13 - Formwork Considerations Related to Fresh Concrete
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Pressure Exerted by Fresh Concrete
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Today, we will talk about how fresh concrete behaves like a fluid. This means it exerts lateral pressure on the formwork, depending on several factors. What do you think those factors might be?
Maybe the height of the concrete?
Correct! The height of the concrete plays a significant role in the pressure exerted. Any other factors?
The temperature of the concrete?
Absolutely! Temperature can affect the viscosity of the mix and consequently the pressure that the formwork must withstand.
So, what happens if the formwork can’t handle that pressure?
Good question! If the formwork fails, it can lead to bulging or even collapse, which may compromise the structural integrity of the concrete.
To remember this concept, think 'PHT'— Pressure, Height, Temperature. These are key factors in understanding the behavior of fresh concrete.
So, it's important to manage those factors to ensure the formwork is safe?
Exactly! Always consider these factors when designing or working with formwork.
To sum up, fresh concrete behaves like a fluid that exerts variable pressure based on height and temperature, and formwork must be designed with these pressures in mind.
Quality of Formwork
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Next, let’s discuss the essential qualities of formwork for fresh concrete. Why do you think it's crucial for formwork to be watertight?
To prevent leakage of the concrete, right?
Exactly! Watertight formwork prevents the loss of materials and maintains workability. What about the strength of the formwork?
It needs to hold the pressure without bending or breaking.
Yes, and if it fails, we may have significant structural issues. Furthermore, why should formwork not absorb water from the concrete?
Because it would reduce the workability of the mix?
Precisely! If the formwork absorbs too much water, it can negatively impact the mix, resulting in a less durable final product.
Let’s use the acronym 'WSR' for 'Watertight, Strong, Resistant' to remember these qualities. Can anyone summarize why these qualities matter?
They help keep the concrete intact and ensure the right properties when it sets!
Exactly! In summary, proper formwork must be watertight, strong, and not absorb water to ensure the integrity and performance of fresh concrete.
Introduction & Overview
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Quick Overview
Standard
The section outlines how fresh concrete acts under pressure, emphasizing that it behaves like a fluid. Key considerations include ensuring that formwork is watertight, strong, and resistant to absorption and deformation during the concrete placement process.
Detailed
Formwork Considerations Related to Fresh Concrete
13.1 Pressure Exerted by Fresh Concrete
Fresh concrete behaves like a fluid and exerts lateral pressure on formwork. This pressure is not uniform and reaches its maximum at the base of the forms. Important factors influencing this pressure include the rate of placing, the height of fresh concrete, and its temperature. Understanding this pressure is essential for designing formwork that can withstand these forces during the initial setting phase.
13.2 Formwork Quality
The quality of formwork is pivotal when dealing with fresh concrete. It must be constructed to be watertight and strong enough to resist the inherent pressure without buckling or bulging. Additionally, formwork must not absorb water from the fresh concrete mix, as this absorption can lead to a loss of workability, adversely affecting the final outcome of the concrete. Proper formwork practices are integral to the successful placement of concrete and achieve high-quality structural outcomes.
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Pressure Exerted by Fresh Concrete
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Chapter Content
• Acts like a fluid; lateral pressure is maximum at the base.
• Depends on rate of placing, height, and temperature.
Detailed Explanation
Fresh concrete behaves similarly to a fluid when it is being poured into formwork. This means that as concrete is placed, it exerts pressure on the sides of the formwork. The most significant pressure occurs at the bottom of the form because the weight of the concrete above creates this force. Additionally, various factors influence how much pressure is applied, including how quickly the concrete is poured (rate of placing), the height of the poured concrete, and the surrounding temperature, which can impact the concrete's consistency.
Examples & Analogies
Imagine filling a large bucket with water. The water pushes against the sides of the bucket, but the pressure is greatest at the bottom because of the water's weight above. Similarly, when pouring concrete into a form, the height of the concrete increases pressure at the base like water in the bucket.
Formwork Quality
Chapter 2 of 2
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Chapter Content
• Should be watertight and strong.
• Should resist bulging under fresh concrete pressure.
• Must not absorb water from concrete (causes loss of workability).
Detailed Explanation
High-quality formwork is essential for supporting the fresh concrete during the curing process. It needs to be watertight to prevent any water loss from the concrete, which can result in poor workability and compromised strength. The formwork should also be robust enough to withstand the pressure exerted by the concrete without bulging or collapsing. Absorbing water is another critical aspect; if the formwork takes in moisture from the concrete, it can hinder the hydration process necessary for the concrete to set properly.
Examples & Analogies
Think of making a cake in a mold. If the mold (formwork) is sturdy and watertight, the cake will hold its shape well and bake evenly. However, if the mold has holes or is weak, the batter could leak out or spill, making a mess and resulting in a poorly shaped cake. Similarly, good formwork ensures that the concrete sets properly and maintains its intended shape.
Key Concepts
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Fresh Concrete Pressure: Fresh concrete exerts lateral pressure on formwork, which varies with height and temperature.
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Formwork Quality: Quality must be watertight, strong, and non-absorbent to maintain concrete integrity.
Examples & Applications
When wet concrete is placed in tall forms, the base experiences maximum pressure due to its height, which requires more robust support.
Using water-resistant coatings on formwork can prevent absorption, ensuring that the concrete maintains its intended workability.
Memory Aids
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Rhymes
Pressure up high, keep it dry, watertight to comply.
Stories
Imagine a tall concrete wall, where each layer adds pressure. The wise builder ensures the formwork is strong and watertight, preventing leaks and bulging—not unlike how a dam holds back water.
Memory Tools
Remember 'WSR' for Watertight, Strong, and Resistant qualities of formwork.
Acronyms
Formwork qualities can be remembered with the acronym 'WSR'
Watertight
Strong
and Resistant.
Flash Cards
Glossary
- Lateral Pressure
The pressure exerted horizontally by fresh concrete against formwork.
- Watertight Formwork
Formwork that does not allow water to penetrate, preserving the mix's workability.
- Bulging
The outward deformation of formwork caused by excessive lateral pressure from fresh concrete.
- Fresh Concrete
Concrete that is in its plastic state and has not yet set or hardened.
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