Cement hydration can generate significant heat, especially in large structures like onshore WTG foundations. This heat can cause internal temperatures to rise above safe levels, potentially leading to delayed ettringite formation, which can compromise the durability of concrete structures.

Cement hydration and high temperatures in WTG foundations

Onshore WTG foundations are massive cast-in-place concrete structures. A key characteristic that distinguishes mass concrete from other types of concrete work is its thermal behavior. This is primarily due to the exothermic nature of the cement-water reaction, known as cement hydration.

How concrete element size influences temperature evolution

When cement mixes with water, cement hydration begins. This exothermic reaction generates heat and produces new compounds that form a solid structure. Over time, these compounds continue to grow and interlock, making the mixture hard and strong.

However, in large concrete masses, the heat generated by this reaction does not dissipate quickly. This can result in a significant temperature rise within the concrete.

The temperature evolution within a concrete element is greatly influenced by its size. Larger volumes have more material insulating the core, causing the temperature to take longer to dissipate and to accumulate as the cement reacts with water. As the cement hydration process continues, heat generation eventually slows down, and the internal temperature gradually decreases.

The animation above was generated using in-house developed software to analyze heat transfer problems by means of the finite difference method. It shows the temperature evolution of concrete at early ages in blocks of different sizes: 1x1 m, 3x3 m, and 6x6 m.

As cement hydration takes place, temperatures rise quickly at the beginning. Later, when most cement has already reacted, the heat generation slows down, so temperatures decrease over time, trending towards the ambient temperature.

We see significant differences in temperature evolution among the three blocks. The larger the size, the greater the maximum temperature and the longer it takes to cool to ambient temperature. In this example, the maximum temperatures for the larger blocks can reach 70 to over 80 ºC, which is a risk zone as we'll see next.

Delayed ettringite formation (DEF)

Delayed ettringite formation (DEF) is a chemical reaction in hardened concrete that leads to expansion and cracking. Ettringite, a hydrated calcium sulfoaluminate, typically forms during the early stages of cement hydration.

However, in DEF, ettringite formation occurs at a delayed stage, long after the concrete has set and hardened. This delayed formation is often triggered by high temperatures (above 70-80°C) during curing, which prevent the normal formation of ettringite during early hydration.

The consequences of DEF include visible cracking, displacement, and an increased risk of secondary deterioration forms such as freeze-thaw damage or reinforcement corrosion. While DEF may not lead to structural collapse, it significantly compromises the durability and integrity of concrete structures.

DEF has been observed in both heat-cured precast concrete components and larger in-situ concrete structures, where the heat of hydration builds up during the early life of the structure.

Example of cement hydration heat in a WTG foundation

Onshore WTG foundations are typically made of large concrete slabs, often reaching depths of 3 to 4 meters at the center.

Based on the temperature evolution observed in the previous example, we can expect the maximum temperatures in WTG foundations to be somewhere between those of the 3x3 and 6x6 concrete blocks.

Here's an example of the heat transfer analysis of an actual WTG foundation. As illustrated, the temperatures reach almost 80 ºC, which is within the danger zone that can lead to DEF.

WTG foundations can be susceptible to DEF under certain circumstances. To prevent DEF, it is important to manage the internal temperature of the concrete during curing. Various concrete standards recommend maintaining the temperature below 70 °C during the early stages to minimize the risk of DEF.

There are several methods to keep the concrete temperatures sufficiently low, depending on the project characteristics and circumstances. If you want to know whether you should be concerned about DEF risk and what you can do to avoid it, don't hesitate and get in touch 📧.