Construction Engineering Services
The use of post-tensioned concrete in building and slab-on-grade construction is popular in many areas. Project team members should be familiar with the advantages and disadvantages of this construction technique, as well as some unique considerations. Post-tensioning is an alternative method for reinforcing concrete structures.
Concrete is strong in compression but weak in tension. Normally, concrete is cast with mild steel (reinforcing bars) to strengthen the side of the member that goes into tension. In post-tensioned concrete, steel cables (or tendons) are cast into a structural concrete member and stretched after the concrete has gained a specified strength. By pulling the tendons and “locking off” the cables after they are stretched, the member is placed in compression. Once in service, as the member is loaded, the “pre-compression” of the concrete is relieved instead of the concrete going into tension and cracking. Post-tensioning can reduce the cross sectional area of the member, reducing costs, and can also be used to increase the span, reduce or prevent “sagging” and reduce or prevent cracking. Post-tensioned concrete is sometimes used in concrete slabs-on-grade to reduce cracking, and also used in foundation mats to reduce thickness.
If post-tensioned concrete is such a great product, why don’t we use it all the time? Good question. There are a few drawbacks to post-tensioned concrete. First, it is harder to design and more difficult to construct. The cables need to be very accurately placed and protected. Also, a nicked or broken cable sheath can let concrete in, preventing the full cable from being stressed. Post-tensioned steel is also more susceptible to damage from salt ions. Cutting penetrations through members after concrete placement risk hitting and de-tensioning one or more cables. High redesign and construction costs are often incurred to remediate the loss of even one cable, along with construction delays. Finally, the post-tensioning process is more dangerous. In most building construction, cable or tendons are stretched using 33,000 pounds of force.
There are several key suggestions for the successful completion of a post-tensioned project:
- A pre-construction meeting with all parties involved with post-tensioning activities should be held. This meeting will help resolve responsibility issues and ensure that everyone is playing off the same page. If everyone understands their responsibilities and duties, the project will run smoother.
- A thorough review of plans by all trades prior to construction helps avoid conflict. The general contractor takes the lead here. Utility and other penetrations are often planned in the post-tensioned members, but the locations of the cables are critical to the structure. Utilities should be planned around the cables, not cables planned around the utilities.
- During concreting operations, the contractor must take extra care to assure that tendons remain in place, and ensure proper consolidation and coverage of concrete around the cables.
- Building codes typically require stressing operations be observed by an independent testing agency. The contractor stressing the cables must use calibrated equipment, and the structural engineer should be informed of the results. Local jurisdictions may have additional requirements with respect to the stripping of form work from post-tensioned concrete. It is important to know the local requirements to achieve trouble-free concrete placement.
As always, early planning and coordination among the members of the construction team is a good investment and it pays real dividends.
We hope this “Lessons Learned” will be helpful to you in planning for and building your next project.
ECS Corporate Services, LLC