Award: Innovative Applications
Owner: North Carolina Department of Transportation
Contractor: PCL Civil Constructors, Inc.
Concrete: Troy Ready Mix, Inc.
Slag Cement: LafargeHolcim (US) Inc.
The historic reinforced concrete open spandrel four arch Lake Tillery Bridge is located in the beautiful Piedmont area of North Carolina on highways NC 24/27/73 at the Stanly - Montgomery County Line just outside of Albemarle. This 1927 structure underwent a major renovation. The 1060 ft (320 m) superstructure above four arch bridge was completely removed and replaced, while keeping the architectural appearance of the original bridge and widening the width by 16 ft (5 m) to accommodate increased safety for travelers.
Not only was the use of 2274 yd3 (1739 m3) of slag cement a highlight of this project, the engineering, demolition, and construction involvement surrounding the unloading and loading of the old arch structure was critical to the project’s success. The balancing methods needed during the dismantling of the old superstructure and reconstructing was an enormous feat. Improper loading of the arches could cause a partial failure of the arch structures and the historic bridge may have become unusable without costly repairs.
The last concrete has been placed on the bridge railing walls and the project team’s patience and hard work have given new life to the historic structure that will allow many to view the inspirational architecture of the past.
Slag cement was used in the deck placements of the bridge due to a mandate by the North Carolina Department of Transportation (NCDOT) to use a supplementary cementitious material in bridge decks to increase durability and to mitigate against alkali silica reaction (ASR) in the concrete.
A slag cement source from Sparrows Point, Maryland was used due to availability and quality. Even though a Grade 100 was acceptable per NCDOT, a Grade 120 was used for the project as an additional safety measure. It was later determined that the safety measure was not needed due to the fact that the 5000 psi (35 MPa) design was easily meeting the design strength in 7 days. This was partially due to the performance of the slag cement, but was also influenced by the prescriptive cementitious contents required in the NCDOT Standard Specifications.
The slag cement mixture performed above expectations. Working with concrete in hot weather conditions, processes can quickly accelerate when the cement starts to hydrate. At the time of this project, NCDOT had a maximum slump of 3.5 in. (89 mm) in bridge deck concrete and the slag cement addition helped with being able to hold the slump longer and the increased workability of the mixture helped the finishers battle the southern heat during the placements.
Due to the loading sequence of the concrete that was required to protect the integrity of the arches, the increased pumpability of the mixture made the concrete easier to place from the long boom lines of the concrete pumps. The early strengths provided by mixture and ambient temperatures enabled the construction team to remove forms early and reset the forms to help speed the construction.
The bridge has a colorful past and is actually the replacement bridge for the bridge known for the “Battle of Swift Island Bridge”, where the U.S. Army made attempts to destroy the bridge by static loading, aerial bombing, artery bombardment, and was finally able to take down the bridge by detonating 2000 lb (900 kg) of explosives placed at the base of the structure. The bridge was only 5-years-old when it was destroyed, but it was necessary to remove the structure to make way for the formation of Lake Tillery when CP&L decided to build a hydroelectric dam downstream. The following link provides some of the actual attempts to destroy the bridge and the Lake Tillery Bridge can be seen in the background in some of the film footage.
Some of the notable aspects of the demolition and reconstruction of the bridge mostly surrounded the loading felt by the arches. Eccentric loading or offloading of the arches could cause cracking of the arches, therefore a symmetrical plan had to be followed to balance the loads. The deck and spandrels had to be removed or replaced using techniques that would keep balanced compression loads on the arches to maintain their integrality.
The formwork for the barrier walls and other architectural details had to be custom made to keep the appearance of the original bridge. This was done at a cost, but the original appearance of the bridge will live on. Rebuilding this bridge saved the taxpayers of North Carolina $4.3 million when compared to the original plan of removing and replacing the bridge. The following link is a meeting with NCDOT to discuss the rehabilitation method for the bridge and the cost compared to complete bridge replacement: https://youtu.be/km331qPBRho .
NCDOT Class AA used for Bridge Deck and Diaphragms
% Slag Cement Replacement
36% Lafarge Sparrows Point Grade 120
% Portland Cement
64% Holcim Holly Hill Type I/II
% Other SCM (if applicable)
#67 granite and C33 pit sand
6310 psi (44 MPa)
10,350 psi (71 MPa)