FALL 2018

ASPIRE is a quarterly magazine published by PCI in cooperation with the associations of the National Concrete Bridge Council. The editorial content focuses on the latest technology and key issues in the Concrete Bridge Industry.

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C O N C R E T E B R I D G E T E C H N O L O G Y 34 | ASPIRE Fall 2018 Post-Tensioned Tendon Grouting on the St. Croix River Crossing The St. Croix River Crossing connects M in n e s o t a S t a t e H ig h w a y 3 6 a n d Wisconsin State Highway 64 over the St. Croix River just east of Minneapolis– St. Paul, Minn. Constructed by Lunda/ Ames Joint Venture (LAJV) from 2014 to 2017, the bridge is a 5080-ft-long pos t-tensioned concrete box gird er structure. The approach spans are 10- to 14-ft-deep precast concrete box-girders, and the approach gore areas and ramps consist of 10- to 18-ft-deep cast-in-place box girders. Approach spans range from 190 to 290 ft in length. The main river spans are 600-ft-long precast concrete extradosed spans composed of side-by- side, 18-ft-deep concrete box girders, which are post-tensioned using high- strength steel, multi-strand tendons inside corrugated plastic or galvanized steel ducts. Protection of a bridge's post-tensioned tendons is extremely impor tant, and grouting, along with duct type, concrete quality, and sound joints, is a key component ensuring a bridge's durability. This ar ticle describes the grou ting practices used for the post-tensioning tendons on the St. Croix River Crossing. Grouting Specifications The project specifications required the use of thixotropic grouts, specific minimum and maximum temperatures for grouting, time limits on the amount of time tendons can be left ungrouted, and experienced personnel to perform the grouting. Thixotropic grou t materials are a recent advancement in grout technology. These materials are highly resistant to the formation of bleed water. They are flowable when injected under pressure into a closed duct system, but they quickly stiffen while at rest. Once at rest, thixotropic grouts are very stable and minimize the air voids that occurred when older cement-water grouts were used. Elimination of bleed water means no excess water is left in the ducts, which can lead to early deterioration of the tendons. Grouting operations were limited to concrete subs trate and ambient air temperatures between 40°F and 100°F. Given these temperature restrictions and Minnesota's climate, grouting typically could not be performed from November through March, and grouting during the summer often had to be performed during early morning or evening hours, when temperatures were cooler. On this project, the maximum allowable time between installation of post-tensioning steel and grouting was 15 calendar days. Precast concrete segment fabrication and erection and cast-in-place concrete operations continued during the late fall and winter months when temperatures were much colder than 40°F. Specifications Preparing to inspect grout at grout vents. Photo: Paul Kivisto, Minnesota Department of Transportation. by Paul Kivisto, Minnesota Department of Transportation, and Paul Towell and Jeff Cavallin, Parsons Transportation Group Grout being batched in a colloidal mixer. Photo: Paul Kivisto, Minnesota Department of Transportation.

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