Calibrating Your Concrete Maturity Sensors in 5 Easy Steps
If you’re reading this article, you’re probably getting ready to use wireless concrete maturity sensors, like SmartRock™, on your jobsite to monitor the strength of your in-situ slab.
Here’s a little refresher on the concrete maturity method.
The maturity method (ASTM C1074) is a non-destructive testing approach that allows you to estimate the early-age and compressive strength of in-place concrete in real-time. The technique is based on the principle that concrete strength is directly related to its hydration temperature history. Though this methodology has been used since the 1970s, it has only recently been gaining popularity on job sites with wireless concrete maturity sensors.
The goal of these sensors is to measure the temperature of your slab in real-time and correlate this data to the concretes’ strength. Wireless sensors are placed within the concrete formwork, secured on the rebar, before pouring. Temperature data is collected by the sensor and uploaded to any smart device within an app using a wireless connection. This information is then used to calculate the compressive strength of the in-situ concrete element based on the maturity equation that is set up in the app. This information is updated every 15 minutes, meaning you will know right away when your concrete is strong enough to move on to the next steps in the construction process. This strength data is considered to be very accurate and reliable as the sensors are subject to the same curing conditions as the in-situ concrete slab. Using this method also avoids inaccuracies associated with testing labs when relying on break tests to obtain compressive strength measurements.
If you just read that summary and have no idea what we’re talking about, or just want to revisit more information on concrete maturity sensors, look at these articles:
- Make Your Construction Jobsite Safer and Faster with Smart Sensors
- Cut Down Your Project Schedule by Testing Concrete Strength On Site
- Eliminate Third-Party Lab Expenses & Cylinder Break Tests with SmartRock™
Through all of your research on the concrete maturity method you’ve probably read a little bit about the calibration process that is required before you start using maturity sensors on site. The goal of the calibration is to associate the strength of your concrete to a maturity index that is at specific times. A maturity calibration only needs to be completed once for a specific mix and its properties. This maturity-strength relationship is developed in the lab, using cylinder break tests, and then correlated to the strength of the in-place concrete on your jobsite.
While this step may seem a little daunting, we promise you it’s not complicated. In fact, the maturity calibration can be done in 5 easy steps:
The Calibration Process for Using Concrete Maturity Sensors On Site
- Make a minimum of 17 cylinders; 2 will be used for temperature monitoring while the other specimens will be used for compressive strength breaks. All cylinders must be cured together in a moist environment according to ASTM C511.
- Select a minimum of 5 break times, for example 1, 3, 7, 14, and 28 days. For each day, obtain the compressive strength of two cylinders, break a third cylinder if the results vary more than 10% from the average. Note the time of the breaks.
- At the time on your breaks, also obtain data from the two cylinders that were used for temperature monitoring and make an average of these values. Input these values into either the Nurse-Saul equation or the Arrhenius method according to ASTM C1074 to obtain a maturity value.
- You will now have a set of at least 5 data points each with a strength associated to a maturity value. Plotting those data points allows you to obtain a curve with a logarithm equation. Here is the formula for calculating the maturity of concrete: Strength=a+ b LOG(maturity)
- Validate your calibration curve by making a couple of additional cylinders on your next pour. Compare the calculated strength obtained from your maturity calculation to the compressive strength obtained in the lab. Up to a 10% difference is acceptable.
Take a look at this video “Real-Time Concrete Strength Monitoring – Maturity Method” for a more thorough explanation on how to perform this calibration process.
Editor’s Note: This post was originally published in November 2017 and has been updated for accuracy and comprehensiveness