Effects of Temperature Segregation on the Denisity and Mechanical Properties of Asphalt Mixtures

Temperature differentials (TD) between the target laydown and actual laydown temperatures of asphalt paving mixtures have been noticed and measured since the late 1990s. Since then, numerous research studies conducted on the phenomena found inconclusive conclusions regarding the effects of TD on the initial pavement quality and the long-term performance. The objective of this study was to evaluate the effects of different levels of TD on the initial quality and the long-term performance of asphalt pavements by evaluating the core density and laboratory measured performance characteristics, respectively. Through the evaluation, it was also aimed to ascertain and establish allowable TD range, which would not adversely affect the quality and performance of the pavements. In addition, impacts of construction related factors such as using different types of material transfer vehicles (MTV), different contractors, ambient temperature, etc. on temperature and density differentials were also evaluated. Seven asphalt rehabilitation projects across Louisiana were selected for this study differing in use of mixture type, laydown temperature, mix layer, month of paving, etc. A multi-sensory infrared temperature scanning bar (IR-bar) system and a hand-held portable thermal camera were used to measure the temperature differentials. Field core samples were collected from thermally segregated areas, which were then evaluated in the laboratory using the Density test, Semi-Circular Bending (SCB) test, Loaded Wheel Tracking (LWT) test, and Indirect Tensile Dynamic Modulus (IDT|E*|) test. Two distinct temperature patterns were observed throughout all projects. Cyclic temperature patterns showing regular high-low temperature fluctuations while irregular patterns caused by work stoppages were present in all thermal profiles. Laboratory test results showed pavement density and SCB Jc values correlated strongly with the temperatures measured prior to compaction. LWT, and IDT|E*| test results showed a decreasing trend in rut depth and stiffness with increasing TD severity level. Furthermore, IR-bar temperature readings were used to measure consistency by defining one qualitative (standard deviation) and one quantitative (%severity levels) parameter. Comparisons with construction factors showed that use of MTV increased consistency in temperature. Based on the results of this study, IR-bar system was found useful to monitor consistency in laydown temperatures. However temperature monitoring at breakdown compaction was observed to present actual effect of temperature differential on pavement performance. To ensure that asphalt mixture gets compacted at target temperature, it was strongly advised to instruct breakdown compactor operator to follow the paver closely. Additionally, the use of tarps over un-compacted portion of asphalt mat is strongly advised to prevent significant temperature loss during paver stops