Flint — On I-475, the highway is closed south of Carpenter Road.

 The southbound lanes are smooth and inky black. This brand new asphalt has only been driven on by the Michigan Department of Transportation (MDOT) and contractors.

 Separated by the new barrier walls is a project that is just getting started — the 50-year pavement design that could change the way roads are built in Michigan.

 The small stretch will look the same as the southbound lane beside it, but it’s designed to last 50 years instead of 20 or 30. It’s expected to be completed by December and is part of an overall $44-million project to repair 18 miles of I-475. The 50-year stretch is a complete rebuild, down to the clay.

 Project Manager John Welch said this is one of four “showcase projects” mandated by Michigan legislature to test the durability of longer-lasting designs. The other project is on I-69 from Ballenger Highway to Fenton Road on the westbound side. This is a 30-year design.

 Roads that are more durable are more expensive and require more material. It is made of the same materials, but more of them. The southbound section, which is a standard “20-year” design has a base of 18 inches of sand, 6 inches of aggregate (crushed stone and concrete) and topped with 7.75 inches of hot mix asphalt (HMA.)

 The 50-year design will have significantly more materials — 24 inches of sand, 12 inches of aggregate and 11 inches of HMA.  

 MDOT and Michigan State University (MSU) will periodically test this 50-year road using the southbound lane for comparison. “The concept is to determine in a real world setting if these longer term designs truly last longer and require less maintenance costs over their lifespan,” Welch said.

 On Thursday, Aug. 29, Dan’s Excavating Inc. began laying concrete storm drains into the ground. In other spots, they rolled the clay base to prepare for the first layer.

 A mini concrete plant is set up farther to the north, where old barrier walls are crushed and ground down to be part of the new road.

 The approximate cost to rebuild a rural freeway is $2 million per lane mile, and $2.5 million for an urban freeway. These roads are designed to last 20 years, and even longer with preventative maintenance.

 This 50-year design costs approximately 37 percent more to build.

 Whether it is cost effective to build these roads will be determined during testing.

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