Break Tests vs. Maturity Method
The strength development of a concrete structure is strongly influenced by the temperature and weather conditions at the job site. In construction projects, knowing this strength is crucial as many further decisions depend on it. Decisions such as when to remove the forms, scheduling of post-tensioning operations, when to open bridges and roads for traffic, and determining when to remove the heating measures against cold weather all depend on having the right concrete strength.
Break tests and the maturity method are two approaches on how to determine concrete strength. In this article we will explain what these methods are, how they work, pros & cons, and how they compare to each other.
Break Tests - what is it and how does it work?
Break tests are the traditional and established way to test the compressive strength of concrete using a destructive approach by crushing concrete cylinders or cubes and measuring the pressure until it breaks.
In this method, test samples are casted and cured either at the job site or in a testing laboratory. When placed on-site, the samples are placed as close as possible to the structure to replicate the same curing conditions. This is also known as field-cured specimens. In testing labs, the samples are cured under controlled conditions, e.g. in water tanks that are kept at a constant temperature, also known as standard curing.
After a certain amount of curing time, break tests are performed to the samples. Each sample is placed in a compression test machine and pressure is applied from top and bottom until the sample breaks. Once the sample fails to withlast the pressure, the compressive strength is calculated by dividing the failing load by the cross sectional area resisting the load.
The results of these tests are used to validate the concrete strength in a wide range of decisions. This could be when to move to the next step in the construction project or as documentation and validation of the 28-day strength.
Advantages of Break Tests
Break testing is one of the most commonly used methods to estimate the compressive strength. It is accepted internationally and has been standardized in almost every country.
Limitations of Break Tests
Do break tests accurately represent the actual strength of a structure?
One of the main limitations of break tests is the difference of mass between the samples and the structure. This mass difference is important because it affects the heat emitted during the cement hydration process. And the amount of emitted heat affects the strength development speed.
Cement hydration, the process where concrete becomes solid and develops strength, is an exothermic reaction, which means that when the reaction occurs, heat is emitted. The amount of emitted heat varies depending on how many components are reacting at the same time. For example, a big structure will have a high mass and consequently there will be many components reacting with each other at the same time. Each of these reactions will emit some heat and when combined, the total heat emission will be very high. In contrast, if there is less mass, there will be fewer components reacting and therefore, less heat emission.
In the graph below, we can see the temperature history of a structure, monitored in each of its four corners, and a test sample (Test Cylinder #3).
Graph 1: Temperature history of the structure’s four corners and a test cylinder
If we observe the test cylinder’s temperature history (represented in purple), we can see that the temperature is much lower than the temperature of the structures’ corners.
This difference is important because the heat emission has an effect on the speed of the strength development. When the temperature rises, the components’ particles will collide more often and with higher energy, causing the reaction speed to increase. In contrast, if the temperature is lower, the components’ particles will collide less and with lower energy, so the strength development is going to be slower.
The effect of temperature in the concrete’s strength development can be seen in the graph below.
Graph 2: Strength development of the structure’s four corners and a test cylinder
If we observe the graph, we can see that the strength development in the test sample (represented in purple) does not follow the same development as the corners of the structure. Instead, this has a slower strength development due to the temperature history of the cylinder being lower, as seen in Graph 1.
Looking again at Graph 2, we can see that the target of 21 MPa was reached around 24 hours later for the test cylinder compared to all of the structure’s four corners. This would have meant that this time could have been saved and instead some processes could have started earlier.
The last thing to take into consideration is that, when using break tests, it is assumed that the structure cures at the same speed everywhere. However, in a structure there are zones which will cure faster than others due to temperature differences. For instance, the structure’s surface is normally more exposed to cold air, winds, and different weather conditions – and these will directly affect the strength development of the concrete at the surface.
For all these reasons, it can be discussed whether break tests are representative of the structure’s actual in-place strength due. The smaller volume and the lower temperatures of samples results in a different rate of strength development when compared to that of a structure.
Can break test results be trusted?
Low breaks or inconsistent compressive test results is 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.
Maturity Method - what is it and how does it work?
The maturity method is a non-destructive method that can be used to estimate the early-age strength development of concrete.
In this method, you start by performing a maturity calibration in a laboratory to find the correlation between time, temperature, and strength. During the maturity calibration, you make some samples with the concrete mixture that will be used in the project and instrument some of them with temperature sensors and recording devices. The samples are then cured under the same conditions and the temperature history is measured using the sensors. Break tests are then performed at different test ages to determine their compressive strength. With the strength data from the break tests, and the maturity from the temperature history, a best-fitting curve (Maturity Curve) is plotted through the data points. This curve represents the strength-maturity relationship for the concrete mix.
After having performed a maturity calibration, the in-place concrete strength can be estimated by placing temperature sensors in the concrete structure and using a maturity system like Maturix. Maturix automatically calculates the maturity from the structure’s temperature history and displays the in-place concrete strength in real-time.
Limitations of the Maturity Method
The maturity method has three limitations. The first one is that it is required to perform a maturity calibration for each concrete mix to estimate the strength. The second is that high variances in the delivered batching can affect the accuracy of the strength estimation, since the mix designs will be different. The third limitation is that many countries still require 28 day compressive strength tests, which the maturity method cannot replace. However, the number of break tests used for other purposes, like determining when to continue with different processes, can be significantly reduced.
Advantages of the Maturity Method over Break Tests
Even though break testing is the most used method to validate concrete strength, the procedure to make the samples is time consuming, and many times the procedure is not done according to specifications resulting in low breaks. Moreover, the samples have a lower mass and, therefore, lower temperatures, which produce a slower strength development when compared to the structure. This means that the samples are not replicating the actual strength development of the structure.
The maturity method overcomes some of these limitations:
Using breaks tests or the maturity method to determine concrete strength both have their pros and cons. In the table below you can see an overview of some of the differences between the two methods:
What type is the method?
How is the concrete strength found?
Compression test results of samples
Temperature history of the structure
When are strength results available?
Only after doing a break test
Continuous information on the in-place strength development
How valid are the test results?
Temperature and curing speed of the samples are not the same as the structure (often the samples have a slower strength development)
Uses the actual temperature history of the structure to give a more accurate estimation of the strength of the structure
What is the strength development in the structure?
Assumes that the entire structure develops strength at the same speed, which is not the case, as temperature variances will result in a different rate of strength development throughout the structure
Possible to monitor and follow the strength development at different locations and parts of the structure, including critical points and cold/hot areas
How is it affecting the project schedule?
Waiting for break tests can cause unnecessary time spent and increases the risk of delays
Results can be followed in real-time, which saves time and improves project efficiency
What is the method best suited for?
Long-term strength validation
Early-age strength estimation
Can I replace my break test with sensors?
There are still some limitations when it comes to a full replacement of the of break tests, as the results of these are often a requirement to comply with concrete standards.
This means, in other words, that break tests are often still necessary. For example, you may need to perform a 28-day strength test in order to validate that the concrete mix you received had the correct mix proportions and has, therefore, achieved the desired concrete strength from the project specifications.
Refer to the country or regional specifications if you want to find out to which degree is it possible to replace your break tests.
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