In this blog, we discuss how concrete maturity and traditional strength testing work together to deliver valuable information sooner – optimizing construction schedules.
The construction materials testing industry is excited by the possibilities of estimating concrete strength via the non-destructive maturity method. This method provides a number of enhancements to the traditional laboratory compression test method. But the maturity method is not an alternative or replacement. Indeed, maturity testing relies completely on the traditional compression test method.
What is Concrete Maturity?
Concrete curing is an exothermic process. In plain speak, that means concrete gets warmer as it hardens, before eventually reaching desired strength and cooling to ambient conditions.
Maturity testing exploits this known warming phenomenon to estimate the early strength of poured concrete based on its temperature. The temperature is measured with temperature reading devices embedded in the concrete when it is placed at site. Done properly, the temperature can then be cross-referenced against the laboratory developed strength-maturity relationship for the concrete mix in question.
So, before you can use the temperature of the in-place concrete, you need to know how the temperature correlates with strength – and that is done under laboratory conditions.
To do this:
- Prepare a batch of concrete test cylinders according to traditional strength testing protocol and of the same concrete mix design that will be used in the field
- Place temperature sensors in at least two of the concrete cylinders
- Compute the Maturity Index by recording temperatures at prescribed time intervals
- At prescribed intervals of elapsed time, break the remaining cylinders according to the traditional compression strength test regimen
- Record compression strength, corresponding temperatures and elapsed time
- Calculate and generate the maturity curve by plotting the average compressive strength versus the average Maturity Index to reveal the strength-maturity relationship.
You can then embed temperature sensors in newly poured concrete, and, armed with the data that reveals the relationship between time, temperature and strength, begin to make informed decisions about your project.
You will need to know the temperature, the time intervals since the concrete was poured, and the strength-maturity relationship. This method can help you make better decisions, sooner than waiting for strength reports exclusively from the lab.
It’s important to remember that this method estimates strength. So, before performing critical operations like form removal or post tensioning, perform additional traditional tests to ensure sufficient strength has developed in the concrete
Should You Use the Maturity Method?
The maturity method can be helpful for a couple of reasons.
#1. Early Strength
Maturity is useful for estimating the early strength of concrete, and importantly, making that estimate at an earlier point in a project timeline than would be done with traditional laboratory methods. This can allow a project to proceed to the next stage sooner.
Concrete in-place is exposed to weather and other ambient conditions that impact early strength. The actual strength of concrete in the field can vary due to different curing conditions. But the laboratory developed strength-maturity relationship can account for these variances and eliminate the need for an extended curing period.
#3. Critical Positions
If you want to know the strength of a critical position like a column, you can’t easily remove a section to conduct a traditional strength test. But maturity sensors can be placed in columns. Maturity sensors in these critical positions allow field technicians ready access to this information.
This enables knowledge of a specific component at any given time. That’s difficult to achieve with traditional methods.
Why You Need More Than Maturity
There’s a flip side to these cases. First, maturity doesn’t work without the initial lab-developed strength-maturity relationship.
Then, while it is true that maturity estimates reflect site conditions impacting early strength, these conditions are less relevant when testing for development of full strength – typically at 28 days. In the early stages of curing, concrete is more vulnerable to site conditions, but less so at 28-days. Maturity is good for estimating (not verifying) early strength, but it cannot determine ultimate strength.
Conversely, some site conditions, such as high early-stage temperatures, may reduce the long-term strength of the concrete, resulting in a lower strength than predicted by the maturity method. In a case like this, additional strength-maturity relationships must be developed in the lab to reflect these unusual conditions.
Concrete poured on-site has to be the same design that the lab used to create the curve. If anything is changed, that curve won’t be valid.
For example, if a batch of the original material design arrives at the site but the field technicians decide the mix isn’t workable, they might add water to improve workability. Or, perhaps they introduce admixtures or accelerants to adjust for air entrainment, or to speed or slow curing. These additions have changed the original mix design, and now the curve calculation is compromised. Maturity can accommodate some site condition variances, but not all.
There are a lot of variances in concrete work – not just final site conditions. And when it comes to critical structures, inaccurate concrete strength can have dire consequences on the integrity of a building. While the maturity method offers a real-time estimate of the strength, that’s all it is: an estimate. Project owners need to verify that the concrete is the designed strength.
At the end of the day, the only way to know the true strength of concrete is by breaking it.
Maturity testing does help speed projects along, but it must be used within its limitations.
Simply put, the maturity method is not valid without traditional strength testing. But it’s clear that the maturity method can add value—when used with traditional testing.
Maturity & Traditional Strength Testing Work Together
Here’s how the maturity method enhances the overall CMT process.
#1. Creating the Strength Curve
The complete construction materials testing process starts and ends with breaking cylinders. Temperature sensors, time and compression testing machines work together to develop the strength-temperature/time relationship – key to deriving the value that maturity testing offers.
This step can be further enhanced with an integrated construction materials testing platform that removes human error from the testing process, provides unalterable test results in context with the batch and mix design, and automates analysis. By automatically capturing test time and strength, without human intervention, the strength-temperature/time curve will be more accurate, giving your maturity testing better results.
#2. Determining Early Strength
Maturity sensors are beneficial as intermittent steps in the construction process. They help projects move forward by providing a quick analysis of what’s going on in the field.
For example, in a multi-floor building construction project, field workers want to know as soon as possible when they can move to the next floor. Maturity sensors give them an instantaneous answer, whereas they would otherwise need to wait for lab results to move to the next step.
But with maturity sensors integrated with mobile applications and access to cloud-based data, the information needed to determine if the concrete is ready is quickly available. Maturity sensors tied to accurate test data from the lab automates the process, giving you information faster and allowing projects to move forward.
#3. Validating Final Results
Ultimately, you need traditional strength testing methods to determine the final strength. Anything else is just an estimate. The only way to truly know is a traditional compressive test.
Use maturity sensors in the field for quick analysis and supplement that information with ongoing (7- and 14-day intermediate tests) and final, 28-day lab test results to verify that everything is okay. Everyone (test labs, project owners, government organizations or regulators, insurers, etc.) needs to be able to trust that construction materials meet expected strengths and tolerances. Your project will be much more defendable if you’ve been checking maturity along the way and validating with break tests at key points.
Luckily, this validation doesn’t have to delay the project. With an integrated CMT platform, as soon as the tests are complete you can be notified if your maturity sensors were on target and your concrete meets the expected strength.
Both testing processes work together to provide more information to construction project managers. The value of all this information together creates better end results and faster project completions. But this is only if (1) the break test data from the lab is accurate and (2) the mix design doesn’t change in the field.
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