Since the Midtown Tunnel was built under the Elizabeth River in 1962, population in the Hampton Roads region of Virginia has increased almost 70%, and tunnel usage has gone up by 600%. Today, this vital link between the cities of Norfolk and Portsmouth carries almost 1 million vehicles a month and is the most heavily traveled two-lane road east of the Mississippi River. Congestion costs millions of dollars in lost time, productivity, and economic development.
Construction of a second Midtown Tunnel began in 2013 to enhance traffic flow, improve safety, and foster connectivity within the region. When completed in 2016, the new 3,800-foot concrete-reinforced tunnel, located adjacent to the existing Midtown Tunnel, will provide a separate road that allows two lanes of traffic in both directions, doubling transportation capacity beneath the Elizabeth River.
Unique all-concrete design
The Parsons Brinckerhoff rectangular profile design of the second Midtown Tunnel is the first deepwater concrete immersed-tube tunnel in North America and only the second all-concrete immersed tunnel in the U.S. Used extensively across Europe, the all-concrete tunnel design allows for a strong, durable structure with substantial economic savings versus a more conventional design using a steel tube encased in concrete.
The tunnel consists of 11 rectangular reinforced concrete elements, each weighing 16,000 tons and measuring about 350 feet long, 55 feet wide, and 28.5 feet high. The design-build team of Skanska USA Civil Southeast, Kiewit, and Weeks Marine — a joint venture dubbed SKW Constructors — decided to fabricate the precast tunnel elements in Sparrows Point, Md., due to the availability of a large dry dock and because there was no site big enough with water deep enough for the job in Hampton Roads. This large site allowed SKW to produce six elements at one time.
The 11 concrete tunnel elements were cast in two cycles, called litters. Litter 1 included the first six elements and litter 2 the final five elements. After each litter was completed, the dry dock was flooded to float the elements so they could be towed the 220 nautical miles down the Chesapeake Bay to the project site.
Developing the optimal mix
To come up with the highest-quality concrete for the tunnel elements, which were designed for a 120-year service life, Lafarge’s quality control team developed and tested more than 100 different high-performance mix recipes over several months in the laboratory and in the field. Lab analysis included tests for compressive strength, flowability, shrinkage, set time, and durability. The material engineers used STADIUM time-step finite-element analysis to simulate the progress of harmful ions — chloride, sulfate, and hydroxide — through the concrete. Field-testing involved multiple sample placements and mockups, including a full-sized tunnel section about 70 feet long.
Agilia self-consolidating concrete (SCC) was selected for the project due to the massive amount of structural steel being used and the consequent need for a very workable mix. This highly fluid concrete places more quickly than standard concrete and flows easily through highly congested reinforcement and embedments while still meeting the stringent durability and strength requirements.
Primary considerations in developing the mix formulation were long-term durability, control of maximum temperature gain, and minimizing temperature differentials between external and internal locations to prevent thermal stresses. To achieve these performance goals, the SCC relied on a mix containing NewCem slag cement and Type I/II portland cement. (STADIUM, Agilia, and NewCem are trademarks of Lafarge.)
The slag cement helps the structures meet the requirements for long-term durability, achieve greater strength, reduce permeability, and increase resistance to sulfate attack and alkali silica reaction. High replacement levels of slag cement in properly proportioned mixes also help control shrinkage, creep, and cracking in mass concrete structures.
The specified compressive strength for the concrete on this project was 6,000 psi; however, the high-performance SCC consistently achieved strengths of 9,000 to 10,000 psi at 28 days.