Nelson Gibson

Viscoelastic, Viscoplastic, and Damage Modeling of Asphalt Concrete in Unconfined Compression

A comprehensive constitutive model for asphalt concrete was calibrated that included viscoelasticity, viscoplasticity, and irreversible micro-structural damage in unconfined compression. Three different types of laboratory tests were designed and performed to calibrate each of these response components. Small-strain dynamic modulus tests were used to calibrate the undamaged linear viscoelastic properties. Cyclic creep and recovery tests to failure were performed to calibrate the viscoplastic properties. Constant-rate-of-strain tests to failure were used to calibrate the damage behavior. These tests were performed at a wide range of temperatures, loading rates, and stress levels. Upon calibration of each individual response, the model was validated by predicting the results of other constant-rate-of-strain tests at temperatures and strain rates different from those used in the calibrations. The predictions for these different conditions indicate that the comprehensive model can realistically simulate a wide range of asphalt concrete behavior.

Three-dimensional image processing methods to identify and characterise aggregates in compacted asphalt mixtures

X-ray computed tomography (CT) is a novel tool to quantify the aggregate characteristics in asphalt pavements. This tool can potentially be used in QA, acceptance, design and forensic applications in pavement engineering. However, there have been challenges associated with the processing of the 3D X-ray CT images, including: (1) segmentation of aggregates that are in close proximity and (2) processing noisy or poor contrast images. This paper describes image processing methods to overcome these challenges and describes methods for computation of size, location, contact points and orientation of the aggregates in HMA. Validations of the algorithms as well as example computations of contact points and orientation have been presented. A significant increase in the number of contact points with increasing compaction level and preferred orientation perpendicular to the direction of compaction in the gyratory compactor were some of the findings presented in this paper.

TIME-TEMPERATURE SUPERPOSITION FOR ASPHALT CONCRETE AT LARGE COMPRESSIVE STRAINS

A study was performed to evaluate whether time-temperature superposition principles would continue to apply to the behavior of asphalt concrete beyond the commonly assumed small strain (<100 με) limits. A series of unconfined uniaxial compression constant crosshead displacement rate tests were performed to large-strain values. The measured axial stress versus axial strain data were cross-plotted to produce stress versus reduced time master curves and corresponding temperature shift functions at various strain levels to determine the maximum strain level at which time-temperature superposition remains valid. The results suggest that asphalt concrete remains a thermorheologically simple material well into the postpeak region (i.e., that time-temperature superposition is valid throughout the useful stress-strain response). The results further suggest that the temperature shift function aT may be only a weak function of strain level. For many practical engineering purposes, however, the differences between the small-strain and large-strain temperature shift relations may be of negligible importance.

Use of small samples to predict fatigue lives of field cores: Newly developed formulation based on viscoelastic continuum damage theory

Fatigue cracking is one of the major distresses in asphalt pavements. Accurate prediction of fatigue life of asphalt pavements can be extremely important both during the design stage and for prediction of remaining service life of in-service pavements. Traditional fatigue life predictions based on bending beam tests can be costly and time-consuming. The uniaxial push–pull (tension–compression) tests run on cylindrical samples have been a novel alternative. However, the traditional sample size for the push–pull tests may prevent its use for thin in-service pavements. This paper presents the results of a study investigating the possibility of using smaller sample sizes for push–pull tests. The viscoelastic continuum damage (VECD) characteristics of regular and small-size samples are compared, and the difference is observed to be negligible. In addition, a practical fatigue life formulation is derived on the basis of VECD theory. Uniqueness of the derived fatigue life (Nf) equation, differing from previously derived VECD-based Nf equations, stems from the fact that it does not force a certain form of equation to fit the damage characteristic curve. Finally, the differences in fatigue lives of different layers of the field sections at FHWAs accelerated loading facility are investigated.

