Ataque por sulfato na perspectiva do projetista de concreto

Sulfate attack from the concrete designer's perspective

June 11, 2024 Roberto Magalhães

Reinforced concrete structures are widely used in civil construction due to their strength and durability. However, these designs are not immune to various forms of deterioration, and a major concern is sulfate attack. In this article we will look at sulfate attack on reinforced concrete structures, paying special attention to structural design aspects.

What is sulfate attack?

Sulfate attack is a chemical reaction that occurs when sulfates in various forms come into contact with concrete. Sulfates can be present in soil, groundwater or even building materials. When these sulfates react with concrete components, they can cause structural damage over time.

Types of Sulfate Attacks

There are two main types of sulfate attack: external sulfate attack and internal sulfate attack.

External sulfate attack

An external sulfate attack occurs when sulfates from the environment, for example from soil or groundwater, penetrate the concrete from the outside. This can happen in areas with high sulfate concentrations in the soil.

Internal sulfate attack

Internal sulfate attack, on the other hand, occurs due to the presence of sulfates in the concrete mix itself. This can happen if sulfate-rich aggregates or contaminated water are used during the construction process.

Effects of sulfate attacks on concrete

Sulfate attack can have several detrimental effects on concrete, including:

Expansion and cracking: Sulfate attack can cause concrete to expand, which in turn leads to cracking and loss of structural integrity.
Loss of strength: The reaction between sulfates and concrete components can weaken the material, reducing its load capacity.
Surface deterioration: Sulfate attack can cause peeling and chipping of the surface, making the concrete unsightly and less durable.

Chemical reactions during sulfate attack

The chemical reactions involved in sulfate attack are complex, but can be summarized as follows:

Sulfate ion reaction: Sulfate ions (SO ₄ ^2- ) in the sulfate source react with calcium ions (Ca ²⁺ ) in the hydrated cement compounds, forming calcium sulfate compounds. These compounds take up more space than the original calcium ions, causing expansion and cracking.
Formation of ettringite: In the presence of aluminum ions (Al^3+), sulfate ions can react to form a compound called ettringite (3CaO·Al). ₂ Ó ₃ ·3CaSO ₄ ·32 hours ₂ Ó). Ettringite has a larger volume than the reactants and contributes to the expansion of concrete.
CSH gel depletion: Sulfate attack can also destroy the hydrated calcium silicate gel (CSH), responsible for the strength of concrete. This gel can be attacked and decomposed by sulfate ions.

Understanding these chemical reactions is critical to developing effective strategies to prevent sulfate attacks and protect concrete structures.

Detect sulfate attack

Detecting sulfate attack in its early stages is critical to mitigating potential structural damage. While some signs are easily visible, a comprehensive assessment may require the expertise of a civil engineer or materials specialist. Here are several methods and indicators to help detect a sulfate attack:

Visual inspection: A thorough visual inspection of the concrete structure can reveal early signs of sulfate attack. Be careful:
- Cracking: Cracks in the concrete surface, especially those with an irregular pattern, can be an early indicator.
- Efflorescence: White, powdery surface deposits caused by sulfate salts are a common visual feature.
- Surface Deterioration: Inspect the concrete for chips, chips or discoloration on the surface, as this may indicate sulfate-related damage.
Core sampling: Core sampling involves drilling into the concrete and collecting cylindrical samples. These samples can then be analyzed for sulfate content and the presence of sulfate-related compounds. By collecting core samples, a more accurate assessment of internal damage is possible.
Ultrasound Examination: Ultrasonic pulse velocity (UPV) testing can determine the integrity of concrete by measuring the speed of sound waves passing through the concrete. Areas with greater sulfate damage may have slower wave speeds. However, this test cannot be used to measure sulfate penetration into concrete, but only to evaluate the condition of the concrete.
Chemical analyses: Laboratory testing of concrete samples can be used to determine the specific sulfate compounds present and their concentrations. This information will help assess the severity of the sulfate attack.
Electrical resistance test: This non-destructive test measures the electrical resistance of concrete. High strength values may indicate sulfate attack, as this often increases the strength of the concrete.
pH Monitoring: Monitor the pH of concrete pore solutions over time to detect changes that indicate sulfate attack. A significant drop in pH may indicate ongoing sulfate reactions.
Infrared thermography: Infrared thermography can be used to identify areas with temperature differences that may indicate moisture or chemical changes associated with sulfate attack.
Microscopic examination: Microscopic analysis of concrete samples can detect the formation of sulfate-related crystals, such as ettringite, within the microstructure of the concrete. Furthermore, it should be noted that the formation of crystals such as ettringite can also have other causes, such as: B. an increase in the temperature of the concrete above acceptable values during the hydration process. Commonly known as delayed ettringite formation .
Data Collection: Continuous monitoring of environmental conditions such as temperature, humidity, and sulfate concentration in soil or groundwater can provide early warning of potential sulfate attack risks.

