Bridge Description

The new bridges are comprised entirely of precast elements: 120-ft-long deck bulb tee beams; stub abutments; prestressed piles; approach, sleeper, and moment slabs; and T-walls. To ensure proper connection between precast elements, Ultra-High Performance Concrete (UHPC) was utilized. This project marks DelDOT’s first use of a UHPC mix that was batched off-site at a concrete plant and transported for placement in a ready-mix truck. Additionally, the profile of US 13 was raised approximately 8.5 ft to address pre-existing sight distance issues and improve driver safety. To accommodate the heavily accelerated construction timeline, Foamed Glass Aggregate (FGA) was chosen as a backfill.

Adjacent bridges on US 13 Northbound and Southbound over Blackbird Creek north of the town of Blackbird, Delaware

Tier 5 (within 3 months)

• ABC: closure 44 Calendar Days (BR 1-488N: 43 days & BR 1-488S: 37 days)
• Conventional: approximately 1.5 Years

• Reduced traffic impacts
• Reduced onsite construction time
• Improved work-zone safety
• Reduced life-cycle cost
• Single construction season
• Minimize business impacts
• Other primary driver – The Department is committed to implementing new and innovative bridge replacement techniques that will decrease construction times, increase commuter and work-zone safety, and minimize user costs due to the decreased road closure duration.

Two 120-ft-long single-span concrete adjacent deck bulb tee beam bridges, 51.5-ft-wide in each direction

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Extended use of contraflow to accommodate phased construction. This would have significantly increased the construction timeline as well as the project limits.

US 13 is 4-lane divided highway, classified as a Minor Arterial. Bridge 1-488S, ranked first on Delaware’s list of deficient bridges, was a 42-ft-long concrete arch bridge built in 1920. Bridge 1-488N was a 40-ft-long concrete rigid frame built in 1933. The existing bridge was identified for replacement because it experienced severe corrosion and loss of fill around the pipe culverts that could have led to an eventual failure. Bridge 1-488S was both structurally and hydraulically deficient. The concrete arch had significant spalling, cracks, and corrosion of exposed bar reinforcement. Bridges 1-488N&S both experienced scour issues due to the stream constriction created by their undersized span lengths. Due to their short timber piles, the bridges were considered scour critical. In addition to their structural and hydraulic deficiencies, both bridges were located at the bottom of a severe vertical sag curve. The roadway profile at the location of these bridges was substandard for stopping sight distance, leading to a high accident rate for vehicles trying to merge onto US 13.

The two new bridges are comprised entirely of precast elements: 120-ft-long deck bulb tee beams; 55-ft-long stub abutments; 50-ft-long prestressed piles; approach, sleeper, and moment slabs; and T-walls. To ensure proper connection between precast elements, Ultra-High Performance Concrete (UHPC) was utilized. Foamed Glass Aggregate was chosen as a backfill.

US 13 is a 4-lane divided highway and one of Delaware’s main thoroughfares. Given the high Annual Average Daily Traffic (AADT) and public exposure at this location, DelDOT used ABC techniques in lieu of conventional methods to replace both bridges. To accommodate the accelerated construction schedule and the need to raise the profile of US 13 approximately 8.5 ft, the new bridges are comprised entirely of precast elements, with UHPC transverse joints connecting the adjacent beams and approach slab segments. DelDOT expects the construction of these new bridges will provide a 100-year service life.

Precast Elements
A single row of eight 16-inch x 16-inch, 50-ft-long piles were used to support the abutments on each side of the bridges. The pile driving took roughly one 10-hour shift per abutment and was finished over the course of four days, allowing the rapid erection of the precast abutment sections.

Each substructure consists of two 55-ft-long precast concrete variable height stub abutments, each set in roughly an hour over four days. Closure pours consisting of Type I, 4.5-ksi concrete with a 2% high-early-strength admixture were used to connect the piles to the abutments. By using this concrete mix, the contractor was able to place the beams four days after performing the closure pour.

With the abutments in place, the contractor then began placing the precast concrete T-Wall segments. The use of the T-Wall system limited impacts to adjacent utilities and wetlands created by the embankment side slopes. The walls are 10-ft high and comprised of 150 precast elements stacked two units high. It took approximately eight days to place the 500-ft length of wall.

The bridge superstructures consist of eight 125-ft-long x 5.5-ft-deep x 6-ft-wide prestressed precast deck bulb tee beams. Utilizing longer beams and significantly extending the spans of the bridges was a cost-effective solution that helped to limit the duration and quantity of the backfill work. Additionally, the use of the deck bulb tee beams eliminated the need for drawn-out cast-in-place deck pours. Placement of the beams took two days per bridge. With the precast beams in place, the UHPC connections were poured, and the remaining precast elements were placed.

The use of fully precast sleeper slabs and approach slabs as part of the bridge replacements marks a first for DelDOT. The sleeper slabs are 51.5-ft-wide and were cast as a single precast element. Each approach slab consists of four 30-ft-long x 12.83-ft-wide x 1.5-ft-thick segments. Grading and placement of the precast sleeper slabs and approach slab segments took place over the course of four days for each element type. The approach slab segments were then connected with UHPC.

