9.13 - Chemical Attack on Concrete
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Sulfate Attack
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Today, we start with the concept of sulfate attack on concrete. Can anyone tell me where sulfates typically come from?
I think they can come from groundwater or sewage?
Exactly! Groundwater, sewage, and even industrial waste can contain sulfates. When they interact with concrete, they can lead to the formation of expansive products like ettringite and gypsum, which can cause cracking. Let's talk about the two types of sulfate attack, can anyone describe them?
There's the external sulfate attack and the internal sulfate attack, right?
That's correct! External sulfate attack occurs when sulfates in the environment react with the concrete, while internal sulfate attack happens within the concrete itself. To prevent these attacks, we can use sulfate-resistant cement (SRC) and ensure proper compaction. Remember, SRC can be a key acronym to recall—Sulfate-Resistant Cement!
What about permeability? Does that play a role?
Great question! Yes, reducing permeability is vital to protect concrete from sulfate penetration. Anyone else have questions about sulfate attacks?
Just to clarify, what can we do to help with compaction?
Use proper curing techniques and pay attention during mixing. Alright, to summarize, sulfate attacks come from various sources, form harmful products, and can be prevented using SRC, reducing permeability, and ensuring good compaction.
Acid Attack
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Next, we will explore acid attacks. What happens to concrete when acids come into contact with it?
The acids react with the concrete, right? Like with calcium hydroxide?
Exactly! Acids like sulfuric acid and hydrochloric acid react with calcium hydroxide and the calcium-silicate-hydrate (C-S-H) gel, resulting in soluble calcium salts that weaken the concrete. Can anyone tell me what signs of acid attack might look like?
I think it can lead to surface erosion or loss of mass?
Spot on! Surface erosion and mass loss are key symptoms. To protect against this, we can apply protective coatings or use pozzolans to reduce free lime content. Anyone know how silica fume could help?
It should improve the acid resistance of the mix, right?
Correct! Silica fume and fly ash can indeed enhance the resistance of concrete against acid attacks. To recap, acid attacks occur due to the reaction with acids leading to significant mass loss, but protective measures can vastly improve resilience.
Alkali-Aggregate Reaction
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Finally, let's discuss alkali-aggregate reactions. Who can explain what happens during an alkali-silica reaction?
I believe it's when alkalis in cement react with reactive silica in aggregates, leading to expansion and cracking?
Exactly right! This is one of the most common types of alkali-aggregate reactions. It's important to understand control measures. Can anyone suggest ways to mitigate this reaction?
We could use non-reactive aggregates?
Great answer! Using non-reactive aggregates is one solution. Additionally, using lithium salts or pozzolanic materials, and opting for low-alkali cement can help control this reaction. Now, how would you summarize today’s topics on chemical attacks?
We learned about sulfate attacks, acid attacks, and alkali-aggregate reactions, along with prevention strategies for each.
Perfect summary! Understanding these chemical attacks allows us to enhance the durability of concrete and ensure it stands the test of time.
Introduction & Overview
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Quick Overview
Standard
Chemical attacks pose significant threats to the integrity of concrete structures. This section outlines the different types of chemical attacks—sulfate attacks, acid attacks, and alkali-aggregate reactions—along with their mechanisms, effects, and prevention strategies to enhance concrete durability.
Detailed
Chemical Attack on Concrete
Concrete is vulnerable to various aggressive chemical environments that deteriorate its matrix and reduce its durability. This section describes several types of chemical attacks:
9.13.1 Sulfate Attack
- Sources: Groundwater, sewage, industrial waste.
- Effect: Forms expansive products like ettringite and gypsum, causing cracking and expansion.
- Types:
- External Sulfate Attack (ESA)
- Internal Sulfate Attack (ISA)
- Prevention:
- Use sulfate-resistant cement (SRC)
- Reduce permeability
- Adequate cover and compaction
9.13.2 Acid Attack
- Mechanism: Acids react with calcium hydroxide and C-S-H gel, forming soluble calcium salts.
- Examples: H₂SO₄, HCl, HNO₃, acetic acid.
- Symptoms: Surface erosion, loss of mass, exposure of aggregates.
- Protection:
- Protective coatings
- Use of pozzolans to reduce free lime content
- Silica fume and fly ash improve acid resistance
9.13.3 Alkali-Aggregate Reaction (AAR)
- Includes
- Alkali-Silica Reaction (ASR): most common
- Alkali-Carbonate Reaction (ACR)
- Effect: Expansion and cracking due to reactive aggregates.
- Control Measures:
- Use of non-reactive aggregates
- Use of lithium salts or pozzolanic materials
- Low-alkali cement
Understanding these chemical attacks and their preventive measures is crucial to ensuring the longevity and safety of concrete structures.
Audio Book
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Introduction to Chemical Attack
Chapter 1 of 4
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Chapter Content
Concrete is vulnerable to various aggressive chemical environments that deteriorate its matrix and reduce its durability.
Detailed Explanation
Concrete is a strong material, but it can be damaged by certain chemicals found in the environment. When these aggressive chemicals come in contact with concrete, they can react with the materials in the concrete and cause degradation. This process weakens the concrete structure, making it less durable over time.
Examples & Analogies
Think of concrete as a protective shell around a fragile treasure. If you expose that shell to corrosive liquids, like acid or saltwater, it can start to break down and compromise the treasure inside. Just like how a shell might crack or degrade, concrete can suffer similar fates when subjected to harmful chemicals.
