Maturity and
strength monitoring

Maturity and Strength Monitoring With Sensor

Why Concrete Maturity?

Using the maturity method enables contractors, precast productions, ready mix companies and concrete labs to estimate the early age strength of fresh concrete. The method has been proven to be a reliable method widely used in projects around the world.

In recent years, the use of sensors has experienced a significant growth due to the potential cost and time savings that real-time data provide. Many leading companies have started to replace or complement their usual methods, such as break tests or drilled cores, with sensors that are able to calculate concrete maturity and strength almost instantly.

What is Concrete Maturity and the Maturity Method?

Concrete Maturity

Concrete Maturity represents the combination of time and temperature. It is calculated based on the temperature history of the structure by applying one of the maturity functions. In its essence, maturity is:

Maturity =  time * temperature

The main concept about concrete maturity is that a specific value of maturity can be obtained with many different combinations of time and temperature. For example, the graphs below represent the temperature history of three different pieces of concrete. All of these have reached the same maturity of 100 °C-hours, but it took a different amount of hours to do so. This is because the temperature has been different throughout that period.

Concrete Maturity Graph 1
Concrete Maturity Graph 2
Concrete Maturity Graph 3

If you look at the first graph (Graph 1), you will see that the temperature is 20 °C and that it has been curing for 5 hours. This gives a maturity of 100 °C-hours.

If we compare this with Graph 2, you will see that the temperature is higher. Since the concrete temperature has been at 40 °C, it only took 2,5 hours for it to reach the same maturity of 100 °C-hours.

Looking at Graph 3, the temperature is the lowest of the examples at 10 °C, and in this case it will take 10 hours to reach the maturity of 100 °C-hours.

Concrete maturity alone does not indicate how strong the concrete is. However, by using the maturity method, you can determine the relationship between maturity and strength development.

Maturity Method

The maturity method is an easy way to estimate the early-age strength development of a concrete mix. The main assumption of the maturity method is that if two samples of the same concrete mix have the same maturity, then they will also have the same strength – even if they were cured under different temperature conditions.

To use the maturity method you will need to perform a maturity calibration, which includes doing tests in a laboratory to find the relationship between the concrete maturity and the concrete’s strength. Once you know this relationship, you can estimate the strength of the in-place concrete by placing temperature sensors in the structure. You can find the maturity by looking at the temperature history measured by the temperature sensors, and then determine the in-place strength using the maturity calibration.

Benefits of maturity and strength monitoring

How can you benefit from using Concrete Maturity?

Many of our clients mention that one key benefit from using the Maturix sensors is the real-time insight into the temperature, maturity and strength development of their concrete. Having this information allows them to do things that they could not before. Such as being able to optimize the use of resources, get insights into their concrete curing, monitor critical spots in the structure, reduce the risk of thermal cracks and create documentation automatically. These benefits have been experienced both by clients doing on-site castings on projects and in precast factories. Below, we have explained more in detail each of the benefits mentioned above:

Get the best out of low carbon concrete

In recent years, we have seen a huge growth in the use of green concrete, which are concrete mixes with a lower co2 footprint. The general experience is that these concrete types have a much slower strength development as well as a lower temperature increase during the hydration process. This means that if a low carbon concrete cement is used, this will have a significantly lower strength after 1 day of curing compared to a traditional basic cement. This can be extremely critical both on job sites and precast factories where keeping daily schedules and production cycles is key to stay competitive in the market. Some research (Lasse Frølich, Portland Open 2021) shows that the new concrete types have a very limited additional strength development after 28 days. This contrasts with the previous experience with normal concrete types where the strength can gradually develop well beyond the 28 day mark. 

These problems can obviously be compensated with use of accelerators and/or adding heating measures to the concrete before casting or in the curing stage. However, the experience obtained working with normal concrete mixes can be difficult to directly apply to these new concrete types. Therefore, it can be beneficial to use sensors to keep track of the curing progress and learn how to get the most out of these new concrete types.

