According to the American Society of Civil Engineers, 42 percent of the 617,000 bridges in the U.S. are at least 50 years old and 46,154 are structurally deficient (1). Moreover, the American Road & Transportation Builders Association states that at the current pace of rehabilitation it will take 40 years and an estimated 42 billion dollars to make all the necessary repairs (2). With climate change making weather conditions more extreme, the magnitude of deterioration will escalate and become an overwhelming challenge.
As the nation’s transportation infrastructure continues to age and deteriorate, owners are paying greater attention to life-cycle costs and relying on more durable preservation and repair materials. This includes long-lasting, high-strength materials such as ultra-high-performance concrete (UHPC). Promoted and supported by the Federal Highway Administration (FHWA), UHPC is steadily gaining traction in regions susceptible to extreme weather environments. It is in such environments that “weak links” of structures often need more frequent repair.
Causes of Deterioration
Bridges subjected to extreme exposure conditions face several key technical challenges, including corrosion resistance, freeze-thaw durability, and surface abrasion.
Corrosion of steel reinforcement embedded within concrete due to the ingress of chlorides from deicing salts or ocean breezes is a major cause of bridge deterioration. As the metal corrodes, the resulting expansive pressure causes the concrete to crack, accelerating the penetration of chlorides, moisture, and other corrosive contaminants. Uncontrolled, corrosion can lead to complete separation and failed structural components, resulting in high maintenance costs and possibly unsafe conditions.
Cyclical freezing and thawing of water is another critical concern. Because water expands when frozen, the expansive forces create hydraulic pressures within the concrete that can result in significant cracking and scaling. If moisture reaches embedded steel reinforcement, the metal corrodes and its volume expands, further fracturing the concrete and accelerating moisture ingress. Deicing salts increase the potential for freeze-thaw damage because they absorb moisture and keep the concrete more saturated.
Abrasion damage occurs when surfaces are unable to resist wear caused by vehicular traffic. Today’s bridges are increasingly subjected to heavier truck loads that threaten to overstress bridge elements and cause fatigue and cracking. Tire chains and studded snow tires also cause considerable wear to surfaces. Compressive strength is the principal factor controlling the abrasion resistance of concrete.
UHPC Performance Advantages
There is more to UHPC than the extremely high strength it can achieve. Its toughness and durability rank at the top of its advantages. Significant benefits of the material include its relative impermeability, low shrinkage, crack resistance, and high abrasion resistance even under the most extreme climatic and chemical conditions.
UHPC is a cementitious composite material with mechanical, durability, and bonding properties that far exceed those of conventional concrete. The mixture has an extremely low water content, contains a high percentage of steel fibers, and can be made without air entrainment. The product’s incredibly low porosity paste matrix is exceptionally durable, and macrocracking will not occur but remain as well distributed microcracks, which will inhibit the penetration of corrosive chlorides. Because the mix is so dense and moisture cannot get in, UHPC will not freeze and expand, thus avoiding freeze-thaw damage.
Currently, there are four major UHPC manufactures in the United States. While formulations are proprietary, mixes commonly consist of Portland cement, silica fume, fine quartz sand, water-reducing admixtures, steel fibers, and a water to cementitious materials ratio of less than 0.25. The material has a minimum specified compressive strength of 17,000 pounds per square inch (120 MPa) with specified durability, tensile ductility, and toughness requirements.
There are a wide variety of bridge locations where UHPC is used to mitigate deterioration, reduce maintenance requirements, and extend the service life of the structure. Below are some widespread problem areas in the superstructure that are addressed with UHPC repairs solutions.
Expansion joints. The leakage of water and deicing chemicals through cracked expansion joints causes deterioration of steel girder ends below the bridge deck. Since UHPC is crack resistant, it offers a maintenance-free, permanent solution to joint problems.
Link slabs. Another approach to addressing leaking joint problems is to eliminate the joint altogether and replace it with a durable UHPC link slab. After the joint is removed by placing UHPC across the joint as a thin slab, the UHPC link slab is ground to match the profile of the existing deck and grooved for skid resistance. This approach is common for linking precast elements as they have a history of durability issues related to longitudinal cracking along key connections. UHPC creates robust and durable connections between prefabricated deck elements that exhibit deterioration or leakage.
Deck overlays. UHPC is a promising solution for enhancing the durability of worn deck surfaces. This thin-bonded UHPC topping layer can be used as a strengthening technique to increase the load-carrying capacity of the structure and provide a surface that is both abrasion resistant and virtually impermeable.
Damaged steel girder ends. To mitigate the deterioration of beams, engineers are encasing the ends of the girders in a jacket of UHPC to strengthen and protect them from further corrosion. This enhances the structural capacity and durability of the girder with minimum traffic interruption and at a lower cost.
