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. (C) 2013 American Society of Civil Engineers.