Timing the tensioning of a post-tensioned slab is a challenging task that has quality, cost and schedule consequences. Basic logic tells you that you should wait until the concrete is strong enough before you tension the slab. Did you know you also can wait too long to tension? And perhaps most obvious, the sooner you tension a slab the faster you can move.
What if you could tension sooner? Or, what if you knew ahead of time that you could tension tomorrow?
The bottom line is the timing on tensioning is critical for both the quality and efficiency of the work – it is critical path. Frequently you can’t start on the next pour until the previous slab has been tensioned. This makes workflow challenging because there are obvious economic benefits to keep the same crew on a job site rather than continually pulling them.
The most traditionally used method to know when the concrete is ready to be tensioned is using cylinders or cubes. It is widely accepted that this method is good for knowing that the delivered concrete meets the engineer’s specifications but is not a good indicator of the strength of the field concrete. In fact, if you following ASTM guidelines closely, the concrete your lab is testing should be cured at optimal conditions (72F and 100% RH or in a water bath), which is certainly not what the field concrete is experiencing.
Beyond cylinders, there are several available options to know when the concrete is appropriate to tension, both destructive and non-destructive to the concrete. Some of the destructive methods include the pull out test, field cores and the CIPPOC method. Of the three that I listed, I like the pull out test the best because it is the least destructive.
All tests have their drawbacks and it is important to be aware of these drawbacks. The most obvious drawback of the destructive tests is that you need to repair your concrete after the test. This can be costly and aesthetically concerning if the concrete is exposed. Also, the destructive tests require training and are prone to human error, such as proper consolidation.
The non-destructive test that is most widely used is the Maturity Method which is described in ASTM C1074, Standard Practice for Estimating Concrete Strength by Maturity Method. This involves testing the concrete mix design ahead of time and correlating curing temperature with a curing rate. The benefit is that if done correctly, you can obtain very accurate estimations of the concrete strength.
The drawback to the Maturity Method is that if you want to be accurate with any mix design at any temperature, you need to properly test the concrete.
At Concrete Sensors, we have a lab that tests for the Maturity Method and our experience has shown that you can be off by as much as 30% by not correctly testing.
When you are relying on a method to know when to tension a slab, having accurate information is not only a safety issue but also a matter of being able to move sooner or not. If you are a contractor that is looking to change from being reactionary to proactive, the Maturity Method allows you to focus on data that will change the way you approach post-tensioned concrete projects.