NEVER HEARD OF PERPETUAL PAVEMENT? YOU WILL!
BY FRED F. FRECKER, P.E.

Today's asphalt pavement already lasts a long time. With surface maintenance it can last more than 30 years. In fact, no full depth asphalt pavement on Ohio's highway system has ever needed to be replaced. Commonly it is a failing joint in the base below some asphalt pavements that leads to major rehabilitation. That's what keeps the orange barrel manufacturers in business, and the rush hour drivers clenching their fists.

"The problem is the industry has designed pavements to fail," said Dr. Marvin Traylor, director of engineering at the Illinois Asphalt Paving Institute. "We design a pavement to handle X number of loadings before it will finally need to be replaced. Perpetual Pavement will be designed never to exceed a critical fatigue level and therefore never to fail. With surface maintenance, these pavements will truly last a lifetime, taking a significant bite out of traffic delay in the future."

This increased durability will be accomplished through the use of mechanistic design. In a mechanistically based pavement, designers analyze how traffic strain will affect the pavement's performance, taking into account material qualities and thickness. By designing the pavement to keep strain below the critical level, fatigue failure is avoided and perpetual performance can be assured. Structural and aeronautical engineers have used mechanistic design principals for years. It's only a matter of time before it's the standard in the paving industry.

Through the use of mechanistic design, each layer in Perpetual Pavement's 3-layer system will be tailored specifically to local climate and traffic conditions. The base layer will be made of flexible fatigue resistant asphalt, designed to resist bottom-up fatigue cracking. The middle layer will be made of high modulus, rut resistant asphalt, formulated to support expected traffic. The surface layer will be made of SMA, OGFC or Superpave, and will be designed to eliminate rutting, reduce splash and spray, absorb noise and hold up to local weather and traffic conditions.

The combination of the three layers will force distress to the surface layer, where it can quickly be maintained at a low cost. The surface will be milled off, recycled, and replaced. This means that improved surface mix design can be incorporated into an existing Perpetual Pavement as it is developed over time.

Perpetual Pavement will also provide a consistently smooth and quiet ride. As traditional pavements approach the need for major rehabilitation the ride quality, or "serviceability," deteriorates into bumps and thumps, until the pavement needs to be overhauled, once again delaying traffic. With Perpetual Pavement, surface maintenance in off-peak hours will keep the serviceability level close to perfect throughout the life of the pavement, without traffic delay. This means drivers will enjoy a perpetually smooth and quiet ride.

While there are currently no Perpetual Pavements in service in Ohio, there are plenty of deep strength asphalt pavements from which we can draw meaningful data, since Perpetual Pavement is essentially an improvement on the existing deep strength asphalt design. Flexible Pavements of Ohio commissioned a study of adjacent deep strength asphalt and concrete pavements. The selection of adjacent pavement sections ensured identical weather and traffic conditions. The study found that the asphalt sections had a lower life-cycle cost, required less maintenance and were a better value to taxpayers. In fact, all of the asphalt pavement sections studied are still in service, some for as long as 40 years.

Deep strength asphalt pavements have been widely used in Europe for years. A recent study conducted by the Transport Research Laboratory in England concluded that in pavements thicker than 8 inches, rutting and cracking was confined to the top layer - where it is easily milled off, recycled and replaced.

These reports show us two things: Thick pavements perform better than thin ones; and when it comes to choosing a pavement that will last a long time, asphalt is the answer. Studies of this nature led the industry's top researchers to deep strength asphalt as the starting point in designing Perpetual Pavement. By incorporating mechanistic design and segmenting the pavement into three distinct layers, Perpetual Pavement begins to take shape.

Engineers from the University of California at Berkeley constructed a segment of Perpetual Pavement and, using an accelerated load tester, were able to simulate years of traffic strain. The results of these tests helped CalTrans engineers evaluate material qualities and thickness for use in the rehabilitation of I-710, the Long Beach Freeway. The partnership between these two organizations led to the development of a strategy to convert this California highway into a Perpetual Pavement.

Dr. Marvin Traylor of the Illinois Asphalt Paving Institute assisted an Illinois Department of Transportation task force in developing a strategy to make Illinois' existing deep strength asphalt pavements perform perpetually. Dr. Traylor's research and recommendations helped the task force develop design plans for I-70 in Clark County and will be used as a road map for other states building Perpetual Pavements. Construction of the first Perpetual Pavement in Illinois is slated to begin in 2003.

Perpetual Pavement is not just a wild-eyed look into the future. It's here today, with projects happening in California, Michigan, Texas and Illinois. Discussions with the Ohio Department of Transportation indicate that a pilot Perpetual Pavement project in our own state may be a reality in the near future. Perpetual Pavement's lifetime guarantee will take asphalt's advantage in cost, convenience and comfort to the next level - something the orange barrel manufacturers may not be so happy about.

Fred Frecker is a Professional Engineer, and is President and Executive Director of Flexible Pavements of Ohio.