Get vital data about the structures’ critical spots

Placing concrete sensors in the concrete structure also enables you to view how the curing is progressing in one or more spots. There are certain places in the structure that cure much faster or slower than the rest. Conventional methods do not provide you with information about these critical spots so you do not know how those are doing. In contrast, the Maturix sensors can be placed anywhere in the structure to keep track of cross sections, corners with low temperature development, temperature outside the castings, strength around hooks, prestressed wires etc. Then, the data about each of the spots can be seen individually, you can see an example of this below:

Data about the structures critical spots

Reduce uncertainty on concrete break tests

Low breaks or inconsistent compressive test results are a common problem in the construction industry. There are many standard procedures describing proper handling and preparation of test samples, however, many times the procedure is not done according to the specifications resulting in inconsistent results.

Therefore, when receiving low break results, it becomes difficult to identify what caused the low result. These results could indicate that the concrete mix was not designed well or that the supplied material was not up to the specifications. But it could also have happened because the samples were not prepared or cured properly, they were damaged during transport or the testing machine was not calibrated properly. 

All of these potential causes will create a lot of uncertainty in the project as it would become unclear how to proceed next, wasting a lot of time waiting and investigating the different possible causes. In contrast, if sensors are placed in the structure, it becomes possible to track the progress continuously and instantly detect if something is not going well. Then, if a low break happens, you will have a much richer information of what might be the cause. 

Optimized use of cooling and heating measures

Keeping track of the curing process with the Maturix sensors enables an optimized:

  1. Usage of covers; as the use can be optimized during the curing stage to fit with the current temperatura and the ambient climate.
  2. Heating usage; many precast facilities add additional heating to the concrete either by water pipes underneath the concrete beds or with infrared heating placed above them. By knowing the curing stage your concrete is on, you will be able to turn off the heating when this is not needed.
  3. Usage of cooling pipes: as explained above, you could see in real time the internal temperatures of your concrete at different spots. This will allow you to make the use of cooling measures much more efficient. 

Reduce CO2 footprint with valuable knowledge

The cement content in the concrete is one of the big determinants of the Co2 footprint. Even a small reduction in the cement content will enable a meaningful Co2 footprint reduction. Using the concrete sensors’ data in conjunction with an ambient monitoring enables you to: 

  1. Improve the mix design of current used concrete 
  2. Apply the most suitable concrete mix for the job. One that still fulfils requirements while having the lowest possible Co2 footprint.

Reduced risk of thermal cracks

Thermal cracking can be one of the most challenging effects to manage in concrete. The risk of cracks and defects increases when the concrete is cured under extreme environments, which might be the case in very hot or cold weather, when producing special concrete elements, or in mass concrete applications. 

In many of these applications, concrete simulations are made beforehand in order to design the structure correctly and choose the appropriate concrete mix. However, these simulations might be inaccurate, as several factors such as the ever-changing weather conditions can be hard to predict in advance.

precise data about in-field concrete

Constant monitoring on-site with the maturity method can provide more precise data about your in-field concrete. Some customers of Maturix even monitor their cooling systems to stay in full control allowing them to always deliver concrete works of excellent quality.

Automatic documentation/compliance

Large concrete jobs like infrastructure, mass concreting, high rises, damps or other utility structures do often have specific requirements for thermal monitoring. Embedding sensors enables automatic documentation, and a lot of labour hours can be saved compared to using a traditional data logger. In Maturix we call this Documentation as you build.

Why not use only break tests?

1. Limited visibility => potential risk of delays

Compression tests are a widely used method, but these are often the reason for unwanted delays and increased costs. Moreover, this type of test only shows a single or few strength data points.

Typical result with a few break tests
Continuous information Full transparency with sensors

As shown above, the break test will only give you a data point occasionally, when a specimen is tested. The use of concrete sensors gives you continuous information during the concrete’s curing process.

