Soohyok Im

Rate- and Temperature-Dependent Fracture Characteristics of Asphaltic Paving Mixtures

Cracking in asphaltic pavement layers causes primary failure of the roadway structure, and the fracture resistance and characteristics of asphalt mixtures significantly influence the service life of asphaltic roadways. A better understanding of the fracture process is considered a necessary step to the proper development of design-analysis procedures for asphaltic mixtures and pavement structures. However, such effort involves many challenges because of the complex nature of asphaltic materials. In this study, experiments were conducted using uniaxial compressive specimens to characterize the linear viscoelastic properties and semi-circular bending (SCB) specimens to characterize fracture behavior of a typical dense-graded asphalt paving mixture subjected to various loading rates and at different temperatures. The SCB fracture test was also incorporated with a digital image correlation (DIC) system and finite-element model simulations including material viscoelasticity and cohesive-zone fracture to effectively capture local fracture processes and resulting fracture properties. The test results and model simulations clearly demonstrate that: (1) the rate- and temperature-dependent fracture characteristics need to be identified at the local fracture process zone, and (2) the rate- and temperature-dependent fracture properties are necessary in the structural design of asphaltic pavements with which a wide range of strain rates and service temperatures is usually associated.

Multiscale testing-analysis of asphaltic materials considering viscoelastic and viscoplastic deformation

Abstract Fine aggregate matrix (FAM) is a phase consisting of asphalt binder, air voids, fine aggregates and fillers. It acts as a primary phase in evaluating the damage and deformation of entire asphalt concrete mixtures. The simplicity, repeatability and efficiency of the FAM testing make it a very attractive specification-type approach for evaluating the performance characteristics of the entire asphalt concrete mixtures. This study explores a linkage in the deformation characteristics between the two length scales: asphalt concrete mixture scale and its corresponding FAM scale. To that end, a simple creep-recovery test was conducted for both mixtures (i.e. asphalt concrete mixture and its corresponding FAM phase) at various stress levels. Test results were compared and analysed using Schapery’s single-integral viscoelastic theory and Perzyna-type viscoplasticity with a generalised Drucker–Prager yield surface. In particular, stress-dependent nonlinear viscoelastic and viscoplastic behaviours were characterised in addition to linear viscoelastic deformation characteristics, because the nonlinear viscoelastic and viscoplastic behaviours are considered significant in asphalt pavements that are subjected to heavy vehicle loads and elevated service temperatures. With a limited scope and test-analysis results at this stage, it was found that there is a strong link between the FAM and asphalt concrete in (linear and nonlinear) viscoelastic and viscoplastic deformation characteristics. This implies that the viscoelastic stiffness characteristics and viscoplastic hardening of typical asphalt concrete mixtures could be estimated or predicted from the simple FAM-based testing-analysis method, which can significantly reduce the experimental–analytical efforts required for asphalt concrete mixtures.

Selection and preliminary evaluation of laboratory cracking tests for routine asphalt mix designs

Cracking has become a primary mode of distress in recent years that frequently drives the need for rehabilitation of asphalt pavements. Meanwhile, asphalt mix designs are becoming more and more complex with the increasing uses of recycled materials, recycling agents, binder additives/modifiers, and multiple warm mix asphalt technologies. Thus, there is an urgent need to identify reliable cracking tests that can be used for routine mix design to eliminate brittle mixes. This paper critically reviewed cracking mechanisms and laboratory tests. A total of 12 cracking tests were discussed at a cracking test workshop held as part of the National Cooperative Highway Research Program Project 9-57. Seven cracking tests were selected for further laboratory evaluation and field validation. Four of the simpler cracking tests from the seven were evaluated in this paper, these being the Texas Overlay Test (OT), Disk-shaped Compact Tension (DCT) test, Semi-Circular Bend test from the Louisiana Transportation Research Center (SCB-LTRC), and SCB test at room temperature from Illinois (SCB-IL). A laboratory sensitivity study was performed, and the results showed that all four cracking tests were generally sensitive to asphalt mix components. However, there were some concerns with the DCT, SCB-LTRC, and SCB-IL. Both the DCT and SCB-IL were found to be not sensitive to asphalt binder content; and both the DCT and SCB-LTRC showed an unexpected increase in cracking resistance when adding RAS to the mix. Additionally, two sets of field test sections were used for preliminary validation of these four cracking tests. It was found that the OT, DCT, and SCB-IL provided rankings which matched the measured field performance for the two sections on US62, Texas; and the OT and SCB-LTRC were valid for six APT test sections. Further validation with different mixes, traffic, and climate is needed.

Mode-Dependent Fracture Behavior of Asphalt Mixtures with Semicircular Bend Test

Cracking in asphalt concrete pavements causes primary failure in the pavement structure. This cracking is considered one of the key issues to be addressed when paving materials are selected and sustainable pavement structures are designed. Given the diverse nature of traffic loads and pavement geometry, the asphalt mixture in the pavement is subjected to complex cracking behavior, such as mixed-mode fracture (i.e., the combination of an opening mode and a shearing mode of fracture). To date, most studies considered Mode 1 (opening) fractures only, because of technical challenges in testing and analysis. For a better understanding of asphalt fracture and more accurate design of pavement structure, mode-dependent fracture behavior needs to be characterized. This paper presents experimental efforts to characterize the mode-dependent fracture behavior of an asphalt mixture. Toward this end, semicircular bending (SCB) fracture tests were incorporated into the results of digital image correlation analysis for a fine aggregate matrix mixture subjected to a 10 mm/min loading rate and an intermediate temperature condition of 218C. To achieve different fracture modes (i.e., opening, sliding, and mixed), the geometric loading configurations of the SCB test were varied through the use of different initial notch inclination angles and different supporting spans. Test results were further analyzed to calculate fracture resistance. Observations from this study, though limited, imply that mixed-mode fracture characteristics exist and need to be considered in the structural design of asphalt pavements with which multiaxial cracking usually is associated.

