The strength development of concrete largely depends on materials properties of cement, water, sand and aggregates and how all of these interact in a temperature-humidity-pressure managed environment.
In the construction industry, knowing how concrete develops real-time helps extremely. When to start using, forms, scheduling, post-tensioning, when to let traffic on roads, or when when to remove the heating applications all depend on having the right concrete strength.
Functional tests like break tests or prediction methods such as maturity are different, cross-functioning and co-supporting approaches on concrete strength determination.
Let’s see how these methods work.
Functional tests
Destuctive functional tests, such as break tests are well established.
- Test samples are casted and cured.
- After curing, tests are performed.
- Results are used to validate the concrete 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
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.
If we observe the test cylinder’s temperature history, we can see that the temperature is much lower than the temperature of field products.
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 strength development in a test sample does not follow the same development as in the structure of a product. Instead, it has a much slower strength development due to the temperature history of the cylinder being lower.
Targets are reached differently for test cylinders compared to the product structure’s sample positions. 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 product cures at the same speed everywhere. However, in a product there are zones which will cure faster than others due to temperature differences. For instance, the product’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 product’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 product.
Break tests. How accurate and reliable can a test be?
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.
What about this new method, maturity functions better?
The maturity method is a non-destructive method that can be used to estimate the early-age strength development of concrete.
With this method, you start by performing a calibrating in a laboratory to find the correlation between time, temperature, and strength.
During calibration, you take a standardized set of procedures, pick the concrete mixture that will be used in the product and embed samples with sensors.
The product is then cured as your processes outline for a product or as per standard. Conditions, such as weather, time, , temperature, sometimes moisture, humidity and pressure history is recorded.
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) can be drawn through the data points.This curve represents the strength-maturity relationship for the concrete mix.
After having performed a maturity calibration, the in-place strength can be estimated by placing temperature sensors in the concrete product and using a prediction system such as concrete.fit or Hilti. These systems automatically calculate the maturity from the product’s temperature history and displays the in-place concrete strength in real-time.
Limitations
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 recipe mix 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 will not replace. However, the number of break tests used for other purposes, like determining when to continue with different processes, can be significantly reduced.
Advantages
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:
- The use of testing facilities and personnel is greatly decreased as the information is mainly gathered by temperature sensors embedded into the concrete. This results in time and cost savings on making, handling, transporting, and testing samples.
- Instead of making guesses about when the strength is sufficient to test the samples, the maturity method indicates when the desired strength has been reached. This eliminates a lot of uncertainty and helps projects to become more efficient, data-driven and proactive which improves decision-making.
- The maturity method can monitor the actual conditions of the structure, including the temperature and strength development at critical zones. This monitoring allows for a much more accurate estimation of the structure’s in-place strength development.
- Using a maturity system like concrete.fit or Hilti you gets real-time and remote data collection. These systems provide continuous monitoring of products which gives a more complete overview of the curing process and the concrete’s strength development. This helps make sure that the production stays within specifications, for example that concrete does not exceed certain temperatures or that differential temperatures between the core and surface stays within the thresholds.