Top 10 Biggest Mistakes Made in Post-Tensioned Concrete Design

After decades of building and inspecting in the Post-Tensioning (PT) concrete construction industry, we’ve seen the best and the worst of PT construction. And we keep a (long and growing) list of the mistakes we’ve encountered along the way. Knowing the ways and the extent to which problems can occur helps us implement the most cost-effective and long-lasting repairs possible. And most importantly, it helps us anticipate and avoid mistakes in our own work to create stronger, longer-lasting structures.

While we’ve encountered many more, here’s a selection of the 10 most common mistakes we’ve encountered in PT construction:

 

10. Not considering long-term cracked deflections. 

PT beams and slabs have an inherent advantage over non-PT members when it comes to deflection. But it is still critical to check and correct for long-term deflection at the design stage. It is especially critical where drainage can be an issue in cases such as roofs, parking garages, and balconies, and with flat plates where long term deflections may affect window installation. 3D software will account for instantaneous deflections but long-term deflections may not be readily evident.  If this is the case, take the time to create a load combination for this output.

 

9. Using rebar for shrinkage and temperature reinforcement in parking garage slabs. 

Rebar reinforcement in this case is technically allowed by code but in our experience, a properly designed post-tensioned concrete slab will minimize cracking due to shrinkage.  We recommend using PT for temperature and shrinkage reinforcement.

 

8. Neglecting to design general anchorage zones or delegating this responsibility.  

PT suppliers are responsible for design of local anchorage zones, and many engineers incorrectly assume they will also take care of the general zone. And while code allows a licensed design professional (LDP) to delegate the design of the general anchorage zone to the contractor, we don’t recommend taking that route. The design of general anchorage zones is critical to the integrity of any post-tensioned concrete structure and we recommend that the LDP complete this task personally.

 

7. Sweeping tendons at low points without proper detailing.

If it is necessary to sweep tendons laterally – and often, it will be as a result of miss-aligned columns – make sure you are detailing these areas properly, whether at low points or at any other point in the slab. Tendons with lateral curvature at low points have a tendency to blowout the bottom of the slab – either immediately upon stressing or many years later. Refer to the PTI Manual, Sixth Edition, 2006, p. 125 for more information on proper detailing for this condition.

 

6. Ignoring the effect of slab folds on tendon profile.  

At slab folds, it is important to check tendon profile at critical points along the span to ensure the cable is positioned properly. It is not uncommon to see a theoretical tendon path falling outside of the slab at the fold – clearly a situation to be avoided. But it is just as important to examine locations where reverse curvature with minimum cover could lead to pop-outs during stressing.

 

5. Overbalancing dead load in your designs.  

There is no specific code requirement for how much load to balance with PT – but checking the balanced load is good practice. Balanced loads exceeding 100% DL are often acceptable and even desirable, so long as the design is serviceable and code-compliant. But we have seen designers balance more than 400% DL in some spans, which almost always leads to sudden failure of the slab.

 

4. Reliance on 3D finite element software for design and drawings. 

Finite element design packages can be incredibly useful in the design of PT structures. We use them frequently ourselves. But in the hands of an inexperienced designer, such programs can lead to lazy design habits, poor quality drawings, and bad designs. Always have an experienced PT designer review the computer’s results and never use drawings produced by a computer program without thorough review.

 

3. Poor specifications related to finishing and inspection of tendon ends.

The finishing process includes cutting strand tails, installing the protective end cap, cleaning the stressing pocket, and patching the pocket with high quality mortar. This is one of the most important steps in ensuring the durability of the structure. When tendon ends are improperly finished, structural durability is compromised. It is critical that the specification include inspection of tendon finishing in the inspector’s responsibilities.

 

2. Failing to specify encapsulated systems. 

Encapsulated PT systems should be specified on all PT buildings (with possible exception of slab on ground construction).  This is now mandated by ACI, and we believe it to be good practice.

 

1. Ignoring restraint to shortening effects. 

This is the source of most problems with new designs – both aesthetic and structural. Stiff vertical elements in the structure such as columns and walls have a tendency to restrain shortening of the PT member. Shortening is caused by elastic shortening, drying shrinkage, and creep.  It is critical to evaluate the design in terms of potential for restraint and include measures to mitigate such effects.

 

In our experience, these mistakes are often the result of an even bigger mistake – a failure to plan ahead! Give yourself and your team enough time to get the job done right. And when in doubt – consult a seasoned expert!