Flexible pavement drainage system effectiveness Masoud Seyed Mohammad Ghavami, Maryam Sadat Hosseini ⇑ , Pablo D. Zavattieri, John E. Haddock Lyle School of Civil Engineering, Purdue University, 550 W Stadium Ave, West Lafayette 47907, IN, USA highlights  The drainage layer is still needed in the flexible pavements.  Drainage layers do effectively lower the subgrade moisture content.  The granular filter shows a better performance than the dense graded asphalt filter.  The materials’ hydraulic properties required for the FEA are obtained in the lab. article info Article history: Received 6 December 2018 Received in revised form 13 April 2019 Accepted 13 May 2019 Keywords: Drainage Asphalt pavement Unsaturated flow Finite element analysis Permeability abstract Moisture can significantly affect flexible (asphalt) pavement performance. As such, it is important to remove moisture as quickly as possible from such pavements, especially to avoid allowing moisture into the pavement subgrade. Using an appropriate drainage system is one method to remove moisture from pavement structures. The objective of this research is to evaluate the effectiveness of flexible pavement drainage systems using currently specified materials to determine if, and when, the drainage system can be safely excluded from the pavements. Moreover, the effects of filter layer and edge drains are also examined. Laboratory experiments were performed to obtain the hydraulic properties of asphalt materials and used in the numerical modeling of pavement sections. The results indicate that drainage layers do effec- tively lower the subgrade moisture content. Also, while the results show that either a dense-graded aggregate or a dense-graded asphalt mixture can be used as a filter layer between the subgrade and the open-graded drainage layer, the subgrade tends to have lower moisture when a granular filter layer is used. Finally, the results indicate edge drains have a positive effect on flexible pavements, especially on pavements that do not contain a drainage layer. Ó 2019 Elsevier Ltd. All rights reserved. 1. Introduction The longer moisture remains in a flexible (asphalt) pavement structure the more likely pavement failure will occur. In particular, the continuous presence of moisture in a pavement subgrade can significantly affect the subgrade’s modulus and reduce pavement performance. Work by Ji and Nantung [8] found that at 2% above optimum moisture content, a pavement’s subgrade resilient mod- ulus can be reduced by as much as 25 percent. Arika et al. [2] found pavement subgrades with moisture contents of 8% above optimum could result in a 50% decrease in asphalt pavement life or could increase construction costs by 32 percent. Conversely, Zaghloul et al. [18] found that reducing the moisture content in an asphalt pavement base course from 45 to 16% can increase the pavement’s service life from 7 to 13 years. In an attempt to increase asphalt pavement life, the Indiana Department of Transportation (INDOT) has used a drainage system in their pavement design since 1993. This system includes an open-graded asphalt drainage layer over a dense-graded asphalt layer (filter) placed immediately on the subgrade, with the drai- nage layer connected to a pavement edge-drain. The filter layer serves as a barrier, thus reducing moisture migration both from the pavement into the subgrade and from the subgrade into the pavement. As a result, moisture moving into the pavement is quickly transported to the edge-drains and removed from the pavement, thus preventing moisture from damaging the pavement system. Over the intervening two decades, since first implementing this drainage system, INDOT has updated its asphalt pavement design https://doi.org/10.1016/j.conbuildmat.2019.05.088 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail addresses: mseyedmo@purdue.edu (M.S.M. Ghavami), mhossei@purdue. edu (M.S. Hosseini), zavattie@purdue.edu (P.D. Zavattieri), Jhaddock@purdue.edu (J.E. Haddock). Construction and Building Materials 218 (2019) 99–107 Contents lists available at ScienceDirect Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat