Ultra-High Performance Concrete Invert Linings

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Culverts are the often hidden pieces of infrastructure tying together the paths of nature and man. The U.S. has over 147,000 culverts classified as bridges due to their role of carrying waterways beneath roads and highways across the country. These passages are critical for allowing rivers to flow, fish to migrate, and storm water to drain unimpeded. However, rushing water and rough silt can cause severe deterioration of the invert on a culvert that is otherwise structurally sound. One of the most prominent issues for corrugated metal pipe (CMP) culverts is deterioration of the base, or invert. Although their out-of-sight location makes them unobtrusive to traffic, it also complicates inspection, repair, and replacement. Frequently buried deep under a roadway, replacement necessitates road closure, redirection of the waterway, and often expensive excavation. As such, robust rehabilitation methods for a variety of site conditions must be available to restore the capacity and functionality of culverts while minimizing cost and disruption to travelers.

While traditional repair methods are plentiful, they each come with their drawbacks. These methods include concrete invert lining, shotcrete coating, and grouted slip lining. Invert linings limit rehabilitation to the base of the culvert where most deterioration occurs. Typically, 3-6 inches of reinforced concrete must be placed to restore the structural capacity of the culvert and provide enough cover for reinforcement. This method has its drawbacks: the thickness of the concrete can change the hydraulic properties of the culvert, which not only affects sedimentation, but also wildlife passage through the waterway. Shotcrete coating has similar limitations, as it has comparable strength to traditional concrete and requires similar thickness of final material to achieve full composite action, but has the advantage of faster application by directly spraying the culvert surface. Grouted slip lining works by inserting a smaller diameter culvert pipe into the deteriorated culvert along the entire length, then pumping grout in between the two culverts to ensure a stable structure.

A promising new material for culvert rehabilitation is ultra-high performance concrete, which has gained recent attention in the structural engineering community for rehabilitation projects. In a partnership between the Connecticut Department of Transportation (CTDOT) and the University of Connecticut (UConn), several methods of placing UHPC invert linings for CMP culverts were tested. The project was developed to support the rehabilitation of Bridge No. 06537, an ellipsoid CMP culvert in Wallingford, CT. It was constructed in 1965 and lies underneath the Warton Brook connector, which connects CT Route 5 and Interstate 91 and carries an average daily traffic of 15,300 vehicles per day. The culvert is 15 feet tall, 13 feet wide and 262 feet long. It is constructed from 7-gauge galvanized steel with corrugations that have a 6-inch pitch and a peak-to-valley depth of 2 inches. The 2022 inspection report categorized this bridge as structurally deficient due to the level of deterioration present on the structure, with approximately 40% of the asphaltic coating experiencing erosion along the entire length of the culvert, concentrated at the bottom of the culvert at and below the waterline. Due to fish migratory patterns and a desire to limit downstream water surface elevations for the 100-year storm surge, it was a priority to minimize the profile of the rehabilitation. This ruled out grouted slip lining or traditional concrete invert linings. UHPC was selected as a long-term repair solution, as it can be cast at only 2 inches thick, required no structural rebar, and would act as both a structural rehabilitation as well as a protective coating for the invert.

The UConn research team worked closely and quickly with CTDOT to design and test several mockup culverts to closely represent the dimensions present on Bridge 06537. Two 12-foot culvert sections were constructed using curved segmental galvanized steel to allow for a total of four UHPC invert pours, each extending 6 feet along the length of the culverts. Following construction, 5 by 5-inch, 10 gauge (0.1 inch) steel wire grid was welded into the inverts along the entire length of the culverts to act as a mechanical connection between the cast-in-place UHPC and the underlying steel. Two top-formed UHPC inverts, one shotcrete UHPC application, and a thixotropic UHPC mix were tested to provide CTDOT with several alternatives for the rehabilitation.

