Raul Velasquez

Modification and Validation of Linear Amplitude Sweep Test for Binder Fatigue Specification

Current asphalt binder specifications lack the ability to characterize asphalt binder damage resistance to fatigue loading. Multiple accelerated testing procedures that attempt to efficiently and accurately characterize the contribution of asphalt binders to mixture fatigue are under investigation. One of these tests, which has received significant acceptance by experts and has been submitted as a draft AASHTO standard, is the linear amplitude sweep (LAS) test. This procedure uses viscoelastic continuum damage mechanics to predict binder fatigue life as a function of strain in the pavement. The LAS test uses cyclic loading with systematically increasing load amplitudes to accelerate damage and provides sufficient data for analysis in less than 30 min. Although results of the current LAS testing protocol are promising, the time and the complex numerical procedures required for the analysis have raised concern. In addition, insufficient damage accumulation was observed when the strain amplitudes proposed in the LAS test were used for a set of polymer-modified binders. This paper presents simplifications of the current analysis procedures and evaluates the ability of extended strain levels to cause sufficient damage for better calculation of the binder fatigue law parameters. The effectiveness of the modified procedure was validated by comparison of the results with the fatigue performance recorded by the Long-Term Pavement Performance program with consideration of the pavement structure. The fair correlations showed the potential for effective use of the modified method for binder specifications.

Measuring the Effect of Moisture on Asphalt–Aggregate Bond with the Bitumen Bond Strength Test

Understanding moisture damage mechanisms in asphalt pavements and evaluating the right combination of materials that are resistant to moisture damage are important. Moisture damage is the loss of strength or stiffness in asphalt mixtures caused by a combination of mechanical loading and moisture. Many test methods have been developed to evaluate loss of adhesion and cohesion in binders. However, a simple procedure to address moisture damage in the asphalt–aggregate interface is not available. The feasibility of the newly developed bitumen bond strength (BBS) test for moisture damage characterization was investigated. An experimental matrix that included various binders, modifications, and aggregates to account for the chemical and physical conditions in the aggregate–asphalt interface was completed. A statistical analysis was performed to verify reproducibility of the BBS test. The results indicated that the bond strength of asphalt–aggregate systems was highly dependent on modification and moisture exposure time. Polymers were found to improve the adhesion between asphalt and aggregate as well as the cohesion within the binder. Results from this study indicated that the BBS test was repeatable and reproducible. To further validate the effectiveness of the BBS test, a comparison of the BBS test results and the modified dynamic shear rheometer strain sweep test was conducted. The comparison showed that the BBS test could rank materials similarly to a more sophisticated and time-consuming test.

Measuring physico-chemical interaction in mastics using glass transition

The current design standard for asphalt mixtures provides guidance on selection of aggregates, asphalt binder and includes requirements for the amount of mineral filler to be included. The amount of filler that can be included is limited to a ratio in the range of 0.6–1.2 by mass of the binder. However, this range is based on experience rather than on scientific evaluation of the interaction between filler and binder. Although many researchers acknowledge the physico-chemical interaction between asphalt binder and the mineral filler, currently a procedure to quantify this interaction, and consider it in selecting favorable filler to binder ratio, is not available. In this paper, the effect of fillers on the glass transition temperature (T g) of the base binder was used to evaluate the physico-chemical interaction in mastics. The total reinforcement of the filler, which is measured in terms of relative viscosity of the filled binder to the unfilled binder, consists of two parts: mechanical and physico-chemical. The mechanical reinforcement part is calculated based on micromechanical models commonly used in the literature that take into account volume filling effects and particle-to-particle interactions. Physico-chemical reinforcement is estimated based on the change in the T g in both Williams-Landel-Ferry (WLF) and Arrhenius time-temperature shift models. The concept introduced in this study is evaluated by viscosity and dilatometric T g testing of three binders mixed with three different fillers, at different concentrations. Results show that the physico-chemical interaction between the mineral filler and the binder can be accurately estimated from the difference in the glass transition temperature of the mastics and the binder.

Modeling Thermal Stress in Asphalt Mixtures Undergoing Glass Transition and Physical Hardening

Asphalt binders have been shown to undergo significant time-dependent stiffening when stored at low temperatures. This physical hardening has a significant effect on the laboratory performance of asphalt binders. However, the importance of isothermal conditioning for asphalt mixtures and its effect on thermal cracking performance have been a subject of significant debate. A theoretical approach accounting for the glass transition and physical hardening in the thermal stress buildup in mixtures was derived from relaxation modulus master curves, the William–Landel–Ferry equation, Boltzmanns superposition principle, and a model describing the isothermal contraction of asphalt as a continuous function of conditioning time and temperature. With the model predictions, it is shown that thermal stress relaxation and stress buildup induced by physical hardening can continuously affect thermal stress throughout the cooling process. The cooling rate also affected the amount of delayed stress buildup that occurred after the temperature had stabilized at isothermal conditions as a result of physical hardening. A relatively simple device was developed and used for verification and support of the thermal stress model. Mixture measurements performed at different cooling rates and isothermal conditions supported the theoretical predictions. The findings clearly show that the effect of physical hardening on stress buildup in mixtures is measurable and important. Therefore, the glass transition of asphalts and their behavior under isothermal conditioning needs to be measured to better predict thermal cracking.

