VAA Virginia Asphalt Fall/Winter 2022

VAASPHALT.ORG 21 So, is pavement design really that complicated, or is it job security for niche engineers like me? The answers are simple: yes and yes. History of Pavement Design Before we can understand the complexities of pavement design, we need to understand how it evolved. For the sake of this article, I plan to cover the quick history lesson; but, I’m no Dr. Al-Qadi, so I will recommend anyone interested in a more detailed history of pavements to visit Pavement Interactive ( Most people have heard about the ancient Roman roads. Before Rome, however, there were roads built by the Mesopotamians and peoples on the Indian subcontinent some 4,000 years ago. These roads were constructed from pavement composed of thick layers of stone, then covered with smaller rocks to lock the materials into place. The pavement’s thickness and the stones’ size protected the natural subgrades and drainage and storage capacity for water. The final surface had to be smooth enough so that large armies could quickly march and drive their chariots across their empire. To my knowledge, these roads were not designed using layered elastic theory or finite elements but they shared our commitment to empirical study: trial, error and observation. Once they found out what worked, the method was replicated over and over, often shared across cultures. Fast forward a few thousand years. While the Roman roads were still being used in the 18th and 19th centuries, the populations of Europe and the United States were growing and expanding outwards, requiring more roads. The roads built by the Romans were costly and labor intensive, so while they worked and persevered over the centuries, that was not the road-building approach used by modern builders. Too many roads were needed for an expanding society, and too quickly. The pavement produced was designed simply to keep travelers out of the mud. Enter British surveyor Thomas Telford and Scottish engineer John McAdam. Both took what they observed from the Roman roads and their experiences with contemporary pavements to make design modifications. Telford designed roads on a flat subgrade with three layers of stone, for a maximum thickness of approximately eighteen inches (450 mm). Drainage was provided on the surface, but this approach allowed the subgrade to be wet. Following Telford was the Macadam method, popularized by John McAdam. He noted two issues with the Telford approach. One, that most surface stones were round and not angular, allowing them to be easily displaced by traffic. Therefore, he proposed replacing the round stones with angular rocks to help lock the surface. Two, the flat subgrade needed to be sloped to aid drainage. His approach still used three layers of stone with the top layer mixed with fines and compacted, but the total thickness was reduced to approximately ten inches (250 mm). The design refined by McAdam was used for decades. By the mid-to-late 1800s, tar and asphalt materials were mixed with aggregates to form a more stable and durable pavement. Macadam projects were then installed in Nottingham, England; Nashville, Tennessee and Washington, D.C. In the 1900s, various locations in the United States had their own set pavement designs. The materials used for the pavements were based on local specifications, and the layer thicknesses were based on experience and performance. It was not until the middle of the 20th century that flexible pavement thickness equations were developed in order to methodize design. Much like the original American Association of State Highway and Transportation Official (AASHTO) nomographs developed from their well-known Road Test in Ottawa, Illinois, these equations were empirically based. In other words, build it and see what happens. Since the AASHTO Road Test, engineers and researchers have been refining the design process. Many large oil companies had pavement research offices like Royal Dutch Shell and Chevron for asphalt pavements. They and entities such as the Asphalt Institute developed Interstate 81 after 40 years of service Just a little too thick continues on page 22 △ PAVEMENT DESIGN: A PHILOSOPHY CONSIDERED