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.