ECS Lessons Learned
OBSERVATIONS AND LESSONS FROM THE SCHOOL OF EXPERIENCE
After more than 30 years in the industry, we’ve amassed some great takeaways pertaining to the work we do. We felt this key information was important to share, so in 1996, Lessons Learned was created as a mailed piece sent out to our peers. Now, we are pleased to offer Lessons Learned electronically once a month, accessible on this page or delivered directly to your inbox when you sign up.
Construction Engineering Services
Post-tensioned concrete in construction is relatively common. It is used in slabs on grade, elevated beams and floor slabs, and mat foundations. Post-tensioning is an alternative method for reinforcing concrete structures.
Concrete is strong in compression, but very weak in tension. Normally, concrete is cast with mild steel (reinforcing bars) to strengthen the tension side of a concrete structural member. In post-tensioned concrete, steel cables (or tendons) encased in a lubricated protective polymer sheath are cast into a structural concrete member. Once the concrete has reached a specified strength, the tendons are pulled resulting in tension and elongation. By locking off the tendons after they are stretched, the tension side of a member is placed in compression.
Post-tensioning can reduce the cross-sectional area of a member, reducing costs, and can also increase the unsupported span, reduce, or prevent “sagging” of the member, and reduce or prevent cracking. The contractor stressing the cables must use calibrated equipment.
If post-tensioned concrete is so great, why isn’t it used all the time? There are a few drawbacks to post-tensioned concrete:
- It is more difficult to design and construct. The tendons need to be very accurately placed, both horizontally and vertically, and protected.
- A nicked or broken tendon sheath can let concrete in, preventing the full tendon from being applied.
- Post-tensioned steel is also more susceptible to damage from salt ions.
- The post-tensioning process is more dangerous. In most building construction, cable or tendons are stretched using 33,000 pounds of force. Sometimes the anchorage points fail, thereby de-tensioning the member and possibly injuring workers.
Here are some components that can lead to successful completion of a post-tensioned project:
- The members of project team should be familiar with the advantages and disadvantages of this construction technique, as well as some unique considerations.
- A pre-construction meeting with all parties involved with post-tensioning activities should be held. This meeting defines and clarifies the responsibilities of the various team members. When everyone understands their responsibilities, the project tends to run smoother.
- 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 tendons are critical to the structure. Utilities should be planned around the tendons, not tendons planned around the utilities. Extreme care for post-pour coring of penetrations needs to be taken to avoid hitting tendons. Damaged and/or de-tensioned tendons can create significant problems and very expensive retrofits.
- During concreting operations, the contractor must take extra care to make sure that tendons remain in place horizontally and vertically, and that proper consolidation and coverage of concrete around the tendons is attained.
- Building codes typically require stressing operations be observed by an independent testing agency. Local jurisdictions may have requirements with respect to the timing of stripping of formwork.
As always, early planning and coordination among the members of the design and construction team is a good investment and that pay real dividends.
We hope this Lessons Learned will be helpful to you in planning for and building your next project. Please contact your nearest ECS office for more info and support.
ECS Group of Companies
The ECS Group of Companies (ECS) is an employee-owned engineering consulting firm with more than 2,000 employees providing geotechnical, construction materials, environmental and facilities consulting services. ECS has grown to over 70 locations and five subsidiaries spread across the Mid-Atlantic, Midwest, Southeast and Southwest. The firm is ranked 68 in Engineering News-Record’s Top 500 Design Firms (April 2021) and 49 in Zweig Group’s 2021 Hot Firms (June 2021).