As the U.S. Army Corps of Engineers (USACE) works to modernize the nation’s infrastructure, it does so at a time when existing infrastructure components are operating well past their original lifespans. In the case of many of the locks supporting inland navigation, new techniques and technologies are critical to make sure lock facilities – some built in the 1930s – continue operating for another 100 years or more.
The key to that mission’s success may already exist in a material used consistently in military applications, private industry and in Europe, but one that has not yet been put to work in our nation’s civil works projects.
Ultra-High Performance Concrete (UHPC) is a class of concrete well known for its strength, durability and sustainability. It is also a material very well understood by engineers and scientists at the U.S. Army Engineer Research and Development Center (ERDC) and is at the center of what may be the future of new construction and rehabilitation of U.S. lock operations.
“We have been using UHPCs for a long time in military applications, such as hardening structures and force protection applications,” said Dr. Stephanie Wood, a research civil engineer at ERDC’s Geotechnical and Structures Laboratory. “Our experience with UHPCs goes back to even before we were ERDC, when the facility was known as the Waterways Experiment Station.
“We have been part of developing those concretes, testing them. Using our extreme weather facility at Treat Island, Maine, we have tested these concretes in that environment for decades. So, we are very confident in our capabilities, and in the performance of this strong and durable material.”
Thanks to years of ERDC research, engineers are now working with USACE to adopt the use of pre-cast UHPC panels for lock wall rehabilitation projects, replacing those made from traditional concrete and then covered with horizontal steel armor. Currently, the steel armor is cast into concrete panels in the precast manufacturing plant as horizontal strips.
The UHPC panels are not only stronger and more durable than those made with conventional concrete, but they do also not require steel armor.
Currently, damaged or deteriorated lock walls are repaired using traditional concrete with strengths ranging from 5,000 to 8,000 pounds per square inch (PSI). USACE District crews remove the damaged concrete down to the sound concrete. A pre-cast concrete is then placed over the sound concrete, and concrete is poured behind the panels to attach to the larger structure. “Those panels have been performing fairly well in the field, but sometimes we have issues with the steel armor,” Wood said. “Through normal operations of barge traffic, the armor and panels will get struck by barges. The metal will get caught, at times, and start pulling away from the concrete, creating a hazard, and in some cases, lock operators can no longer use that part of the lock chamber until repairs are made.”
In 2018, the Rock Island District issued a statement of need, asking if UHPC could be a substitute for current methods. Over the next three years, ERDC tested UHPC, discovering techniques that not only created a stronger and more resilient material, but also developed processes and guidance on how UHPC panels could be created using materials already on hand at concrete manufacturing facilities throughout the country.
During small-scale and large-scale testing, engineers also showed panels created with UHPC could be made thinner than those using conventional concrete.
“Panels using conventional concrete are traditionally 6-to-8 inches thick, which makes them pretty heavy and cumbersome. This requires USACE teams to have a very large crane on site to put them in place,” Wood said. “Not only does it take up a large footprint on site, but often times the crane requires just as many truck loads to haul it to the site as the panels themselves, which was the impetus of the statement of need from Rock Island.”
UHPC panels are no thicker than three inches and cure to a strength of 22,000 PSI in just 28 days. Using tests and data from more than three decades of research, ERDC tested the UHPC panels, including simulating what would happen if the panels were struck by a barge. The results validated the exceptional strength and durability of the UHPC panel.
“We have eliminated the coarse aggregate, or the rock, out of this mixture,” Wood said. “By eliminating the coarse aggregate, we have also eliminated the interfacial transition zone, which is where the cementitious paste meets the aggregate particle. This zone is typically the weakest location in conventional concrete, and we don’t have that in UHPC.”
Now that research proves the panels are stronger and more durable, the goal is to increase their usage in USACE projects.
“The biggest obstacles to overcome are the unfamiliarity with the product and with the technology, and the cost,” Wood said. “For most people, the only thing they do know about UHPC is that it is more costly up front.”
But that may no longer be an issue, Wood said.
The costs associated with the steel armor, along with the more complex logistics of casting it into the face of conventional concrete panels, increase project costs. The elimination of the steel, in addition to the fact you need less concrete to produce a much stronger panel, makes the use of UHPC more cost comparable.