Accuracy of Current Complex Modulus Selection Procedure from Vehicular Load Pulse: NCHRP Project 1-37A Mechanistic-Empirical Pavement Design Guide

The Mechanistic-Empirical Pavement Design Guide (MEPDG) uses the complex modulus to simulate the time and temperature dependency of hot-mix asphalt (HMA). To account for the time dependency of HMA, MEPDG recommends calculation of the frequency of the applied load as a function of the vehicle speed and the pavement structure. By this approach, the Odemark method of thickness equivalency is first used to transform the pavement structure into a single-layer system, and it is then assumed that the stress distribution occurs at a constant slope of 45° in the equivalent pavement structure. Concerns were raised that the current MEPDG methodology may be overestimating the frequency, which would result in underconservative distress predictions. Therefore, to evaluate the MEPDG methodology for calculation of the loading time, the results of the MEPDG procedure were compared with those of an advanced three-dimensional (3-D) finite element (FE) approach that simulates the approaching-leaving rolling wheel at a specific speed. The model developed accurately simulated actual tire rib sizes and the applicable contact pressure for each rib. In addition, laboratory-measured viscoelastic properties were incorporated into the FE model to describe the constitutive behavior of HMA. Comparison of these two methods shows that the frequencies calculated on the basis of the MEPDG procedure are greater than the ones determined by the 3-D FE method, which indicates that the loading time determined from MEPDG is not conservative. Ultimately, this would result in underestimation of the pavement response to a load and, therefore, greater errors in calibrations of the pavement response to field distress. Correction factors are thus presented to ensure the correctness of the loading time calculation in MEPDG. Adoption of the proposed factors within the MEPDG software does necessitate a recalibration of the performance models.

Viscoplasticity Modeling of Asphalt Concrete Behavior

A constitutive model based on an extended form of the Schapery continuum damage formulation is currently being evaluated and developed as a comprehensive material model for asphalt concrete. This model considers the viscoelastic, damage, and viscoplastic components of asphalt concrete behavior over the full range conditions of interest for the mechanistic prediction of flexible pavement distresses. The focus of the present paper is limited to the viscoplastic response component at intermediate and high temperatures. The results confirm earlier findings that asphalt concrete in compression is a thermorheologically simple material well into the large strain viscoplastic regime at elevated temperature. The study demonstrates that the proposed viscoplastic model component provides a good representation of the viscoplastic response of asphalt concrete in uniaxial unconfined compression. Given the validity of time-temperature superposition for viscoplastic response, the viscoplastic material parameters can be calibrated from a limited number of uniform time and uniform load creep and recovery tests. Typical viscoplastic material parameters are derived for a representative asphalt concrete mixture.

Laboratory Evaluation of Asphalt Binders and Mixtures Containing Polyphosphoric Acid

Four modified asphalt binders were investigated for performance grade, multiple stress creep and recovery (MSCR), mixture dynamic modulus, and mixture fatigue resistance: polyphosphoric acid (PPA) only, PPA plus Elvaloy, styrene–butadiene–styrene (SBS) only, and SBS plus PPA. MSCR data indicated that the binder modified with PPA only had the highest nonrecoverable compliance and lowest percentage of recovery, whereas the binders modified with PPA plus Elvaloy and with SBS plus PPA were best, with the lowest nonrecoverable compliance and highest percentage of recovery, depending on whether the extracted or laboratory binder was evaluated. The dynamic modulus test results illustrated a smaller difference between mixtures, except where the binder modified with PPA plus Elvaloy had a more desirable variation in stiffness (e.g., softer at high frequencies and low temperatures, and slightly stiffer at low frequencies and high temperatures). The fatigue life ranking was different before the data were normalized for controlled strain conditions with the use of viscoelastic continuum damage principles. Without normalization, data from the two SBS-modified mixtures (with and without PPA) had the highest average fatigue life; however, with normalization, the data for mixtures modified with PPA plus Elvaloy exhibited the highest average fatigue life. Implications of the results are that PPA modification strategies can provide adequate resistance to rutting and moisture damage and that modification with PPA only is not the same as (and is statistically less resistant to fatigue cracking than) modification with polymer or with polymer plus PPA. Also, comparable fatigue cracking resistance can be achieved with the use of SBS alone or SBS plus PPA, which uses less polymer in conjunction with PPA.

