One of the most critical periods in a concrete construction project is the first few weeks after the concrete is mixed and poured. During this time, chemical processes happen inside the concrete that determine its final properties. These processes are very sensitive to temperature and moisture level, and if the conditions aren’t ideal, the strength, permeability, and durability of the concrete will suffer. Uneven temperature and moisture during this time can also cause surface cracking.
For this reason, special concrete curing techniques are used to maintain the optimal conditions after concrete is placed. In this article, we’ll talk about why curing is important, and what techniques are typically used. (As a side note- the term “concrete curing” can mean the techniques that are used, or the chemical process happening within the concrete.)
An important aspect of curing is continuous monitoring of the conditions- particularly the temperature- in and around the concrete element. This allows adjustments to be made to the curing procedures if needed. Industrial data loggers are ideal for this, because they can continuously measure temperature and moisture levels at multiple points, and upload data remotely.
What happens during concrete curing?
After concrete is blended, a series of fairly complicated chemical reactions take place between water and the cement, known as hydration reactions, where water is incorporated into the cement structure. This causes it to harden and develop strength. On a microscopic level, crystals form that incorporate and connect the aggregate particles.
The end of the curing process is tricky to define, because hydration continues to take place even years after the concrete is in place. As a rule of thumb, the American Concrete Institute recommends that curing procedures are used until the concrete reaches about 70% of its final strength. For concrete slabs on the ground, or structural concrete, this usually requires a minimum of 7 days when the temperature is above 40 °F, but that can vary with the size and shape of the part, and the type of concrete used.
It’s important to distinguish curing from drying, which is a physical process. Curing is a chemical process where water isn’t lost from the bulk material.
What factors affect the curing process?
Curing is primarily affected by moisture and temperature. Maintaining a high moisture level is critical, so that there is enough water present to react completely with the cement and the optimal strength is achieved.
It’s also important that the moisture level is relatively constant throughout the concrete, so that reaction and shrinkage happens at a constant rate, preventing surface cracking. Several of the curing strategies we’ll talk about in the next section are focused on adding moisture to the outside of the concrete, where it is quickly lost through evaporation.
Temperature is important because the hydration reactions speed up at higher temperatures. Higher temperature leads to faster curing, but at the expense of lower final strength.
We also have to think about the size and shape of the element. Since hydration reactions are exothermic, concrete heats up during curing.The center of a large element will heat up more, because the outside releases heat to the environment more quickly. This can result in thermal stresses and cracking if it isn’t controlled. For this reason, the design of large concrete elements usually includes a thermal control plan to minimize temperature differentials.
The weather conditions can also have a significant effect on curing. We’ll talk more about that in the next section.
How can the curing process be controlled?
Three approaches are commonly used to optimize curing:
- Adding water to the surface of the concrete, by “ponding” on flat surfaces, or by misting or spraying. Saturating wood forms, or wrapping bare concrete in wet cloths, can also be used to add moisture. It’s important to keep the surface wet constantly, and to keep the temperature differential between the concrete and surface water low to avoid thermal stresses.
- Sealing the concrete surface to prevent water loss. This can be done with solid barriers like plastic sheeting, or by applying a sealing compound to the surface.
- Accelerating the curing process by adding heat, or a combination of heat and water. Heating blankets, coils, or heated forms can be used, or the concrete can be exposed to hot steam. These strategies can also be used to prevent freezing when the concrete is used in cold weather conditions.
When concrete is used in very hot or cold conditions, these strategies become even more important. In cold weather, strength development is slower, and extra efforts must be made to prevent freezing. In hot weather, fast moisture loss and accelerated chemical reactions can lead to cracking or poor final strength. The American Concrete Institute publishes a number of guides and specifications that can be used to adapt to these challenging situations.
What role can data loggers play?
Curing can be affected by a number of variables, including weather conditions, size and shape of the elements, and the composition of the mix. For this reason, an optimized curing procedure should be able to detect and respond to variable and unexpected conditions.
This is where industrial data loggers come in. With data loggers, continuous temperature and moisture level measurements can be automated, reducing costs and eliminating the risks of human error that come with manual temperature measurement and data collection.
For example: while concrete is being poured, data loggers could be placed to measure ambient temperature and humidity, as well as temperature inside the concrete using embedded thermocouples. These loggers can be configured to remotely transmit data, by WiFi or bluetooth, or through wired connections when that’s more convenient.
During curing, the loggers will continuously transmit monitoring data, so that the curing strategy can be adjusted as needed. If conditions fall outside of a safe range, targeted alerts can be sent remotely, via phone, SMS text, or email, so that major issues can be dealt with before there's a risk to the integrity of the concrete. After the curing period is over, the digital records collected by the loggers can be maintained as a form of QA.
To return to the question at the top of this article: yes, data loggers can definitely be a valuable tool for monitoring and controlling concrete curing. Curing is a sensitive process that can be affected by unpredictable variables, and the automated, continuous monitoring provided by data loggers is an efficient way to deal with these variables.
For more information on industrial data loggers, contact the experts at Dickson.