2020 PROJECT OF THE YEAR The Austonian: Slab Edge Repairs SUBMITTED BY PIVOT ENGINEERS HIGH RISE CATEGORY AUSTIN, TEXAS Fig. 1: The Austonian building Fig. 2: Slab edges at the Austonian (red lines highlight typical exposed edges) T he Austonian is a 59-story, 683 ft (208 m) tall high-rise condominium that was completed in 2010 and is located in downtown Austin, Texas (Fig. 1). The building is comprised of reinforced concrete structural elements and unbonded, post-tensioned (PT) concrete slabs. The majority of the slab edges in a typical fl oor are covered with aluminum cladding, while the remaining slab edges are exposed to the elements. The building has approximately 1.75 linear miles (2.8 km) of exposed slab edges distributed over the entire height of the structure (Fig. 2). Six years after construction, a 5 ft (1.5 m) long concrete segment (Fig. 3) detached from a balcony edge, fell more than 200 ft (60 m), and almost hit a sta member. Fortunately, no one was injured, and the spalled concrete caused limited property damage. Immediately after the incident, a visual survey identifi ed 31 slab edge locations with evidence of severe cracking and distress (Fig. 4). The distressed slab edges were distributed throughout the building and were not concentrated in any particular area. To mitigate the risk of falling hazards, temporary protective measures were installed at the distressed locations to secure the concrete in place until permanent repairs could be performed. Understandably, the residents were concerned about the possible danger to people and property from falling debris. FORENSIC INVESTIGATION A comprehensive forensic investigation of the conditions at the exposed slab edges was performed to: 1) determine the cause and extent of the observed distress; and 2) develop an appropriate repair plan. The investigation examined a statistically representative and spatially distributed sample that included more than 15% of the exposed slab edges in the building, which ensured a high confi dence level in the investigation results. The investigation used several non-destructive and destructive evaluation techniques to estimate the in-situ concrete cover and evaluate the risk of ongoing corrosion activity. For the concrete cover survey, a cover meter (Fig. 5) and surface penetrating radar were used to locate the steel reinforcement and estimate the concrete cover. WWW.ICRI.ORG 12 CONCRETE REPAIR BULLETIN NOVEMBER/DECEMBER 2020
Concrete Repair Bulletin November/December 2020: Page 12