Effect of long-term ageing on RAP mixtures: laboratory evaluation of plant-produced mixtures

As the use of reclaimed asphalt pavement (RAP) in asphalt concrete mixtures increases, it is important to understand how the addition of already aged asphalt binders affects the overall properties and performance of the mixture. In this study, four plant-produced mixtures containing 0%, 20%, 30%, and 40% RAP were long-term oven aged in the laboratory to three levels. Mixture testing included uniaxial complex modulus and fatigue. Recovered binder testing included performance grading (PG) and rheological characterisation. The mixture testing showed that the RAP mixtures stiffen due to laboratory ageing at a slower rate than virgin mixtures and that the impact of the presence of RAP on material properties decreases with ageing time. The relative fatigue performance of the mixtures changes dramatically in stress versus strain control, indicating the importance of linking mixture and pavement design. The recovered binder properties show increase in the high continuous PG, minimal impact at the intermediate continuous PG and a slight increase in the low continuous PG with ageing time. The complex shear modulus shows an increase in stiffness and a decrease in phase angle with ageing. The binder and mixture properties generally show the same qualitative trends with respect to RAP content and ageing level; quantitative comparisons of several parameters show some correlation, indicating that binder properties may be used to evaluate the expected mixture properties measured in the lab.

Performance evaluation of REOB-modified asphalt binders and mixtures

This study evaluated a limited number, but well-controlled group of asphalt binders of the same performance grade made with a wide range of recycled engine oil bottoms (REOB) contents; 0%, 2.5%, 6% and 15%. The most practical indicator of the possible presence of a considerable quantity of REOB with measureable changes to the rheological and performance characteristics was the difference between the bending beam rheometer (BBR) m-value temperature grade and the BBR S stiffness temperature grade, ΔTcritical. When a binder exhibited a large ΔTcritical, it was associated with larger differences in performance losses depending on the binder tests and to a lesser extent the mixture test. The ΔTcritical performance disruption was made worse by oxidative ageing by means of double pressure ageing vessel conditioning as well as holding the binder at extended low temperatures before testing. The impact of REOB on moisture damage resistance showed higher moisture sensitivity with increasing REOB content, but did not interfere with liquid anti-strip additives. Mixture cracking test results were mixed. Low-temperature relaxation, strength and fracture measured with thermal stress restrained specimen test showed the fracture strength had slight increases or decreases with 2.5% and 6% REOB and could be interpreted as unaffected. Ageing also improved the average strength of these mixes. However, the strength of the highest 15% REOB mix was measurably decreased and made worse by ageing. The impact of REOB on intermediate temperature fatigue cracking performance depended on the ageing condition and whether stress-control or strain-control performance was considered. To minimise risks, a best practice needs to be developed which includes a maximum use level, taking into account both the variability of REOB and the effects on asphalt binders from different sources. Asphalt binder specifications might be refined by placing lower limits to BBR stiffness or a maximum allowable ΔTcritical. More focus should be placed on intermediate REOB levels such as near 10% to the 6% used in this study to better identify performance pitfalls and performance benefits from REOB and the corresponding ΔTcritical.

Characterizing Cracking of Asphalt Mixtures with Fiber Reinforcement

This study characterized the cracking resistance of two independent sets of mixtures from the FHWA full-scale accelerated loading facility and a Pennsylvania Department of Transportation trial section. Both sets had the same selection of three types of comparative materials: an unmodified control mixture, a mixture with a binder modified with styrene–butadiene–styrene (SBS), and the same control mixture modified with synthetic fiber reinforcement. Two methods of cracking characterization that can be conducted with the Asphalt Mixture Performance Tester were evaluated: simplified viscoelastic continuum damage cyclic fatigue and direct tension monotonic strength. Dynamic modulus results showed that fiber modification had less of an effect than did polymer modification. Cyclic fatigue test results predicted that both SBS- and fiber-modified mixes performed better than did the control mixes in both sets of materials. Furthermore, the cyclic fatigue tests also indicated that the SBS-modified mix performed better than did fiber under smaller fatigue strains, but the fiber-reinforced mix performed better at higher strains. Recent performance data from the FHWA full-scale accelerated loading facility agreed with the laboratory observation. The pattern where the fiber mixtures exhibited a strain-dependent performance benefit was also observed when the same continuum damage models were used but with data from a different testing methodology by means of monotonic direct tension tests. When all test data are considered, the performance benefits of fiber modification for crack resistance appear to be subtle when observed in the laboratory, but benefits are likely at relatively higher strains.