As our communities in-fill and become more dense, more multi-level structures will be introduced. Post-tensioned slabs are becoming a larger part of the design and construction mix here. We’re seeing these systems become a more economical and viable option.
Is Post-tensioning right for your job?
When dealing with multilevel structures–whether residential, parking, office or retail–obviously no one type of construction system fits all. Site access, local resources, site constraints and the building code are important factors.
Specific to residential, there has been an increase in post-tensioned construction in mid to high-rise structures over the last few years, which require tighter construction tolerances.
Structure Magazine explains it this way: “As slabs become thinner and spans become longer in residential units, the need to counterbalance the forces and stresses in the slab is provided with post tensioning…PT cables can play a significant role in deflection issues within the flat plate construction. From the start of concrete pouring and stressing, deflections need to be evaluated for the construction loads.”
Right now, we are finishing up a multilevel post-tensioned structure, and we’ve seen the same principles applied. I asked our project superintendent on this job to share a few “boots on the ground” lessons learned:
- Before each post-tensioned cable pour, confirm the cable count and check profile elevation of the cables.
- Make an extra set of concrete cylinders. Confirmation of early concrete strengths will allow early stressing of cables. Early shoring and form removal can be use for future pours, reducing cost of shoring and form work.
- Set-up a clean plan and line of communication with your cable designer, your structural engineer and the testing company. This will allow for quick flow of information and test results that will allow removal of shoring and formwork. Also, this line of communications will be available in case of stressing deviations.
- Use a “high early” concrete mix design for post tensioned slabs – a mix design that will yield 70% + of design criteria in 3 days will allow for early stressing of cables – ie early form and shoring removal and increased productivity for your next concrete pour.
- Have extra cable stressing machines on site in case of equipment failure. This could delay cable stressing/ form and shoring removal, impacting cost.
Are you using post-tensioning? Do you have advice to share?