Post Tensioning Benefits
The use of floating slabs – ground-supported, reinforced concrete foundation slabs – first gained popular acceptance after WW II, when economical, affordable housing was sought for the young families started by post-war GIs.
As pre-stressed concrete began catching fire with engineers and designers in the late 1950s and early 1960s for bridges and other structures, post-tensioning began to be used for the construction of residential and light commercial slabs in an effort to improve crack control, further reduce the cost of the slab, and to decrease slab deflection.
Benefits of Post Tensioned Slabs over Conventional Slabs
Instead of using welded wire or conventional rebar to reinforce the slab-on-grade, post-tensioning tendons are cast in the middle of the slab, running in both directions. The thickness of the slab and spacing of the tendons depends largely on the size of the structure and the type of soil the slab is built upon.
The embedded tendons are then stressed to an effective force of about 25,000 pounds with the use of hydraulic jacks. The internal compressive force thus generated in the concrete results in numerous advantages over conventional slab-on-grade construction, including:
- Increased crack resistance under load
- Decreased cracking due to temperature variations or concrete shrinkage
- Reduced footing thickness resulting in reduced excavation, and less material costs
- Fewer reinforcing elements result in faster installation
- Improved durability
- Reduced maintenance costs
- Flatter surface
In situations where the soils is medium expansive or higher (expansive index 50+)- Post Tensioned Slabs often become the construction method of choice for both quality and cost Each of the above benefits can be argued, of course: Decreased cracking may not occur if too much water is added to the mix. Reduced maintenance costs may not occur if tendons are not installed properly.
But if done properly, Post Tensioning is a superior slab construction method.
Post Tensioned Slab Construction is used in:
- Residential Slabs-on-Grade
- Parking Structures
- Condominiums & Apartments
- Office Buildings
- Industrial Floors
FACTS ABOUT POST-TENSIONED SLABS
- Post-tensioned slabs were approved by the V.A. and F.H.A. in 1967. National building codes also allow post-tensioned foundations.
- Post-tensioned slabs control cracks much better than reinforced slabs.
- Post-tensioned slabs can be installed much faster than reinforced slabs.
- As steel and concrete increase in price, the savings with a post-tensioned slab will increase because the quantity of steel and concrete require for a P.T. slab is less than for a conventional reinforced slab.
- Large National buildings companies use post-tensional slabs in many areas in the United States.
- The F.H.A., V.A. and most city codes require that a rational post-tensioned design be performed by a registered engineer.
- Large concrete areas, such as tennis courts, need no expansion joints, which eliminate maintenance. Most tennis court contractors now recommend and use post-tensioning cables.
CPML Contractors Limited post-tensioning systems utilizes ½”, 7/16” and 3/8” diameter steel stressing cables. As recommended for use in residential and commercial slabs, CPML Contractors Limited, slab on grade post-tensioned system provides a simple and economical foundation, which has the following advantages over other reinforcing systems:
- Require less concrete in slab
- Ease of installation.
- Speeds slab construction.
- Improves control over project scheduling.
- Reduced member size, cutting from cost.
- Efficient long span design.
Perhaps the greatest advantage of the CPML Contractors Limited, post-tension systems is the flexibility of design for a specific site. CPML Contractors Limited offers the following services:
- Feasibility studies.
- Preliminary design layout.
- Design and specifications assistance.
- Post-tensioning drawings and stressing calculations.
- Installation of cables.
- Stressing of cables at job site.
- Technical assistance at job site.
Designing the Post Tensioned Slab
Details for the slab will be developed by the design engineer or architect on the project. The shop drawings for Post Tensioned Slabs are then developed by the post-tensioning subcontractors design team or an outside Post Tension Slab designer hired by the builder.
The designer needs three things to prepare an accurate design:
- The soils report from the Geotechnical Engineer outlining soils conditions on the site.
- The outline or footprint.
- Load points and locations of hold down anchors from the structural engineer
These shop drawings are then submitted for review and approval by the project’s design engineer or architect prior to submittal to the building department.