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Tips for Crack-Free Concrete Slabs - Continued

Expansion Joints

If the slab is contained by walls, you must provide expansion joints (also known as isolation joints) around the inside perimeter of the foundation to allow the slab to move independently of the wall. The same thing holds true for structural columns — if they are inside the area of the slab, they should be surrounded with expansion joints. Residential expansion joints are often made with 1/2-inch-thick fiberboard strips (Figure 3). The material can be flush with the surface of the slab if appearance is no concern. For a more finished look, run the length of fiberboard on a table saw to score it 1/2 inch below the top edge (which will be at the surface elevation) so you can easily remove the top 1/2 inch after the pour. Then use a caulk to seal the slab edge.

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Figure 3. Fiberboard expansion joint material permits the slab to move independently of the wall (top). For a finished appearance at the slab edge, score the fiberboard on a table saw so the top 1/2 inch can be easily snapped off after the pour. Then finish the joint with sealant (above).

Remember that the slab must be isolated from the footing, too. Either sand or 15# felt between the slab and footing works well as a bond breaker (Figure 4).

Figure 4. Breaking the bond between the footing and the slab with felt paper or sand will help prevent cracking as the slab moves.

Placing the Steel

Proper placement of reinforcement is critical. At the very least, reinforcing mesh, pipes, and cables should be suspended so that none lie on the bottom of the poured slab. Wire mesh should be in the middle of the slab if it is to do its job properly. Pipe or conduit laid directly on the ground will dramatically weaken the slab; it has the same effect as scoring a ceramic tile before snapping it. However, that same conduit suspended with 1 inch of concrete underneath will have no effect (Figure 5).

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Figure 5. Placing rebar and pipes so concrete can flow underneath them prevents stress risers.

Make use of steel or plastic rebar chairs to get the rebar in the right place. Do not use brick, because it will pull moisture out of the surrounding concrete too quickly and create a stress point.