Reinforced Concrete

What is the main reason concrete structures develop cracks shortly after construction?

• Concrete is too flexible.

• Concrete is weak in compression.

• Concrete is weak in tension.

• Concrete expands too much.

Answer explanation

Concrete is inherently weak in tension, which makes it prone to cracking shortly after construction when it experiences tensile stresses. This is why the correct answer is that concrete is weak in tension.

What happens to the bottom edge of a concrete block under tension?

• It breaks immediately.

• It stretches.

• It remains unchanged.

• It compresses.

Answer explanation

Under tension, materials like concrete can experience stretching. The bottom edge of a concrete block will elongate as the tensile forces pull it apart, making 'It stretches' the correct answer.

What is the role of steel reinforcement bars in concrete?

• To handle tensile forces.

• To prevent water absorption.

• To increase the weight of the structure.

• To make concrete more flexible.

Answer explanation

Steel reinforcement bars, or rebar, are used in concrete to handle tensile forces. Concrete is strong in compression but weak in tension, so rebar provides the necessary tensile strength to prevent cracking and structural failure.

What is the main advantage of reinforced concrete over plain concrete?

• It is more expensive.

• It requires no maintenance.

• It is lighter.

• It can bear significantly higher loads.

Answer explanation

The main advantage of reinforced concrete is its ability to bear significantly higher loads compared to plain concrete, making it ideal for structures that require enhanced strength and durability.

What is the purpose of pre-compressing concrete?

• To change its color.

• To make it more brittle.

• To reduce tensile stress when an external load is applied.

• To increase its weight.

Answer explanation

Pre-compressing concrete helps to reduce tensile stress when an external load is applied, enhancing its durability and resistance to cracking. This process improves the overall performance of the concrete under stress.

How is pre-compression achieved in concrete?

• By heating the concrete.

• By stretching the reinforcement bars before pouring concrete.

• By manually compressing the concrete.

• By adding more water to the concrete mix.

Answer explanation

Pre-compression in concrete is achieved by stretching the reinforcement bars before pouring the concrete. This process helps to counteract tensile forces, enhancing the concrete's strength and durability.

What is post-stressing in concrete?

• Compressing the concrete after it has solidified.

• Compressing the concrete before it hardens.

• Adding more cement to the mix.

• Cooling the concrete rapidly.

Answer explanation

Post-stressing involves compressing the concrete after it has solidified to enhance its load-bearing capacity and reduce tensile stresses. This technique improves the overall performance of concrete structures.

Why are wedges used in post-stressing?

• To keep the cables in tension after the hydraulic jack is removed.

• To add weight to the structure.

• To make the concrete more flexible.

• To prevent water from entering the concrete.

Answer explanation

Wedges are used in post-stressing to maintain tension in the cables after the hydraulic jack is removed, ensuring the structural integrity and performance of the concrete.

In a cantilever beam, where do tensile stresses develop?

• In the bottom fibers.

• Evenly throughout the beam.

• In the top fibers.

• In the middle fibers.

Answer explanation

In a cantilever beam, tensile stresses develop in the top fibers due to bending. The top fibers experience tension as the beam deflects downward, while the bottom fibers are under compression.

What is the main reason for using reinforcement techniques in concrete?

• To make concrete waterproof.

• To reduce the cost of construction.

• To address concrete's inherent weakness in tension.

• To make concrete more colorful.

Answer explanation

The main reason for using reinforcement techniques in concrete is to address its inherent weakness in tension. Concrete is strong in compression but weak in tension, so reinforcement helps improve its tensile strength.