Tech Talk – How Do You Address Expansion Cracks in Concrete?

Lenny Hall – Endurance Floor

An expansion crack in concrete is a joint that’s been put there for the sole reason of not having a large span of concrete crumble on its own pressure when it expands and contracts. However, if you’re going to lay a wood floor over it, you have to address the fact that the floor system above the crack may move if the floor is bonded permanently to that system. You could use a floating subfloor system over the entire concrete surface so that the subfloor system basically is bridging across that expansion gap and then your hardwood is installed on top of your floating subfloor. If you can’t do that and you’re using a glued down system to install a hardwood floor, then you would have to use a bridging membrane, which is what is done in the ceramic and tile industry. A bridging membrane is a sheet goods type material that’s bondable. It will be bonded to the concrete slabs on both sides of the expansion joint and then your hard surface is glued down over the top of that. It doesn’t mean that you will have control over vertical shifting of each slab section on either side of the joint, but the lateral expansion and contraction would not be transferred to the hardwood floor.

Brett Miller – NWFA

As the NWFA installation guidelines indicate, wood flooring that is secured to a slab should not bridge moving joints without allowing for a breaking point. When concrete decides to move, it is going to move. Expansion cracks in concrete have purpose. These joints are placed or cut into the concrete in an effort to encourage cracking to follow an orderly, predetermined pattern. There are several types of cracks to be aware of when installing over a concrete slab, including Construction (or Cold joints), Control (or Contraction joints), Isolation joints, Acoustical joints, and expansion joints. During installation, these cracks must be honored. When gluing down a wood floor, the safest way to honor these cracks is to use specific bridging products made by the adhesive manufacturers to help isolate the floor from the slab in those areas.

The other option would be to leave expansion space in the floor at those locations. This expansion space may be accompanied with t-moulding or some sort of elastic caulking/cork that would allow the floor to move independently on either side of the crack, if, and when, the concrete slab decides it is time to move.

The third option is to either use the “floating wood subfloor over concrete” method, or specify a floating wood floor. Both of these methods greatly minimize any negative effects caused by movement in the slab at these expansion joints.

Daniel Saucedo – Daniel Wood Floors

The way that I manage cracks on a foundation depends on size and length. If it is a bigger crack, I usually leave it to foundation companies. With small cracks, I do use a grinder with a diamonds blade cutting dip, not more than 1”, vacuum well, and wire brush, and apply Sika, which comes in tubes in two different sizes. It’s the only way that it works perfect for me.

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