In this blog post, we’ll explore how reinforcing bars and concrete complement each other’s weaknesses and have become core materials in modern construction structures.
Most of the buildings around us are made of steel structures and concrete structures. A steel structure refers to a structure that uses steel materials—such as structural steel, steel plates, and steel pipes—as its primary materials, while a concrete structure refers to a structure that uses concrete as its primary material. Although not visible from the outside, rebar is always embedded within concrete structures. In this post, we will examine the characteristics of concrete and explore why rebar and concrete are considered the ultimate combination.
Concrete is a material made by mixing cement, aggregate, and water in appropriate proportions. When cement comes into contact with water, it undergoes a chemical reaction and hardens; this is called the hydration reaction. Aggregate refers to materials such as sand and gravel, which are classified as coarse aggregate and fine aggregate based on particle size. Concrete produced in this way is an excellent structural material and is commonly referred to by the slang term “gongguri” on construction sites. The process of pouring the mixed concrete into formwork is called “placing,” and the process of applying vibration after placing to remove air from the concrete and ensure the hydration reaction proceeds smoothly is called “compaction.” Furthermore, high strength can only be achieved by properly managing the concrete mix ratio, the compaction condition, and the temperature and humidity during the curing process. Concrete is generally transported by ready-mix concrete trucks and poured into the formwork on-site using a concrete pump truck.
For concrete poured in this manner to fulfill the structural function of a structure, it must possess the strength appropriate for the structure’s intended use. In other words, it must be capable of sufficiently withstanding the loads acting on the structure. For example, in the case of bridges, road bridges carrying vehicular traffic are subjected to much greater loads than pedestrian bridges, and therefore require higher strength. The compressive strength of typical structural concrete ranges from approximately 20 to 40 MPa, meaning it can withstand very large compressive forces. In contrast, its tensile strength is only about 10% of its compressive strength. In other words, concrete has the disadvantage of being a material that is very strong under compressive forces but very weak under tensile forces, which are pulling forces. The material used to compensate for this weakness is steel reinforcement. Steel reinforcement possesses high strength under both tensile and compressive forces, and products with a yield strength of approximately 400 MPa or higher are widely used. When steel reinforcement is placed inside the formwork before the concrete is poured, the concrete and reinforcement act as a single structural unit as the concrete hardens, and the steel reinforcement bears the load in place of the concrete where tensile forces are applied.
Take a beam as an example: when a load is applied to a beam supported at both ends, the beam sags downward. During this process, the top of the beam is subjected to compression, while the bottom is subjected to tension. Since a structure experiences both compressive and tensile forces simultaneously when under load, concrete alone cannot sufficiently withstand the tensile forces. Therefore, in beams, rebar is primarily placed on the underside, where tensile forces occur. This principle applies not only to beams but also to various reinforced concrete structures such as columns and slabs.
For the reinforcing bars and concrete to behave as a single structural unit, the reinforcing bars must be sufficiently bonded to the concrete. To achieve this, the surface of the reinforcing bars is textured with ridges, which enhance the bond strength between the bars and the concrete. The physical and chemical properties of the two materials are also well-suited to each other.
First, the thermal expansion coefficients of rebar and concrete are very similar, so even when temperature changes occur, significant cracking caused by differing expansion and contraction does not occur. Additionally, concrete protects the rebar from the external environment, thereby reducing corrosion. Conversely, while rebar has high thermal conductivity and tends to lose strength rapidly at high temperatures, concrete’s low thermal conductivity allows it to act as an insulator during a fire, slowing the rapid rise in rebar temperature and helping to delay the collapse of the structure.
Concrete allows for the relatively economical construction of structures in various shapes using formwork, making it one of the most widely used construction materials today. If concrete were not used in conjunction with rebar, its low tensile strength would have made it difficult to utilize it in such a wide variety of structures as we see today. However, because rebar effectively compensates for concrete’s weaknesses and the physical and chemical properties of the two materials complement each other well, reinforced concrete has established itself as the premier structural material in modern construction. Furthermore, thanks to continuous advancements in materials and construction techniques, reinforced concrete has achieved even greater strength and durability; based on these advancements, various large-scale structures—such as super-tall buildings and long-span bridges—are being constructed around the world.
