Hasan M. Faisal

Effects of Dwell Time and Loading Rate on the Nanoindentation Behavior of Asphaltic Materials

In a nanoindentation test, a test sample is indented or loaded by an intender tip and then unloaded, and the load-displacement data are recorded. Load-displacement data are then analyzed using a common method to determine elastic modulus and hardness of materials. The slope of the unloading curve is positive and typically follows the slope of the loading curve for hard samples such as aluminum and silicon. The analysis method is not applicable if the unloading curve follows a negative slope, which is the case for soft viscous materials, such as asphalt binders and polymers. However, past studies have not attempted to examine the effects of dwell time and loading rate on viscous materials, such as asphalt binders, which are performed in this study. It is shown in this paper that an increase in dwell time shows a decrease in the bowing out or nose effect of the unloading portion of the load-displacement curve. At very low loading rates, asphalt binder shows a high viscous effect, which creates a large negative slope of the unloading curve.

Modeling Nanoindentation Creep Behavior of Asphalt Binder

Researchers have used the Oliver-Pharr method to analyze nanoindentation test results for viscoelastic materials without considering the viscous effect. This study develops procedure to analyze nanoindentation creep results of asphalt binder considering the viscous effect. In particular, models such as Voigt, Burger, and others, which use spring, dashpot, and rigid body are used to fit the laboratory data. The spring-dashpot-rigid (SDR) element model uses the loading, holding, and unloading time-displacement data to predict the modulus, hardness, and indentation viscosity of the material. Modulus and hardness from the Oliver-Pharr model are much less than those from SDR model. In addition to modulus and hardness, the SDR model and other Voigt and Burger models can provide viscous parameters that are very important for the advanced modeling of asphalt concrete. In the study, the model parameter retardation time shows a clear decreasing trend with increase in loading rate, however, no clear trend is found between retardation time and dwell time (the time where maximum load is kept constant for a specific period of time). In the study, the nanoindentation test results are analyzed with both the nonlinear SDR model, as well as the linear SDR model. However, the nonlinear SDR model showed higher efficiency in prediction compared to the linear SDR model.

Characterisation and modelling of vapour-conditioned asphalt binders using nanoindentation

Nanoindentation tests were conducted on unconditioned and vapour-conditioned asphalt binder samples to determine damage processes by characterising contact creep data and using viscoelastic mechanical models. For vapour-conditioned asphalt binders, a thin film of asphalt binder was prepared on a glass slide and subjected to relative humidity (RH) of 25%, 49% and 71% inside enclosed desiccators using three aqueous solutions: potassium acetate, potassium carbonate and sodium chloride, respectively. Based on the nanoindentation contact creep test data, it was observed that vapour-conditioned asphalt binders showed larger indentation viscous depth than the unconditioned binders. Indentation test data were modelled using viscoelastic mechanical models such as Burgers and Maxwell models. The models showed that elasticity increased and viscosity decreased in the vapour-conditioned asphalt binders. In 71% RH-conditioned binders, elastic components of Burgers model, E1 and E2 increased about 10% and 15%, respectively, compared to the unconditioned binder. The relaxation time decreased about 35% and retardation time increased about 69% in 71% RH-conditioned binders compared to unconditioned binders.

Nanoindentation Characterization of Moisture Damage in Different Phases of Asphalt Concrete

Traditional microscale testing cannot be performed on asphalt binder, mastic, or aggregate, while they are an integral part of asphalt concrete (AC). Recently, nanoindentation has created an opportunity to characterize mastic and asphalt binder while they reside in an AC sample. In the study, laboratory nanoindentation testing is carried out to characterize moisture-induced damage in different phases of AC. A moisture-induced sensitivity testing (MIST) device is used for moisture conditioning of AC. In the MIST device, an AC sample is fully submerged under water and all-around cyclic pressure is applied through the pores inside an AC sample to cause damage. Damaged AC samples are indented for an extended dwell time of 200 s using an unloading rate of 0.02 mN/s to minimize viscous effects of asphalt on test results. The indentation load-displacement curve is analyzed by the Oliver–Pharr method to obtain elastic modulus and hardness. When comparing wet and dry sample indentation test results, it is observed that the modulus of the wet mastic reduces to 60 % of dry mastic modulus. Overall moisture conditioning reduces the modulus of AC by 70 %. In addition, the creep response of the mastic phase is modeled by a viscoelastic Burger model. The creep compliance value of wet mastic is 42 % higher than that of dry mastic.

Finite Element and Mechanical Modeling of Fatigue Behavior of Partial Vapor-Conditioned Viscoelastic Material

Past studies have shown that vapor conditioning to 100% Relative Humidity (RH) reduces the fatigue life of viscoelastic materials such as asphalt binder. However, it is not known how partial vapor conditions such as RH 25%, 49%, and 71% affect asphalt binder’s fatigue behavior. In addition, it is unknown which viscoelastic material parameter (i.e. viscus or elastic parameter) is responsible for damage in asphalt binder or Asphalt Concrete (AC) in general and what steps can be taken to reduce fatigue damage. In this study, films of asphalt binders were prepared and partially vapor-conditioned in enclosed chambers containing potassium acetate (25% RH), potassium carbonate (49% RH), and sodium chloride solutions (71% RH). Creep nanoindentation tests were performed on the vapor-conditioned asphalt film samples. The nano-creep test data are fitted using Burgers models. The Burgers model shows that elasticity increases and viscosity decreases as RH% increases. To this end, a Finite Element Method (FEM) model is developed in ABAQUS to examine the fatigue performance of the asphalt binder at 49% vapor-conditioned only. Using the spring and dashpot elements of Burgers model as FEM inputs, simulations are run. Results indicate that an increase in binder viscosity would reduce permanent deformation in the viscoelastic material.Copyright