Alkali Silica Reaction Concrete: Prevention Strategies

If you’re investigating ‘alkali silica reaction concrete,’ you are likely facing questions on how to prevent or address this pervasive problem impacting concrete durability. ASR triggers expansion and cracks in concrete when alkalis react with silica, a common aggregate component. This article sets out to demystify ASR, equipping you with practical knowledge on identifying risks, spotting symptoms, and implementing the most effective prevention and repair methods.

Key Takeaways

ASR is a detrimental chemical reaction in concrete initiated by alkalis reacting with reactive siliceous aggregate in moist conditions, leading to expansion and cracking. Knowing the contributing factors and symptoms is critical for management and prevention.
Preventive strategies to inhibit ASR include using nonreactive aggregates, controlling cement alkali content, employing Supplementary Cementitious Materials (SCMs), and reducing the water-to-cement ratio to lower permeability and mitigate gel expansion.
Managing ASR in existing structures involves structural repairs, moisture control, and nationwide and international specifications and standards to guide the prevention and control of ASR, including performance-based and prescriptive approaches.

Understanding Alkali-Silica Reaction Concrete

ASR is a deleterious chemical reaction that occurs between alkalis in cement and reactive silica present in the siliceous aggregates used, culminating in expansion and cracking in hardened concrete. The main catalysts for ASR are sodium hydroxide (NaOH) and potassium hydroxide (KOH), formed from the dissolution of Na2O and K2O oxides during the pyrolysis of cement kiln. ASR occurs under specific conditions – a high concentration of hydroxyl ions in the concrete pore solution, reactive siliceous aggregate, and moist conditions are needed for this destructive reaction to take place. In some cases, alkali carbonate reactions can also contribute to the degradation of concrete structures.

The aftermath of the alkali silica reaction concrete is the formation of an alkali-silica gel, a substance that swells upon moisture absorption, leading to volume expansion and resultant cracking in concrete. Contributors to the potential for ASR include soluble sodium and potassium salts that indirectly source calcium hydroxide ions in the hardened cement paste pore water. Acknowledging these key factors is a stepping stone towards developing effective prevention strategies.

Keep Reading: ASR Testing Concrete: Everything You Need to Know.

Reactive Aggregates

Reactive aggregates are essential players in the initiation of alkali aggregate reactivity (alkali silica reaction concrete). These are types of aggregates that contain minerals capable of reacting with alkali hydroxides in concrete. Some examples of natural reactive aggregates include:

Chert
Strained quartz found in sands
Some quartzites
Graywackes
Recycled waste glass

Certain types of andesite containing reactive phases like opal, cristobalite, and volcanic glass are highly reactive aggregates that are particularly prone to causing ASR. Comprehending the characteristics of these reactive aggregates is pivotal in preventing ASR. Identifying these aggregates and their reactivity potential enables us to manage their use in concrete production proactively, thus reducing the risk of ASR.

Factors Influencing Alkali Silica Reaction Concrete

The degree and rate of alkali silica reaction concrete in concrete are primarily controlled by factors such as the total alkali content in cement, the quantity of reactive silica in aggregates, and the alkali hydroxide concentration in the concrete pore solutions. Temperature also plays a significant role in influencing ASR. At higher temperatures, the solubility of ettringite, a mineral often found in cement, increases. This raises the sulfate ion concentration in the pore solution and alters the pH, subsequently impacting ASR expansion rates.

Interestingly, the pessimum temperature for ASR-related expansion is about 40°C. This is a critical point where the heightening of the thermal environment may significantly exacerbate the expansion behavior. Therefore, thorough understanding of these influencing factors is essential in preventing and managing ASR in concrete.

Symptoms and Detection of ASR

Early detection of ASR plays a significant role in curbing its impacts. Some visible signs of this reaction include:

Irregular surface cracking, often accompanied by a gel material seeping from the cracks
Internal swelling of concrete, resulting in characteristic cracking
Expansion, surface pop-outs, and a mix of cracking and localized crushing of concrete
Surface discoloration and gel deposition

By recognizing these field symptoms, you can identify and address ASR in its early stages.

Detecting ASR can be achieved through a combination of laboratory tests, field inspections, and petrographic analysis. Some methods for detecting ASR in concrete include:

Laboratory tests such as ASTM C1778 and AASHTO R 80
Petrographic analysis of concrete samples
Using reagents like those in Gilson’s ASR Detect Kit, which allow for the detection of initial or advanced stages of ASR by color staining following a reaction completed in less than five minutes, with permanent staining as a result

It should be noted that the manifestation of ASR activity in concrete may take years, and early signs of expansion and cracking don’t always indicate compromised structural integrity.

