Q: I read with interest your article about self-leveling concrete. A contractor friend recommended this material to us, and based upon what you wrote, I would like to get your thoughts.
We have a small basement, about 4 feet by 12 feet, in our 1939 home that is used as a laundry area. The concrete was poured on dirt and while the foundations are very strong, the floor has cracked and it’s slightly tilted on one side. We planned to use this self-leveling concrete to give us a smooth floor.
Without seeing the area, I’m sure you can’t give a strong opinion, but do you think this sort of application would be appropriate for self-leveling concrete?
A: Actually, we do have a strong opinion. Dad preached, "If you’re going to do a job, do it right!" Self-leveling concrete is not the answer.
We recommend demolishing the existing cracked and tilted slab and replacing it with a new reinforced slab on a properly prepared base. With moderate skills this can be a do-it-yourself project, to a point. You can remove the old slab yourself. With moderate skills and a fair amount of hard work you can place the base, build the forms and set the rebar. But, unless you have experience with concrete, we suggest that you hire a contractor to place and finish the mud.
Because the slab is relatively small at about 50 square feet and has no reinforcing steel, as evidenced by the cracks, it should break up easily. But be aware that breaking the slab into pieces and getting it out of the basement will be labor intensive.
You could use a sledgehammer, but we suggest renting an electrically powered jackhammer. This tool lets you break the slab into relatively small pieces and saves wear and tear on the old body. You could take the debris to the landfill, but why not recycle the pieces by using them as steppingstones in your lawn or for building a raised planter bed?
With the old slab out of the way, place several inches of compacted road mix down as a solid base for the new slab. Road mix is a mixture of small rocks and sand. Compact the material by soaking it with water and tamping it with a hand tamper. An alternative is to rent a commercial compactor, but in our view the area is too small and the weight the slab will have to bear is too light to justify the effort and expense.
Once the base is down, form the slab with 2-by-4s. Drive 1-by-2 wood stakes to hang the form members. Use 1-by-2 stakes driven into the ground at a 45-degree angle and nailed to the top edge of the form boards to hold the form in place. Make sure the forms are level. Try to get it square, although in this case close to square is good enough.
Next is the most important step to make sure the new slab lasts. Concrete has great compressive resistance but lousy tensile strength. Translated, that means it’s tough to crush, but doesn’t bend. Make an 18-inch grid of 1/2-inch rebar to keep the concrete from cracking. Lay bars on the base at 18-inch intervals in the horizontal direction, then lay bars in the vertical direction to form a crosshatch pattern. Tie the joints together with wire. Support the rebar about 2 inches above the base with some of the rubble from the old slab.
For the final step we recommend employing a concrete contractor. Though this is a small job, a pro has the know-how to finish the slab smoothly. Once poured, the slab will be fully cured in 28 days. But you can safely replace the washer-dryer within a day so after the pour.