Investigation and Repair of Posttensioned Concrete Slabs -A Case History

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Title: Investigation and Repair of Posttensioned Concrete Slabs -A Case History

Author(s): Stephen Hom and Garrison Kost

Publication: Concrete International

Volume: 5

Issue: 7

Appears on pages(s): 44-49

Keywords: beams (supports); concrete slabs; control joints; cracking (fracturing); epoxy resins; parking facilities; post-tensioning; pressure grouting; prestressed concrete; repairs.

Date: 7/1/1983

Abstract:
Prestressed concrete floor systems that are rigidly restrained against movement have been experiencing severe structural cracking in many buildings that were constructed 10 to 15 years ago. A case history of the investigation of such a floor system in a three-story underground parking garage that was designed and constructed in the late 1960s is presented. The floor system consists of post-tensioned beams and slabs rigidly anchored to the exterior walls. The indications of a distress condition were the excessive amount of movement at an expansion joint separating two portions of the garage and the multiplicity of cracks in the slabs and columns. A finite-element analysis was performed to confirm that concrete shrinkage and creep were the main contributors to cracking. The crack pattern generated for the analysis compared very well with the actual crack pattern of the slab. To insure the structural integrity of the slabs, the cracks were pressure grouted with epoxy adhesive, and a subsequent load test was performed on a portion of the ‘slab. The analysis, repair, and testing of the slab are presented and discussed.