2. Cubes/Cylinders risk to show wrong results due to curing conditions

A challenge that can occur both on job sites and in precast facilities is incoherence between the in-place strength and the one from the specimen cured on-site, next to the concrete structure. The reason for this incoherence normally is due to the different curing temperatures.

A small concrete cube/cylinder has smaller mass than the structure so the heat development will be lower in it. Moreover, the often much larger concrete structure is exposed to a significantly higher internal heat development caused by the hydration process. The higher internal heat will in turn make the curing process faster. This mean that when comparing a specimen to the actual structure this has: lower mass, lower internal heat and therefore, lower strength development.

Temperature
Maturity hours
Compressive strength

In the graphs shown above, you can see the graphs showing the temperature, maturity and strength development of two structures. The green ones show the development under cold conditions while the orange one shows it under hot conditions. If you observe the graphs, you can see that when the temperature rises, the maturity is gained faster, see red line. Moreover, if the temperature is comparatively higher, the strength development will also be gained faster. 

If you are only using the compressive strength of your cube/cylinder, then you may not know for sure if you have achieved the desired strength. What you are in fact validating is that the concrete used on the job site is able to gain a certain strength after a certain amount of days. In other words, the 28-day water bath cured compressive strength just validates the concrete used is able to gain that strength.

3. Cheaper and faster results with sensors

Concrete sensors are obviously available in different price classes, but in most cases are significantly cheaper and faster to provide a return on investment. A break test often requires extensive labor work, transport to concrete lab, waiting time to get results – all while the concrete is still curing. With Maturix, you only need to embed a thermocouple into the concrete with averaging costs on the embedded to around 1-10€. It is rare that you can do a compression test for those costs – and also get the added benefit of seeing the results in real time!

4. Avoid low break test delays​

Low compression test results can require a contractor to simply wait for the next planned specimen testing (for example wait for a 14-day cube testing) in order to be able to move on with the project. 

The reason why a break test suddenly is significantly lower than expected or even required might be many, but some of the most common issues are:

Using sensors reduces risk of an unexpected low break test. Sensors provide continuous vital information about concrete temperature and strength. So if there is something wrong with the concrete you will be able to see it early on and correct it right away. 

So for all these reasons, it is a good idea to use the maturity method in conjunction to break tests. This way you can comply with the standards and requirements while also getting continuous information that can be used to make better decisions.

How does Concrete Maturity work?

The maturity method has three main steps, which you can read more about below. (from “What is the Maturity Method?”

Step 1: Make a Maturity Calibration

A Maturity Calibration determines the relationship between the maturity and strength development of a specific concrete mix.

To find this relationship, you make some samples with the concrete mixture that you will use in your project and instrument some of them with temperature sensors. The samples are then cured under the same conditions and the temperature history is measured using the sensors. Then, you need to perform break tests of the samples at different test ages to determine their compressive strength. Once that is done, plot the strength data from the break tests and the maturity from the temperature history in a graph. Lastly, find the best fitting curve through your data points, also known as the Maturity Curve.

Maturity Calibration

Read our detailed article about Maturity Calibration.

Step 2: Estimate the in-place strength

Once you have performed a maturity calibration for your concrete mixture, you can estimate the in-place concrete strength by placing temperature sensors inside your structure. These will calculate the Maturity Index in your concrete and relate it to a certain strength from the Maturity Curve.

Process to estimate concrete strength

Read our detailed article Estimate In-place Strength with the Maturity Method to learn more.

Step 3: Validating the Maturity Calibration

Validating the calibration and maturity curve regularly is important as there might be small variations in materials, batching equipment, and conditions which might affect the accuracy.

To validate your maturity calibration, make some samples during the next batch and compare the strength estimated using the Maturity Method with the strength obtained from other testing methods.

ASTM C1074 strongly recommends not to perform critical operations without verification of the maturity calibration or without strength validation using other test methods.

Read more about Validating the Maturity Calibration.

Where and for what can you use Concrete Maturity?