Laboratory Tests

When it comes to laboratory tests, ASTM C 1260 and ASTM C 1293 are the standardized test methods for assessing the alkali-silica reactivity (ASR) potential in aggregates and concrete mixes. ASTM C1778-22, on the other hand, provides a standardized procedure for identifying potentially alkali-silica reactive (ASR) aggregates and assessing potential alkali silica reactivity. These tests utilize standard laboratory equipment and share similar equipment setups and procedures. The testing durations can vary, with some tests, excluding sample preparation and reporting, taking 16 or 30 days to complete.

An innovative method proposed to improve ASR testing accuracy is the Alkali-Wrapped Concrete Prism Test. This standard test method maintains a constant alkali content and moisture level, offering better insights into the effects of temperature on ASR expansion.

Field Inspection

Field inspection is another vital component in detecting and managing ASR. Concrete that is directly exposed to moisture is more likely to display severe symptoms of ASR. Therefore, conducting regular field inspections, especially in moist environments, is crucial. However, it is essential to distinguish non-ASR-related forms of distress from those caused by ASR when surveying the condition of concrete structures.

A well-trained eye can discern the subtle signs of ASR and distinguish them from other forms of concrete deterioration.

Prevention Strategies for ASR

With a clear grasp of ASR’s characteristics, we can formulate strategies to prevent its development. These strategies include avoiding reactive aggregates, controlling cement alkali content, or restricting water – the three elements of what is known as the critical triangle for ASR. The use of Supplementary Cementitious Materials (SCMs) such as fly ash, silica fume, and slag may reduce ASR by lowering the alkali content and altering pore solution alkalinity. SCMs are versatile in concrete mix design, with the ability to replace varying levels of cement, which can be tailored based on specific performance requirements.

Using a lower water-to-cement ratio in concrete mixes can reduce the permeability of the hardened concrete, consequently limiting water and alkali movement and minimizing the gel expansion characteristic of ASR. The utilization of recycled aggregates, especially in optimal blends, provides a sustainable alternative to natural aggregates, contributing to resource conservation and possibly enhancing the concrete’s durability.

Nonreactive Aggregates

Nonreactive aggregates, due to their lack of reactive silica, play a central role in preventing ASR. The use of these aggregates directly addresses one of the three necessary components for ASR occurrence, thus being a key aspect of proactive ASR prevention. Selecting nonreactive aggregates when designing new concrete structures is recognized as the most common method to minimize the risk of expansion due to ASR.

Securing a supply of nonreactive aggregates might require comprehensive testing of materials and procure from quarries with a history of nonreactive materials.

Supplementary Cementitious Materials

Supplementary cementitious materials (SCMs) can mitigate the effects of ASR in concrete. Using fly ash and blast furnace slag as partial substitutes for Portland cement can reduce the concrete’s permeability and the severity of ASR. When nonreactive aggregates are not available, using a blend of reactive and nonreactive aggregates along with SCMs can help reduce the risk of ASR.

Alternatives to low alkali cement for controlling ASR include employing SCMs or surface sealers to manage moisture ingress.

Low Alkali Cement

To prevent ASR, builders use low alkali cement with less than 0.60% equivalent Na2O in some concrete mixtures. However, highly reactive aggregates can still cause ASR, affecting the surrounding cement paste despite using low alkali cement.

Moreover, adhering to environmental regulations has resulted in inherently higher alkali content in cement, potentially undermining the effectiveness of low alkali cement as a long-term ASR mitigation strategy.

Managing Alkali Silica Reaction Concrete in Existing Structures

Even with prevention strategies in place, ASR can still occur in existing structures. ASR cracking provides pathways for water and chemicals, promoting corrosion of reinforcing steel and shortening the useful service life of the concrete structure itself. After ASR damage has occurred, the affected concrete becomes more susceptible to other deterioration processes, such as steel corrosion and freeze-thaw damage.

Damaged sections of concrete due to ASR can be repaired, and measures can be taken to slow the ingress of water, even though the ASR process may continue. Some possible solutions include:

Removing and replacing the affected concrete
Applying a surface treatment to slow down the reaction
Installing a waterproofing system to prevent water penetration

However, it is important to note that the repair and replacement of structures affected by ASR represent significant economic burdens.

