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My driveway was poured in sections with lengths of 2x4 which remained between the slabs as expansion joints. Due to heaving and moisture many of these are in poor shape. I replaced one once by carefully carving a 2x4 on a table saw to fit the oddly shaped gap, but I'd like a better solution.

Is there some pourable product I can use to replace these joints? Should I just grout them now with additional concrete?

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3 Answers

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I have been caulking expansion joints in concrete for 15 years. This is what you do:

  • Get the wood out
  • Clean the top 3/4" of the interior walls of the joint with a wire brush
  • Install "closed cell" backer rod into the joint. You want it to be about 1/8"-1/4" larger than the joint. Roll it down in from the side is easiest. The top of the rod should be about 3/4" below the top of the concrete.
  • Blow the crack out with a leaf blower. (the backer rod should fit tight enough to stay in place as long as you dont blast it head on)
  • Use professional grade urethane or silicone caulk for concrete. The best thing is probably Dow 888 silicone but its very expensive. Urethanes are cheaper and almost as good. Sika brand Sikaflex 1a or Sonneborn brand NP1 are great
  • Caulk depth in the center of the joint should be 1/2 the width of the joint but no deeper than 1/2". People are tempted to think deeper is better but it will cause bubbling and premature failure. If it is too thick it will not stretch easily and therefore pull away from the concrete.
  • Tooling the caulk is necessary. It will force the caulk into the pores of the concrete and it will smooth out lumpiness. Use a metal or rubber spatula or a tablespoon.
  • Self levelling urethanes are available but the have the consistency of pancake batter so your joints have to be pretty level to use it. You'll also have to dam up the ends of your joints. Plus you will have to be on the lookout for rain, bugs, and blowing leaves for a couple of hours after you finish.
  • 2x4s are a lousy choice for expansion joint because they are so wide. You are going to use a lot of caulk.
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A quick Google search for concrete joint sealer turns up loads of products, most of which seem to be Polyurethane Epoxy. I haven't done enough of this type of work to recommend one brand over another, though I'm sure an employee of your local hardware store will have an opinion.

When sealing the joint, you'll want to pay attention to the depth to width ratio. The manufacturer of the product you've chosen should recommend the proper ratio for the product, which will usually be between 1:2 and 3:2.

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You can use foam "backer rod" or "filler rope", to achieve the proper depth to width ratio.

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Definately do not use concrete to fill the gap. Concrete expands and contracts depending on the temperature so using a hard substance will cause further cracking, expansion joints are critical between slabs.

The options are:

  • Replace the wood with wood
  • Use a Joint Sealer

With either option, try and pick a time of the year when it is colder. Hotter days the concrete will expand, so you want to do the work when the concreate is at its "smallest".

If you go the wood option, get a product that has been rated for ground contact as it should last longer.

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