Low concrete cover. The American Concrete Institute recommends a minimum cover thickness of 2 to 2.5 inches over steel-reinforced concrete in harsh marine and weather environments. A UHPC overlay will provide a more durable cover to prevent the ingress of moisture and other corrosive agents.
Beyond repairing the weak links in the bridge superstructure discussed above, there are also promising substructure rehabilitation applications of UHPC. This includes retrofitting structures to enhance their seismic performance and upgrading deteriorated bridge columns and piers. For more information on these applications, see the FHWA document on UHPC bridge preservation and repair solutions (3).
The proven performance of UHPC in withstanding the extreme environments bridges experience has increasingly made it a material of choice for keeping this vital infrastructure in good condition. Since its first application more than 15 years ago, 35 states plus the District of Columbia have employed UHPC in bridge projects, and 13 have relied on it for bridge preservation and repair (4).
In recent years, state departments of transportation (DOTs) have used multiple techniques and strategies to repair or strengthen bridges with UHPC (5). While many of these projects used UHPC to repair connections between precast slab units, other successful applications included bridge deck overlays, link slabs, and beam-end repairs.
Thin-bonded UHPC overlays, which were first applied in Iowa in 2016, have generated much interest to rehabilitate and provide long-term protection to bridge decks. In 2020 and 2021, Delaware and New Jersey implemented this solution to extend the service life of multiple bridges throughout both states.
New York was the first state to use a UHPC link slab in 2013 and, to date, has completed the most UHPC link slabs in the U.S. As of 2020, UHPC link slabs have been installed on at least 35 additional bridges in six states.
Connecticut was the first state to use UHPC to repair deteriorated steel girder ends on an I-91 bridge in 2018. UHPC repairs of damaged bridge beams—aimed at tackling the ever-growing challenges of bridge maintenance—soon followed in Rhode Island, Michigan, Florida, and Texas.
A well written material specification includes testing criteria for verifying target performance levels. Regardless of the UHPC application, it is important to specify the use of ASTM C1856 modified test methods. This guidance document details necessary alterations to standard quality-control tests for measuring the properties of fresh UHPC and for making and testing specimens of hardened UHPC. The use of AASHTO-accredited laboratories that employ trained and certified personnel with extensive experience in performing ASTM C1856 modified test methods is highly recommended. Additionally citing the standard will key the local labs into the different requirements for testing this material and, hopefully, prevent them from going into the project blindly.
Product placement is an additional consideration when working with UHPC. Cleaning and abrading the surface to ensure a good bond between the UHPC and the substrate is critical to ensuring high-quality results. Specialized training is not necessary as the fluid material is self-consolidating and requires no vibration. In essence, the manufacturer manages all the production and batch mixing requirements on the job site and provides oversight of the small-volume pours.
While UHPC has many performance advantages, its cost is substantially higher than conventional concrete in a direct volume comparison. As such, UHPC repair applications are focused on critical bridge locations to mitigate deterioration concerns and improve life-cycle costs. The initial cost increment in using UHPC connections for prefabricated bridge elements, for example, is relatively low because the quantity of material required is small. However, the potential long-term cost savings are high when factoring in extended service life, reduced maintenance requirements, and fewer traffic disruptions.
Moving Forward with UHPC
For those who own and maintain bridges throughout the U.S., it makes good engineering and financial sense to use advanced preservation materials that are stronger, are more durable, and offer life-cycle cost advantages. As such, many state DOTs are increasingly specifying UHPC for bridge repair projects to lower maintenance requirements, reduce traffic delays, and extend service life.
In 2020, the FHWA selected UHPC as one of the technologies to be included in its Every Day Counts (EDC) initiative. Under the EDC banner, the FHWA will provide assistance in using UHPC in bridge preservation and repair projects.
1. “2021 Report Card for America’s Infrastructure”, American Society of Civil Engineers, pp 18–25.
3. “Design and Construction of UHPC Based Bridge Preservation and Repair Solutions”, FHWA, publication no. FHWA-HRT-22-065, May 2022, pp 11–13.
4. Haber, Zachary B., “Improving Bridge Preservation with UHPC”, Turner-Fairbank Highway Research Center, FHWA.
5. “UHPC for Bridge Preservation and Repair”, EDC-6 fact sheet, FHWA.
Benjamin Birch, P.E., is a senior engineer and laboratory engineer for concrete and mortar at the CTLGroup. He is an expert in the mechanical properties of concrete and a member of the American Concrete Institute, Transportation Research Board, American Society of Civil Engineers, American Concrete Pavement Association, and ASTM International. He received his BS and MS in civil engineering from the University of Illinois at Urbana-Champaign and is a licensed professional engineer in 10 states. CTLGroup’s team of engineers and material scientists can provide additional assistance in evaluating the cause of problems, developing UHPC repair strategies, and producing material test results that are accepted by state DOTs. For more information, visit www.CTLGroup.com.