What is Alkali Aggregate Reaction

Alkali-Aggregate Reaction (AAR), also known as Alkali-Silica Reaction (ASR) or Alkali-Carbonate Reaction (ACR), is a chemical reaction that occurs in concrete structures.

It involves the interaction between certain components in the concrete mix:

Alkalis: Alkalis are compounds found in cement, primarily sodium oxide (Na2O) and potassium oxide (K2O).

Aggregates: Aggregates are the solid materials like sand, gravel, or crushed stone used in concrete.

In the context of AAR:

Alkalis in the cement react with reactive components in the aggregates. These reactive components can be silica (in ASR) or carbonates (in ACR) found in some aggregates.

The result of this reaction is the formation of expansive products, such as alkali-silica gel or alkali-carbonate gel. These products can absorb water and expand over time, causing the concrete to crack and potentially leading to structural problems.

Which one is more common, ASR or ACR?

Alkali-Silica Reaction (ASR) is generally more common and widely recognized compared to Alkali-Carbonate Reaction (ACR) in concrete structures. ASR occurs when the reactive silica in certain aggregates reacts with the alkalis (sodium and potassium) present in the cement, leading to the formation of expansive alkali-silica gel. ASR has been extensively studied and documented in the construction industry.

On the other hand, ACR, which involves the reaction between alkalis in the cement and certain types of carbonate rocks or aggregates containing reactive carbonates, is less common and often less understood in comparison to ASR. ACR is more region-specific and typically associated with specific types of aggregates.

Understanding the Alkali-Aggregate Reaction Mechanism

Alkali-Aggregate Reaction (AAR), also known as Alkali-Silica Reactivity (ASR), is a chemical reaction that occurs between the active silica components in aggregates and alkalis, specifically Na2O and K2O, present in cement.

The forms of reactive silica are typically found in aggregates sourced from materials like traps, opaline or chalcedonic cherts, andestite and andesite tuffs, rhyolites and rhylotic tuffs, siliceous limestones, and certain types of sandstones.

When the right conditions are met, this reaction leads to the formation of expansive alkali-silicate gels. These gels can cause concrete to swell, resulting in the development of distinctive pattern cracks and, ultimately, the failure of concrete structures.

It’s important to note that not all aggregates exhibit this reaction; it primarily depends on the presence of reactive silica in a specific proportion and fineness.

This reaction occurs when the soluble alkalies in cement dissolve in the mixing water, transforming it into a highly caustic liquid. This caustic liquid then interacts with the reactive silica found in certain aggregates.

The result?

The formation of a highly expansive substance known as alkali-silica gel.

This gel has a significant impact as it alters the boundaries of the aggregate itself. The expansion continues because the gel keeps growing due to a constant supply of water and the right temperature conditions.

Now, why is this expansion a concern?

As the silica gel grows, it exerts internal hydraulic pressure on the surrounding set-cement gel. This pressure is generated through a process called osmosis. This pressure can lead to pattern cracking, especially in thinner concrete sections like pavements. These cracks, in turn, can lead to a loss of strength and elasticity in the affected concrete.

But that’s not all. The formation of cracks due to the alkali-aggregate reaction can also accelerate other deterioration processes, such as carbonation.

In essence, it’s a domino effect, where one problem can lead to others if not managed properly.

Understanding this mechanism is crucial for engineers and builders to prevent and address AAR effectively, ensuring the long-term stability of concrete structures.

Several factors contribute to the promotion of alkali-aggregate reactions:

1. Reactivity of the Aggregate:

The potential reactivity of an aggregate can be assessed through petrographic examination of thin rock sections.

Methods such as the mortar bar expansion test and chemical tests, as described in IS: 2386 (Part-VII)–1963, help determine an aggregate’s potential reactivity.

The reactivity of the aggregate is influenced by factors like particle size and porosity, as they affect the surface area available for the reaction.

2. Alkali Content in Cement:

The total alkali content in cement is typically expressed as Na2O equivalent (Na2O + 0.658 K2O).

Many specifications limit the alkali content to less than 0.6, designating such cement as low-alkali cement.

The degree of expansion due to the reaction is also influenced by the fineness of the cement.

3. Availability of Moisture:

The progress of the alkali-aggregate reaction depends on the presence of non-evaporable water in the cement paste.

The reaction and subsequent deterioration are more prominent on the concrete’s surface and less significant in its interior.

Applying waterproofing agents to the concrete surface can reduce deterioration by preventing additional water penetration.

The reaction is accelerated in conditions involving alternating wetting and drying.

4. Temperature Conditions:

The ideal temperature range for promoting alkali-aggregate reactions is typically between 10°C to 40°C.

In summary, the alkali-aggregate reaction is a complex chemical process that can impact the durability of concrete structures. Understanding its causes and factors that contribute to its progression is crucial for ensuring the long-term stability of construction projects.

Controlling Alkali-Aggregate Reaction (AAR)

The management of AAR involves several strategies:

1. Avoiding Reactive Aggregates:

The most straightforward approach is to avoid using aggregates known for their reactivity.

However, in cases where it’s economically challenging to avoid such aggregates, other measures can be taken.

2. Low-Alkali Cement:

Using low-alkali cement with an alkali content of less than 0.6%, or even lower at 0.4%, can reduce the likelihood of AAR. This type of cement is effective in mitigating the reaction’s impact.

3. Air-Entraining Agents:

To counteract the osmotic pressure caused by the formation of expansive silica-gel in AAR, air-entraining agents can be employed. These agents help absorb the pressure.

4. Controlled Moisture Availability:

Since continuous moisture is essential for AAR, managing the availability of water is crucial. Preventing excessive water contact can help control the reaction.

5. Optimum Temperature:

Ensuring that the temperature remains within an optimal range can also influence the progress of AAR.

6. Using Pozzolanic Additives:

One effective method for controlling AAR involves making aggregates less reactive by disrupting the ideal conditions for silica’s specific proportion and fineness.

This is achieved by incorporating pozzolanic additives like crushed stone dust, diatomaceous earth, surkhi, fly ash, etc., into the mix. These additives inhibit AAR effectively and offer a practical solution.

7. Adding Reactive Silica:

Another approach to reduce expansion due to AAR is by adding fine reactive silica in powdered form to the concrete mix.

This increases the surface area, altering the calcium hydroxide-alkali ratio at the aggregate boundaries. Consequently, this can lead to the formation of a non-expanding calcium-alkali-silicate product.

A common guideline suggests adding 20 grams of reactive silica for each gram of alkali in excess of 0.5% of the cement’s mass.

Incorporating these control measures helps ensure the structural integrity of concrete in the presence of AAR, making construction projects more durable and dependable.