What is Alkali Aggregate reaction?
Alkali-Aggregate Reaction (AAR) is a chemical reaction that occurs in concrete when the alkali hydroxides in cement paste come into contact with delirious minerals in the aggregates and form a gel that absorbs water and expands, leading to cracking.
The most common form is Alkali-Silica Reaction (ASR), with Alkali-Carbonate Reaction (ACR) and Alkali-Silicate Reaction being less frequent.
AAR can compromise the concrete's durability, appearance, and structural integrity over time, particularly under conditions of high moisture and alkali content.
Understanding and mitigating AAR is therefore crucial for the durability and longevity of concrete structures.
What causes Alkali Aggregate reaction on concrete structures?
The most common form of alkali aggregate reaction, ASR, occurs when aggregates containing certain forms of silica react with the concrete's alkali hydroxides, forming a gel that draws water from the surrounding cement paste. ACR, though less common, happens with certain dolomitic rocks and can also cause significant expansion.
Several factors contribute to the occurrence of AAR. First, there must be sufficient moisture since the gel formation that leads to expansion requires water. Aggregates that are alkali-reactive, like opal, tridymite, chert, and siliceous limestone, can trigger the reaction.
The concrete must also have high alkali content, from the cement itself, or the aggregates used.
Controlling these factors to prevent AAR, by using low-alkali cement, selecting non-reactive aggregates, or incorporating supplementary cementitious materials like silica fume, fly ash, and GGBS to reduce the alkali content are vital.
What are the signs of Alkali Aggregate reaction on concrete structures?
Volumetric expansion.
Disintegration.
How can I identify Alkali Aggregate reaction on concrete structures?
Visual survey | Non-destructive testing | Destructive testing |
Expansion monitoring |
How can I prevent Alkali Aggregate reaction on concrete structures?
To prevent AAR in concrete structures, a multi-faceted approach is essential. Selection of non-reactive aggregates and use of low-alkali cement can significantly reduce the risk. Cement replacement with low alkali mineral alternatives like silica fume, fly ash, or ground granulated blast furnace slag not only reduce the overall alkali content but modify the properties of the AAR gel, making it non-expansive and less harmful.
Managing moisture is equally important, as AAR necessitates water to progress, limiting water ingress can prevent the conditions necessary for it to occur. Specific lithium compounds can mitigate the effects of AAR by counteracting the expansive nature of the gel formed during the reaction.
Regular monitoring and assessment of concrete structures for signs of AAR enables early detection and intervention, potentially saving significant repair costs and extending the lifespan of the infrastructure.
Advanced testing methods, including petrographic analysis, can assess the reactivity potential of aggregates and the condition of the concrete, guiding the choice of materials and mitigation strategies.
How can I repair the damage from Alkali Aggregate reaction on concrete structures?
Since the mechanism for AAR lies in the volume of the structure itself, once it has been recognized, it is very difficult to bring solutions.