The two top-formed invert casts used a traditional self-leveling UHPC mix, batched on-site in a ready-mix truck. The first cast used a wooden formwork with curved plywood and dimensional lumber applying a radial stress to the plywood. This proved unsuccessful due to the uplift pressure from the UHPC splitting the top pieces of plywood away from the side bulkheads. The high density and flowability of UHPC can create an uplift pressure of 1 pound per square inch for each foot of head, totaling several thousand pounds of uplift force over the surface of the formwork. The issue was resolved in a second cast using engineered steel formwork with trusses braced against the culvert walls. A single curved steel plate was reinforced with a welded truss and held down with braces and brackets. In theory, this would allow it to be modular, since it is not directly welded to the culvert. The braces could be removed, the form slid further into the culvert, and braces reinstalled for segment casts. However, during casting, a small amount of UHPC spilled over the edge of the truss and bonded it in place, making demolding very difficult. The entire truss had to be cut apart and folded inwards with winches before being lifted out with a telehandler. While demolding was difficult, a very consistent 2-inch thickness was achieved, marking this method a successful option for future use with the caveat of ensuring UHPC does not spill over the top of the form.

Two new methods of casting UHPC were also tested in this project: thixotropic and shotcrete. Both are desirable as they eliminate the need for formwork and can be shaped to the culvert without significant labor but have not been successfully used for culvert rehabilitation in the United States to date.

The shotcrete UHPC cast was conducted under the close supervision of the material supplier, with all aspects of the process controlled by representatives from the supplier. A custom mixing station and pump were used to minimize loss of moisture in the UHPC between mixing and casting, and the concrete was dispensed from a hose operated by a representative of the company. Despite several hours of effort, the shotcrete method could not achieve the desired 2-inch thickness due to poor flow characteristics after pumping. Thin layers were sprayed on to the base steel, supported by integral steel fibers and the welded wire mesh previously installed in the culvert. After the first layer, however, subsequent layers ran down and began pooling in the base of the culvert. After several layers could not stack up, casting was suspended, and this method was considered unready for use in a real project.

The final mix tested was a thixotropic UHPC, with chemical admixtures added to reduce flowability and accelerate setting time. A standard, low viscosity UHPC was mixed in a ready-mix truck, as was previously done for the top-formed casts, but following initial mixing, it was poured into a smaller planetary paddle mixer for the addition of admixtures. After the admixtures were added, the UHPC was manually transported in buckets to the culvert and poured by hand, where it was troweled. Although troweling was time-consuming and required significant effort to achieve a smooth finish, a consistent thickness was eventually achieved, and a successful invert was cast. This method required minimal preparation beyond the installation of wire mesh, which was required for all casts. The mesh served the additional purpose of holding the UHPC in place after casting, and it is recommended to ensure a mechanical bond between the UHPC and underlying culvert.

The proposal for a UHPC culvert repair has been considered for a long time, with tests for a sprayable UHPC being conducted in Florida. This study marks one of the first that directly compared the ease of construction of different UHPC casting methods for invert repair, as well as directly leading to the first large-scale culvert rehabilitation with UHPC in the United States. Following the success of this mockup project, a thixotropic UHPC mix was selected for the rehabilitation of Bridge 06537 in Wallingford CT. Although the UConn trials showed a steel top form as creating the best and most consistent surface and thickness, the time it took for engineering design of the formwork, construction for modular re-use, and demolding were found to be impractical for the actual implementation.

UHPC offers a compelling solution to the problem of CMP deterioration in the United States, particularly for locations with limited access or restrictions on a culvert’s capacity. As infrastructure owners seek long-term solutions with reduced lifecycle costs, UHPC provides a competitive structural rehabilitation option with minimal thickness, multiple application methods, and excellent resistance to a corrosive environment. For practicing engineers, this work highlights the importance of material behavior under field conditions and the need for iterative, full-scale validation when deploying advanced materials. With continued research and refinement, UHPC has the potential to become a standard tool in the culvert rehabilitation toolbox—ensuring the resiliency of our infrastructure without compromising ecological or hydraulic performance. ■

About the Authors

Brian Lassy is a Ph.D candidate at the University of Connecticut researching and developing methods to rehabilitate corroded bridges with ultra-high performance concrete encasement.

Dr. Alexandra Hain is an assistant professor at the University of Connecticut with research focuses on advanced imaging techniques for structural evaluation, bridge repair with novel materials, and large-scale experimental testing.

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