In this blog post, we’ll examine the causes of thermal expansion and cracking that occur when buildings are exposed to heat, explore the role of expansion joints in preventing these issues, and use the principles of architectural engineering to reflect on human relationships as well.
The built environment is an inseparable part of our lives, not only in South Korea but everywhere in the world.
Last week, I attended the wedding of a senior from my department. As with any wedding, toward the end of the ceremony, the bride and groom exchanged vows of trust in each other, and the professor who officiated the ceremony left a particularly strong impression on me. Referring to their vows, the professor said, “Marriage is a promise to acknowledge that, since two different people have come together, there will always be a gap that prevents them from becoming completely one.” In this way, a married life that allows for a certain degree of freedom and leeway for each other will not lead to a catastrophic collapse, even if it creates minor cracks.
The same is true for a building, where different materials come together to withstand the test of time. Let’s take concrete—a representative building material—as an example. Concrete is created when cement, water, sand, and gravel combine to form a material with new properties. The process of pouring the mixture into a fixed formwork and maintaining it for a certain period to allow the materials to harden sufficiently and bond firmly together is called “curing.” During curing, water is used in the hydration reaction of the cement or partially evaporates, while the cement, sand, and gravel bond together to form a single structure. Once the formwork is removed, the new material—concrete—is finally complete.
However, over time, concrete begins to develop microscopic cracks due to various factors, including differences in the physical properties of the cement and aggregates, drying shrinkage, and temperature changes. It is not easy for materials with different properties to perfectly maintain a single, uniform property.
One of the primary causes of these limitations in concrete is thermal expansion. Most objects on Earth expand in volume as their temperature rises. In everyday life, it is common to see railroad tracks laid at regular intervals. This is because the steel used in the tracks expands in length when exposed to intense sunlight and high temperatures; without this spacing, the tracks could bend, potentially causing derailments.
This phenomenon also occurs in buildings. However, unlike metal—which is a single material—concrete is a composite material consisting of cement, aggregate, and reinforcing bars, so its behavior in response to temperature changes is more complex. When exposed to high temperatures or repeated temperature fluctuations over a long period, thermal expansion and contraction occur repeatedly within the concrete. If the stress generated during this process exceeds the material’s tensile strength, cracks may form.
Concrete is a material that is very strong in compression but relatively weak in tension. Therefore, if it cannot sufficiently withstand tensile stresses caused by temperature changes or drying shrinkage, cracks will form. Even a small crack, if it continues to widen, can develop into a large gap wide enough for a person’s foot to fall through, and in bridges or large structures, this can compromise structural safety. Furthermore, if various foreign substances—such as water and salt—penetrate the interior through microscopic cracks, corrosion of the reinforcing bars can occur, significantly reducing the durability and service life of the building.
Therefore, devices are installed in structures such as buildings, bridges, and roads to mitigate these problems. If you look at a concrete-paved road, you can easily spot joints at regular intervals where metal or elastic materials are inserted; such devices are also installed in buildings and bridges. In other words, they provide a certain amount of space to allow the structure to move naturally in response to temperature changes. In civil engineering, this is called an expansion joint. In other words, it is a device that allows a structure to expand and contract on its own in response to temperature changes.
So, let’s examine why metal is often used in expansion joints. As explained earlier, metal has relatively uniform material properties, so the amount of deformation can be predicted fairly accurately using its coefficient of thermal expansion when the temperature rises. If it were impossible to predict temperature-induced deformation, structural design itself would become extremely difficult.
For this reason, metal offers sufficient strength and durability while allowing for expansion and contraction within a predictable range. For example, standard structural steel has a coefficient of thermal expansion of approximately 12×10⁻⁶/℃; therefore, a 1-meter-long steel bar will expand by about 1.2 millimeters when the temperature rises by 100℃. Thanks to this predictability, metals are widely used in expansion joints and various structural connection members.
In life, we often face stress, both internal and external. Sometimes, conflicts arise due to small cracks that form between ourselves and our friends. Just as with these cracks, while we can minimize the gaps between people with different personalities, we cannot completely eliminate them. As we’ve seen, architects and structural engineers install expansion joints to prevent catastrophic cracks by accounting for a building’s potential to expand. If we apply this principle to human relationships, why not set aside a little breathing room—a “mental expansion joint”—in advance to allow for mutual understanding and consideration in our interactions with those around us?
