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The New I-35: How Long Will It Last?

Back to Volume 8, Number 7


It's a bit hard to believe, but the new I-35 mainlanes we ride on through the Waco District are made up of 15 inches of concrete. And that thickness is just part of the "recipe" for the new pavements, which are designed to last at least 30 years, until about 2050.

"A lot goes into pavement design because a lot can go wrong. And that's what we are trying to avoid," says TxDOT's Stephen Kasberg, who oversaw the I-35 reconstruction pavement design. Kasberg, who is currently the Gatesville Area Engineer in the Waco District, was previously the construction materials and pavement engineer responsible for getting those recipes right.

photo from 1966 shows I-35 near Georgetown

How I-35 is built has changed over the years. This photo from 1966 shows I-35 near Georgetown, which was designed, literally, by hand. Today, engineers use computer models to design and test concrete mixtures before ever pouring a cubic foot on the roadway.


Called "continuously reinforced concrete pavement," the concrete has two layers of steel reinforcing bars throughout each section that help decrease the chance of excessive cracking — the primary cause of pavement failure. The reinforcing steel assists the pavement by keeping any cracks that do form as small as possible. Below the concrete are a hot-mix asphalt layer and flexible-base layer, which have to be specifically designed for the soil conditions. And those soil conditions vary dramatically in the Waco District, from Salado to Hillsboro.

Although Kasberg had nothing to do with the original I-35 pavements built in the 1960s, he's surprised that some of those pavements lasted as long as they did. "The original I-35 was built with no idea how much traffic we would have today. So in that respect, with all the additional stresses from all the trucks, it's remarkable that some of those sections of roads lasted so long."

slip-form paver

In 2014, workers use a slip-form paver to pour concrete during the I-35 Reconstruction Project.


Kasberg thinks the original I-35 was built to withstand 75 million equivalent single-axle loads (ESALs) — a term that reflects the number of loadings that a road can withstand. His new pavement was built for 268 million ESALs — 3.5 times as much as the old pavement.

"The biggest change we face today is traffic loading. For us, that really is multiplied because of the huge number of trucks on the road. And predicting that amount of freight 20 to 30 years in the future is a hard thing to do," he says.

Kasberg points out that a low-volume concrete rural road in Tehuacana, Texas, in Limestone County was built in 1919 — nearly 100 years ago — and is still an effective pavement today. So, it really is mostly the traffic loadings that determine lifespan. So why not build them even thicker with better materials to make them last even longer than 30 years?

layers of materials necessary to build a safe roadway

Roads aren't as simple as they seem. The display above shows the various kinds of layers of materials necessary to build a safe roadway that also lasts a long time and is flexible enough to work with Mother Nature.


"We could build roads that have double the lifespan, but unfortunately everything has a price tag," he says. "Today, a large percentage of our I-35 road users are 18-wheelers, and they really do most of the damage. That's why the mainlanes' pavement layer is so substantial."

Today, designing the right concrete pavement mixture is done by computer. Years ago, it was all done by hand. "That's another reason why we marvel at how good some of those early engineers were," Kasberg says.

"Today, we have new strategies, new formulas, and years of research to rely on to make the best road possible. But like I said earlier, a lot of care has to be taken in pavement design, because there are so many variables. Even with the best road, it eventually wears out," Kasberg explains. "With a little luck on our side, it would be great if this new pavement lasts 40 to 50 years, just like some sections of the original pavement."

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