt pavement is illustrated in Figure 1. Moreover, various factors such as aggregate characteristics (aggregate source, shape, size, aggregate type and gradation), bitumen type (unmodified and modified), mix type (dense-graded, gap-graded), and mix properties (binder content, air voids, voids in mineral aggregates (VMA), voids filled with bitumen (VFB)) have strong influence on the rutting performance of asphalt mixtures. Therefore, a thorough understanding of the mechanical behaviour and performance properties of asphalt mixtures is essential for ensuring the longevity and performance of road pavements.

Need for Performance Tests

Widely, asphalt mixes have been designed based on volumetric properties (air voids, VMA, VFB etc.). Countries like the US have incorporated performance tests on asphalt mixes satisfying the volumetric properties. However, the implementation of these performance tests was limited to a few special projects, considering the practical problems. Recent investigations have revealed that volumetric properties alone are insufficient for evaluating the impact of mix variables such as recycled asphalt pavement (RAP), additives (such as polymers, rubber, rejuvenators, warm mix additives, bio-binders, fibers, waste plastic, etc.), quality of asphalt binder (including source variability), and various other innovative materials. These factors notably influence the performance properties of asphalt mixtures. Recently, the balanced mix design (BMD) concept has been gaining the momentum, in which asphalt mixes are designed based on performance criteria such as rutting, cracking, and moisture damage rather than solely depending on volumetric criteria (West et al. 2018). Thus, incorporating performance tests with appropriate acceptance criteria can help to understand performance behaviour and differentiate between poor and better-performing asphalt mixtures.

Laboratory Tests to Capture Rutting Resistance and Limitations

Many laboratory test methods such as asphalt pavement analyzer (APA), the Hamburg wheel tracking test (HWTT), flow time (FT), flow number (FN), dynamic modulus (DM), and repeated load permanent deformation (RLPD) tests are available to evaluate the rutting potential of asphalt mixtures. These test methods are most widely used to evaluate the rutting resistance of asphalt mixtures during the mix design phase. Moreover, these test methods had a good correlation with field rutting performance and established the threshold criteria to produce the high rutting resistance asphalt mixtures (West et al., 2018; Zhou et al., 2020). However, various practical problems associated with these performance tests such as higher equipment cost, relatively complex test methods, longer testing hours, and requirement of additional tools (i.e., coring and cutting equipment) for sample preparation limit their use during the mix design and plant production phases. Therefore, these test methods are not practical/suitable for use during the quality control (QC)/quality assurance (QA) of asphalt mixtures (Zhou et al. 2020).

Simple Performance Tests

To overcome limitations or constraints associated with the current test methods, there is a need to have a test method with low equipment cost, simple (no coring, cutting, or notching), accurate, quick, practical (less time for sample preparation/testing), repeatable (low variability), and should have good relationship with the field performance. The recent focus on balanced mix design (BMD) has further emphasized the need for simple performance tests based on monotonic loading, which can help the QC and QA of the asphalt mixes. Therefore, researchers have explored simplified alternative tests based on monotonic loading rates at appropriate high temperatures to evaluate the rutting resistance of asphalt mixtures, which can be performed using existing available loading frames in many laboratories. Notably, the high temperature indirect tensile strength (HT-IDT) test, ideal shear rutting (IDEAL-RT) test, and Marshall rutting test (Marshall-RT) were developed to capture the rutting resistance of asphalt mixtures. Several researchers showed a fair to good correlation between these simple monotonic tests and different laboratory rutting tests/field rutting performance (Vamsikrishna and Singh 2023b). All three fixtures (i.e., IDEAL-RT, HT-IDT, and Marshall-RT) with samples are shown in Figure 4.

Implementation and Way Forward

A strategic approach integrating various aspects such as testing, analysis, and establishing the mixture performance criteria is pivotal in implementing simple performance tests based on monotonic loading. Considering the implementation of simple performance tests, test temperature and loading rate are among the most critical factors. As mentioned earlier, these tests can be performed using existing available loading frames and the jigs in many laboratories to perform the Marshall stability, indirect tensile strength, moisture damage, and cracking tests. The IDEAL-RT test setup requires an additional shear fixture to supplement the existing loading frame. Selecting the suitable test temperature based on the geographical location and climatic conditions is important. Since the test samples in simple performance tests are vertically oriented at high temperatures, test samples are expected to be susceptible to creep and damage during handling. Hence, performing the test and capturing realistic data makes it difficult. Therefore, based on the experience, researchers suggested performing the simple performance tests at a temperature 10°C lower than the yearly, 7-day average maximum pavement temperature 20 mm below the pavement surface as determined by Long-Term Pavement Performance Bind (LTPPBind) (Boz et al. 2023; Vamsikrishna and Singh 2023a).

It is important to understand the sensitivity of the simple performance tests in capturing the performance of asphalt mixes to changes in mixture properties. The properties include recycled materials, additive, mix types (des dense-graded, gap-graded), aggregate characteristics (aggregate source, shape, size, aggregate type and gradation), binder type (unmodified and modified (i.e., polymer modified, rubber modified, and highly modified etc.)), asphalt mix properties (binder content, air voids), aging conditions etc. Notably, the selected test should capture and be capable of identifying differences in the performance of the asphalt mix depending on the material properties and tolerance limits.

Another important aspect is that any test method to be considered in the specification should have a good relationship with field pavement performance. It helps in establishing the performance acceptance criteria. The direct and best way to establish acceptance criteria is to construct and monitor the field performance. However, this process is time-consuming to get the actual field data. An alternative approach to developing preliminary acceptance criteria for simple performance tests is establishing correlations with well-known existing performance tests (such as HWTT, APA, FN, etc.) and their associated criteria, until meaningful field performance data is obtained. This method can help expedite the process by leveraging existing data while awaiting meaningful field performance data.

Overall, simple performance tests are practical, efficient, and can be performed using existing available loading frames and the jigs in many laboratories. However, implementation of simple performance tests is still in the developing stage. Therefore, a two-phase approach can be followed to adopt these tests for better implementation of mix design and plant production phases such as QC/QA of the asphalt mixes. During the mix design phase, simple performance tests and well-known existing performance tests (HWTT, APA, and FN) can be used to establish the benchmark for the asphalt mixtures. Only simple performance tests can be used as quality assurance (QA) tools during the plant production phase.

References

• Boz, I., Habbouche, J., Diefenderfer, S. D., Coffey, G. P., Ozbulut, O. E., & Seitllari, A. (2023). Simple and practical tests for rutting evaluation of asphalt mixtures in the balanced mix design process. Virginia Transportation Research Council.
• Vamsikrishna, G. and Singh, D., 2023a. Comparison of Rutting Resistance of Plant Produced Asphalt Mixes Using Hamburg Wheel Tracker and Surrogate Simple Performance Tests: IDEAL-RT and HT-IDT. Journal of Materials in Civil Engineering, 35(12), p.04023471.
• Vamsikrishna, G. and Singh, D., 2023b. Exploring potential of Marshall-RT as simple performance test to evaluate rutting resistance of asphalt mixtures. International Journal of Pavement Engineering, 24(1), p.2265030.
• West, R., C. Rodezno, F. Leiva, and F. Yin. 2018. Development of a framework for balanced mix design. Final Rep., Project No. NCHRP 20-07/Task 406. Washington, DC: Transportation Research Board of theNational Academies.
• Zhou, F., Hu, S. and Newcomb, D., 2020. Development of a performance-related framework for production quality control with ideal cracking and rutting tests. Construction and Building Materials, 261, p.120549.