Why Concrete Needs Reinforcement

Summary

In this *Practical Engineering* episode, Grady explores concrete's greatest weakness—its inability to resist tension. He demonstrates how concrete can easily break under tension, but when reinforced with steel rebar, its tensile strength improves, making it much stronger and more reliable. Grady explains the difference between passive reinforcement, which requires concrete to crack before working, and active reinforcement, like prestressed concrete, which applies tension before or after curing to enhance concrete’s performance. The video also highlights methods like post-tensioning, offering greater control over concrete's durability, and concludes with a plug for Skillshare, the episode’s sponsor.

Takeaways

• 😀 Concrete is strong in compression but weak in tension, making it unsuitable for structural applications that experience pulling forces.
• 😀 Concrete's inability to resist tension was demonstrated with two concrete cylinders: one failed under compression, while the other failed under much lower tension.
• 😀 In structural applications like beams, concrete experiences both compressive and tensile stresses, which can lead to failure if the material is unreinforced.
• 😀 The failure of an unreinforced concrete beam happens where tensile stress is highest—at the bottom of the beam.
• 😀 Reinforcement, usually in the form of steel rebar, is added to concrete to resist tensile stress and improve its performance in structural applications.
• 😀 Rebar helps transfer tensile loads to the steel, which is strong in tension, making concrete a composite material that can resist both compressive and tensile stresses.
• 😀 Rebar reinforcement also changes the failure mode of concrete from brittle to ductile, providing more warning before catastrophic failure.
• 😀 Steel rebar is a passive reinforcement, meaning it only starts to resist tension after the concrete has cracked and the steel stretches.
• 😀 In some situations, you may want to avoid cracking or excessive deflection. This is where prestressed concrete comes into play.
• 😀 Prestressed concrete involves applying a pre-existing tension to the steel reinforcement, either before (pre-tensioning) or after (post-tensioning) the concrete is poured, to improve its strength and prevent cracking.
• 😀 Prestressed concrete is commonly used in large structures like bridges, where higher strength and reduced cracking are required.

Q & A

Why is concrete considered strong in compression but weak in tension?

Concrete is strong in compression because it can withstand forces that push its particles together. However, it is weak in tension because it lacks the ability to resist forces that pull its particles apart. This makes it prone to cracking when subjected to tensile stresses.

What is the significance of stress in material science, and how does it relate to concrete?

Stress in material science refers to the internal forces within a material when it is subjected to external loads. Concrete, while strong under compressive stress, is weak under tensile stress, meaning it performs well when compressed but poorly when pulled apart.

What was demonstrated with the two concrete cylinders in the video?

The two concrete cylinders were tested to show the difference in how concrete handles compressive and tensile stress. The compressive test showed that the cylinder could withstand about 1000 lbs of load before breaking, while the tensile test demonstrated that concrete fails under much less force, with only about 80 lbs causing failure.

Why should concrete not be used alone in structural members?

Concrete should not be used alone in structural members because it is weak in tension. Most structural members experience a combination of compressive and tensile stresses, and without reinforcement, concrete will fail when subjected to tensile forces.

What happens when a beam made of pure concrete is subjected to stress?

When a pure concrete beam is subjected to stress, the top experiences compressive stress while the bottom experiences tensile stress. As the load increases, the concrete fails first at the bottom, where tensile stress is highest, causing the beam to crack and eventually break.

How does reinforcement improve the performance of concrete?

Reinforcement, typically made of steel rebar, enhances concrete's performance by providing tensile strength, which concrete lacks. The reinforcement takes on the tensile stresses, while the concrete resists compressive stresses, making the composite material much stronger and more durable.

What is the advantage of using steel rebar in concrete reinforcement?

Steel rebar provides a strong bond with the concrete and helps resist tensile stress. Its deformation, or ridges, ensures better adhesion between the steel and concrete, enabling the concrete to handle both compressive and tensile stresses effectively.

What is the difference between passive reinforcement and active reinforcement in concrete?

Passive reinforcement, like regular rebar, begins to work only after the concrete cracks and stretches under tension. Active reinforcement, such as prestressed concrete, involves applying stress to the reinforcement before the concrete is placed in service, enabling it to resist tensile stress more effectively from the start.

How does prestressed concrete differ from traditional concrete reinforcement?

Prestressed concrete is a type of active reinforcement where the steel reinforcement is put under tension before the concrete is placed. This creates a compressive stress in the concrete that helps prevent cracking under tensile forces. This differs from traditional reinforcement, where rebar only helps after cracking occurs.

What are the two methods of prestressing concrete mentioned in the video?

The two methods of prestressing concrete discussed in the video are pre-tensioning, where tension is applied to the steel tendons before casting the concrete, and post-tensioning, where tension is applied to the reinforcement after the concrete has cured.