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RE: Debonding of precast plank topping

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Apparently there was little of no provision to provide a mechanical bond to augment the adhesion.  I have used a similar application in the past and have required both a raked surface and a truss embedded into the precast to provide a positive bond to the topping specifically to mitigate the problem you have identified. 
 
Investigate the possibility of creep, shrinkage and strand bond relaxation.  If the bond in the precast strand is adequate, the precast concrete is sound, and topping concrete is sound then it appears that it a repair may be possible.  You can calculate the anticipated precast deflection assuming zero bond on the precast.  This should verify your hypothesis.  It is a bit of a guess because you will have to assume an initial elevation. 
 
Possible repair:
1.  Calculate the horizontal shear between the topping and precast assuming zero bond.  Design new shear connectors in lieu of bond. 
2.  Jack and shore the slabs into place with allowance for DL deflection of the composite section. 
3.  Drill holes through the topping and through the precast at a workable uniform spacing (locate using NDT and avoid the strands). 
4.  Using a plate with a welded headed stud in the hole, place a repair concrete in the hole to establish the required bond between the precast and topping.  Calculate the required hole size assuming no contribution from the headed stud and all shear is resisted by the concrete.  (This is very conservative, but may be warranted).  The headed stud can be wired in place from the top and the plate on bottom will serve as a form to hold the repair concrete in place.  If you use rectangular plates and cut rectangular holes, you may be able to do all of the repair from the top.  (Exception: Obviously the shoring will require access from the bottom.) 
5.  Cure the concrete, remove the shoring, measure the final deflections to verify the adequacy of the repair. 

Regards,
Harold Sprague



From: tiger(--nospam--at)palaunet.com
To: seaint(--nospam--at)seaint.org
Subject: Debonding of precast plank topping
Date: Wed, 2 Jan 2008 17:51:54 +0900

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.

 

Regards,

T. Gillham

 

 

Terangue *Tiger* Gillham, PE
GK2, Inc.
PO Box 8061, Koror, Palau 96940

tiger(--nospam--at)palaunet.com <
mailto:tiger(--nospam--at)palaunet.com>

 



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