Sulfate Attack
Chapter 2 of 4
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Chapter Content
9.13.1 Sulfate Attack
- Sources: Groundwater, sewage, industrial waste.
- Effect: Formation of expansive products like ettringite and gypsum causing cracking and expansion.
- Types:
- External Sulfate Attack (ESA)
- Internal Sulfate Attack (ISA)
- Prevention:
- Use sulfate-resistant cement (SRC)
- Reduce permeability
- Adequate cover and compaction
Detailed Explanation
Sulfate attack occurs when sulfates in the environment, like those found in groundwater or industrial waste, penetrate concrete. When these sulfates react with the components of concrete, they form new compounds such as ettringite and gypsum, which can expand. This expansion creates internal pressures that lead to cracks and structural damage. There are two types of sulfate attack: external, where sulfates come from outside, and internal, where sulfates are generated inside due to certain aggregates. To prevent sulfate attack, it's essential to use sulfate-resistant cement and ensure a tightly packed, low-permeability concrete mix.
Examples & Analogies
Imagine putting a sponge into saltwater. Over time, the salt crystals might expand, causing the sponge to crack and break apart. Similarly, when sulfate-containing water enters concrete, it causes compounds to expand and ultimately disrupt the integrity of the concrete structure.
Acid Attack
Chapter 3 of 4
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Chapter Content
9.13.2 Acid Attack
- Mechanism: Acids react with calcium hydroxide and C-S-H gel, forming soluble calcium salts.
- Examples: H₂SO₄, HCl, HNO₃, acetic acid.
- Symptoms: Surface erosion, loss of mass, exposure of aggregates.
- Protection:
- Protective coatings
- Use of pozzolans to reduce free lime content
- Silica fume and fly ash improve acid resistance
Detailed Explanation
Acids can significantly harm concrete by reacting with its components, particularly calcium hydroxide, which is a major part of the concrete's structure. This reaction produces soluble salts, leading to surface erosion and loss of concrete mass. For instance, strong acids like sulfuric acid or hydrochloric acid can make the surface of concrete look pitted and worn down, while exposing the aggregates within. To protect concrete from acid attacks, special coatings can be applied, and using supplementary materials like silica fume or fly ash can also help improve resistance.
Examples & Analogies
Consider how lemon juice, which is quite acidic, might erode a metal surface if left on for too long. In the same way, when acids in the environment interact with concrete, they can wear it down, leading to structural damage over time.
Alkali-Aggregate Reaction (AAR)
Chapter 4 of 4
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Chapter Content
9.13.3 Alkali-Aggregate Reaction (AAR)
- Includes:
- Alkali-Silica Reaction (ASR): most common
- Alkali-Carbonate Reaction (ACR)
- Effect: Expansion and cracking due to reactive aggregates.
- Control Measures:
- Use of non-reactive aggregates
- Use of lithium salts or pozzolanic materials
- Low-alkali cement
Detailed Explanation
Alkali-Aggregate Reaction occurs when alkalis in cement react with certain reactive types of silica and carbonate found in some aggregates. This reaction can result in significant expansion within the concrete, leading to cracks. The most common type of this reaction is the Alkali-Silica Reaction (ASR). To mitigate these reactions, it's important to use aggregates that are known to be non-reactive, or to incorporate lithium salts or other pozzolanic materials that can counteract these reactions, along with using low-alkali cement.
Examples & Analogies
Imagine a sponge that expands when exposed to water. If that sponge were confined in a tight space, its expansion might lead to cracks in the walls of that space. Similarly, when reactive aggregates in concrete swell due to alkali reactions, they can create stress within the concrete structure, resulting in visible cracking.
Key Concepts
-
Sulfate Attack: Chemical attack that expands and cracks concrete through the formation of sulfate products.
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Acid Attack: Deterioration of concrete caused by acids, leading to mass loss and erosion.
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Alkali-Aggregate Reaction: Reaction of concrete aggregates with alkali substances, causing expansion and cracks.
Examples & Applications
A concrete bridge subjected to groundwater with high sulfate content suffers from cracking and spalling due to sulfate attack.
A sewage treatment facility experiences acid attack, leading to visible surface erosion of concrete channels and structures.
Memory Aids
Interactive tools to help you remember key concepts
Rhymes
Sulfates in the ground can make concrete sound, but with SRC, it won't break down.
Stories
Imagine a brave hero, Sulfate Slayer, protecting a castle made of concrete from the evil army of acidic invaders and alkali gremlins with special armor and magical pozzolans.
Memory Tools
S.A.A. - Sulfate, Acid, and Alkali - remember the three types of chemical attacks.
Acronyms
P.A.A.C. - Prevention Against Acid Caustics, identifying protective methods against acid attacks.
Flash Cards
Glossary
- Sulfate Attack
Chemical deterioration of concrete due to reaction with sulfates in the environment, leading to expansion and cracking.
- Acid Attack
Degradation of concrete caused by acidic substances reacting with the cement matrix, typically resulting in loss of mass and surface erosion.
- AlkaliAggregate Reaction (AAR)
Chemical reaction between alkalis in cement and reactive aggregates, leading to swelling and cracking of concrete.
- Ettringite
A crystalline compound formed in concrete due to sulfate attack, which causes expansion.
- CSH Gel
Calcium-Silicate-Hydrate gel, the primary binding agent in hydrated cement paste, crucial for concrete strength.
- Pozzolans
Materials that, when mixed with lime and water, form compounds with cementitious properties, used to enhance concrete durability.
Reference links
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