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Re: Concrete Overlay

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            Brian,

                        The “common practice for adding a concrete overlay to a slab” is to follow the horizontal shear provisions of ACI 318 Chapter 17. Since you’re not looking for composite behavior, that’s not required.

                        My recommendation is to prepare the surface as you describe and provide some expansion bolts into the existing slab with a nut at the very end of the projection of the bolt, so it looks like a stud head in the new pad. Why? For the time when a corner cracks and the concrete bond, despite your preparation, fails, there’s something mechanical to help hold it together.

                        You should take this with a grain of salt, as I would not pour a slab-on-grade without some reinforcing – lots of scrap pieces of WWR are better than nothing when a crack occurs. Anything that keeps the aggregate interlocked – or provides some shear friction steel (ACI 318 11.7.

                        The other case is an isolated machine pad. I have less familiarity with them, but have seen equipment on a separate foundation, with a joint filled with soft material around it.

            Jim Getaz

            Winchester, Virginia