Mitchell K. Carr, Mississippi Department of Transportation and Bruce Strickland, Sika Corporation
One of the South’s most traveled scenic highways and a major Gulf Coast artery is U.S. Route 90. When Hurricane Katrina destroyed the existing four-lane bridge across Biloxi Bay between Biloxi and Ocean Springs, the Mississippi Department of Transportation (MDOT) realized that the bridge had to be rebuilt quickly and be as durable as possible. The new bridge would need to have sufficient elevation to clear water levels expected from a catastrophic hurricane. The services of an ocean engineering firm were used to develop a computer model to predict future storm surges plus maximum wave height.
With this information in hand, MDOT advertised the project and subsequently awarded a $338 million design-build contract to GC Constructors, a joint venture of Massman Construction Co., Kansas City, MO; Traylor Bros., Inc., Evansville, IN; and Kiewit Southern Co., Fort Worth, TX; Standard Concrete Products of Columbus, GA, was contracted to supply the precast, prestressed concrete beams for the project.
The New Bridge
At the highest point above the navigation channel, the new 1.66-mile (2.67-km) long bridge rises 95 ft (29 m) above the normal high tide level, affording unobstructed views to travelers while allowing large ships to pass beneath without lowering their masts. Constructed as two side-by-side superstructures, each structure carries three lanes of traffic. In addition, the eastbound structure has a 12-ft (3.7-m) wide path for cyclists and pedestrians. The project also included twin 800-ft (244-m) long bridges over the railroad in Ocean Springs. Awarded in June 2006, the project proceeded at a very fast pace with the north lanes for westbound traffic opening just 16 months later and 12 days ahead of schedule. At its peak, the project employed 450 people and required 18 floating cranes.
Span lengths for the bridge vary from about 86 ft (26 m) at the low level approaches to 250 ft (76.2 m) for the navigation channel span. The superstructure utilizes precast, prestressed concrete bulb tees at a typical spacing of 12 ft (3.7 m). The bulb-tee depths are 54 in. (1.37 m) for 86-ft (26-m) spans, 72 in. (1.83 m) for 120-ft (36.6-m) spans, and 78 in. (1.98 m) for 150-ft (45.7-m) spans. The navigation span of 250 ft (76.2 m) is flanked by two spans of 200 ft (61.1 m). Each line of girders for these three spans was constructed as a five-segment, post-tensioned, spliced girder unit consisting of haunched girders over the piers adjacent to the navigation channel, two end span segments, and a main span drop-in segment. The end span and drop-in segments utilize 78-in. (1.98-m) deep bulb tees, while the haunched segment varies in depth from 78 in. (1.98 m) at the ends to 144 in. (3.66 m) over the piers. To accommodate the post-tensioning ducts, the web thickness of the bulb tees was increased from 7 to 9 in. (178 to 229 mm).
Self-Consolidating Concrete
“Two major factors contributed to the speed with which we were able to complete this project,” said Peter Pieterse, division manager for Standard Concrete Products. “MDOT accepted our recommendation to use self-consolidating concrete to cast the beams. A debonded rather than draped strand pattern for the prestressing strands allowed much faster production so the beams could be ready for delivery as needed.”
Polycarboxylate technology was utilized to produce the self-consolidating concrete. This provided a number of benefits including extended slump retention, higher concrete strengths for greater design flexibility and structural economies, more durability, lower permeability, and fewer surface defects.
The use of self-consolidating concrete reduced the time required to cast the beams by approximately 50%, virtually eliminated the need for vibration equipment, reduced associated noise, and reduced the costs of final finishing by an estimated 60%. The specified concrete compressive strengths were 6500 psi (45 MPa) at release of the strands and 8500 psi (59 MPa) at 28 days. Actual average strengths were 7000 psi (48 MPa) within 15 to 18 hours of being placed, and 13,000 psi (90 MPa) at 28 days. The specifications required a slump flow of 24 to 28 in. (610 to 710 mm), an air content of 3 to 6%, and a maximum temperature at placement of 95 °F (35 °C). The average measured slump flow was 25 in. (635 mm) and the average measured air content was 4%.
Static-segregation and J-Ring tests were conducted for every 1000 yd3 (765 m3) of concrete. The concrete and strand also easily passed the specified strand-bond test often called the Moustafa Test.
“The concrete met all of the specifications, and the beams looked great,” concluded Pieterse. “As a result of the reduced concrete waste, speed of construction, and increasing acceptance of self-consolidating concrete, we’re using it whenever and wherever possible.”
Lessons Learned
The use of self-consolidating concrete not only reduced production costs through faster placement but it also allowed for placement with less skilled workers. Concrete was deposited at fewer points along the forms and flowed approximately 35 ft (10.7 m). Placing self-consolidating concrete in this manner created a smooth surface without signs of bleeding or discoloration. When producing self-consolidating concrete, slump flow and visual stability index (VSI) testing should be performed on the first few batches at the beginning of the placement. Further slump flow tests and VSI testing were done per the specifications. It should also be noted that moisture content of the aggregate and variations in aggregate gradations have a greater impact on self-consolidating concrete than conventional concrete.
Typically, self-consolidating concrete requires a higher cementitious materials content. The requirement to be highly flowable dictated the use of a high-range water-reducing admixture. The lower water content and higher cement content resulted in higher compressive strengths than conventional concrete.
Further Information
For further information about this article, contact the second author at [email protected].