Slaying the post-tensioning management monster

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.

Available options

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.

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Learn how to go from reactionary to proactive!

That data from the concrete is going to hurt

What you don’t know really can still hurt you: the growing need for data in construction

As construction managers, we have certain resources we like to hold on to, like skilled workers, good-quality suppliers—and information about our project. Sometimes things get off course, or some part is delayed. We figure that’s no one’s problem but ours. We can still get the job done on schedule, at good quality. Telling others will just mean micromanagement, interference, and second guessing.

And we’re experienced and have a lot of tricks up our sleeve, so we usually get by. But sometimes we end up with a real problem, one with performance penalties and maybe even danger to others.

But I’ve often seen people—smart, successful managers, mind you—who affirmatively don’t want to know too much, just so they don’t have to decide whether or not to communicate it.

This isn’t just head in the sand thinking—it’s unsophisticated thinking that has real consequences. And it’s going to stop. Not because I say so. Because the industry is changing, and if you want to compete, you’re going to have to understand the increasing role of information in our industry.

Information is an essential tool on the job site

One thing I looked forward to when I stopped being a construction manager and started a technology company was seeing all the different ways companies in our industry operate.

Somewhat to my surprise, I found that, no matter where they were or how big their projects, there were a lot of ways they were the same. And one of those ways was their resistance to using information. A common attitude is "If no one’s specifically asking for that information, and it could potentially cause trouble, please don’t tell it to me.”

Put that way, it sounds kind of dumb. Because it is dumb. Why would you deliberately refuse to know something about your project – it could be crucial information?

Sure, the information might be unpleasant. It’s that new pain in a tooth, the unusual spot on the driveway under your car, the teenager who’s not where they said they’d be. Maybe there’s one or another of these that you do take care of promptly. But you know the attitude, and you know it never leads anywhere good.

I’ve seen this with everyone from Project Engineers to VPs, and from small local contractors to global behemoths.

Unintended Consequences

So what do you get when you excavate a pit for your head?

Mistakes that are impossible to hide

The concrete doesn’t care whether you didn’t learn it got too hot. But the structural compromises that happen because it got too hot are there and will have real world consequences. Wouldn’t rather have learned about that at the time when you could have done something about it?

Poor productivity

We lag every other industry in productivity. In my corner of the industry, we think about it a lot. Take a look at this graph from a McKinsey article on productivity.

 
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You can’t improve processes you’re not tracking. You can’t prevent mistakes you pretend you’re not making. You can’t efficiently time when you strip forms and lay down flooring if you don’t know when the concrete is strong enough, and have to wait around an extra day to be sure. Wouldn’t you rather do your job more quickly, realizing efficiencies in labor and raw materials?

Quality and safety compromises

These are often separated, but I think these two things go together. Poor quality work tends to be unsafe work. More data about what works and what doesn’t helps both.

I recently heard of a bunch of projects that ended up with concrete that included bad aggregate from a ready-mix supplier with no visibility into its supply chain. Don’t you want to know exactly what you’re pouring, how strong it’s going to be, and that your people aren’t at risk?

If you don’t do it, someone else will

I’m not trying to scare you...well, actually, I am. Someone’s going to be doing this. They’ll start getting information about every part of their workflow, about what can be made more efficient, safer, and easier. They’ll get done more quickly with their jobs, at higher quality. Their employees will be happier, and excited about developing new skills.

So turn that them into you.

Set the stage at the beginning of project. Construction managers: tell the owner and design team what you are doing and what data to expect, and that sometimes it will tell an unhappy story. Owners and design teams, let construction managers manage. Learn from each project. You’ll find that you detect problems earlier, solve them more efficiently, and have a better handle on every part of your operation.

Make clear at the kick-off that this isn’t just rhetoric. It’s the way things are getting done.


At Concrete Sensors, we’re from construction, and we’re seeing how much better we get at our jobs when we know what’s going on. So get the sand out of your ears, and start hearing what your project is telling you.


Cylinders are crushing productivity and quality

I’m sure you’ve been there—pacing around, wondering if the concrete break test will tell you that you can finally strip forms or tension slabs. I certainly have. You worry that the bad weather a day ago affected the pour, or that the test will tell you still have another day to wait. But you have a deadline to meet!

Those cylinders are the harsh masters of our schedule. The ACI code and engineers that have to meet its requirements use them to be sure the concrete off the truck meets the stated specs. Construction managers use them to confirm that the concrete is strong enough to move on with the next step, and to monitor quality.

But Moses didn’t come down from Mount Sinai with two concrete cylinders. What if that cylinder test has some problems?

Conditions have a huge effect

Concrete in a cylinder will cure differently than concrete does in the field. How differently? Well in cold weather, you could be more than 100 percent off at any point in the first week!

We see this frequently in our own testing. For example, the time it takes for the same mix-design to get to 500 psi could be eight hours at a warm temperature and 24 hours at a cool temperature. This inaccuracy can really hit your schedule and quality.

Weather, heat from the hydration process, a more-exposed part of your pour, day/night cycles, freezing and thawing...your pour goes through all sorts of things those cylinders in the lab don’t.

Don’t reach the wrong conclusion

Part of the problem is that we’re using cylinder break tests to answer questions they’re not designed for. They’re intended to confirm that concrete delivered matches concrete specified.

But, lacking a better measure, we’ve increasingly relied on them to tell us the real-time strength of the concrete, or whether the pour is curing appropriately. Cylinders are just too unreliable to answer those kinds of questions. Relying on the cylinders may actually give you a false positive that everything in the field is ok. There are other ways to get these indicators.

The frustration of the lagging indicator

You have a limited number of test cylinders, and the labs test them at prearranged times. If you’re trying to be aggressive in your schedule, you’ll inevitably be frustrated. And sometimes let down, if the test shows that the concrete might have been ready 12 hours or even a day ago.

And how good is the quality control, anyway?

Concrete laboratories have been devalued over many decades forcing them to hire technicians at low wages. If concrete laboratories were ever the place top engineers competed to work, that day is long gone.

Frustrated by the low quality standards of the available labs, New York City has opened its own lab to ensure reliability and quality. I know a quality-control engineer at a medium-sized ready-mix company who estimates that he gets at least 100 calls and meetings a year resulting from poor lab QC.

As a construction manager you can at least ensure your end of things is handled properly. Prepare and consolidate properly, carefully handle the samples in the first 24 hours, and cover the samples. Find out about the curing conditions in the lab, their capping procedure, and how they maintain their crushing machines. You can’t just assume it’s being done properly.

So what should you do?

Use cylinders only to confirm that the mix-design that was delivered meets the specified strength. Don’t rely on them to give you detailed information about curing. Attempts have been made to replicate curing conditions with climate-controlled boxes but with varying results. Focus the use of cylinders on the specification strength to keep your quality control high.

Contractors vary in their trust of cylinders. One approach I dub the “head in sand” approach is to test the concrete mix once and then assume that what they get later is the same. Putting aside the obvious code issues, this is a comically dangerous way to proceed.

On the other end of the scale, some managers catalogue, test, and retest, using the Bill Belichick strategy of being prepared for every possible downside. I’ve been impressed by how much they know about various mix designs. But I think they’re working much harder than they need to—which means they are costing their projects more than they should. Most are in-between, trusting the lab because they have too many other things to worry about. Secretly, most are not that happy about it.


At Concrete Sensors, we’re bringing new information to the process. What we provide complements what the labs do, and we have labs as clients. I’ve known labs to use our data to detect errors in their own processes. That’s a lab you want to use!