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Basement Slab

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>> Basement Slab

Basic Info
The basement slab provides a smooth durable surface for the basement floor. The slab also supports the bottom of the foundation wall against earth pressures (see graphic below).
Control joints are often not placed in the basement floor. The reasoning is that the concrete will crack (as all concrete does) but that it will be covered with carpet and the cracks will go unseen.
The basement slab also provides lateral support for the bottom of the basement walls. As earth pressure pushes against the bottom of the wall the slab provides support. the top of the foundation walls are supported by the framed floor system.

  • Steel wire mesh is often used to reinforce concrete slabs. The mesh is usually comes in 6'-8' rolls and has a 6" x 6" wire grid. However is it becoming more common to use fiber admixture in the concrete mix.
  • The concrete is poured to a thickness of 4".
  • The basement slab is poured following sub rough plumbing and HVAC inspection.

Pouring the slab
Before the concrete truck arrives the level of the slab is marked on the side of the foundation walls with a chalk line. Screed boards are placed in the middle of the floor at the same height to ensure that the slab is level. The concrete is then poured, leveled and finished.
Care must be taken not to drive the concrete truck too close to the foundation and especially not on top of the backfill that was recently placed against the foundation walls. Many foundations have been cracked, bowed or have collapsed due to the pressure of concrete trucks, bull dozers, and excavators.

The process
After the concrete is placed it is leveled with a screed board. A straight 2 x 4 is often used as a screed board.
Once the floor is screeded, it is tamped with grated mesh tool called a "jitterbug". The tamping pushes the large aggregate down into the slab and allows the "cream" (sand, water and cement) to rise to the surface. The cream allows the surface to be finished hard and smooth.
The surface is then floated with a large metal float. The float called a bull float is pushed across the floor to smooth out any imperfections and once again push the large aggregate down. As the cement hardens, it is finished by toweling with wood and metal trowels.

If hiring a contractor for flatwork, check previous work for flatness and smoothness.

Conditions that are generally found in a contract:

  • Price for the job (usually figured by the square foot).
  • Price for change orders.
  • Hourly charge for work not covered by the contract.
  • Description of work to be performed.

    Specs to include:
  • Type and strength of concrete mix to be used. (compressive strength, slump, etc).
  • Size and type of wire mesh or reinforcement to be installed.
  • Type of finish (smooth, broom, etc.).

Owner builder managment
Because concrete sets up fast, there is no time for changes in the project once the concrete is being placed. Preparation is the key when pouring concrete. Once it sets, it is expensive to remove and replace. Concrete crews like to water-down the concrete so that is takes less effort to spread. Adding a little water is okay as long as the concrete retains the correct slump. Remember that adding water significantly reduces the concrete strength.

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