Structural Repairs

Structural repairs have a significant impact on managing ASR in existing structures. One effective repair method involves the removal of deteriorated concrete until sound concrete is reached, followed by pinning and recasting. Applying cement-based patches over areas affected by ASR can help to decelerate the deterioration process, although it will not halt it entirely.

Strategic periodic realignment of anchors can manage the slow ASR-induced expansion in concrete structures, thereby helping to maintain their structural strength. Additionally, renewing closed joints by saw cutting to reopen them can be a viable repair technique when dealing with very slow ASR-induced expansion and when the concrete’s structural integrity is not compromised.

Moisture Control

Controlling moisture is another crucial element in managing ASR in present structures. Applying waterproofing membranes and wrapping concrete elements can control the ingress of water and mitigate ASR progression. Coating concrete with sealers can maintain the internal relative humidity below 80%, effectively stopping further gel growth and ASR.

Regular cleaning of concrete structures can prevent ASR by eliminating salts that could otherwise dissolve and penetrate the concrete, thereby reducing the risk of ASR development. Some strategies for preventing ASR in concrete structures include:

Regularly cleaning the concrete to remove salts
Addressing moisture issues in massive structures like dams
Drying components before installation
Installing a watertight membrane to slow down or stop ASR development

By implementing these strategies, you can effectively prevent ASR in your existing concrete structures here.

National and International Specifications

National and international specifications provide guidelines for preventing and managing ASR in concrete. ASTM C1778-22 is an international standard that guides the reduction of the risk of deleterious Alkali-Aggregate Reaction (AAR) in concrete. This standard provides recommendations for identifying the potential for deleterious AAR and selecting preventive measures based on prescriptive-based or performance approaches.

The PARTNER Project, funded by the European Community, aimed to unify testing procedures to evaluate the potential alkali-reactivity of aggregates across European regions. Outcomes of the PARTNER Project were intended to be implemented by CEN, the European Committee for Standardization, in the form of new standard test methods and specifications.

In addition to these standard test methods, there is a guide and test method – ASTM C295/C295M – for petrographic examination of aggregates and two practices, AASHTO R 80 and ASTM C1778, for determining the reactivity of concrete aggregates and selecting appropriate preventive measures.

Performance-based Specifications

Performance-based testing in standard structures seeks to reduce ASR-related expansion risks to tolerable levels. The PARTNER Project, a significant initiative in this regard, produced a state-of-art report compiling existing national standards and requirements from European countries that aim to prevent alkali aggregate reaction (AAR).

Prescriptive Specifications

ASTM C1778-22 prescribes concrete material usage, crucial in mitigating alkali-silica reaction (ASR) risks. ASTM C1778-22 outlines precautions for using potentially reactive aggregates in concrete. It mandates preventive measures for their incorporation.

The prescriptive approach mandates the selection of appropriate mitigation measures for ASR aggregates to effectively minimize the likelihood of concrete expansion.

Summary

Understanding and managing ASR in concrete requires knowledge of chemical interactions. Use nonreactive aggregates, supplementary materials, and low-alkali cement. Also, implement structural repairs and moisture control measures in existing structures. ASR management is a multifaceted endeavor. It involves various strategies to mitigate its effects. National and international specifications provide crucial guidelines that help navigate this complex process. Knowledge and effective strategies ensure our concrete structures’ longevity. This safeguards against the significant threat of ASR. Protecting our built environment is crucial.

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Frequently Asked Questions

How do you control alkali silica reaction concrete?

Use supplementary cementitious materials like silica fume, fly ash, and slag. Prevent water access. Keep humidity below 80%. Applying a sealer like paint or a moisture barrier to the concrete surface can help achieve this.

Which minerals cause silica alkali reaction in concrete?

The minerals that cause alkali silica reaction concrete are reactive forms of silica in the aggregate, such as chert, quartzite, opal, and strained quartz crystals. This reaction occurs due to the interaction between the hydroxyl ions in the alkaline cement pore solution and the reactive silica in the various aggregate particles.

What does alkali silica reaction concrete do to cement?

Alkali in cement can cause a deleterious swelling reaction called alkali-silica reaction (ASR), resulting in expansion, cracking, and structural damage in concrete. This can lead to major problems and even require demolition.

What is the effect of alkali aggregate reaction in concrete?

Alkali-aggregate reaction (AAR) can harm concrete expansion and cracking when the alkali hydroxides in the pore solution react with certain aggregate constituents. This can compromise the structural integrity of the concrete.

How can ASR be prevented?

To prevent ASR, use nonreactive aggregates, control cement alkali content, and restrict water – these are known as the critical elements for preventing ASR.