Permanent Repair on the San Antonio Y Interchange

Context

The San Antonio Y Interchange in the center of San Antonio, Texas, USA, is a Y shaped interchange between US Highway 35 from the northeast and US Highway 10 from the northwest that merge together moving south. The viaducts of this elevated intersection were built in the 1980s utilizing precast concrete segmental type construction. The 755 sections were post-tensioned together using longitudinal and transversal tendons, and the bridge has a total length of approximately 23km (14.3mi).

When the interchange was built, it was not yet known how important it is to completely grout the tendons, and at the time industry standards did not place a strong emphasis on this component of the construction. As the grouting operations were not carried out thoroughly, voids often occurred that allowed moisture and oxygen to infiltrate the system, thereby causing corrosion of the post-tensioning strands. During a cursory evaluation of the Y Interchange, the Owner, TXDOT, found voids in some of the tendon ducts. Furthermore, the ducts of many tendons were damaged, and most anchors had no caps, which are important for protecting the strand tails and act as an additional level of corrosion protection.

DYWIDAG was selected as the subcontractor by the Owner and Contractor to perform all of the remediation work associated with this section of the elevated highways. One of the biggest challenges was determining the number of the originally installed tendons that had to be repaired because an in-depth evaluation and survey was not possible in advance.

Solution

DYWIDAG engineered all test inspection and repair protocols as part of their construction plan and carried out the work through the use of bore scopes to identify the condition of the strands within the voided areas. In addition, when DYWIDAG identified voids, specialized vacuum grouting techniques were employed to measure the void volumes and then fill the voids with grout.

Since the original construction of the Y Interchange was done in several phases and by different contractors, the tendons had also been supplied by several different Post-Tensioning companies, resulting in many different anchorage types. DYWIDAG designed, produced and installed custom caps for each type of anchorage.

For those ducts that had incurred major damage over the years in the form of splitting and cracking, DYWIDAG designed and installed a special split duct system for permanent repair. All of the existing voids inside the post-tensioning tendons were filled by DYWIDAG using a vacuum or vacuum-assisted grouting method.

The largest challenge to overcome on the project was the access to the work areas inside of the precast segments. This was primarily done from directly underneath the bridge segments through integrated hatches that were many times located directly above heavy traffic areas of street level roadways and parking lots. This demanded considerable traffic control with lane closures and the need to complete a significant amount of the work at night.

Despite the large project footprint and other challenges encountered, DYWIDAG completed all repair measures within the contract timeframe of just over two years. DYWIDAG’s crews worked safely on these structures, logging over 75,000 man hours to complete the work.