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Re: Slab on grade[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: seaint(--nospam--at)seaint.org
- Subject: Re: Slab on grade
- From: rlewis(--nospam--at)techteam.org (Richard Lewis)
- Date: 08 Oct 1998 18:47:49 GMT
I am responding to the original post and several responses, but to save space I am not copying the original text and responses into this note. Regarding Mr. Horning's comments. First, polypropylene fibers are NOT flexural reinforcement. They are only shrinkage reinforcement. Once the concrete has shrunk the fibers provide very little value in flexure. You can call any supplier and talk to their engineer and they will tell you not to count on fibers for flexural reinforcing. If you need that type of reinforcing then put the steel in. One of the advantages of WWF is that it does provide some, an admittedly low amount, of flexural reinforcement. It can help tie the slab together for minor cracks. But its main purpose is also for shrinkage stresses. If you need flexural reinforcement, put it in, not WWF. Polypropylene fibers will temporarily leave a fuzzy surface on the concrete, but it does wear off. Polypropylene fibers are best used where WWF would have a possible rusting environment, such as an outdoor exposure. Also, shrinkage reinforcing is usually placed about 2 inches down from the top of slab. Since you are putting #4 at 12 EW in the middle in the middle for flexure, you will probably be okay. But if your main intent is shrinkage control, you may want to move it up slightly. The suggestion of spacing your joints in feet at 3 to 5 ties the slab thickness in inches is a good rule of thumb. And, if you are going to put control joints in the slab you should reduce the reinforcing at that location to effectively weaken the slab. This means you have to have planned the joint layout as you lay out the reinforcing. One critical item not mentioned is the thicken slab. I am not sure why you are doing it. You said you are not attaching the slab to the foundation, so why are you thickening it? Why not just float it at the edge? But, if you do thicken it you have to adjust your jointing. Thickening the slab creates a point of resistance for the horizontal movement that occurs as the concrete shrinks. First, let's consider a typical concrete floor slab that is jointed on all 4 sides. How does the concrete shrink? Shrinkage causes the perimeter edge of the panel to move towards the center. This is what opens up the joints. Therefore, the slab movement is greatest at the perimeter edge and is zero in the middle. Everything shrinks towards the middle. Now when you thicken a slab you create resistance to horizontal movement so that the point of zero movement is now at the thicken slab and the greatest movement is at the other edge parallel to the thicken slab. You have effectively doubled the size of the panel. The thicken slab shrinkage behave is the same as an ordinary panel twice its size. So, the location of the first shrinkage joint parallel to the thicken slab is 1/2 the spacing of a typical joint. Given your 27' x 105', I would locate the first joint 9 feet from the thicken slab and then have an 18 feet panel to the center construction joint. I would space the transverse joints at 17 to 18 feet on center. Curing a slab this large is very critical. And, it doesn't matter if the roof is on or not. Curing should start immediately after finishing is completed. You should either wet cure it, or use a high solids curing compound. A 6 inch slab is quite substantial for such a light plane. If you can limit the area where you will park the plane and not put the dump truck or tractor there, you might save some money by going to a 4 inch slab. Someone previously mentioned expansive clays. If you do have them in your area then you must design for them, or you will have major problems. One last item. Concrete does shrink, and it does crack. You have not failed if you do find a crack. Your goal is to either spread them out as micro cracks which are not visible to the eye, or control where they occur, at the joints. __________________________________________________ Richard Lewis, P.E. Missionary TECH Team rlewis(--nospam--at)techteam.org The service mission like-minded Christian organizations may turn to for technical assistance and know-how.
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