Because of the need to tighten the embankment limits, precast moment slabs were selected in lieu of steel guardrail. Each moment slab section is 24.5-ft long. In total, 14 moment slab segments were placed over three days.

Foamed Glass Aggregate Backfill (FGA)
With so much backfill required to adequately raise the vertical profile of U.S. 13 (approximately 12,000 yd³), DelDOT needed to use a material that could safely and efficiently accommodate the expedited construction schedule. As such, the backfill work was a critical path for opening the roadways on schedule. To facilitate this work, DelDOT opted to use FGA because of its unique material properties.

FGA is produced from 100% recycled glass and is approximately 85% lighter than typical backfills, alleviating concerns for settlement of the existing soils and the proposed embankments. Its permeability, high friction angle, and low unit weight made it an ideal backfill material for the replacement. The free-draining characteristics of FGA mean that it can be placed in almost any weather condition, except for ponding water. This allowed the contractor to continue to place the material during rain events. The high friction angle and low unit weight improved stability of the slopes outside of the T-Walls, reduced lateral earth pressure on the wall, and decreased the load on the existing soils. Installation of the FGA took place over 14 days and occurred concurrently with the installation of the bridge elements and UHPC.

Ultra-High Performance Concrete (UHPC)
DelDOT first used UHPC as part of a bridge replacement project in 2015. Since then, the use of UHPC for the connection of adjacent superstructure elements is standard DelDOT practice, and numerous projects utilizing the material have been completed.  Because of DelDOT’s continued commitment to the use of UHPC, surrounding precasters have invested in reusable formwork for the superstructure shear key. While reusable formwork represents a significant upfront cost to the fabricator, it also is a huge step towards normalizing the use of UHPC.

The successful use of the decked bulb tee superstructure and precast approach slabs was predicated heavily on the use of UHPC. Without UHPC, the closure pours and formwork for the joints would have needed to be much larger and far more labor intensive. Additionally, not utilizing UHPC would most likely have resulted in reflective cracking.

For this project, the decked bulb tee beams have large precast end diaphragms that act as the backwall and seats for the precast approach slabs. Because of the complexity of these diaphragms, it is common to field-cast them. However, given the time constraints of the project, the end diaphragms were precast with the beams and connected transversely with UHPC. Instead of holding up other operations while the contractor prepared formwork for the shear key joints, the UHPC for the end diaphragms was poured separately from the longitudinal joints. All the work associated with the UHPC end diaphragm joint pour was performed over four days: one day per bridge to prepare the surface and install formwork, and approximately 1.5 hours of pouring per bridge.

Backfilling operations for the FGA, as well as construction of the temporary walkways outside of the bridge parapets, were dependent on achieving adequate strength in the UHPC. Backfilling against the abutments and attaching the overhang brackets and walkways to the beams required that the UHPC reach a compressive strength of 10 ksi. It took the UHPC approximately two to three days to gain the required strength.

By sequencing the pours, the contractor was able to give the UHPC in the end diaphragm joints a head start on gaining strength while they worked to assemble the formwork for the longitudinal joints. This allowed the other construction operations to continue alongside the UHPC pour for the longitudinal shear keys. All the work associated with the longitudinal UHPC joint pour was performed over six days: two days per bridge to prepare the surface and install formwork, and approximately 4 hours of pouring per bridge.

Once the UHPC pours were completed for the decked bulb tee beams, the contractor placed the precast approach slab segments. The use of fully precast approach slabs was made possible, in no small part, because of the connection that UHPC can provide. For this project, the joints between the approach slab segments were designed to match FHWA’s shear key for adjacent box beams. Because this shear key is only 3/8-inch-wide, the contractor was able to utilize backer rod in lieu of wooden formwork. All work associated with the UHPC approach slab joint pour was performed over three days: one day per bridge to prepare the surface and install formwork, and approximately 1.5 hours of pouring per bridge.

For the first time in Delaware, the UHPC was mixed off-site, at a concrete plant. The UHPC manufacturer, Cor-Tuf UHPC, trained and certified members of Heritage Concrete to mix the material using standard mixers. Utilizing typical mixing machines allows for an increased level of control that cannot easily be achieved in the field. It is also worth noting that this mixing process allows for larger quantities of UHPC to be mixed in a single batch. For this project, representatives from Cor-Tuf UHPC and DelDOT’s Materials & Research section oversaw the mixing process performed by Heritage Concrete. The mixing process took approximately 1.5 hours to complete. Once mixed, testing samples were taken, and the UHPC was loaded into a standard ready-mix truck. It took another 30 minutes for the material to be delivered to the construction site. By slowly rotating the mixing barrel on the concrete truck, the UHPC was able to remain workable for approximately 4 hours.

