Here is a concise summary of the provided text:
**Title:** The Importance of Reinforcement in Concrete
**Summary:**
* Concrete's greatest weakness is its low tensile strength, making it prone to cracking and failure under tension.
* Demonstrations show:
1. Unreinforced concrete cylinders fail under relatively low tensile stress (~80 lbs) compared to compressive stress (~1000 lbs).
2. A homemade concrete beam fails instantly under load due to tensile stress at the bottom.
* Adding reinforcement (e.g., rebar) improves tensile strength and changes the failure mode from brittle to ductile.
* Two types of reinforcement are discussed:
1. **Passive Reinforcement** (e.g., traditional rebar): effective but can allow cracking before taking up tensile stress.
2. **Active Reinforcement** (e.g., pre-stressed or post-tensioned concrete): applies stress to the reinforcement before concrete is in service, reducing cracking and deflection.
**Key Takeaway:** Reinforcement is crucial for concrete structures to add tensile strength and prevent sudden, brittle failures.
Here are the key facts extracted from the text, keeping each fact a short sentence and avoiding opinions:
1. **Concrete's strength varies by stress type**: Concrete is very strong in compression but weak in tension.
2. **Concrete's compressive strength can be measured**: A concrete cylinder broke at approximately 1,000 pounds (450 kg) of compressive load in a test.
3. **Concrete's tensile strength is relatively low**: A concrete cylinder broke at approximately 80 pounds (36 kg) of tensile load in a test, <10% of its compressive strength.
4. **Structural members rarely experience only one type of stress**: Most structures experience a mixture of compressive, tensile, and shear stresses.
5. **The classic beam demonstrates mixed stress**: The top of a beam experiences compressive stress, while the bottom experiences tensile stress when a force is applied.
6. **Unreinforced concrete beams can fail suddenly**: Failure often occurs with little warning, characterized as a brittle mode.
7. **Adding reinforcement changes the failure mode**: Reinforced concrete can exhibit a ductile mode, where cracks form before complete failure, providing warning.
8. **Common reinforcement materials include deformed steel (rebar)**: Rebar provides strength against tensile stress, complementing concrete's compressive strength.
9. **Rebar is a passive reinforcement**: It only begins to resist tension after it has stretched, often after the concrete has cracked.
10. **Pre-stressing (pre-stress concrete) is an alternative method**: This involves applying stress to the reinforcement before the concrete is in service.
11. **Methods of pre-stressing include**:
* **Pre-tensioning**: Applying tension to steel reinforcement tendons as concrete is cast.
* **Post-tensioning**: Developing stress in reinforcement after the concrete has cured.
12. **Many concrete bridge beams are pre-stressed**: This method is commonly used for such structures.
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