What is "expansion joint" (between isolated slabs)

On page 41 of Rod's book, he describes a procedure for laying isolated concrete slabs. He says that the space between the slabs should be filled with "expansion joint" and then caulked to seal out moisture. What kind of material is meant by this? I'm in the UK so names of specific products would be great if UK-available. Otherwise just the type of thing being referred to, and I'm sure I can track it down. Thanks.

a quick google:

Expansion Joints Expansion joints permit volume change movement of a concrete structure or member. These are usually constructed by installing pre-formed, or pre-molded elastic/resilient material of approximately 1/4" to 1/2" thickness as wide as the concrete is thick, before the concrete is placed. Expansion joints should never be less than 1/4" wide. Pre-molded expansion joints for installation in residential, commercial, or industrial slabs may be of fiber, sponge rubber, plastic, or cork composition. Such materials must be highly resilient, and non-extruding in hot weather, or brittle in cold weather. An expansion joint should always be utilized where a concrete member will join or abut an existing structure of any type. This would include a junction of sidewalks, sidewalk with a driveway, building, curb, or other similar members, as well as where a floor slab joins a column, staircase, etc. The square formed by the intersection of two sidewalks should have pre-molded expansion material enclosing the perimeter. Normally, expansion joints are not provided in sidewalks other than where the walk abuts an existing structure. Expansion joints should also be provided in a building floor slab where the slab abuts walls or footings. Sealing of expansion joints is desirable in many outdoor or industrial/commercial applications.

your concreter will know anyway ;)

Thanks John. Presumeably the important thing is the elasticity of it, since it must act as a spring in not transmitting vibrations from one slab to the other? I'm reluctant to trust it to the discretion of a concreter, as he may not be aware of this necessity...

It's hard to get to the bottom of this. In this thread for example: viewtopic.php?f=2&t=3621&hilit=expansion+joint words like celltex and styrofoam are mentioned. However Rod in the same thread says that he actually recommends taking the expansion joint out after pouring! What I wonder about then is what happens between the slabs, in terms of preventing moisture from rising from the Earth. I actually used this technique in my current building. It was all a bit of a bodge because I had agreed with my builder to leave the space open. He then used polyurethane expanding foam in it without consulting me (I think because he was panicked about damp control). We ended up digging most of it out and just leaving a bit in the bottom, where we could. I have to say the isolation of my room is very good, though I'm not completely sure about the damp. I'd like to get to the bottom of this and really know what I'm doing before building my next place. Given the popularity (and in my case at least, soundproofing success) of this technique, I'm surprised there isn't a clearer consensus about what to do, or how to seal the gap.

The illusion is that you are talking about an "isolated" slab. This has nothing to do with acoustics. It is the description of a slab that is alone, isolated. An expansion joint is part of the design of concrete in general to aid in the prevention of compression and lifting of the slab. You can use anything you want to border the slab(s) and create the separation, celotex, styrofoam or even 3/4 inch pressure treated material. In any concrete pour, a layer of heavy plastic(vapor barrier) that completely covers the excavated area is the next to the last thing to go in, the last thing being the wire or rebar. If you have never formed and poured a slab, contract it out, it is not something for the do it yourself'er to handle. "in terms of preventing moisture from rising from the Earth." Backer rod and an approved caulking for a concrete expansion joint area. This is not something you can read out of a self help book and do on a weekend. It is the most important part of a project so if you mess it up...it never gets any better ;)

xspace - I have every intention of contracting it out. I certainly wouldn't try and do it myself. But given that the pouring of isolated slabs is (at least as we talk about it here, or it's described in Rod's book) for the purpose of restricting sound travel between them, I need to know how to achieve this so I can tell him. An ordinary non-specialist builder is not going to know, or be trustable to use the right material.

The expansion joint material that Rod has spec'd for years is no longer being manufactured. (I found out the hard way) He may have found a replacement density material by now, but when I was ready for my pads to be poured, he hadn't at that time. So, he had me use standard 1/2" x 2'x4' drop ceiling tiles, glued with the finish sides to the outside. I glued them together with a standard construction adhesive, then slit/scored the edge 1" from the top. Once the pour was cured, I went back and dug the top 1" slit out. Then filled the gap with type SL1, self leveling sylomer caulk, to prevent moisture from getting in. In a couple of places, the joints had "opened up" small cracks and the SL1 just sucked down in between the joint and the pad. I ended up having to take a few slivers of joint material and stuffing them in the cracks to plug the holes. I detailed the whole thing in my blog from about Nov of 2007 to Jan of 2008, if you wanna go dig around and show your contractor. Oh yeah, be sure you have the contractor put the mesh and re-bar on saddles or lifts to get it in the middle of the slab when it's poured. Rod rightfully, chewed my ass about that one! :D As a side note, the cheeks on the slab are extremely important. This is what your walls are going to sit on, and you damn sure don't want them cracking your slab along that perimeter where they turn up into the slab proper. IIRC, my cheeks were 16" wide before they turned up into the 6" slab. Instead of a standard 45' angle, I used a slightly slower angle of 30'. That way the stress of the mass was more evenly spread across the slab.