State of the Practice in 2006 for Open-Graded Asphalt Mix Design

This document presents a survey performed in 2006 of the materials and procedures used to produce asphalt mixes with an open aggregate structure in the United States and in Europe. In the U.S. these types of materials are known as “open-graded mixes” and they are utilized mostly to improve permeability and, consequently, wet-weather friction, which is related to the frequency of traffic accidents. In Europe, these materials are known as “porous asphalt mixes” and they are used for the same reasons as in the U.S., as well as to lessen tire/pavement noise. In general, European porous mixes have higher air-void contents than those in the U.S., ranging between 20 and 25 percent; open-graded mixes in the U.S. generally have air-void contents of less than 20 percent. European countries have specifications for horizontal and vertical permeabilities, while there are no permeability specifications in the U.S. However in both Europe and the U.S., a maximum particle loss of 50 percent from the LA abrasion test is specified. In Europe, the tensile strength ratio for porous mixes is specified as at least 50 percent; in the U.S. however, only a few states specify a tensile strength ratio. The required value is usually above 80 percent for those states that have a tensile strength ratio specification.