Home FAQs Cracking & Joint Problems in Concrete I am adding onto my house. Should I use a felt expansion joint where the old and new slabs meet?
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I am adding onto my house. Should I use a felt expansion joint where the old and new slabs meet?

Q: I'm just about to add on to my house using a slab on grade. Is wire mesh enough or should I use 3/8 rebar? Also should I use a felt expansion joint where the old and new slabs meet?


A:  The answer to your question depends on a couple of factors.
If your slab is to be placed on a consistent, supportive base material, wire mesh should be fine. In fact, for this type of application I would recommend the use of fibers in the concrete mix in place of the wire. The dosage rate for the fibers should be 1 1/2 pounds per cubic yard.
If your slab is to be placed on a poor base that has soft spots, or if the slab will be bridging over areas such as plumbing ditches, etc. then you need to consider rebar. I would normally use 1/2 inch rebar as opposed to 3/8 inch rebar, and the spacing of the rebar is also very important. Many contractors use rebar, but spread the bars too far apart to provide much benefit. For simple crack control with rebar, a spacing of 18 inches on center is usually adequate, but to provide support for bridging over soft areas, the spacing would probably need to be reduced to at least 12 inches on center, and perhaps closer depending on the size of the area to be spanned.
With regard to the expansion joint, I would suggest using some type of expansion material between the two slabs. This material will function as a bond-breaker rather than an expansion joint - concrete does not expand past its original volume except in very specific and somewhat unusual circumstances - so the term expansion joint is really a misnomer. There are several types of materials, including closed cell foam, that could be used for this purpose. Just make sure the material you use extends the full depth of your existing slab to prevent any of the new concrete from bonding to the old slab.