Concrete sensors are widely all over the globe for:

Each type of concreting job is different and the Maturix sensors can be adapted to them as they are very versatile. The needs and requirements might be different but we have experienced the success of our clients using the Maturix system in almost every type of construction project, country and weather conditions.

Case studies

Maturity Monitoring in hot- and cold weather concreting

Concreting under changing and difficult weather conditions whether it is cold- or hot weather makes it difficult to cure concrete properly. The concrete might be exposed to high temperature differences in cross section, too long curing times in cold weather etc. 

Managing the concrete curing with sensors during these challenging conditions is an effective way to keep track of what is happening and ensure an optimal temperature and strength development. Having the sensor placed in the concrete is like placing a set of eyes inside the concrete. You know exactly what is happening and which actions need to be taken adjusted to this. This can be placing covers in the right time or adjusting the effect on cooling pipes for example.

Most asked questions on Concrete Maturity

How can temperature-controlled specimens be compared to my structure?

Calculating the in-place temperature into a maturity age enables a comparison with the concrete specimens done from the concrete lab. With the in-place temperature of the structure it is possible to calculate its maturity age (clock time * temperature). Then, the calibration curve is used to determine the relation between a maturity age and the compressive strength. For every new temperature measured, a new maturity age can be calculated – and thereby a new and increased strength can be estimated.

Why do I need a concrete calibration curve?

A concrete calibration curve is used to have the relation between maturity age and the compressive strength. Concrete sensor solutions will not be able to estimate the strength without the calibration curve.

For how long is my concrete calibration valid?

Theoretically, the concrete calibration would be valid as long as the concrete mix design or materials going into the concrete mix remain the same. But in the real world, a material supplier might change, the material provided by the same supplier might be different, the mixing machine may change in tolerances etc. Therefore, it makes sense to set up a running schedule for validating the concrete mix calibration. This could be for XX amount of concrete used or one time every second month or so. 

Some clients of Maturix use this calibration validation to extend the calibration curve data set with more data. If they don’t exceed 10% difference, then they will just count as part of the natural daily variations you have when concrete is mixed. If differences between the previously done calibrations or the fitted curve exceeds 10%, it is recommended to redo the calibration curve.

How reliable is the method?​

The maturity method has over many decades shown great reliability. The biggest challenge experienced when you are new to sensors and maturity is to make the (1) correct calibration curve and (2) valid strength comparison. When this is in place, most people will have a good experience with the reliability of the strength estimations.

Correct calibration curve

One of the most common mistakes is not measuring the correct maturity age, when making the compression test to type into Maturix or a similar program. Typing in either clock hours or just rough numbers on the maturity age will have the potential impact of wrong strength estimations. The reason is that the correlation between the maturity age and compressive strength will be wrong. Maturix and other maturity systems won’t be able to calculate better than the data provided. A key is therefore to ensure a correct maturity age during the compression tests.

Valid strength comparison

Another common mistake is a wrong estimation of the in-place strength using sensors in comparison with break tests. The reason for this is often differences in curing conditions. This is described in section “2. Cubes/cylinders risk to show wrong results due to curing conditions”. 

Some key limitations and requirements for making a reliable estimation of maturity strength using the concrete method concrete maturity are:

Limitations of Concrete Maturity

The maturity method has proven to be a reliable and easy to use method to estimate the development of early concrete strength. Every test method has its limitations, so below you can see a list of the most relevant limitations we have experienced working with concrete sensors on +1000 projects across 6 continents (we are still missing Antarctica). 

  1. A calibration curve needs to be done prior to concrete casting for every concrete mixture that you plan to use. 
  2. Low moisture content and availability of water during the curing process can affect the precision of the strength estimations. 
  3. High temperatures can affect long term strength estimations. This is due to something called the cross-over effect. This effect causes the strength to increase faster than expected at the beginning but then results in a lower than expected 28-days.

 

Do you want to know all the limitations? Please check out our in-depth article here.

Concrete Maturity terminology

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