Why Concrete Needs Reinforcement

Summary

In this video, Grady from Practical Engineering explores the weaknesses and solutions to concrete’s inability to resist tensile stress. Concrete is strong in compression but weak in tension, which leads to cracks in structural elements like beams. To address this, reinforcement methods such as rebar and prestressed concrete are introduced. Rebar enhances concrete’s strength by providing resistance against tension, while prestressing techniques, like post-tensioning, apply stress to the reinforcement before or after curing. These methods transform concrete’s failure mode from brittle to ductile, offering better durability in construction.

Takeaways

• 😀 Concrete is strong in compression but weak in tension, making it unsuitable for structural applications without reinforcement.
• 😀 The tensile strength of concrete is much lower than its compressive strength, demonstrated through practical tests with concrete cylinders.
• 😀 When concrete beams experience stress, the top experiences compression and the bottom experiences tension, leading to failure at the tensile side.
• 😀 Concrete alone is not a good material for structural members that experience tension, as it cannot resist tensile forces effectively.
• 😀 To overcome concrete's weakness in tension, reinforcement like steel rebar is added to create a composite material that performs better under stress.
• 😀 Steel rebar helps concrete resist tension, and its presence allows concrete to fail in a more ductile way, providing warning signs before catastrophic failure.
• 😀 Rebar is a passive reinforcement, meaning it only starts to help resist tension once the concrete cracks and the rebar stretches.
• 😀 Some applications, such as bridges, may require prestressed concrete, where the reinforcement is put under tension before concrete is placed into service.
• 😀 Prestressed concrete, achieved through methods like pre-tensioning and post-tensioning, improves concrete's strength and reduces the chances of early cracking.
• 😀 The video emphasizes the complexity of reinforced concrete and encourages further exploration of the topic, inviting questions for future videos.

Q & A

What is the main weakness of concrete that is discussed in this video?

Concrete is very strong in compression but weak in tension. This weakness makes it unsuitable for use in structural members that experience tensile stress.

What are the three fundamental types of stress that materials experience?

The three fundamental types of stress are compression (pushing together), tension (pulling apart), and shear (sliding along a line or plane).

Why did the concrete cylinder fail at a much lower load in the tensile test than in the compressive test?

The concrete cylinder failed at a much lower load in the tensile test because concrete is much weaker in tension. It took only about 80 lbs (36 kilos) to break the tensile sample, which is less than 10% of its compressive strength.

What happens when a concrete beam experiences a load?

When a concrete beam is subjected to a load, the top experiences compressive stress and the bottom experiences tensile stress. The failure of the beam occurs at the bottom where the tensile stress is highest.

What is the role of reinforcement in concrete structures?

Reinforcement, such as rebar, is added to concrete to improve its tensile strength. The concrete resists compressive forces, while the reinforcement resists tensile forces, creating a composite material.

How does the addition of rebar affect the failure mode of concrete?

Rebar changes the failure mode of concrete from brittle (with no warning) to ductile, where cracks form gradually before complete failure, giving a chance to address the issue before catastrophe occurs.

What is the disadvantage of rebar in concrete reinforcement?

The main disadvantage of rebar is that it is a passive reinforcement. It only starts to resist tensile stress after the concrete cracks and stretches the rebar, which can result in excessive cracking or deflection.

What is prestressed concrete and why is it used?

Prestressed concrete involves applying a stress to the reinforcement before the concrete is placed into service. This helps the concrete resist tension and can prevent cracking, improving its overall performance in structures like bridges.

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

The two methods of prestressing concrete mentioned are: (1) putting tension on the steel reinforcement tendons as the concrete is cast, and (2) post-tensioning, where stress is applied to the reinforcement after the concrete has cured.

Why does post-tensioning result in a stronger concrete beam compared to conventional reinforcement?

Post-tensioning increases the strength of the concrete beam by applying tension to the reinforcement after the concrete cures. This compressive stress from the tensioned rods helps prevent cracking and delays failure.