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Chicago General Contractor Discusses How to Successfully Cut a PT Slab Opening

A common misconception leads some to believe that the creation of an opening in an existing post-tension (PT) slab is either extremely complex or nearly impossible. In fact, the penetrations of PT slabs are possible when observing proper methods.

PT Slab Opening for Springhill Suites

SpringHill Suites, in Chicago’s Chinatown neighborhood, is an explicitly unique project that required PT slab openings. The 4-story hotel is being constructed over a 2-story existing garage with the second story garage being a PT slab. Due to the uniqueness of the project, Summit had to construct one elevator shaft and two stairwell shafts without affecting the structural integrity.

The process involved quite a bit of research, value engineering, and expert feedback. With the help of our extremely skilled concrete subcontractors, the slab opening was a success. Below are the steps we took.

Steps Taken to Complete the PT Slab Openings

1. The surveyor marked the location of the stair and elevator shaft’s openings.

2. Concrete Scanning Company, with the help of ground-penetrating radiation (GPR), a geophysical method that uses electromagnetic radiations to image the subsurface, located/approximated the location of PT cables.

3. Catch decks were built at the location of the opening to ensure safety prior to the cutting of the slabs. We demolished a patch of the 2nd-floor concrete to check the existence of the PT cable and fortunately, the scannings were accurate.

4. The slabs were unbounded post-tension (PT) slab which means the post-tension systems are fixed to the structure at the end anchorage but are otherwise free to move independently of concrete being greased and encased in plastic sheathing.

5. We then proceeded towards making the cut, leaving some extra length of PT cable from the edge of the opening. This extra length will be helpful to grab onto later in the procedure and restress the cable. This releases the tension in the existing cable causing it to deflect less than 1 inch (imagine holding a string at both ends and cutting the center).

6. Next, we placed the encapsulated anchor and wedge through the PT cable, drilled holes into the outside edge of the slab, placed reinforcement in the perpendicular direction of the cables, and poured the concrete, leaving us with a cleaned finished edge.

7. Stressing equipment is introduced in the anchor and the PT cable is restressed to the desired tension.

8. The above steps are performed on both ends of the opening where the main PT cables run.

9. The inside of the opening is now completely free of any stress and can be cut like any other slab opening.

10. Normally the grease inside the sheaths slows the release of energy when strands are cut and the wedges do not disengage from the wedge. Nevertheless, the former cannot be guaranteed and necessary precautionary measure needs to be considered so as not to cause any snapping of the PT cables at its ends.

Thanks to Jose and Miguel of Tor Construction, both were the superheroes who executed this job with the highest precision, safety, and most cost-effectively.

Do you have an upcoming construction project that might involve cutting openings in PT Slabs? Let our team of construction experts help!