It is important to note that early detection is essential for effective remediation. When sulfate attack reaches an advanced stage, repair becomes more difficult and expensive.

Therefore, regular inspections coupled with appropriate testing methods can help ensure the long-term durability of reinforced concrete structures in sulfate-rich environments. If in doubt, contact experienced professionals specialized in concrete analysis and structural assessment.

Precautions

To reduce the risk of sulfate attack, several preventive measures can be taken:

Correct selection of materials: Choosing sulfate-resistant materials, such as low-alkaline cement and sulfate-resistant aggregates, is essential.
Water quality: It is important to ensure that the water used to mix the concrete is free of sulfates.
Construction practices: Implementing good construction practices such as: Other measures, such as proper curing and minimizing sulfate exposure, can help prevent sulfate attack.
Design considerations such as sufficient armor coverage

Sulfate resistant materials

The incorporation of sulfate-resistant materials into concrete mixes is a fundamental strategy to protect concrete structures from sulfate attacks. These materials are specifically designed to resist the corrosive effects of sulfates, ensuring the longevity and durability of the structure. Here are some important sulfate-resistant materials and considerations:

Sulfate resistant cement:

The main component of concrete is cement, and the use of sulfate-resistant cement is the basis for preventing sulfate attacks. This type of cement contains a lower proportion of tricalcium aluminate (C ₃ A), which has high reactivity with sulfates.

Helps mitigate chemical reactions that cause expansion and cracking. Sulfate-resistant cement is designated as a code of conduct. These types of cement or cement combinations are intended to be used for construction.

Sulfate resistant aggregates:

Aggregates make up a significant proportion of concrete and the use of sulphate-resistant aggregates is equally important.

These aggregates react only slightly with sulfates and do not contribute to the expansion of concrete. To ensure its effectiveness, it is important to use aggregates that have been tested and certified as sulphate resistant.

Low alkaline cement:

Alkalis like sodium and potassium can make sulfate attack worse. Low alkali cement has a lower alkali content and is therefore more resistant to the effects of sulfates. This type of cement is suitable for use in areas where sulfate pollution is a problem.

Fly ash and slag:

Complementary cementitious materials such as fly ash and slag can improve sulfate resistance. These materials react with sulfates to form stable compounds, reducing the risk of expansion and cracking. They also contribute to improving long-term durability.

Chemical additives:

To further improve sulfate resistance, sulfate-resistant chemical additives can be added to the concrete mix.

These additives work by altering the hydration process and reducing the permeability of the concrete. Common additives include calcium nitrate and calcium thiocyanate.

Seals and coatings:

In addition to using sulfate-resistant materials in the mix, applying sealers and coatings to the concrete surface can provide an additional layer of protection.

These products create a barrier that limits the penetration of sulfates into concrete.

Plastic modified concrete:

Polymer modified concrete contains polymers in the mix that improve the concrete's resistance to various forms of degradation, including sulfate attack. These polymers improve the cohesion and impermeability of concrete.

Fiber concrete:

By adding fibers such as polypropylene or steel fibers to the concrete mix, the resistance to cracking caused by sulfate attack can be increased. These fibers improve the tensile strength of concrete and help control shrinkage cracks.

Quality control and testing:

To ensure the effectiveness of sulfate-resistant materials, quality control measures and testing protocols must be implemented during concrete production. Regular laboratory testing and adjustments to the concrete mix can help optimize sulfate resistance.

Proper construction practices:

In addition to material selection, implementing proper construction practices is crucial. Ensuring proper curing, proper setting of concrete, and minimizing exposure to sulfate-rich environments during and after construction are important aspects of preventing sulfate attacks.

By combining these sulfate-resistant materials and processes, concrete structures can resist sulfate exposure and maintain their structural integrity over time. It is important for engineers and builders to work together and consider both material selection and construction techniques to create durable concrete structures in sulfate-rich environments.

Structural design considerations

The project plays a crucial role in protecting reinforced concrete structures against sulfate attacks. Here are some important considerations:

Reinforcement Placement: Proper placement of rebar and ensuring adequate concrete coverage can prevent sulfate penetration.
Durability design: Civil engineers must consider durability principles that take sulfate exposure into account in their calculations.
Maintenance regulations: By incorporating maintenance precautions into the design, regular inspections and repairs are possible to counteract sulfate attacks as they occur.