Free. That's what University of Michigan Professor of Civil and Environmental Engineering Sherif El-Tawil’s team will charge for the non-proprietary ultra-high performance concrete (UHPC) blend it has created.
At the request of Michigan’s Department of Transportation (MDOT), U-M engineers created the new UHPC formula. UHPC has been available from several companies but the high cost of the material has kept DOTs from using much of it.
“Regular concrete will have a strength of about 4,000 pounds per square inch (psi)… that’s maybe the weight of an SUV on every square inch,” said Sherif El-Tawil, a U-M professor of civil and environmental engineering. “However, UHPC can support at least 22,000 psi or six times as much.”
El-Tawil's UHPC is a new class of concrete that utilizes a low water content as well as steel fibers mixed in for extra density. Once set, the concrete is able to take on significantly more impact load (over 6-7 times more) than regular concrete before it fails.
The U-M blend also improves the concrete's durability. As measured in freeze-thaw cycles, ordinary concrete tested in the lab showed deterioration at 28 freeze-thaw cycles. The U-M team’s UHPC blend showed virtually no deterioration after 90 cycles
According to the Michigan Engineer report, DOTs looking to replace a bridge or rebuild a crumbling road, might spend $100 to $120 per cubic yard of regular concrete. To upgrade to a brand-name UHPC, that same local government would be looking at paying $3,000 – $4,000 per cubic yard.
“While the durability properties of UHPC are very attractive, mixes are currently prohibitively expensive for widespread use and require specialized equipment and curing practices,” MDOT’s research report stated. “But if costs were reduced, UHPC could have potential including thin overlays for decks that now need to be replaced every few decades.”
Reaching that extreme level of strength and durability requires modification to the traditional concrete recipe. The formula for regular concrete contains some fairly simple ingredients – cement, gravel, sand and water.
Instead, El-Tawil removed the gravel in favor of different types of sand. Using ideas from the pharmaceutical industry, the particle sizes of the sands are carefully selected so that the resulting UHPC mixture has a structure that is densely packed.
Higher density leads to fewer voids inside, reducing the opportunities for water to penetrate and cause problems during the freeze-thaw process. The dense packing is also the reason why UHPC is so strong.
To reduce the carbon footprint of the material, a byproduct of the steelmaking industry (ground slag) is used to replace a significant portion of the cement and, at the same time, help increase the packing density.
A key ingredient in UHPC that is not in regular concrete is steel fibers. Isaya Miyata, a senior studying civil engineering, is among those who make sample batches of UHPC in a mixing lab on U-M’s North Campus.
“Steel fibers add strain-hardening properties to the concrete,” he said. “So basically, once it yields the concrete will take a lot more load before it ultimately fails.”
While the fibers themselves are small, roughly a half-inch in length, they have an outsize impact on the final price of UHPC. They come from chopped up thin wires such as those used in steel-belted tires. There aren’t many other uses for them, therefore they are quite expensive.
Because of the fibers, the UHPC created at U-M is still more expensive than regular concrete. However, the U-M UHPC cost 70 percent less than UHPC brands currently on the market.
“Our UHPC brings down the cost of long-term maintenance,” El-Tawil said. “It’s still more expensive than regular concrete, but if you consider the effect over the lifetime of a bridge, then the cost becomes very competitive.
“If you have a concrete deck on a bridge that lives for 200 years with little to no maintenance, imagine the cost savings.”