Global warming and increase in fuel prices in the world induced the pavement committee to consider alternative construction techniques to conventional Hot Mix Asphalt (HMA). Warm Mix Asphalt (WMA), which is the general term used for the asphalt pavements produced and placed at lower temperatures, is introduced to the pavement industry to overcome the environmental and economical challenges of HMA. WMA technologies reduce the overall mixture viscosity at lower temperatures to increase the workability of the loose mixtures and provide improved (better) compaction. WMA also facilities aggregate coating at low temperatures. These technologies have been extensively used in Europe after 1995 and after 2004 in USA. However, there are still many unknowns related to their long term performance in the field. Foam-based WMA's are the most commonly used techniques; however there is no specification or method to evaluate the quality of foam generated by different techniques. Although producing foamed binder is relatively simple process, where hot binder is mixed with a limited amount of water (typically 2-3% by weight of the binder), the rheology of the foamed binder is not very simple. The quality of the foamed binder depends various factors such as the binder type, grade and modification, the foaming technology used, amount of water, and temperature. Moreover, the quality of the binder plays a crucial role during the mixing, laying and compaction stages of WMA pavement production. Asphalt foams used in base stabilization applications were typically characterized using following three parameters: Expansion Ratio (ER), Half-life (HL) and Foam Index (FI). However, there is no available method to measure these parameters precisely. Therefore, an accurate and repeatable procedure is needed for the measurement of reduction in height of foamed asphalt in order to calculate the foam binder quality parameters. Asphalt Foam Collapse Test (AFCT), an automated test to measure the reduction in the height of the foam binder over time via image analysis, is developed during this study. The height reduction data is used to accurately calculate the commonly used foam quality parameters. In addition, two new parameters, Bubble Size Distribution (BSD) and Surface Area Index (SAI), are introduced as quality parameters in this study. It is found that these parameters are strong candidates for evaluating the workability and coating, as well as the performance of the pavements. A nozzle-based laboratory foamer was utilized in this study to determine the effect of water content and air pressure on the foam quality individually and in combination. Results revealed that the water content and air pressure have significant influence on ER, HL, FI, BSD, and SAI. It was observed that the low water content and low pressure produced foams with relatively small bubbles as compared to foams made with high water content and pressure.The current WMA pavement design procedures are based on limited empirical data and recommendations of the WMA technology suppliers. WMA design procedures do not consider the foam quality since its importance has not been fully understood. Therefore, the long term performance of the WMA mixtures prepared with foamed binders prepared with various injected water content and air pressures was evaluated via laboratory performance tests and compared with the foamed binder quality parameters. It was concluded that a WMA mix design should consider the foam quality, which is currently ignored. In summary, this study identified/developed various parameters as foam binder quality indicators, developed a practical laboratory device to measure these parameters, validated the device by comparing with nondestructive 3D imaging methods (i.e., x-ray microtomography), investigated the relationship between the binder quality indicators and the mixture performance tests, investigated foaming characteristics of different kinds of binders, and investigated the effects of air pressure and water content on the foamed binder and mixture performance.
Read
