# Maturity and strength monitoring

## 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 significant growth due to the potential cost and time savings that real-time data provide. Many leading companies have replaced or complemented their usual methods, such as break tests or drilled cores, with sensors that can instantly calculate concrete maturity and strength.

## 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 central concept of concrete maturity is that you can obtain a specific value of maturity 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.

If you look at the first graph (Graph 1), you will see that the temperature is 20 °C, and 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.
In Graph 3, the temperature is the lowest of the examples at 10 °C; 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, you can determine the relationship between maturity and strength development using the maturity method.

### Maturity Method

The maturity method is an easy way to estimate the early-age strength development of a concrete mix. The central 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 you cured them under different temperature conditions.
To use the maturity method, you will need to perform a maturity calibration, which includes 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.

## How can you benefit from using Concrete Maturity?

Many of our clients mention that one key benefit of using the Maturix sensors is the real-time insight into their concrete’s temperature, maturity, and strength development. This information allows them to do things they could not do before. Such as optimising the use of resources, getting insights into their concrete curing, monitoring critical spots in the structure, reducing the risk of thermal cracks and creating documentation automatically. These benefits have been experienced 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:

In recent years, we have seen a massive 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 and 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 one day of curing compared to traditional basic cement. This can be extremely critical both on job sites and precast factories, where keeping daily schedules and production cycles are key to staying competitive in the market. Some research (Lasse Frølich, Portland Open 2021) shows that the new concrete types have 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.
You can compensate for these problems by using 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 apply directly 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 lets you view how the curing is progressing in one or more spots. Certain places in the structure cure much faster or slower than the rest. Conventional methods do not provide 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, the temperature outside the castings, strength around hooks, prestressed wires, etc. Then, you can see the data about each of the spots individually; you can see an example of this below:

#### Reduce uncertainty on concrete break tests

Low breaks or inconsistent compressive test results are common problems in the construction industry. There are many standard procedures describing proper handling and preparation of test samples; however, the process is often 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 appropriately cured, they were damaged during transport, or the testing machine was not calibrated correctly.
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, placing the sensors in the structure makes it possible to track the progress continuously and instantly detect if something is not going well. Then, if a low break happens, you will have much richer information about 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 temperature and the ambient climate.
2. Heating usage; many precast facilities add additional heating to the concrete by water pipes underneath the concrete beds or with infrared heating placed above them. Knowing the curing stage your concrete is on, you can 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 significant determinants of the Co2 footprint. Even a slight reduction in the cement content will enable a meaningful Co2 footprint reduction. Using the concrete sensors’ data in conjunction with ambient monitoring allows you to:

1. Improve the mix design of currently used concrete
2. Apply the most appropriate 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 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.

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 complete control, allowing them to deliver concrete works of excellent quality continuously.

#### Automatic documentation/compliance

Large concrete jobs like infrastructure, mass concreting, high rises, damps, or other utility structures often have specific thermal monitoring requirements. Embedding sensors enable automatic documentation, and you can save a lot of labor hours 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.

As shown above, the break test will only occasionally give you a data point when a specimen is tested. Concrete sensors give 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 is typically due to the different curing temperatures.
A small concrete cube/cylinder has a smaller mass than the structure; thus, the heat development inside will be lower. 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 means that when comparing a specimen to the actual structure, this has: lower mass, lower internal heat, and, therefore, lower strength development.

In the graphs shown above, you can see the graphs showing the temperature, maturity, and strength development of two structures. The green ones show 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, maturity is gained faster; see the 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, you may not know if you have achieved the desired strength. You are validating that the concrete used on the job site can gain a particular strength after a certain number of days. In other words, the 28-day water bath cured compressive strength validates that the concrete used can gain that strength.

#### 3. Cheaper and faster results with sensors

Concrete sensors are available in different price classes but, in most cases, are significantly cheaper and faster in providing a return on investment. A break test often requires extensive labor work, transport to the concrete lab, and waiting time to get results – all while the concrete is still curing. With Maturix, you only need to embed a thermocouple into the concrete, averaging costs on the embedded to around 1-10€. You can rarely do a compression test for those costs and 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) 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 the risk of an unexpected low break test. Sensors provide continuous vital information about concrete temperature and strength. So if something is wrong with the concrete, you will be able to see it early on and correct it immediately.
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 you can use 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 a specific concrete mix’s maturity and strength development.
To find this relationship, you make some samples with the concrete mixture you will use in your project and instrument some with temperature sensors. The samples are then cured under the same conditions, and the sensors measure the temperature history. Then, you need to perform break tests of the samples at different test ages to determine their compressive strength. Once you complete that process, 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, known as the Maturity Curve.

### 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.

### Step 3: Validating the Maturity Calibration

Validating the calibration and maturity curve regularly is vital as slight variations in materials, batching equipment, and conditions 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 performing critical operations without verification of the maturity calibration or without strength validation using other test methods.

## 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 differ, but we have experienced the success of our clients using the Maturix system in almost every type of construction project, country, and weather condition.

### Maturity Monitoring in hot- and cold weather concreting

Concreting under changing and difficult weather conditions (whether cold or hot) 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 effectively keeps track of what is happening and ensures optimal temperature and strength development. Having the sensor placed in the concrete is like placing a set of eyes inside the concrete. You know precisely what is happening and which actions need to be taken to adjust to this. This can be placing covers at 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 relationship between maturity age and 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 relationship between maturity age and compressive strength. Concrete sensor solutions cannot estimate the strength without the calibration curve.

### For how long is my concrete calibration valid?

Theoretically, the concrete calibration would be valid if 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 once every second month.
Some clients of Maturix use this calibration validation to extend the calibration curve data set with more data. If they don’t exceed the 10% difference, they will be part of your natural daily variations when mixing concrete. If differences between the previously done calibrations or the fitted curve exceed 10%, it is recommended to redo the calibration curve.

### How reliable is the method?​

The maturity method has, over many decades, shown excellent 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 potentially impact wrong strength estimations. The reason is that the correlation between 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 compared to 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 reliable and easy to use to estimate early concrete strength development. Every test method has limitations, so below, you can see a list of the most relevant limitations we have experienced working with concrete sensors on +1000 projects across six continents (we are still missing Antarctica).

1. A calibration curve must be done before concrete casting for every concrete mixture you plan to use.
2. Low moisture content and water availability 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 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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Casper Harlev

You can contact Casper Harlev by phone, email, or LinkedIn if you want help finding out whether Maturix is the right solution for you.