Once on site, the UHPC was loaded into a concrete hopper and lifted by crane. The hopper was smoothly guided along the joints, quickly filling them as it went. The concrete hopper, which had a capacity of 2 yd³, had a spring gate that allowed the manufacturer to easily control the flow of UHPC. On-site mixing procedures typically use wheelbarrows or concrete buggy machines during placement. This equipment offers a capacity of between 6-14 ft³, only approximately 10-25% of the capacity of the concrete hopper. The concrete hopper eliminated the need for wheelbarrows and buggies, and a smaller crew was easily able to complete the UHPC pour, installation of top forms, maintenance of the pressure head system, and ensure joint integrity. The decreased site congestion increased the speed and efficiency of the UHPC pour. It also improved the cleanliness of the construction site compared to an on-site UHPC mixing operation.

Timeline
Utilizing ABC techniques to replace both bridges took a total of 44 calendar days. The detour of US 13 Northbound and Southbound was staggered by 1 week. US 13 Northbound closed on September 8, 2025 and was reopened in 43 days. US 13 Southbound closed on September 15, 2025 and was reopened in 37 days.

The replacement of the bridges was deemed a monumental success and provided the Department and the contractor with valuable insight and experience. In many ABC applications, it is common to trade a shorter construction duration for an increased replacement cost. However, given the complexities of normal construction in this location, the ABC replacement actually proved to be slightly less expensive, while still opening the road in a fraction of the time. The Department’s continued commitment to the use of ABC methods has helped to stabilize and sometimes diminish costs as better details are developed, ABC practices become more mainstream, and contractors gain more experience. While further advancements in accelerated bridge construction techniques can be made in Delaware, the success of this project proves the merit of using such techniques, especially for high-volume roadways. Given the high-profile nature of this project, the Department closely monitored the progress of construction. Some of the major lessons learned from the replacement include:

• While Delaware is one of the states on the forefront with accelerated bridge construction techniques, the Department recognizes the benefits of seeking out assistance from other states and federal agencies. During the design phase, the design team reached out to multiple states while developing details and specifications. This allowed DelDOT to learn and improve on previously implemented techniques when utilizing them for this project.
• Communication is a key to the successful implementation of any ABC project. Maintaining an open line of communication between the precast fabricator and the design team helps to minimize fit-up concerns and avoid conflicts. It is important for the contractor and the Department to work collaboratively to quickly solve any issue that arises in the field.
• The use of precast elements can greatly increase the speed of construction. However, with the use of precast elements, in-field adjustability is greatly limited. Tolerances become tighter, and the contractor does not have the flexibility associated with typical cast-in-place methods. This requires an increased level of detail in the plans and shop drawings.
• On ABC projects, the connection between precast elements is vital. UHPC offers a strong connection and smaller joint sizes.
• Having a contractor that is both experienced and open-minded in the use of UHPC is vital. The contractor for this job has become one of Delaware’s more knowledgeable UHPC installers. They continue to look for ways to improve delivery and implementation of the material, and they are open to suggestions throughout the construction process.
• Care should be used when sealing the beams and building the formwork for the UHPC joint pours. The contractor’s approach for the formwork helped increase pour speed and eliminate material loss due to leakage.
• Mixing UHPC at a concrete plant instead of on-site provides many advantages that can have a positive ripple effect on the project as a whole. Mixing off-site eliminates the need for large mixers within the work zone, allows for larger UHPC batches, and provides an added level of quality control. It is important for concrete plant staff to be trained and certified in the UHPC mixing process, and it is also recommended that a manufacturer’s representative be present to oversee the mixing process.
• During the off-site mixing, the contractor did express concerns with waste associated with the process. The concrete plant mixing the UHPC was only set up to batch the material in one-cubic-yard increments. If smaller increments were achievable, it could help to reduce material waste. This is an issue that could be resolved as this process becomes more mainstream and concrete plants adjust their procedures.
• The elimination of top forms has the potential to help greatly speed up the installation of UHPC, but it is important that considerations should be given in regard to the slope of the joints.  The contractor found success when no top forms were used on flatter joints. However, difficulties were encountered when trying this technique on joint slopes greater than 0.5%. It is possible, with more time and planning, that steps could be taken to mitigate issues on joints with steeper slopes.
• The ability to install UHPC with a concrete hopper helped to improve speed and efficiency. It decreased the need for a large allocation of personnel for the operation and minimized site congestion.
• As off-site mixing and delivery of UHPC in ready-mix trucks becomes more prevalent, the efficiency of the installation will continue to increase. Seeing the merit to this approach, it is likely that more UHPC suppliers will modify their mix designs to suit this approach. This will lead to more concrete plants becoming proficient in the mixing of UHPC and further advancement of the industry as a whole.
• The Department, contractors, and even the manufacturer are still learning the best practices when applying UHPC. In discussions during the UHPC pour, the manufacturer’s representative explained that this specific mix design is capable of being pumped. Being able to pump UHPC would continue to revolutionize the material’s applications. Pumping could allow for faster installation and the ability to install UHPC in less accessible areas.

UHPC joint mix (off-site mixing method); Foamed Glass Aggregate (FGA) backfill