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RE: Debonding of precast plank topping[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: <seaint(--nospam--at)seaint.org>
- Subject: RE: Debonding of precast plank topping
- From: "Stuart, Matthew" <mStuart(--nospam--at)cmxengineering.com>
- Date: Wed, 2 Jan 2008 08:36:48 -0500
Title: GK2 Standard Letter
As it is very likely that the precast planks were designed to support the dead load of the field cast topping with composite action counted on for live loads only, I find it odd that the planks, as you indicate below, do not check for the dead load of the topping. It would be unusual for precast plank to have to be shored to support the topping during the initial construction phase, but I will admit I’m not that familiar with western precast concrete practices from the 1980’s.
Is it possible that the strands (I assume that the reinforcement you refer to is prestressed tendons) are experiencing long-term slippage (i.e. debonding of the concrete from the strands) which would explain a loss of prestress force and subsequent excessive deflections.
D. Matthew Stuart, P.E., S.E., F.ASCE, SECB
Senior Project Manager
200 Route 9
Manalapan, NJ 07726
732-577-9000 (Ext. 1285)
I am currently performing an investigation into excessive deflections that are manifesting in the roof of a local hospital.
The structural system consists of precast gabled frames spaced at approximately 20 feet on center with a composite precast slab system spanning between them. The composite slab is composed 4 foot wide 4 inch thick conventionally reinforced planks (shored during construction) with a 2 inch topping reinforced with heavy gage WWF.
The facility was constructed in the late 1980’s and sits directly adjacent to the ocean. The first observation of excessive deflections started about 7 years after initial construction, and were limited to noticeable cracks and staining at the joints between the eave planks. Nothing was done at the time, and the problems worsened to the point that some of the planks are exhibiting up to .75 inches of deflection, necessitating the use of shoring jacks. The location of planks with excessive deflections is seemingly random; planks with excessive deflections are abutted by spans with planks that don’t have noticeable deflections.
Cores have been taken of spans both with and without excessive deflections and sent to a lab of petrographic analyses. The results show that the concrete for both the topping and the planks is of good quality and strength. Additionally, a total of 8 planks were removed and replaced at the entrance to the hospital as the shoring jacks were blocking traffic. Inspection of the topping and planks showed that the reinforcement in both was NOT corroded.
During the coring operations it was found that the topping has debonded from the planks in the slabs experiencing excessive deflections, resulting in a vertical gap between the two. Additionally, the topping and planks debonded during coring operations for both slabs with and without excessive deflections. Both of these observations point to the possible conclusion that debonding is the culprit. Once the composite action is lost due to debonding, the 4 inch slab with 2 inches of concrete dead weight trying span 18 feet clear just doesn’t work.
I have looked for any information on other instances of precast composite slab systems that experienced this type of problem but have had no luck (full web search, ACI search, ASCE search). I am wondering if anyone on the listserver has had any experience with a situation like this one, and can point me to some resources that can shed some light of possible causes of the debonding. Most of the excessive deflections are confined with the roof eaves at this point, which have a total area of about 29,000 sf. However, a few instances of excessive deflections have been observed at the interior portion of the facility, with a total roof area of 92,000 sf. Given the potential scope of work to address the problems, I need to make sure I look at all possible causes and solutions.
Terangue *Tiger* Gillham, PE
- Debonding of precast plank topping
- From: Terangue Gillham
- Debonding of precast plank topping
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