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RE: Cracks in an existing composite floor deck[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
- Subject: RE: Cracks in an existing composite floor deck
- From: "Sprague, Harold O." <spragueho(--nospam--at)bv.com>
- Date: Thu, 1 Apr 2004 15:04:06 -0600
It is common practice to NOT put in control joints in slabs on metal deck. It is also common experience to have shrinkage cracking. The cracks are most pronounced in composite beam / girder systems with composite decks. I generally use rebar over girders and stagger the ends of the bar. Rebar is placed on chairs, and the rebar can then support the mesh. There is no reason to repair the cracks unless you are going to place a flooring that could reflect the cracks. In that case, just inject them with epoxy ($6.00 per lineal foot), and be done with it. The cracks are NOT active if they are indeed shrinkage cracks. Bear in mind that that the beam rotation and resulting cracks are significantly less when the concrete has age (shrinkage is time dependent) and the strength to work as a composite. I had this same question and asked Walt Schultz, the director of R&D for Vulcraft in Norfolk, NE. He said that the mesh is strictly for shrinkage, and that shrinkage cracks in the slab do not reduce the load carrying capacity of the composite slab. You will note that the SDI shrinkage steel is less than the ACI shrinkage steel. I will often bump the steel to meet ACI requirements. It is not that costly. Although with steel price rises, I might have to revisit that. You can also mitigate concrete shrinkage to a degree by adjusting your slab concrete mix. Regards, Harold O. Sprague -----Original Message----- From: Stephen L. Fisher [mailto:slf(--nospam--at)idgcorp.net] Sent: Thursday, April 01, 2004 2:11 PM To: seaint(--nospam--at)seaint.org Subject: RE: Cracks in an existing composite floor deck My firm is evaluating an existing composite concrete floor deck (Verco W3 deck with 3 1/4" lightweight concrete topping, total 6 1/4") to receive new equipment loading. The structure (ie deck, beams, girders, etc) was already evaluated and determined to be adequate for the proposed new loading. The contractor has removed the existing flooring, cleaned the flooring adhesive and lightly bead blasted the concrete topping. With the concrete topping exposed there are numerous cracks evident and the question was raised by the client whether or not the floor was structurally adequate. I have gone out to inspect the floor and noticed the following: 1. The cracks occur, almost without fail, parallel to the girders and beams. This makes sense because the deck is in negative bending with the top surface of the concrete topping in tension. If the WWF was not place properly this could very easily occur. Even with the WWF placed properly this cracking might have occured. 2. The cracks are between 1/16" and 1/8" wide and appear to be on the order of 5/16" deep. There are a few locations where the cracks are as wide as 1/4" and as deep as 1/2", but those incidents are less frequent. 3. There was no evidence of sawcut or cast in control joints anywhere in the concrete topping slab. 4. The area of the building involved in this remodel is approximately 80 feet by 90 feet. The columns are spaced at 40 feet on center in one direction and 30 feet on center in the other direction. My current conclusions are as follows: 1. Since the majority of the cracks are narrow and shallow they likely occured as a resulting combination of shrinkage cracking, no prepared control joints, and possibly poorly placed reinforcing which lead to tension cracks over the supporting beams. 2. These cracks should not affect the performance of the composite floor system and therefore the new equipment can still be located as planned. 3. Any attempt to fill the cracks would mearly be cosmetic because they would open right back up in areas of negative bending when new loading (from the equipment) is placed. 4. Some of the cracks may open up a little more as a result of the originally expected live load being partially replaced with real dead load. My questions to the List are the following: 1. Does anybody have any other thoughts or concerns about the situation I described? 2. Has anybody seen this type of cracking and had any repair methods worth pursuing? Sincerely, Stephen Fisher,SE Sacramento, CA ******* ****** ******* ******** ******* ******* ******* *** * Read list FAQ at: http://www.seaint.org/list_FAQ.asp * * This email was sent to you via Structural Engineers * Association of Southern California (SEAOSC) server. To * subscribe (no fee) or UnSubscribe, please go to: * * http://www.seaint.org/sealist1.asp * * Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you * send to the list is public domain and may be re-posted * without your permission. Make sure you visit our web * site at: http://www.seaint.org ******* ****** ****** ****** ******* ****** ****** ********
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