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High-performance concrete used for underwater tunnel segments

Source: | Updated: Jan 29, 2016

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.

Quality control at the forefront

To ensure a high degree of consistency, reliability, and quality, the project team mobilized two concrete batch plants. At the fabrication site, the two portable Lafarge plants produced 72,000 cubic yards of SCC, testing every load for air content, unit weight, slump, and temperature.

Lafarge kept its Sparrows Point slag cement processing facility and other operations open whenever the materials were needed, which was important because the structural concrete placements during the summer typically started at midnight. Following strict quality control guidelines and sometimes working around the clock, Lafarge’s cement and aggregate production teams, along with the company’s local crew of more than 50 delivery drivers, supplied the slag cement and aggregate for the concrete mix.

Keeping it cool

Due to the high ambient temperatures during the summer (up to 96° F) and the concern for the temperature rise that occurs during concrete curing, the mix was injected with liquid nitrogen to cool the concrete to a temperature between 60° F and 70° F before it was placed. SKW also installed a heating and cooling system throughout the invert, walls, and roof of each segment, and pumped a glycol coolant solution through the hoses during curing to help control the concrete temperatures. Thermocouples were used in all pours in conjunction with a maturity system to monitor temperature data and predict concrete strengths for form removal timing.

The first production pour of element 1/segment 1 in April 2013 went smoothly. After three days, the formwork was stripped and rolled ahead on the rail into position for the next pour. All 11 segments of the massive tunnel elements are now complete.

Float-out and immersion

The first six massive tunnel elements (litter 1) arrived at the Portsmouth Marine Terminal in summer 2014, after being towed one at a time by a tug fleet down the Chesapeake Bay. In March 2015, the new Midtown Tunnel reached the halfway mark with the immersion of the sixth tunnel element and its connection to the five previously placed elements.

At the same time, the remaining five concrete tunnel elements (litter 2) were completed at the Sparrows Point fabrication dry docks and prepared for float-out to the Virginia jobsite. This preparation process included sealing and waterproofing the elements, constructing four temporary, interior ballast tanks in each to provide stability during floating and towing, and installing temporary bulkheads on each end to allow the elements to float. For the voyage, each tunnel section was outfitted with entry hatches, navigational lights, GPS survey towers, and fenders to protect against any wayward debris. The second litter of Elements 7 to 11 reached Portsmouth in April, and the elements were placed across the Elizabeth River at the rate of about one every five weeks. The last tunnel element was immersed on July 14, 2015, connecting the shorlines of Portsmouth and Norfolk.

Broad and lasting benefits

The new Midtown Tunnel will offer broad and lasting benefits to the Hampton Roads transportation system, its economy, and the lives of those who live and work in the region. When the tunnel is completed in 2016, the existing 54-year-old tunnel will be rehabbed, after which the side-by-side tunnels will carry two lanes of traffic in each direction. The average round-trip user will save about 30 minutes a day, roadway safety will improve with the elimination of bidirectional traffic, and increased capacity will support the movement of goods in and out of various port facilities in the region.

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