Un-bonded Post-Tensioned Slabs Development and Repair Systems using CFRP


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Title: Un-bonded Post-Tensioned Slabs Development and Repair Systems using CFRP

Author(s): Pinaki Chakrabarti, Uksun Kim and David Naish

Publication: Special Publication

Volume: 287


Appears on pages(s): 1-18

Keywords: Development, Un-bonded post-tensioning, Existing Structures, Continuous Slab, Flat-Plate/Slab, Repair System, CFRP

Date: 5/1/2012

Un-bonded post-tensioned slabs were developed and principally flourished in the USA since the mid 1950’s. The continuous un-bonded one-way post-tensioned slabs became popular due to their predominant use in parking structures all over the country which is true even now. Two-way un-bonded post-tensioned slabs, mainly flat-plates and flat-slabs gained popularity since mid 1960’s. The use of a Banded System of placement of un-bonded post-tensioning tendons, introduced in the early 1970’s, made flat-plate and flat-slabs more competitive. Flat-plate/slab and shear-wall system became and remains very popular for mid-rise and hi-rise buildings. A brief discussion on the development of un-bonded post-tensioned slabs and its relevance in current design and construction is made in part-1 of this paper. Many of these slabs are now between 30 to 60 years old. These structures need routine repairing and retrofitting work. Existing methods are labor intensive and expensive. An alternative method could be repair work with composite materials. Use of composites (mainly CFRP) as a repair material for concrete structures is becoming very common. Most of the repair procedures are based on researches with reinforced concrete specimens and in some cases with pre-tensioned specimens. Research work using un-bonded post-tensioned specimens, especially two-way slabs is practically non-existent. A testing program was developed with the goal of finding the cracking and ultimate strength behavior of un-bonded post-tensioned slabs (before and after repair with CFRP) with different boundary conditions. A total of six slabs were tested. In the first phase two two-way simply supported un-bonded post-tensioned slabs were tested. In these tests CFRP repair configurations were varied. In one case CFRP was placed across the cracks in another case it was orthogonal (parallel to the edges). In the second phase, four more slabs were tested (three one-way slabs and one two-way slab). One one-way slab had two ends fixed, another one had one end fixed and one end simply supported and the third one had both ends simply supported. The CFRP placement configuration in these three slabs varied. CFRP was placed across the cracks in the supports and mid spans. The fourth slab was a two-way slab with simple support. In this case the CFRP repair configuration was similar to the first slab (but CFRP had 2 inches/5 cm overlaps). Sketches of different cracking patterns and CFRP configurations are shown inside. The repair and testing of slabs is discussed in part-2 of this paper.