Laboratory tests and finite element simulations to model thermally induced reflective cracking of composite pavements

Abstract This study presents a mechanistic pavement modelling approach to predict the performance and damage characteristics of composite pavements at low-temperature conditions. To meet the research objective, laboratory tests were incorporated with mechanistic finite element modelling. A typical composite pavement structure where an asphalt overlay is placed on cement concrete layer was selected and modelled by considering environmental conditions and paving materials of individual layers. Thermally induced reflective cracking of asphalt overlay was predicted and analysed by conducting finite element simulations incorporated with cohesive zone fracture. Parametric analyses were also conducted by varying pavement geometry and material properties, which could lead to helping pavement designers and materials engineers understand the mechanical sensitivity of design variables on the overall responses and performance characteristics of pavement structures. This better understanding is expected to provide roadway engineers with more scientific insights into how to select paving materials in a more engineered way and to potentially advance the current structural pavement design practices.

Development of a Simple Fatigue Cracking Test for Asphalt Binders

Current Superpave® PG specification uses parameter |G*|sin(δ) to quantify asphalt binder fatigue resistance. The parameter’s effectiveness has been debated for a long time. AASHTO recently adopted the linear amplitude sweep test as a provisional standard, AASHTO TP 101-12. The authors evaluated the sensitivity of this standard to different aging conditions: unaged original binders, rolling thin-film oven-aged binders, and 20- to 80-h pressure aging vessel–aged binders. Test results showed, in many cases, longer predicted fatigue lives for more-aged binders. Thus this study developed a simple fatigue cracking test for asphalt binders. In this new test, the pure linear amplitude sweep (PLAS) test, peak shear strain was increased linearly from 0% to 30% over a course of 3,000 oscillatory cycles. A new fatigue parameter, the fatigue resistance energy index (FREI), was derived with fracture mechanics. The PLAS test and FREI parameter were sensitive to both binder aging conditions and rejuvenator type and dosage. Four laboratory mixtures were employed to evaluate the correlation between this new binder fatigue test and the two mixture cracking tests: the Texas overlay test and the Illinois flexibility index test. The results showed that the PLAS and FREI correlated well with the mixture cracking tests. Additionally, the proposed method was preliminarily verified with the FHWA accelerated loading facility test, and a fair relationship with the full-scale fatigue test data was observed. It is obvious that the PLAS and associated FREI need further validation through more field test sections.

New and Simpler Cracking Test Method for Asphalt Mix Designs

Because of environmental conservation and sustainability concerns, reclaimed asphalt pavements and recycled asphalt shingles are increasingly used in the asphalt paving industry to replace virgin asphalt and aggregate materials. However, these recycled materials are often highly aged and can cause cracking issues for asphalt pavements. Additionally, other factors such as binder additives, modifiers, and multiple warm-mix asphalt technologies can alter the performance of the mixtures both positively and negatively. The volumetric mix design alone is not sufficient for evaluating the potential cracking behavior of asphalt mixes. Although many cracking test methods are available, there is no widely accepted performance-related cracking test method that is practical enough for routine use in asphalt mix designs. This paper presents a newly developed, simple, and practical cracking test method for asphalt mix designs. The new cracking test method is repeatable, time- and cost-effective, easily implemented, sensitive to mix compositions, and well correlated to field performance. The new cracking test is performed at an intermediate temperature of 25°C and a loading rate of 50 mm/min. Furthermore, a unitless index is proposed as the cracking resistance indicator for evaluation of the cracking resistance of asphalt mixes. Additionally, the effectiveness of the new cracking test was validated with the test results from FHWA’s accelerated loading facility.

Development of an IDEAL cracking test for asphalt mix design and QC/QA

The focus in recent years has been to make asphalt mixes more affordable, and this has led to the increased use of recycled materials and binder modifications. Consequently, one often-heard complaint is that the recent mixes are more susceptible to cracking. There is an urgent need for a practical cracking test for routine use in the process of mix design, quality control, and quality assurance testing. This paper develops an indirect tensile asphalt cracking test (IDEAL-CT). The IDEAL-CT is typically run at the room temperature with 150 mm diameter and 62 mm high cylindrical specimens with a loading rate of 50 mm/min. The IDEAL-CT is a simple (no instrumentation, cutting, gluing, drilling, or notching of specimens), practical (minimum training needed for routine operation), and efficient test (test completion less than 1 min). The test can be performed with regular indirect tensile strength test equipment. As described in this paper, the IDEAL-CT is sensitive to key asphalt mix components and volumetric properties including reclaimed asphalt pavement and recycled asphalt shingles content, asphalt binder type, binder content, ageing conditions, and air voids. The proposed test also has a much lower coefficient of variation than traditional repeated load cracking tests. Furthermore, the IDEAL-CT results were compared with field cracking data collected from the Federal Highway Administration’s accelerated load facility, Texas SH15 and SH62, and MnROAD. The IDEAL-CT characterisation correlated well with field performance in terms of fatigue, reflective, and thermal cracking. Last but not the least, the ruggedness test performed in this study indicated that the IDEAL-CT, after combining both statistical and practical views, could be considered as rugged with all four variables: specimen thickness, loading rate, test temperature, and air voids.