Design and construction of concrete subfloor

There are two different types of concrete floors: level slab and structural. Sloped slab is a floor placed directly on a prepared subgrade to provide a clean, durable surface for building occupants. The durability of the slab depends on the strength and finish of the concrete, but the strength of the slab depends on the strength of the supporting subgrade.

The structural slab, which must span between supporting walls, columns or beams, must be capable of supporting its own dead weight and the superimposed load it was designed to carry. Therefore, its strength depends solely on the strength of the concrete and the amount of reinforcing steel that has been used.

Concrete slab on slope

Sloped concrete slabs may be placed before any other part of the building has been constructed on a prepared subgrade to provide a level surface upon which the structural frame is erected. When the frame is supported by footings and slope beams, the floor can be laid after the building has been closed.

In the first instance, wooden or metal side forms are placed, leveled and staked around the perimeter of the building, and screed strips are placed at convenient intervals to provide guides for leveling the concrete.

Slabs-on-grade often provide less than adequate results when placed in service due to excessive cracking, dusting, and chipping. To ensure that the slab provides a durable and useful surface, the Portland Cement Association presents the following recommendations:

• The subgrade must be uniform and have adequate bearing capacity
• Concrete must be of uniform quality
• Slab thickness should reflect anticipated loads and should be consistent without abrupt changes in thickness due to poor subgrade preparation.
• Provide proper slab bonding
• Provide good work force
• Provide adequate surface finish for anticipated loads
• Use proper repair procedures when necessary.

Structural Concrete Floor Slabs

A concrete floor supported by a structural frame may be one of several floor types now available. Which one will be used for any particular building depends on a number of factors, including:

1. The intended use of the building
2. The type and magnitude of the loads to which the floor will be subjected
3. The length of the section between supports
4. The type of building frame
5. The number of floors in the building.

Types of concrete structural floor slabs include the following:

• One-way solid slabs
• Two-way solid flat slabs/li>
• Bidirectional flat plate slab
• One-way beam slab
• Waffle slab (two-way beam)

Post-tensioned concrete slabs

Adding post-tensioning strands to concrete slabs produces sections that can extend greater distances than unprestressed sections and yet are capable of supporting greater loads with less deflection and virtually no cracks.

High-strength steel strands, or tendons, encased in a protective plastic coating are distributed throughout the slab at predetermined intervals. The following figure illustrates a typical tendon with slab anchor, wedge anchor and protective coating. The coating ensures that the concrete does not adhere to the post-tensioning tendon.

Once the concrete has sufficiently cured, the tendons are tensioned to a predetermined tension and anchored. The figure below illustrates the strands being placed for a two-way post-tensioned flat plate slab. Note that the tendons rise higher up the form in the vicinity of the supporting column than in the middle of the slab. This arrangement ensures that the compressive forces induced in the concrete slab by the tendons counteract the tensile stresses produced in the slab by the anticipated loads.