Concrete is a composite material composed of cement, water, coarse and fine aggregates, and often chemical admixtures. Cement is the binding agent that holds the other materials together to form a solid, strong, and durable material.
The coarse and fine aggregates typically consist of gravel, crushed stone, sand, or other materials, which provide bulk and strength to the concrete. Chemical admixtures, such as water reducers, accelerators, and retarders, can be added to the concrete mixture to improve its workability, strength, or durability.
Concrete is widely used in construction due to its versatility, strength, and durability. It is commonly used for foundations, walls, floors, pavements, bridges, and other structures. The properties of concrete can be customized to meet specific design requirements, such as strength, durability, and resistance to environmental factors like water, fire, and chemicals.
Despite the merits and utility of concrete, it has one particular weakness. To understand it, one would have to understand the different forces that act on building materials.
Tension, compression, and shear forces are three of the most common types of forces that act on building materials. These forces can cause stress and deformation in the materials and must be carefully considered during the design and construction of buildings and structures.
Tension: Tension is a pulling force that stretches or elongates a material. In building materials, tension is often caused by external forces, such as wind or seismic activity, that create a pulling force on a building’s components. For example, tension can cause steel cables or rods to stretch or elongate, creating stress and potential damage. Concrete is relatively weak in tension and tends to crack or fail when subjected to tensile forces. This is because concrete is brittle and cannot stretch or deform significantly before cracking.
Compression: Compression is a squeezing force that compresses or shortens a material. In building materials, compression is often caused by the weight of the building itself or other external forces, such as live loads or snow loads. For example, compression can cause concrete columns or walls to compress, creating stress and potential damage. Concrete is strong in compression and can support heavy loads without significant deformation or failure. Most of the weight of a building is supported by the concrete foundation and walls, which are designed to withstand compressive forces.
Shear: Shear is a force that causes two parts of a material to slide past each other in opposite directions. In building materials, shear is often caused by external forces that create a lateral or twisting force on a building’s components. For example, shear can cause a steel beam to twist or a concrete slab to crack, creating stress and potential damage. Concrete is also relatively weak in shear, as it tends to crack and fail when subjected to lateral or twisting forces.
The weakness of concrete under tension and shear forces significantly reduces its utility as a construction material. This is because construction materials are often subjected to a mixture of all three forces. This would make the use of concrete in construction catastrophic if its weaknesses are not dealt with. To remedy this, engineers modify concrete by reinforcing it with steel to form reinforced concrete.
Reinforced concrete is a composite material that combines the strength of concrete with the tensile strength of steel reinforcement. The steel reinforcement is embedded in the concrete, creating a material that is strong in both compression and tension.
To create reinforced concrete, steel reinforcing bars, known as rebar, are placed in the formwork before the concrete is poured. As the concrete sets and hardens, it bonds to the rebar, creating a strong and durable composite material.
Reinforced concrete is commonly used in construction due to its strength, durability, and versatility. It can be used to create a variety of structures, including foundations, walls, floors, beams, columns, and bridges. Reinforced concrete structures can be designed to withstand a wide range of forces, including wind, seismic activity, and heavy loads.
The use of reinforced concrete has revolutionized the construction industry, enabling engineers and architects to create larger and more complex structures than was previously possible with traditional materials such as wood and masonry. By combining the strength of concrete and steel, reinforced concrete has become one of the most widely used and important building materials in modern construction
Besides increasing the utility of concrete by increasing its resistance to tension and shear forces, reinforcement in concrete also changes the mode of failure of concrete from a brittle and instantaneous one, to a ductile and slow one. That is if a concrete structure or member is overwhelmed and begins to fail, concrete that isn’t reinforced would fail instantly and snap like breaking glass. This implies that if it’s used to construct a building, for instance, failure can bring down the entire building in a matter of seconds. For reinforced concrete on the other hand, if it’s to fail, it occurs slowly. Before the failure, the structure shows warning signs in the form of cracks. This gives one time to either remedy a failure before it becomes severe or safely evacuate an impending catastrophic failure.
However, the weakness of reinforced concrete is that the steel reinforcements have to first stretch before they can begin to resist tension. This implies that a structure or concrete member would inevitably undergo some minor deformation or even develop cracks before the reinforcements begin to work. Engineers remedy this by using prestressed and post-stressed concrete reinforcement.
Prestressed and post-tensioned concrete are two methods of reinforced concrete that use tensioned steel to improve the strength and durability of the material.
Prestressed concrete involves the use of pre-tensioned steel reinforcement, which is tensioned before the concrete is poured. The steel reinforcement is anchored at both ends, and the concrete is then poured around it. Once the concrete has hardened, the steel is released, causing it to contract and place the concrete in compression. This compressive force improves the strength and durability of the concrete, making it more resistant to deformation and cracking.
Post-tensioned concrete, on the other hand, involves the use of steel cables or tendons that are tensioned after the concrete has been poured and has partially cured. The tendons are placed in ducts or sheaths within the concrete and are then tensioned using hydraulic jacks. Once the tendons have been tensioned, they are anchored at both ends and the concrete is allowed to fully cure. The compressive force generated by the tensioned steel helps to strengthen the concrete and resist deformation and cracking.
Both prestressed and post-tensioned concrete offer significant advantages over traditional reinforced concrete, including greater strength, durability, and resistance to deformation and cracking. They are commonly used in the construction of large-scale infrastructure projects such as bridges, parking garages, and high-rise buildings, where the strength and durability of the concrete are critical to the safety and stability of the structure.
In conclusion, Reinforcing concrete significantly improves the properties of concrete as a construction material by channeling the strength of steel under tension to remedy the weakness of concrete under tension and shear forces.
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