Tips for Crack-Free Concrete Slabs - Continued
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
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).
4. Breaking the bond between the footing and the slab
with felt paper or sand will help prevent cracking as the slab
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).
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.