What is concrete setting?
The setting is defined by ASTM C125 as “the process, due to chemical reactions, occurring after the addition of mixing water, that results in a gradual development of rigidity of a cementitious mixture.” In other words, it is the process a concrete mixture goes through, from being a liquid mixture to gaining certain stiffness.
There are two crucial moments in the concrete setting process:
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Why it is important to know the setting time
The setting time might not seem important, but this can be very useful for many purposes. The initial setting time tells you how much time you have from when you mix water with cement until this starts solidifying. Knowing this helps you plan when to mix the concrete and how much time you have available for transporting and placing the concrete. The final setting time indicates when the concrete has turned solid and starts to gain strength. After the final setting time, it is not recommended to do consolidating activities as these will disturb the concrete’s strength development.
Ideally, you want the initial setting time to be long enough to transport the concrete and pour it while it is still fluid. However, you do not want the initial setting time to be too long, as the concrete finishers and the rest of the crew will need to wait (sometimes up to 5 hours) until the concrete is ready to be consolidated.
The same applies to the final setting time, where you want it to be long enough to be able to finish the concrete, but not so long that the concrete takes forever to harden, as this prolongs the project schedule.
When does concrete setting happen?
During the cement hydration process, the concrete mixture goes from a liquid to a solid state. This change does not happen suddenly; instead, it occurs in different phases:
The initial setting time of concrete, indicated in the graph above, occurs when the reaction starts to gain speed and the heat emission increases at the end of the dormancy period.
During the third phase, the concrete starts gaining stiffness quickly and heat is released due to the chemical reactions. A few hours after the beginning of phase 3, the concrete becomes solid, marking the final setting time.
Read more about the cement hydration phases here.
Factors affecting the concrete setting time
Different factors can affect the setting times. This can be, for example, water/cement ratio, temperature conditions, and admixtures.
The water/cement ratio has a big impact on the setting time of your concrete and its overall performance and quality.
Concrete mixes with lower water/cement ratios generally have faster setting times. However, these may be harder to work with as they start hardening earlier; thus, it it necessary to do all the finishing operations faster. On the other hand, concrete mixes with higher water/cement ratios have slower setting times. This can be an advantage as you have more time before the concrete starts hardening. However, too much water affects the final strength of the concrete and may cause delays as the concrete will need more time before the concrete finishers can start working on it.
Admixtures are substances added to the concrete mix to alter one or more of the concrete properties. Two main types of admixtures can be used to modify the setting time: Accelerators and retarders – accelerators to increase the rate of cement hydration, which will shorten the setting time, and retarders to delay the setting time of concrete, giving more time for concreting works.
Temperature can affect the setting times of concrete as this affects how fast the phases in the cement hydration happen. If the temperature is high, the cement hydration reactions will occur faster, and the setting time will occur earlier. If the temperatures are low, the cement hydration process will slow down, and the setting time will happen later.
Smart, wireless systems such as Maturix can help you monitor the temperature in your concrete in different spots in the structure and allow you to access this information from anywhere, at any time. This will give you a clear picture of what is happening inside the concrete.
How to determine concrete setting times
It is important to understand that there are two types of tests related to setting time; cement setting time and concrete setting time.
The cement setting time tests are relevant for cement manufacturers but are hard to use in construction projects. This is because, for the tests for the cement setting times, only pure cement and water are used neglecting the effect of aggregates and admixtures. Therefore, the most relevant test to do is the concrete setting time. The standard procedure to do it is:
ASTM C403: Standard Test Method for Time of Setting of Concrete Mixtures by Penetration Resistance. You can see the required materials, equipment, and test steps in this video.
Use Maturix to monitor concrete temperature and strength
The reality is that the concrete setting test is not widely used. This is because even if the information it provides is interesting, there is normally not enough time or personnel to do it. So in many cases, a better solution will be to use something like Maturix. This system offers the opportunity to: