Two measurement methods

Measuring concrete strength

The conventional method for monitoring concrete strength involves pressing cubes that cure outside the construction itself. These cubes have a different temperature than the construction concrete so their strength develops at a different rate, making this a poor strength indicator.

ConSensor does it differently. In the lab, ConSensor starts by making calibrations of strength in relation to maturity and conductivity. On site, the contractor uses these calibrations to accurately determine the concrete strength. 

The main difference between ConSensor and other systems is that ConSensor uses 2 measurement methods:

  1. Weighted maturity (NEN 5970).
  2. Electrical conductivity, which diminishes proportionally with the hardening of the concrete.

Why does ConSensor offer two methods? First, as a failsafe, and second, to monitor the long-term integrity of concrete.

When you turn on the measuring device sometime after the pour, the system measures too little weighted maturity, giving you an estimate of the concrete’s strength that is too low.

In contrast, when using the conductivity method, turning on the device too late isn’t an issue—it gives you a correct strength value based on the actual conductivity at that moment.

Used together, the two measurement methods offer more certainty about the strength of the concrete and the setting process.

And in the long term, measuring the concrete’s conductivity serves as an indication of the concrete’s condition. An increase in conductivity after many years indicates an ingress of water and possibly ions, signaling the need to investigate the concrete’s integrity. 

Let’s compare the methods for creating a calibration curve.

 
 
 
 

Weighted maturity

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The hydration of concrete is the chemical reaction of cement and water. Hydration accelerates as the concrete’s temperature rises, and vice versa. So the temperature multiplied by the time—shown as the red area in the top graph—is a measure of the amount of hydration. This red area represents the concrete’s maturity. 

The red datapoints in the middle graph represent the concrete’s maturity at the time of compression tests on days 2, 4, 8, 16 and 28.  

The bottom graph combines the maturity and compression test values. The datapoints plot the maturity value at the time of each compression test on the X-axis with the compression test value for strength on the Y-axis. 

ConSensor automatically calculates a regression line through these points and a lower calibration line as a safeguard against variation in the compression tests. 

Electrical conductivity

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When you mix concrete, the cement dissolves in the water, creating an electrically conductive solution. As the cement and water react, the concrete hydrates and its conductivity decreases, as shown in the top graph. The datapoints mark the conductivity at the time of the compression tests in the lower graph. 

Conductivity also indicates the setting time where the line transitions from concave to convex. 

The bottom graph combines the (inversed) conductivity on the X-axis and the compression test values (on the Y-axis). ConSensor automatically calculates the regression line through these datapoints and a lower calibration line as a safeguard against variation in the compression tests. 

One of ConSensor’s great features is that you can leave the sensor in the concrete to measure its conductivity over its lifespan. If the conductivity decreases, it means the hydration and strengthening of the concrete continue. On the other hand, if it starts increasing, it’s a signal that the amount of water and ions in the concrete has increased—seldom good news. It’s much better to learn this at an early stage before any damage has occurred. If diagnosed at a later stage, you run the risk of more disruption and expense. With ConSensor you gain the time to investigate the issue further and deal with it. 

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