Sarah Mildred Long Bridge
Slag Cement: LafargeHolcim
Award: Innovative Applications
The Sarah Mildred Long Bridge project is an important regional crossing linking Maine and New Hampshire across the Piscataqua River. The new 2800 ft (850 m) concrete vertical-lift bridge replaced the existing steel vertical-lift bridge that was built in 1940. The project is part of a “Three Bridge Agreement” between Maine and New Hampshire. Engineers believe that the $158.5 million project may be the first vertical-lift bridge to use precast concrete segmental towers.
As the Construction Manager/General Contractor, Cianbro Construction precast on site the 88 tower segments needed for the four towers with each segment weighing 80 tons. The towers reach a height of 215 ft (66 m) and were erected in post-tension increasing its vertical resting position by 56 ft (17 m) allowing for 68% fewer bridge openings. The span was also increased to 300 ft (90 m) allowing larger ships to access the port. With a focus on safety, performance, durability, economy, and aesthetics the ridge features concrete roadway for vehicle traffic above and railroad superstructure for heavy rail below that services the nearby Portsmouth Naval Submarine Facility.
Slag cement was used to reduce concrete permeability and lower the water-cementitious materials ratio which helped improve the workability and durability of the concrete. Slag cement was used in the drilled shafts, totaling 8000 yd (7000 m), up through the project for another 20,000 yd (18,000 m). Replacement levels ranged from 35 to 50% throughout the project.
The project team incorporated slag cement into their mixtures to achieve the durability and design requirements required by the Maine and New Hampshire Department of Transportation (DOT).Notes from the project team:
- Slag cement and ice helped mitigate thermal cracking concerns during large Summer placements.
- Compressive strengths exceeded design strengths at 28-days, using lower than typical cementitious factors.
- Use of slag cement helped plastic concrete temperatures stay within tight specification during slow placements involving crane/bucket.
- Owner: Maine and New Hampshire Departments of Transportation
- Contractor: Cianbro Construction
- Concrete: The Redimix Companies, a Division of CRH
- Slag Cement: LafargeHolcim