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Crack control in slab

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I think you are probably thinking of control (contraction) joints.  Control
Joints are normally on the order of 15 to 20 feet in both directions for
slabs less than 10 inches thick.  One of the key design factors is that the
joint (saw cut or formed) needs to be at least a 1/4 of the slab thickness.
For an 8 inch slab that would be 2 inches.  If you need to transfer shear
loads across your joint then you will also need to put in a key joint or
dowels.  Recent thinking is that key joints are not that effected however
if you go with dowels then you need to be very careful that they are level
and straight in both the vertical and horizontal plane.  True expansion
joints are not that different than the control joint above with the
exception that you need a full depth void space for movement and they are
generally spaced 75 to 100 feet.

Thomas Hunt, S.E.
Duke/Fluor Daniel

----- Forwarded by Tom Hunt/DFD on 03/13/02 07:08 AM -----

                    Samir Ghosn
                    <sghosn@harris-       To:     seaint(--nospam--at)
          >            cc:
                                          Subject:     Crack control in slab
                    03/11/02 09:15
                    Please respond
                    to seaint

Hi friends

I am interested in some feed back on spacing of expansion joints in a slab
on grade supporting 65,000 Lb Gross Vehicle Load, the slab is 8 inches
thick, reinforced to account for loading using beam on elastic foundation
theory, using K=180 pci.  My initial thought was to have a spacing about 10
t0 12 feet.  Am I on the right track here?.
The Slab will have a 4 inches  of class II aggregate per Caltrans
specification over an 18 inches of subgrade compacted to 95 percent.
Your input is greatly appreciated.
Samir Ghosn, P.E.
Harris & Associates
(800) 827-4901
fax( 949) 655-3995
e-mail " sghosn(--nospam--at)"

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