Field Evaluation of Different Rollers Effect on the Compaction of Hot Mix Asphalt Overlays Using an Electromagnetic Densitometer Seyed Rasool Fazeli 1,* , Hamidreza Sahebzamani 2 , and Omid Ferdosian 3 1 Corresponding author; School of Civil Engineering, University of Tehran, Tehran, Iran. rasoolfazeli@alumni.ut.ac.ir 2 School of Civil Engineering, University of Tehran, Tehran, Iran 3 School of Civil Engineering, Iran university of Science and Technology, Tehran, Iran ABSTRACT. Paving asphalt overlays, from raw materials extraction, production of asphalt mixtures, transporting the asphalt mixture to the job site, and laying hot mix asphalt with paving machines should constantly be tested. One of the main controls after asphalt paving is the compaction of the new layer and its accurate density. Inaccurate compaction will reduce the pavement life. Core sampling is the most known method for determining newly paved roads. Using this method has its defects. Destruction of the new paved layer for sampling will be the weakness of the new layer. As a result, there will be a limitation for measuring the asphalt pavement density. On the other hand, contractors cannot achieve the accurate number of rollers passes and density of the implemented HMA layer. Samples should be tested at the laboratory, and insufficient density will face contractors to penalties or even reconstructions. These concerns led to non-destructive methods to measure in-place density. There are two non-destructive methods for measuring the in-place density of asphalt overlays. The electromagnetic and the nuclear densitometer. The main concern of using these non-destructive methods is the accuracy of measurements. In this research, authors tried to evaluate the effectiveness of using an electromagnetic densitometer for measuring the density of new asphalt overlays in two sites. Device results after site calibration were compared to core samples. Determination of sufficient roller pass using an electromagnetic densitometer was the other purpose of this study. The most common rollers that use for hot mix asphalt compaction are pneumatic and steel wheel rollers. The effectiveness of using pneumatic and steel wheel rollers on HMA layer compaction was the other issue evaluated in this study. Results showed that electromagnetic densitometer is a not reliable non-destructive method for quality assurance of asphalt layer in-place density. Although, a sufficient number of roller passes could be determined for any project by this method. The other important conclusion was that the steel wheel roller has a greater role in gaining density than the pneumatic roller. Moreover, in the case of using a pneumatic roller without a steel wheel roller, the need for passes will increase. Keywords. Electromagnetic Densitometer, Asphalt Compaction, Pneumatic and Steel Wheel Roller. Introduction In the first of the 90s, there were two major methods for determining Bulk specific gravity of newly paved asphalt pavement, core sampling, which was a destructive method, and the nuclear densitometer, which was the non-destructive method. In the nineties, a new method was proposed. This new method was non-destructive, and it was much safer, faster, lighter, and cheaper than the nuclear method. The primary model of this apparatus was fabricated in 1991 as a pavement quality indicator (PQI), and Troxler made the next model in 1998. This apparatus measures the compaction of HMA by measuring the dielectric constant of the components of the asphalt layer. It passes the electromagnetic waves through the asphalt layer. These waves move from transmitters to receivers in an isolated field. Impedance or electrical resistance will be used for dielectric constant by passing through the asphalt layer. Accumulation of dielectric constants is a proportion of density (Hausman and Buttlar, 2002).