Effect of Healing on Fatigue Law Parameters of Asphalt Binders

Asphalt binder has the ability to self-heal during rest periods when repetitive loading is applied. Studying the effect of rest on fatigue law parameters provides useful insight into the healing capabilities of asphalt binders. Currently, standard testing and analysis procedures to quantify asphalt binder healing capability are limited and difficult to implement in practice. Fatigue is known to depend on both traffic loading and pavement structure. Power law relations (e.g., Nf = Aγ−B) are commonly used for fatigue analysis of pavement materials. Power laws are used to estimate fatigue life (i.e., number of cycles to failure, Nf) as a function of load amplitude (e.g., strain, γ), which is a reflection of the pavement structure. In this study, testing consisted of strain-controlled time sweeps in the dynamic shear rheometer with a single rest period inserted at a specified damage level. With the selected test, the effect of healing on the relationship between fatigue life and strain was investigated. Nine neat and modified binders were tested. Healing testing was conducted at multiple age levels and strains. Healing that resulted from a single rest period had an insignificant effect on fatigue performance compared with modification and oxidative aging. Although this paper highlights the challenges of using few rest periods to predict healing potential, preliminary results of testing with multiple rest periods show the importance of healing. Further investigation is needed to verify the effect of multiple rest periods on binder fatigue.

Critical factors affecting thermal cracking of asphalt pavements: towards a comprehensive specification

Thermal cracking remains one of the most challenging distresses to predict and reduce in pavements constructed in cold regions. The research community after more than 40 years of extensive research has gained significant understanding of the mechanisms and factors affecting this distress. Current specifications and testing methods used for the selection of materials are a good starting point for the minimisation of this distress. However, there are critical factors and properties not addressed in current specifications that if considered could further improve the performance of asphalt pavements to thermal loading. This paper provides a critical review of the main factors affecting low-temperature cracking and the current analyses and testing methods used for the design. Analysis of thermo-volumetric and fracture properties from a multi-scale perspective (i.e. Binder→Mastic→Mixture) is used to suggest the addition of key performance parameters that can complement current specification parameters (e.g. S and m-value at 60 s in the bending beam rheometer). Furthermore, this paper discusses the pros and cons of the traditional methods used for thermal stress–strain analysis when designing pavements to resist thermal cracking: (a) continuous approach and (b) fracture mechanics approach (i.e. inclusion of a pre-determined flaw). Based on recent findings and testing results from major research activities as part of the Asphalt Research Consortium and two pooled-fund studies on low-temperature cracking (Phase I and Phase II), recommendations are made with regard to the analysis framework and to the mechanical characterisation of asphalt materials. Suggestions are made for the development of a more comprehensive specification that includes fracture and thermo-volumetric properties not accounted for in current practice.

Relationship between binder and mixture damage resistance at intermediate and low temperatures

The importance of binder performance in mixture response to accelerated fatigue loading and thermal cracking was investigated. Binder fatigue performance was measured by means of the linear amplitude sweep test, and fatigue properties of the mixtures were investigated by performing a strain sweep test. The low-temperature properties of the binders were investigated by measuring the glass transition temperature and fracture properties with the single-edge notched beam (SENB) test. The mixtures fracture properties were investigated by using the FENIX test. The experimental matrix for this study included unmodified and modified binders and limestone aggregates. A good correlation between binder and mixture fracture energy was observed at low temperatures. This good correlation indicates the importance of the fracture response of the binder to the overall low-temperature cracking performance of the mixture. Experimental results suggest that a significant part of the variation of the fracture energy of the mixture can be explained by the binder fracture properties. Good correlations were also obtained for the displacement at maximum load in the SENB and FENIX tests. Similar accelerated fatigue responses for binders and mixtures were observed when the stresses and strains were normalized. Significant reduction in stress occurred at about the same normalized strain in the binder and mixture. The mixture had remaining strength after reaching peak stress, probably as a result of the aggregate structure.

Asphalt thermal cracking analyser (ATCA)

The Asphalt Thermal Cracking Analyser (ATCA) is a device that can simultaneously test two asphalt mixture beams while undergoing selected thermal history. The first beam is unrestrained and thus the change of its length with temperature can be used to obtain glass transition temperature (Tg) and coefficients of thermal expansion or contraction. The second beam is restrained at the ends and can be used to measure the thermal stress build-up as a function of time and temperature. The measures of length change and stress in beams can be used to get a comprehensive evaluation of the low temperature performance including change in strain, stress as a function of time and temperature.

Effect of Water Conditioning for Extended Periods on the Properties of Asphalt Binders

Selecting materials with a low susceptibility to moisture damage (i.e., stripping) is critical to guarantee the performance of asphalt pavements. Stripping in the asphalt mix can accelerate damage and consequently reduce pavement life. Most current research on moisture damage considers the effect of water on the mastic, the aggregate, and the adhesive bond between the aggregate and the binder. However, limited research has been conducted to determine the effect of extended water exposure on the properties of asphalt binders. This study investigated the influence of extensive water exposure on the stripping potential of asphalt binders by measuring the rheological properties, bond strength, and wettability of a Colombian binder before and after immersion. Thin films (2-mm height) of asphalt were immersed in water for 3, 6, and 9 months, and comparisons were made between the experimental results of unconditioned and conditioned binders. Master curves for complex modulus and phase angle were obtained by using a frequency sweep test in a dynamic shear rheometer. The bond strength between the binder and aggregates was measured with the recently developed binder bond strength test. The wettability potential of the conditioned and unconditioned binder was estimated by using the sessile drop method, and dynamic modulus testing of mixes prepared with the unconditioned and conditioned binder was conducted. Experimental results showed significant changes in the properties of the binder after 9 months of water conditioning. Further, dynamic modulus of the mixes prepared with the binder conditioned for 9 months was significantly higher than the modulus of the unconditioned mix.